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The Wikipedia article on "Atmosphere of moon" says:

One source of the lunar atmosphere is outgassing: the release of gases such as radon and helium resulting from radioactive decay within the crust and mantle. Another important source is the bombardment of the lunar surface by micrometeorites, the solar wind, and sunlight, in a process known as sputtering.

[...]

Gases can:

  • be re-implanted into the regolith as a result of the Moon's gravity;
  • escape the Moon entirely if the particle is moving at or above the lunar escape velocity of 2.38 km/s (1.48 mi/s)
  • be lost to space either by solar radiation pressure or, if the gases are ionized, by being swept away in the solar wind's magnetic field.

This seems to imply that the solar wind hits the surface of the moon. My question is, given that atmosphere of the moon is so thin, with the molecules normally missing each other completely, does the solar wind blow through it, i.e. do the particles of the solar wind usually hit the moon before/without hitting any particle of the exophere.

Nilay Ghosh
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Matthew Christopher Bartsh
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1 Answers1

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When the moon is exposed to the solar wind the charged particles of the solar wind, called a plasma, mostly pass through the exosphere of the moon. The number density of the solar wind is typically ~5-10 cm-3 while the lunar exosphere neutral number density can be upwards of ~105 cm-3. Even though the lunar exosphere is ~4 orders of magnitude denser than the solar wind, it's still more tenuous than nearly all the lab vacuums humans can create.

Does the solar wind blow through the moon's atmosphere/exosphere?

As noted in this answer, the mean free path of particle-particle collisions is on the order of ~105 km while the scale height of the lunar exosphere is much smaller.

So yes, the solar wind mostly propagates through the lunar exosphere without much interaction.

This seems to imply that the solar wind hits the surface of the moon.

Yes, the solar wind continuously bombards the lunar surface in regions that don't have remnant, crustal magnetic field anomalies. The result can be sputtering – the ejection of particles from a solid due to the impact of energetic particles. This can create what are called pick-up ions.

...do the particles of the solar wind usually hit the moon before/without hitting any particle of the exophere.

Yes, they usually do.

honeste_vivere
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