A renowned international team of researchers led by Benton Clark from the Space Science Institute in Boulder, Colorado, recently looked into one of the key questions in astrobiology—whether life is likely to have arisen on Mars. Their analysis, published in the journal Life, is the most comprehensive assessment of that question I’ve seen to date, and is based not only on current thinking about how life originates, but also on the latest results from the Mars rover investigations.
Their conclusion: The chances that life arose on Mars are at least as high as they were on Earth, because the requirements—as we currently understand them—were all there, including liquid water, organic compounds, essential elements and minerals, and access to energy sources.
The authors give several reasons why the chances of life starting up on the Red Planet might actually have been higher than on Earth: (1) Mars should have received a higher influx of organic material from impacting asteroids and comets; (2) Sulfur, an important element for biology, is more common on Mars; (3) Mars had a head-start for the origin of life, because Earth was hit by a large impactor that created the Moon just 20 to 100 million years after our planet formed, which certainly sterilized the surface; (4) Mars experienced many wet-dry and freeze-thaw cycles, which are critical for concentrating organic compounds. In fact, these cycles were likely more common on the Red Planet early in its history. While 95 percent of Earth’s land was submerged under oceans, only five percent of Mars was, meaning that Mars would have had three times more land area despite its smaller size.
Wet-dry cycles and freeze-thaw cycles occur best if there is exposure to air, which favors the “hydrothermal pond” scenario for life’s origin. But if life on Earth began on the ocean floor, as some researchers have suggested, it would have been less likely to arise on Mars. That’s not to say there were no opportunities at all. For example, the Nili Fossae region on Mars is considered by some researchers to have been similar to the “Lost City” hydrothermal field in the Atlantic Ocean. And there are still other scenarios for the origin of life—under cold conditions or in salty brines, which both have the potential to concentrate organic compounds. Either of these would again favor Mars, given its much colder environment and the ubiquitous presence of brines.
One scenario the authors did not consider is that life on our own planet could have started in tidal flats due to Earth’s unusually large moon, which together with the Sun exerts immense tidal forces. These forces were much stronger four billion years ago when the Moon was closer to Earth. Mars has no large moon, however, and if that turns out to have been the critical factor, life on Mars seems doubtful.
What we really need to know is which of these scenarios for the origin of life Earth is the right one. And of course, there is still the real possibility that life started first on Mars, and was brought here by meteorites later.