Yesterday NASA revealed that it has found evidence for present day liquid water on Mars. In this blog post I’ll explain how they made this discovery, why it is significant and what it means for the search for life on our neighbouring planet. NASA’s announcements about water on the martian surface are sometimes met with cynicism as it can seem like it’s constantly being repeated that water is being found on the red planet. However, liquid water is the key requirement for life as we know it, so any discovery that informs about water in the past or present on Mars is hugely significant. Previous discoveries have revealed much about the role of water in Mars’ past, such as the presence of rivers, lakes and potentially an ocean. It has also been found that in the present day water is stored in subsurface ice or in minerals that have water in their chemical structure, as well as in Mars’ tiny polar ice caps.
The hugely exciting discovery of present day liquid water on Mars was made using an orbiting spacecraft called Mars Reconnaissance Orbiter, or MRO for short. MRO is a long lived mission that has been analysing the martian surface since 2006. Two key instruments on board MRO are the High Resolution Imaging Science Experiment (HiRISE) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). HiRISE has sent back thousands of incredibly detailed images of the martian surface and you can see the photo catalogue here, which is being constantly updated. While HiRISE’s role is to photograph features as small as a metre across, the job of CRISM is to analyse how the martian surface interacts with visible and infrared light as this can inform about the mineralogy of the rocks being imaged.
The incredible resolution of HiRISE images enabled the discovery of small features called recurring slope lineae (RSL). RSL are thin dark streaks that annually appear on some slopes on Mars during the martian summer and fade away once summer ends. A dynamic present day process on Mars is of great interest to scientists and understanding what might be forming the RSL has been a priority for the MRO team. Observations by HiRISE and CRISM have led NASA to the conclusion that RSL are formed by liquid water and this finding is supported by data from missions to the surface of Mars. The most significant of these was the Phoenix lander in 2008. Phoenix discovered a chlorine containing salt known as perchlorate when it performed a chemical analysis of martian soil. Perchlorate is extremely rare on Earth as it dissolves readily in water but Mars is dry enough for it to be common on the surface. Perchlorate on Mars has mostly been seen as a problem as it can interfere with the heating experiments rovers and landers use to look for organic compounds, which might inform about ancient or present day life on Mars. However, perchlorates have a good side too. Salty water freezes at a lower temperature than fresh water, which is one reason why Earth’s oceans don’t turn into giant ice cubes every winter. When perchlorate salts dissolve in water they lower the freezing point of the resulting brine significantly, low enough that at times a liquid perchlorate brine could potentially survive on the cold surface of Mars. Curiosity rover has also found perchlorate salts at its study site in Gale Crater supporting the theory that perchlorates are widespread on Mars. It had been theorised that liquid perchlorate brines might exist on Mars but until yesterday direct evidence for these fluids had not been found.
The resolution of MRO’s CRISM instrument (which can inform about mineralogy) is not as sharp as that of HiRISE so to counter this scientists aimed MRO’s instruments at areas of Mars where there were so many RSL bunched together that CRISM could analyse RSL mineralogy. CRISM found that the RSL showed evidence for hydration, i.e. the minerals in the RSL had water in their structure. When the RSL vanished after summer ended the hydration signature also disappeared. Either the formation of RSL added water to pre-existing minerals in the study areas or the RSL themselves were transporting and depositing minerals that have water in their structure, which then dehydrated when the RSL faded away. Both of these processes require a release of liquid water on the surface of Mars. CRISM found that the mineralogy of these regions was likely typical martian soils mixed with chlorine containing salts, including perchlorates. Surface releases of perchlorate brines are therefore a realistic explanation for RSL formation.
Where this liquid water might be coming from remains unclear. Perchlorate is a hygroscopic mineral, which means it likes to absorb water vapour from the atmosphere onto its chemical structure. If the atmosphere around perchlorate deposits on the surface of Mars became sufficiently humid then the salts could potentially absorb so much water that perchlorate brines form (a process called deliquescence). However, it’s uncertain as to how the martian atmosphere might become sufficiently enriched in water vapour this way every year to form RSL. Melting of subsurface ice is another possibility, but many RSL are equatorial and at these locations ice exists very deep down in the subsurface. It’s likely that at different RSL forming regions there are different processes operating and further investigations are certainly required.
What is critical is that NASA has identified regions on Mars where liquid water occurs in the present day. These observations place these RSL hosting sites at the top of the list for regions that might host martian life, as life as we know it needs liquid water to exist. How long this liquid water may persist (at the surface or in the near-surface) is a key question that remains to be answered. It also needs to be considered what kind of microbes might thrive in perchlorate rich brines. It will be interesting to see if the scientists deciding on landing sites for the upcoming rovers ExoMars and Mars 2020 will consider landing near an RSL rich region and investigating it up close.
This research was published by lead author Lujendra Ojha in Nature Geoscience and is unfortunately behind a paywall but a readcube version is available here. If you have any questions please feel free to comment below or message me on twitter.