by Paul Gilster | May 15, 2019 | Deep Sky Astronomy & Telescopes |
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.
by Paul Gilster | May 14, 2019 | Missions |
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.
by Paul Gilster | May 10, 2019 | Outer Solar System |
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.
by Paul Gilster | May 9, 2019 | Outer Solar System |
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 Hi?iaka 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).
by Paul Gilster | May 8, 2019 | Exoplanetary Science |
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).
