What you need to know

• Evidence of carbon deposits in the crust of Mars has been found, suggesting the presence of a carbon cycle.
• Mars once had a far warmer climate with liquid water and a thick carbon dioxide atmosphere.
• A broken cycle may have contributed to Mars becoming uninhabitable.

Scientists studying soil samples from NASA's Curiosity rover have discovered that a carbon cycle similar to the one that sustains life on Earth once played out on the red planet.

While it's unclear whether Mars ever supported life, its current harsh environment may be due to an "imbalanced" carbon cycle.

"Mars seems to have been habitable for its first billion years and that waned very quickly," Ben Tutolo, a space researcher at the University of Calgary, Canada, told DW.

Mars once had a thick atmosphere full of carbon dioxide capable of trapping heat in a Martian "greenhouse effect." This enabled liquid water to exist on a warm surface.

But today, Mars is a dry and cold planet with water concentrated in the form of its frozen polar ice caps. Mars scientists have long asked themselves a question: Where did the carbon go?

The Tutolo-led analysis of surface samples taken by Curiosity, published Thursday in the journal Science, goes a step toward one explanation. It has identified iron carbonate — known as siderite — in far higher quantities than previously identified by orbital sensors.

That suggests a history of chemical reactions between water, carbon dioxide and sediments to form these carbon-based deposits, similar to the natural carbon cycling that takes place on Earth.

Carbon cycles: the basics

On Earth, carbon exists in the atmosphere as carbon dioxide, in living organisms as an essential molecule in DNA and the proteins it produces to create genetically distinctive life, as well as in "sinks" like oceans, rocks and soils.

Over timescales ranging from thousands to millions of years, carbon cycles through the atmosphere, sediments and rocks, and living organisms.

Plate tectonics — the shifts and collisions between massive, moving geological structures beneath the Earth's surface — causes earthquakes and volcanic eruptions that inject carbon back into the atmosphere as part of this natural process.

The amount of carbon on Earth doesn't change throughout this cycle, but its place within each reserve does — as shown by the burning of fossil fuels, like oil, coal and gas. These fuels are derived from decomposed plants and animals, and when burned release huge amounts of carbon into the atmosphere and rapidly warm the Earth.

A curious find in an ancient lake

In its journey across Mars, the Curiosity rover drilled into four areas of the Gale Crater, which was once an ancient lake.

Tutolo and his collaborators found up to a 10th of the material recovered contained siderite, or iron carbonate.

It's a surprise finding — previous analysis of Mars' surface by orbital satellites had found only trace amounts of siderite, and not enough to explain why the planet has such a thin atmosphere today.

"We were completely surprised to find carbonates in this deposit," said Tutolo.

The siderite appears to be masked by abundant deposits of magnesium sulfate occurring across the planet, which would explain why it was previously undetected.

It suggests carbon became stored in the ground when ancient oceans in places like the Gale Crater would have reacted with atmospheric CO2 and underlying sediments to produce siderite.

Carbon makes life on Earth, but life on Mars remains elusive

Mars is a very different planet from Earth, and its carbon cycle is also unique.

While plate tectonics are a crucial part of Earth's system, Mars does not have this geological foundation.

"There's no plate tectonics on Mars, there's no good mechanism for bringing that CO2 back into the atmosphere," said Tutolo.

He described Mars as having an "imbalanced" carbon cycle as a result — while atmospheric carbon can be sequestered in the ground, the absence of plate tectonics makes it difficult to trigger eruptions that would send it back. 

It's likely a deciding factor in whether Mars can support life. Even as different planets may harbor certain conditions required to foster life as we know it, missing pieces could prevent it from evolving.

"Mars has a very different type of carbon cycle and that brings into light the potential that plate tectonics is required to maintain habitability," said Tutolo.

"If planets outside of our own never develop plate tectonics, as our has done, it's possible that they may lose their habitability after initially starting to be warm and wet."

Edited by: Carla Bleiker

Sources:

Carbonates identified by the Curiosity rover indicate a carbon cycle operated on ancient Mars