Regolith - Moon

Moon

Regolith covers almost the entire lunar surface, bedrock protruduing only on very steep-sided crater walls and the occasional lava channel. This regolith has formed over the last 4.6 billion years from the impact of large and small meteoroids, from the steady bombardment of micrometeoroids and from solar and galactic charged particles breaking down surface rocks.

The impact of micrometeoroids, sometimes travelling faster than 60,000 mph (96,000 km/h), generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates, reminiscent of tektites found on Earth.

The regolith is generally from 4 to 5 metres thick in mare areas and from 10 to 15 metres in the older highland regions. Below this true regolith is a region of blocky and fractured bedrock created by larger impacts, which is often referred to as the "megaregolith".

The term lunar soil is often used interchangeably with "lunar regolith" but typically refers to the finer fraction of regolith, that which is composed of grains one centimetre in diameter or less. Some have argued that the term "soil" is not correct in reference to the Moon because soil is defined as having organic content, whereas the Moon has none. However, standard usage among lunar scientists is to ignore that distinction. "Lunar dust" generally connotes even finer materials than lunar soil, the fraction which is less than 30 micrometers in diameter. The average chemical composition of regolith might be estimated from the relative concentration of elements in lunar soil.

The physical and optical properties of lunar regolith are altered through a process known as space weathering, which darkens the regolith over time, causing crater rays to fade and disappear.

During the early phases of the Apollo Moon landing program, Thomas Gold of Cornell University and part of President's Science Advisory Committee raised a concern that the thick dust layer at the top of the regolith would not support the weight of the lunar module and that the module might sink beneath the surface. However, Joseph Veverka (also of Cornell) pointed out that Gold had miscalculated the depth of the overlying dust, which was only a couple of centimeters thick. Indeed, the regolith was found to be quite firm by the robotic Surveyor spacecraft that preceded Apollo, and during the Apollo landings the astronauts often found it necessary to use a hammer to drive a core sampling tool into it.