What The Air On Mars Was Like Four Billion Years Ago

Martian air samples analyzed by the Curiosity rover provide the most detailed picture yet of how the Red Planet's atmosphere has changed over time.

The findings, discussed in two papers published on Thursday, July 18, in the journal Science, suggest Mars' atmosphere today is very similar to what it was like around 4 billion years ago.

Mars' atmosphere is mostly a mix five gases: carbon dioxide, argon, nitrogen, oxygen, and carbon monoxide.

In this case, scientists are interested in the different possible versions of each gas, called isotopes. All isotopes of the same element have the same number of protons and electrons, but differ in the number of neutrons.

"If each gas is a finger on your hand, for example, the isotope ratios are going to tell us about the fine fingerprints," lead author Christopher Webster of NASA's Jet Propulsion Laboratory said in a podcast with Science.

Isotopes are important because they are the window into the whole time history of Mars.

"Isotopes of particular forms of gas have a way of remembering the temperature history of the events that they went through," explains Webster.

Previous atmospheric measurements from the Viking lander missions in the 1970s were too crude to provide a definitive map of how Mars' atmosphere evolved over time. Curiosity's readings are more accurate than Viking, and for the first enabled scientists to "nail down the true Mars isotope ratios," says Webster. What's more, the rover's measurements match those that come from meteorites that landed on Earth, which confirms the belief that these rocks are from Mars.

Measurements from Curiosity's tunable laser spectrometer (TLS) and its quadrupole mass spectrometer (QMS) reveal that Mars went through two stages of evolution.

In particular, the D/H isotopic ratio — which tells us where Earth's water came — shows a "significant early loss of water to space (before 3.9 billion years ago), followed by only modest loss to space during the past 4 billion years," according to the study.

This indicates that carbon dioxide and water reservoirs on Mars were formed around 4 billion years ago, after the major loss of atmospheric mass. Most atmospheric species probably did not survive this period, the authors write.

A comparison of ancient meteorites from Mars with the modern Martian atmosphere, showing similar isotopic ratios, suggests that Mars has not changed much over the last 4 billion years.