  Boulders on the surfaces of planets, satellites and small bodies, as well as
their geological associations, provide important information about surface
processes. We analyzed all available images of the surface of Mercury that have
sufficient resolution and quality to detect boulders, and we mapped all the
boulders observed. The lower size limit of detectable boulders was ~5 m. All
boulders found on Mercury are associated with fresh impact craters hundreds of
meters in diameter or larger. We compared boulder population on Mercury with
population of boulders of the same size on the Moon, and found that boulders on
Mercury are ~30 times less abundant than in the lunar highlands. This exact
quantitative estimate is inherently inaccurate due to the limitation in the
source data; however, the significant relative rarity of boulders on Mercury
can be firmly and reliably established. We discuss possible causes of the
observed difference. Higher thermal stresses and more rapid material fatigue
due to diurnal temperature cycling on Mercury may cause rapid disintegration of
the upper decimeters of the boulder surface and thus contribute to more rapid
boulder obliteration; however, these factors alone cannot account for the
observed difference. A proposed thicker regolith on Mercury is likely to
significantly reduce boulder production rate. A higher micrometeoritic flux on
Mercury is likely to result in micrometeoritic abrasion being a dominant
contributor to boulder degradation; this high abrasion rate likely shortens the
boulder lifetime. A combination of these factors appears to be able to account
for the relative rarity of boulders on Mercury.
