  The surface of the Martian moon Phobos exhibits two distinct geologic units,
known as the red and blue units. The provenance of these regions is uncertain
yet crucial to understanding the origin of the Martian moon and its interaction
with the space environment. Here we show that Phobos' orbital eccentricity can
cause sufficient grain motion to refresh its surface, suggesting that space
weathering is the likely driver of the dichotomy on the moon's surface. In
particular, we predict that blue regions are made up of pristine endogenic
material that can be uncovered in steep terrain subject to large variations in
the tidal forcing from Mars. The predictions of our model are consistent with
current spacecraft observations which show that blue units are found near these
regions.
