by Paul Gilster | Sep 21, 2018 | Missions |
The image below intrigues me. It’s the first image of the Earth and the Moon together taken from a CubeSat, one of a pair of such tiny spacecraft NASA has despatched to Mars as part of a mission called MarCO (Mars Cube One), which will work in conjunction with the InSight lander. Taken on May 9, the photo was part of the process of testing the CubeSat’s high-gain antenna. But to me it’s a reminder of how far miniaturized technologies continue to advance.
Image: The first image captured by one of NASA’s Mars Cube One (MarCO) CubeSats. The image, which shows both the CubeSat’s unfolded high-gain antenna at right and the Earth and its moon in the center, was acquired by MarCO-B on May 9. Credit: NASA/JPL-Caltech.
As of this morning, we are 66 days away from InSight’s landing on Mars, at a distance of 65 million kilometers from Earth and 16 million kilometers to Mars. I don’t usually focus on Mars and lunar missions because this site’s specialty is deep space, which for our purposes means Jupiter and beyond, and of course the overall theme here is interstellar. But experimental technologies that bring us greater performance from very small payloads are certainly relevant.
Anything we can do to shrink payload size pays off as we look at ever more distant targets, and the cruise velocities and propellant needed to reach them. And CubeSats are a way of exploring small payloads. The standard 10×10×11 cm basic CubeSat is a ‘one unit’ (1U) CubeSat, but larger platforms of 6U and 12U allow more complex missions. With fixed satellite body dimensions, the CubeSat format creates a highly modular and integrated system.
What we have with the two MarCO spacecraft is the application of what had been a low-Earth orbit satellite technology to a planetary mission, with a useful goal. Trailing InSight by thousands of kilometers, they’ve already demonstrated their ability to operate in the interplanetary environment. At Mars, the intention is for them to relay data on InSight’s landing, a job consigned to Mars orbiters, but one this mission may show CubeSats are able to perform.
Image: Illustration of one of the twin MarCO spacecraft with some key components labeled. Front cover is left out to show some internal components. Antennas and solar arrays are in deployed configuration. Credit: NASA/JPL-Caltech.
Each of the MarCOs has its own high-gain antenna and the necessary radio equipment for data relay, with propulsion systems that have already made two steering maneuvers enroute. No one would claim the diminutive space travelers are as complex as conventional interplanetary craft, but I can see two goals here, the first of which leverages the ‘traditional’ CubeSat role of acting as low-cost entry-level ways to reach orbit.
“Our hope is that MarCO could help democratize deep space,” said Jakob Van Zyl, director of the Solar System Exploration Directorate at NASA’s Jet Propulsion Laboratory in Pasadena, California. “The technology is cheap enough that you could envision countries entering space that weren’t players in the past. Even universities could do this.”
Fair enough, as we’ve learned how satellites can be ‘piggybacked’ to open up access to space, lowering launch costs even as the cost of the CubeSats offers opportunities for inexpensive missions. Moreover, the fact that CubeSats can be built with standardized parts and systems, with key components provided by commercial partners, underscores their efficiency.
But let’s move beyond today’s current CubeSat. If we can build these craft strong enough to handle relay operations from Mars, we can contemplate future CubeSats capable of a wider range of science return and consider propulsion technologies like solar sails for ‘swarm’ missions to targets beyond Mars. Of particular interest is coupling CubeSats with solar sails for propulsion. Remember, for example, The Planetary Society’s LightSail-1, launched in 2015, which demonstrated sail deployment despite a series of major software glitches.
LightSail-2 is designed to demonstrate controlled solar sailing in the CubeSat format, with a sail of 32 square meters. A key goal of this mission is to raise the orbit apogee after sail deployment at 720 kilometers. I should also mention LightSail-3, which could take this technology out to the L1 Lagrangian point, where it would remain to monitor geomagnetic activity on the Sun.
NASA’s own future plans for CubeSat work take in BioSentinel, which would take living yeast (S. cerevisiae) into space to study DNA lesions caused by energetic particles, with operations expected to last 18 months at distances well beyond low Earth orbit. The NEA (Near-Earth Asteroid) Scout mission would take a CubeSat/sail to a small asteroid, exploring its rotational properties, spectral class, regional morphology and regolith, while Lunar Flashlight would achieve lunar orbit to study ice deposits on the Moon for the use of future explorers.
Image: Engineer Joel Steinkraus uses sunlight to test the solar arrays on one of the Mars Cube One (MarCO) spacecraft. Credit: NASA/JPL-Caltech.
I might likewise mention such European Space Agency efforts as GOMX-3, a CubeSat mission exploring the telecommunications capabilities of such craft. GOMX-3 was deployed from the International Space Station in October of 2015 and operated for a year before re-entering the atmosphere.
The list of upcoming missions under ESA’s In-Orbit Demonstration is extensive (you can see it here), and it’s noteworthy that the agency inserts at the top of its list of potential applications the fact that CubeSats can serve “As a driver for drastic miniaturisation of systems, ‘systems-on-chips’, and totally new approach to packaging and integration, multi-functional structures, embedded propulsion.”
So we can keep an eye on the MarCOs as a harbinger of CubeSat operations to come. All three of the future NASA CubeSat missions I’ve mentioned are designed to be launched as secondary payloads on a future Space Launch System (SLS) mission. But however we get such missions into space, they point toward further exploration of small payloads, a parameter space Breakthrough Starshot is pushing to the max in its plans for a centimeter-sized, gram-scale payload to be driven by laser propulsion at 20 percent of lightspeed to another star.
by Paul Gilster | Sep 20, 2018 | Outer Solar System |
We’ve only orbited one object in the Solar System known to exhibit cryovolcanism, but Ceres has a lot to teach us about the subject. Unlike the lava-spewing volcanoes of Earth, an ice volcano can erupt with ammonia, water or methane in liquid or vapor form. What appear to be cryovolcanoes can be found not only on Ceres but Titan, and the phenomenon appears likely on Pluto and Charon. Neptune’s moon Triton is a special case, with rugged volcanic terrain in evidence, as opposed to much smoother surfaces without obvious volcanoes elsewhere.
Activity like this can be a good deal less dramatic than what we see on Earth, or spectacularly on Io. The eruption of an ice volcano involves rocks, ice and volatiles more or less oozing up out of the volcano to freeze on the surface, a process thought to be widespread on Ceres. But what happens to cryovolcanoes as they age? Ahuna Mons, an almost five-kilometer tall mountain that is no more than 200 million years old, raises the question. Why is it so tall, and why, assuming other cryovolcanic activity on this surface, are there no other mountains this imposing?
Image: Ceres’ unusual mountain Ahuna Mons is seen in this mosaic of images from NASA’s Dawn spacecraft. On its steepest side, this mountain is about 5 kilometers high. Its average overall height is 4 kilometers. The diameter of the mountain is about 20 kilometers. Dawn took these images from its low-altitude mapping orbit in December 2015. Credit: NASA/JPL/Dawn mission.
Tackling the question of cryovolcano evolution is Michael Sori (University of Arizona), working with colleagues at UA and elsewhere to advance a theory they call ‘viscous relaxation.’ The idea here is that the rock and ice making up Ceres can flow in much the way that glaciers flow on Earth, with the ice/rock balance affecting the flow rate. Also a key factor: The temperature, which on Ceres never warms higher than -35 degrees Celsius. A cryovolcano at the poles, in other words, would ‘relax’ at a much slower rate than a warmer mountain at the equator.
Image: Each curve represents a cryovolcano of a different age and shows that mountains at low latitudes close to the equator flatten out at younger ages than mountains located at high latitudes at the poles, where they will maintain their height and width for eons. Credit: Michael Sori.
The youthful Ahuna Mons would have been active in the geologically recent past. In a paper published in 2017, Sori’s team used numerical models to predict the flow velocity of the cryovolcano, which they derive as 10-500 meters per million years given the variables of ice content, rheology (the deformation and flow of matter), grain size, and temperature. Flows at the slow end of this range can relax a cryovolcanic structure over 108 to 109 years, a rate sufficient to allow Ahuna Mons to remain identifiable today.
Older cryovolcanoes, having gone lengthier periods of relaxation, become shorter, wider and more rounded with the passage of time. The trick is to identify ancient cryovolcanic structures on today’s surface. In a paper just released in Nature Astronomy, the scientists used their computer simulations as the context in which to scan Dawn data on Ceres’ topography.
22 surface features matched the simulation’s predictions, including Yamor Mons, an old polar mountain five times wider than it is tall. Mountains elsewhere on the surface of the dwarf planet have lower aspect ratios; that is, they are much wider than they are tall. Sori’s team was able to use the simulation/data match to estimate the age of many volcanoes and their volume.
Viscous relaxation as modeled here allows the researchers to derive the rate at which cryovolcanoes form, which turns out to be one every 50 million years. Ceres shows signs of cryovolcanism throughout its history, making the process an important factor in its geological evolution, though less so than the kind of volcanism found on terrestrial planets. Depending on latitude, modeled volcanoes begin tall and steep, but grow short and wide over time.
This passage from the 2017 paper summarizes the process of cryovolcano evolution. Note the acronym FEM, standing for ‘finite element method,’ a reference to software code originally developed for the study of the movement of ice on Earth and used in these simulations
Viscous flow of topography is a modification mechanism that can describe the presence of a single, young, prominent cryovolcano (Ahuna Mons). Flow velocities are fast enough to heavily modify structures on timescales of tens to hundreds of million years, shallowing their topographic relief and providing an explanation for why Dawn observations have not yet identified a plethora of cryovolcanoes. Flow velocities are not so fast that the observed near-pristine state of Ahuna Mons becomes problematic. Domes must be more compositionally ice rich than the average Cerean crust to flow, which is a reasonable property for a cryovolcanic construct. Our hypothesis works with any volumetric fraction of ice > ~40%, but we favor the lower end of this range because past studies do not favor a pure-ice composition for Ahuna Mons. Cerean cryovolcanism involving cryomagma that is approximately half water by volume fits both Dawn observations and our FEM models.
Image: A topographic map of Ceres (top), with the dark red representing the highest elevations and dark blue being the lowest. The topography of Ahuna Mons is shown on the bottom left, and Yamor Mons is shown on the bottom right. The bottom center image shows the topography of an ancient cryovolocano located north of Ceres’s equator. Credit: Michael Sori.
Cryovolcanoes aren’t dramatic. We substitute an ooze of cryomagma, a mixture of rock, salty ice and volatiles, for explosive eruptions. Freezing on the surface, the cryomagma eventually rises into peaks like Ahuna Mons before the inevitable relaxation as geological ages pass. How and why the cryovolcanic activity occurs just where it does is a question for future research, and one that may be illuminated by studies of the ice volcanoes elsewhere in the Solar System.
The paper is Sori et al., “Cryovolcanic rates on Ceres revealed by topography,” Nature Astronomy 17 September 2018 (abstract). The earlier paper is Sori et al., “The vanishing cryovolcanoes of Ceres,” Geophysical Research Letters Vol. 44, Issue 3 (16 February 2017) 1243-1250 (abstract).
by Paul Gilster | Sep 19, 2018 | Outer Solar System |
There are two senses in which we are ‘looking back’ at Titan in today’s post. On the one hand, the New Horizons spacecraft has already taken sensors well beyond Pluto in preparation for the encounter with MU69. From its perspective, anything in the Solar System inside the Kuiper Belt is well behind. What with our Pioneers, Voyagers and now New Horizons, the human perspective has widened that far.
But we’re also looking back in terms of time when we revisit the Cassini mission and what it had to tell us about Saturn’s moons. Below is the final view the spacecraft had of Titan’s lakes and seas, a view of the north polar terrain showing the abundance of liquid methane and ethane. The view was acquired on September 11, 2017, a mere four days before Cassini was sent to its fiery end in Saturn’s atmosphere as a way of avoiding any potential future contamination.
Image: This view of Titan’s northern polar landscape was obtained at a distance of approximately 140,000 kilometers (87,000 miles) from Titan. Image scale is about 800 meters (0.5 miles) per pixel. The image is an orthographic projection centered on 67.19 degrees north latitude, 212.67 degrees west longitude. An orthographic view is most like the view seen by a distant observer looking through a telescope. Credit: NASA/JPL-Caltech/Space Science Institute.
Just above the center of this mosaic is Punga Mare, 390 kilometers (240 miles) across, with Ligeia Mare (500 kilometers, or 300 miles wide) below center. The large body of methane/ethane at left is Kraken Mare, some 1200 kilometers across (730 miles). Note the scattering of small lakes around the seas, especially at the right side of the mosaic.
Also interesting is the small amount of cloud cover, though a few do appear just below center in the image, as well as several above Ligeia Mare. We would expect clouds, since Titan’s methane cycle involves rainfall, surface runoff, collected methane in large bodies like these, and evaporation, much like Earth’s water cycle. And in fact, cloud activity was found during southern summer over the south pole. But here we’re in northern spring and summer, with few clouds.
“We expected more symmetry between the southern and northern summer,” said Elizabeth (“Zibi”) Turtle of the Johns Hopkins Applied Physics Lab and the Cassini Imaging Science Subsystem (ISS) team that captured the image. “In fact, atmospheric models predicted summer clouds over the northern latitudes several years ago. So, the fact that they still hadn’t appeared before the end of the mission is telling us something interesting about Titan’s methane cycle and weather.”
Remember that among the many things Cassini found on Titan are two different kinds of methane/ethane-filled depressions that create the features we see in this mosaic. Some are responsible for the collection of the large seas, which are not only hundreds of kilometers across but evidently several hundred meters deep, all fed by branching channels like rivers on Earth. The much smaller lakes show terrain with steep walls and rounded edges. They do not appear to be associated with incoming channels, meaning they are filled by rainfall or by upwelling from below, and it’s known that some of them fill and dry out during Titan’s seasonal cycle.
Image: An earlier, colorized mosaic from NASA’s Cassini mission shows Titan’s northern land of lakes and seas. Here the data were obtained from 2004 to 2013. In this projection, the north pole is at the center. The view extends down to 50 degrees north latitude. In this color scheme, liquids appear blue and black depending on the way the radar bounced off the surface. Land areas appear yellow to white. Credit: NASA/JPL-Caltech/Space Science Institute.
In a 2015 study, Thomas Cornet (European Space Agency) and colleagues went to work on Titan’s lakes, noting that they are reminiscent of ‘karstic’ landforms on Earth. On our planet, these result from erosion due to groundwater and rainfall affecting dissolvable rock such as limestone and gypsum, creating both caves and sinkholes or, in desert climates, salt pans.
Cornet’s team worked out how long it would take to create features like this on Titan, assuming a surface covered in organic material, with the main dissolving agent being liquid hydrocarbons operating in a climate based on current models. The result: 50 million years to create a 100-meter (330 ft) depression in the polar regions. Says Cornet:
“We compared the erosion rates of organics in liquid hydrocarbons on Titan with those of carbonate and evaporite minerals in liquid water on Earth. We found that the dissolution process occurs on Titan some 30 times slower than on Earth due to the longer length of Titan’s year and the fact it only rains during Titan summer. Nonetheless, we believe that dissolution is a major cause of landscape evolution on Titan and could be the origin of its lakes.”
Thus we find a process of erosion dependent on rock chemistry, rainfall rate and surface temperature that has similarities with what we see on Earth on what Cornet calls a “relatively youthful billion-year-old surface,” a comparatively slow surface transformation that is even slower at lower latitudes, where the rainfall is less frequent. For more on this, see New Insights into Titan.
The Cornet paper on Titan’s surface features is “Dissolution on Titan and on Earth: Towards the age of Titan’s karstic landscapes,” Journal of Geophysical Research – Planets 25 April 2015 (full text).
by Paul Gilster | Sep 18, 2018 | Exoplanetary Science |
I like nautical metaphors as applied to the stars, my favorite being the words attributed to Antoine de Saint-Exupéry, French writer/aviator and author of poetic works about flight like Wind, Sand and Stars (1939), and a work familiar to most American students of French, Vol de nuit, published in English as Night Flight (1931). I think the Saint-Exupéry quote captures what it takes to contemplate far voyaging:
“If you want to build a ship, don’t drum up the men to gather wood, divide the work and give orders. Instead, teach them to yearn for the vast and endless sea.”
Image: Antoine de Saint-Exupery, whose work inspired, among many other things, my own decision to take up flying.
I had to track down the quote because the last time it appeared in these pages, a reader wrote to tell me he had never found it in Saint-Exupéry. I hadn’t either, which bothered me because I am a huge fan of the man’s work. It certainly sounded like him. So I did some digging and turned up a passage in Saint-Exupéry’s posthumously published Citadelle (1948) that comes close. The quote above is a much abbreviated paraphrase but does capture the spirit of the original (you’ll find a translation of the original at the end of this post).
Looking out over the ocean is much like looking out into the stars, triggering that same sense of immensity and, among some at least, the drive to explore. I get the same triggering effect by looking at images from spacecraft missions, like the views below from the Transiting Exoplanet Survey Satellite (TESS), which has now provided us with a ‘first light’ look at the southern sky.
Image: The Transiting Exoplanet Survey Satellite (TESS) took this snapshot of the Large Magellanic Cloud (right) and the bright star R Doradus (left) with just a single detector of one of its cameras on Tuesday, Aug. 7. The frame is part of a swath of the southern sky TESS captured in its “first light” science image as part of its initial round of data collection. Credit: NASA/MIT/TESS.
So do images of the stars make us yearn for deep space as Saint-Exupéry’s passage makes us yearn for the sea? I suspect so, and it would explain how often we resort to the sea in describing the stars. This morning I can point to Paul Hertz, who is astrophysics division director at NASA Headquarters, a man who likewise resorts to a maritime theme in describing what TESS will do:
“In a sea of stars brimming with new worlds, TESS is casting a wide net and will haul in a bounty of promising planets for further study. This first light science image shows the capabilities of TESS’ cameras, and shows that the mission will realize its incredible potential in our search for another Earth.”
That bounty should reveal numerous nearby targets for the investigation of the James Webb Space Telescope as well as later space- and ground-based instruments. The full four-camera image that is shown below was captured on August 7, and took in constellations from Capricornus to Pictor, and both the Large and Small Magellanic Clouds. This view of the southern sky includes more than a dozen stars already known to have transiting planets.
Image: The Transiting Exoplanet Survey Satellite (TESS) captured this strip of stars and galaxies in the southern sky during one 30-minute period on Tuesday, Aug. 7. Created by combining the view from all four of its cameras, this is TESS’ “first light,” from the first observing sector that will be used for identifying planets around other stars. Notable features in this swath of the southern sky include the Large and Small Magellanic Clouds and a globular cluster called NGC 104, also known as 47 Tucanae. The brightest stars in the image, Beta Gruis and R Doradus, saturated an entire column of camera detector pixels on the satellite’s second and fourth cameras. Credits: NASA/MIT/TESS.
Whereas the Kepler mission ‘stared’ at a single field of stars at distances up to 3,000 light years, TESS puts the same transit detection strategy to work on much closer targets, 30 to 300 light years away, and up to 100 times brighter. The spacecraft will monitor 26 sectors of the sky for 27 days each, ultimately covering 85 percent of the sky, with the first year of operations dedicated to southern stars before beginning the second year-long survey of the northern sky.
The science investigations will include those requested through the TESS Guest Investigator Program, which allows the scientific community at large to use the spacecraft for research.
“We were very pleased with the number of guest investigator proposals we received, and we competitively selected programs for a wide range of science investigations, from studying distant active galaxies to asteroids in our own solar system,” said Padi Boyd, TESS project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “And of course, lots of exciting exoplanet and star proposals as well. The science community are chomping at the bit to see the amazing data that TESS will produce and the exciting science discoveries for exoplanets and beyond.”
Here’s a square from the third camera; note the Small Magellanic Cloud just right of center.
Image: The Transiting Exoplanet Survey Satellite (TESS) captured this square of stars and galaxies in the southern sky with its third camera during one 30-minute period on Tuesday, Aug. 7. Bright objects are labeled. Credit: NASA/MIT/TESS.
A sea of stars indeed, with the TESS targets bright enough for high-grade spectroscopic follow-up. All of this is coming together as part of the extension of our studies to the atmospheres of small, rocky worlds. Yesterday we looked at a paper from Anthony Del Genio et. al. on Proxima Centauri b. The Del Genio paper makes the point that “The population of potentially habitable rocky exoplanets in M star systems has now suddenly reached the point at which it will soon be possible to assess the demographics of this class of planet.”
Exoplanet demographics! We’ve come so far since the first detection of planets around a main sequence star back in 1995. TESS will be a huge part of extending our catalog.
And returning to the Saint-Exupéry passage with which I opened, here is a translation of the passage in La Citadelle that I found online.
One will weave the canvas; another will fell a tree by the light of his ax. Yet another will forge nails, and there will be others who observe the stars to learn how to navigate. And yet all will be as one. Building a boat isn’t about weaving canvas, forging nails, or reading the sky. It’s about giving a shared taste for the sea…
Good stuff, but I admit to liking the abbreviated version better. It has more punch.
by Paul Gilster | Sep 17, 2018 | Exoplanetary Science |
Proxima Centauri b is back in the news, although I’ll confess that in my case, it’s rarely out of my thoughts — I’ve been obsessed with the Alpha Centauri system since my youth. The latest comes through work by Anthony Del Genio and colleagues (NASA GSFC), who describe in Astrobiology their new simulations with regard to potential habitability.
You’ll recall the issues here. A planet this close to its host star may well be tidally locked, with one side always facing the M-dwarf Proxima Centauri. Martin Turbet (Sorbonne Universités, Paris) and colleagues described possible climates on Proxima b in a 2016 paper, using a 3D climate model (GCM) to simulate the atmosphere and water cycle of the planet for its two possible rotation modes, a 1:1 and a 3:2 spin resonance (in other words, gravitational forces could keep Centauri b locked to Proxima or rotating 3 times for every 2 orbits of the star).
The Solar System offers analogues: The Moon is in a 1:1 spin resonance with the Earth, while Mercury is in a 3:2 spin resonance with the Sun. What Del Genio and company bring to the table is a model that incorporates these resonances and a global climate model but also includes the effects of an ocean that can transfer heat from one side of the planet to another.
The addition of this dynamic ocean modeling gets us to an interesting outcome: Rather than, as with the Turbet paper, finding an entirely frozen dark side, with a star-facing side that has at least some potential for a sea, Proxima b may have conditions allowing for an equatorial zone of liquid water even on the dark side, with large open ocean possible elsewhere.
This result improves the odds on habitability, as Del Genio is quoted as saying in an article called The Closest Exoplanet to Earth Could Be “Highly Habitable.” An ocean-covered Proxima b with an atmosphere like Earth’s could have open oceans that extend into the dark side, at least at low latitudes, and this turns out to be true both for synchronous rotation and a 3:2 spin-orbit resonance with a somewhat eccentric orbit.
Image: An image of the closest star to the Sun, Promixa Centauri, and its surrounding field of stars. Note that Centauri A and B both appear as a single ‘star’ here, their light combined in the bright object at the left. The bright star on the right is Beta Centauri, which is not a part of the Alpha Centauri system and is much further away from Earth. Credit: Marco Lorenzi.
But in going through the paper, I think it’s important to note that the researchers also derive much colder temperatures than previously suggested, the result of the transport of oceanic heat around the surface. Let’s look at how the paper treats this outcome with reference to Earth:
Because of its weak instellation, however, Proxima Centauri b’s climate is unlikely to resemble modern Earth’s. “Slushball” episodes in Earth’s distant past with cold but above-freezing tropical oceans (Sohl et al, 2017) are better analogs. The extent of open ocean depends on the salinity assumed. Elevated greenhouse gas concentrations produce some additional warming, but this is limited for any M-star planet by the reduced penetration of near-infrared starlight to the surface, and for Proxima b in particular, by its existence near a possible dynamical regime transition.
Unlike Turbet and team, Del Genio’s paper rules out a single ocean at the substellar area — this is sometimes called the ‘eyeball Earth’ scenario — because their dynamic ocean models show enough heat transport to allow water to exist on a wider basis. But note the effect of salinity, which determines the temperature at which seawater freezes. Modeling different levels of salinity in their various simulation scenarios (which also varied atmospheric composition), the scientists found that at high salinity, broad regions of open ocean occur and little ice.
We may have a Proxima b that is habitable but in terms that are starkly different from Earth:
…if we think more broadly about what constitutes a habitable planet (Cullum et al., 2016), it is reasonable to imagine a cold but inhabited Proxima Centauri b with a very salty shallow remnant of an earlier extensive ocean in which halophilic bacteria are the dominant life form, especially if the ocean is in contact with a silicate seafloor (Glein and Shock, 2010).
In an interesting discussion of other exoplanets at the end of the paper, the researchers consider TRAPPIST-1 e, LHS 1140 b, GJ 273 b and other worlds in the context of their models. The point could not be clearer: We are steadily building our inventory of potentially habitable rocky exoplanets around M-dwarfs. Where this paper points is toward continued refinements to climate modeling as we anticipate our first atmospheric detections and characterizations.
Calling the Del Genio paper “a very exciting result indeed,” Guillem Anglada Escudé (Queen Mary University of London), who led the team that discovered Proxima b, likewise pointed to nearby M-dwarf systems and the likelihood that their planets would be rich in water. By ‘rich,’ we are talking about 10-30% in water content, as opposed to < 1 percent for Earth.
The problems of atmosphere retention through early tidal heating and later erosion due to stellar flares and a strong stellar wind are serious and we cannot rule out the possibility that they would make habitability impossible. Anglada Escudé noted these issues in an email this morning, but continued:
“The paper (necessarily) needs to make a large number of assumptions, but this is very important work as it points towards the observational probes that we should soon be able to apply to these planets (temperature, thermal environment, atmospheres rich in water vapor, or whatever). As usual, we would like to see a few more of these simulations by independent teams to obtain a better perspective on the landscape of possibilities.”
Where to next with this research? Anglada-Escudé adds:
“As a shopping list of things that I would like to see from our modeller colleagues, it would be great to see some of these models coupled with erosive effects from stellar activity on the upper atmosphere, and the resulting exotic atmosphere we might expect. That is, even if large chunks are eroded due to a particularly violent coronal mass ejection or flare, the ocean would possibly outgas a new one quickly.”
The paper adds that we have some analogues in our own Solar System to seas of high salinity like those modeled here. Europa, for example, is thought to contain such a sea, as is Enceladus. Interestingly, we can’t observe life signatures within these bodies without going there, setting up a situation in which we may detect — within a few decades — biosignatures on an exoplanet before we find signs of life beyond Earth in our own Solar System.
The paper is Del Genio, “Habitable Climate Scenarios for Proxima Centauri b with a Dynamic Ocean,” published online at Astrobiology 5 September 2018 (preprint). The Turbet paper is “The habitability of Proxima Centauri b II. Possible climates and observability,” Astronomy & Astrophysics Vol. 596, A112 (December, 2016). Abstract
