coma may also be back-scattered to the nucleus
surface and recondense, contributing to the frost
formation (24, 25). From the shadow travel speed
and the extent of the frost fronts, we calculate an
ice permanence time of about 3 min (12). From
thermal modeling using an intimate mixture, we
estimate a water-ice sublimation rate of about
4 g m−2 min−1, from which we infer a total ice
content of about 12 g m−2 for the frost layer, which
is extremely thin (thickness ~10 to 15 mm of equivalent solid ice) (table S3). If we assume an areal
mixing, the sublimation rate and total ice content
are lower by a factor of 8. In the absence of direct
temperature measurements of the frost, it is impossible to discriminate whether areal or intimate
mixture represents the correct thermal model (1).
In either case, the exposed ice is depleted on short
time scales. In the case of geographical mixing,
however, sublimation rates at the time of the observations would be so low that a frost permanence time of 3 min would imply a frost layer on
the order of ~1 mm solid-ice equivalent. Because
it is questionable whether such a thin layer would
be optically thick, it appears more likely that the
frost patches are intimately mixed with the refractory material. Similar to the observed diurnal
color variations, the recondensation of frosts is
also a periodic phenomenon that takes place
close to topographic shadows, as indicated in
Figs. 3 and 4.
The long-term observations of 67P provide information on the composition of the outermost
layers of the nucleus. They reveal that ice is abundant just beneath the surface on the whole nucleus,
which, most of the time, is covered by a thin layer
of dust. Hence, the apparent surface composition
is globally dominated by anhydrous refractory materials (6). The increasing cometary activity while
approaching the Sun progressively thins the dust
surface layer, partially exposing the ice-richer subsurface, yielding nucleus’s colors that are bluer
relative to those at large heliocentric distances.
OSIRIS observations show that mixtures having high abundance (up to ~30%) of water ice are
occasionally locally present on the nucleus and
that the lifetime of exposed ice is short, on the
order of a few minutes to a few days. However,
no dust-free ice patches were observed even during the peak of activity near perihelion, indicating that water ice and dust are well mixed within
the resolution limit of the images (~2 m pixel−1).
Most of the bright features are observable only
at high spatial resolution and under particular
insolation conditions—i.e., close to the morning
shadows. Similar phenomena presumably take place
on other comets, explaining why cometary nuclei
are so dark even if they have important water ice
abundance. The extended bright patches observed
in the Anhur/Bes regions indicate a local enrichment
of water ice, pointing to compositional heterogeneities in the uppermost layers of comet 67P.
REFERENCES AND NOTES
1. J. M. Sunshine et al., Science 311, 1453–1455 (2006).
2. J. M. Sunshine et al., The Distribution of Water Ice on Comet
103P/Hartley 2. Proceedings of the conference held May
16–20, 2012, in Niigata, Japan; Lunar and Planetary Institute
(LPI) Contribution no. 1667, id.6438 (2012).
Fig. 3. Spectral slope changes and frost sublimation. (A and B) Spectral slope maps of Imhotep
region taken 40 min apart. The Sun is toward the top. (C and D) Zoom in radiance factor (I/F) of the regions
indicated by the white rectangle on (A) and (B) showing morning frosts (evidenced by the yellow arrows),
disappearing and moving with shadows. The yellow rectangular region in (C) indicates the area where the
same frost structures are seen 2 weeks later and analyzed in Fig. 4.
Fig. 4. Reflectance of frost fronts. (A) Bright fronts seen close to shadows on the Imhotep region (see
Fig. 3 for the context). (B and C) Reflectance normalized at 535 nm and normal albedo of three selected
regions [magenta points in (A)]. The dashed line represents the best-fit model, with uncertainties in gray,
including 17 ± 4% of water frost linearly mixed to the comet dark terrain.