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Is High Definition Astrometry Ready to Fly?

In a white paper submitted to the Decadal Survey on Astronomy and Astrophysics (Astro2020), Philip Horzempa (LeMoyne College) suggests using technology originally developed for the NASA Space Interferometry Mission (SIM), along with subsequent advances, in a mission designed to exploit astrometry as an exoplanet detection mode. I’m homing in on astrometry itself in this post rather than the mission concept, for the technique may be coming into its own as an exoplanet detection method, and I’m interested in new ways to exploit it.

Astrometry is all about refining our measurement of a star’s position in the sky. When I talk to people about detecting exoplanets, I find that many confuse astrometry with radial velocity, for in loose explanatory terminology, both refer to measuring the ‘wobble’ a planet induces on a star. But radial velocity examines Doppler effects in a star’s spectrum as the star moves toward and then away from us, while astrometry looks for tiny changes in the position of the star in the sky, especially the kind of periodic shift that implies an otherwise unseen planet.

The problem has always been the level of precision needed to detect such minute variations. Long-time Centauri Dreams readers will recall Peter Van de Kamp’s work at Swarthmore on what he believed to be a planet detection around Barnard’s Star, but we can also mention Sarah Lippincott’s efforts at the same observatory on Lalande 21185. Kaj Strand, likewise working at Sproul Observatory at Swarthmore, thought he had detected a planet orbiting 61 Cygni all the way back in 1943. Instrumentation-induced errors unfortunately made these detections unlikely.

But astrometry has major advantages if we can reach the necessary accuracy, which is Horzempa’s point. The Gaia mission makes the case that astrometry is already in play in the exoplanet hunt. Thus the mission’s detections of velocity anomalies at Barnard’s Star, Tau Ceti and Ross 128. Gaia also found evidence for a possible gas giant around Epsilon Indi. For more on this, see Kervella et al., “Stellar and Substellar Companions of Nearby Stars from Gaia DR2,” Astronomy & Astrophysics Volume 623, A72 (March 2019). Abstract.

Image: Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky. This technique can also be used to identify planets around a star by measuring tiny changes in the star’s position as it wobbles around the center of mass of the planetary system. ESA’s Gaia mission, through its unprecedented all-sky survey of the position, brightness and motion of over one billion stars, is generating a large dataset from which exoplanets will be found, either through observed changes in a star’s position on the sky due to planets orbiting around it, or by a dip in its brightness as a planet transits its face. Credit: ESA.

What is significant here is that an era of astrometry 50 to 100 times more precise than Gaia may be at hand. Recall that minutes of arc are used to describe small angles. A circle of 360 degrees can be divided up into arcminutes and then arcseconds (1/60th of an arcminute). A milliarcsecond, then, is one-thousandth of an arcsecond, and a microarcsecond is one millionth of an arcsecond. You can see from this the precision needed for space-based astrometry.

Horzempa believes we are ready for 0.1 micro-arcsecond (μas), or 100 nano-arcseconds (nas) astrometry, a level anticipated for NASA’s canceled Space Interferometry Mission, and one for which hardware had already been constructed. Incorporating advances in laser metrology engineering since 2010, the Star Watch mission he is proposing would leverage the SIM work and make astrometry a valuable tool for complementing radial velocity and transit work. High-definition astrometry at 100 nas, he argues, is the best way to achieve the goal stated in a National Academy of Sciences’ 2018 report, which calls for developing ways to detect and characterize terrestrial-class planets in orbit around Sun-like stars.

Gaia is able to measure the position of brighter objects to a precision of 5 μas, with proper motions established down to 3.5 μas per year. For the faintest stars (magnitude 20.5), reports the European Space Agency, several hundred μas is the working range. Bear in mind that, according to the NAS report, 20 μas is a level of precision “sufficient to detect gas giant planets amenable to direct imaging follow-up with GSMTs” [giant segmented mirror telescopes].

If 10 microarcseconds is the size of a euro coin on the Moon as viewed from Earth, imagine what might be done with 100 nano-arcseconds, a high-definition astrometry that would offer many advantages over conventional radial velocity studies. Radial velocity can only offer a minimum mass determination because we cannot know the angle of inclination of unseen planets, and thus don’t know whether we are seeing a system edge-on or at any other angle.

Horzempa’s case is that astrometry can be used to tune up such radial velocity detections and tease out other worlds in the same systems that are undetected by RV. A case in point is Tau Ceti, which raises questions based on radial velocity work alone. From the white paper, where the author notes that this is a system that has been thought to hold 4 super-Earths::

This would be the case only if the RV m(sin i) mass values are the true values. However, if one assumes that the invariant plane for the system is the same as that of the observed disk (40 degrees), then the masses would range from 2.5 to 10 Me, putting them in the sub-Neptune to Neptune class. That would still be an interesting system but it would not be a system of super Earths. High-Definition astrometry will bring clarity, as these inner worlds of tau Ceti, if they exist, would generate signals that ranged from 1 to 15 uas, well within the reach of the next-generation 100 nano-arcsecond Probe. As a bonus, the Star Watch mission would be able to detect any true terrestrial worlds in the tau Ceti system.

Horzempa cites other intriguing possibilities, such as Zeta Reticuli, a star not known to have any planets through transit or radial velocity methods. Orbital inclination could rule out a transit detection in any case, while smaller planets might be too difficult a catch for RV around this binary system of two G-class stars in a wide orbit. 100-nas astrometry would be able, however, to detect Earth-class planets if they are there around either or both stars. If we are seeing ‘face-on’ systems, astrometry may be the only detection method applicable here.

There is no question about the advantages astrometry presents in being able to measure the true mass of planets it detects as opposed to a minimum mass, and as Horzempa writes, “…it will provide knowledge of the coplanarity of exoplanet systems, and it will discover extremely rare and valuable) planetary system ‘oddballs.’” All this shows the refinement of a method that has disappointed in the past. From the NAS report:

Astrometry has a spotty history involving exoplanet false positive detections and the cancellation of the Space Interferometry Mission. Only two “high-confidence” exoplanets have been discovered with this technique (Muterspaugh et al., 2010), neither of which has been independently confirmed. Despite its intrinsic merit, astrometry has not been viable as a search technique, given that a space mission is needed for large samples and low-mass planets.

It is a space mission that Horzempa now calls for, one building on technology developed for the Space Interferometry Mission and taking astrometry to unpreceded levels of precision. It is far too early to know how the idea will be received in the space community, but given the movement of astrometry toward the 100-nas level, a case can be made for supplementing existing methods with a strategy that at long last is beginning to deliver on its exoplanet promise. We’ll follow the progress of both astrometric measurement and this Star Watch mission concept with interest.

The paper is Horzempa, “High Definition Astrometry,” submitted to Astro2020 (Decadal Survey on Astronomy and Astrophysics) and available as a preprint.

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Insulating a Plutonian Ocean

An ocean inside Pluto would have implications for many frozen moons and dwarf planets, not to mention exoplanets where conditions at the surface are, like Pluto, inimical to life as we know it. But while a Plutonian ocean has received considerable study (see, for example, Francis Nimmo’s work as discussed in Pluto: Sputnik Planitia Gives Credence to Possible Ocean), working out the mechanisms for liquid ocean survival over these timeframes and conditions has proven challenging. A new paper now suggests a possible path.

Shunichi Kamata of Hokkaido University led the research, which includes contributions from the Tokyo Institute of Technology, Tokushima University, Osaka University, Kobe University, and the University of California, Santa Cruz. At play are computer simulations, reported in Nature Geosciences, that offer evidence for the potential role of gas hydrates (gas clathrates) in keeping a subsurface ocean from freezing. At the center of the work, as in so much recently written about Pluto, is the ellipsoidal basin that is now known as Sputnik Planitia.

Image: The bright “heart” on Pluto is located near the equator. Its left half is a big basin dubbed Sputnik Planitia. Credit: Figures created using images by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Gas hydrates are crystalline solids, formed of gas trapped within molecular water ‘cages’. The ‘host’ molecule is water, the ‘guest’ molecule a gas or a liquid. The lattice-like frame would collapse into a conventional ice crystal structure without the support of the trapped molecules.

What is illuminated by the team’s computer simulations is that such hydrates are highly viscous and offer low thermal conductivity. They could serve, in other words, as efficient insulation for an ocean beneath the ice. The location and topography of Sputnik Planitia lead the researchers to believe that if a subsurface ocean exists, the ice shell in this area is thin. Uneven on its inner surface, the shell could thus cover liquid water kept viable by the insulating gas hydrates.

We wind up with a concept for Pluto’s interior that looks like the image below.

Image: The proposed interior structure of Pluto. A thin clathrate (gas) hydrate layer works as a thermal insulator between the subsurface ocean and the ice shell, keeping the ocean from freezing. Credit: Kamata S. et al., “Pluto’s ocean is capped and insulated by gas hydrates.” Nature Geosciences, May 20, 2019 (full citation below).

Methane is implicated as the most likely gas to serve within this insulating lattice, a provocative theory because of what we know about Pluto’s atmosphere, which is poor in methane but rich in nitrogen. What exactly is happening to support this composition? From the paper:

CO2 clathrate hydrates at the seafloor could have acted as a thermostat to prevent heat transfer from the core to the ocean. Primordial CO2, however, may have been converted into CH4 through hydrothermal reactions within early Pluto under the presence of Fe–Ni metals. As CH4 and CO predominantly occupy clathrate hydrates, the components that degassed into the surface–atmosphere system would be rich in other species, such as N2.

We arrive at an atmosphere laden with nitrogen but low in methane. This notion of a thin gas hydrate layer as a ‘cap’ on a subsurface ocean is one that could serve as a generic mechanism to preserve subsurface oceans in large, icy moons and KBOs. “This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” adds Kamata.

The authors point out that freezing of the ocean, causing the ice shell to thicken, would cause the radius and surface area of Pluto to increase, producing faults on the surface. New Horizons was able to observe these, and recent studies have shown that the fault pattern supports the global expansion of the dwarf planet. Thus we have a scenario consistent with a global ocean, perhaps one that is still partly liquid. Analyzing surface changes would offer constraints on the thickness of any potential layers of clathrates that could firm up the liquid water hypothesis.

The paper is Kamata et al., “Pluto’s ocean is capped and insulated by gas hydrates,” Nature Geosciences May 20, 2019 (abstract).

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New Horizons: Results and Interpretations

Another reminder that the days of the lone scientist making breakthroughs in his or her solitary lab are today counterbalanced by the vast team effort required for many experiments to continue. Thus the armies involved in gravitational wave astronomy, and the demands for big money and large populations of researchers at our particle accelerators. So, too, with space exploration, as the arrival of early results from New Horizons in the journals is making clear.

We now have a paper on our mission to Pluto/Charon and the Kuiper Belt that bears the stamp of more than 200 co-authors, representing 40 institutions. How could it be otherwise if we are to credit the many team members who played a role? As the New Horizons site notes: “[Mission principal investigator Alan] Stern’s paper includes authors from the science, spacecraft, operations, mission design, management and communications teams, as well as collaborators, such as contributing scientist and stereo imaging specialist (and legendary Queen guitarist) Brian May, NASA Planetary Division Director Lori Glaze, NASA Chief Scientist Jim Green, and NASA Associate Administrator for the Science Mission Directorate Thomas Zurbuchen.”

New Horizons still has the capacity to surprise us, or maybe ‘awe’ is the better word. That’s what I felt when I saw the now familiar shape of Ultima Thule highlighted on the May 17 Science cover in an image that shows us what the object would look like to the human eye. The paper presents the first peer-reviewed scientific results and interpretations of Ultima a scant four months after the flyby.

As to the beauty of that cover image, something like this was far from my mind during the 2015 Pluto/Charon flyby, when the occasional talk ran to an extended mission and a new Kuiper Belt target. I wouldn’t have expected anything with this degree of clarity or depth of scientific return from what was at that time only a possible future rendezvous, and one that would not be easy to realize. Another indication of how outstanding New Horizons has been and continues to be.

Image: This composite image of the primordial contact binary Kuiper Belt Object 2014 MU69 (nicknamed Ultima Thule) – featured on the cover of the May 17 issue of the journal Science – was compiled from data obtained by NASA’s New Horizons spacecraft as it flew by the object on Jan. 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute//Roman Tkachenko.

I’d be hard-pressed to come up with a space-themed cover from Science that’s more memorable than that. For that matter, here’s an entertaining thought: What would the most memorable space-themed Science cover of, say, 2075 look like? Or 2250? Let’s hope we’re vigorously exploring regions from the Oort Cloud outward by the latter year.

Meanwhile, New Horizons is helping us look back toward our system’s earliest days.

“We’re looking into the well-preserved remnants of the ancient past,” says Stern. “There is no doubt that the discoveries made about Ultima Thule are going to advance theories of solar system formation.”

Indeed. The two lobes of this 36-kilometer long object include an oddly flat surface (nicknamed ‘Ultima’) and the much rounder ‘Thule,’ the two connected by a ‘neck’ that raises the question of how the KBO originally formed. We seem to be looking at a low velocity merger of two objects that had originally been in rotation. Their merger is important; remember, we’re dealing with an ancient planetesimal. From the paper:

For MU69 ’s two lobes to reach their current, merged spin state, they must have lost angular momentum if they initially formed as co-orbiting bodies. The lack of detected satellites of MU69 may imply ancient angular momentum sink(s) via (i) the ejection of formerly co-orbiting smaller bodies by Ultima and Thule, (ii) gas drag, or both. This suggests that contact binaries may be rare in CCKBO [Cold Classical Kuiper Belt Object] systems with orbiting satellites. Another possibility, however, is that the lobes Ultima and Thule impacted one another multiple times, shedding mass along with angular momentum before making final contact. But the alignment of the principal axes of MU69 ’s two lobes tends to disfavor this hypothesis.

Also interesting is the possibility of tidal locking of the two lobes before their merger:

In contrast, tidal locking could quite plausibly have produced the principal axis alignment we observe, once the co-orbiting bodies were close enough and spin-orbit coupling was most effective… Gas drag could also have played a role in fostering the observed coplanar alignment of the Ultima and Thule lobes… Post-merger impacts may have also somewhat affected the observed, final angular momentum state.

And let’s consider what the science team has to say about Ultima Thule’s color. Like many other objects found in the Kuiper Belt, it’s red, much redder than Pluto. This obviously has significant interest because of what it implies about the organic materials on the surface and how they are being modified by solar radiation. Here the mixture of surface organics differs from other icy objects previously examined, though not all:

There are key color slope similarities between the spectrum of MU69 and those of the KBO (55638) 2002 VE 95 (45) and the escaped KBO 5145 Pholus… Also, these objects all exhibit an absorption band near 2.3 mm, tentatively attributed to methanol (CH3OH) or perhaps more complex organic molecules intermediate in mass between simple molecular ices and tholins… Similar spectral features are also apparent on the large, dark red equatorial region of Pluto informally called Cthulhu… suggesting similarities in the chemical feedstock and processes that could operate there and on MU69.

The paper points out that we see evidence for H2O ice in the form of shallow spectral absorption features, indicating low abundance in the object’s upper surface, as compared with planetary satellites and even some Kuiper Belt objects with clearer H2O detections. As expected, we do not see the spectral signatures of volatile ices such as carbon monoxide, nitrogen, ammonia or methane, which were observed at Pluto. These were not expected at MU69 “…owing to the ready thermally driven escape of such ices from this object over time.”

We’re left with a lot of unanswered questions, including the nature and origin of surface features like ‘Maryland crater,’ the largest (8 kilometer wide) scar on the KBO, likely the result of an impact, and the various bright spots and patches. How much of the pitted surface can be related to outgassing via sublimation, or material falling into already existing underground spaces? Bear in mind that the data return from New Horizons is to continue until late summer of 2020. Meanwhile, the spacecraft continues to measure the brightness of other Kuiper Belt objects while also mapping the charged-particle radiation and dust environment of its surroundings.

The paper is Stern et al., “Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object,” Science Vol. 364, Issue 6441 (17 May 2019). Abstract.

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A Neutrino Beam Beacon

If you want to look for possible artifacts of advanced civilizations, as do those practicing what is now being called Dysonian SETI, then it pays to listen to the father of the field. My friend Al Jackson has done so and offers a Dyson quote to lead off his new paper: “So the first rule of my game is: think of the biggest possible artificial activities with limits set only by the laws of physics and look for those.” Dyson wrote that in a 1966 paper that repays study today (citation below). Its title: The Search for Extraterrestrial Technology.” Dysonian SETI is a big, brawny zone where speculation is coin of the realm and the imagination is encouraged to be pushed to the limit.

Jackson is intrigued, as are so many of us, with the idea of using the Sun’s gravitational lens to make observations of other stars and their planets. Our recent email conversation brought up the name of Von Eshleman, the Stanford electrical engineer and pioneer in planetary and radio sciences who died two years ago in Palo Alto at 93. Eshleman was writing about gravitational lensing possibilities at a time when we had no technologies that could take us to 550 AU and beyond, the area where lensing effects begin to be felt, but he saw that an instrument there could make observations of objects directly behind the Sun as their light was focused by it.

Claudio Maccone has been working this terrain for a long time, and the complete concept is laid out in his seminal Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer Praxis, 2009). There is much to be said about lensing and space missions, and it’s heartening to see interest in scientists within the Breakthrough Starshot project — a sail moving at 20 percent of lightspeed gets us to 550 AU and beyond relatively quickly. By my back of the envelope figuring, travel time is just a little short of 16 days.

There would be no need for Starshot to approach 550 AU at 20 percent of c, of course. The focal line runs to infinity, but as Jackson explains when running through gravitational lensing’s calculations, we can assume beam intensity gradually diminished by absorption in the interstellar medium, though all of this with little beam divergence. Just how to use the Sun’s gravity lens (a relay for returning data from a star mission, I assume) and how to configure mission parameters to get to the lensing region and use it are under debate.

Transmitting Through a Gravitational Lens

But back to Al Jackson’s paper, which offers us a take on gravitational lensing that I have never before encountered [see my addendum at the end of this post for a correction]. What he is proposing is that an advanced civilization of the kind Dyson is interested in would have the capability of using a gravitational lens to transmit data. He’s turned the process around, from observation to beacon or other form of communication. And he’s working with neutrinos, where attenuation from the interstellar medium is negligible.

A gravitational lens does not, of course, need to be a star, but could be a higher mass object like a neutron star. In Jackson’s thinking, a KII civilization could place a neutrino beam transmitting station around a neutron star. We make neutrino beams today via the decay of pi mesons, as the author reminds us, when large accelerators boost protons to relativistic energies that strike a target, producing pions and kaons that decay into neutrinos, electrons and muons. What counts for Jackson’s purposes is that pions and kaons can be focused to produce a beam of neutrinos.

For a stellar mass gravitational lens and 1 Gev neutrinos, the wavelength is about 10-14 cm, the gain is approximately 1020! The characteristic radius of main region of concertation is about one micron; however there is an effective flux out to about one centimeter.

And as mentioned above:

This beam intensity extends to infinity only diminished by absorption in the interstellar medium, encounters with a massive object like a planet or star and a very small beam divergence.

Image: This is Figure 6 from the paper. Caption: A schematic illustration of a possible neutrino accelerator-transmitter, the accelerator and lens (nothing to scale). Credit: A. A. Jackson.

A one-centimeter beam creates the problem of focusing on a specific target, one whose phenomenal pointing accuracy could only be left to our putative advanced civilization. Even so, increase the number of transmitters and detection becomes easier. Thus Jackson:

Suppose that a K2 type civilization capable of interstellar flight can reach a neutron star it should have the technological capability to build a beacon consisting of an array of transmitters in a constellation of orbits about the neutron star. Let this constellation consist of 1018 ‘neutrino’ transmitters 1 meter in characteristic size ‘covering’ the area of a sphere 1000 km in radius with 1018 particle accelerators in orbit… At the present time there is the development of plasma Wakefield particle accelerators that are meters in size [21, 22]. It is probable that a K2 civilization may construct Wakefield electron accelerators of very small size.

Jackson has heeded Dyson, that’s for sure. Remember the latter’s injunction: “…think of the biggest possible artificial activities with limits set only by the laws of physics and look for those.”

What emerges is a ‘constellation of neutrino beam transmitters,’ 1018 in orbit at 1000 neutron star radii, increasing the probability of detection, so that the detection rate at 10,000 light years becomes approximately 5 per minute. The transmitters must be configured to appear as point sources to the gravitational lens, again another leap demanding KII levels of performance. But if a civilization is trying to be noticed, a neutrino beam that takes neutrino detection well out of the range produced through stellar events in the galaxy should stand out.

Thus we have a method for producing directed neutrino signal transmission. Why come up with such? Harking back to Dyson, we can consider the need to examine the range of possible ETI technologies, hoping to create a catalog that could explain future anomalous observations. Jackson’s beam could be used as what he calls a ‘honey pot’ to attract attention to an electromagnetic transmitter broadcasting more sophisticated information. But we would not necessarily be able to understand what uses an advanced civilization would make of such capabilities. We might have to content ourselves merely with the possibility of observing them.

As Jackson points out, a KII civilization “…would likely have the resources to finesse the technology in a smarter way,” so what we have here is a demonstration that a thing may be possible, while we are left to wonder in what other ways a neutrino source can be used to produce detections at cosmic distances. Going deep to speculate on technologies far beyond our reach today should, as Dyson says, remind us to stay within the realm of known physics while simultaneously asking about phenomena that advanced engineering could produce. We hope through the labors of Dysonian SETI to recognize such signatures if we see them.

Appreciating Von Eshleman

And a digression: When Al brought up Von Eshleman in our conversations, I began thinking back to early gravitational lensing work and Eshleman’s paper, written back in 1979 but prescient, surely, of what was to come in the form of serious examination of lensing capabilities for space missions. Let me quote Eshleman’s conclusion from the paper, cited below. I have a lot more to say about Eshleman’s work, but let’s get into that at another time. For now:

It has been pointed out that radio, television, radar, microwave link, and other terrestrial transmissions are expanding into space at 1 light-year per year (2). Another technological society near a neighboring star could receive the strongest of these directly with substantial effort and could learn a great deal about the earth and the technology of its inhabitants. The concepts presented here suggest that on an imaginary screen sufficiently far behind that star, the short-wavelength end of this terrestrial activity is now being played out at substantial amplifications. Properly placed receivers with antennas of modest size could in principle scan the earth and discriminate between different sources, mapping such activity over the earth and learning not only about the technology of its inhabitants, but also about their thoughts. It is possible that several or many such focused stories about other worlds are now running their course on such a gigantic screen surrounding our sun, but no one in this theater is observing them…

I learned about Eshleman’s work originally through Claudio Maccone and often reflect on the tenacity of ideas as we go from a concept originally suggested by Einstein to a SETI opportunity realized by Eshleman to a mission concept detailed by Maccone, and now a potential actual mission seriously discussing using gravitational lensing as a relay opportunity for data return from Proxima Centauri (Breakthrough Starshot). As it always has, the interstellar field demands long-term thinking that crosses generations in support of a breathtaking goal.

[Addendum]: Although I hadn’t heard of discussions on using gravitational lensing for transmission, an email just now from Clément Vidal points out that both Claudio Maccone and Vidal himself have looked into this. In Clément’s case, the reference is Vidal, C. 2011, “Black Holes: Attractors for Intelligence?” at Towards a scientific and societal agenda on extra-terrestrial life, 4-5 Oct, Buckinghamshire, Kavli Royal Society International Centre. Abstract here. This quote is to the point:

“For a few decades, researchers have proposed to use the Sun as a gravitational lens. At 22.45AU and 29.59AU we have a focus for gravitational waves and neutrinos. Starting from 550AU, electromagnetic waves converge. Those focus regions offer one of the greatest opportunity for astronomy and astrophysics, offering gains from 2 to 9 orders of magnitude compared to Earth-based telescopes…It is also worth noting that such gravitational lensing could also be used for communication…Indeed, it is easy to extrapolate the maximal capacity of gravitational lensing using, instead of the Sun, a much more massive object, i.e. a neutron star or a black hole. This would probably constitute the most powerful possible telescope. This possibility was envisioned -yet not developed- by Von Eshleman in (1991). Since objects observed by gravitational lensing must be aligned, we can imagine an additional dilating and contracting focal sphere or artificial swarm around a black hole, thereby observing the universe in all directions and depths.”

The author of The Beginning and the End: The Meaning of Life in a Cosmological Perspective (2014), Vidal’s thinking is examined in The Zen of SETI and elsewhere in the archives.

I’m glad Clément wrote, especially as it jogged my memory about Claudio Maccone’s paper on using lenses as a communications tool, where the possibilities are striking. See The Gravitational Lens and Communications for more. I wrote about this back in 2009 and thus have no good excuse for letting it slip my mind!

The paper is Jackson, “A Neutrino Beacon” (preprint). The Dyson paper is “The Search for Extraterrestrial Technology,” in Marshak, ed. Perspectives in Modern Physics: Essays in Honor of Hans Bethe, New York: John Wiley & Sons 1966. The Eshleman paper is “Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances,” Science Vol. 205 (14 September 1979), pp. 1133-1135 (abstract).

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Survivors: White Dwarf Planets

The term ‘destruction radius’ around a star sounds like something out of a generic science fiction movie, probably one with lots of laser battles and starship crews dressed in capes. It’s a descriptive phrase as used in this University of Warwick (UK) news release, but let’s go with ‘Roche radius’ instead. Dimitri Veras, a physicist at the university, probes the term in the context of white dwarfs in a new paper for Monthly Notices of the Royal Astronomical Society. Veras and collaborators are looking at what happens after the challenging transition between red giant and white dwarf, a time when planets will be in high turmoil.

The idea is to model the tidal forces that occur once a star collapses into a super-dense white dwarf, blowing away its outer layers in the process. We see the clear potential for dragging planets into new orbits, with some pushed out of their stellar systems entirely. The Roche radius, or limit, is the distance from the star where a self-gravitating object will disintegrate because of tidal forces, forming a disk of debris around the star. It’s not limited to white dwarfs, of course, and can be applied to the tidal forces acting between any two celestial bodies.

White dwarfs are generally comparable in size to the Earth, while their Roche limit extends outwards to about one stellar radius. They’re also known to have metallic debris in their photosphere, presumably from objects pulled within the Roche radius. Learning about the tidal interactions of such worlds will help us calculate a planet’s inward and/or outward drift, and determine whether an object in a particular orbit will reach the Roche radius and be destroyed.

Image: An asteroid torn apart by the strong gravity of a white dwarf has formed a ring of dust particles and debris orbiting the Earth-sized burnt out stellar core. Credit: University of Warwick/Mark Garlick.

I’m not aware of a lot of work on tidal effects in white dwarf scenarios, but Veras believes that now is the time to begin such investigations, for the number of planets around white dwarfs is sure to rise with the advent of new extremely large telescopes (ELTs). We learn here that according to the authors’ models, the more massive a planet, the more likely it will be destroyed by these tidal forces. We do have to factor in the fact that some simplifications needed to run these calculations will need to be further developed to help us characterize such systems.

“Our study, while sophisticated in several respects, only treats homogenous rocky planets that are consistent in their structure throughout,” adds Veras. “A multi-layer planet, like Earth, would be significantly more complicated to calculate but we are investigating the feasibility of doing so too.”

How likely are planets or asteroids to be pulled into the Roche limit? Tidal forces modeled here imply that an object of low viscosity — think of Enceladus as a local example — is ultimately absorbed by the star if it orbits at separations within five times the distance between the center of the white dwarf and the Roche limit. By contrast, a high viscosity world is likely to be eventually swallowed into the star only if it is found at distances closer than twice the separation between the center of the white dwarf and Roche radius. Viscosity counts. Observationally, we might be on the lookout for disruptions in white dwarf disks just outside the Roche limit.

By ‘high viscosity world,’ the paper refers to planets with a dense core of heavy elements, such as the iron- and nickel-rich SDSS J122859.93+104032.9, recently found in a star-grazing 2-hour orbit around a white dwarf by astronomers at the same university (for more, see White Dwarf Debris Suggests a Common Destiny — Veras was on the team that did this work).

The current work suggests that this object has avoided falling into the star because of its small size. Or as the paper puts it: “Because the magnitude of the stellar tides scales as the mass of the perturber, the orbital dynamics of the asteroids in the WD 1145+017 and SDSS J1228+1040 systems are unaffected by stellar tides.” The takeaway: The best chance for survival near the white dwarf is to be small and dense, packed with heavy elements. And a little distance doesn’t hurt. A distance of just .13 AU, a third of the distance between Mercury and the Sun, is enough to ensure survival for the kind of rocky, homogeneous planet this paper models.

We learn that massive super-Earths are more readily disrupted than lower-mass planets, while as seen above, planets of higher viscosity are more likely to survive than their low-viscosity counterparts. We’re dealing with a challenging environment indeed for surviving objects, but one that repays investigation considering its ubiquity. As the paper points out, almost every known exoplanet currently orbits a star that will become a white dwarf. The authors continue:

Exo-asteroids are already observed orbiting two white dwarfs in real time… Planets which then survive to the white dwarf phase play a crucial role in frequently shepherding asteroids and their observable detritus on to white dwarf atmospheres, even if the planets themselves lie just outside of the narrow range of detectability. Further, planets themselves may occasionally shower a white dwarf with metal constituents through post-impact crater ejecta and when the planetary orbit grazes the star’s Roche radius…

The paper’s goal is to create a computational framework for future observations that can grow out of this early study of a two-body system comprising an idealized solid planet and a white dwarf. Of note: “…the boundary between survival and destruction is likely to be fractal and chaotic,” reinforcing the challenge of characterizing these maximally stressed stellar systems.

An interesting question: Can second- or third-generation planets form from white dwarf debris disks? The paper briefly considers the possibility and notes that such worlds would be in near-circular orbits and possibly be out of the reach of detection by current technologies.

The paper is Veras et al., “Orbital relaxation and excitation of planets tidally interacting with white dwarfs,” Monthly Notices of the Royal Astronomical Society Vol. 486, Issue 3 (July, 2019), pp. 3831-3848 (abstract).

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Toward a High-Velocity Astronomy

Couple the beam from a 100 gigawatt laser with a single-layer lightsail and remarkable things can happen. As envisioned by scientists working with Breakthrough Starshot, a highly reflective sail made incredibly thin — perhaps formed out of graphene and no thicker than a single molecule — could attain speeds of 20 percent of c. That’s good enough to carry a gram-scale payload to the nearest stars, the Alpha Centauri triple system, with a cruise time of 20 years, for a flyby followed by an agonizingly slow but eventually complete data return.

A key element in the concept, as we saw yesterday, is the payload, which could take advantage of microminiaturization trends that, assuming they continue, could make a functional spacecraft smaller than a cell phone. The first iterations of such a ‘starchip’ are being tested. The Starshot work has likewise caught the attention of Bing Zhang, a professor of astrophysics at the University of Nevada, Las Vegas. Working with Kunyang Li (Georgia Institute of Technology) Zhang explores in a new paper the kind of astronomy that could be done by such a craft.

For getting to Proxima Centauri for the exploration of its interesting planet involves a journey that could itself provide a useful scientific return. The paper’s title, “Relativistic Astronomy,” flags its intent to study how movement at relativistic speeds would affect images taken by its camera. As Zhang explains in a recent essay on his work in The Conversation, when moving at 20 percent of lightspeed, an observer in the rest frame of the camera would experience the universe moving at an equivalent speed in the opposite direction to the camera’s motion.

Relativistic astronomy, then, explores these different spacetimes to observe objects we are familiar with from our Earth-based perspective as they are seen in the camera’s rest frame. Zhang and Li consider this “a new mode to study astronomy.” Zhang goes on to say:

…a relativistic camera would naturally serve as a spectrograph, allowing researchers to look at an intrinsically redder band of light. It would act as a lens, magnifying the amount of light it collects. And it would be a wide-field camera, letting astronomers observe more objects within the same field of view of the camera.

Image: Observed image of nearby galaxy M51 on the left. On the right, how the image would look through a camera moving at half the speed of light: brighter, bluer and with the stars in the galaxy closer together. Zhang & Li, 2018, The Astrophysical Journal, 854, 123, CC BY-ND.

Such observations become intriguing when we consider how light from the early universe is red-shifted as a result of the expansion of the cosmos. Zhang and Li point out that a camera moving at the relativistic speeds of the Proxima Centauri probe sees this redshifted light becoming bluer, counteracting the effect of the universe’s expansion. Light from the early universe that would have had to be studied at infrared wavelengths would now be susceptible to study in visible light. The camera, then, becomes a spectrograph allowing the observation of everything from remote galaxies to the cosmic microwave background.

Moreover, other relativistic effects come into play that add value to the fast camera. From the paper on this work:

…unique observations can be carried out thanks to several relativistic effects. In particular, due to Doppler blueshift and intensity boosting, one can use a camera sensitive to the optical band to study the near-IR bands. The light aberration effect also effectively increases the field of view of the camera since astronomical objects are packed in the direction of the camera motion, allowing a more efficient way of studying astronomical objects.

Let me depart for a moment from the Zhang and Li paper to pull information from a University of California at Riverside site, a page written by Alexis Brandeker, and presumably illustrated by him. In the figure below, we see only the effect of aberration at a range of velocities. Notice how the field becomes squeezed at we move from 0.5 c to 0.99 c. At 0.99 c, almost all visible radiation from the universe is confined to a region 10 degrees in radius around the direction of travel.

Image: This figure shows aberration effects for the ship travelling towards the constellation of Orion, assuming a 30 degrees field of view. The field of view is kept constant, only the speed is changed from 0 to 0.99c, showing dramatic effects on the perceived field. No radiative effects are considered, only geometrical aberration. Credit: Alexis Brandeker/UC-R.

But to get the overview, we have to fold in Doppler effects as infrared radiation is shifted into the visible. If we combine these effects in a single image, we get the startling view below.

Image: Both relativistic effects switched on. Credit: Alexis Brandeker/UC-R.

But back to Zhang and Li, whose camera aboard the probe is a spectrograph, a lens, and a wide-field camera all in one. The authors make the case that fast-moving cameras can likewise be used to probe the so-called ‘redshift desert’ (at 1.4 ≲ z ≲ 2.5) that coincides with the epoch of significant star formation (the name comes from the lack of strong spectral lines in the optical band here). Lacking data, we have no large sample of galaxies in a particular range of redshifts, which hinders our understanding of star formation.

Zhang and Li consider relativistic observations of gamma-ray bursts (GRBs) at extreme redshifts, as well as tracing the electromagnetic counterparts to gravitational wave events. Thus a Breakthrough Starshot payload enroute to Alpha Centauri offers a new kind of astronomy if we can master the construction of a camera that can withstand a journey through the interstellar medium without damage from dust as well as one that can transmit its data back to Earth.

What struck me as I began reading this paper is that when it comes to relativistic effects, 20 percent of lightspeed is actually on the slow side, making me wonder how much better the kind of observations the authors describe would be at higher velocities. But Zhang and Li move straight to this question, describing the relativistic effect of a Starshot probe as ‘mild,’ and noting that a Breakthrough laser infrastructure might be used for faster, dedicated astronomy missions.

If one drops the goal of reaching Alpha Centauri, cameras with even higher Doppler factors may be designed and launched. A Doppler factor of 2 and 3 (which gives a factor of 2 and 3 shift of the spectrum) is available at 60% and 80% speed of light, respectively. More interesting astronomical observations can be carried out at these speeds.

While probes in this range would demand ever more powerful acceleration from their laser energy source, they might actually be easier to build, for the need for cosmic ray shielding on a long cruise or data transmission at interstellar distances would be alleviated by sending them on missions closer to home. Of course, pushing probes to speeds much higher than 20 percent of c is even more problematic than the Centauri mission itself. Beyond Starshot, the authors argue that relativistic astronomy will repay the effort if we continue to push in the direction of beamed laser probes with an eye toward ever faster, more capable missions.

The paper is Zhang & Li, “Relativistic Astronomy,” Astrophysical Journal Vol. 854, No. 2 (20 February 2018). Abstract.

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Recent tests of a ‘wafer-craft’, an early prototype for what may one day be the ‘starchip’ envisioned by scientists involved with the Breakthrough Starshot project, have been successful. The work grows out of a NASA-funded effort led by Philip Lubin (UC Santa Barbara), whose investigations into large scale directed energy systems began in 2009. Lubin went on to perform multiple studies for NASA’s Innovative Advanced Concepts program developing the idea that would become known as DEEP-IN (Directed Energy Propulsion for Interstellar Exploration). His NIAC Phase 1 report studied as one option beamed propulsion driving a wafer-scale spacecraft.

Renamed Starlight, the proposal went on to Phase II funding as well as support from the private sector. A subsequent review by Breakthrough Initiatives led to endorsement of the concept within its Breakthrough Starshot effort. Breakthrough is devoting $100 million to studying the viability of sending a ‘starchip’ to a nearby star such as Proxima Centauri, a mission that, moving at 20 percent of lightspeed through laser-beamed propulsion, would arrive at its target within 20 years as opposed to the tens of thousands of years required for chemical propulsion.

But back to that prototype wafer-scale spacecraft, whose launch was conducted in collaboration with the United States Naval Academy. The craft rose into the stratosphere above Pennsylvania via balloon on April 12, 2019 — the 50th anniversary of the Gagarin flight — reaching an altitude of 32 kilometers. The test is part of what Lubin calls “a long-term program to develop miniature spacecraft for interplanetary and eventually for interstellar flight.”

Nic Rupert, a development engineer at UC Santa Barbara, describes the wafer-craft in its current incarnation:

“It was designed to have many of the functions of much larger spacecraft, such as imaging, data transmission, including laser communications, attitude determination and magnetic field sensing. Due to the rapid advancements in microelectronics we can shrink a spacecraft into a much smaller format than has been done before for specialized applications such as ours.”

Image: An artist’s concept of the wafer-craft. Credit: UC Santa Barbara.

The good news is that the chip performed exceptionally well, returning over 4,000 images of Earth by way of testing what may eventually emerge as a space technology that could turn interstellar. The process is iterative, working with off-the-shelf components that can be pushed to increasingly difficult conditions that will test the wafer-craft’s viability under extreme conditions of temperature and radiation, as well as its potential to survive impact with dust particles.

In other words, to get to ‘starchips,’ we must first get to ‘spacechips,’ and that begins with balloon flights well within the atmosphere to shake out early data on performance. The goal: A one-gram chip that contains within itself a functional spacecraft. At UC-Santa Barbara, an undergraduate group drawing on students from physics, engineering, chemistry and biology is conducting the balloon flights that may result in future, mass-produced interstellar probes.

This news release from UC-Santa Barbara notes that the ramping up of testing points to a suborbital flight next year. Early applications of the technology should involve missions closer to home; indeed, a laser beaming infrastructure would have applications for fast interplanetary travel as well as planetary defense against asteroids and other space debris. Thus testing funded by NASA and private foundations examines the viability of miniaturized spacecraft that, given the beaming resources, could one day give us a close-up look at Proxima Centauri b.

For more on the Starlight effort, visit its website.

Tomorrow I’ll keep the focus on fast interstellar missions in the form of some interesting ideas from a paper by Bing Zhang, an astrophysicist from the University of Nevada, on what he is calling ‘relativistic astronomy’ and its possibilities on the way to distant destinations.

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Europa’s Oxygen and Aerobic Life

Few destinations in the Solar System have excited the imagination as much as Europa. Could a deep ocean beneath the ice support a biosphere utterly unlike our own? If so, we could be looking at a second emergence of life unrelated to anything on Earth, with implications for the likelihood of life throughout the cosmos. But so much depends on what happens as Europa’s surface and ocean interact. Alex Tolley, a fixture here on Centauri Dreams, today looks at new work suggesting the deeply problematic nature of Europa’s ocean from the standpoint of astrobiology. He also offers an entertaining glimpse at what Europa might become.

by Alex Tolley

Image: Plume on Europa’s Surface. Credit: NASA

With the abundance of newly discovered exoplanets, a fraction of them being both rocky and in their habitable zones (HZ), the excitement at finding life on such worlds is increasing. Given the ambiguous results of the attempt to detect life on Mars with the Viking experiments in 1976 and the subsequent NASA missions to look for proxies rather than direct detection, it is only to be expected that astrobiologists turned their attention to these exoplanets.

We tend to think of life primarily in terms of metazoa rather than unicellular organisms, so the search for life generally focuses on finding evidence of free oxygen (O2), even though aerobic metazoa only appeared on Earth within the last billion years. But detecting proxies for life is not the same as studying it directly. Therefore a search for metazoa in our own solar system would offer the opportunity to sample extraterrestrial life well before we get such an opportunity from an exoplanet.

The discovery of ecosystems in the near lightless abyssal depths of Earth’s oceans around “hot smokers” has stimulated new hypotheses concerning abiogenesis and extended the known environments for extremophile life. Anaerobic bacteria feeding on the chemical brew from the vents become the primary food of aerobic metazoans living around these smokers. On Earth, all but a few metazoa are aerobic [5], as the higher energy from this mode of respiration allows faster growth and reproduction, as well as active behaviors. With the discovery that some icy moons around Jupiter and Saturn have subsurface oceans and active geologies, it seemed possible that these moons might harbor life too, and therefore offer a local, solar system destination to discover life and return samples. Because terrestrial metazoans are aerobes, the presence of oxygen in the icy moons would be a positive indication that there may be metazoan life forms as well as microbes.

Of the icy moons that might have oxidizing oceans Europa is the clear favorite.

Fictional Interlude 1

The Europa Oxygen and Life Surveyor (EOLiS) probe swung by Europa. Earthside Mission Control had done all it could to ensure the craft had successfully reached its target for orbital insertion. At 13:31:07 UTC the lander released itself from the orbiter, deployed its own magnetic radiation shield, and fired its braking rockets. Now the lander’s onboard AI became fully autonomous as it guided the craft towards the preselected surface destination, fusing its sensor data, radar and vision, to locate its surface landing point. The mother craft would remain in orbit and release 4 more communication relay satellites to maintain uninterrupted communications with Earth. The lander quickly reached the surface, mere meters from its preferred landing spot, and in the smoothest terrain within its target radius.

One of the mission objectives was to determine the depth profile of oxidants in the surface ice, a key variable for the oxygen levels in the subsurface ocean, and a factor for the evolution of metazoa. A nuclear fission-heated probe slowly melted its way down through the ice. Measurements of free O2, H2O2, and other oxidizing molecules were continually taken. Initial readouts indicated that the very high oxygen levels on the surface were not maintained below a few meters of the surface.

After 60 orbits around Jupiter, the lander had transmitted its findings back to mission control. Oxygen was mostly in the top meter of ice and snow, but in much lower concentrations down to at least the 3 kilometers its probe had penetrated. Despite this, the oxygen levels were still of the order of grams per cubic meter of ice. The planetologists inferred that the Europan ocean was likely still anoxic. The astrobiologists had to content themselves that maybe anaerobic microbes were still possible. The lander had performed well in its primary mission, so the extended mission included boring down to the base of the surface ice and into the ocean below.

Fact

Europa’s surface ice is subjected to about 0.125 W/m2 of ion radiation, radiolytically producing oxygen on the surface and a very thin atmosphere (the mechanism shown in figure 1).

Figure 1. A simplified sequence for radiolytic production and destruction of water, H2O2, and molecular oxygen. Some H2O2 and O2 become sequestered below the heavily processed surface. UV, ultraviolet. [Hand et al – [3]]

Hand estimated up to 7.6% oxidant contaminants in the surface ice, and up to 53% of the surface in the form of clathrate cages containing oxygen and other oxidants [3]. Most important for life is this trapping of oxygen that might find its way to the subsurface ocean. The relatively young surface of Europa is criss-crossed by ridges due to liquid or slush being pushed up from below, freezing and overlaying the surface. These clathrates would descend and oxygenate the subsurface oceans due to the resurfacing. The base of the ice crust melts into the ocean, a cycle that takes between 20 and 500 million years.

Hand’s estimates placed the rate of production of O2 at 2E-7 to 8E-7 kg/m2/yr.

Greenberg was very much more positive about ocean oxygenation, suggesting that the oceans could reach Earth levels of O2 saturation (around 10 mg/L) well within the rapid resurfacing rate times for the clathrates to reach the oceans, in about 10 million years [6].

In 2016, Vance and Hand continued to use Hand’s earlier O2 production rates of 3E-7 to 3E-4 kg/ m2/ yr.

These estimates were based on assumptions about how the tenuous atmosphere was maintained. If it was primarily due to radiolysis, then subsurface radiolysis to produce O2 would result in trapping of the O2 for transport to the ocean. This would support the calculations by Vance of a rapidly oxidizing ocean that could support aerobic life, if not a rich as Earth’s, then at least within striking distance of abyssal life density.

These estimates may have been optimistic.

In recent papers [1,2], R E Johnson and A Oza call into question this model. They simulate the atmosphere and find that the best explanation for the atmosphere is thermal release of the O2 from the surface ice by desorption. This implies that far less O2 is trapped in the ice grains that can be subducted to the oceans.

Johnson:

This assumes that Europa’s ice regolith is permeated with trapped O2, which could also affect our understanding of the suggestion that the radiolytic products in Europa’s regolith might be a source of oxidants for its underground ocean.

While the O2 is produced within the top meter of ice, gas diffusion prevents loss of O2, and regolith subduction and mixing draw down the O2 into the lower depths. Gardening only allows mixing to about 10 meters, but resurfacing due to upwelling at the ridges results in the O2 to be drawn down to the base of the ice sheet and enter the oceans below on timescales of tens to hundreds of billions of years.

Johnson et al:

Although direct diffusion to the depth of the ocean is likely problematic, geologic mixing and subduction of oxygen rich ice has been suggested as a possible source of oxidants for putative ocean biology.

Oza and Johnson’s previous paper [2] estimated production of O2 on Europa was just 0.1-100 kg/s, or about 3E6 to 3E9 kg O2 /yr (Earth year) or 1E-7 to 1E-4 kg/m2/yr. Their mechanism is explained in figure 3 below. They argue that thermally desorbed O2 from the ice best explains the atmospheric dynamics over a Europan day, and therefore the O2 at depth is less than previous estimates and models suggest.

Figure 2. Schematic diagram of O2 trapping and thermal desorption: 1) Primary origin of O2 (and H2) is magnetospheric ion radiolysis. 2) Due to preferential loss of H2, the regolith becomes oxygen rich enhancing the production of O2. Formed and returning O2 can become trapped at incomplete (dangling) H bonds (shown) as well as in voids (as shown and observed by Spencer & Calvin 2002). 3) The accumulated O2 can then be thermally desorbed from the weak dangling bonds due to solar heating, maintaining a quasi vapor pressure equilibrium (Oza et al. 2018a), with a smaller gas-phase contribution from direct sputtering of O2. A fraction of the trapped O2. is likely subducted. [Johnson et al – [1]]

Fictional Interlude 2

The probe ran the samples from the bore hole through a battery of molecule and life detectors. While the usual mix of carbon compounds that could be found on any icy body, including comets and asteroids were present, none registered anything definitive for life. Asymmetry in organic molecules’ chirality was absent, as were odd lipid chain lengths. None of the growth experiments registered any change. Like the previous disappointment with Mars, the hopes of the early 21st century astrobiologists to find life in the icy moons were frustrated. Europa had so far proven sterile. Neither was there any unambiguous evidence of prebiotic chemistry.

The top kilometer of ocean below the ice crust proved still rather anoxic compared to the ice above it. The sheer volume of the ocean, plus the reducing nature of the vent emissions kept the oxygen levels well below that of the terrestrial oceans. Coupled with the absence of any signatures of microbial life, it was clear that there could not be multicellular life in that ocean.

While disappointing to the biologists, this finding indicated that there would be no violation of a putative “Prime Directive” should colonization be attempted.

Fact

Whatever the amount of O2 trapped in the ice, it is the production rate of O2 that determines the steady state in a biosphere, even if accumulation can create highly oxic conditions in the oceans suitable for aerobic life to exist.

Therefore the key question is just how much O2 is produced by radiolysis? Let me put that in perspective, given the earlier conclusions, especially the optimistic ones of Greenberg.

On Earth, photolytic O2 production is insignificant compared to that from photosynthesis. Earth’s environment was largely anoxic for billions of years, with aerobic, multicellular life only appearing in the fossil record less than a billion years ago and flowering in the Cambrian as O2 levels increased. This was a result of the evolution of photosynthesis, which is the dominant source of Earth’s O2.

On Earth, net primary production (carbon fixation by photosynthesis minus plant respiration) creates about 3E14 kg O2/yr, or about 0.65 kg /M2/yr averaged over the total Earth’s surface. It is about a tenth as much if all respiration from heterotrophs and saprophytes is included.[7].

Therefore the rate of O2 production on Europa is 3-6 orders of magnitude lower than Earth’s net primary production of released O2. The difference between Earth’s and Europa’s O2 production is somewhat larger than Vance’s suggestion that Europa’s O2 production is about 1% of Earth’s. Therefore, even without any other sinks, Europa’s O2 production is 1/1000th that of Earth, at best, on an area based comparison, and possibly just a millionth at worst. Johnson’s analysis of likely lower O2 concentrations in the surface would further reduce the subduction rate of O2.

The implication for life in Europa is that the production of oxygen via radiolysis is clearly insufficient to replace photosynthetic organisms that produce the oxygen in quantities to support aerobic life on Earth, even those most adapted to low concentrations, such as sessile invertebrates.

While Greenberg has suggested that photosynthetic life might reach just below the surface to add primary production to the oceanic organisms below the surface, it is more likely that if life exists at all, it is going to be anaerobic bacteria, like those of the Archaean in Earth’s history. If that is correct, any ocean vents may have bacteria, but aerobic metazoa will not be present around them as they are on Earth now.

How does this impact the search for life in Europa? If life is either absent or anaerobic, the fanciful suggestion by Freeman Dyson that we might look for fish remains ejected from the ocean is likely futile. As all but a few terrestrial metazoa are aerobic, the lack of significant O2 production seems to diminish any likelihood that Europa hosts large animals as suggested by Clarke [9]:

Suddenly, a vast bulk broke through the surface of the ocean and arched into the sky. For a moment, the whole monstrous shape was suspended between air and water.

The familiar can be as shocking as the strange – when it is in the wrong place. Both captain and doctor exclaimed simultaneously: ‘It’s a shark!’

Image: Europa deep ocean vent visited by a robotic submersible. Credit: NASA/JPL

However, there is the possibility that without a sink via consumption, free O2 could just accumulate over the eons and possibly jumpstart the greening of Europa’s oceans.

The O2 level in Earth’s oceans is saturated around 10 mg/L at 0 degrees C and declines with rising temperatures. Active vertebrates like fish need around 4 mg/L, much more than the 1% saturation required by sessile invertebrates like sponges.

Using Oza and Johnson’s estimated range of 0.1 – 100 kg/s O2 production on Europa by radiolysis, and ignoring issues of reduced surface concentration levels, and other sinks for O2, Europa’s ocean would reach saturation at 10 mg/L in 3 million to 3 billion years. The time required is due to the immense volume of the estimated 100 km deep ocean, 2-3x as great as Earth’s oceans.

However, the rich O2 levels in the ice might range from 0.01 to 10 kg/m3. Melted, this ice would provide for a more than adequate level of oxygen saturation for terrestrial fish. Adding terrestrial life to such lakes would quickly deplete the O2 levels. New oxygen would have to be added by either maintaining a rate of ice melting or adding photosynthetic organisms.

If this analysis is correct, while it seems to rule out a rich aerobic ecology today, it does not preclude one tomorrow, if the production rates of O2 could be enhanced.

Fictional Interlude 3

“Sub One operating nominally,” intoned a somewhat bored Thomas Roberts. He was lead eco-engineer at Nagata base on Callisto, well clear of the intense, deadly radiation from Jupiter that was the key to the greening of Europa. Roberts team was monitoring the newly created subsurface lake christened Dodon Lake, known more colloquially as “dee-el” by his co-workers. It was situated in the Conamara Chaos and radar imaging had indicated it was now about 1 kilometer long and half a kilometer wide, with a maximum depth of 10 meters, laying just 20 meters below Europa’s surface near what appeared to be an old plume vent. The shallow depth of the lake beneath Europa’s surface ensured both sufficient radiation shielding as well as relatively easy access via the fractured ice in the vent.

The lake had started out as a natural fracture below the surface. Nuclear generators had melted the ice at the base of the fracture, creating a freshwater lake that was saturated with oxygen, and with more than enough extra oxygen to fill the void above it with breathable air. Preliminary tests indicated the water column was now mostly freshwater, not unlike that of L. Vostok in Antarctica, although with more dissolved CO2 and SO4. The dissolved O2 was at saturation. All that was missing was enough nitrogen and phosphorus, as well as trace minerals, to make this a living lake like those in Northern Canada, albeit without the summer mosquitoes. It was as dark as any subterranean cave on Earth, although that would soon change. 10 submersibles with high intensity LED lamps had been lowered down from the surface and had swarmed out across the surface of the lake, guided by the swarm intelligence of their onboard AI. The juice needed to power the motors and lights came from small nuclear reactors which fed waste heat to the lake bottom to increase the O2 release.

When the first sub powered up its lights, for the first time since its formation, the lake became a wonderland, illuminated with purple light, whose red and blue wavelengths were suited to maximize the photosynthesis that was to come. A cocktail of single cell algae originally sourced from subsurface Antarctic and Greenland lakes and cryogenically stored during transit, was released from the subs and soon began to photosynthesize and reproduce near the lights.

After a week, the crystal clear water started to become faintly cloudy as the density of algae increased to become the needed food for the large variety of invertebrates that followed. After a month, Thomas was certain that there would be no need to tweak the nutrients that had been added to the lake. The relatively simple starter ecosystem was on the predicted growth path that would reach its stable state cycle in 2 years. Within a decade, it was expected that a stable ecology would be established with sufficient oxygen production to maintain the first seeding of fish. But that was a job for the next crew of engineers to baby. The first steps to the greening of Europa had begun.

References

1. Johnson, R.E. et al (2019) “The Origin and Fate of O2 in Europa’s Ice: An Atmospheric Perspective,” Space Sci Rev (2019) 215:20 DOI 10.1007/s11214-019-0582-1

2. Oza A P et al (2019) “Dusk Over Dawn O2 Asymmetry in Europa’s Near-Surface Atmosphere,” Planetary and Space Science 167 23-32

3. Hand, K. P., Chyba, C. F., Carlson, R. W., & Cooper, J. F. (2006). “Clathrate Hydrates of Oxidants in the Ice Shell of Europa,” Astrobiology, 6(3), 463–482. doi:10.1089/ast.2006.6.463; Davis, J C, (1975) “Minimal Dissolved Oxygen Requirements of Aquatic Life with Emphasis on Canadian Species: a Review,” J. Fish Res. Bd. Can. Vol. 32(12)

4. Danovaro, R., Dell’Anno, A., Pusceddu, A., Gambi, C., Heiner, I., & Kristensen, R. M. (2010). “The first metazoa living in permanently anoxic conditions,” BMC Biology, 8(1), 30. doi:10.1186/1741-7007-8-30

5. Greenberg, R., (2010) “Transport rates of radiolytic substances into Europa’s ocean – Implications for the potential origin and maintenance of life,” Astrogiology Vol. 10, Number 3, 2010. DOI: 10.1089/ast.2009.0386

6. Huang J, et all (2018) “The global oxygen budget and its future projection,” Science Bull.. v63:18 pp1180-1186 https://doi.org/10.1016/j.scib.2018.07.023

7. Vance, S. D., K. P. Hand, and R. T. Pappalardo (2016), “Geophysical controls of chemical disequilibria in Europa,” Geophys. Res. Lett., 43, 4871–4879, doi:10.1002/2016GL068547.

8. Clarke, A C. 2061: Odyssey 3. Ballantine Books, 1987.

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Haumea: Probing an Outer System Ring

I rarely get the chance to talk about the exotic dwarf planet Haumea, but it’s a personal favorite when it comes to the outer Solar System. That’s because of its odd shape (a bit like an American football), evidently the result of a catastrophic collision, which makes it an interesting object for close study if we can get a probe to it to examine its composition. Back in 2009, Joel Poncy and colleagues at Thales Alenia Space in France went to work on a fast orbiter mission, an extraordinarily tough challenge that would push our propulsion technologies hard.

But Haumea would surely repay close study. A rapid rotator (3.9 hours, itself a likely indicator of a turbulent past), it’s a dwarf world with a ring as well as two moons, the larger of which, Hi’iaka, is some 300 kilometers in diameter. Add to this the fact that Haumea is quite reflective, indicating a surface covered with crystalline water ice. We know we can get a probe to Haumea, but orbiting it is an order of magnitude tougher. But imagine what we might learn about planetesimal differentiation! For more, see Fast Orbiter to Haumea and Haumea: Technique and Rationale. I provide the citation below for the appearance of Poncy’s later paper in Acta Astronautica.

Meanwhile, we study Haumea from afar. Othon Cabo Winter (São Paulo State University) and colleagues home in on Haumea’s ring in a new paper in Monthly Notices of the Royal Astronomical Society. The ring may be evidence of Haumea’s violent past, but we’re learning that rings are anything but uncommon in the Solar System. Beyond Saturn we have identified many ringed objects, including Uranus, Neptune and Jupiter, as well as the asteroids Chariklo and Chiron, which orbit between Jupiter and Neptune. Doubtless we’ll find more.

Image: Haumea and its satellites, imaged on June 30, 2015 by the Hubble Space Telescope. The moon Hiʻiaka is above Haumea, with the other moon Namaka below. The ring is too narrow and tenuous to be visible. Credit: NASA / STScI.

The ring around Haumea has yet to be directly observed, but astronomers reported its existence in 2017 after studying the result of an occultation as Haumea passed in front of a distant star. You’ll recall how useful occultations turned out to be for the New Horizons mission, accurately determining the shape of Ultima Thule long before the spacecraft made its successful flyby. The thought after the Haumea occultation was that the ring was in a 1:3 resonance region, with the ring particles making one revolution every three times the dwarf planet rotates.

But how significant is this resonance? Winter and team examine the question by simulating the trajectories of particles in the ring region, showing that for the resonance to define the ring particle orbits, a degree of eccentricity would be demanded that is not found — the ring is both narrow and practically circular. Thus the resonance does not define the ring’s orbit; rather, the ring is in a stable orbit the authors identify near the resonance region. Says Winter:

“Our study isn’t observational. We did not directly observe the ring. No one ever has. Our study is entirely computational. Based on simulations using the available data on Haumea and the ring, subject to Newton’s law of gravitation, which describes the motions of the planets, we concluded that the ring isn’t in that region of space owing to the 1:3 resonance but owing to a family of stable periodic orbits.”

A bit more on the terminology here. The authors describe Haumea’s ring as being in a family of “first-kind periodic orbits.” The paper relies upon techniques developed by Poincaré to analyze the dynamics of the region in which the Haumea ring is located. Using Poincaré’s methodology, they explain that orbits of the first kind are those “originated from particles initially in stable circular orbits.” This distinguishes them from periodic orbits of the second kind, which are those in which “…the particles are in eccentric orbits in a mean motion resonance, the so-called resonant periodic orbits.” Stability arises either from a highly eccentric orbit forced by resonance or a low eccentricity orbit, likewise stable, but not forced by the resonance.

We wind up with an orbit for ring particles that is of the first kind in what the authors call an ‘island of stability.’ Other factors could influence the orbit, but the paper notes that a ring of only 70 kilometers in width would have to be extremely massive to itself reduce the eccentricity produced by the 1:3 resonance.

And the authors continue:

Collisions between the ring particles were also not considered in this work. They could allow large orbital eccentricities of particles at the 1:3 resonance to be damped and fit within the radial range of the ring. Nevertheless, particles at the ring borders would not remain confined, still needing a confining mechanism. However, independently of collisions, particles associated with first-kind periodic orbits define regions of stability that fit very well in size and location with Haumea’s ring. Therefore, this analysis suggests that Haumea’s ring is in a stable region associated with a first-kind periodic orbit instead of the 1:3 resonance.

Haumea’s ring was the first discovered around a Trans-Neptunian Object, and we should bear in mind how challenging these calculations have to be to account for the non-spherical nature of the parent body. We know that the ring plane lines up with Haumea’s equatorial plane as well as the orbital plane of the outer moon Hi’iaka. What Winter and colleagues have shown is that the ring particles are on circular orbits that are near but not actually inside the 1:3 resonance.

The paper is Winter et al., “On the location of the ring around the dwarf planet Haumea,” Monthly Notices of the Royal Astronomical Society Volume 484, Issue 3 (April 2019), pp. 3765–3771 (abstract). Also of interest: Araujo et al., “Rings under close encounters with the giant planets: Chariklo vs Chiron,” accepted for publication at MNRAS (preprint). Joel Poncy’s paper on a Haumea mission is “A preliminary assessment of an orbiter in the Haumean system: How quickly can a planetary orbiter reach such a distant target? Acta Astronautica Vol. 68, Issues 5-6 (March-April 2011), pp. 622-628 (abstract).

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Planetary Interiors a Key to Habitability

Interdisciplinary approaches to new data offer a robust way to see past the conventions of a specialized field, noting connections that provide perspective and deepen understanding. That idea is sound across many disciplines, but it is getting new emphasis with an essay in Science asking whether we have not been too blinkered in our approach to astrobiology. After all, reams have been written about studying exoplanet atmospheres for biomarkers, but shouldn’t we be studying how atmospheres couple to planetary interiors?

“We need a better understanding of how a planet’s composition and interior influence its habitability, starting with Earth,” says Anat Shahar (Carnegie Institution for Science), one of the paper’s four authors. “This can be used to guide the search for exoplanets and star systems where life could thrive, signatures of which could be detected by telescopes.”

Thus the paper’s call for merging data from astronomical observations, mathematical modeling and simulations, and laboratory experiments on planetary interiors. We can assume key building blocks of rocky planets like those similar to Earth, knowing to expect silicon, magnesium, hydrogen, iron, oxygen and carbon. But each planet will have its own specific abundances, its own history shaped by its position in its stellar system and its interior chemistry, all of which will help to determine whether or not it has oceans, their size, and the nature of its atmosphere.

Shahar, along with Carnegie’s Peter Driscoll, Alycia Weinberger, and George Cody, proceed to explain the significance of understanding these factors if we want to make the call on habitability, citing the range of outcomes possible from different compositions:

Composition determines the internal material properties associated with heat and mass transport, like melting temperature, thermal and electrical conductivity, viscosity, and the abundance and partitioning of radiogenic isotopes. These properties control the heat budget and thermal evolution of a planet. The amount of water accreted during formation will affect the ocean volume at the surface, which in turn is influenced by water cycling between the surface and the deep Earth. The composition and subsequent partitioning of elements in the interior will determine the oxidation state of the mantle and therefore whether the species that are outgassed to the atmosphere are enriched or reduced (11). The physical parameters of high-pressure phases of rock that might exist in deep exoplanetary mantles control their water capacity, rate of heat transfer, likelihood of global convection, and rate of core cooling.

This figure from the paper illustrates the significance of plate tectonics:

Image Credit: N. Desai/Science.

The contingent nature of planetary evolution is clear as we study what can happen to a world over billions of years in the evolution from protoplanet through differentiation of the interior, impact history and the emergence of plate tectonics and development of a magnetic field. What the authors are arguing is that coherent research on these matters is not the work of a single discipline. Indeed:

Observations of stellar, disk, and planetesimal compositions must be combined with experimental studies of mineral physics and melting behavior to serve as inputs to planet formation and geodynamic models. In turn, the results of those modeling efforts will provide feedbacks into the observations and experiments by making predictions and identifying the compositions and material properties that are most important for habitability.

So as we learn about exoplanetary atmospheres, and we are on the edge of great strides in this area with the next generation of large ground- and space-based telescopes, we’ll need to put what we learn in the context of planetary interiors and their role in evolving a life-sustaining atmosphere. The idea that habitability is hugely influenced by planetary interiors is sensible, even obvious — think of the Earth without plate tectonics — but our approach to these habitability questions will surely be enriched by crossover studies of the kind the authors describe.

After all, as opposed to straight characterization of an atmosphere, learning about the interior planetary processes needed for life will be difficult. We can make the first call based on our evaluation of planet densities, available through combined transit and radial velocity studies. But density gives us only a crude insight into planetary composition. Our best recourse, then, is the combination of modeling, experimentation, and observations that will help us learn whether planets unlike our own may still have internal processes that can support and sustain life.

The paper is Shahar et al., “What makes a planet habitable?” Science Vol. 364, Issue 6439 (03 May 2019), pp. 434-435 (full text).

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