Rotationally induced surface slope-instabilities and the activation of CO2 activity on comet 103P/Hartley 2

Abstract

Comet 103P/Hartley 2 has diurnally controlled, CO 2 -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 CO 2 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 CO 2 -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 CO 2 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 tip of the small lobe. 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 2 (rotation period increased) from a 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 terrains on the imaged portion of the nucleus.

Local Manifestations of Cometary Activity

“…Transient events associated to such collapse have also been observed on 67P Grün et al, 2016;Pajola et al, 2017). Other comets show similar link between activity and morphology , and also with landslides in areas covered with fine dust like on comet 103P (Steckloff et al, 2016).…”

“…the sudden release of a large quantity of gas and dust, driven by mechanisms that are still debated. While outbursts have usually been interpreted as akin to an explosive decompression, this interpretation is challenged by EPOXI and Rosetta data and the associated models which suggest that the transient release of material can be well explained by avalanches (Steckloff et al, 2016) or topographic collapses (pits, cliffs, Vincent et al (2016)). Spacecraft observations have shown that such events are more likely to occur around perihelion and in the outbound orbit, but not always.…”

Cometary spin-down

“…The formation of these new synapses, and the loss of some old ones, mean that spicule movement comes under the is now causing the strong torque. A similar sequence of events could have occurred in comet 103P/Hartley 2, which was visited by the Deep Impact Extended Investigation (DIXI) space mission 8,10 in 2010.…”

“…If a comet is spun up to a rotation rate at which the centrifugal force near the equator surpasses gravitational and cohesive forces, landslides and partial or even catastrophic fragmentation can occur [6][7][8] . Such events would be accompanied by strong sublimation (transformation of ice into gas) and dust production from newly exposed areas, which is one possible cause of sudden increases in brightness called outbursts.…”

The Rocky‐Like Behavior of Cometary Landslides on 67P/Churyumov‐Gerasimenko

“…On 67P, multiple landslide deposits are identified in close association with cliffs. These observations suggest that cliff collapse is an important process in reshaping cometary surfaces (Britt et al, ; Pajola et al, ; Steckloff et al, ; Steckloff & Samarasinha, ). OSIRIS observations show a direct evidence of the occurrence of a cometary landslide, with the resulting production of a newly formed boulder talus located at the base of the scarp (see the Aswan case; Pajola et al, ).…”