To learn about life that might live deep under Mars, we can look at very old life forms deep in Earth.
Mars is red, but it’s also wet. On August 12, U.S. scientists found signs of a big pool of liquid water deep in Mars’ rocky ground.
NASA’s Mars robot felt over 1,300 Mars-shakes in four years. Scientists looked at these shakes. They think the shakes went through wet rocks.
Mars looks dry on top. But the team thinks there’s lots of water in rocks deep down. This water might be 7 to 12 miles under the ground.
Martian life, if it exists, could well be subterranean
Karen Lloyd, who studies tiny life deep in the ground, says this is very big news.
Water under Mars means life might be there too. We’ve found lots of life deep in Earth. The same could be true on Mars. If there’s life on Mars, it might be living under the ground.
The deep biosphere
For over 30 years, we’ve known life exists deep in Earth. Scientists dug deep into the ground and sea floor. They found tiny life forms in old dirt and rocks.
Most of these deep-living things are very small. They’re bacteria and archaea. These are the oldest life on Earth. They’ve been here for over three billion years, long before plants or animals.
In the last 20 years, we’ve learned that many types of life live deep down. Cara Magnabosco, who studies life in rocks, says there are lots of different tiny life forms underground.
Bacteria come in big groups called phyla. We know about a few dozen groups. But we think there are about 1,300 in total. Magnabosco says we can find almost all these groups underground.
A 2023 study found two main types of bacteria live under land. But they also found rare, new kinds.
These tiny life forms can’t use sunlight for food. They live in total dark. Lloyd says they don’t need the Sun at all.
They also don’t get food from above. Magnabosco says many of these deep places are cut off from the surface.
These tiny life forms make food from chemicals, not sunlight. They use stuff from rocks and water around them. Some use gases like methane. Lloyd says there are many ways they make food from chemicals.
We don’t see these life forms much because they live deep down. But they’re some of the oldest life on Earth. Some people think the first life on Earth made food this way.
Like the slow microbes living deep under Earth’s oceans, Martian microbes may be clinging to life despite scant nutrients
Some tiny animals live deep down too. In 2011, scientists found small worms in old water deep in mines. The worms might have lived there for thousands of years.
In 2015, they found more tiny animals deep down. These animals eat the tiny life forms on rocks.
Living deep underground is hard. Not many things live there. But there’s a lot of space in all that rock.
In 2018, scientists counted how many tiny life forms live under land. They think there are more of these tiny life forms than there are stars in the sky we can see.
Most of Earth’s tiniest life forms live underground. About 7 out of 10 are down there.
We don’t know how deep life can go. Some tiny life forms can live in very hot places. One kind can live at 122°C (252°F).
Deep down, the push of rock is a problem. The type of rock matters too. It changes what food tiny life can make.
We haven’t dug deep enough to find the bottom of where life can live. One study thinks life might be 6 miles under the sea floor.
Some parts of life move very slowly. “There are large areas deep below the Earth’s surface, especially under the oceans, where almost nothing happens for millions of years,” says Lloyd. Without fresh nutrients from above and no way to leave, the microbes in these areas have very little food. “This means they don’t have the energy to create new cells,” she says. Instead, they slow down their metabolism and enter a nearly inactive state. “It’s quite possible for a single cell to live for thousands of years or more.”
This type of life, depending on chemical reactions between rocks and water, and with a very slow metabolism, might exist in the water-rich rocks deep beneath the surface of Mars.
Martian microbes
There is no solid proof of life on Mars yet, even after many missions to the planet. The surface is dry and cold, and no living thing has been seen by Mars rovers.
But features like canyons suggest that Mars had flowing water billions of years ago. Some of that water might have escaped into space, but Wright’s team thinks much of it is still underground.
“We know that water is needed for life as we know it,” says Lloyd. So maybe the surface of Mars was once habitable, and now only the underground areas are.
Lloyd believes that if life exists on Mars, it might be buried. Like the slow-moving microbes deep under Earth’s oceans, Martian microbes might be surviving with very little food. “The same things happening under Earth’s surface could also 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 interesting sign of life on Mars so far is the methane gas in its air, which changes with the seasons. On Earth, microorganisms often produce methane, so this gas could be a waste product from underground life. But Lloyd warns us to be careful. “There are many non-life reasons for methane plumes,” she says.
There are also many challenges for life beneath Mars’ surface. “Life doesn’t just need water,” says Lloyd. “It also needs energy and a place to live, a habitat.” We don’t know if the pores in Martian rocks are big enough for microbes. The chemicals in the deep rocks are also important because they provide energy.
For Magnabosco, the biggest question about life on Mars is whether it ever started there. We don’t know how the first living things formed from non-living material, so we can’t say if Mars ever had the right conditions for life to begin. “If life did develop on Mars,” she says, “it has a good chance of still being there today.”
If there is life deep underground on Mars, how could we find it? The obvious idea is to drill into Mars, but we would need to drill down 10 kilometers (6.2 miles) or more. That’s a tough job even on Earth. Doing it on Mars, where there’s no breathable air or running water, is “much, much more difficult,” says Magnabosco.
But we can still gather evidence. NASA’s planned Mars Sample Return mission could bring Martian rocks back to Earth. These samples might hold traces of life.
“Following the methane could be really helpful,” says Lloyd. Right now, we don’t know where the gas comes from. “If we find that the water pockets are connected to the methane plumes,” it could suggest life, she says.
If Mars has water moving around, we could use that. On Earth, features like hot springs bring deep underground water to the surface. “Mars has mud volcanoes,” says Lloyd. “There are places on Mars where deep subsurface material has been brought up to the surface.”
It may take decades to get a clear answer. And that answer might be disappointing: Mars is much less active than Earth, so life might be very rare or not present at all. “We could be searching for life that hasn’t been alive for a long time,” says Lloyd. In that case, we might only find fossils, not living organisms. “But even so, it’s still life on Mars,” she says.
