  The so-called "early activity" of comet 67P/Churyumov-Gerasimenko has been
observed to originate mostly in parts of the concave region or "neck" between
its two lobes. Since activity is driven by the sublimation of volatiles, this
is a puzzling result because this area is less exposed to the Sun and is
therefore expected to be cooler on average (Sierks et al. 2015).
  We used a thermophysical model that takes into account thermal inertia,
global self-heating, and shadowing, to compute surface temperatures of the
comet. We found that, for every rotation in the August--December 2014 period,
some parts of the neck region undergo the fastest temperature variations of the
comet's surface precisely because they are shadowed by their surrounding
terrains. Our work suggests that these fast temperature changes are correlated
to the early activity of the comet, and we put forward the hypothesis that
erosion related to thermal cracking is operating at a high rate on the neck
region due to these rapid temperature variations. This may explain why the neck
contains some ice --as opposed to most other parts of the surface (Capaccioni
et al. 2015)-- and why it is the main source of the comet's early activity
(Sierks et al. 2015).
  In a broader context, these results indicate that thermal cracking can
operate faster on atmosphereless bodies with significant concavities than
implied by currently available estimates (Delbo' et al. 2014).
