Anyone expecting the moon’s Sea of Tranquility to live up to its name will have been disappointed with the expanse of dry, barren rock revealed when Apollo 11 landed there 49 years ago. The naming of this expanse of lunar territory by the Italian Jesuit astronomer Giovanni Battista Riccioli in 1651 acknowledged Galileo’s contention that the moon is a world topographically like ours, although Ricciolo thought it was uninhabited.
Likewise the dark streaks of “canali,” or channels, described in 1877 on Mars by another Italian astronomer, Giovanni Schiaparelli. Although excitedly translated as “canals” and thereby helping to inspire HG Wells’ The War of the Worlds, these proved bone dry. Such linguistic slippage attests to our long-standing impulse to imagine water on other worlds.
But the martian lake just discovered from radar soundings—taken by the European Space Agency’s Mars Express spacecraft in orbit around the planet—is no mirage or euphemism. It appears indeed to be a shallow body of liquid water about 20 km wide, close to the martian South Pole.
That there seems to be a reservoir of water (and not just ice) on the planet surely ranks as one of the most astonishing finds on Mars for a long time. It seems only fitting that Italian astronomers, led by Roberto Orosei of the National Institute of Astronomy in Bologna, are once again behind the discovery.
There seems no doubt that Mars once hosted a great deal of water: rivers and even shallow oceans. Billions of years ago it is thought to have had a warmer climate and a thicker atmosphere, so that much of the water now frozen along with carbon dioxide (aka “dry ice”) in the polar caps was liquid. Evidence of that remains, most strikingly in the sinuous canyons carved out of the rocky martian surface, which can hardly be anything but the work of water erosion. There is also a record of Mars’ watery past in its rocks, some of which have a chemical composition that testifies to their having been formed in water. Gale Crater, just south of the martian equator and formed by a meteorite impact around 3.5-3.8bn years ago, has a central mound thought to be made from sedimentary minerals deposited in what was once a freshwater lake—as NASA’s rover Curiosity verified when it landed there in 2012.
The surface water evaporated or froze as the martian atmosphere thinned and the planet cooled. But Mars’s wet past offers some prospect that microbial life might once have arisen there—as it did on the early Earth. If so, there might still be fossil traces.
It seemed less likely that any substantial body of surface water still exists. But the lake seen by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board Mars Explorer is not on the surface. It is sealed underneath the ice of the South Pole.
That makes it comparable to the subglacial lakes that exist hundreds of metres beneath the Antarctic ice sheet on our planet. Those lakes were also first seen by radar, although now their water has been sampled by drilling through the ice—and found to containrich microbial ecosystems.
“Evidence of water remains, most strikingly in the sinuous canyons carved out of the rocky martian surface"Reflection of radar signals depends on the way in which the material they encounter interacts with electric fields, since radar is an electromagnetic wave with centimetre-scale wavelength. So a sudden change in this responsiveness to the radar waves as theytravel from dusty martian ice into liquid water causes a bright reflection in the radar map, and the strength of the reflection supplies a way to estimate what the reflecting material might be. Radar soundings of the South Polar ice cap were taken previously by MARSIS in 2007, but seemed to show only ice. The more detailed radar mapping carried out between 2012 and 2015 called the Planum Australe now reveals what can apparently only be a layer of liquid water.
It is extremely cold underneath the martian ice—estimated to be around minus 68oC, well below the ordinary freezing point of water. Nevertheless the lake may be kept from freezing both by the high pressure of the ice above, which lowers the freezing point, and especially by the large amount of salts that are probably dissolved in the water: chemical compounds such as magnesium, calcium and sodium perchlorate. It’s because salt can lower the freezing point of water that roads are salted in winter. At the martian surface there’s enough of these salts to lower the freezing point to minus 74oC, and droplets of salty water were detected by NASA’s Phoenix lander which touched down in 2008.
So it’s thought that the subsurface lake sandwiched beneath 1.5 km of ice in the Planum Australe is filled with very strong brine. That makes it comparable to the Antarctic lakes too, which have a salinity up to six times greater than seawater.
The paper and accompanying commentaryare scrupulous about avoiding any discussion of the possibility of life in this water. That’s probably the right approach, but since I am unfettered by such propriety, let me say that the feasibility depends partly on how much salt there is—which the scientists can’t yet estimate accurately. But if it’s similar to the Antarctic lakes, microbial life is at least possible.
Of course, saying that such an environment could support life is very different to suggesting that it does. One school of thought has it that life cannot persist tenuously—in tiny, fragile pockets—on any world: it either colonises (and probably transforms) the planet, as on Earth, or soon dies out. But the tantalising possibility of martian life has been raised too by Curiosity’s measurements of the chemistry of Mars’s thin atmosphere, which contains very small amounts of methane. On Earth, nearly all of the methane in the atmosphere is generated by microbes. There are conceivable ways methane could be produced by purely geological processes, but such an explanation is complicated by the fact that the methane levels seem to changewith the martian seasons. Another possibility is that there are underground methane reservoirs—natural-gas deposits, basically—produced from long-extinct martian microbes.
In the end I remain rather sceptical about martian life. But the presence of liquid water surely alters the odds.
