Mars is the most explored planet in the solar system besides Earth. With all of our robotic visitors there, we have discovered that it is too dry, cold and irradiated a world to support the intriguing humanoids or invading tentacles that science fiction ever imagined.
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 launch its latest rover, Perseverance, on a seventh-month trip to Mars. Like its predecessor, Curiosity, Perseverance will land on the remains of an ancient Martian lake bed. What it finds there, along with missions launched by China and the United Arab Emirates, could help us understand what Earthlings were like as a young planet about four billion years ago, and whether life flourished on its surface.
How habitable was early Mars?
It is a serene image: a river that flows into an expansive lake that fills a crater basin. Waves licking the shoreline; sediment accumulated in a delta. A clay-covered lake bed.
This is the kind of aquatic environment life could endure, and it was once a familiar sight on Mars.
“The evidence for the lakes and rivers is incontrovertible,” said Ken Farley, a Perseverance project scientist and geochemist at the California Institute of Technology.
Although Mars was once a wet planet, there is substantial debate about the origins, range, and lifespan of its long-lost bodies of water.
For example, early Mars could have been heated by gas burps from active volcanoes, which thickened its atmosphere and caused melting of Martian permafrost. Cataclysmic asteroid impacts could also have unleashed 900-foot mega-tsunamis that flooded the planet’s terrain. There is even disputed evidence that an ocean once covered its northern lowlands.
“Were they strange, short and transitory events, or was there an ocean?” Dr. Farley said. “I would say there is no consensus. There are a lot of ideas out there, and we really need a lot more information to solve it. ”
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 forming improve the longer stable bodies of water persist.
During Curiosity’s eight-year journey through Gale Crater, an ancient lake bed, the rover discovered sediments suggesting that water was present for at least a few million years. Curiosity also detected organic compounds, 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 “inhabited”. The surface of Mars is exposed to damaging solar and cosmic radiation, which could have reduced the chances of complex, multicellular life being formed.
“If life existed on Mars, there would be a strong evolutionary force to be resistant to radiation,” said Dr. Sumner.
There are microbial extremophiles on Earth that can withstand intense radiation, often curing their own DNA on the fly. Therefore, it is not unreasonable to imagine that there could be Martian microbes that could tolerate a radiation attack. Furthermore, they may have been able to retreat underground if conditions became particularly hostile on the surface.
“The great lesson about life, from the DNA power revolution, is that life can go everywhere,” said Dr. Farley. “It’s amazing. It will fill every niche he can get into, and he will do it in a relatively short period of time.”
Why did Mars become less habitable?
Mars’ past oases are now mirages of the distant past, and modern Mars is a dry shell. Earth, by contrast, has been habitable for microbes for most of its lifespan and has positively exploited in the veins with biodiversity for eons. Why did these brother worlds experience such different results?
Like baby planets, Mars and Earth were wrapped in two protective blankets: a relatively thick atmosphere and a strong magnetic field. The Earth has clung to both comforts. Mars has none.
Mars mysteriously lost its magnetic mojo 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.
This is great news for Earthlings, as we need those processes to survive. However, the absolute mortality of Mars in the last billions of years could facilitate the reconstruction of its early history.
“Life has been so successful on Earth that it is difficult to trace its origin,” said Dr. Sumner. “On Earth, everything is covered with organic matter from modern life.”
“One of the really interesting and exciting things about Mars is that, because it doesn’t have plate tectonics, large parts of its surface have these super old rocks,” he continued. “It is a good place to try to understand what a primitive planet would be like.”
Could Mars harbor life now?
Robot explorers on Mars have revealed countless ideas about the red planet, but have never found clear signs of creatures currently residing there. Life, at least as we know it on Earth, simply doesn’t seem likely on the Martian surface.
“If there is life on Mars now, it needs at least a little liquid water,” said Dr. Sumner. “The surface of Mars is now very dry. Simply incredibly dry. If there is life on Mars now, it would be deep underground. “
There is some evidence that liquid water is locked in underground reservoirs, so there may be no sunless ecosystems lurking there. If these habitats exist, they are beyond the direct reach of our rovers and landers.
Recent detections of methane and other gases in what remains of Mars’s atmosphere are “a tantalizing potential signature,” Dr. Farley said, reinforcing speculation about underground Martians. Many microbes on Earth produce methane, so it is possible that gas surges on Mars can be traced back to extraterrestrial life forms deep underground.
Curiosity, which is equipped with a methane-sensitive spectrometer, has compounded the mystery by recording strange gas spikes on the Martian surface that remain unexplained.
Unfortunately, the satellites orbiting the red planet have been unable to provide support for these readings, and new NASA and Chinese explorers on the red planet may not be able to solve the puzzle.
Methane can also be created by a wide range of natural processes that have nothing to do with life. Some experts, such as Dr. Sumner, say that the presence of the gas on Mars “is not a surprise” because it has all the geological processes it needs to produce the lifeless gas.
The discovery of life on Mars, whether in the form of ancient fossils or underground deposits, would be one of the most important advances in human history. At last, we would have another example of a living planet, even if it only flourished in the past, implying that life can at least strike twice in the universe.
But even if we never find Martians, “Mars is a place we can go to answer some of the questions about life on Earth,” said Dr. Sumner. The red planet is still a mysterious time capsule from the time when life first emerged in our own world, and the direction it might have gone if all the factors that made our world possible had not turned out the way correct.
