Into the Exosphere!

Last time I talked about the different ways rocks breakdown in the space environment. This time we will look at what happens to some of these products of the erosion and weathering of the materials go.

Whilst many of the smaller bodies in the solar system don’t have an atmosphere (which would stop most space weathering), there is still atoms/ions/dust particles floating around above the surface. Whilst this material is too spread out to interact with each other in the way that a gas does in an atmosphere, these materials are still trapped, bound by the gravity of the body – this is called the exosphere.

Unlike other bodies, these exospheres are predominantly comprised of material generated from space-weathering; sputtering releasing atoms, and melting which releases volatile molecules. Some are released from surfaces by heating at sunrise. As the solar wind of charged particles moves over a body, its surface can become charged, dust particles gain a similar charge and so are repelled floating above the surface. The amount of dust is linked to the number of micrometeorite impacts, the intensity of the solar wind, and any magnetic fields. This dust can refract light; giving the appearance of a diffuse sunrise which would otherwise not be possible on an airless world.

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Lunar Horizon Glow on the Moon, generated by dust in its exosphere, taken by Nasa’s Clementine mission (NASA)

For rocker bodies, such as Mercury and the Moon atoms of  Ca, Mg, & K have been detected. For some of the other bodies in the solar system, beyond the frost line – water ice covers many of the bodies like Europa and Enceladus, when the surfaces are broken down by radiation, the hydrogen escapes, leaving oxygen and hydroxide around them.

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The Calcium and Magnesium tails of Mercury as taken by MESSANGER (NASA/Johns Hopkins/ Carnegie institution

These materials don’t stay in the exosphere forever, s0me can be reabsorbed by the surface of the planet, often on the night side of a planet, and the dust and atoms can settle down. Atoms can become charged by sunlight (photo-ionisation), or interaction with charged particles in the solar wind. These ions are caught by solar winds or by magnetic fields and expelled out into interplanetary space or fired at high speed into the ground close to the poles. The force of sunlight hitting atoms or the rare collisions between atoms can slowly add momentum until some material eventually gain enough velocity and can escape the exosphere.

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