# How thick does lunar regolith ever get? Has the thickness been measured and mapped by satellite?

Discussion in comments below the question Couldn't we just clear away the Moon Dust? have lead me to wonder if

1. thickness of the lunar regolith is fairly well mapped by some combination of high temporal resolution radar and thermal inertia measurements by satellite.
2. If so, how thick does it get?

My feeling is that it never gets more than say 20 cm and is usually a lot thinner, but I have no idea.

• My first google came up with: "The thickness of the regolith varies from about 5 m on mare surfaces to about 10 m on highland surfaces." - Source: curator.jsc.nasa.gov/lunar/letss/regolith.pdf – Christoph Apr 5 '19 at 8:58
• @Christoph It would be excellent if you considered posting an answer drawing upon that presentation! There is a lot of information about the various methods used to deduce density versus depth, as well as the mechanisms that can continue to produce small particles beyond the meteor impacts we've all heard about. Thanks! – uhoh Apr 5 '19 at 9:28
• @Christoph I recognize some of those plots as plots in this answer. – uhoh Apr 5 '19 at 9:30

First, we have to define 'regolith'. NASA uses the term for all unconsolidated debris, including large boulders:

The lunar surface is covered by a layer of unconsolidated debris called the lunar regolith (fig. 53). The thickness of the regolith varies from about 5 m on mare surfaces to about 10 m on highland surfaces.

Lumps smaller than 1 cm are called "lunar soil".

The stuff you're probably after is the fraction fine enough to be called "dust".

Lunar dust generally connotes even finer materials than lunar soil. There is no official definition of what size fraction constitutes "dust"; some place the cutoff at less than 50 μm in diameter, while others at less than 10 μm.

All of these categories are mixed together, with smaller particles filling the space between larger particles.

At the surface, you tend to get a layer of dust.

Since the Moon lacks any sort of an atmosphere, the upper few millimeters of the regolith is exposed to the bombardment of micrometeorites and to solar wind irradiation. The extensive bombardment by micrometeorites, which continues today, breaks up soil particles and melts portions of the soil.

Lunar dust builds up at a rate of 1 mm/1000 y. Impacts can break up rock, but can also consolidate particles by melting:

The impact of micrometeoroids, sometimes travelling faster than 96,000 km/h (60,000 mph), generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates,[12] reminiscent of tektites found on Earth.

We have some data on regolith depth, but it's incomplete. The best data is the samples taken by the Apollo missions, but those are very localized. Whole-surface data relies mostly on photos and various estimation techniques.

This is a Ground Penetrating Radar map made by Chang'e 3's rover Yutu.

Radar measurements from Earth can give an indication of surface grain size:

• This is interesting! I had assumed the Moon's surface was binary; solid rock with ~10cm of power on top. Perhaps an occasional random boulder here and there. I didn't realize there was thought to be such a gradual transition over such an extended range of depths! I'm also surprised that existing surface-penetrating radar data is only from Earth, and not from any spacecraft orbiting the Moon! – uhoh Apr 5 '19 at 9:21
• "This is a Ground Penetrating Radar map from Yutu" What's Yutu? – Steve Linton Apr 5 '19 at 10:33
• The rover part of the Chinese Chang'e 3 lunar lander mission – Hobbes Apr 5 '19 at 10:36
• Only Yutu, or Yutu-2 too? – uhoh Apr 5 '19 at 10:40

Lunar regolith is approximately 1.35 $$g/cm^3$$ for the top 30 cm, and it is approximately 1.85 $$g/cm^3$$ at a depth of 60 cm. It is as thin as 5 cm at some places. It can cause lung cancer if ingested, and is similar to asbestos.

• Hi @Matthew and Welcome to Space! When writing answers in Stack Exchange it's important to include links or citations to the sources of information that you quote. It' helps to verify the accuracy and makes your answer more informative to future readers. – uhoh Apr 5 '19 at 9:15