Dried-up riverbeds, water-weathered minerals, underground reservoirs of ice: the evidence for Mars’ watery past keeps building. In fact, the eight missions that have successfully landed on the red planet have been mostly focused on finding hints of Mars’ watery past because it’s considered a strong indication that the planet may have hosted life before it became the cold, barren planet we know today.

But water is just one of several factors that would support the evolution of life. Other necessary physical and chemical characteristics, like temperature and radiation levels, might be more difficult to find in the rock record because the nature of these conditions means they are not as well-preserved, says Pamela Conrad, an astrobiologist with NASA’s Planetary Environments Laboratory in Greenbelt, Maryland.

"People have often focused on the water, and there’s a lot more going on with Mars," says Rory Barnes, an astrobiologist at the University of Washington in Seattle who was not involved in this study. "Conrad’s paper provides a sort of sanity check on what we’re trying to do on Mars and what we still need to find out. It’s a complicated web of issues that affect planetary habitability."

One of the physical factors Conrad points to is temperature variation; today, even near the equator where temperatures are more stable, a Martian day can be about 70 degrees Fahrenheit but drop to negative 100 degrees at night. Even if other factors for bacterial life like the right nutrients and water were present, the extreme temperatures would make survival challenging for early life. Although Barnes points out that these daily temperature changes would be more drastic if a Martian day lasted longer, it’s probably still too cold for life to thrive.

Radiation is another important factor. Mars’ thin atmosphere doesn’t do a good job of protecting its surface from the sun’s damaging rays. "If you have too much radiation, it overwhelms the repair mechanisms that we have in our DNA," Conrad said. With high radiation, life would be unlikely to survive.

Wind can also affect burgeoning life forms because of how it deposits iron. Simple and complex organisms need iron for many biological processes, such as making DNA and transporting oxygen and carbon dioxide within the body. Most primitive organisms aren’t very mobile, so they would need to grow in a place where the wind consistently deposits iron for them to use.

These necessary characteristics are affected by Mars’ thin atmosphere. Around 4 billion years ago, a barrage of broken-up asteroids and comets slammed into the early solar system in an event called the Late Heavy Bombardment. Because Mars is so small (it has only 11 percent of Earth’s mass), its gravity isn’t as strong; scientists think that so many objects hit the planet that most of its atmosphere was blasted away. "On Mars, it would be really important to understand if the planet maintained its atmosphere when it was warm and wet," Conrad says, because those conditions together may have been the best chance for life to occur.

Although the rovers have answered a lot of scientists’ questions about Mars, they can’t synthesize information the way humans can. "You can get a broader context by having humans present — one can respond in real time and one can look underneath and behind rocks that appear to have morphological and chemical clues about the evolution of the Martian surface," Conrad said. Both Conrad and Barnes have high hopes for what humans will be able to discover in NASA’s rumored crewed mission to Mars, possibly in 2030.

There’s still a remote chance that we’ll find life preserved in the Martian rock record, Conrad says. The bacteria would have to be abundant enough for us to find it, preserved well at the time, not destroyed by radiation in the aeons since they were fossilized, and we would need the right tools to know what we were looking at. But even if they don’t find life, Conrad believes that the missions to Mars have been worthwhile because of how much scientists have learned about the conditions that may or may not support life.

"If we can find life that originated on another planet, that will do an enormous amount for us to understand the origins of life on Earth," Barnes says. And by understanding how life started both here and on Mars, Conrad says, we can get a better sense for where to look elsewhere.