The Red Planet today is a frigid desert, but overwhelming evidence shows that Mars, billions of years ago, had abundant flowing water on its surface. This remains a surprising fact given that the Sun was much dimmer at the time. So, how was the planet warm enough to keep its liquid water for so long?
“To have made the planet warm enough for liquid surface water, its atmosphere would likely have needed an immense amount of greenhouse gas, carbon dioxide specifically,” Chris Tino, a graduate researcher at the University of California Riverside, said in a statement.
Planetary scientists believe that the answer lies in how its atmosphere was back then. Unfortunately, we cannot time travel back to that period to study it, so researchers have to come up with different strategies. And one presented in the latest issue of Science Advances has a cosmic connection down here on Earth.
An international team has looked at the Nordlinger Ries Crater, a meteor impact site that formed 15 million years ago in modern-day southern Germany. The site is very similar to Mars' Jezero Crater, the landing place of the Mars 2020 rover, which will launch this summer. The team believes that this location can be used as an analogous site to what might be found on Mars.
The crater has incredibly well-preserved rocks, making it a truly outstanding geological treasure throve. The analysis of these rocks allowed the team to study the local conditions, focusing on both the pH (acidity) of the water that was present here as well as how alkaline it was.
“Ries crater rock samples have ratios of nitrogen isotopes that can best be explained by high pH,” co-first author Eva Stüeken, from the University of St. Andrews in Scotland, explained. “What’s more, the minerals in the ancient sediments tell us that alkalinity was also very high.”
These findings have similarities with some expectations of Mars. The researchers state that the water likely had a neutral pH, but it was also highly alkaline. This might sound confusing. How can the water be alkaline and neutral at the same time?
"When CO2 is very high, one would usually expect an acidic pH. However, the pH can be maintained neutral if the water contains high levels of alkali and alkali earth elements (Na, K, Ca, Mg). If CO2 is relatively low while the water is alkaline, the pH can be quite high. Previous studies have demonstrated high alkalinity on Mars, but not much is known about pH," Dr Stüeken explained to IFLScience.
To constrain the pH and thus the atmospheric composition, the team suggests using nitrogen. This approach has shown success for the Ries Crater rocks and the team believes it will work equally well for Mars. The Mars 2020 rover will collect samples with the intention of eventually sending them to Earth, and while it might take some time for those precious collections to get here and be analyzed, scientists can work on refining this and other methods to open a window on the climate of ancient Mars.