Low activity main belt comet 133P/Elst-Pizarro: New constraints on its Albedo, Temperature and Active Mechanism from a thermophysical perspective

Abstract

133P/Elst-Pizarro is the firstly recognized main-belt comet, but we still know little about its nucleus. Firstly we use mid-infrared data of Spitzer-MIPS, Spitzer-IRS and WISE to estimate its effective diameter $D_{\rm eff}=3.9^{+0.4}_{-0.3}$ km, geometric albedo $p_{\rm v}=0.074\pm0.013$ and mean Bond albedo $A_{\rm eff,B}=0.024\pm0.004$. The albedo is used to compute 133P's temperature distribution, which shows significant seasonal variation, especially polar regions, ranging from $\sim40$ to $\sim200$ K. Based on current activity observations, the maximum water gas production rate is estimated to be $\sim1.4\times10^{23}\rm~s^{-1}$, being far weaker than $\sim10^{26}\rm~s^{-1}$ of JFC 67P at similar helio-centric distance $\sim2.7$ AU, indicating a thick dust mantle on the surface to lower down the gas production rate. The diameter of the sublimation area may be $<\sim200$ m according to our model prediction. We thus propose that 133P's activity is more likely to be caused by sublimation of regional near-surface ice patch rather than homogeneous buried ice layer. Such small near-surface ice patch might be exposed by one impact event, before which 133P may be an extinct comet (or ice-rich asteroid) with ice layer buried below $\sim$40 m depth. The proposed ice patch may be located somewhere within latitude $-50\sim50^\circ$ by comparing theoretical variation of sublimation temperature to the constraints from observations. The time scale to form such a thick dust mantle is estimated to be $>100$ Myr, indicating that 133P may be more likely to be a relatively old planetesimals or a member of an old family than a recently formed fragment of some young family.