  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.
