Centauri Dreams
Imagining and Planning Interstellar Exploration
Upwelling Oceans: Modeling Exoplanet Habitability
We usually talk about habitability in binary form — either a planet is habitable or it is not, defining the matter with a ‘habitable zone’ in which liquid water could exist on the surface. Earth is, of course, the gold standard, for we haven’t detected life on any other world.
But it is conceivable that there are planets where conditions are more clement than our own, as Stephanie Olson (University of Chicago) has recently pointed out. The work, presented at the just concluded Goldschmidt Geochemistry Congress in Barcelona, models circulatory patterns in oceans, some of which may support abundant life if they exist elsewhere. The emphasis here is not so much on surface ocean currents but upwelling water from deep below. Says Olson:
“We have used an ocean circulation model to identify which planets will have the most efficient upwelling and thus offer particularly hospitable oceans. We found that higher atmospheric density, slower rotation rates, and the presence of continents all yield higher upwelling rates. A further implication is that Earth might not be optimally habitable–and life elsewhere may enjoy a planet that is even more hospitable than our own.”
All this has implications for how we use the term ‘Earth-like,’ and reminds us to be careful, as Olson told a Los Angeles Times interviewer in 2018:
“The phrase Earth-like does not refer to a planet that necessarily resembles modern-day Earth at all… It’s actually a very broad term that encompasses a broad variety of worlds. It includes hazy worlds like the Archean; it includes icy worlds like the ‘snowball Earth’ intervals; it includes anoxic worlds with exclusively microbial ecosystems; it includes worlds with complex and intelligent life; and it includes worlds that we haven’t even seen yet.”
Image: Geophysicist Stephanie Olson. Credit: University of Chicago.
Stephanie Olson makes the case that life has to be far more common than what we can detect at our current stage of technology. An ecosystem beneath the surface of an icy moon may defeat our methods, as could microorganisms deep within a planet’s mantle. So what we need to do, in this scientist’s view, is build our target lists for future study around a subset of planets, those that meet the habitability demands of forms of life that are global, active and detectable. This also builds the list of those worlds for which a non-detection would be the most telling.
In general, our developing models for habitability have tracked our interest in finding atmospheric biosignatures, for we are closing in on the capability of doing this for small, rocky worlds circling nearby M-dwarf stars. The complexities of ocean dynamics have been left out of the picture other than when used as a mechanism for climate regulation or heat transport.
In her conference abstract at the Goldschmidt conference, Olson argues that the implications of circulatory patterns in oceans should be folded into the habitability question. Cycles of ocean upwelling driven by winds can recycle nutrients from the deep ocean back to shallower waters where they can play a role in stimulating photosynthesis. From the abstract:
Photosynthesis,,,provides energy in the form of chemical disequilibrium that sustains life more broadly on our planet. Ocean circulation is thus a first-order control on the productivity and distribution of life on Earth today and throughout our planet’s history. Moreover, ocean circulation patterns, sea ice coverage, and sea-to-air gas exchange kinetics modulate the extent to which biological activity within the ocean is communicated to the atmosphere. The chemical evolution of Earth’s atmosphere has ultimately been an imperfect reflection of the evolution of Earth’s marine biosphere owing to these oceanographic phenomena.
Models of Habitability
Olson’s tool for exploring ocean dynamics on a range of modeled, habitable exoplanets is a global circulation model (GCM) called ROCKE-3D. The software is designed to examine different periods in the evolution of terrestrial-class planets, with the goal of finding what kind of techniques might flag the presence of life in these environments. You can have a look at ROCKE-3D in action in this NASA page on the simulation of planetary climates. Different parameters can be selected on a form to create maps of a number of climate variables.
Below is an example of one of these maps, as created by the ROCKE-3D software.
Image: The discovery of the planet Proxima Centauri b orbiting the star closest to Earth has generated much research about whether it has a chance to be habitable. With ROCKE-3D we have imagined Proxima Centauri b as an “aquaplanet” covered by water. Because the planet is close to its star, it may show the same face to the star all the time, as the Moon does to the Earth. If so, the dayside remains a few degrees above freezing (yellow colors). Elsewhere, the ocean is perpetually covered by ice (dark blue colors), except near the equator where winds and ocean currents push sea ice eastward onto the dayside where it breaks up and melts (pale blue to light yellow colors). Credit: NASA Nexus for Exoplanet System Science (NExSS) / NASA Goddard Institute for Space Studies (GISS).
Three-dimensional planetary general circulation models have been used to project climate change into future decades, but have matured to the point that they can probe habitability questions such as how a planet can become habitable under variations in stellar radiation and atmospheric chemistry. The NASA Nexus for Exoplanet System Science (NExSS) effort works on these matters in a cross-disciplinary effort to parse habitability in terms of the factors that make it happen, from host stars to protoplanetary disks and rocky planet atmospheres.
ROCKE-3D stands for Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics, now developing as a collaborative investigation within NExSS. At NASA GSFC, the Goddard Institute for Space Studies (GISS) developed ROCKE-3D to run global circulation model simulations deploying and manipulating past climates of Earth and other planets by way of analyzing climates and ocean habitats. The idea is to produce model spectra and phase curves for future observations. Let me quote from the GISS website:
Our project uses solar radiation patterns and planetary rotation rates from simulations of spin-orbit dynamical evolution of planets over Solar System history provided by our colleagues at the Columbia Astrobiology Center and at other institutions that are part of our NExSS team. In turn, the synthetic disk-integrated spectra we produce from the GCM will be used as input to a whole planetary system spectral model that emulates observations that candidate future direct imaging exoplanet missions might obtain…
Here you can see the direction of this work. What these teams are trying to do is model what future observatories may see when we become capable of directly imaging rocky exoplanets. We need to learn what kind of signals may be detectable as we allocate precious observing time to those targets most likely to repay the effort. Here theory about the kind of spectral details that life may produce is the foundation for later direct observational data.
Olson’s Oceans
Back to Olson, who wants to fold ocean dynamics into this effort and consider how they may be manifested on habitable exoplanets. Can features of ocean circulation that we cannot observe be inferred from atmospheric properties we can see? Olson’s work is an attempt to link ocean circulation with key planetary parameters, invoking the biological constraints differing ocean habitats may place on worlds around a variety of stars.
We can’t say how this work will develop, but there is the real prospect for the telescope design of future missions — think LUVOIR (Large UV/Optical/IR Surveyor) or HabEx (Habitable Exoplanet Observatory) — to be affected as we learn more about what we need to look for. Adds Olson:
“Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life. Life in Earth’s oceans depends on upwelling (upward flow) which returns nutrients from the dark depths of the ocean to the sunlit portions of the ocean where photosynthetic life lives. More upwelling means more nutrient resupply, which means more biological activity. These are the conditions we need to look for on exoplanets.”
Immense effort is going into modeling planetary climate and evolution to guide our investigation of habitability. It will be fascinating to watch the trajectory of these studies as we begin to deploy advanced space-based resources to probe for biosignatures. My guess is that we will see early detections of potential biosignatures — these will receive huge press coverage — but we will not find anything that is unambiguous.
That may seem like a letdown when it happens, but ruling out abiotic mechanisms for possible biosignatures is equally a part of global circulation modeling, and this work will take time.
JUICE: Targeting Three Icy Moons
Because Europa Clipper has been on my mind, what with the confirmation of its next mission phase (see Europa Clipper Moves to Next Stage), we need to continue to keep the mission in context. What is playing out is a deepening of our initial reconnaissance of the Jovian system, and the JUICE mission (Jupiter Icy Moons Explorer) is a significant part of that overall effort. The European Space Agency has the spacecraft under development, with Airbus Defence and Space as the primary contractor.
We saw last week that while Europa Clipper will use flybys of Ganymede and Callisto for gravity maneuvers intended to refine its orbit, the latter two moons are not science priorities. JUICE, on the other hand, focuses on all three, each thought to house liquid water beneath the surface. JUICE is slated for a June, 2022 launch, reaching Jupiter in 2029 with the help of five gravity assists along the way, so its operations will overlap with Europa Clipper, the NASA craft launching in 2023. The coming decade will be busy indeed as we journey to and explore these compelling icy targets.
The orbital maneuvers chosen for JUICE are intriguing, for after its first flyby of Europa in 2030, the spacecraft is to enter a high-inclination orbit to study Jupiter’s polar regions and magnetosphere. Repeated flybys of Europa, Ganymede and Callisto are planned, and following a Callisto flyby in 2031, the spacecraft will actually enter orbit around Ganymede, making it the first spacecraft to orbit a moon other than our own. I’m simplifying these complicated orbital maneuvers for the sake of brevity, but the point is that JUICE will greatly expand our datasets on all three moons.
In June of 2019, engineers at Airbus Defence and Space’s site in Toulouse tested the navigation camera that will be essential for radio tracking and position and velocity information of the spacecraft relative to the moon it is currently studying. Given the powerful radiation found near Jupiter, the spacecraft will, like Europa Clipper, be radiation-hardened, allowing it to operate between 200 and 400 kilometers from its targets at closest points of rendezvous. The pointing accuracy demanded of NavCam during fast and close approaches like these is critical to the mission’s success.
The June tests looked at the NavCam engineering model in real sky conditions, the point being to stress the hardware and software interfaces to validate their design, as well as to prepare the image processing and onboard navigation software that JUICE will use to acquire its images. The engineers observed Earth’s own moon and a variety of sky objects included Jupiter itself as part of these tests, running NavCam in ‘imaging mode’ and ‘sky centroiding mode’ as part of fine tuning attitude control software.
Image: The Navigation Camera (NavCam) of the Jupiter Icy Moons Explorer (JUICE) has been given its first glimpse of the mission’s destination while still on Earth. The camera was mounted to an equatorial mount and pointed towards different targets, including bright stars, Jupiter and its moons in order to exercise its ‘Imaging Mode’ and ‘Stars Centroiding Mode’. The integration time was optimized for capturing the stars and moons acquisition, so Jupiter appears saturated. In this annotated image the size of Jupiter is indicated. Credit & Copyright: Airbus Defence and Space.
“Unsurprisingly, some 640 million kilometres away, the moons of Jupiter are seen only as a mere pixel or two, and Jupiter itself appears saturated in the long exposure images needed to capture both the moons and background stars, but these images are useful to fine-tune our image processing software that will run autonomously onboard the spacecraft,” says Gregory Jonniaux, Vision-Based Navigation expert at Airbus Defence and Space. “It felt particularly meaningful to conduct our tests already on our destination!”
Image: Impressions of how the Jupiter Icy Moons Explorer will see moons Europa (left), Ganymede (middle) and Callisto (right) with its Navigation Camera (NavCam). To generate these images, the NavCam was fed simulated views – based on existing images of the moons – to process realistic views of what can be expected once in the Jupiter system. Credit & copyright: Airbus Defence and Space.
The actual flybys will provide close inspection of surface features on Europa, Ganymede and Callisto. In the suite of tests, NavCam also received simulated views of the three moons in order to process the kind of imagery it will acquire in Jupiter space. NavCam will capture imagery that will be greatly augmented by the high-resolution camera suite that will give us our best views of the icy surfaces below.
By the end of 2019, the test NavCam will be augmented with a full flight representative performance optics assembly that will support onboard tests of the complete spacecraft. Meanwhile, a test version of the spacecraft’s 10.5-m long magnetometer boom developed by SENER in Spain has undergone testing at ESA’s Test Centre in the Netherlands, as part of what ESA is describing as “… the most powerful remote sensing, geophysical, and in situ payload complement ever flown to the outer Solar System.”
Image: Magnetometer boom built for ESA’s mission to Jupiter by European Space Agency. Credit: ESA-G. Porter, CC BY-SA 3.0 IGO.
Going Deep into Jupiter’s Storms
Having just looked at events that may have shaped Jupiter’s core, it seems a good time to note the new Hubble image of the planet, taken on June 27, 2019. A couple of things to focus on in the image below: The vast anticyclonic storm we call the Great Red Spot, about the diameter of the Earth, is evident as it rolls counterclockwise between bands of clouds moving in opposite directions toward it.
We still don’t know why, but the storm itself continues to shrink. Smaller storms show up vividly as white or brown ovals, some of which dissipate within hours, while others may be as long lasting as the Great Red Spot, which has dominated Jupiter’s face for at least 150 years. Note the cyclone showing up south of the Spot, visible as a worm-shaped feature. You can also see other anticyclones, appearing as white ovals.
Image: The NASA/ESA Hubble Space Telescope reveals the intricate, detailed beauty of Jupiter’s clouds in this new image taken on 27 June 2019. It features the planet’s trademark Great Red Spot and a more intense colour palette in the clouds swirling in the planet’s turbulent atmosphere than seen in previous years. This image was captured by Hubble’s Wide Field Camera 3, when the planet was 644 million kilometres from Earth. Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley).
But let’s move beyond Hubble. New work on the storm clouds of Jupiter swirling through the planet’s atmospheric belts has just appeared, drawing not only on space-based resources but also a mix of optical and radio telescopes that have gone into recent tracking of their activity. In January of 2017, Australian amateur astronomer Phil Miles observed the bright plume of a storm that was subsequently picked up by observations with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile. The latter work was led by UC-Berkeley astronomer Imke de Pater, producing a paper that has been accepted at the Astronomical Journal (citation below).
Things happen quickly enough on Jupiter that we can track them by daily observation, and Hubble images taken a week after the ALMA work showed that what had been a single plume had spawned a second plume and left visible downstream changes in Jupiter’s south equatorial belt. Moreover, four bright spots seen three months earlier in the north equatorial belt had disappeared, while the belt itself had widened as well as changed colors, from a striking white to orange-brown. This may be the result of gas from plumes now depleted of ammonia falling back into the lower atmosphere.
“If these plumes are vigorous and continue to have convective events, they may disturb one of these entire bands over time, though it may take a few months,” says de Pater. “With these observations, we see one plume in progress and the aftereffects of the others.”
The paper posits that plumes like these emerge about 80 kilometers below the cloud tops, in a region where clouds of liquid water droplets are common. Jupiter’s atmosphere is primarily hydrogen and helium, with trace amounts of methane, ammonia, hydrogen sulfide and water. What we’re seeing at the top-most cloud layer, with its brown belts and white zones, is largely made up of ammonia ice. A layer of solid ammonium hydrosulfide particles is found below this in the upper cloud deck.
Image: ALMA’s view of Jupiter at radio wavelengths (top) and the Hubble Space Telescope’s view in visible light (bottom). The eruption in the South Equatorial Belt is visible in both images: a dark spot in radio, a bright spot in visible. Credit: ALMA image by Imke de Pater and S. Dagnello; Hubble image courtesy of NASA.
The radio telescopes of ALMA are able to see beneath the upper ammonia clouds that are opaque in visible frequencies, but de Pater’s team also brought data from Hubble, the Very Large Array, the Gemini, Keck and Subaru observatories in Hawaii and the Very Large Telescope (VLT) in Chile into the mix, homing in on the storm seen above as it emerged from the lower cloud levels into the upper ammonia ice clouds.
Image: A closeup of the two bright white plumes (center) in the South Equatorial Belt of Jupiter and a large downstream disturbance to their right. Credit: Imke de Pater, UC Berkeley; Robert Sault, University of Melbourne; Chris Moeckel, UC Berkeley; Michael Wong, UC Berkeley; Leigh Fletcher, University of Leicester.
The storm clouds, reaching Jupiter’s tropopause, where the atmosphere is at its coldest, spread out in much the same way as the anvil-shaped formations of thunderstorms we see in Earth’s atmosphere. The ALMA data were sufficient to show that high concentrations of ammonia gas are forced upward during an eruption like this. Convection, the scientists believe, brings both ammonia and water vapor high enough for the water to condense into liquid droplets, releasing heat along the way.
Now we have a plume with enough momentum that, as heat is released from condensing water, can break out above the clouds of the upper deck, where the ammonia will freeze to create the white plume of these images.
“We were really lucky with these data, because they were taken just a few days after amateur astronomers found a bright plume in the South Equatorial Belt,” adds de Pater. “With ALMA, we observed the whole planet and saw that plume, and since ALMA probes below the cloud layers, we could actually see what was going on below the ammonia clouds.”
Image (click to enlarge): An illustration of “moist convection” in Jupiter’s atmosphere shows a rising plume originating about 80 kilometers below the cloud tops, where the pressure is five times that on Earth (5 bar), and rising through regions where water condenses, ammonium hydrosulfide forms and ammonia freezes out as ice, just below the coldest spot in the atmosphere, the tropopause. Credit: Adapted from illustration by Leigh Fletcher, University of Leicester.
This useful analysis was made possible because of simultaneous observations at different wavelengths, in this case homing in on transient events and showing us how the atmosphere at different levels, from cloud tops to deep below, responds to them. This is new ground in the study of Jupiter’s weather, as the paper notes:
These data are the first to characterize the atmosphere below the cloud layers during/following such outbreaks. Aided also by observations ranging from uv to mid-infrared wavelengths, we have shown that the eruptions are consistent with models where energetic plumes are triggered via moist convection at the base of the water cloud. The plumes bring up ammonia gas from the deep atmosphere to high altitudes, where NH3 gas is condensing out and the subsequent dry air is descending in neighboring regions. The cloud tops are cold, as shown by mid-infrared data, indicative of an anticyclonic motion, which causes the storm to break up, as expected from similarities to mesoscale convective storms on Earth. The plume particles reach altitudes as high as the tropopause.
The paper is de Pater et al., “First ALMA Millimeter Wavelength Maps of Jupiter, with a Multi-Wavelength Study of Convection” (preprint).
Giant Jovian Impact Could Explain Juno Data
Impacts seem to have run rampant in the early Solar System, to judge from what we keep uncovering as we survey today’s evidence. The Moon is widely considered to be the result of Earth’s impact with a Mars-class object, while Mercury’s big iron core may show what happens when a larger world is stripped of much of its mantle in another ‘big whack.’ Then there’s Uranus, spinning lopsidedly in the outer system.
We also know that impacts continue to make their mark. They’re shown up on Jupiter at a fairly brisk pace, with Shoemaker-Levy striking the gas giant in 1994, and another evident impact from an asteroid earlier this month, creating a definitive flash.
For that matter, we have a Hubble image from 2009 showing an impact, an expanding spot twice the length of the United States. That one was discovered by Australian amateur astronomer Anthony Wesley. Later observations allowed scientists to estimate the impactor’s diameter at 200 to 500 meters, with an explosion thousands of times more powerful than the Tunguska event in 1908. Juno mission scientist Ravit Helled (University of Zurich) jokes that when planetary scientists lack a solution, they tend to invoke a giant impact. If so, it seems to be an understandable assumption.
Image: Hubble’s view of the 2009 impact event on Jupiter. Credit: NASA, ESA, and H. Hammel (Space Science Institute, Boulder, Colo.), and the Jupiter Impact Team.
But Helled and colleagues in the National Centre of Competence in Research PlanetS (Bern, Switzerland) aren’t joking when it comes to results from Juno that are forcing them to change their views of Jupiter’s core. The gravity data demand explanation, which may emerge in a massive impact early in the history of Jupiter’s formation:
“Instead of a small compact core as we previously assumed, Jupiter’s core is ‘fuzzy,’” Helled explains. “This means that the core is likely not made of only rocks and ices but is also mixed with hydrogen and helium and there is a gradual transition as opposed to a sharp boundary between the core and the envelope.”
In a paper just published in Nature, Helled and an international team led by Shang-Fei Liu (Sun Yat-sen University in Zhuhai, China) present the results of simulated collisions between an early Jupiter and planetary embryos. They worked with software code developed by PhD student Simon Müller that probed planetary evolution. Particularly puzzling is the thermal evolution of the planet after the impact. Could the diluted core Juno found really persist for billions of years until today?
Giant impacts, the authors argue, are most likely to occur not long after runaway gas accretion. This is when the gravitational effects of the growing planet increase 30-fold in the space of a few million years, destabilizing the orbits of nearby embryos. The team’s simulations show that Jupiter’s gravitational effect on nearby planetary embryos would have been profound, with at least a 40 percent chance that a large embryo would hit Jupiter within the first few million years.
The scientists worked through tens of thousands of simulations to model this effect, and went on to use separate computer code to investigate what these impacts would do to Jupiter’s internal structure. To produce the diluted core we see today, heavy elements in the core and the embryo need to mix with the surrounding gas envelope.
Analyzing heat transport and heavy element mixing, the team finds that it would take an impactor with about 10 times Earth’s mass to stir Jupiter’s core, mixing denser layers with less dense layers above. The team’s 3D models show the effects of a major hit below.
Image: This is Figure 3 from the letter in Nature. Caption: Three-dimensional cutaway snapshots of density distributions during a merger event between a proto-Jupiter with a 10M? rock/ice core and a 10M? impactor. a, Just before the contact. b, The moment of core–impactor contact. c, 10 h after the merger. Owing to impact-induced turbulent mixing, the density of Jupiter’s core decreases by a factor of three after the merger, resulting in an extended diluted core… Credit: Shang-Fei Liu/Sun Yat-sen University.
The authors go on to compute the thermal evolution following the impact of a 10 Earth mass impactor until the present day, a span of 4.56 billion years. There is only one solution that produces a diluted core like that found by Juno. From the paper:
We conclude that Jupiter’s diluted-core structure could be explained by a giant impact event, but only under specific conditions including a head-on collision with a massive planetary embryo, a post-impact central temperature of about 30,000 K or an initial thermal structure created by the accretion shock during the runaway phase. Indeed, the hydrodynamic simulation suggests that most of the impact energy is not deposited in the deep interior, and therefore the central temperature is unlikely to increase substantially, supporting the diluted core solution.
Interestingly, such an impact would demand the collision be head-on, for grazing impacts would not produce the core-density profile that Juno has now measured. Even a grazing embryo of 10 Earth masses, in this scenario, would be disrupted while sinking to the center of the planet. Meanwhile, smaller impactors (1 Earth mass or less) disintegrate in the envelope of the gas giant before they ever reach its center.
How definitive is this impact solution? The paper points out in its conclusion that a gradual accretion of planetesimals along with runaway gas accretion could produce a disrupted core, but the authors question whether this would allow a diluted core to be preserved to the present day. They also note that giant impacts like the one they model here may be producing an observational signature in extrasolar gas giants, in the form of the high metallicity found in some of these worlds. And this is interesting:
Since impacts of planetary embryos are expected to be frequent after a gas giant’s runaway gas accretion phase, such an event with different impact conditions (such as a small impactor or an oblique collision) may have also happened to Saturn, and could in principle explain the differences between the internal structures of Jupiter and Saturn.
Shang-Fei Liu et al., “The formation of Jupiter’s diluted core by a giant impact,” Nature 572 (15 August 2019), pp. 355–357 (abstract).
Europa Clipper Moves to Next Stage
Europa Clipper stays on my mind, with the intent of digging deeper into the spacecraft as development moves forward. We are talking about a craft that is by necessity radiation-tolerant as it will make a series of close flybys of Europa during its long orbit of Jupiter. 45 such flybys are in the cards, at altitudes varying from 2700 to 25 (!) kilometers, with flybys of Ganymede and Callisto in the mix as well. The latter are considered gravitational maneuvers intended to refine Europa Clipper’s orbit, and while they should be productive, they are not science priorities.
Image: Because Europa lies well within the harsh radiation fields surrounding Jupiter, even a radiation-hardened spacecraft in near orbit would be functional for just a few months. Studies by scientists from the Jet Propulsion Laboratory show that by performing several flybys with many months to return data, the Europa Clipper concept would enable a $2B mission to conduct the most crucial measurements of the cancelled $4.3B Jupiter Europa Orbiter concept. Here we see how the mission can achieve global coverage during successive flybys. Credit: NASA/JPL.
NASA has now announced confirmation of Europa Clipper’s next mission phase, which means we proceed to completion of the final design, which will in turn be followed by construction and testing of the spacecraft and its science payload. NASA associate administrator for the Science Mission Directorate Thomas Zurbuchen frames Europa Clipper within the sequence of outer system missions that most recently has included Cassini’s operations at Saturn:
“We are all excited about the decision that moves the Europa Clipper mission one key step closer to unlocking the mysteries of this ocean world. We are building upon the scientific insights received from the flagship Galileo and Cassini spacecraft and working to advance our understanding of our cosmic origin, and even life elsewhere.”
Image: This artist’s rendering shows NASA’s Europa Clipper spacecraft, which is being developed for a launch sometime in the 2020s. Credit: NASA/JPL.
As the concept evolves, we’ll see how closely it tracks the image above, in which the ice-penetrating radar antennae are attached to the solar arrays extending from the spacecraft. The magnetometer boom and round high-gain antenna are visible on the side of the spacecraft, with a remote-sensing palette housing the rest of the instrument payload on the left.
The instruments NASA has selected to study Europa include nine of the thirty-three originally proposed. As you would imagine, they include a thermal instrument that will search the surface for recent eruptions of warmer water even as other instruments look for tiny particles in the thin atmosphere around the moon. It was back in 2012 that Hubble data indicated water vapor above the south polar region, giving us the possibility of water plumes linked to the subsurface ocean. As at Enceladus, that would open sampling options without drilling through the ice.
Ice-penetrating radar will be used to determine the thickness of the ice shell while also looking for the kind of subsurface lakes found beneath Antarctica. The matter has been the subject of controversy for years and clearly determines what is and is not possible in terms of ocean sampling from the surface, although collection of materials near Europa’s chaos regions, where the surface has been deformed, may one day allow a lander to study frozen ocean brines.
Long linear cracks on the surface seem to be the result of tidal forces causing the ice shell to flex. The constant gravitational interaction with Jupiter could provide enough heat energy to enable chemical reactions in the interior that, through volcanoes or hydrothermal vents, recycle nutrient-rich water between the ocean and the rocky interior. How well Europa’s different layers move material between them may determine whether living organisms can flourish here.
Image: This view of the Conamara Chaos region on Jupiter’s moon Europa taken by NASA’s Galileo spacecraft shows an area where the icy surface has been broken into many separate plates that have moved laterally and rotated. These plates are surrounded by a topographically lower matrix. This matrix material may have been emplaced as water, slush, or warm flowing ice, which rose up from below the surface. One of the plates is seen as a flat, lineated area in the upper portion of the image. Below this plate, a tall twin-peaked mountain of ice rises from the matrix to a height of more than 250 meters. The matrix in this area appears to consist of a jumble of many different sized chunks of ice. Though the matrix may have consisted of a loose jumble of ice blocks while it was forming, the large fracture running vertically along the left side of the image shows that the matrix later became a hardened crust, and is frozen today. Credit: NASA/JPL.
Europa Clipper will also carry cameras and spectrometers to produce high-resolution images and map surface composition, along with a magnetometer to measure the moon’s magnetic field, offering insights into the depth and salinity of the ocean. NASA announced in March that it was going to replace the earlier magnetometer designed for the mission — Interior Characterization of Europa Using Magnetometry, or ICEMAG — with a less complex (read ‘expensive’) instrument. The current list of instruments can be accessed here.
LHS 3844b: Rocky World’s Atmosphere Probed
These days we have a keen interest in small red dwarf stars (M-dwarfs) not only because they’re ideal for study, with deep transits of worlds in their habitable zones and the prospect of future analysis of their atmospheres, but also because they are so plentiful. Comprising perhaps 80 percent of all stars, they may well be home to the great majority of planets in the galaxy. And while they are common, they’re also long-lived, so that life would have plenty of opportunity to develop.
Now we have word of new work using both the Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope. TESS is, of course, a transit hunter, looking for the telltale dips in light from a parent star when a planet passes in front of it. The planet in question is LHS 3844b, about 48.6 light years out, and discovered by TESS in 2018. Follow-up observations in the infrared with Spitzer have detected light from the surface of this newly discovered world, allowing study of its atmosphere and composition. Note: This is not direct imaging; see below for more on the techniques used.
LHS 3844b orbits its star in 11 hours, making it almost certainly tidally locked; i.e., with one side always facing the star. The Spitzer data show that the dayside here reaches 770 degrees Celsius, while the nightside temperature is consistent with 0 Kelvin. In other words, the researchers could detect no heat being transferred from one side to the other, a process we would expect in the presence of an atmosphere.
Heat transfer is a mechanism that could ameliorate the effects of tidal lock, spreading warmth to the dark side and moderating global temperatures, but it takes an atmosphere to do that. We learn, then, that LHS 3844b is an object something like the Moon, or at any rate, a large version of it. Laura Kreidberg (Harvard-Smithsonian Center for Astrophysics), lead author of the paper that appears in Nature, says that this planet “…matches beautifully with our model of a bare rock with no atmosphere.” The scientist continues:
“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present.”
Image: This artist’s illustration depicts the exoplanet LHS 3844b, which is 1.3 times the mass of Earth and orbits an M dwarf star. The planet’s surface may be covered mostly in dark lava rock, with no apparent atmosphere, according to observations by NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).
This is painstaking analysis indeed, drawing on phase curve data from the planet’s transits. Phase curves are a combination of reflected light and thermal emission from the planet. Unable to resolve the planet from the host star, astronomers must work with their combined light, and observe light variations of exquisite subtlety as planets go through phase changes as they orbit. A phase curve, then, is the time-dependent change in the brightness of a planet as seen from Earth during one orbital period.
Image: Detecting Light from Exoplanet LHS 3844b. Credit: NASA/JPL-Caltech/L. Kreidberg (CfA | Harvard & Smithsonian).
Learning about the atmosphere (or lack thereof) of a small rocky world — LHS 3844b has a radius 1.3 times that of Earth — is therefore something of a coup, and bodes well for future discovery. The authors infer from the planet’s reflectivity (albedo) that it is covered with basalt, much like the mare of the Moon, which is probably an indication of volcanic activity in the distant past. From the paper:
We modeled the emission spectra of several rocky surfaces and compared with the measured planet-to-star flux… We considered multiple geologically plausible planetary surface types, including primary crusts that form from solidification of a magma ocean (ultramafic and feldspathic), secondary crust that forms from volcanic eruptions (basaltic), and a tertiary crust that forms from tectonic re-processing (granitoid). Governed by the reflectivity in the visible and the near-infrared and the emissivity in the mid-infrared, the surface types have distinct emission spectra. The measured planet-to-star flux for LHS 3844b is most consistent with a basaltic composition. Such a surface is comparable to the lunar mare and Mercury, and could result from widespread extrusive volcanism.
But this is a world much larger than the Moon, so what happened to its atmosphere? M-dwarf flare activity is thought to erode early planetary atmospheres, especially given how closely worlds like this orbit their star. The researchers rule out an atmosphere of over 10 bars (Earth’s atmospheric pressure at sea level is about 1 bar), and largely rule out one between 1 and 10 bars. They believe stellar winds and flares are the culprit. Modeling atmospheric escape over time, they assess how an early atmosphere dissolves within a magma ocean during planet formation or photolyzes into hydrogen and oxygen because of the intense bombardment of X-rays and UV from flares.
Thus a thick atmosphere is ruled out by the data, while stellar winds could account for further erosion of a thin atmosphere, all leading to the conclusion that LHS 3844b is a bare rock unless a thin atmosphere is replenished over time. Should we assume that hot terrestrial planets orbiting well inside the habitable zone of M-dwarfs are all devoid of atmospheres? Perhaps, but these stars may still be of astrobiological interest:
The results presented here motivate similar studies for less-irradiated planets orbiting small stars. Cooler planets are less susceptible to atmospheric escape and erosion, and may provide a friendlier environment for the evolution of life. In coming years this hypothesis can be tested, thanks to the infrared wavelength coverage of the James Webb Space Telescope and the influx of planet detections expected from current and future surveys.
The paper is Kreidberg et al., “Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b,” Nature 19 August 2019 (abstract / preprint)
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