Research Gives New Insight Into How Much Atmosphere Mars 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 at Mars today (left). (Image: NASA’s Goddard Space Flight Center)

A key tracer used to estimate how much atmosphere Mars lost can change depending on the time of day and the surface temperature on the Red Planet, according to new observations by NASA-funded scientists. Previous measurements of this tracer — isotopes of oxygen — have disagreed significantly.

An accurate measurement of this tracer is important in order to estimate how much atmosphere Mars once had before it was lost, which reveals whether Mars could have been habitable and what the conditions might have been like.

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Mars is a cold, inhospitable desert today, but features like dry riverbeds and minerals that only form with liquid water indicate that long ago it had a thick atmosphere that retained enough heat for liquid water — a necessary ingredient for life — to flow on the surface.

It appears that Mars lost much of its atmosphere over billions of years, transforming its climate from one that might have supported life into the desiccated and frozen environment of today, according to results from NASA missions, such as MAVEN and Curiosity, and going back to the Viking missions of 1976.

However, many mysteries about the Red Planet’s ancient atmosphere remain. Timothy Livengood of the University of Maryland, College Park, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, lead author of a paper on this research published online in Icarus August 1, said:

One way to estimate how thick the original atmosphere of Mars was is to look at isotopes of oxygen. Isotopes are versions of an element with different mass due to the number of neutrons in the atomic nucleus. Lighter isotopes escape to space faster than heavier isotopes, so the atmosphere that remains on the planet gets gradually enriched in the heavier isotope.

In this case, Mars is enriched compared to Earth in the heavier isotope of oxygen, 18O, versus the lighter and much more common 16O. The measured relative amount of each isotope can be used to estimate how much more atmosphere there was on ancient Mars, in combination with an estimate for how much faster the lighter 16O escapes, assuming that the relative amount of each isotope on Earth and Mars was once similar.

The problem is that measurements of the amount of 18O compared to 16O on Mars, the 18O/16O ratio, have not been consistent. Different missions measured different ratios, which results in different understandings of the ancient Martian atmosphere.

The new result provides a possible way to resolve this discrepancy by showing that the ratio can change during the Martian day. Livengood said:

This range of isotope ratios is consistent with the other reported measurements. Livengood added:

The team thinks the change in ratios during the day is a routine occurrence due to ground temperature, in which the isotopically heavier molecules would stick to cold surface grains at night more than the lighter isotopes, then are freed (thermally desorb) as the surface warms up during the day.

Since the Martian atmosphere is mostly carbon dioxide (CO2), what the team actually observed were oxygen isotopes attached to carbon atoms in the CO2 molecule. They made their observations of the Martian atmosphere with the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, using the Heterodyne Instrument for Planetary Winds and Composition developed at NASA Goddard. Livengood said:

The new work will help researchers refine their estimates of the ancient Martian atmosphere. Because the measurements can now be understood to be consistent with the results of such processes in other planets’ atmospheres, it means they are on the right track for understanding how the Martian climate changed. Livengood explained:

Provided by: Bill Steigerwald and Nancy Jones, NASA’s Goddard Space Flight Center [Note: Materials may be edited for content and length.]

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