  Comet 103P/Hartley 2 has diurnally controlled, CO2-driven activity on the tip
of the small lobe of its bilobate nucleus. Such activity is unique among the
comet nuclei visited by spacecraft, and suggests that CO2 ice is very near the
surface, which is inconsistent with our expectations of an object that
thermophysically evolved for ~45 million years prior to entering the Jupiter
Family of comets. Here we explain this pattern of activity by showing that a
very plausible recent episode of rapid rotation (rotation period of ~11 [10-13]
hours) would have induced avalanches in Hartley 2's currently active regions
that excavated down to CO2-rich ices and activated the small lobe of the
nucleus. At Hartley 2's current rate of spindown about its principal axis, the
nucleus would have been spinning fast enough to induce avalanches ~3-4 orbits
prior to the DIXI flyby (~1984-1991). This coincides with Hartley 2's discovery
in 1986, and implies that the initiation of CO2 activity facilitated the
comet's discovery. During the avalanches, the sliding material would either be
lofted off the surface by gas activity, or possibly gained enough momentum
moving downhill (toward the tip of the small lobe) to slide off the nucleus.
Much of this material would have failed to reach escape velocity, and would
reimpact the nucleus, forming debris deposits. The similar size frequency
distribution of the mounds observed on the surface of Hartley 2 and chunks of
material in its inner coma suggest that the 20-40 meter mounds observed by the
DIXI mission on the surface of Hartley 2 are potentially these fallback debris
deposits. As the nucleus spun down from a rotation period of ~11 hours to 18.34
hours at the time of the DIXI flyby, the location of potential minima, where
materials preferentially settle, migrated about the surface, allowing us to
place relative ages on most of the imaged terrains of the nucleus.

Earth and Planetary Astrophysics