Experiments Indicate Regolith is Looser in the Lunar Polar Regions than at the Lunar Landing Sites

TL;DR

Experiments and remote sensing data suggest lunar polar regolith is less compacted and looser than at landing sites, primarily due to reduced thermal cycling, impacting rover mobility.

Contribution

This study confirms thermal cycling as a key mechanism affecting regolith compaction and explains regional differences observed in lunar soil properties.

Findings

Thermal cycling effectively compacts lunar regolith.

Polar regions exhibit less soil compaction due to reduced thermal cycling.

Looser regolith in polar regions may challenge rover mobility.

Abstract

Since the Apollo program or earlier it has been widely believed that the lunar regolith was compacted through vibrations including nearby impact events, thermal stress release in the regolith, deep moon quakes, and shallow moon quakes. Experiments have shown that vibrations both compact and re-loosen regolith as a function of depth in the lunar soil column and amplitude of the vibrational acceleration. Experiments have also identified another process that is extremely effective at compacting regolith: the expansion and contraction of individual regolith grains due to thermal cycling in the upper part of the regolith where the diurnal thermal wave exists. Remote sensing data sets from the Moon suggest that the soil is less compacted in regions where there is less thermal cycling, including infrared emissions measured by the Diviner radiometer on the Lunar Reconnaissance Orbiter (LRO). Here, we performed additional experiments in thermal cycling simulated lunar regolith and confirm that it is an effective compaction mechanism and may explain the remote sensing data. This creates a consistent picture that the soil really is looser in the upper layers in polar regions, which may be a challenge for rovers that must drive in the looser soil.