Mars is the most explored planet in the solar system other than Earth. With all of our robotic visitors there, weve discovered that it is a world far too dry, cold and irradiated to support the scheming humanoids or tentacled invaders once imagined by science fiction.

But our trips to Mars have opened a window into the deep past of the red planet, when conditions were far more conducive to life.

This summer, NASA will launch its latest rover, Perseverance, on a seventh-month journey to Mars. Like its predecessor, Curiosity, Perseverance will touch down in 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 Earthlings understand what Mars was like as a young planet some four billion years ago, and whether life ever blossomed on its surface.

Its a serene image: A river flowing into an expansive lake that fills a crater basin. Waves lapping at the shoreline; sediment piling into a delta. A lake bed caked with clay.

This is the type of aquatic environment that might support life, and it was once a familiar sight on Mars.

The evidence for the lakes and rivers is incontrovertible, said Ken Farley, project scientist on Perseverance and a geochemist at the California Institute of Technology.

Although Mars was once a wet planet, there is substantial debate about the origins, extent and life span of its long-lost bodies of water.

For instance, early Mars might have been warmed by the gassy belches of active volcanoes, which thickened its atmosphere and caused Martian permafrost to melt. Cataclysmic asteroid impacts might have also unleashed 900-foot mega-tsunamis that flooded the planets terrain. Theres even disputed evidence that an ocean once covered its northern lowlands.

Was it weird, short, transient events, or was there an ocean? Dr. Farley said. I would say theres no consensus. Theres a lot of ideas out there, and we really need a lot more data to sort it out.

One major question concerns the longevity of Mars liquid water. Nobody knows how much time is required for life to emerge on a planet, including on Earth. But the odds of life forming get better the longer that stable bodies of water persist.

During Curiositys eight-year journey across Gale Crater, an ancient lake bed, the rover discovered sediments that suggest water was present for at least a few million years. Curiosity also detected organic compounds, key ingredients for life as we know it.

What weve 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 odds of complex, multicellular life ever forming.

If life did exist on Mars, there would be a strong evolutionary force toward being resistant to radiation, Dr. Sumner said.

There are microbial extremophiles on Earth that can endure intense radiation, often healing their own DNA on the fly. So its not far-fetched to imagine that there might be Martian microbes that could tolerate an onslaught of radiation. Plus, they may have been able to retreat underground if conditions became particularly hostile at the surface.

The big lesson about life, from the revolution of being able to use DNA, is life is able to go everywhere, Dr. Farley said. It is amazing. It will fill every niche it can get itself into, and it will do it in a relatively short period of time.

The bygone oases of Mars are now mirages of a distant past, and modern Mars is a dried-up husk. Earth, in contrast, has been habitable to microbes for most of its life span and has positively burst at the seams with biodiversity for eons. Why did these sibling worlds experience such different outcomes?

As baby planets, Mars and Earth were each swaddled in two protective blankets: a relatively thick atmosphere and a strong magnetic field. Earth has held on to both comforts. Mars has neither.

Mars mysteriously lost its magnetic mojo billions of years ago. With no magnetic sheath to protect it from solar wind, the Martian atmosphere was stripped away over time, though it still maintains a thin shell of its past skies.

These changes have left Mars relatively inert for billions of years, while Earth reinvents itself through tectonic activity, atmospheric shifts and the ingenuity of life.

This is great news for Earthlings, as we need those processes to survive. Yet the sheer deadness of Mars over the past few billion years could make it easier to reconstruct its early history.

Life has been so successful on Earth that its hard to trace back its origin, Dr. Sumner said. On Earth, everything is covered with organic matter from modern life.

One of the really cool and exciting things about Mars is that, because it doesnt have plate tectonics, large parts of its surface have these super-old rocks, she continued. Its a good place to go to try to understand what an early planet would be like.

Robot explorers on Mars have turned up countless insights about the red planet, but they have never found clear-cut signs of creatures currently residing there. Life, at least as we know it on Earth, simply does not seem probable on the Martian surface.

If theres any life on Mars now, it needs at least some liquid water, Dr. Sumner said. The surface of Mars now is very dry. Just incredibly dry. If theres life on Mars now, it would be in the deep subsurface.

Theres some evidence that liquid water is locked away in subterranean reservoirs, so perhaps there are 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 whats left of Mars atmosphere are a tantalizing potential signature, Dr. Farley said, bolstering speculation about subterranean Martians. Many microbes on Earth produce methane, so it is possible that whiffs of the gas on Mars could be traced to alien life-forms deep underground.

Curiosity, which is equipped with a methane-sensitive spectrometer, has compounded the mystery by recording weird spikes of the gas at the Martian surface that remain unexplained.

Unfortunately, the satellites orbiting the red planet have not been able to provide backup for these readings, and the new NASA and Chinese rovers 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, like 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 gas without life.

The discovery of life on Mars, either in the form of ancient fossils or subterranean reservoirs, would be one of the most momentous breakthroughs 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 the very least, life can 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, Dr. Sumner said. The red planet remains an eerie time capsule of the era when life first sprouted on our own world, and the direction it could have gone had all the factors that made our world possible not turned out just the right way.

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3 Great Mysteries About Life on Mars - The New York Times