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It’s always good to have eyes and ears on the ground at events I can’t get to, so I was pleased when Aleksandar Shulevski contacted me with the offer to send back notes from the European Space Agency’s Advanced Concepts Team Interstellar Workshop in Noordwijk in the Netherlands. Born and raised in Bitola, Republic of Macedonia, Aleksandar is a science fiction reader and amateur astronomer who followed up electrical engineering studies in Skopje with an MSc in astronomy at Leiden University (Netherlands), dealing with calibration issues on the LOFAR radio telescope. He received a PhD from the University of Groningen, doing research on active galactic nuclei radio remnants observed with LOFAR. After working at the Netherlands Institute for Radio Astronomy (ASTRON) as junior telescope scientist, he is now a research scientist at the Anton Pannekoek Institute for Astronomy at the University of Amsterdam, specializing in low-frequency radio transients and pursuing his interest in SETI. Aleksandar could only attend half of the workshop, and unfortunately no video is available, but I’m told a fall issue of Acta Futura should include papers on many of the topics covered in Noordwijk.

by Aleksandar Shulevski

I have attended only the first day of the Interstellar Exploration workshop, organized by the Advanced Concepts Team (ACT) at the ESTEC ESA site in Noordwijk. Even though I follow the goings on in the community, I must admit that I almost missed the workshop; maybe the organizers should have advertised it more widely. Obviously this is just a personal opinion.

The program indicated potentially an interesting gathering, and I am pleased to say that I was not disappointed. The auditorium was packed, I estimate that more that the total number of participants was above sixty. The technical director of ESA opened the workshop, remarking that interstellar topics are more and more in the limelight, especially since Breakthrough Initiatives launched Breakthrough Starshot: “It’s never too early to start discussing interstellar matters”.

Michael Hippke started the morning session with a popular talk on interstellar communication. He gave a thorough overview on the history of thought dealing with the problem of communicating with other worlds. The radio window was de-emphasised on account of more exotic techniques like X-ray lasers, neutrinos etc. Michael thus established the “out of the box” thinking needed for such a meeting.

Rob Swinney (BIS) gave a historical overview of fusion propulsion concepts, covering the design of the Daedalus craft, as well as the successor design effort (Icarus). His remarks were in line with his enthusiasm: “We tend to underestimate what we can do on long timescales”.

The ongoing effort funded by NASA to outline and design a mission concept for a probe to 1000 AU was the topic of the presentation by Pontus Brandt. The tentative launch date is to be sometime in the 2030s. Pontus went over past missions which are on their way out of the Solar System (Pioneer, Voyager, New Horizons), tracing their origins to a strategy for Solar System research outlined in the Simpson committee report of 1960. The committee was chaired by John A. Simpson and James A. Van Allen as part of the contribution of the Space Studies Board, which was established in 1958 to focus on space research for the National Academies of Sciences, Engineering, and Medicine.

Long term thinking is key, along with lessons learned from past experience. In a set of stunning visuals, Brandt outlined current plans. Most likely the probe will form its trajectory by a Jupiter gravity assist, burning its final fuel supply there to achieve final escape velocity considerably greater than that of Voyager 2, reaching interstellar space in 15 years. Mission goals are observations of the infrared background as well as taking the first comprehensive “outside view” of the Solar system.

Michael Waltemathe discussed the philosophical and religious aspects of interstellar travel. What if any ETIs in existence are morally superior? Will humanity export the concept of original sin among the stars? Issues like planetary protection in the interstellar context were raised, and obviously, there were few definite answers.

Phil Lubin’s presentation dealt with directed energy propulsion, discussing detailed concept designs on Breakthrough Starshot, specifically the emitter array. Excellent beam handling was reported, and no issues (apart from funding) are show-stoppers on the path of scaling up to the operational laser system. However, an issue that warrants serious research effort is the communication subsystem. Laser comms will suffer from unattainable pointing accuracy requirements. The lack of deceleration in the target system may be problematic, but potentially overcome by launching a huge number of StarChips in succession, thus replacing the need for orbiting craft. The beamer system has tangential benefits as well, like Solar System exploration applications, and planetary defense. Long term R&D commitment is needed to realize the full potential of this effort.

[PG: I notice that MIT Technology Review just posted an article looking at some of the issues involved in getting this kind of laser array operational, while reviewing other issues re Starshot that Aleksandar mentions].

The morning session was followed by a less structured period for the duration of which multiple discussion groups were formed upon the suggestion of interested attendees who proposed topics. The organizers suggested most of the questions that needed to be addressed at these sessions. I proposed to discuss the propagation effects ISM plasma would have on the radio link used for communication with a Breakthrough Starshot style StarChip. There are multiple papers on the topic and we went over David Messerschmitt’s concepts of signal conditioning, discussed at length in his book featured on Centauri Dreams in the past [for more on Messerschmitt, see for example, Is Energy a Key to Interstellar Communication?]

The discussion period was followed by a panel on which each discussion topic had a representative, and the audience had a chance to ask questions. The afternoon plenary session began with a talk by Andreas Hein on worldships, in which he presented results from his latest paper in Acta Futura. Everything else being equal, and assuming economic growth continues unabated, Hein believes Earth will be able to build a worldship in a few hundred years, although the concept of a worldship may be by then outpaced by more practical developments.

Angelo Vermeulen presented the research done by him and collaborators at TU Delft and elsewhere on evolving spacecraft which borrow from biology to convert asteroids to interstellar craft. The study focused on the life support aspects of the problem and went on to model at length various constraints like (for example) the impact of the mineral content of the asteroid being used on mission viability.

Jeffrey Punske focused on language development on generation ships. Illustrating the various (and not always obvious) reasons for language drift, he concluded that it is inevitable that the language of a generation ship will evolve so far from Earth standard in a matter of a few hundred years as to render communication between the two unintelligible, something which mission planners should take into account. He dismissed the notion that a universal translator device is a viable development. If communication becomes a ritual performed by a priestly caste on the ship using the archaic language of the predecessors, maybe communication will still be possible in some form even after the divergence occurs.

Elke Hemminger addressed the sociology of interstellar exploration. Using an interactive approach, she engaged the audience in discussing philosophical topics. What if the values we hold most dear in our current societal organization are incompatible with the mission requirements of a colony ship? What are we willing to sacrifice to make the mission a successful one? Individual freedom? Is it worth it to go to the stars if we make such sacrifices?

These are more than abstract topics and may have more immediate concern on the human condition. It can easily turn out that dealing with climate change will require modification of our values. It may well be that totalitarian societies are more suited to dealing with emergencies than democracies. How do we balance our social values against the imperative of survival?

ESA’s Advanced Concepts Team closed the session by outlining the results of the latest Global Trajectory Optimisation Competition, this time dealing with Galaxy colonization, in which three types of interstellar generational vessels are imagined to be sent around the Milky Way in a quest 90 million years long. This was the 10th edition of the GTOC competition, hosted this year by the Mission Design and Navigation section at the Jet Propulsion Laboratory. China’s solution won, in a very interesting and difficult problem posed by JPL. An August workshop will allow top teams from the competition to present papers at the Astrodynamics Specialist Conference in Maine.

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Titan and Astrobiology

Night launches are spectacular, that’s for sure, especially with a rocket as muscular as the SpaceX Falcon Heavy. Less spectacular, at least at this point in my life, is staying up until 0230, but delays are part of the rocket business, and what counts is a launch successful in everything but the return of the center booster (both side boosters landed upright at Cape Canaveral). Prox-1, the carrier vehicle for The Planetary Society’s LightSail 2, was released at 720 kilometers, with deployment of the sail itself scheduled for July 2.

While we wait for LightSail developments and also follow the fortunes of NASA’s Deep Space Atomic Clock, launched as one of 24 satellites deployed by this bird, the 2019 Astrobiology Science Conference in Seattle draws attention this morning with new information about Saturn’s tantalizing moon Titan. I’m still having to adapt to not having Cassini in Saturn space, but without its presence scientists are proceeding with laboratory studies that re-create conditions like those on the surface of Titan. Among their findings: Compounds and minerals not found on Earth, including a ‘co-crystal’ composed of solid acetylene and butane.

I had to look this one up. A co-crystal (often written without the hyphen) is a molecular crystal with multiple components. An evidently authoritative source (G.P. Stahly, writing in a journal called Crystal Growth & Design in 2007), says that co-crystals “consist of two or more components that form a unique crystalline structure having unique properties.” In the case of the work presented by JPL’s Morgan Cable at the Astrobiology Science Conference, both acetylene and butane appear as gases on Earth, but on Titan, they are solid and combine to form crystals.

The mineral Cable’s team identified in the lab may account for a surface feature found in Cassini imagery in the form of orange shadings around the edges of Titan’s seas and lakes. We know that some of these areas show signs of evaporated material left behind as the liquid receded. These bright features show up in data from the spacecraft’s Visual and Infrared Mapping Spectrometer (VIMS) and Synthetic Aperture Radar (SAR) around lakes in the north polar region. The thinking is that organic molecules generated in the atmosphere and deposited on the surface dissolve in liquid methane and ethane and precipitate when it evaporates.

Image: A false-color, near infrared view of Titan’s northern hemisphere collected by NASA’s Cassini spacecraft shows the moon’s seas and lakes. Orange areas near some of them may be deposits of organic evaporite minerals left behind by receding liquid hydrocarbon. Credit: NASA / JPL-Caltech / Space Science Institute.

To investigate, the scientists used a cryostat filled with liquid nitrogen, gradually warming the chamber until the nitrogen turned to gas, and then adding methane and ethane along with other carbon-containing molecules. Benzene crystals emerged from the mix, creating a co-crystal with ethane molecules. The acetylene and butane co-crystal, likely more common on Titan given the moon’s composition, was then discovered. Other undiscovered hydrocarbon crystals may exist, say the researchers, although finding them will require a mission to Titan’s shorelines.

At surface temperatures in the range of 90 K, the authors believe, the acetylene-butane co-crystal “…may be ubiquitous in these regions of Titan’s surface.” Cable and colleagues are interested in chemical gradients that could potentially be exploited by life, as their presentation notes:

The catalytic hydrogenation of acetylene has been proposed as a possible energy-yielding reaction for metabolism (Schulze-Makuch et al. 2005; McKay et al. 2005; Tokano et al. 2009). It is possible that acetylene-based co-crystals might be a mechanism for storing acetylene, in a manner similar to how carbon dioxide is stored in carbonate deposits on Earth, where it might be more readily accessible to a putative microbial community.

What a complex place we’re dealing with, one whose atmosphere undergoes chemical reactions that produce organic molecules that wind up on the surface, where chemical reactions may occur as the landscape is manipulated by frigid winds and rain. A liquid water ocean beneath the surface may exchange materials with the surface over long timeframes. Being discussed at the conference is the question of whether conditions on Titan could produce the chemical complexity and particularly the energy life would demand. We’re a long way from the answer.

The presentation is Cable et al., “The Acetylene-Butane Co-Crystal: A Potentially Abundant Molecular Mineral on Titan,” with abstract here.

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Into the Uranian Rings

Both DSAC (the Deep Space Atomic Clock) and LightSail 2 are on the line when a SpaceX Falcon Heavy launches on Monday evening. Both missions portend interesting developments in our push to deep space, with DSAC testing our ability to extend navigational autonomy, and LightSail 2 a solar sail that will use the power of solar photons to raise its orbit. You can follow the launch (now scheduled for 2330 Eastern (0330 UTC) on NASA Live. Also lifting off with the Falcon Heavy from the Kennedy Space Center will be almost two dozen other satellites, a nod both to the Falcon Heavy’s capabilities but also to increasing spacecraft miniaturization.

And speaking of interesting missions, here’s something good about one whose anniversary we’re about to celebrate. My friend Al Jackson, who served as astronaut trainer on the Lunar Module Simulator in the Apollo days, passed along a link to the Air-to-Ground Loop and the Flight Director’s Loop from Apollo 11. Give yourself 20 minutes or so and don’t miss this:

https://www.firstmenonthemoon.com/

On to today’s topic.

Return to the Planet of Doubt

I first mentioned Stanley Weinbaum’s story “The Planet of Doubt” back in 2011, certainly the most memorable monicker Uranus has ever received, even if Weinbaum’s Uranus doesn’t hold a candle to later imaginings from Geoff Landis and, in particular, Gerald Nordley (everyone should read Landis’ “Into the Blue Abyss” and Nordley’s brilliant “Into the Miranda Rift.”). And then there is Kim Stanley Robinson’s The Memory of Whiteness (1985), a vivid reading memory from a trip with my oldest son to Maine, where I couldn’t put it down. This is how it begins:

Dear Reader, two whitsuns orbit the planet Uranus; one is called Puck, the other, Bottom. They burn just above the swirling clouds of that giant planet, and with the help of the planet’s soft green light they illuminate all that dark corner of the solar system. Basking in the green glow of this trio are a host of worlds — little worlds, to be sure, worlds no bigger (and many smaller) than the asteroid Vesta — but worlds, nevertheless, each of them encased in a clear sphere of air like little villages in glass paperweights, and each of them a culture and society unto itself. These worlds orbit in ellipses just outside the narrow white bands of Uranus’s rings; you might say that the band of worlds forms a new ring in the planet’s old girdle: the first dozen made of ice chunks held in smooth planes, the newest made of an irregular string of soap bubbles, filled with life. And what holds all these various worlds together, what is their lingua franca? Music.

That’s quite a start, and it certainly had me hooked that summer up near Blue Hill. The Memory of Whiteness is one of the ultra-rare pieces of fiction in which the Uranian rings appear, which is why I quote it, because we’re learning more about the rings through the work of a team of scientists from UC-Berkeley and the University of Leicester in the UK, who have released updated views of the system. Here we get a glimpse of the thermal glow of these dim structures, a tough catch in visible light as well as near-infrared for any but the largest telescopes. The instruments in play are the Atacama Large Millimeter/submillimeter Array (ALMA) as well as the Very Large Telescope (VLT).

Results from both sets of observations appeared this week in The Astronomical Journal; note that the VLT work was done in 2017 using the VISIR instrument, a spectrometer working at infrared wavelengths that is now, in upgraded form, in use by the NEAR project in the search for planets around Centauri A and B. This is the first time that the temperature of these rings has been measured, at 77 Kelvin, which is the boiling temperature of liquid nitrogen. Prior images have all shown the rings in reflected sunlight. The new work offers millimeter (ALMA) and mid-infrared imaging (VLT/VISIR) to produce what you see below.

Image: Composite image of Uranus’s atmosphere and rings at radio wavelengths, taken with the ALMA array in December 2017. The image shows thermal emission, or heat, from the rings of Uranus for the first time, enabling scientists to determine their temperature: a frigid 77 Kelvin (-320 F). Dark bands in Uranus’s atmosphere at these wavelengths show the presence of molecules that absorb radio waves, in particular hydrogen sulfide gas. Bright regions like the north polar spot (yellow spot at right, because Uranus is tipped on its side) contain very few of these molecules. Credit: UC Berkeley. Image by Edward Molter and Imke de Pater.

We also learn a good deal about the composition of the rings, which differs from what we see elsewhere in the Solar system. Imke de Pater (UC-Berkeley), a co-author of the paper on this work, points to the range of particle sizes from meters down to micron-size in Saturn’s innermost D ring up to tens of meters in the main rings. At Uranus, the smaller size particles are missing in the epsilon ring, the brightest and densest of the Uranian rings, which appears to be made up of particles no smaller than golf balls. A dust component does show up between the main rings.

Graduate student Edward Molter is lead author:

“We already know that the epsilon ring is a bit weird, because we don’t see the smaller stuff. Something has been sweeping the smaller stuff out, or it’s all glomming together. We just don’t know. This is a step toward understanding their composition and whether all of the rings came from the same source material, or are different for each ring.”

These rings are also dark as charcoal, as well as being quite narrow compared to the rings of Saturn. The epsilon ring is the widest, varying from 20 to 100 kilometers in width, while Saturn’s rings can range from hundreds to tens of thousands of kilometers wide. 13 rings have thus far been found around Uranus, which Voyager flew past in 1986, likewise finding a scarcity of dust-sized particles. This sets Uranus apart from the other ice giant, for Neptune’s rings are largely dust, while gas giant Jupiter’s are mostly made up of particles of micron size.

Image: The Uranian ring system captured at different wavelengths by the ALMA and VLT telescopes. The planet itself is masked since it is very bright compared to the rings. Credit: Edward Molter, Imke de Pater, Michael Roman and Leigh Fletcher, 2019.

While the observations here are not sensitive to dust down to micron-size, the results are consistent with studies of the rings in optical and near-infrared reflected light that indicate no dust of this size is present. Here’s the paper’s conclusion:

The consensus between our millimeter and mid-infrared observations and literature visible-wavelength observations shows that the properties of the main rings remain the same at any observed wavelength despite the fact that our observations are not sensitive to micron-sized dust. This finding confirms the hypothesis, proposed based on radio occultation results (Gresh et al. 1989), that the main rings are composed of centimeter-sized or larger particles. A simple thermal model similar to the NEATM model for asteroids was applied to determine that the ring particles display roughly black-body behavior at millimeter/mid-infrared wavelengths at a temperature of 77 ± 2 K, suggesting the particles’ thermal inertia may be small enough and rotation rate slow enough, to induce longitudinal temperature differences between their dayside and nightside.

The image below, taken from earlier work, offers a near-infrared view of the rings that shows some of the interleaved dust between them, with the epsilon ring at the bottom.

Image: Near-infrared image of the Uranian ring system taken with the adaptive optics system on the 10-meter Keck telescope in Hawaii in July 2004. The image shows reflected sunlight. In between the main rings, which are composed of centimeter-sized or larger particles, sheets of dust can be seen. The epsilon ring seen in new thermal images is at the bottom. Credit: UC Berkeley image by Imke de Pater, Seran Gibbard and Heidi Hammel, 2006.

The paper is Molter et al., “Thermal Emission from the Uranian Ring System,” accepted at The Astrophysical Journal. Preprint.

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Ring Imagery from Cassini’s Deep Dive

Cassini’s productivity at Saturn continues to provide fodder for scientific papers and encouragement for the builders of complex missions, who have seen enough data gathered by this one to guarantee continuing insights into the ringed planet for years to come. The June 14 issue of Science offers up four papers (citations below) that show results from four of the spacecraft’s instruments, including startling views of the main rings.

The data examined in the Science papers were gathered during Cassini’s ring-grazing orbits from December, 2016 to April of 2017 as well as during the ‘Grand Finale’ between April and September of 2017, when Cassini flew closer than ever before to the giant planet’s cloud tops. Consider the image below, showing an infrared view as captured by the spacecraft’s Visible and Infrared Mapping Spectrometer (VIMS), with (at the left) the natural color view taken as a composite by Cassini’s Imaging Science Subsystem.

Imaging the rings in visible and near-infrared wavelengths, VIMS surprised scientists by finding weak water ice bands in the outer parts of the A ring, a seeming contradiction given the highly reflective nature of this area, which has been taken to indicate less contaminated ice and stronger water ice bands. While water ice is dominant in the rings, the spectral map shows no ammonia or methane ice, nor does it see organic compounds. This is an oddity given that organics have been found flowing out of the D ring into Saturn’s atmosphere.

“If organics were there in large amounts — at least in the main A, B and C rings — we’d see them,” says Phil Nicholson, Cassini VIMS scientist of Cornell University in Ithaca, New York. “I’m not convinced yet that they are a major component of the main rings.”

Image: The false-color image at right shows spectral mapping of Saturn’s A, B and C rings, captured by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). It displays an infrared view of the rings, rather than an image in visible light. The blue-green areas are the regions with the purest water ice and/or largest grain size (primarily the A and B rings), while the reddish color indicates increasing amounts of non-icy material and/or smaller grain sizes (primarily in the C ring and Cassini Division). At left, the same image is overlaid on a natural-color mosaic of Saturn taken by Cassini’s Imaging Science Subsystem. Credit: NASA/JPL-Caltech/University of Arizona/CNRS/LPG-Nantes. Saturn image credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic.

The rings, now believed to have formed much later than the planet itself, continue to deliver complexities that will feed into future models of ring evolution. The last phase of Cassini’s mission is, in the words of project scientist Linda Spilker (JPL), author of one of the papers in Science, “like turning the power up one more notch on what we could see in the rings,” a higher resolution that even as it answers some questions raises still more.

At the outer edge of the main rings, for example, impact-generated streaks appear in the F ring that are of approximately equal length and similar orientation, suggesting a string of impactors striking at the same time. The implication: The F ring, at least, is shaped by materials already orbiting Saturn rather than by materials the planet encounters in its orbit of the Sun. Small moons embedded in the rings can be considered in that sense to be sculpting them, according to Cassini scientist Matt Tiscareno (SETI Institute). Here we might see useful analogs in young exoplanet systems, where stellar systems form out of circumstellar disks that are in turn shaped by the emerging planets within them, processes out of which our own Solar System formed.

We also see how remarkably ring textures can differ even in segments close to each other, as seen in the image below. Teasing out the interactions that shaped the patterns found here will take us deeply into chemistry and temperature changes as mapped by Cassini.

Image: New images of Saturn’s rings show how textures differ even in close proximity of one another. The image on the right has been filtered so that the newly visible straw-like textures and clumps are more visible. Credit: NASA/JPL-Caltech/Space Science Institute.

Now have a look at Daphnis, one of the embedded moons, which is here shown creating three waves in the outer edge of the Keeler Gap, at the outer edge of the A ring. The spacecraft was at a shallow 15 degree angle above the rings when the image was taken. The images here were taken in visible light using Cassini’s narrow-angle camera at a distance of about 28,000 kilometers from Daphnis and at a Sun-Daphnis-spacecraft phase angle of 71 degrees.

Image: This enhanced-color image mosaic shows Daphnis, one of the moons embedded in Saturn’s rings, in the Keeler Gap on the sunlit side of the rings. Daphnis is seen kicking up three waves in the gap’s outer edge. Three wave crests of diminishing sizes trail the moon. In each successive crest, the shape of the wave changes as the ring particles within the crest interact and collide with each other. A thin strand of ring material to the lower left of Daphnis is newly visible in this image, and there are intricate features that also hadn’t been previously observed in the third wave crest downstream. Credit: NASA/JPL-Caltech/Space Science Institute / Tilmann Denk (Freie Universität, Berlin).

And finally, this image showing the third wave crest, which displays multiple strands of ring material, an indication of the effects of Daphnis on the ring. Here we are again in visible light, now at a distance of 23,000 kilometers and at a phase angle (Sun-Daphnis-spacecraft) of 94 degrees, with an image scale of 160 meters per pixel.

Image: A closeup of the third wave crest (toward the far left in the color image) focusing on detail scientists hadn’t seen before, with multiple strands of ring material visible. The image shows how the ring material behaves after losing the structure Daphnis triggered and goes back to interacting with itself. The ribbon of material that appears to be protruding into the gap in the right-hand portion of the image is probably actually soaring above the ring plane. Toward the left-hand part of the image, that same ribbon of material dives below the ring plane and becomes obscured by the main part of the rings, which is why it disappears from view. Credit: NASA/JPL-Caltech/Space Science Institute.

I had never thought of Saturn as providing a kind of astrophysical laboratory that could illuminate processes at work in the early Solar System, but this is what the four papers in Science suggest, for here we see the kinds of interactions that take place in disk structures that contain larger accreted objects. The Cassini imagery and data will always symbolize the era of early Solar System reconnaissance on which we are embarked. Imagine what we might do with a next-generation craft, a Cassini follow-up around Uranus or Neptune.

The four papers on the Cassini ‘Grand Finale’ all appear in the June 14 issue of Science, Vol. 364, Issue 6445, and can be accessed here. They are Tiscareno et al., “Close-range remote sensing of Saturn’s rings during Cassini’s ring-grazing orbits and Grand Finale”; Buratti et al., “Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus”; Militzer et al., “Measurement and implications of Saturn’s gravity field and ring mass”; and Spilker, “Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery.”

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Can We Catch the Next ‘Oumuamua?

Ever since the passage of interstellar interloper ‘Oumuamua, we’ve become aware of the opportunities presented by objects entering our system from interstellar space, at the same time wishing we had the resources at hand to investigate them close-up. Andreas Hein and colleagues at the Initiative for Interstellar Studies have examined the possibilities for reaching ‘Oumuamua through Project Lyra (see Project Lyra: Sending a Spacecraft to 1I/’Oumuamua), a study that also takes in the kind of future infrastructure that could allow us to react to the next such object.

Now comes the interesting news that the European Space Agency is developing a mission called Comet Interceptor, one capable of visiting a long-period comet coming into the inner system from the Oort Cloud, but just as capable of reaching an interstellar visitor. The idea revolves around not a single spacecraft, but a combination of three. The composite vehicle would be capable of orbiting the L2 Lagrange point 1.5 million kilometers from Earth until it finds a suitable target. At that point, it would journey to the object and separate into three modules.

Image: Comet Interceptor has been selected as ESA’s new fast-class mission. It will be the first spacecraft to visit a truly pristine comet or other interstellar object that is only just starting its journey into the inner Solar System. The spacecraft will wait at the Sun-Earth Lagrange point L2, which is 1.5 million kilometres ‘behind’ Earth as viewed from the Sun. It will travel to an as-yet undiscovered comet, making a flyby of the chosen target when it is on the approach to Earth’s orbit. The mission comprises three spacecraft that will perform simultaneous observations from multiple points around the comet. Credit: ESA.

Each module will be equipped with a science payload that complements the instrumentation on the others, offering insights into cometary gas and dust and the plasma environment near the object through a mass spectrometer along with dust, field and plasma instruments. Thus we get ‘multi-point’ measurements offering insights into cometary interactions with the solar wind, the stream of plasma from the Sun that itself is constantly changing in velocity and intensity.

This is a fundamentally different concept from previous missions like Giotto and Rosetta. Giotto flew within 600 kilometers of Comet 1P/Halley in 1986, with another pass by Comet Grigg-Skjellerup in 1992. Rosetta targeted Comet 67P/Churyumov-Gerasimenko in a highly successful mission in 2014. Both comets are short-period objects with periods of less than 200 years, with 67P/Churyumov-Gerasimenko orbiting every 6.5 years and Halley every 76.

In both cases, the comet’s frequent passage into the inner system has meant changes to the surface. What Comet Interceptor is looking for is a first-time visitor, one whose materials should be relatively unprocessed since the earliest days of the system. But ESA is also thinking about interstellar objects like ‘Oumuamua as potential destinations, for the mission has the luxury of being able to choose its target from its stable vantage point at L2. Given the success of the Pan-STARRS effort at finding new comets and the construction of the Large Synoptic Survey Telescope in Chile, slated to reach first light in 2020, we should have no shortage of targets.

ESA director of science Günther Hasinger describes the mission in context:

“Pristine or dynamically new comets are entirely uncharted and make compelling targets for close-range spacecraft exploration to better understand the diversity and evolution of comets. The huge scientific achievements of Giotto and Rosetta – our legacy missions to comets – are unrivalled, but now it is time to build upon their successes and visit a pristine comet, or be ready for the next ‘Oumuamua-like interstellar object.”

Image: Kuiper Belt and Oort Cloud in context. Credit: ESA.

In official terminology, Comet Interceptor is an F-class mission, the ‘F’ standing for ‘fast,’ as in ‘fast implementation’ — the total development time from selection of the mission to readiness to launch is to be eight years. But we might also consider it in terms of ‘fast response,’ just what is needed to reach objects that appear with no prior warning. This category of mission will have a launch mass of less than 1,000 kilograms. Comet Interceptor is now seen going into space along with exoplanet hunter ARIEL in 2028, both missions being delivered to L2.

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Breakthrough Listen: SETI Data Release

On Monday I was talking about the rise of open access scientific journals, using the European Space Agency’s Acta Futura as just one example. The phenomenal arXiv service, not itself a journal but a repository for preprints of upcoming papers, is already well known in these pages. Now we have the largest public release of SETI data in the history of the field, a heartening follow-through on a trend that broadens the audience for scientific research.

Breakthrough Listen is presenting two publications in the scientific literature (available as full text, citation below) describing the results of three years of radio and optical observations, along with the availability of a petabyte of data from its work at the Green Bank instrument in West Virginia and the Parkes Radio Telescope in Australia. This covers a sample of 1327 nearby stars (within 160 light years from Earth) and builds on the team’s results on 692 stars as presented in 2017.

No signs of extraterrestrial civilizations turn up in the analysis, says Parkes project scientist Danny Price, who emphasizes that the search will continue:

“This data release is a tremendous milestone for the Breakthrough Listen team. We scoured thousands of hours of observations of nearby stars, across billions of frequency channels. We found no evidence of artificial signals from beyond Earth, but this doesn’t mean there isn’t intelligent life out there: we may just not have looked in the right place yet, or peered deep enough to detect faint signals.”

Image: The Green Bank site in West Virginia, where Breakthrough Listen observations continue. Credit: NRAO/AUI.

There are reasons why making such data public benefits the SETI effort. Both within the public and the astronomical community, those interested can now download the results of these observations and examine them independently. Those with programming skills may well develop algorithms for the detection of signals and filtering out of background noise that improve on the current model. And there may be information within the datasets that will prove useful in the investigation of unrelated astrophysical phenomena.

The existing tools developed by the Breakthrough Listen science team at the Berkeley SETI Research Center (BSRC) include both radio frequency searches as well as optical scans and algorithms designed to flag unexplained astrophysical phenomena. Go to this UC-Berkeley page for the overview, including the two just released papers. Likewise available to the public are software tools used in the analysis such as blimpy (for loading raw format data files), and turboSETI (for running Doppler drift searches). The datasets are examined in the analysis paper by Dr. Price and made available at the Breakthrough Listen Open Data Archive and via BSRC (more search options available at the latter).

For those wanting to get into data crunching themselves, the second paper (lead author Matt Lebofsky at Berkeley) goes into the intricacies of the current analysis, the tools used, the data formats and the archival systems now in play. “While we have been making smaller subsets of data public before in varying forms and contexts,” says Lebofsky, “we are excited and proud to offer this first cohesive collection along with an instruction manual, so everybody can dig in and help us search. And we’re just getting started – there’s much more to come!”

Considering the complexities involved in creating a search ‘pipeline’ that can scan through billions of radio channels, the more eyes on search algorithms and filtering techniques, the better. Thus far the detected signals have come from human technologies, with the Breakthrough Listen team filtering for narrow-band signals showing a Doppler drift, meaning they change in frequency with time because of their motion with respect to the telescope.

A second filter in the pipeline removes signals that do not appear to originate from a fixed point on the sky. The application of both techniques reduces millions of signals down to a comparative few, all of which have been examined and found to be human-generated frequency interference. From the Price paper, which notes that in its search for narrowband signals showing Doppler drift, 51 million hits emerged, with 6154 that cleared the automated filtering process, leading to a final round of manual inspection and cross-referencing against known sources of interference:

…these observations constitute the most comprehensive survey for radio evidence of advanced life around nearby stars ever undertaken, improving on the results of Enriquez et al. (2017) in both sensitivity and number of stars. Together with other recent work from the resurgent SETI community, we are beginning to put rigorous and clearly defined limits on the behavior of advanced life in the universe. We note that significant additional observational and theoretical work remains to be done before we are able to make general statements about the prevalence of technologically capable species.

Be aware that the archive also includes data from Breakthrough Listen observations of the first repeating fast radio burst ever detected, FRB 121102, as well as scans of the ‘Oumuamua object, along with optical data from the Automated Planet Finder at Lick Observatory. The search of nearby stars continues while also being expanded into the galactic disk at Parkes, and a one-million star sample with the MeerKAT telescope in South Africa is forthcoming.

The papers are Price et al., “The Breakthrough Listen Search for Intelligent Life: Observations of 1327 Nearby Stars over 1.1-3.4 GHz,” submitted to The Astrophysical Journal (preprint) and Lebofsky et al., “The Breakthrough Listen Search for Intelligent Life: Public Data, Formats, Reduction and Archiving,” submitted to Publications of the Astronomical Society of the Pacific (preprint).

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We rarely talk about Teegarden’s Star when mentioning interesting objects near the Solar System, probably because the star was only discovered in 2003 and until now had not been known to host planets. Today we learn, however, that an international team led by the University of Göttingen has found two planets close to Earth mass in what it considers to be the habitable zone around the tiny star. Interestingly, from where the system is located, any local astronomers would be able to see the planets of our Solar System in transit across the face of the Sun, about which more in a moment.

One of the reasons that this comparatively nearby star has been so late to be discovered is its size. We are dealing with an M-class red dwarf, this one in the constellation Aries, and no more than 12.5 light years from us. It took three years of patient radial velocity monitoring to track down planets around a star that is only about 2700 degrees Celsius in temperature, and fully 10 times lighter than the Sun. What we now have are two planet candidates, each with a minimum mass 1.1 times that of Earth, with orbital periods of 4.91 and 11.4 days respectively.

These are, according to the paper, “…the first Earth-mass planets around an ultra-cool dwarf for which the masses have been determined using radial velocities.”

Because no transits have been detected, the scientists have no information on planetary radii, and therefore estimated them based on various possible compositions, from rocky to gaseous mini-Neptune, finding that the resulting radii differ by a factor of about three. The other stellar and planetary parameters were plugged into the Earth Similarity Index (ESI), which compares key parameters to those of Earth. If these worlds are not mini-Neptunes, we get this (from the paper):

Except for the case of a mini-Neptune composition, the two planets have a high ESI. For a potentially rocky composition, the ESI value is 0.94 and 0.8 for planets b and c, respectively. This makes Teegarden’s Star b the planet with currently the highest ESI value. However, the ESI is only an estimate, and different weighting of the parameters may lead to changing ESIs. This ESI definition, for example, does not take into account the stellar spectral energy distribution and the resulting planetary atmospheric composition, which very likely have an effect on habitability.

So we push at the boundaries of what we still don’t know. Lead author Matthias Zechmeister (University of Göttingen) noted the resemblance between these two worlds and the inner planets of our Solar System, saying “They are only slightly heavier than Earth and are located in the so-called habitable zone, where water can be present in liquid form.” I asked Dr. Zechmeister to amplify on the habitability zone finding, to which he replied:

You may have noted that PHL (http://phl.upr.edu/press-releases/pr_draft_tee4321) ranks Teegarden b now as the exoplanet most similar to Earth. The conditions are good for liquid water on the surface, given a similar insolation and mass as Earth’s. Still, we cannot be sure to 100%. We have measured “only” the mass (which is a minimum mass, but true masses are statistically only ~16% higher). So we do not know the true chemical composition, though a rocky composition is probable (see Fig. 12 in the paper for other compositions).

Dr. Zechmeister also made note of the fact that Teegarden’s Star is about 8 billion years old, roughly twice the age of the Sun, allowing plenty of time for interesting things to develop if life ever took hold there. And he raised a caution re the habitability issue, noting that the diminutive star is what he refers to as ‘an extreme host,’ a type of star about which we still have a great deal to learn.

So small is Teegarden’s Star that it is not far above the upper size limit for brown dwarfs, often considered to be somewhere between 60 and 90 Jupiter masses, and at magnitude 15, it demands a large telescope to see it at all. In fact, it was actually discovered in 2003 via stored data in the Near-Earth Asteroid Tracking (NEAT) program, and had been logged in our data earlier, turning up on photographic plates from the Palomar Sky Survey taken in 1951.

Image: Comparison habitable zone in Teff [effective temperaeture] – HZ diagram. Credit: C. Harman.

Now ponder this: Radial velocity detections have produced more than 800 exoplanets, but few have been found around old, cool M-dwarfs. In fact, we have only two other planet hosts with effective temperatures cooler than 3,000 K, and one of these is Proxima Centauri, while the other is TRAPPIST-1, around which fully seven transiting planets are known to exist. The authors of the paper on the Teegarden’s Star work consider the lack of planet detections around very late-type stars the result of observational bias owing to the faintness of the objects at visible wavelengths.

From the paper:

Both planets have a minimum mass close to one Earth mass, and given a rocky, partially iron, or water composition, they are expected to have Earth-like radii. Additionally, they are close to or within the conservative HZ, or in other words, they are potentially habitable. Our age estimate of 8 Gyr implies that these planets are about twice as old as the solar system. Interestingly, our solar system currently is within the transit zone as seen from Teegarden’s Star. For any potential Teegardians, the Earth will be observable as a transiting planet from 2044 until 2496.

A later note from co-author Guillem Anglada-Escudé, the discoverer of Proxima Centauri b, unpacks this further. The transit that would be visible from this system in 2044 would have occurred in our Solar System in 2032, factoring in the 12 year light travel time. There have been SETI discussions regarding the possibility of conducting communication attempts of stars whose planets would see our own world in transit around the Sun, so any such attempt would need to take place in 2032 or later, with the earliest potential response expected around 2056.

Image: Top 19 potentially habitable exoplanets, sorted by distance from Earth. Credit: A. Mendez (PHL).

The team behind this work used data from CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs), an effort using two separate spectrographs located at the 3.5m telescope at the Calar Alto Observatory in Almeria, Spain. The goal of the project, conducted by a consortium of German and Spanish institutions, is to carry out a survey of approximately 300 late-type main-sequence stars with the goal of detecting low-mass planets in their habitable zones.

We have not, in other words, heard the last from CARMENES.

The paper is Zechmeister et al. “The CARMENES search for exoplanets around M dwarfs – Two temperate Earth-mass planet candidates around Teegarden’s Star,” accepted at Astronomy & Astrophysics 2019 (abstract).

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ESA Advanced Concepts Team Interstellar Workshop

Given the difficulties that persist in retrieving many good papers from behind publisher firewalls, I’m always glad to see open access journals plying their trade. Let me call your attention in particular to Acta Futura, which comes out of the scientists working with the European Space Agency’s Advanced Concepts Team. Acta Future defines itself as multidisciplinary in scope with a focus on the long-term development of space science.

Hence the list of topics is wide, as the website notes, “…ranging from fundamental physics to biomimetics, mission analysis, computational intelligence, neuroscience, as well as artificial intelligence or energy systems,” and this does not exhaust the range of possibilities. If you’re interested in browsing through or searching the archives, click here for a page with the appropriate links as well as information on how to submit papers to Acta Futura.

I’ve had ESA’s Advanced Concepts Team on my mind this weekend because long-time Centauri Dreams reader David Wojciech passed along news of the upcoming ACT interstellar workshop, beginning this week on the 20th, and carrying over to the following day. The conference anticipates an upcoming issue of Acta Futura devoted to interstellar exploration. The venue will be Erasmus Highbay at ESTEC, Noordwijk, Netherlands.

Topics to be covered in depth are:

  • Advanced propulsion technologies for interstellar probes
  • Communication for interstellar exploration
  • Concepts and ideas surrounding world ships: sociology, ethics, anthropology, language development
  • Technologies and measure to keep humans alive, healthy and productive during long duration spaceflight

And I note that registration is free of charge until June 18, so if you’re fortunate enough to be in range of the meeting, be sure to check in at the conference webpage. I’m seeing plenty of good material here, from Pontus Brandt’s “A Pragmatic Interstellar Probe in the 2030s,” to Andreas Hein’s “World ships: feasibility and rationale” and Michael Hippke’s “Interstellar communication.” But look at the whole list (available here), which includes Philip Lubin on laser propulsion, Rob Swinney on fusion, Angelo Vermeulen on evolvable spacecraft and Ugo Lafont on self-healing materials. Most of these will be familiar names to regular Centauri Dreams readers.

Image: A Bussard ramjet in flight, as imagined for ESA’s Innovative Technologies from Science Fiction project. Credit: ESA/Manchu.

There was a time not so long ago when posting news of an upcoming interstellar conference would be relevant only to those within immediate geographic range. The entire field of interstellar studies, in fact, was once an afterthought at conferences largely devoted to other matters, usually discussed only at the end. These days, a rising interest in interstellar possibilities leads to conferences whose papers will be readily accessible in an open access journal like Acta Futura. We’ll have a look at these when the materials reach publication stage.

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Giant Planets Less Likely around Sun-like Stars

We’re getting first results from the Gemini Planet Imager Exoplanet Survey (GPIES), a four-year look at 531 young, nearby stars that relies on the instrument’s capabilities at direct imaging. Data from the first 300 stars have been published in The Astronomical Journal, representing the most sensitive, and certainly the largest direct imaging survey for giant planets yet attempted. The results of the statistical analysis are telling: They suggest that planets slightly more massive than Jupiter in outer orbits around stars the size of the Sun are rare.

The Gemini Planet Imager (GPI), located at the Gemini South Telescope in Chile, can achieve high contrast at small angular separations, making it possible to see exoplanets directly, as opposed to the indirect methods that have dominated the field, such as transits and radial velocity analysis. As successful as the latter have been, they are most effective with planets closer to their stars, whereas an instrument like the GPI can find planets in regions outside the orbit of Jupiter. The GPI can directly image exoplanets a millionth as bright as the host star.

Bruce Macintosh (Stanford University), principal investigator for the GPI, calls this effort “…the most sensitive direct imaging survey for giant planets published to date.” The question it raises is significant: Just how representative is our Solar System in having gas giants like Jupiter and Saturn in outer orbits around a G-class star? We’re only beginning to learn the answer, but surveys like this one are on track to tell us. The answer may have astrobiological consequences, as Franck Marchis (SETI Institute), a co-author of the just published report, explains:

“We suspect that in our solar system Jupiter and Saturn sculpted the final architecture that influences the properties of terrestrial planets such as Mars and Earth, including basic elements for life such as the delivery of water, and the impact rates. A planetary system with only terrestrial planets and no giant planets will probably be very different to ours, and this could have consequences on the possibility for the existence of life elsewhere in our galaxy.”

Image: Close-up Picture of Gemini Planet Imager currently located at Gemini South Observatory in Cerro Pachon. Photo by J. Chilcote.

Out of the 300 stars in the thus far released Gemini survey data, 123 are more than one-and-a-half times more massive than the Sun. What the data show is that the hosts of the planets thus far detected are all among the higher mass stars. This despite the fact that given the differential between stellar light and that of a planet, a giant planet orbiting a fainter star more like the Sun is actually easier to see. This relationship of mass to giant planet frequency has been discussed in the literature and is now strengthened by the results of the GPI survey.

The results of the Gemini survey pick up on the theme that other planetary systems tend to be different from our own, despite the assumption that gas giants in outer orbits and rocky worlds on inner orbits would be a fairly standard pattern. Both the GPIES and other exoplanet surveys point to the rarity of giant planets around stars as small as the Sun. Worlds several times more massive than Jupiter and above (the GPIES is not sensitive enough to pick up planets of as low a mass as Jupiter itself) tend to be hosted by stars more massive than the Sun. Our own wide-orbit Jupiter, then, may be a statistical outlier, although that is yet to be determined.

From the paper’s conclusion:

From the first 300 stars observed out of the planned 600-star survey, reaching contrasts of 106 within 1′′ radius, GPIES is one of the largest and deepest direct imaging surveys for exoplanets conducted to date. Our analysis of the data shows that there is a clear stellar mass dependence on planet occurrence rate, with stars >1.5 M [i.e. 1.5 times Solar mass] more likely to host giant planets (5–13 MJup ) at wide separations (semimajor axes 10–100 au) than lower-mass stars.

The paper reports the imaging of six planets and three brown dwarfs, with a sensitivity to planets of several Jupiter masses at orbital distances comparable to those beyond Saturn (at least 12 gas giants had been expected based on earlier models). The only previously unknown planet was 51 Eridani b, which was discovered via GPI as far back as 2014, a gas giant of two-and-a-half Jupiter masses in a Saturn-like orbit around a young star some 97 light years away, and one that had been previously unknown despite attempts to observe the star because no other instrument was able to sufficiently suppress the starlight to make the planet visible.

Image: Results of the survey of 531 stars and their exoplanets in the southern sky are plotted to indicate their distance from Earth. Gray dots are stars without exoplanets or a dust disk; red are stars with a dust disk but no planets; blue stars have planets. Dots with rings indicated stars imaged multiple times. Credit: Paul Kalas, UC Berkeley; Dmitry Savransky, Cornell; Robert De Rosa, Stanford.

Another useful find: A brown dwarf labeled HR 2562 B, 30 times more massive than Jupiter in a Uranus-like orbit. This brown dwarf and the other two imaged in the study shed light on planet vs. brown dwarf formation at wide separations from the host star. The question of brown dwarf vs. planet formation is long-standing. Whereas stars have been considered to form through the gravitational collapse of large clouds of gas and dust, planets are thought to have formed largely through core accretion, as small rocky bodies undergo collision and accumulation of mass.

Eugene Chiang (UC-Berkeley) is a co-author of the paper:

“What the GPIES Team’s analysis shows is that the properties of brown dwarfs and giant planets run completely counter to each other. Whereas more massive brown dwarfs outnumber less massive brown dwarfs, for giant planets, the trend is reversed: less massive planets outnumber more massive ones. Moreover, brown dwarfs tend to be found far from their host stars, while giant planets concentrate closer in. These opposing trends point to brown dwarfs forming top-down, and giant planets forming bottom-up.”

So our Solar System evidently doesn’t resemble many other systems that we’ve observed. Gas giants in outer orbits seem to be more common around significantly larger stars. Putting together a catalog of gas giants in the outer systems of other neighboring stars is going to take time — it’s telling that even the GPI can’t detect Jupiter-mass planets in these orbits — but the GPIES is the beginning of that process, and one that will soon publish additional results. Observations in the survey wrapped up in January with an examination of its 531st star. Moving toward their report on the complete dataset, the team is now following up candidate planets at the same time that it begins an upgrade on the Gemini Planet Imager itself.

The paper is Nielsen et al., ”The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10 to 100 au,” Astronomical Journal Vol. 158, No. 1 (12 June 2019). Abstract.

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What Sodium Chloride Means for Europa’s Ocean

We have priceless data on Europa from the Voyager and Galileo missions, but we’re updating earlier interpretations thanks to new work with both the Hubble Space Telescope and the Keck Observatory on Mauna Kea (Hawaii). Thus the discovery that the yellow color visible on parts of Europa’s surface in visible light is most likely sodium chloride (NaCl), familiar as table salt and the principal component of sea salt. That’s an interesting result, given that it suggests a Europan ocean chemically more similar to Earth’s than we had previously assumed.

The re-thinking of the spacecraft data stems from the fact that Galileo was equipped with the Near-Infrared Mapping Spectrometer instrument, useful for analyzing the surface of a planetary body. What Galileo lacked, however, was a visible spectrometer to complement its near-infrared device. The problem: Chlorides are not apparent in the near-infrared. While Galileo had found water ice, it identified a substance believed to be magnesium sulfate salts on the surface.

But spectral data from the Keck instrument showed none of the expected sulfate absorptions. Caltech graduate student Samantha Trumbo is lead author of the paper on this work:

“No one has taken visible-wavelength spectra of Europa before that had this sort of spatial and spectral resolution. The Galileo spacecraft didn’t have a visible spectrometer. It just had a near-infrared spectrometer, and in the near-infrared, chlorides are featureless.”

The researchers used spectra obtained with the Hubble instrument to detect a 450-nm absorption indicating irradiated sodium chloride on the surface. Moreover, this feature correlates with the interesting ‘chaos’ terrain that seems to show interactions with the ocean below, making it appear that there is an interior source for the sodium chloride, as discussed in the paper’s conclusion:

As chaos terrain is geologically young, extensively disrupted, and potentially indicative of locations of subsurface upwelling or melt-through…, and as the leading hemisphere chaos regions are shielded from the sulfur implantation of the trailing hemisphere, the composition of these regions may best represent that of Europa’s endogenous material. However, their spectra are categorically smooth at higher spectral resolution, lacking any identifiable infrared spectral features other than those of water ice. Nevertheless, the unique geology and 1.5- to 4-μm spectra of leading hemisphere chaos terrain suggest a salty composition. Chloride salts provide a potential explanation…

At the Jet Propulsion Laboratory, co-author Kevin Hand put ocean salts to the test under conditions of radiation similar to Europa’s, finding that sodium chloride produced color changes under irradiation that could be identified through analysis of the visible spectrum. Hand likens the substance to invisible ink — radiation is what makes it apparent to the observer. In the laboratory, it turns a shade of yellow similar to the Europan region known as ‘Tara Regio.’ The 450-nm absorption in the visible spectrum detected by Hubble matches the irradiated salt in the laboratory, firming up the idea that Tara Regio’s color comes from irradiated NaCl.

Image: This color composite view combines violet, green, and infrared images of Jupiter’s intriguing moon, Europa, for a view of the moon in natural color (left) and in enhanced color designed to bring out subtle color differences in the surface (right). The bright white and bluish part of Europa’s surface is composed mostly of water ice, with very few non-ice materials. In contrast, the brownish mottled regions on the right side of the image may be covered by hydrated salts and an unknown red component. The yellowish mottled terrain on the left side of the image is caused by some other unknown component. Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometers long. Credit: NASA/JPL/University of Arizona.

We could be looking at sodium chloride as one of the various materials found in the moon’s outer shell, but the prospect that it is derived from the subsurface ocean means that we may have an insight here into the chemistry going on beneath the ice. From the paper:

The presence of NaCl on Europa has important implications for our understanding of the internal chemistry and its geochemical evolution through time. Whereas aqueous differentiation of chondritic material and long-term leaching from a chondritic seafloor can result in a system rich in sulfates, more extensive hydrothermal circulation, as on Earth, may lead to an NaCl-rich ocean. The plume chemistry of Enceladus, which is perhaps the best analog to Europa, suggests an NaCl-dominated ocean and a hydrothermally active seafloor. However, the compositional relationship between Europa’s ocean and its endogenous material is unknown, and the surface may simply represent the end result of a compositional stratification within the ice shell… Regardless of whether the observed NaCl directly relates to the ocean composition, its presence warrants a reevaluation of our understanding of the geochemistry of Europa.

Image: In a laboratory simulating conditions on Jupiter’s moon Europa at NASA’s Jet Propulsion Laboratory in Pasadena, California, plain white table salt (sodium chloride) turned yellow (visible in a small well at the center of this photograph). The color is significant because scientists can now deduce that the yellow color previously observed on portions of the surface of Europa is actually sodium chloride. The JPL lab experiments matched temperature, pressure and electron radiation conditions at Europa’s surface. Credit: NASA/JPL-Caltech.

A more active seafloor than we have assumed? If so, Europan geology becomes still more interesting. A geologically young icy shell with striking areas showing past activity amid evidence of NaCl makes the assumption that these salts derived from the ocean plausible.

The paper is Trumbo et al., “Sodium chloride on the surface of Europa,” Science Advances Vol. 5, No. 6 (12 June 2019). Full text.

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