How Likely Is Life on Mars?

Mars is by far the most Earth-like planet in the solar system…but that’s not saying much.

We know that Mars once hosted oceans, rivers, lakes, streams. It had a thick, carbon-dioxide rich atmosphere. Billions of years ago, Mars and Earth looked like siblings. Yes, the red planet was likely cooler than the Earth, which meant it probably sported much larger polar ice caps and was much drier, but all conditions were DEFINITELY in the range for supporting life. And at that same time, billions of years ago, we know that microorganisms roamed – or, floated, or flagellated, or whatever it is that microorganisms do – the Earth.

So was Mars alive in the distant past? Have any remnants survived to the present day? Or is there at least a fossil record remaining that we could find someday?

Since Mars is the closest thing any world of the solar system comes to the Earth, we have the best chances of finding the closest analogs to Earth-like life here. And so in our assessment of Mars as a past or present abode for life, we need to evaluate conditions based on four criteria:

One, we need water. Liquid water. The universal solvent. It can’t be frozen or evaporated. It has to be available in abundance for life to use. And it has to have the right solinity and pH – too salty, not salty enough, too acidic, too basic, and life can’t function. Yes, many extremophiles on Earth can live in a wild range of conditions, but there’s still a limit to what basic biochemistry can handle.

Two, life needs the right chemical environment. It needs lots of the good stuff, the elemental building blocks, like carbon, hydrogen, nitrogen, and oxygen. And it needs those elements in high enough concentrations and in the right places: as in, on or near the surface near bodies of water. And simultaneously, life needs LOW concentrations of the bad stuff, the heavy metals like zinc and nickel. Life needs some of those to function, for sure, but too high concentrations lead to a toxic environment.

Next, life needs energy. Just straight-up energy. A power source. This can be the Sun, it can also be geochemical, like hydrothermal vents. It also needs oxidants and reductants – it needs chemical pathways so that it can store, transform, and use energy.

Lastly, the physics has to be right. It can’t be too hot or too cold. The pressure can’t be too high or too low – life isn’t going to have a great time in the vacuum of space or the core of a planet. There can’t be too many cosmic rays, which are really good at destroying vulnerable molecules. It needs a substrate, something to anchor on to. It needs transportation so that it can encounter fresh resources.

Life is…needy. And while we know for sure that past Mars certainly ticked off SOME of these boxes, we don’t know if it ticked off ALL these boxes. And even if all the conditions were met in the past, we don’t know if life is guaranteed to happen, or has a chance of not making it.

As for today, the environments on Mars that could potentially support life are few and far between, and it’s all about following the water.

Mars still has plenty of water, but it’s almost all frozen in either the polar ice caps or under the surface. But over the years there has been a fair amount of evidence that there may be pockets of liquid. For example, in 2018 scientists discovered what they believed to be a liquid water lake buried under the southern polar ice cap, kept liquid by the pressures of the cap above it. This discovery was based on radar mapping taken with the Mars Express orbiter. However, further research has cast doubt on whether liquid water is the best interpretation of the radar signals.

And then there’s everyone’s favorite, the recurring slope lineae, which are tapered dark streaks that appear on some crater walls in the summer months, which were discovered in 2015 using the Mars Reconnaissance Orbiter. One possibility is that these are caused by underground water thawing and comping to the surface, although there are other possibilities like moisture being pulled out of the atmosphere, or just sulfates and chlorine salts combining to create grains that roll downhill.

The last place where we might find water on Mars – and hence where life could find a home – is deep underground. In 2024 scientists working with NASA’s Insight Lander concluded that strange seismometer readers were consistent with liquid water in the crust at depths of 10 to 20 kilometers. This wouldn’t be a vast underground ocean (that’s coming up), but hydrated minerals: little bits of water tucked into crevices and cracks, kept warm by the residual heat remaining down there.

We know that life on Earth ekes out an existence under similar conditions, so perhaps ancient Martian life migrated down there as the planet cooled and dried, much like Venusian life may have made its way to the clouds.

NASA’s Perseverance rover is currently crawling around Jezero Crater at the edge of an ancient lake, looking for signs of past life, while the European Space Agency’s Rosalind Franklin rover is hoping to reach the surface in 2028.