But our trips to Mars have opened a window into the deep past of the red planet when conditions were much more conducive to life.
This summer, NASA will send Perseverance, its newest scout vehicle, on a seven-month trip to Mars. Like Curiosity, its predecessor, Perseverance will land on the remains of the ancient Martian lake bed. What you find there, along with the China and UAE missions, could help us understand what Mars was like some 4 billion years ago, when it was a young planet, and whether life ever flourished on its surface.
How habitable was Mars in its earliest times? It is a serene image: a river flows into an expansive lake that fills the basin with a crater. Waves lapped the shoreline and sediment accumulated in a delta. The lake bed was covered in clay.
This is the kind of aquatic environment that could be conducive to life and was once a familiar sight on Mars.
“The evidence for lakes and rivers is incontrovertible,” said Ken Farley, a Perseverance project scientist and geochemist at the California Institute of Technology. Although Mars was a wet planet, there is substantial debate about the origins, range, and lifespan of its water bodies that long ago disappeared.
For example, it is possible that Mars, in its beginnings, warmed up thanks to the gaseous emanations of active volcanoes that thickened its atmosphere and caused the fusion of Martian permafrost. Cataclysmic asteroid impacts could have triggered 274-meter megatsunamis that flooded the planet’s territories. There is even controversial evidence that the ocean once covered its northern lowlands.
“Were they strange, short and transient events or was there an ocean?” Farley said. “I think there is no consensus. There are many ideas and we really need more information to be able to solve them ”.
An important question concerns the longevity of Mars’ liquid water. No one knows how long it takes for life to emerge on a planet, even on Earth. But the chances of life being generated improve the longer stable bodies of water last.
During Curiosity’s eight-year journey through Gale Crater, which is the ancient lake bed, the vehicle discovered sediment suggesting that the water was present for at least a few million years. Curiosity also detected organic compounds that are key ingredients for life as we know it.
“What we have learned from Curiosity suggests that Mars was habitable,” said Dawn Sumner, a planetary geologist at the University of California, Davis, and a member of the Curiosity science team.
Of course, “habitable” does not necessarily mean that it has been “inhabited”. The surface of Mars is exposed to damaging solar and cosmic radiation, which could have reduced the chances of complex, multicellular life being generated.
“If life existed on Mars there would be a strong, radiation-resistant evolutionary force,” Sumner said.
Why did Mars become less habitable?
The old oases of Mars are now mirages of a distant past, the modern planet is a dry shell. Earth, by contrast, has been habitable for microbes for most of its life and has taken advantage of streaks of biodiversity for aeons. Why did these brother worlds experience such different results?
When they were baby planets, Mars and Earth were wrapped in two protective blankets: a relatively thick atmosphere and a strong magnetic field. Earth has clung to both conditions. Mars no longer has any.
Mars mysteriously lost its magnetic field billions of years ago. Without a magnetic cover to protect it from the solar wind, the Martian atmosphere was stripped away over time, although it still maintains a thin layer of its past heavens.
These changes have left Mars relatively inert for billions of years, while Earth reinvents itself through tectonic activity, atmospheric changes, and the ingenuity of life.
Could Mars harbour life today?
The robotic explorers that we have sent to Mars have developed countless ideas about the red planet, but have never found clear signs that there are creatures that currently reside there. Life, at least as we know it on Earth, simply doesn’t seem likely on the Martian surface.
“If life exists on Mars, it needs at least a little liquid water,” Sumner said. “The surface of Mars is very dry. Incredibly dry. If there is life on Mars, it would be deep underground.”
There is some evidence that liquid water is encapsulated in underground reservoirs, so there may be ecosystems there that do not need sunlight. If these habitats exist, they are beyond the direct range of our vehicles and landers.
Recent detections of methane and other gases in what remains of Mars’ atmosphere are “an attractive potential signature,” Farley said, reinforcing speculation about underground Martians. On Earth, many microbes produce methane, so it is possible that the smell of gas on Mars could lead to extraterrestrial life forms deep underground.
The discovery of life on Mars, whether in the form of ancient fossils or in underground deposits, would be one of the most far-reaching discoveries in human history. At last, we would have another example of a living planet, even if it only flourished in the past, implying that at least life can arise twice in the universe.
But even if we never encounter Martians, “Mars is a place we can go to answer some of the questions about life on Earth,” Sumner said. The red planet is still a mysterious time capsule from the time when life first emerged in our own world and shows us what could have happened if all the factors that made our world possible had not turned out the right way.