Since time immemorial, Mars has enticed earthlings with the possibility that it harbors life. But like a bad soap opera, exploring it created astrobiological cliffhangers that mostly turned out to be dead ends.
Today, planetary scientists are convinced that for a brief period of 500 million years in its early history, Mars had liquid water running across its surface, with rivers, lakes, deltas and perhaps even an ocean. The current reality, however, is that the Martian surface is even more inhospitable than Earth’s driest deserts.
The surface is completely inhospitable, Jennifer Stern, a planetary scientist at NASA’s Goddard Space Flight Center in Maryland, told me. Coupled with the fact that there is no liquid water on the surface, Mars also suffers from a continuous stream of cosmic and solar radiation, she says. It’s really hard to imagine that any kind of organism could have adapted to these really bad conditions, Stern says.
From what we’ve measured on the surface at least, everything points to abiotic and non-biological processes, Stern said in a presentation last week at the 241st meeting of the American Astronomical Society (AAS) here in Seattle. The best hope for future Mars exploration lies underground, she says.
The near unexplored subsurface could have maintained a habitable environment longer than the surface, Stern notes in his AAS summary. And records of organic molecules can persist beyond the range of ionizing radiation from galactic cosmic rays, she writes.
How has the habitability paradigm on Mars changed since the era of Viking Mars landers?
When NASA sent the two Viking landers, they really threw everything at the question of life, Stern says.
But when they didn’t detect any life, it kind of spelled the end of NASA’s exploration of Mars for the next 25 years, she says.
Then, when we got back, we started thinking about the most basic thing life needs, Stern asks. And it’s pretty indisputable that this water must be present for life as we know it to exist, she says.
But to date, there is no hard evidence of life on the surface of Mars.
With NASA’s Mars rovers detections of complex organic compounds in the 3.8-billion-year-old mudstones of Gale Crater and Jezero Crater, we now know that organic carbon is likely widely distributed on Mars, noted Stern in his AAS presentation.
Certain organic molecules, like amino acids and lipids, are more indicative of life and things than we see in the fossil record on Earth, Stern says. But we don’t see these molecules on Mars, she said. And we don’t see any evidence of microfossils, Stern says.
Are we halfway in understanding the history of Mars’ habitability?
I think we’re pretty advanced, Stern said. With the Curiosity and Perseverance rovers, we have confirmed that there are carbon-carrying organic molecules on Mars, she says.
Does Mars currently have flowing water?
There’s plenty of evidence of past water, but we don’t have any evidence of water flow today, Stern says. The minerals we find are minerals that can only be made in water, she says.
Why did Mars go horribly wrong so quickly?
“I don’t know if it’s that Mars got so bad or maybe Earth really got lucky in its habitability,” Stern said.
Even so, Mars had the misfortune of being just smaller than Earth, meaning it cooled much faster, Stern says. Mars likely lost its internal dynamo early, which would normally have generated a global magnetic field that would have shielded the Red Planet from the ravages of charged particles from the Sun.
NASA’s MAVEN spacecraft showed that Mars was likely stripped of its atmosphere by charged particles from the solar wind.
We know there’s ice underground on Mars and there may well be liquid water underground, we just don’t know that, Stern says. On Mars, the subsoil is completely unexplored, but we know that Earth’s subsoil is a habitable environment, she says. There is life on Earth about three miles deep, Stern says.
Does Mars also have life under its surface?
I think all bets are off because we haven’t been there yet, so we don’t know, Stern said. If there can be life deep within Earth, there very well could be life deep within Mars, she says.