American researchers tested whether an Earth microorganism can not merely survive but actively reproduce in conditions close to those that may exist beneath the surface of Mars. The work was published on November 5 in the journal Applied and Environmental Microbiology.
The focus was Haloferax volcanii, a microbe classified as an extremophile, that is, an organism capable of tolerating extremely harsh conditions on Earth. Scientists chose it because it prefers environments with a very high salt concentration, similar to the hypothesized small subsurface reservoirs on Mars.
The cultures were kept for 160 days without oxygen, in an atmosphere with a high carbon dioxide content. The pressure was only a small fraction of Earth's, and the solution was seven times saltier than seawater. Instead of oxygen, the microbes were offered nitrate or perchlorate - compounds that may occur on Mars and be used for respiration. In control samples where neither was present, the microbes remained alive but did not grow. This helped separate survival from reproduction.
In the experimental media, the microorganisms reproduced, although noticeably more slowly than in analogous cultures at normal Earth pressure. In addition, dense structures - biofilms - were observed in the growing cultures at low pressure. They may have helped retain water in the salt solution: during the experiment, almost no liquid was lost.
The study did not reproduce all Martian conditions. The cultures were kept at 21 degrees Celsius, whereas most of subsurface Mars is far colder. The nutrients were added by the scientists themselves. And the pockets of liquid water modeled in the laboratory have not yet been confirmed on Mars - they exist only in theory.
"The environment we simulate in this experiment has not been observed on Mars, but it is assumed that it may exist," explained the head of the work, Adam Robinson of the University of Florida.
This distinction is fundamental: the results do not prove that life exists on Mars today. However, they suggest that future missions searching for life beneath the planet's surface should consider the possibility of active microbial processes, and not only look for traces of ancient life.
Co-author Scott Perl of the Natural History Museum of Los Angeles County and the Blue Marble Space Institute of Science noted that scientists should reconsider ideas about what makes an environment habitable. Microorganisms, if they arose on Mars, had billions of years to adapt to changing conditions. "What is hostile to us is not necessarily hostile to these microbes," he said.
According to Robinson, the work adds arguments in favor of considering the possibility that life exists on Mars at present.