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The mole needs friction from the soil to dig itself further, but it hasn't been able to get past the "cement-like duricrust" on Mars, so it just bounces in place.

InSight seems to have been unlucky enough to land in a place where the soil is compacted into a harder material called a duricrust, says Matthew Golombek, a planetary scientist at the Jet Propulsion Laboratory.

The duricrust is both harder and possibly thicker than expected.