A Relatively Nearby Earth-Sized Planet

by Paul Gilster on November 11, 2015

Given my abiding interest in red dwarf stars and the planets that circle them, I always keep an eye on what’s happening with the MEarth project. Two arrays of robotically controlled telescopes are involved in MEarth (pronounced ‘mirth’), one at the Fred Lawrence Whipple Observatory on Mt. Hopkins (AZ), the other a cluster of eight at the Cerro Tololo Inter-American Observatory in Chile. Both these arrays are controlled from MEarth’s offices in Cambridge (MA). MEarth is all about observing nearby M-dwarfs in the hunt for Earth-class planets.

My fascination in these stars is simply a result of the numbers. We’ve learned that M-dwarfs comprise as much as 80 percent of the stars in the Milky Way. Earth is not, in other words, orbiting the most common type of star out there. We also know that M-dwarfs host planets. If we learn that conditions on such worlds can support life, then we’ve dramatically expanded the search space for astrobiology. The prospect of a living world, probably tidally locked to its star, conjures images strange and wonderful, a world where shadows are permanent and half of the planet is an ice-covered waste, as Stephen Baxter recently portrayed in a planet called ‘Per Ardua’ that circles Proxima Centauri in his novel Ultima (Roc, 2015).

The latest news from MEarth comes out of MEarth-South, whose 40-centimeter instruments have detected an interesting light curve around the star GJ 1132, finding a dip of approximately 0.3 percent in the starlight. The signal, confirmed by other instruments in Chile, flags a planet that is roughly 1.2 times the size of the Earth, in a tight 1.6-day orbit. Given the star’s radius and the amount of light it blocks, researchers led by Zachory Berta-Thompson (MIT) calculate this is a planet with about 1.6 times Earth’s mass, a world that may well be rocky.


Image: In this artist’s rendering of GJ 1132b, a rocky exoplanet very similar to Earth in size and mass, circles a red dwarf star. GJ 1132b is relatively cool (about 226 degrees C) and could potentially host an atmosphere. At a distance of only 39 light-years, it will be a prime target for additional study with Hubble and future observatories like the Giant Magellan Telescope. Credit: Dana Berry.

The world is probably tidally locked. And life on GJ 1132b looks to be unlikely, given an estimated average temperature of 500 K (226 degrees Celsius). Says Berta-Thompson:

“The temperature of the planet is about as hot as your oven will go, so it’s like burnt-cookie hot. It’s too hot to be habitable — there’s no way there’s liquid water on the surface. But it is a lot cooler than the other rocky planets that we know of.”

That’s a useful fact because while surface conditions appear inimical to life, the planet is cool enough to retain a substantial atmosphere. GJ 1132 is a mere 39 light years from Earth, making it the closest Earth-sized exoplanet yet discovered. Recently we’ve been discussing the next generation of space telescopes, and now we find a world that will surely be a target for scrutiny, especially by the James Webb Space Telescope, which should be able to analyze the chemical constituents of the planet’s atmosphere and even detect the patterns of its winds.

“If we find this pretty hot planet has managed to hang onto its atmosphere over the billions of years it’s been around, that bodes well for the long-term goal of studying cooler planets that could have life,” adds Berta-Thompson. “We finally have a target to point our telescopes at, and [can] dig much deeper into the workings of a rocky exoplanet, and what makes it tick.”

Berta-Thompson points out that some 500 star systems are known to be closer to us than GJ 1132, and instruments like TESS (Transiting Exoplanet Survey Satellite) and CHEOPS (Characterizing Exoplanets Satellite), both scheduled for launch in 2017, will help us study many more targets. Learning how to analyze the atmospheres of nearby worlds is critical for our investigations into exoplanetary life. Planets like GJ 1132b will be useful in refining our tools as we begin to turn them to worlds with better prospects for living things.

The paper is Berta-Thompson et al., “A rocky planet transiting a nearby low-mass star,” Nature 527 (12 November 2015), 204–207 (abstract).



Quantifying KIC 8462852 Power Beaming

by Paul Gilster on November 10, 2015

Plasma physicist James Benford, CEO of Microwave Sciences, is well known here on Centauri Dreams. Today he is joined by his son Dominic, whose work focuses on the development of ultrasensitive technologies for far-infrared through millimeter-wave astronomy. The younger Dr. Benford is Program Scientist for NASA’s WFIRST mission, which is designed to conduct major surveys in the near-infrared to answer fundamental questions on the nature of dark energy, the distribution of dark matter, the occurrence of planets around other stars, and even to enable the direct imaging of planetary systems. Previously, Dominic was Chief Scientist for the Cosmic Origins Program Office, as well as Deputy Mission Scientist for WISE, the Wide-field Infrared Survey Explorer. In today’s entry, the Benfords look at the SETI Institute’s recent observations of KIC 8462852 and analyze the detectability of power beaming at these distances.

by James and Dominic Benford


The recent report from the SETI Institute of radio observations of the anomalous star KIC 8462852 has immediate implications. That report concluded that, using the Allen Array, no narrowband radio signals were found above a few hundred Janskys in 1 Hz channels and no “wideband” signals above 100 Janskys are seen in 100 kHz channels. This is for observations taking place for 2 weeks, observing half the time. This implies about 180 hours of observations, although only about 1% of the time is spent at any individual frequency.

The purpose of the observations is to see whether the anomalous star is the site of a super-civilization that might be incidentally radiating sufficient power that we can observe, i.e., leakage radiation. They might even be intentionally producing signals for us to detect. The easiest way to do that is to ‘piggyback’, to put a message onto the power beams.

The thresholds they have reported, above which no signals are present, have implications for the presence of power beams in the anomalous star system. Beaming power on astronomical scales has been a frequent topic on this site and it has long been pointed out that the beaming of power for various purposes could be observable at astronomical distances.

The missions suggested for power beaming involve Earth–to–space applications such as launching spacecraft to orbit or raising satellites from a lower orbit to a higher one. Several workers have studied interplanetary missions, meaning space–to–space transfers of cargo. Finally, launch into the outer solar system and for interstellar precursors and ultimately for starships has also been quantified.

We have examined the thresholds in light of concepts proposed for beaming power in and around our solar system. By comparing the reported thresholds set by the SETI Institute, the non-observation of leakage signals at their stated thresholds implies the following:

  • Orbit raising missions, which require lower power, are not detectable at the thresholds of the Allen Array.
  • Launch from a planetary surface into orbits would be bright enough to be seen by the 100 kHz observations. However, the narrow bandwidth 1 Hz survey would not see them.
  • Interplanetary transfers by beam-driven sails should be detectable in their observations, but are not seen. This is for both the narrow 1 Hz and for the “wideband” 100 kHz observations.
  • Starships launched by power beams with beamwidths that we happen to fall within would be detectable, but are not seen.

These results must be qualified by noting:

  • Power beaming is not an isotropic endeavor, and so the geometry of the transmitter and the intended recipient will produce a conjunction from our point of view only episodically. The observations were conducted for only a limited time and further observations would provide a more stringent constraint.
  • Even the “wideband” observation is actually quite narrow compared with the kinds of sources that would be used in power beams, based on our current understanding of microwave physics. For the applications discussed here, the 100 kHz bandwidth observed would be about 10 to 100 millionths of the center frequency of the Beamer. But high-power devices are inherently not designed for such narrow bandwidths.
  • The frequencies we would use for power beaming are in the millimeter band, so are outside the microwave range the Allen Array observed.

Therefore the observations by the Allen Array are not sufficiently broad to produce firm conclusions about realistic Beamers.

Readers are encouraged to consult the original paper: Harp et al., “Radio SETI Observations of the Anomalous Star KIC 8462852” (preprint). Previous discussions on this matter can be found in the following reports:

“A Path Forward for Beamed Sails”: http://www.centauri-dreams.org/?p=20962

“Seeing Alien Power Beaming”: http://www.centauri-dreams.org/?p=34133

“Microwave Beaming: A Fast Sail to Mars”: http://www.centauri-dreams.org/?p=1176

“The Case for Beamed Sails”: http://www.centauri-dreams.org/?p=20924



Alpha Centauri Planet Reconsidered

by Paul Gilster on November 9, 2015

Finding a habitable world around any one of the three Alpha Centauri stars would be huge. If the closest of all stellar systems offered a blue and green target with an atmosphere showing biosignatures, interest in finding a way to get there would be intense. Draw in the general public and there is a good chance that funding levels for exoplanet research as well as the myriad issues involving deep space technologies would increase. Alpha Centauri planets are a big deal.

The problem is, we have yet to confirm one. Proxima Centauri continues to be under scrutiny, but the best we can do at this point is rule out certain configurations. It appears unlikely, as per the work of Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), that any planet of Neptune mass or above exists within 1 AU of the star. Moreover, no ‘super-Earths’ have been detected in orbits with a period of less than 100 days. This doesn’t rule out planets around Proxima, but if they are there, so far we don’t see them.


Image: The Alpha Centauri stellar system, consisting of the red dwarf Proxima Centauri and the two bright stars forming a close binary, Centauri A and B. Credit: NASA.

Centauri B, the K-class star in close proximity to G-class Centauri A, was much in the news a few years back with the announcement of Centauri Bb, a candidate world announced by Swiss planet hunters. This is radial velocity work based on data gathered by the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. The signal that Xavier Dumusque and team drew out of the data was 0.5 meters per second, a fine catch if confirmed.

What we thought we had in Centauri Bb was a mass just a little over the Earth’s and an orbit of a scant 3.24 days. As the blistering first planet detected around one of the Centauri stars, it would be a significant find even if it’s a long way from the temperate, life-sustaining world we’d like to find further out. The putative Centauri Bb supported the idea that there might be other planets there, and we’ve known since the work of Paul Wiegert and Matt Holman back in the 1990s that sustainable habitable zone orbits are possible around both the primary Alpha Centauri stars.

But Centauri Bb has remained controversial since Artie Hatzes (Thuringian State Observatory, Germany), using different data processing strategies, looked at the same data and found a signal he considered too noisy, indicating that what might be a planet might also be stellar activity on Centauri B itself. Debra Fischer’s team at Cerro Tololo Inter-American Observatory has also been studying Centauri Bb using the CHIRON spectrometer but has not been able to confirm it. And while a transit search using the Hubble Space Telescope did find a promising lightcurve (about which more in a moment), it couldn’t confirm Centauri Bb.


Image: Of the three stars of Alpha Centauri, the dimmest, Proxima Centauri, is actually the nearest star to the Earth. The two bright stars, Alpha Centauri A and B form a close binary system; they are separated by only 23 times the Earth – Sun distance. This is slightly greater than the distance between Uranus and the Sun. The Alpha Centauri system is not visible from much of the northern hemisphere. The image above shows this star system and other objects near it in the sky. Credit/copyright: Akira Fujii / David Malin Images.

Now we have a new paper from Vinesh Rajpaul (University of Oxford) and colleagues that makes Centauri Bb look more unlikely than ever. Rajpaul praises the thorough work of Xavier Dumusque and the team at the Geneva Observatory, but notes that their attempts to filter stellar activity out of their data evidently boosted other periodic signals that had nothing to do with a planet. The signal grows out of the time sampling, or ‘window function,’ of the data.

What is left behind is what the paper calls ‘the ‘ghost’ of a signal’ that was present all along. The paper argues that when a signal is sampled at discrete times (and the Dumusque team had to use the La Silla instrument only when it was not otherwise booked), periodicities can be imposed on the signal. Rajpaul was able to simulate a star with no planets, generating synthetic data out of which the exact same 3.24-day planetary signal emerged. The problem is particularly acute when working with planetary ‘signals’ as weak as these. From the paper:

D12’s data set [i.e., the data gathered by Dumusque and team] was particularly pathological because the window function happened to contain periodicities that coincided with the stellar rotation period of α Cen B, and its first harmonic; when these signals were filtered out, the significance of the 3.24 d signal was preferentially boosted.

All this is going to be quite useful if it helps us refine our techniques for identifying small planets. Rajpaul proposes that his team will carry out a new study of the spurious but coherent signals that can emerge from noisy datasets that should help us learn how to mitigate the problem:

We alluded to a number of other tests we believe worth carrying out when considering the reliability of planet detections from noisy, discretely-sampled signals. These include using the same model used to detect the planet instead to fit synthetic, planet-free data (with realistic covariance properties, and time sampling identical to the real data), and checking whether the ‘planet’ is still detected; comparing the strength of the planetary signal with similar Keplerian signals injected into the original observations; performing Bayesian model comparisons between planet and no-planet models; and checking how robust the planetary signal is to datapoints being removed from the observations.

Xavier Dumusque praises the Rajpaul team in this story in National Geographic, saying “This is really good work… We are not 100 percent sure, but probably the planet is not there.” We’re going to get a lot out of this investigation even though we lose Centauri Bb.

But back to that HST transit study run by Brice-Olivier Demory (University of Cambridge). I mentioned that it could detect no transit of Centauri Bb, which certainly fits with what we’ve just seen, but there was an interesting lightcurve suggesting a different possible planet, this one in an orbit that might range from 12 to 20 days. If this planet exists, radial velocity confirmation would be even more challenging than for Centauri Bb. Its signal, as Andrew LePage notes in The Discovery of Alpha Centauri Bb: Three Years Later, would be only half that of Centauri Bb.

LePage’s work at Drew ex Machina is definitive, and he has devoted a good deal of attention to Alpha Centauri. Here he explains why that second ‘planet’ is going to be so hard to spot:

Unfortunately with such a poorly constrained orbit, three weeks of nearly continuous photometric monitoring of α Centauri B will be required to confirm this hypothesis. HST is too busy to accommodate a dedicated search of this length and no other space telescope currently available is capable of making the needed observations. In addition, since the radial velocity signature for this planet would be expected to be maybe half that of α Centauri Bb, this method has little likelihood of providing independent confirmation of this sighting any time soon. Once again, we will have to wait for a few more years for new telescopes to become available such as NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite) which are both scheduled for launches in 2017 and may be capable of making the required observations of such a bright target.

Alpha Centauri is frustrating in many ways because you would expect the closest stellar system to have revealed more of its secrets by now. One of the problems, though, and a huge one, is that the angular separation (as viewed from Earth) of the primary Centauri stars has been decreasing as they move through their orbits. It won’t be until December of this year that they’ll reach minimum separation as seen from Earth. We’ll need to give Alpha Centauri a little time, in other words, before we can hope to get data on other possible worlds around Centauri B.


Image (click to enlarge): Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time [10] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days. Credit: Wikimedia Commons.

The Rajpaul paper is Rajpaul, Aigrain & Roberts, “Ghost in the time series: no planet for Alpha Cen B,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint). The Hatzes paper is “Radial Velocity Detection of Earth-Mass Planets in the Presence of Activity Noise: The Case of α Centauri Bb”, The Astrophysical Journal, Vol. 770, No. 2, (2013) (preprint).



SETI: No Signal Detected from KIC 8462852

by Paul Gilster on November 6, 2015

I’ve mentioned before that I think the name ‘Tabby’s Star’ is a wonderful addition to the catalog. It trips off the tongue so much more easily than the tongue-twisting KIC 8462852, and of course it honors the person who brought this unusual object to our attention, Yale University postdoc Tabetha Boyajian. 1480 light years away, Tabby’s Star is an F3 with a difference. It produces light curves showing objects transiting across its face, some of them quite large, and the search is on to find an explanation that fits within the realm of natural causes.

Five articles about Tabby’s Star have already appeared in these pages, with the most likely explanation being some kind of cometary activity, an answer that seems to satisfy no one. We’ve also consulted both Boyajian’s paper on the subject and a paper by Jason Wright and colleagues out of the Glimpsing Heat from Alien Technologies project at Penn State. The light curves we’re looking at do fit the scenario of a ‘Dyson swarm,’ a cluster of power-collecting surfaces that an advanced civilization might create to extract maximum energy from its star.

Thus we can’t rule out the possibility of an extraterrestrial civilization, but no one is claiming that we’ve found one. The point is that in terms of Dysonian SETI, which looks for signs of another civilization’s activity in our astronomical data, Tabby’s Star is the most interesting target we’ve found, so it only makes sense to investigate it. Assuming we do deduce a natural cause for its signature, we will have learned something about an unusual astrophysical process, and that is all to the good. The sole driver here is to investigate and find out what is happening.

The SETI Institute has a natural interest in all this and has been deploying the Allen Telescope Array on Tabby’s Star for more than two weeks. Now we have an update on what the Institute has found. The effort used the Array’s 42 antennas north of San Francisco to look for narrow-band signals (approximately 1 Hz in width) that could be part of an interstellar beacon. In general, SETI at radio and optical frequencies (SETI, that is, of the non-Dysonian kind) looks for this kind of signal, a deliberate attempt by a civilization to declare its presence.


Image: Allen Telescope Array. Credit: Seth Shostak, SETI Institute.

But the SETI Institute also looked for broadband signals, an interesting choice. Here we are asking whether, if there really is an enormous astro-engineering effort going on around this star, there would be spacecraft sent out to service it. Our own investigations into quick travel around the Solar System point to microwave beaming as a feasible solution, the basis for an interplanetary infrastructure. Such intense microwave beams might well be visible, a kind of ‘leakage’ from the civilization’s activities that implies nothing about communication.

Here’s the result, from the SETI Institute’s paper on the work (Jy stands for jansky, a unit of density used in radio astronomy):

The observations presented here indicate no evidence for persistent technology-related signals in the microwave frequency range 1 – 10 GHz with threshold sensitivities of 180 – 300 Jy in a 1 Hz channel for signals with 0.01 – 100 Hz bandwidth, and 100 Jy in a 100 kHz channel from 0.1 – 100 MHz.

So no clear evidence for either kind of signal between 1 and 10 GHz. The paper goes on:

These limits correspond to isotropic radio transmitter powers of 4 – 7 1015 W and 1020 W for the narrowband and moderate band observations. These can be compared with Earth’s strongest transmitters, including the Arecibo Observatory’s planetary radar (2 1013 W EIRP [effective isotropically radiated power]). Clearly, the energy demands for a detectable signal from KIC 8462852 are far higher than this terrestrial example (largely as a consequence of the distance of this star).

What this initial search does is to place upper limits on anomalous emissions from Tabby’s Star. It tells us that we can rule out omnidirectional transmitters broadcasting narrow-band signals at approximately 100 times today’s total terrestrial energy usage, as well as broadband emissions of ten million times terrestrial energy usage. These numbers are high, as the Institute notes, but the paper goes on to say that required transmitter power for narrow-band signals could be reduced considerably if a signal were being beamed in our direction intentionally. It’s worth remembering, too, that any civilization of K2 status (capable of building a Dyson swarm) should have approximately 1027 watts to work with, the energy output of its star.

In any case, says Institute astronomer Seth Shostak, we keep looking:

“The history of astronomy tells us that every time we thought we had found a phenomenon due to the activities of extraterrestrials, we were wrong. But although it’s quite likely that this star’s strange behavior is due to nature, not aliens, it’s only prudent to check such things out.”

Exactly so. The authors add that the star will be the subject of observations for years to come.

Addendum: The Boquete Optical SETI Observatory in Panama is also going to be brought into the search, as per this story.

The paper is Harp et al., “Radio SETI Observations of the Anomalous Star KIC 8462852” (preprint). A SETI Institute news release is also available.



A 3D Look at GJ 1214b

by Paul Gilster on November 5, 2015

An old friend used to chide me about the space program, asking good-naturedly enough why it mattered to travel nine years to get to a place like Pluto (this was not long after the New Horizons launch). ‘Just another rock,’ he would say. ‘Why go all that way to look at just another rock?’ Although we had many disagreements, Abe was one of the shrewdest people I’ve ever known. I had met him when he was in his sunset years, but in his prime he had run a large financial operation, been the subject of a story on the front page of the Wall Street Journal and had made a serious fortune in real estate speculation.

So what about this ‘just another rock’ meme? Abe died a few years back but I think about him in relation to things like yesterday’s story on Charon. The point is, it’s not just another rock. It’s this particular rock. And maybe it’s not a rock at all; maybe it’s a ball of icy slush. And maybe, as we’ve learned, it’s a seriously interesting thing that surpasses expectation. Each time we get to one of these places, or study a planet with transits and radial velocity methods, we’re seeing something never seen before. It may be able to teach us about conditions far different from those we experience and explain deep questions about the history of our own solar system.

Beyond that, each new ‘rock’ is part of a process of building understanding. We need to know what is around us because while we cannot solve all mysteries, we are compelled to solve the ones in front of us, the ones we can get to or develop the tools to investigate. The thing is, each time we go looking we seem to find something surprising, and then we need an answer for the conundrum. It’s a process that presumably began early in the development of our species.

I used to kid Abe by responding, ‘What does another dollar matter? It’s just another dollar.’ He got the point. Abe had a natural touch with money and knew how to make it multiply. Each new business venture was for him a kind of exploration. We just differed in the direction of our passions. I’ll take Charon or GJ 1214b over stock exchanges in New York or Tokyo, and that’s probably while you’ll never be reading about me on the front page of the Wall Street Journal.

An Exoplanet’s Clouds

So let’s talk about exoplanets. GJ 1214b is likewise ‘just another planet’ when viewed with the wrong filter. Viewed as we view things on Centauri Dreams, it’s one particular planet, and like so many we’ve found, it has its own set of things to teach us. Its star, an M-dwarf, is 42 light years away in the constellation Ophiuchus. Consider this planet a ‘mini-Neptune,’ one of the first discovered. What makes GJ 1214b so interesting is that it’s so close to our own system and gives us such a rich transit signature every 1.6 days.


Image: Comparing the sizes of exoplanets GJ 436b and GJ 1214b with Earth and Neptune. Credit: NASA / ESA / A. Feild and G. Bacon, STScI.

We can use what is called transmission spectroscopy to study the atmospheres of places like this even though we can’t actually see the planet other than through its light curve. A planet passing in front of its star as seen from Earth offers us the opportunity to parse starlight that has been filtered through its atmosphere. The spectrum thus produced tells us about the composition of that atmosphere, its molecules and dust grains. It’s a method that has been used with great effect on worlds like HAT-P-11b and the ‘hot Jupiter’ HD 189733b.

The problem is, when we apply the techniques of transmission spectroscopy to GJ 1214b, nothing much happens. Studies using the Hubble instrument see little variation, a ‘flat spectrum’ that rules out an atmosphere of hydrogen, water, carbon dioxide or methane. Something in the planet’s atmosphere is evidently blocking out light. In a new paper, Benjamin Charnay (University of Washington), working with the university’s Victoria Meadows and several other researchers, has been attacking the problem, setting up models of varying atmospheric temperatures and composition that simulate a three-dimensional cloud structure.

Using a climate model developed by Charnay’s former research group in Paris, the scientist applied models previously used to study Titan to this intriguing exoplanet. GJ 1214b is close enough to its star that atmospheric temperatures are high, exceeding the boiling point of water. Clouds on a world like this would, Charnay believes, most likely be made of potassium chloride (KCl) or zinc sulfide (ZnS), lifted high into the atmosphere to produce such a flat spectrum. The model relies on a robust atmospheric circulation to boost these clouds to altitude.

But there is another potential source of GJ 1214b’s flat spectrum: A photochemical haze. Charnay is now looking at modeling hazes that could produce the same kind of spectrum. Data from the James Webb Space Telescope, scheduled to be launched in 2018, will be needed to rule out alternatives. Assuming 0.5 µm particles (necessary to produce the flat spectra observed), the paper finds that potassium chloride clouds produce a constant reflectivity at visible wavelengths, while zinc sulfide clouds do not absorb at 0.5 µm, producing a peak of reflectivity. Organic haze, on the other hand, strongly absorbs at short visible wavelengths.

A stratospheric thermal inversion should show up at infrared wavelengths. From the paper:

The observation of a few primary/secondary eclipses or full orbits by JWST could provide very precise spectra and phase curves revealing GJ1214b’s atmospheric composition and providing clues on the size and optical properties (i.e. absorbing or not) of clouds. Non-absorbing clouds would suggest KCl particles. Absorbing clouds would favor ZnS particles or organic haze. In that case, the best way for determining the composition of cloud particles would be direct imaging or secondary eclipses/phase curves of reflected light in the visible. The different clouds/haze have characteristic features in visible reflectivity spectra. Future large telescopes such as ELT may have the capabilities for measuring this.

Charnay’s model shows how potassium chloride or zinc sulfide clouds would be created and lifted into the upper atmosphere, while also predicting the effect the clouds would have on planetary weather. The atmospheric circulation in this model is strong enough to carry KCl particles to high altitude regions while producing a minimum of cloud cover at the equator.

Just another rock? Look what we’re doing here: We’re studying layers of atmosphere on a planet we cannot see by using the spectroscopic signatures produced by a star 42 light years from the Sun. In astronomical terms, of course, that’s close, and GJ 1214b’s proximity makes it ideal for the study of mini-Neptunes. Moreover, it may be useful for understanding how our own planet’s atmosphere has changed over time, as Charnay notes in this UW news release:

“Worlds like Titan and this exoplanet have complex atmospheric chemistry that might be closer to what early Earth’s atmosphere was like. We can learn a lot about how planetary atmospheres like ours form by looking at them.”

The trick is in knowing what to look for and developing the tools to investigate — later resources, both space- and ground-based, will then be brought to bear to further the analysis. Given the wild multiplicity of exoplanets, we’re sure to be applying lessons learned at GJ 1214b to other mini-Neptunes, and generalizing from there to broader models of atmospheric evolution. Uncovering things, learning, pushing deeper is a compulsive process. We all have our obsessions, but I can’t think of a better one than the drive to explain other worlds.

The paper is Charnay et al., “3D modeling of GJ1214b’s atmosphere: formation of inhomogeneous high clouds and observational implications,” Astrophysical Journal Letters, Volume 813, Number 1 (abstract / preprint).



Unusual Crater on Charon

by Paul Gilster on November 4, 2015

Another surprise from New Horizons, in a year which will surely see a few more before it ends. After all, we have a long flow of data ahead as the spacecraft continues to return the information it gathered during the July flyby of Pluto/Charon. Now we focus on Charon and the crater being called Organa, which produced an anomaly when scientists studied the highest resolution infrared compositional scan of the moon available. This crater and some of the surrounding materials show infrared absorption at about 2.2 microns, indicating frozen ammonia.

Not far away on Charon’s Pluto-facing hemisphere is Skywalker crater, which under infrared scrutiny shows the same composition as the rest of Charon’s surface. Here water ice — not ammonia — dominates. As this JHU/APL news release notes, ammonia absorption was first detected on Charon as far back as 2000, but what we’re seeing here is unusually concentrated. In any case, why is Organa so different from Skywalker and the rest of Charon’s craters?


Image: This composite image is based on observations from the New Horizons Ralph/LEISA instrument made at 10:25 UT (6:25 a.m. EDT) on July 14, 2015, when New Horizons was 81,000 kilometers from Charon. The spatial resolution is 5 kilometers per pixel. The LEISA data were downlinked Oct. 1-4, 2015, and processed into a map of Charon’s 2.2 micron ammonia-ice absorption band. Long Range Reconnaissance Imager (LORRI) panchromatic images used as the background in this composite were taken about 8:33 UT (4:33 a.m. EDT) July 14 at a resolution of 0.9 kilometers per pixel and downlinked Oct. 5-6. The ammonia absorption map from LEISA is shown in green on the LORRI image. The region covered by the yellow box is 280 kilometers across. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Organa and Skywalker are roughly the same size, about 5 kilometers in diameter, and both show the same ‘rays’ of ejected material, although Organa’s central areas are darker. But there seems to be no correlation: The ammonia-rich material extends beyond the dark area. One possibility is that the impactor that created Organa was rich in ammonia. An alternative posited by Will Grundy (New Horizons Composition team lead, Lowell Observatory) is that the crater could have been the result of an impact into a pocket of ammonia-rich subsurface ice.

“This is a fantastic discovery,” says Bill McKinnon (Washington University, St. Louis), deputy lead for the mission’s Geology, Geophysics and Imaging team. “Concentrated ammonia is a powerful antifreeze on icy worlds, and if the ammonia really is from Charon’s interior, it could help explain the formation of Charon’s surface by cryovolcanism, via the eruption of cold, ammonia-water magmas.”

Meanwhile, New Horizons has now completed the third of its four trajectory-adjusting maneuvers needed for intercept of Kuiper Belt object 2014 MU69. This was a 30-minute burn with the craft’s hydrazine thrusters and all indications are that it was successful. The fourth and final targeting maneuver is scheduled for today (November 4), although adjustments will be made later in the mission as more information about the KBO’s orbit is obtained. Remember that we only found 2014 MU69 in 2014, after a long search for a Kuiper Belt candidate.


Image: Projected route of NASA’s New Horizons spacecraft toward 2014 MU69, which orbits in the Kuiper Belt about 1.6 billion kilometers beyond Pluto. Planets are shown in their positions on Jan. 1, 2019, when New Horizons is projected to reach the small Kuiper Belt object. NASA must approve an extended mission for New Horizons to study the ancient KBO. Credit: JHU/APL.



Exoplanetology Beyond Kepler

by Paul Gilster on November 3, 2015

Useful synergies continue to emerge among our instruments as we ponder the future of exoplanet studies. Consider the European Space Agency’s PLATO mission (PLAnetary Transits and Oscillations of stars). Operating from the L2 Lagrangian point, PLATO will use 34 telescopes and cameras on a field of view that includes a million stars, using transit photometry, as Kepler did, to find planetary signatures. Working at optical wavelengths, PLATO will look for nearby Earth-sized and ‘super-Earth’ planets in the habitable zone of their stars.


This mission is scheduled to be launched in 2024, an interesting date because it’s also the year that the European Extremely Large Telescope (E-ELT) is scheduled to see first light. Huge new installations like these, although ground-based, are so powerful that they should be able, with the help of adaptive optics, to study planetary atmospheres on the PLATO-discovered planets. Thus we get the best of both worlds, with repairable and upgradable ground telescopes fleshing out the data gathered by our space instruments, just as today we can use Kepler data to find planet candidates and then confirm them using radial velocity studies from the ground.

The TESS mission (Transiting Exoplanet Survey Satellite) launches earlier (probably in 2018) but offers the same kind of synergies with other instruments. Both TESS and PLATO, for example, will hand off data to the James Webb Space Telescope, scheduled for a 2018 launch. Here again we can look for deepened studies of the targets other missions have found. And in the process, we can be assured that we’ll enrichen our catalog of extrasolar worlds.

Screenshot from 2015-11-03 12:15:19

Just what we might find is the subject of new work by Michael Hippke (Institute for Data Analysis, Neukirchen-Vluyn, Germany) and Daniel Angerhausen, a postdoc at NASA GSFC. Writing in The Astrophysical Journal, the duo explain in one recent paper that while planets with sizes and orbits similar to Mars or Mercury will be out of reach (around solar-class stars, at any rate), planets the size of Venus or Earth should show up readily for TESS and PLATO. The optimum target for life-hunters, of course, is an Earth-class world in an Earth-like orbit, and both instruments are believed to be capable of finding these. From the paper:

In this work, we have shown that future photometry will be able to detect Earth- and Venus-analogues when transiting G-dwarfs like our Sun… Larger sized planets (> 2R) will be detected in a single transit around G-dwarfs, in low stellar noise cases, and assuming one can find them in the first place. The search techniques for such single transits will require further research and validation, and will likely be performed remotely, due to the large storage requirements.

But Hippke and Angerhausen’s interests extend beyond planets. They believe that a large planet like Jupiter that has several large moons should produce a characteristic signature, allowing its ‘exomoons’ to be detected. The detection would be marginal, as Angerhausen explains: “We wouldn’t have a clear detection, and we wouldn’t be able to say whether the planet had a single large moon or a set of small ones, but the observation would provide a strong moon candidate for follow-up by other future facilities.”

Let me drop back quickly to one of Hippke’s papers from early 2015, which explored exomoon candidates from the Kepler data, using what is known as the orbital sampling effect, which stacks numerous planet transits and tries to extract an exomoon signature. In this paper, Hippke used numerical simulations to inject exomoon signals into real Kepler data. This is useful because it shows us that there is a size limit to what we can find, one that TESS and PLATO should be able to improve on. The paper finds that for suitable planets with orbits between 35 and 80 days, an exomoon’s detectable radius is approximately 2120 kilometers, or about a third the radius of Earth, while for longer period planets, even larger moons are the minimum.

Such moons go beyond what we generally see in our system, but as the paper notes:

…our solar system might not be the norm – we have no Hot Jupiters, warm Neptunes, or Super-Earths in our solar system, and thus no reference for typical moons around such planets. Also, there is a strong selection bias, based on the detection limits…, and in addition the simple fact that the strongest dips are most significant. The first moons to be found will likely be at the long (large/massive) end of exomoon distribution, as was the case for exoplanets.

Hippke is surely right that the first moons found will be at the larger end of the size range, just as the first exoplanets we detected were massive worlds in close orbits that were the easiest to see with our instruments. For more on Hippke’s work and the methods he employs, see the article he wrote for Centauri Dreams, Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak.

But back to the Hippke and Angerhausen paper I started with. It notes that while the detection of moons will remain problematic for planets analogous to those in our own system, moons around planets orbiting quiet M-dwarf stars should be easier to detect. This paper, “Photometry’s bright future: Detecting Solar System analogues with future space telescopes,” focuses in directly on the capabilities of instruments like TESS and PLATO in offering datasets beyond Kepler’s.

Here again the authors deploy the Orbital Sampling Effect:

The OSE can be used to detect a significant flux loss before and after the actual transit (if present), which might be indicative of an exomoon in transit. The basic idea is that at any given transit the moon(s) must be somewhere: They might transit before the planet, after the planet, or not at all – depending on the orbit configuration. But by stacking many such transits, one gets, on average, a flux loss before and a flux loss after the exoplanet transit.

And bear in mind that moons are only one of the things we might expect to extract from TESS and PLATO data. Right now we have one detected ring system, around the planet J1407b, a massive ring more than 200 times larger than Saturn’s. The authors show that a transiting planet with a ring system produces a definitive signal. Even Trojan asteroids, which lead and follow a planet by 60 degrees in its orbit, should be in range for detection. In a third paper, the authors use Kepler data, injecting synthetic Trojan light curves to search for the limits of detectability. From the paper:

Our result gives an upper limit to the average Trojan transiting area (per planet) corresponding to one body of radius < 460km at 2σ confidence. We find a significant Trojan-like signal in a sub-sample for planets with more (or larger) Trojans for periods >60 days.

The authors call these results tentative and suggest that improved data from TESS and PLATO should help us refine them. “As good as the Kepler data are, we’re really pushing them to the limit, so this is a very preliminary result,” adds Hippke in this NASA news release. “We’ve shown somewhat cautiously that it’s possible to detect Trojan asteroids, but we’ll have to wait for better data from TESS, PLATO and other missions to really nail that down.”

All of which tells us that we have much to expect from TESS and PLATO and the instruments that will subsequently home in on the targets they have provided. The papers are Hippke, “On the detection of Exomoons: A search in Kepler data for the orbital sampling effect and the scatter peak,” The Astrophysical Journal Vol. 806, No. 1 (abstract / preprint); Hippke and Angerhausen, “Photometry’s bright future: Detecting Solar System analogues with future space telescopes,” accepted at The Astrophysical Journal (preprint); and Hippke and Angerhausen, “A statistical search for a population of Exo-Trojans in the Kepler dataset,” accepted at The Astrophysical Journal (preprint).



Voyager Update: Probing the Boundary

by Paul Gilster on November 2, 2015

I always feel that my day starts right when a story involving the Voyagers crosses my desk. The scope, the sheer audacity of these missions in their day cheers me up, and the fact that they are still communicating with us is a continual cause for celebration. With Voyager 1 now moving beyond the heliosphere, we’ve got an interstellar craft on our hands, one that’s telling us a good deal about the perturbed regions through which it moves. Every day that the Voyagers stay alive is a triumph for an inquisitive and exploring species, and one day we’ll be launching their successor, targeting the local interstellar medium with instruments designed for the task.


Image: This artist’s concept shows NASA’s Voyager spacecraft against a backdrop of stars. Credit: NASA/JPL-Caltech.

The heliosphere is that ‘bubble’ blown by the particles of the Sun’s solar wind in surrounding interstellar space. As such, it’s a moving and malleable thing, flexing, flowing, contracting here, expanding there according to the stream of particles filling it from our star. Now we’re finding, not surprisingly, I think, that the magnetic field just outside the heliosphere is perturbed. Voyager 1 data show that the magnetic field here is more than forty degrees at variance from observations of the interstellar magnetic field that have been produced by other spacecraft.

Remember that while Voyager 1 is the first spacecraft to reach such distances from the Sun, we’ve had other missions exploring the outer regions of the system even though they operate well within it. The Interstellar Boundary Explorer (IBEX) is the outstanding example. Launched in 2009, IBEX is in Earth orbit (apogee 322,000 kilometers, perigee 16,000 kilometers), using instruments that track the solar wind’s interactions with ionized interstellar material. Like the Ulysses spacecraft before it, IBEX is also measuring inflowing neutral particles that penetrate the heliosphere, in the process creating a map of the Solar System’s elastic boundaries.


Image: An artist’s rendition of a portion of our heliosphere, with the solar wind streaming out past the planets and forming a boundary as it interacts with the material between the stars. Credit: Adler Planetarium/IBEX team.

IBEX has already told us a lot, including the fact that our Sun is located in a region of space where dust and gas are much more dispersed than they were when the Sun was formed. With IBEX data, scientists can map the distribution of elements like hydrogen, helium, neon and oxygen as they enter the heliosphere, and can use the neon to oxygen ratio in the Sun to trace element ratios in the distant past. We see less oxygen than expected in the interstellar medium today, indicating the changes to the medium since solar formation.

We also see that our Sun is close to the boundary of a local cloud of gas and dust, but still within it — this work challenges earlier Ulysses findings that found the Sun to exist between two clouds, though close to the boundary of the ‘Local Cloud.’ Within a few thousand years, we will be moving out of the Local Cloud and into a somewhat different galactic environment.

IBEX, then, is helping us understand the general region of space through which we move, while Voyager 1 is reporting on conditions just outside the heliosphere boundary. In 2009, IBEX data showed what principal investigator David McComas (SwRI) called a ‘very narrow ribbon that is two to three times brighter than anything else in the sky’ at the interstellar boundary. While this circular arc is still under study, the current view is that it is produced by neutral hydrogen atoms from the solar wind that were reionized in interstellar space and then became neutral again by picking up an additional electron. (For more on IBEX, see IBEX: The Heliosphere in Motion).

This is where the recent Voyager findings come in. Nathan Schwadron (University of New Hampshire) and colleagues have reanalyzed magnetic field data from Voyager 1, discovering that the direction of the magnetic field has been turning ever since the craft crossed into interstellar space. The work, published in Astrophysical Journal Letters confirms that the magnetic field direction at the center of the IBEX ‘ribbon’ is aligned with the magnetic field in the interstellar medium. Voyager is, in other words, now moving through a distorted region. By 2025, the magnetic field around it should align with the field direction found by IBEX.

At that point, we’ll be able to say that Voyager 1 has reached a more settled part of the interstellar medium, less perturbed by the ‘churn’ of the heliosphere. “This study provides very strong evidence that Voyager 1 is in a region where the magnetic field is being deflected by the solar wind,” says Schwadron in this JPL news release. A Voyager follow-up mission will be built from the outset as an interstellar probe, carrying instruments optimized for exploring this active boundary. We can hope that one day even more ambitious missions will use the data thus gathered to chart the regions through which they’ll fly on their way to another star.

The paper is Schwadron et al., “Triangulation of the Interstellar Magnetic Field,” Astrophysical Journal Letters Vol. 813, No. 1, L20. (abstract).



Science Fiction and the Symposium

by Paul Gilster on October 30, 2015

Science fiction is much on my mind this morning, having just been to a second viewing of The Martian (this time in 3D, which I didn’t much care for), and having just read a new paper on wormholes that suggests a bizarre form of communication using them. More about both of these in a moment, but the third reason for the SF-slant is where I’ll start. The 100 Year Starship organization’s fourth annual symposium is now going on in Santa Clara (CA), among whose events is the awarding of the first Canopus Awards for Interstellar Writing.

Screenshot from 2015-10-30 12:13:56

A team of science fiction writers will anchor what the organization is calling Science Fiction Stories Night on Halloween Eve. Among the writers there, I’m familiar with the work of Pat Murphy, whose novel The Falling Woman (Tor, 1986) caught my eye soon after publication. I remember reading this tale of an archaeological dig in Central America and the ‘ghosts’ it evokes with fascination, although it’s been long enough back that I don’t recall the details. Joining Murphy will be short story writer Juliette Wade, novelist Brenda Cooper and publisher Jacob Weisman, whose Tachyon publishing is a well-known independent press.

As to the Canopus Awards, they’re to be an annual feature of the 100 Year Starship initiative aimed at highlighting “the importance of great story telling to propel the interstellar movement” (I’m quoting here from their press materials). In case you’re looking for some reading ideas, here are the Canopus finalists going into the event.

In the category of “Previously Published Long-Form Fiction” (40,000 words or more):

Other Systems by Elizabeth Guizzetti

The Creative Fire (Ruby’s Song) by Brenda Cooper

InterstellarNet: Enigma (Volume 3) by Edward M. Lerner

Aurora by Kim Stanley Robinson

Coming Home by Jack McDevitt


In the category of “Previously Published Short-Form Fiction” (between 1,000 and 40,000 words):

“Race for Arcadia” by Alex Shvartsman

“Stars that Make Dark Heaven Light” by Sharon Joss

“Homesick” by Debbie Urbanski

“Twenty Lights to the Land of Snow” by Michael Bishop

“Planet Lion” by Catherynne M. Valente

“The Waves” by Ken Liu

“Dreamboat” by Robin Wyatt Dunn


In the category of “Original Fiction” (1,000-5,000 words):

“Landfall” by Jon F. Zeigler

“Project Fermi” by Michael Turgeon

“Everett’s Awakening” by Ry Yelcho

“Groundwork” by G. M. Nair

“His Holiness John XXIV about Father Angelo Baymasecchi’s Diary” by Óscar Garrido González

“The Disease of Time” by Joseph Schmidt


In the category of “Original Non-Fiction” (1,000-5,000 words):

“Why Interstellar Travel?” by Jeffrey Nosanov

“Finding Earth 2.0 from the Focus of the Solar Gravitational Lens” by Louis D. Friedman and Slava Turyshev

Of Martians and Wormholes

This will be the first 100YSS symposium I’ve missed and I’ll regret missing the chance to meet Pat Murphy and see Mae Jemison, Lou Friedman, Jill Tarter and many others who have made past events so enjoyable. I imagine Jack McDevitt will be there as well; he usually goes to these. His Canopus Award entry Coming Home (Ace, 2014) is another in the Alex Benedict series, featuring a future antiques dealer among whose many artifacts are ‘antiques’ that were crafted far in our own future. I mention Jack because I admire him, have read all the Alex Benedict novels, and thought Coming Home was one of his best.

As to The Martian, it’s a movie I loved for its attention to detail and the sheer bravura of its proceedings. For people who remember Apollo, the idea of a Mars exploration program of similar audacity is a wonderful morale-booster, a reminder that the spirit that took us to the Moon is still alive. It also makes for a jolting comparison between those days and today’s public apathy and budgetary dilemmas, all of which make Mars a target that always seems to be, like fusion, somewhere in the future. Movies like The Martian could do something to reach younger generations, and perhaps ignite interest in both government and private attempts to get to the Red Planet. But be aware that the regular version offers far more verisimilitude than the 3D, whose effects seem contrived and often distracting.

I don’t have time to dig deeply into Luke Butcher’s new paper on wormholes, but I do at least want to mention this effort as one that has caught the interest of wormhole specialist Matt Visser, and should intrigue science fiction authors for its plot possibilities. Working at the University of Cambridge, Butcher has been studying how to keep wormhole mouths open, the problem being that although people like Kip Thorne have speculated on using negative energy to do the trick, wormholes appear to be utterly unstable, closing before they can be used.

If wormholes do exist and we could find a way to use them, we might have a way to cross huge distances without contradicting Einstein’s limits on travel faster than light, using the wormhole’s ability to shortcut its way through spacetime itself. Butcher looks at negative energy in terms of Casimir’s parallel plates sitting close together in a vacuum. What if a wormhole’s own shape could generate such Casimir energies, thus holding it open long enough to use?


Image: Imagining a wormhole. Here we see a simulated traversable wormhole that connects the square in front of the physical institutes of University of Tübingen with the sand dunes near Boulogne sur Mer in the north of France. The image is calculated with 4D raytracing in a Morris–Thorne wormhole metric, but the gravitational effects on the wavelength of light have not been simulated. Credit: Wikimedia Commons.

Butcher can’t find ways to keep wormholes open for long, but he does offer the theoretical possibility that we might be able to keep one open long enough to get a beam of light into it. Get the picture? Communications moving through the wormhole, with the same effect as if they were moving faster than light, with all the interesting causal issues that raises. From the paper:

…the negative Casimir energy does allow the wormhole to collapse extremely slowly, its lifetime growing without bound as the throat-length is increased. We find that the throat closes slowly enough that its central region can be safely traversed by a pulse of light.

So there you are, science fiction writers, another plot possibility involving communications between starships or, for that matter, between planets in, say, a galaxy-spanning civilization of the far future. Make of it what you will. The delight of science fiction is that it can take purely theoretical constructs like this one and run down the endless chain of possibilities. In our era of deep space probes, astrobiology and exoplanet research, science fiction has truly moved out of the literary ghetto in which it once saw itself enclosed. Canopus Award winners take note: You’re starting to go mainstream.

The paper is Butcher, “Casimir Energy of a Long Wormhole Throat,” submitted to Physical Review D (preprint).



Where We Might Sample Europa’s Ocean

by Paul Gilster on October 29, 2015

No one interested in the prospects for life on other worlds should take his or her eyes off Europa for long. We know that its icy surface is geologically active, and that beneath it is a global ocean. While water ice is prominent on the surface, the terrain is also marked by materials produced by impacts or by irradiation. Keep in mind the presence of Io, which ejects material like ionized sulfur and oxygen that, having been swept up in Jupiter’s magnetosphere, eventually reaches Europa. Irradiation can break molecular bonds to produce sulfur dioxide, oxygen and sulfuric acid. And we’re learning that local materials can be revealed by geology.

A case in point is a new paper that looks at infrared data obtained with the adaptive optics system at the Keck Observatory. The work of Mike Brown, Kevin Hand and Patrick Fischer (all at Caltech, where Fischer is a graduate student), suggests that the best place to look for compounds indicative of life would be in the jumbled areas of Europa called chaos terrain. Here we may have materials brought up from the ocean below.

“We have known for a long time that Europa’s fresh icy surface, which is covered with cracks and ridges and transform faults, is the external signature of a vast internal salty ocean,” says Brown, and our imagery of these areas taken by Galileo shows us a shattered landscape, with great ‘rafts’ of ice that have broken, moved and later refrozen. The clear implication is that water from the internal ocean may have risen to the surface as these chaos areas shifted and cracked. And while a direct sampling of Europa’s ocean would be optimal, our best bet for studying its composition for now may well be a lander that can sample frozen deposits.


Image: On Europa, “chaos terrains” are regions where the icy surface appears to have been broken apart , moved around, and frozen back together. Observations by Caltech graduate student Patrick Fischer and colleagues show that these regions have a composition distinct from the rest of the surface which seems to reflect the composition of the vast ocean under the crust of Europa. Credit: NASA/JPL-Caltech.

Brown and team, whose work has been accepted at The Astrophysical Journal, examined data taken in 2011 using the OSIRIS spectrograph at Keck, which measures spectra at infrared wavelengths. Keck is also able to bring adaptive optics into play to sharply reduce distortions produced by Earth’s atmosphere. Spectra were produced for 1600 different locations on the surface of Europa, then sorted into major groupings using algorithms developed by Fischer. The results were mapped onto surface data produced by the Galileo mission.

The result: Three categories of spectra showing distinct compositions on Europa’s surface. From the paper:

The first component dominates the trailing hemisphere bullseye and the second component dominates the leading hemisphere upper latitudes, consistent with regions previously found to be dominated by irradiation products and water ice, respectively. The third component is geographically associated with large geologic units of chaos, suggesting an endogenous identity. This is the first time that the endogenous hydrate species has been mapped at a global scale.

We knew about Europa’s abundant water ice, and we also expected to find chemicals formed from irradiation. The third grouping, though, being particularly associated with chaos terrain, is intriguing. Here the chemical indicators did not identify any of the salt materials thought to be on Europa. The paper continues:

The spectrum of component 3 is not consistent with linear mixtures of the current spectral library. In particular, the hydrated sulfate minerals previously favored possess distinct spectral features that are not present in the spectrum of component 3, and thus cannot be abundant at large scale. One alternative composition is chloride evaporite deposits, possibly indicating an ocean solute composition dominated by the Na+ and Cl ions.


Image: Mapping the composition of the surface of Europa has shown that a few large areas have large concentrations of what are thought to be salts. These salts are systematically located in the recently resurfaced “chaos regions,” which are outlined in black. One such region, named Western Powys Regio, has the highest concentration of these materials presumably derived from the internal ocean, and would make an ideal landing location for a Europa surface probe.
Credit: M.E. Brown and P.D. Fischer/Caltech , K.P. Hand/JPL.

The association with chaos areas is significant. Because these spectra map to areas with recent geological activity, they are likely to be native to Europa, and conceivably material related to the internal ocean. In this Caltech news release, Brown speculates that a large amount of ocean water flowing out onto the surface and then evaporating could leave behind salts. As in the Earth’s desert areas, the composition of the salt can tell us about the materials that were dissolved in the water before it evaporated. Brown adds:

“If you had to suggest an area on Europa where ocean water had recently melted through and dumped its chemicals on the surface, this would be it. If we can someday sample and catalog the chemistry found there, we may learn something of what’s happening on the ocean floor of Europa and maybe even find organic compounds, and that would be very exciting.”

So we’re learning where a Europa lander should be able to do the most productive science in relation to astrobiology and the ocean beneath the ice. Keep your eye on the western portion of the area known as Powys Regio, where the Caltech team found the strongest concentrations of local salts. Powys Regio is just south of what appears to be an old impact feature called Tyre. The image below, with the concentric rings of Tyre clearly visible, reminds us that an ocean under a mantle of ice is vulnerable to surface activity and external strikes that would break through the ice and deposit ocean materials within reach of the right kind of lander.


Image: The feature called Tyre, showing signs of an ancient Europan impact. Credit: NASA/JPL-Caltech.

The paper is Fischer, Brown & Hand, “Spatially Resolved Spectroscopy of Europa: The Distinct Spectrum of Large-scale Chaos,” accepted at The Astrophysical Journal (preprint).