Could MMX collect a sample of Mars’s ancient atmosphere?

When the Martian Moons eXploration (MMX) mission returns to Earth in 2029, the spacecraft will bring home a sample capsule containing material from the martian moon, Phobos. This material is expected to not only reveal the composition and history of the moons, but also contain grains ejected from Mars over the red planet’s history. New research now suggests that the sample may also include traces of Mars’s ancient atmosphere.

Mars today is enveloped by an atmosphere too thin to allow water to exist on the planet surface. But features of the Martian terrain suggest that rivers once flowed across the red world, pointing to the presence of a much thicker atmosphere around the early Mars. But what was this atmosphere like? Intriguing research published in Nature Geoscience by a US team led by Quentin Nénon at the University of California, Berkeley suggests the answer may be embedded in the regolith of Phobos.

Using data from the NASA Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, the team observed ions (atoms with an electric charge) close to the orbit of Phobos that originated from the Martian atmosphere. They proposed that ions from Mars’s atmosphere may have been continually swept up to Phobos over the planet’s history, embedding in the moon’s regolith to form a veritable history of atmospheric content.

Proof that a moon could catalogue the past atmosphere of its planet has been found before. The JAXA Kaguya (SELENE) lunar mission found evidence that atoms escaping from Earth’s atmosphere had implemented themselves in the surface of the Moon’s regolith. And as Phobos orbits Mars sixty times closer than the Moon orbits the Earth, the expected transmission rate of material is significantly greater.

Atmospheric ions such as oxygen, hydrogen, nitrogen, carbon dioxide and argon may all have become embedded in the Phobos regolith. Once there, the ions may migrate to the moon’s surface and escape back into space. However, the time scale for this process depends on the atom. Hydrogen atoms may quickly exit the regolith, but atoms of oxygen or noble gases such as nitrogen or argon, may remain trapped for millions to billions of years. This would make these ions evidence for the theory that Mars’s ancient atmosphere was able to create a habitable planet.

The Martian atmosphere is not the only source of ions hitting the moon. The solar wind contains similar elements that can also drive into the Phobos regolith. However, the high energy of the solar wind means that this contribution will likely be embedded more deeply in the regolith than ions from Mars. Moreover, the isotopic composition (that is, the number of neutrons found in the nucleus of the ions) is likely to differ between the two sources, and should allow their origin to be determined.

Like our own Moon, Phobos is tidally locked to Mars with one side perpetually facing the planet. This fixed orientation means that the Mars-facing side of the moon is expected to experience a bombardment of Martian ions higher by a factor of 15 – 100 compared to the farside. This will be an important consideration for the MMX mission, when the spacecraft begins scanning the moon surface for suitable landing locations.

The Martian atmosphere ions are expected to be embedded within the top layer (few hundred nanometers) of regolith on the moon’s surface. If MMX is able to land on the Mars-facing side of Phobos, these ions could be scooped up by the spacecraft’s P-Sampler, which will use a pneumatic system contributed by NASA to collect material from the upper layers of the moon. The C-Sampler will aim to collect material from a depth down to >2cm, digging deeper than the exogenous additions to retrieve material related to the moon’s own composition.

In addition to embedding Martian atmospheric history, the bombardment of ions from Mars may also change the evolution of Phobos’s surface. Space weathering alters the surface of airless bodies, such as moons and asteroids, and its sources must be understood to correctly interpret the moon’s evolution and properties such as its age. While particles from the solar wind are expected to buffer the moon’s surface fairly evenly, greater weathering may be expected on the Mars-facing side due to the barrage of ions coming from the planet. Such a change may be detectable by the MMX spacecraft, as the spacecraft MIRS and the OROCHI camera aim to map the surface of Phobos at high resolution.

Phobos’s own formation and evolution is a time capsule of information about the formation of our Solar System. But as we learn more about the history of the Martian system, it is clear that the tiny moon is a time capsule of secrets waiting for us to unpack.

Further reading:

Journal paper: Implantation of Martian atmospheric ions within the regolith of Phobos

DOI: 10.1038/s41561-020-00682-0

Authors: Q. Nénon*, A. R. Poppe, A. Rahmati & J. P. McFadden
(*corresponding)

Affiliations:
Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA, USA.