CO2 as the driving force of comet Hartley 2’s activity—An experimental study

• We show experimentally that CO2, released from ice, can break it and propel ice grains.
• This can account for the observations of Comet Hartley 2.
• We show experimentally that CO2 is trapped in ice 103–104 times more efficiently than CO, CH4 and Ar.

In our experimental study we observed massive ice grain ejection driven by CO2, during several temperature ranges when the ice is warmed up. A'Hearn et al., 2011. Science 332, 1396–1400 and Meech et al., 2011. The Astrophysical Journal Letters 734, L1 among others, found that CO2 is the major driving force of the activity of Comet 103P/Hartley 2. In our experimental studies we found that CO2 is trapped in low temperature “cometary” amorphous water ice 3–4 orders of magnitude more efficiently than gases such as CO, CH4 and Ar. Thus, the ice grains which agglomerated to form the nuclei of comets where CO dominates CO2 in the coma seem to have formed in regions highly depleted in CO2. In the experiments we observed ice grains ranging from submicron to hundreds of microns, accompanied by jets of CO2. The experimentally observed cm size fragments, which were formed by cracking the overlying ice layer, could also be ejected under the cometary microgravity. These size ranges were also observed on Comet 103P/Hartley 2. The smooth terrain observed on Hartley 2’s “waist” is similar to the smooth terrain observed on comet Temple 1 and could have been formed by settling of ice grains ejected nearby, as observed in our experiments.