To understand the life that might survive deep below Mars' surface, we can look to some of the deepest, and oldest, forms of living organism on our own planet.
Mars isn't just the red planet: it's also a wet planet. On 12 August, US researchers reported evidence of a vast reservoir of liquid water, deep in the rocky crust of the planet.
The data came from Nasa's Mars Insight Lander, which recorded more than 1,300 Marsquakes over four years. Researchers led by Vashan Wright, a geophysicist at the University of California San Diego's Scripps Institution of Oceanography, studied the seismic waves that reached the lander and concluded that they had passed through layers of wet rock.
While the surface of Mars is a barren desert, Wright's data suggests considerable volumes of water are locked up in rocks between 11.5 and 20km (7.1-12.4 miles) down.
Martian life, if it exists, could well be subterranean
"If they are correct," says Karen Lloyd, a subsurface microbiologist at the University of Southern California in Los Angeles, "I think this is a game-changer."
Underground water on Mars opens up the possibility of underground life on Mars. The last few decades have revealed that there is an enormous biosphere hidden deep within the Earth.
It now seems the same could be true on Mars. Martian life, if it exists, could well be subterranean.
The deep biosphere
For over 30 years, biologists have accumulated evidence that life persists deep underground on Earth. Researchers have drilled deep into the sea floor and the continents, finding life in buried sediments and even amongst layers and crystals of solid rock.
Most of these dwellers in the dark are single-celled microorganisms, specifically bacteria and archaea. These two huge groups are the oldest known forms of life on Earth: they have existed for over three billion years, long before animals and plants.
Within the last 20 years, it has also emerged that the deep biosphere is highly diverse.
"There's actually quite a lot of different types of organisms living deep underground," says Cara Magnabosco, a geobiologist at ETH Zurich in Switzerland.
Bacteria are divided into large groups called phyla: only a few dozen of these groups have been formally identified, out of an estimated 1,300. "Pretty much all of these phyla can be found underground," says Magnabosco.
Not that they are evenly distributed. A 2023 meta-analysis found that most ecosystems under land were dominated by two phyla, Pseudomonadota and Firmicutes. Other types of bacteria were much rarer, but they included phyla that had never been seen before.
Because it is pitch-black, these microbes cannot get energy directly from sunlight, as photosynthetic organisms at the surface do.
"The really important thing to note is that they don't depend, by and large, on the Sun," says Lloyd.
They also aren't receiving any other inputs such as nutrients from above. Many of these deep ecosystems are "completely disconnected from the surface", says Magnabosco.
Instead, these ecosystems are based on chemosynthesis. The microbes get their energy by performing chemical reactions, taking in chemicals from the surrounding rocks and water.
For instance, they may use gases such as methane or hydrogen sulphide as their source material. "The subsurface has many, many different chemical reactions," says Lloyd. "A lot of us spend a lot of time finding new reactions that support life."
Chemosynthetic microbes may seem alien because they are rare in the sunny surface areas where we spend our time and are confined to the depths of the sea and the solid underground. But they are some of the oldest kinds of living organisms on Earth.
Some hypotheses about the origin of life suppose that the first life on Earth was chemosynthetic.
Like the slow microbes living deep under Earth's oceans, Martian microbes may be clinging to life despite scant nutrients
While single-celled microbes dominate the subsurface, there are a few rare animals. A 2011 study identified nematode worms in fracture water from 0.9-3.6km (0.6-2.2 miles) down in South African mines.
The water seemed to have been there for at least 3,000 years, suggesting the nematode population could be millennia old. A 2015 follow-up found flatworms, segmented worms, rotifers and arthropods in fissure water 1.4km (0.9 miles) down: the water there was up to 12,300 years old. The animals were feeding off a thin film of microbes on the rock surface.
To us, the deep underground seems like an extremely challenging place to live. Compared to the surface, the microbial populations are sparse – but there is also an awful lot of rock to live in.
In 2018, Magnabosco and her colleagues estimated the scale of the biomass living under continents, by combining data on numbers and diversity of cells from drilling sites around the world.
They estimated that there are 2 to 6 × 10^29 cells living underneath Earth's continents. In comparison, there are only about 10^24 stars in the observable universe.
"We have a very numerically large number of cells beneath our feet," says Magnabosco. In fact, she says, about 70% of all the bacteria and archaea on Earth are underground.
Quite how deep the biosphere extends is not yet clear. Life presumably has an upper temperature limit but we don't know exactly where it lies.
Nothing can live on the surface of molten lava, but some microbes can endure surprising heat: an archaean called Methanopyrus kandleri can survive and reproduce at 122C (252F).
Go far enough underground and pressure also becomes an issue. The type of rock is also significant, because it affects the chemical reactions that can occur and thus the types of chemosynthetic microbes that can live there.
"But I can't give you a number [on how far down life exists] because we haven't hit it yet, because we just haven't drilled that deep," says Lloyd. The limit may be surprisingly deep: a 2017 study of samples from a mud volcano suggested life could exist 10km (6.2 miles) below the seabed.
Some of this life is lived extremely slowly. "There are definitely large parts of the subsurface, primarily underneath our oceans, where nothing really happens for millions of years," says Lloyd. With no new nutrients coming in from above, and no way to escape, the microbes in these places have very little food.
"That means they just don't have the energy necessary to make new cells," she says. Instead they slow their metabolisms and are almost in stasis. "It's actually quite reasonable that a single cell could live for thousands of years or longer."
It's this kind of life – reliant on chemical reactions between rocks and water, and possibly with an extremely slow metabolic rate – that might plausibly be found in the water-rich rocks deep beneath the surface of Mars.
Martian microbes
So far there is no solid or direct evidence of life on Mars, despite decades of uncrewed missions to the red planet. The surface is dry and cold, and no living organism has ever wandered into shot of a Mars rover camera.
However, features like canyons strongly suggest that Mars did have running water on its surface billions of years ago. Some of that water was probably lost to space, but Wright's team concluded that much of it is underground.
"We know that water is a prerequisite for life as we know it," says Lloyd. So perhaps the Martian surface used to be habitable, and now only the subsurface is. "I've always preferred the notion that life would be buried somehow," she says.
Like the slow microbes living deep under Earth's oceans, Martian microbes may be clinging to life despite scant nutrients. "The same sort of processes that happen in our subsurface can happen on Mars," says Magnabosco.
If life was able to develop on Mars, it has a very good chance of still surviving and being on Mars today – Cara Magnabosco
The most suggestive evidence of life to date is the plumes of methane in the Martian air, which vary with the seasons. On Earth, methane is often made by microorganisms – so the gas could be a waste product from underground life. However, Lloyd urges caution. "There are many non-life reasons why there could be plumes of methane," she says.
Furthermore, there are many other obstacles to life in the Martian subsurface. "Life doesn't just need water," says Lloyd. "It needs energy and a place to be, so it needs a habitat." We don't yet know if the pores in the Martian rock are large enough for microbes.
Likewise, the chemical makeup of the deep rocks is crucial, as they would be the source of chemical energy.
For Magnabosco, "the biggest uncertainty" about life on Mars "is whether or not it emerged". Because we don't know how the first living things formed from inanimate material, we don't know if conditions on Mars were ever suitable for the emergence of life.
"If life was able to develop on Mars," she says, "it has a very good chance of still surviving and being on Mars today."
If this Martian deep biosphere exists, how could we find it? The obvious idea is to drill into Mars, but we would need to drill down 10km (6.2 miles) or more – a heavy lift even on Earth. Doing that on a planet that lacks breathable air or running water? "It's much, much more difficult," says Magnabosco.
However, it should be possible to build up the supporting evidence. Nasa's planned Mars Sample Return mission would bring Martian rocks back to Earth: such samples might contain traces of life.
"Chasing the methane would be really helpful," says Lloyd. At present, we don't know where the gas is coming from. "If we find that the water pockets are associated with the methane plumes," that would be suggestive of life, she says.
Finally, if Mars really does have water moving around, we could take advantage of that. On Earth, features like hot springs bring water from deep underground to the surface. "Mars has mud volcanoes," says Lloyd.
"There are places on Mars that you can go where you actually have deep subsurface samples that have been exhumed and brought up to the surface."
It may well be decades before we get a definitive answer. That answer might be frustrating: Mars is much less tectonically and hydrologically active than Earth, which suggests that life is either sparse or non-existent.
"We could be looking for life that has not been alive for a long time," says Lloyd. In that case, all we might find is fossil
evidence, rather than living organisms. "Either way, it's life on Mars," she says.
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