Space update special: the lake on Mars

An artist’s impression of Mars Express probing Mars using its MARSIS instrument superimposed on a radar cross-section of the southern polar layered deposits. The continuous white line at the top of the radar results above marks the beginning of the South Polar Layered Deposit; a layered accumulation of water ice and dust. The blue spots are areas of very high reflectivity and thought to be water. Credit D. Coero Borga/ESA / INAF

Whether or not liquid water exists on Mars has long been a source of study with regards to the Red Planet. There are many signs that the surface of Mars was once affected by free-flowing liquid – most likely water – in the planet’s ancient past. Curiosity, NASA’s Mars Science Laboratory rover, has confirmed the crater it has been exploring was home to several lakes during the early part of Mars’ history. What’s not clear is whether and how much of the remnants of that water still survives in liquid form today under the planet’s surface. Now a group of European scientists believe they have found direct evidence a sub-surface lake of liquid water on Mars.

The news comes via a paper published on Wednesday, July 25th in the Journal Science by a team of researchers involved in analysing the data from the European Space Agency’s Mars Express orbiter – something of a “forgotten mission” around Mars, given the volume of US missions on and orbiting the planet.

Mars Express arrived at Mars on December 25th, 2003. Since then, it has been quietly working away, observing Mars, gathering data about the planet’s atmosphere, surface and sub-surface, using a range of instruments including the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) package capable of “seeing” what lies under the surface of the planet. It is data from MARSIS which points to the presence of the subsurface lake.

An artist’s impression of Mars Express over Mars. The long booms (40m in length) extending from the vehicle are the deployed radar emitters for MARSIS. Credit: ESA

The story actually begins in 2007, when data from a MARSIS survey of a region near the Martian south pole revealed very strong differences in the radar returns (“echoes”) of two subsurface layers. On Earth, such a strong differentiation in returns typically tends to be the result of one of the layers being subsurface water. Analysis of the data from MARSIS initially suggested the “bright” return from the region on Mars might be caused by a layer of carbon dioxide ice. However, as further data was gathered on the region, researchers noticed something odd: the measurements of the layer kept changing over relatively short periods of time, rather than remaining relatively constant as might be expected from a body of subsurface ice.

Investigations of the apparent fluctuations in different radar returns of the same area revealed something unexpected: such was the volume of data point being collected by MARSIS, the software aboard Mars Express to initially process the returns was effectively averaging things out – giving the impression whatever the radar was encountering beneath the surface of Mars was somehow fluctuating: present in quantity during one pass, all but gone a few passes later, only to suddenly return in volume.

A composite image showing the south pole of Mars (r) and the region where the MARSIS radar reflections have been found (l). Credit: ESA

With their understanding of what was happening with the MARSIS processing, researchers were able to work out a means of compensating for it, and began a campaign of gathering data from the region, which ran for three years between mid-2012 mid mid-2015. It is the Earth-based analysis of this data over the last couple of years that has led to the conclusion that not only had MARSIS discovered something under the surface of Mars, but that it is very likely liquid water sitting under a covering of relatively cleat ice.

It is unclear if the body, some 20 km (12 mi) across and at least 2 metres deep and lying some 1.6 km (1 mile) beneath the surface, is actually an ice-covered body of water, or if it is an aquifer created by water filling interconnected pores in Martian rock beneath the ice.

However, given the extremely low temperatures on Mars, any water under the surface of the planet would require high concentrations of salt held in suspension within it, because salt helps reduce the temperature at which water freezes (a 20% solution lowers the freezing point of water to -16oC (-2oF), for example).  The data gathered by MARSIS is consistent with the liquid containing high concentrations of salts.

The discovery also has possible repercussions for the idea of Martian life.

For Life to get started, it needs three things: liquid water, an energy source such as minerals leeching into the water, and a biological seed. As noted at the top of this article, the evidence for water once having existed on Mars is strong. What’s more, NASA’s Curiosity rover has already found evidence for the second requirement – an energy source in the form of leeching minerals – was present at the time the planet had liquid water on its surface. So, if the third element – the biological seed – was available, then it is possible that microbial life may have started on Mars. Thus, there is the tantalising question of whether those Martian microbes might have followed the water into places like the south polar lake. However, we’re still a very, very long way from answering this particular question.

From what I think we have learned about this sub-glacial lake, the most likely analogue for this environment is the sub-glacial lake that was recently discovered in Canada… in which the lake itself is in contact with a deposit of salt, and so it is very, very salty. There are micro-organisms that are capable of surviving well below zero even without being in contact with water, and there are micro-organisms that can use the salt, presumably the salt in the water on Mars… for their metabolism.

– Roberto Orosei, MARSIS instrument co-investigator, and co-author of the lake study

While the MARSIS team are confident of their findings, they also acknowledge the need for independent verification. This should have been obtainable via the SHARAD (SHAllow RADar) instrument carried aboard NASA’s Mars Reconnaissance Orbiter. But for some reason, SHARAD doesn’t work as effectively over the south polar region of Mars as it does elsewhere, and thus far has been unable to “see” any similar returns presented by MARSIS. One possible explanation for this is that sub-surface irregularities within the region, coupled with SHARAD’s much higher operating frequency (20 MHz compared to the 5 MHz used by MARSIS), could be scattering its signals so that it cannot obtain meaningful returns from the depth of the lake.

Given this, it is unlikely that any confirmation of the MARSIS finding will come before 2021, when China’s planned Mars Global Remote Sensing Orbiter, Lander and Small Rover is due to arrive at Mars. This will be equipped with a ground penetrating radar system operating at a frequency between those of MARSIS and SHARAD, and so might be able to “see” the layer and offer a means to verify the MARSIS data.

In the meantime, the MARSIS team are now extending their studies to include other subsurface “bright spots” the instrument has located around the south pole of Mars to try to determine if they might also be indicative of  subsurface liquid water. If it can be shown there are numerous subsurface bodies of water in the region, then it rises a further intriguing question: are they, like the lakes beneath the ice of Antarctica, connected by a sub-surface network of channels that can carry water between them? This is unfortunately a question MARSIS cannot answer, as it is incapable of detecting channels that may only be a couple of metres or so across.

If it can be shown that subsurface bodies of water or aquifers are present on Mars, it could help towards confirming the cause of the seasonal recurring slope lineae (RSL) features seen on the slopes of crater walls. As I reported in October 2015, analysis of these strange dark streaks which appear on many Martian slopes each spring / summer, has shown they are rich in hydrated salts, suggesting they are the result of very briny water somehow brought to the surface of the planet from underground water tables. So the discovery of such a water table in the south polar regions could be a further confirmation of this theory.

Recurring slope lineae (RSL) features images on the slope of a Martian crater in 2007 and 2012 by NASA’s MRO, appear to be evidence of liquid activity on Mars and are rich in hydrated salts, leading to the suggestion that subsurface on Mars is kept liquid with the help of high concentrations of salt – as would need to be the case with the new lake. Credit: NASA

Another point of interest here is that the analysis of RSLs on Mars indicates they are particularly rich in perchlorate salts – magnesium perchlorate and sodium perchlorate. Such perchlorates lower the freezing point of liquid water to as much as -23oC (-10oF), something that would clearly help keep the MARSIS water table liquid, if they are present within it.

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