Daily Changes In Oxygen Make It Difficult To Estimate How Much Atmosphere Mars Has Lost

This artist’s concept depicts the early Martian environment (right) – believed to contain liquid water and a thicker atmosphere – versus the cold, dry environment seen on Mars today (left). NASA’s Goddard Space Flight Center

Mars used to be very different from how it is today. Forget the red frigid desert and imagine flowing water and active volcanos. However, the planet's past climate is still unclear and we're also not sure about the density of its atmosphere. 

A clever way to track how much atmosphere the Red Planet used to have is to look at the level of oxygen. Mars' atmosphere is mostly carbon dioxide and researchers are interested in the oxygen in these molecules. Oxygen can be found mainly in two forms, or isotopes, oxygen-16 (which has eight protons and eight neutrons in its nucleus) and oxygen-18, which has two extra neutrons. This is a good tracer for atmospheric loss because oxygen-16 is lighter and more easily escapes into space.

But in science, nothing is very straightforward. Different missions studying the Red Planet have discovered that the ratio between the two isotopes of oxygen is not consistent. But the fault is not in the measurements. As reported in Icarus, it’s the actual atmosphere that is changing.

Measurements at noon local time showed that the ratio between the two forms of oxygen was about 9 percent depleted in heavier isotopes. Measurements taken at 1.30pm showed that the air was enriched by 8 percent more oxygen-18 than we'd see in normal Earth oxygen ratios. This is the first measurement to show the variation using only a single method and it also shows correlations with surface temperature.

“Our measurements suggest that the previous work all may have been done correctly but disagreed because this aspect of the atmosphere is more complex than we had realized,” lead author Timothy Livengood of the University of Maryland said in a statement. “Depending where on Mars the measurement was made, and what time of day on Mars, it is possible to get different values.”

While the complexity is a roadblock to a detailed estimate of the Martian past, it does tell us that there are no unexpected phenomena stealing away certain isotopes while leaving others behind. The models will have to be refined but they don’t have to be overhauled.

“It shows that the atmospheric loss was by processes that we more or less understand,” added Livengood. “Critical details remain to be worked out, but it means that we don’t need to invoke exotic processes that could have resulted in removing CO2 without changing the isotope ratios, or changing just some ratios in other elements.”

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