When mankind first noticed Mars in the sky there were surely plenty of people who wondered if life was roaming around on its surface. In more recent years we got a good look at the surface and realized nobody is home, but the old sci-fi cliche of an underground Martian society hiding out has remained. Now, a new study reveals that life might well have thrived under the surface of the planet, but that time has long since passed.

The research, which was published in Earth and Planetary Science Letters, suggests that while life on Mars has long since died off — if it ever existed there to begin with — the conditions underground were once ideal for it to take root.

“We showed, based on basic physics and chemistry calculations, that the ancient Martian subsurface likely had enough dissolved hydrogen to power a global subsurface biosphere,” Jesse Tarnas of Brown University, and lead author of the study, said in a statement. “Conditions in this habitable zone would have been similar to places on Earth where underground life exists.”

In part, the paper focuses on the wealth of hydrogen that would likely have been present in the Martian soil some four billion years ago, as well as the existence of water on the surface. The researchers used climate models and readings from Mars missions to estimate how large a subterranean habitable zone might have been. They now believe a layer as thick as several kilometers would have been ripe for life.

The conditions would have been much like those that allow tiny organisms on Earth to thrive. What’s even more important is that even if radiation from the Sun was making it hard for life to exist above ground it would have been protected beneath the ground.

Tarnas is quick to point out that this doesn’t necessarily mean life was actual present on ancient Mars. A planet being comfortable for life and a planet actually hosting life are two very different things, and we’ll need to wait for future Mars missions before we’ll know if there is any evidence of past life there or not.

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