The past decades of solar system exploration have revealed that Mars used to have abundant water like Earth. In 1971 the Mariner 9 satellite revealed ancient flood-carved valleys. Recently, the Curiosity rover visited a dried up river bed in Gale crater. But these observations have left the most pressing question in astronomy unanswered: was there, or is there, life on Mars and other planets? Water was vital for life to evolve on Earth, but water alone isn’t enough. So, did the water on Mars contain other key ingredients for life? Answering this question is challenging, since Mars has lost its oceans, lakes, and rivers over the past three billion years. What can the present, dry Mars tell us about the composition of its water in the past?
The Martian rock record holds clues to Mars’ wet past. Satellite images alone reveal over 400 craters that contained lakes over three billion years ago. Water in these lakes deposited thick layers of sediment into the crater basins. Bits of mud and sand suspended in the standing water slowly fell, settled, and collected at the lakes’ murky depths. These layers of sediments are like pages in a book, or rings in a tree, each providing a record of the environmental conditions in which they formed.
Some of these layers are made of salt minerals that settle or precipitate when water dries up. Sodium chloride, common table salt, is abundant in Earth’s oceans, and is one such mineral that we spot on the surface of Mars. Other salt minerals also form when water evaporates. The rover Opportunity stumbled upon sequences of a mineral called gypsum, or calcium sulphate, that is one of the first minerals to crystallize as water slowly disappears. Not only does gypsum form in water – it even traps and incorporates H2O in its crystal structure, providing even more evidence for Mars’ wet past. But neither table salt nor gypsum can form if the water lacks the appropriate elements. Salt minerals, therefore, are the direct product of water chemistry.
And water chemistry influences life! Sulphur and oxygen, two elements found in gypsum, are “CHNOPS” elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulphur) – the top six most abundant elements in biological molecules. Many other elements found in salt minerals are electrolytes, playing a vital role in osmosis, or the balancing of water in and out of the cell. Certain microorganisms thrive in salty environments on Earth, and scientists extrapolate that these minerals are necessary for life to thrive on other planets. Determining the water chemistry of Mars’ past lakes is key to assessing if they were once habitable.
The assessment is still out, and the search for life on Mars continues. New data from rovers like Curiosity, and images from satellites like the Mars Trace Gas Orbiter (one of many modern descendants from the old Mariner 9 spacecraft) continue to roll in from our neighbouring planet. Although the Opportunity rover has recently lain to rest, Curiosity continues to explore the remnants of the lake that used to fill Gale Crater. The next generation of Mars rovers, including NASA’s Mars2020 and ESA’s ExoMars rovers will each target sites of past water to look for evidence of life. While ExoMars will be drilling deep into the hardened bedrock of Oxia Planum, Mars2020 is destined for Jezero Crater – an ancient lake bed with layers of clays and a hidden story to tell. With two new rovers landing on Mars in 2021, we will have increasing opportunities to study the ghosts of Mars’ past lakes.