There's been liquid water on the Martian surface throughout the planet's history, measurements by NASA's Phoenix Mars Lander suggest.
The water's been near freezing, though, implying hydrothermal systems similar to Yellowstone's hot springs on Earth have always been rare.
The results come from a new analysis of 2008 measurements of stable isotopes of carbon and oxygen in the CO2 of the Martian atmosphere. It shows that the proportion of carbon and oxygen isotopes is similar to that in Earth's atmosphere, suggesting Mars is much more geologically active than previously thought.
In fact, the new results suggest that Mars has replenished its atmospheric carbon dioxide comparatively recently, implying that there's been recent geological activity such as volcanoes. The carbon dioxide has then reacted with liquid water on the planet's surface.
"Atmospheric carbon dioxide is like a chemical spy," said Paul Niles,
a space scientist at NASA's Johnson Space Center in Houston and lead author of the paper. "It infiltrates every part of the surface of Mars and can indicate the presence of water and its history."
According to Niles, the isotopic signature indicates that liquid water has been present on the Martian surface recently and in great enough quantity to affect the composition of the current atmosphere.
The results provide supporting evidence that the watery conditions associated with carbonate formation have continued even under Mars' current cold and dry conditions. "This shows that the carbonates formed under the influence of water and the atmosphere in the recent geologic past," Boynton said.
"The findings do not reveal specific locations or dates of liquid water and volcanic vents, but geologically recent occurrences of those conditions provide the best explanations for the isotope proportions we found."