NASA has announced that it has come one step closer to ascertaining the presence of liquid water on Mars, and released new evidence obtained by one of its orbiters that has renewed hopes for a direct discovery in the future. At a press conference at the NASA HQ in Washington, D.C., a panel comprising the agency’s administrative chiefs, technical experts as well as a PhD student from Georgia Tech released the news, which follows from a discovery made in 2011. Finding if liquid water could exist on the surface of Mars today, and so support alien life, was one of the principal mysteries NASA set out to explore with its fleet of orbiters and rovers.
Recent discoveries, including one in March this year, suggested that some 20% of the red planet’s surface was covered in liquid water some time in its past. However, all that remains today is as ice around Mars’s poles, which makes today’s announcement more significant. The signs of liquid water flows have been found far from the polar ice and near the planet’s equator, where the surface temperature is higher and no ices are thought to be present.
“Under certain circumstances, liquid water has been found on Mars,” said Jim Green, the director of planetary science at NASA HQ, at the conference. Michael Meyer, lead scientist for the Mars Exploration Program also at NASA HQ, joined Green in attesting that the agency had orbital evidence for the first time of the conditions that encouraged the flow of liquid water on the Martian surface.
Well before the press conference itself, people were able to guess what the announcement would be about by its sensational headline as well as, and more so by, the inclusion of the youngest member of the panel: Lujendra Ojha, a PhD student in planetary sciences at Georgia Tech. It was Ojha who had, as a grad student at the University of Arizona, discovered the signs of briny water on Mars in 2011, spotting ‘finger-like’ structures across which the substance could’ve flowed in warmer climes. These structures, resembling narrow channels carved as if by flowing water, were called recurring slope lineae (RSL). Ojha had found them in images captured by the High Resolution Imaging Science Experiment (HiRISE) camera on board the Mars Reconnaissance Orbiter (MRO).
The interesting RSL resurfaced in 2014, when the orbiter turned in yet more evidence for liquid water-flows on Mars, this time observed with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument. The press statement from the time emphasises that the MRO still hadn’t spotted direct evidence of liquid water – i.e. liquid water itself – but only surface features one of whose causes could be the flow of liquid water. In fact, it also mentions another probable cause. From 2014:
One possible explanation for these changes is a sorting of grain sizes, such as removal of fine dust from the surface, which could result from either a wet process or dry one. Two other possible explanations are an increase in the more-oxidised (ferric) component of the minerals, or an overall darkening due to moisture. Either of these would point to water, even though no water was directly detected. The spectral observations might miss the presence of water, because the [RSL] are much narrower than the area of ground sampled with each CRISM reading. Also, the orbital observations have been made only in afternoons and could miss morning moisture.
Here’s Ojha discussing the results on RTV.
In today’s announcement, complemented with a paper in Nature Geoscience, NASA has said it has found the presence of hydrated salts at four different locations at the same time when RSL are also widely present there, during warmer seasons. Then, the salts appear in lower concentrations when the RSL fade in colder times – strongly suggesting that the formation of the two is related. According to the HiRISE team, “RSL are located in many places: equatorial and as far north as Acidalia Planitia,” which is a plain situated about 50º N and 340º E.
Analysing data obtained by the CRISM instrument, Ojha & co. found that the salts were magnesium perchlorate, magnesium chlorate and sodium perchlorate. On Earth, these compounds typically condense out of water, so their presence acts as a wetness marker.
The salts are also hygroscopic: they absorb water vapour from the atmosphere and then dissolve quickly in the resulting solution. The name ‘perchlorate’ refers to the molecules containing the the ClO4– anion. They form naturally in dry conditions, although on Earth they are present in very low concentrations. However, in 2009, scientists found that the soil surrounding Mars’s northern polar regions, in the plains called Vastitas Borealis, contain perchlorates to the tune of 0.4-0.6%. Such quantities are dangerous to humans should they ever land on Mars as they’re known to interfere with how the human body processes iodine. Perchlorates are also very soluble in water and reduce its freezing point – sodium perchlorate by 40 kelvin and magnesium perchlorate by upto 70 kelvin – a property that could be keeping water liquid on the chill Martian surface.
As Ojha said during the presser, “The source of molecular water in the perchlorates is either due to features of the RSL itself or some other property that created the RSL”, but then adding that “the features are probably due to the presence of liquid water” itself.
Even so, if the RSL is all the water’s doing, where could it be coming from? Ojha and his team have considered three unlikely possibilities. Melting ice is doubtful because the RSL were situated around equatorial regions, where ice is not likely to exist. That the water could be coming from a local aquifer is ruled out because no RSL have been found extending to mountaintops. There’s the off chance that some compounds in the dirt – like the perchlorates – could be absorbing moisture out of the atmosphere and condensing it on the ground. Mary Beth Wilhelm of NASA’s Ames Research Centre in Moffett Field, California, calls this her “favourite scenario”.
This possibility was considered prominently in 2013, when the Curiosity rover found similarities in the chemical composition of the Martian atmosphere and soil, with the dirt thought to be acting like a sponge to absorb water vapour and other compounds. Though scientists don’t think the thin Martian atmosphere has enough moisture to contribute to the formation of RSL every year, the rover’s Sample Analysis on Mars instrument also found that Martian soil was 2% water by weight. Nonetheless, the question of the water’s source remains a mystery.
These questions all warrant as much attention because of what they tell us about the red planet’s history, how its near-surface features and atmosphere evolved, its contemporary hydrology as well as if it harbours alien life. As Doug McCuistion, a former director of NASA’s Mars Exploration Program, told The Guardian before the presser began, “If they are announcing that they have found easily accessible, freely flowing liquid water under the surface, which is one of the theories we have been hearing for years and years, that has massive implications both for the potential for life on that planet and sustainability of humans.”
On Earth, we’ve found life of some kind wherever we’ve been able to ascertain the presence of liquid water – even in the arid Atacama Desert along South America’s Pacific coast. However, scientists are not so sure if Martian microbes could be swimming around in a briny liquid containing perchlorates, and which doesn’t stay wet on the Martian surface for more than a year. In fact, American physicist and former astronaut John Grunsfeld said, “If I were a microbe, I wouldn’t exist near one of the RSLs. I would probably exist farther up north near one of the freshwater glaciers.” NASA doesn’t yet have evidence that these glaciers exist but, according to Grunsfeld, suspects that they do.
Talking about future missions that could be better equipped to investigate the presence of alien life, Green, Meyer and Grunsfeld were all careful to emphasise the importance of planetary protection – not contaminating Mars with life from Earth inadvertently carried there by rovers and landers. With that in mind, Meyer said that the agency was taking a “measured approach”, with a mission planned in 2020 to “cache samples” taken from the red planet’s surface as well as from underground and bring them to Earth for analysis. Grunsfeld also chipped in that the next planetary decadal survey, a document prepared by the US National Research Council identifying the biggest challenges in planetary exploration and how they might be handled, “might recommend a Mars mission with life-detection capability”.
In the meantime, there’s no doubt that with the signs of contemporary liquid water piling up, Ojha and others will continue working on obtaining direct proof. The day could be near when Mark Watney has one less thing to worry about.