As summer winds down and we prepare for the fall semester, I finally found the time to read the recent announcement about finding sub-surface water on Mars using the MARSIS radar onboard Mars Express.
Although evidence for liquid water on Mars has been reported for a long time, these reports are almost always about ancient flows or very modest, salty trickles (and the presence of water often turns out to be illusory). By contrast, if this most recent report holds up to scrutiny, there could be 10 billion liters of liquid water under the martian south pole.
That’s not much on the scale of the Great Lakes (the smallest one, Lake Erie, contains 10,000 times more water), but it’s more than a thousand times the volume of tanks at the Georgia Aquarium, which hosts more than 100,000 aquatic animals. So the martian lake could easily host a microbial zoo (although no direct evidence for that as of now).
As is common in polar regions on Earth, the martian water lies under kilometers of polar ice and is probably so cold it requires some kind of geological anti-freeze to keep from freezing solid (the kinds of mineral salts that can do the job are actually pretty common on Mars). The overburden pressure from all the ice also helps keep the water liquid.
But the fact that the lake sits underneath so much ice raises an obvious question: how did the scientists spot it in the first place? The answer is related to why the recent wildfires in the west, in addition to fouling the air, have given us very lovely sunsets.
When it first leaves the surface of the Sun, sunlight is colored white. But as it passes through the atmosphere, the light (which is a wave of electric and magnetic fields, after all) interacts electromagnetically with the atmospheric gas molecules, which themselves contain electric charges.
The closer the wavelength of the light ray is to the sizes of the molecules, the stronger this electromagnetic interaction and the more the ray can be diverted from its straight path.
Since blue light has a wavelength (500 nanometers) closer to the size of the atmospheric molecules than red light (700 nanometers, it is more readily diverted or scattered. At dusk, as the sun sets, its light has to pass through more and more of the Earth’s atmosphere. So more and more blue light is scattered away, leaving behind more red light and making the Sun look red. If you sprinkle in lots of smoke from a wildfire, you can enhance the coloration.
What does all this have to do with martian lakes? The MARSIS instrument used to find the subsurface lake uses very red radar light, with wavelengths tens to hundreds of meters long. Similar to red sunlight, such long wavelengths can easily pass through even solid rock since they’re much larger than the rocky molecules that make up the martian surface.
This explanation simplifies things a lot, but the upshot is that MARSIS could see the lake as a very unusual bright patch underneath all that polar ice.
What’s next? It’s possible that continued data collection and analysis will turn up other subsurface lakes on Mars. If Mars’ south pole is brimming over with these icy lakes, it could be an especially good habitat for martian microbes. So maybe the effort to find martian life should explore using the same ice drilling technology being considered for exploring the oceans of Jupiter’s moon Europa.
Dr. Colin Dundas of the USGS’s Astrogeology Group based in Flagstaff spotted these buried ice cliffs during his daily scan of the regularly collected images taken by the HiRISE camera onboard Mars Reconnaissance Orbiter (MRO) currently circling Mars. (The camera itself is pretty stunning – it produces orbital images of Mars at high enough resolution that you could almost read the headline on a martian newspaper, assuming they had newspapers.)
In scanning through the daily haul of images, Dundas spotted striking blue strata in the walls of steep cliffs just a few meters below the dusty martian surface that sure look like water ice. Follow-up spectral observations by CRISM instrument on MRO confirmed the cliffs were indeed almost completely pure water ice, with no more than with less than 1% dust.
Ice on Mars isn’t particularly surprising – astronomers have known (or at least suspected) there is water ice at the poles of Mars for more than 100 years, and a mountain of data has indicated vast stores of ice in Mars’ subsurface, especially near the poles. But key questions about this ice have persisted: Was the ice recently deposited, and how much dust is mixed in?
Since these newly discovered cliffs are so pure, though, Dundas and colleagues suggest that they were probably deposited as snow before being buried. Mars’ current climate isn’t really conducive to water snow, and so the ice was probably deposited millions of years ago, when Mars’ axis had a very different tilt resulting a very different climate from now. The fact that the ice cliffs occur much nearer to the equator than might be expected also points to formation during a previous climatic epoch.
The implications of these cliffs for Mars’ climate history aren’t entirely clear, but their importance for exploration of Mars is hard to overstate. As Dundas et al. say in their paper, the cliffs would very likely serve as a resource for future human visitors. The water could be combined with gases in the martian atmosphere to make rocket propellent and even oxygen.
So there are large deposits of ice in the subsurface of Mars? Maybe “Total Recall” wasn’t so much science fiction as science prophesy.
The earliest phase in the Earth’s history, called the Hadean, was hellish. During long stretches, the surface was probably molten, the atmosphere was toxic, and there were no oceans. Eventually, though, the Earth transitioned to its current hospitable condition, and life got a toehold by at least 3.5 billion years ago.
But today in the Boise State Geosciences seminar, we heard about possible evidence for life going back almost a billion years earlier from Dr. Elisabeth Bell of UCLA’s Earth, Planetary, and Space Sciences Dept.
Dr. Bell’s work focuses on the hardiest of all mineral grains, the mighty zircons. These little rocks can be smaller than the width of a human hair, but they provide some of the strongest clues about conditions on the early Earth.
The mineral from which they’re made is very tough, and so weathering processes that usually break down other minerals barely affect zircons at all. Consequently, zircons that formed billions of years ago still retain their integrity, and, like the amber from “Jurassic Park”, often contain treasures in the form of other minerals. Dr. Bell and her team analyze these mineral time-capsules to learn what the early Earth was like.
One of the most exciting finds from her work comes from the Jack Hills geological formation in western Australia, where rocks almost as old as the Earth can be found. Inside these rocks are even older zircons, dating back to 4.1 billion years ago, which themselves have trapped small grains of graphite.
By analyzing the isotopic composition of these graphite grains, Dr. Bell has found tantalizing evidence for chemical processing of carbon that resembles biology. Although the evidence is still tentative, the results suggest life on Earth started chugging away smack in the middle of the Hadean, much earlier than has previously been believed.
Taking a step back, such a result suggests that life may get started on an Earth-like planet very quickly, which could mean that life is an almost inevitable outcome of the evolution of Earth-like planets. Given how common Earth-like planets may be, that could mean the universe may be replete with at least simple life.
The traditional definition of a habitable planet is “a world that can maintain stable liquid water on its surface”, but, as astrobiologists have explored for decades, this definition involves a vast flotilla of assumptions and very narrowly focuses our search for Earth-like life.
Even with all its limitations, this definition provides a very useful and practical starting point – at the first order, whether a world can host stable liquid water on its surface depends on the amount of sunlight it receives and whether it has a sufficiently thick (but not too thick) atmosphere.
Having found countless worlds outside our solar solar, astronomers are able to assess whether those worlds satisfy these conditions using observations we can already make, and a few dozen (probably) do.
In his review paper, Robinson discusses the observational and theoretical techniques astronomers can employ in the near future to take the next steps in deciding whether a world really has liquid water. Among the different approaches he describes, one is the most striking is the search for the glint from an alien ocean.
Robinson points out that Galileo was the first person to propose how to look for an ocean on another world. In his controversial Dialogue Concerning the Two Chief Worlds Systems, he says that, if the Moon had seas, “the surface of the seas would appear darker, and that of the land brighter”, just as on Earth.
Seas can also appear very bright compared to land, given the right observing geometry: seas exhibit specular reflection – this is the same effect you see looking out a plane’s window when the Sun reflects off the ocean. So looking for the glint provides a way to find large bodies of water on a distant world. Indeed, the first extraterrestrial oceans were found on Saturn’s moon Titan using this method, and one of the sea glints now frequently observed by the Cassini mission is shown in the figure at left.
Of course, we don’t have spacecraft orbiting any extrasolar worlds (yet), so we can’t resolve individual points on their surfaces. But, as discussed by Robinson, as they orbit their host stars, some of those worlds line up the right way that we could see a spike in the total amount of light coming from the planets. Observing such a spike over and over again whenever the planet was in the right geometry would be a strong hint that it had a large body of liquid reflecting sunlight. Given a little more information about planetary conditions, we could confidently infer such a planet had liquid water on its surface.
Amazingly, astronomers have used Earthshine reflected from the Moon to indirectly observe sea glints from the Earth. And so we’ve actually detected oceans on two worlds using distant spacecraft (if you let me call the Moon a “spacecraft”). As Robinson’s review implies, astronomers are probably on the cusp of finding oceans on extrasolar worlds. From there, it’s just a hop, skip, and a jump to finding life.
Canfield starts by exploring how cyanobacteria generate oxygen during photosynthesis and how the process evolved. For instance, chloroplasts, the photosynthetic power plants of cyanobacteria, were once free-floating cyanobacteria themselves that took up residence in and eventually merged with eukaryotes that evolved into present-day cyanobacteria.
Subsequently, Canfield discusses Earth’s oxygen removal and renewal processes – decomposition of organics burns up oxygen, while their rapid burial preserves it.
The next several chapters present the geochemical evidence for changes in oxygen throughout Earth’s history, including variations in the ratios of different isotopes, sensitive to biological and abiological processes. Since I teach a class on astrobiology at Boise State, I focused a lot of attention on these parts, trying the piece together the interplay between biology and geology betokened by the isotopic variations.
One element of that story especially relevant to my class: variations in the carbon-13 isotope. As it turns out, one enzyme in cyanobacteria, RuBisCO, helps convert atmospheric C02 into organic carbon compounds, but it preferentially selects the lighter carbon-12 isotope 2.5% more often than the carbon-13 isotope. The organic compounds built using RuBisCO are therefore slightly depleted in carbon-13 relative to the atmosphere and when they are later incorporated into geological strata, the slight depletion gives a measure of how much life was around when the stratum was laid down.
The book contained lots of other appealing details. For instance, it’s not exactly clear what caused an enormous variation in oxygen on Earth 580 million years ago, a sea change in Earth’s history matter-of-factly called the Great Oxygenation Event. But one explanation has to do with the evolution of a new kind of poop:
The idea is that zooplankton [newly evolved 580 million years ago] produce fast-sinking fecal pellets. These would decompose less in the upper layers of the ocean as they sink […] when compared to the smaller, slowly settling microbial biomass [that had previously predominated]. (pp. 135-136)
Since the old sinking biomass took a long time to sink to the ocean floor, it had a long time for bacteria to decompose it, using up a lot of oxygen in the process. But the new, faster-sinking poop made it to the ocean floor before it decomposed much and so left the oxygen dissolved in the ocean instead.
I did have to spend a lot of time reading and re-reading the discussions of geochemical cycles and signals because it’s been a long time since high school chemistry for me, but I was willing to struggle through these parts because I found the underlying story so interesting.
So a really fascinating and challenging read about the complex (and poop-filled) evolution of Earth’s bio-geo-atmosphere.
In this paper, the authors consider a very wide range of evolutionary scenarios for Proxima b to explore the resulting range of outcomes and decide how habitable the planet is, really.
They incorporate lots of potentially important effects, including the evolution of the host star’s luminosity and its influence on the planet’s surface temperature.
M-dwarf stars, like Proxima Centauri, get dimmer early in their lifetimes. As a consequence, the surface temperature of a planet orbiting such a star can drop over time.
Or, put another way, the habitable zone (HZ) around the star can move inward, meaning planets that start out interior to the HZ (i.e., planets that might be too hot to be habitable) may eventually enter the HZ.
Figure 11 from Barnes et al. (2016) shows that this is probably what happened to Proxima b: it started out way too hot for habitability and, as its host star dimmed, it entered the HZ.
As Barnes et al. show, such a history could potentially drive away all the planet’s water (assuming it started with any), leaving behind a dried husk of a planet. But the fact that the planet is CURRENTLY in the HZ could fool us into thinking it’s habitable.
This result shows that planetary habitability is a complicated idea and that the current conditions on a planet can depend in a complex (and hard-to-determine) way on its history. Time (and lots more data) will tell whether Proxima b is actually an extraterrestrial oasis for life or a barren wasteland.
Saturn’s moon Enceladus has inspired fascination since Herschel found it in the late 1700s. The discovery of active
cryovolcanoes geysers** on its surface by the Cassini mission in 2006 raised that fascination to a feverous intensity.
Although the source of energy powering the
volcanoes geysers** was not (and is still not) understood, their implication was clear: Enceladus could have a sub-surface ocean, like his big sister Europa.
The follow-up discovery of salty particles in the geysers all but clinched the existence of a sub-surface water source. Salt in the eruptions would probably require liquid water to dissolve rocky materials to produce the salt. But it wasn’t clear whether the sub-surface source is a small pocket of water directly beneath the geysers or a more global-scale ocean.
In our journal club today, we discussed a paper that points to the existence of a global ocean. The recently published study from Peter Thomas and colleagues analyzed Enceladus’ rotation to study its internal structure. By tracking the motion of hundreds of control points on Enceladus’ surface, they found that its outer icy crust oscillates back and forth during its rotation much more than it should if the moon were solid all the way through.
Thomas and colleagues show that the large oscillations they found (called libration) require a large layer of fluid within Enceladus to lubricate the space between its outer, icy shell and its rocky interior. Otherwise, the moon would oscillate a lot less. Difficult to say exactly, but Thomas and colleagues estimate the ocean could be as thick as 30 km beneath an 20-km thick icy crust.
Just as Europa’s ocean, a sub-surface ocean in Enceladus could represent an enormous harbor for life. Even on this tiny moon, such a deep ocean is only about a tenth the size of the Earth’s oceans*.
Although Enceladus’ surface would be a tough neighborhood for life, sub-surface biota (if they ever evolved) would be protected by a thick layer of ice from the vacuum of space and interplanetary radiation. In fact, this same radiation could impinge on the surface and produce a steady supply of biologically useful oxidants, which could then trickle down into the subsurface ocean and help power the alien biosphere.
Thomas and colleagues suggest a more detailed analysis of Enceladus’ surface geology, newly inspired by their discovery, might help unravel the history of the ocean. Similar analyses of Europa’s complex tangle of surface ridges and cracks helped piece together that moon’s geological history.
Journal club attendees today included Jennifer Briggs, Hari Gopalakrishnan, Tyler Gordon, and Jacob Sabin.
*Earth’s oceans are, on average, 4 km deep, much smaller than Earth’s radius of 6,400 km. Therefore, I approximated their volume as pi*(6,400 km)^2 (4 km) ~ 500 Mkm^3. A similar calculation for Enceladus’ ocean — pi*(50 km)^2 (30 km) ~ 24 Mkm^3.
**Dr. Thomas kindly pointed out that there is some debate over the nature of the eruptions on Enceladus, and volcanoes is probably not the term to use.
Exciting discovery reported last week of a planet a little bigger than Earth orbiting a star very like our Sun.
The planet, Kepler-452 b, was discovered by the Kepler mission and has a radius 60% larger than the Earth’s. It receives about 10% more light from its star than we do here on the Earth, and it’s probably about 2 billion years older. Together, these qualities mean it may be the most Earth-like exoplanet found to date (although there are lots of other similar planets).
Unfortunately, the host star is so distant, 1,400 lightyears from Earth*, that the usual method for directly estimating the planet’s mass, radial velocity observations, is not feasible. Instead, the planet’s discoverers constrain the planet’s mass by considering a range of compositions, calculating the radius expected for each of those compositions, and comparing it to the observed radius. Based on this analysis, they estimate at least a 49% probability that the planet is rocky, like the Earth.
Based on the amount of light it receives from its host star, there’s a good chance Kepler-452 b is habitable. This means, given a long-list of assumptions about the planet and its atmosphere, liquid water would be stable on its surface. Thus, Kepler-452 b joins a short but rapidly growing list of planets that might host life.
With our success finding potentially habitable planets, it’s probably only a matter of time (maybe just a few more years) before we find a planet that’s not just habitable but inhabited. Children in school right now might be the first generation to grow up in a universe where they know we’re not alone.
Today’s journal club attendees included Jennifer Briggs, Hari Gopalakrishnan, and Jacob Sabin.
*This website is the only reference I can find that gives the distance to Kepler-452 b from Earth. The paper itself doesn’t say 1,400 light years. The exoplanet.eu catalog gives a stellar magnitude V = 13.7 (also not given in the discovery paper). Converting that V magnitude to a flux and then using the stellar parameters given in the paper, I estimate a distance of 2,400 light years.