Measurements by NASA's Phoenix Mars Lander have determined that liquid water has had a presence throughout Mars' history. 

In 2008, the Phoenix performed measurements of stable isotopes of oxygen and carbon in the carbon dioxide of the atmosphere on Mars. These measurements were taken by the Evolved Gas Analyzer, which is part of the Phoenix's Thermal and Evolved Gas Analyzer (TEGA). TEGA was built at the University of Arizona and has a mass spectrometer capable of better analyses of carbon dioxide than those on NASA's Viking landers (these were the only others with instruments that could compile results on Mars' isotopic composition). To perform its task, the TEGA instrument opened a "pin-point-sized" hole while a puff of Mars' atmosphere was sucked into its chamber with a vacuum.

"We use the TEGA instrument as a crime scene investigator," said William V. Boynton, a professor at the Lunar and Planetary Lab in the UA's department of planetary sciences and co-author of the paper. "Like a chemical fingerprint, isotopes tell us what process is responsible for making the material we are studying."

Figuring out the ratios of isotopes in Mars' carbon dioxide offers new information on the complete history of volcanic activity and water on Mars' surface. Based on the measurements, liquid water has existed on Mars' surface at freezing temperatures, which means that hydrothermal systems (much like hot springs at Yellowstone National Park here on Earth) have played a small part on Mars' surface throughout the planet's history, but has been present nonetheless.

"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."

Other results from the TEGA analysis concluded that both Mars' carbon dioxide has proportions of oxygen and carbon isotopes much like the carbon dioxide found in Earth's atmosphere. The analysis noted two key clues that shows that Mars is both "geologically active" and that water has been present on the planet throughout its history. 

First, Mars has recently replenished its atmospheric carbon dioxide because an older atmosphere would contain more of the carbon-13 isotope, and Mars has seen a significant loss in this particular isotope. The results suggest that the atmosphere was replenished with carbon dioxide from volcanoes, meaning Mars is more active than previously thought.

Second, the measurements were compared to Martian meteorites that fell to the Earth after being hurled into space, and the meteorites had carbonate materials that could only form in the presence of carbon dioxide and liquid water. One meteorite in particular crystallized about 170 million years ago, which is considered recent in Mars' geological time, and contains carbonates with "isotopic proportions that match the atmospheric measurements by Phoenix." This tells us that water has been present on Mars recently and in the past, and there's enough of it to manipulate the composition of the planet's atmosphere. And for water to exist under Mars' cold and dry conditions, it has remained near its freezing point.

"The findings do not reveal specific locations or dates of liquid water and volcanic vents," said Niles. "But geologically recent occurrences of those conditions provide the best explanations for the isotope proportions we found."

The study was published in the September 10 issue of Science.