Scientists have known for years that Mars once had a thick atmosphere and could actually keep liquid water on its surface. Then it lost that atmosphere to space, and now only has an atmosphere that is 1% the density of Earth’s. One possibility for studying that lost atmosphere is by sampling the surface layer of Mars’ moon, Phobos, which has been bombarded by the escaping atmospheric ions. Those ions of oxygen, carbon, nitrogen, and argon could be preserved in the uppermost layers of rock.

And to make the sampling easier, Phobos is tidally locked to Mars, so the same side faces the planet all the time. So long as a sample is taken from the facing side, it should show evidence of that atmosphere, if it has been preserved as predicted. Even better, JAXA has a mission called the Martian Moons Exploration probe that is scheduled for a 2024 launch and includes collecting samples from Phobos as part of its mission plan.

As scientist Quentin Nénon said: With a sample from the near side, we could see an archive of the past atmosphere of Mars in the shallow layers of grain, while deeper in the grain we could see the primitive composition of Phobos.

The reason we think this idea will work is that it already has with the Moon. Samples taken from the Moon during the Apollo missions recorded atoms coming from the Sun and Earth, creating a historical record of part of the early solar system. With no atmosphere or erosion, the Moon’s surface provides a well-preserved archive for us to research, and the same should be true of Phobos.

More Information

NASA Goddard press release

“Implantation of Martian atmospheric ions within the regolith of Phobos,” Q. Nénon et al., 2021 February 1, Nature Geoscience