In 1976, NASA’s Viking 1 and 2 missions landed on Mars and began conducting the first astrobiology studies on another planet. This involved the analysis of soil samples for possible indications of organic molecules and biological processes (aka. “biosignatures”). The results of these studies were inconclusive and led to a general sense of pessimism towards the idea that Mars ever hosted life. However, the presence of features that could only have formed in the presence of flowing water – flow channels, delta fans, hydrated minerals, etc. – led to renewed astrobiology efforts by the 1990s.
Since then, no less than 25 missions (a combination of orbiters, landers, and rovers) have been sent to Mars to learn more about its past and resume the search for biosignatures. These efforts have been bolstered by the discovery of Recurring Slope Lineae (RSL), which refers to dark linear features on steep slopes on Mars. These features appear to be seasonal in nature, appearing in summer and fading away during winter, which suggests the presence of liquid water. In a recent paper, Vincent Chevrier of the University of Arkansas (UArk) presents the most compelling evidence to date that seasonal brines occur on Mars.
Between the extreme variations in temperature and Mars’ very low atmospheric pressure (less than 1% that of Earth), water cannot exist in a stable form on the surface. As such, the existence of RSLs remains a controversial issue for scientists. These “brines” are believed to result from seasonal melts mixing with the natural perchlorates in Martian soil. Assuming they can exist, these patches could host life in the form of single-celled microbes. According to the latest research by
Vincent Chevrier, an associate research professor at UArk’s Center for Space and Planetary Sciences,
Vincent Chevrier, an associate research professor at the University of Arkansas’ Center for Space and Planetary Sciences. Credit: UArk
Seasonal frosts are common on Mars and present the best chance for finding liquid brines. However, because of Mars’ thin atmosphere, water tends to transition directly from ice and vapor without becoming a liquid (aka. sublimates). To investigate the possibility of liquid existing periodically in the form of brines, Chevrier consulted meteorological data collected by the Viking 2 mission, which landed in the Utopia Planitia region on September 3rd, 1976. Located in Mars’ Northern Lowlands, this region is known to have permafrost and is believed to have once been covered by a planetwide ocean.
This was combined with data from the Mars Climate Database and computer modeling to determine if brines could form from melting frost for brief periods. Chevrier selected the Viking 2 data because it is the only mission to have clearly observed, identified, and characterized frost on Mars. Chevrier has spent the last 20 years studying Mars for signs of liquid water and has long suspected that perchlorates are the most promising salts for brine formation because of their extremely low salt-water melting point.
This includes brines composed of water and calcium perchlorate, which solidifies at -75 °C (-103 °F), whereas average surface temperatures on Mars range from 20 °C (68 °F) during the day to -153°C (-243°F) at night. Based on the climate modeling data, Chevrier determined there is a brief window lasting for one Martian month (roughly two months on Earth) between late winter and early spring when temperatures are right for the formation of brines. He further concluded that ideal temperatures are present between early morning and late afternoon, and are either too hot or too cold at other times.
These brines would be scarce, since calcium perchlorate accounts for about 1% of Martian regolith, and frosts that form in the Northern Lowlands are extremely thin.
While these findings are not conclusive proof that brines exist on Mars, they do indicate that Mars could conceivably support life adapted to much colder, drier conditions. What’s more, they offer a tantalizing prediction of what future missions to Mars could find and suggest that similar processes may occur in other frost-bearing regions, such as the mid-to-high latitudes.
The paper that describes his findings was recently published in Nature Communications Earth and Environment.
Further Reading: University of Arkansas, Nature Communications Earth and Environment