panel of Fig. 27). This material is six times brighter than the mean comet
reflectance, and shows a spectrum which is flat in the visible and near
infrared range, but with a flux enhancement in the ultraviolet region,
consistent with the spectra of laboratory water frost (_Wagner et al._ ,
1987). _Fornasier et al._ (2016) reproduced the spectral behavior by linear
mixing of the cometary dark terrain with 17$\pm$4 % of water frost, and
estimated the frost layer is extremely thin, with depth of only 10-15 microns.

Figure 27: Seasonal [top], and diurnal [center and bottom panels] colors
evolution of comet 67P/CG from OSIRIS observations (_Fornasier et al._ ,
2016). Top panel: Seasonal evolution of the spectral slope from August 2014
(when the comet was at 3.6 AU inbound) to August 2015 (just after the
perihelion passage). Middle panel: Diurnal evolution of the spectral slope in
the Imhotep region. The two images shown were acquired on 27 June 2015 and
only 40 minutes apart. The corresponding heliocentric distance was 1.37 AU,
the spatial resolution 3.2 m/pixel, and the phase angle 90∘. The Sun is toward
the top. Bottom panel: radiance factor (I/F) of the regions indicated by the
white rectangle on the middle panels showing morning frosts (evidenced by the
yellow arrows), disappearing and moving with shadows.

###  4.2 Seasonal composition evolution

By orbiting in formation with 67P/CG nucleus, Rosetta had the unique
opportunity to follow the evolution of the surface and coma properties as a
function of the heliocentric distance and seasonal changes. Due to the 52.4∘
obliquity of the rotation axis (_Jorda et al._ , 2016), the summer season on
the north and south hemispheres of the nucleus is reached in different orbital
periods: the northern regions are continuously illuminated by the Sun during
the 5.5 years-long northern summer season. This includes the aphelion passage
which occurs at heliocentric distance of 5.68 AU (beyond Jupiter’s orbit). The
inbound equinox was reached in May 2015 when the comet was orbiting at 1.7 AU
from the Sun: this date marks the beginning of the 10 months-long and intense
summer on the southern hemisphere while the north hemisphere was moving in
night and was no more illuminated by the Sun. The perihelion passage occurred
on August 13 2015 when the comet reached the minimum heliocentric distance of
1.24 AU. During this phase the southern hemisphere was experiencing its brief
summer season when the Sun becomes circumpolar (never going below the line of
the horizon during a comet’s day) and the surface was receiving the maximum
solar flux resulting in the maximum activity and erosion of a surface’s layer
of thickness $\approx$ 4 m (_Fulle et al._ , 2019). Depending from models, a
percentage between 20$\%$ (_Keller et al._ , 2017) to 50$\%$ (_Hu et al._ ,
2017) of the dust flux lifted from the south hemisphere at perihelion falls
back on the northern hemisphere. The grain size distribution of the fall-back
flux is dominated by tens cm-size dehydrated aggregates containing only a very
small fraction of water ice (_Keller et al._ , 2017). Along the outbound
orbit, the equinox is repeating again in March 2016 when the southern
hemisphere returned in nightside and the northern in dayside. In this phase,
with the progressive settling of the activity on the southern regions, the
layering of the dust on the surface blankets again the exposed pristine icy
material and the nucleus surface returns darker and redder. This condition is
the same observed at the beginning of the Rosetta’s mission.

Nucleus’ surface colors have been routinely monitored by VIRTIS onboard
Rosetta allowing to catch the changing color and brightness occurring on
different regions, as more pristine, water-ice rich material was progressively
exposed on the surface (_Filacchione et al._ , 2016a; _Ciarniello et al._ ,
2016). On average, the comet’s surface appears dark and red where dehydrated
dust made out of a mixture of minerals and organic matter, prevails. Localized
active regions (as discussed in Pajola et al., chapter 10) and the occasional
ice-rich exposures are characterized by higher albedo and more neutral colors.
VIRTIS and OSIRIS observations show that global changes are noticeable, with
an overall trend of the comet becoming bluer and more water-ice-rich while
approaching perihelion (_Fornasier et al._ , 2016; _Filacchione et al._ ,
2020; _Ciarniello et al._ , 2022).

The heliocentric distance is also driving the long-term stability of the
water-ice-rich BAPs discussed in section 3.1 which starts to sublimate when
the solar exposure becomes substantial (_Raponi et al._ , 2016). These patches
are buried below a dust layer when the comet is at relatively large
heliocentric distances ($\gtrsim$3.4 AU) and for this reason, they are
scarcely recognizable at 1.5-2 $\mu$m where the diagnostic absorption bands of
water ice are located. Approaching the Sun the activity rapidly starts to
remove dust particles from the surface gradually exposing the ice beneath it.
The patch keeps sublimating thanks to the steady increase of the solar flux
until the ice deposit is completely lost. This trend is shown in Fig. 28 where
we report the 6-months temporal sequence of the water ice band area for
heliocentric distances between 3.44 and 2.23 AU along the inbound orbit.
During this period the Imhotep region is moving from winter to spring season.
Long-term ice stability on the surface of 67P/CG has been reported for many
other ice deposits observed by OSIRIS (_Oklay et al._ , 2017).

Figure 28: Temporal evolution of the water ice band area at 2.0 $\mu$m for BAP
1 (discussed in Fig. 13). Each panel represents the extension and intensity of
the water ice band for heliocentric distances between 3.443 and 2.230 AU along
the inbound orbit corresponding to a period of 6 months. The feature steadily
increases on the first six panels, and then decreases in the last three. In
the last panel, the feature is very weak or absent. The dark areas are in
shadow. All acquisitions were taken with similar illumination conditions
(average incidence angle between $53^{\circ}-67^{\circ}$ corresponding to
early morning local time). The white areas outside the edge of the map are out
the field of view of the acquisitions. From _Raponi et al._ (2016).

Spectral modeling of synthetic spectra of water ice and dark terrain mixtures,
in both areal and intimate mixing, has been exploited by _Raponi et al._
(2016) to correlate visible slope and 2 $\mu$m band area, that are the two
widely-used spectral indicators sensitive to water ice abundance on 67P/CG
spectra.

Theoretical trends from models are compared with experimental data collected
at 95∘ phase angle. The choice of the phase angle reflects the fact that
Rosetta was passing great part of its time orbiting along terminator orbits. A
similar relationship is useful to infer water ice abundance by visible colors
in absence of infrared data. Noteworthy, since both slope and band area are
phase-dependent, this correspondence is valid only at the specific 95∘ phase
angle. As computed by _Ciarniello et al._ (2015), the slope changes with phase
by +0.007 $\mu$m-1 deg-1.

Figure 29: The theoretical abundance of water ice as a function of the slope
at a phase angle of 95∘ for different mixing modalities (color lines) compared
with VIRTIS data (points in black). From _Raponi et al._ (2016).

Few studies have focused their analyses on the variability of the comet’s
environment at different heliocentric distances with the aim to correlate the
spectral changes occurring on the nucleus and in the coma. The evolution of
the nucleus and coma spectral properties evidences the presence of an orbital
cycle of water ice which can be observed by measuring multiple spectral
indicators, like the integrated radiance, the spectral slopes, and the
wavelength of the radiance’s peak measured across the visible spectral range
(_Filacchione et al._ , 2020). Comets’ gaseous activity is strongly dependant
on solar heating which provides the energy input to sublimate volatile
species. The sublimation of volatile species is responsible for the removal of
the dust grains from the surface, a process which depends on multiple factors,
such as the heliocentric distance, the local orientation of the surface with
respect to the solar direction, the composition, and physical properties of
the subsurface (_Gundlach et al._ , 2015; _Tubiana et al._ , 2019). As a
consequence of the activity, both the nucleus (_Filacchione et al._ , 2016a;
_Ciarniello et al._ , 2016; _Fornasier et al._ , 2016; _Ciarniello et al._ ,
2022) and the coma (_Hansen et al._ , 2016; _Bockelée-Morvan et al._ , 2019)
change their appearance along the orbit. The light scattered by dust grains in
the coma, taking into account for grain size distribution, shapes (_Güttler et
al._ , 2019), and viewing geometry, allows to infer their composition:
carbonaceous-organic grains are recognizable by their red-color (_Jewitt and
Meech_ , 1986) while magnesium silicate (_Zubko et al._ , 2011; _Hadamcik et
al._ , 2014) and water ice (_Fernández et al._ , 2007) grains by their
distinctive blue color.

By modeling the light scattering properties of the dust particles in 67P/CG
coma _Filacchione et al._ (2020) have found different populations of grains
along the orbit: during the inbound orbit water ice grains are the dominant
group, at perihelion submicron sized organic material and carbon-rich
particles are the more abundant, and along the outbound orbit water ice (or
possibly magnesium silicate which has similar color) return to be dominant.
Such dust composition end-members and grain sizes are reconcilable with other
Rosetta studies: 1) during the emission of outbursts _Agarwal et al._ (2017)
have observed the release of submicron-size water-ice grains mixed with 100’s
$\mu$m size refractory grains; 2) at the same time, _Bockelée-Morvan and
Biver_ (2017) have reported a broad emission feature centered at 3.5 $\mu$m
visible on the outbursts that is compatible with the sublimation of organic
materials caused by the grains’ heating by the solar flux; 3) similar dust
grains composition has been inferred by _Frattin et al._ (2017) which modeled
resolved dust grains color images by OSIRIS with three composition classes:
organic material grains corresponding to high spectral slope, mixtures of
silicate and organic material grains with intermediate slope, and water ice
grains with flat slope; 4) the presence of elemental carbon, silicon and
magnesium in dust grains has been inferred by COSIMA (see Table 3 from _Bardyn
et al._ (2017)); 5) predictions on the dust properties (composition and size)
from the theoretical "cometary model" of _Fulle_ (2021).

Figure 30: Time-series of spectral indicators as measured in 67P/CG coma
during inbound, perihelion, and outbound orbital phases. Panel a): visible
integrated radiance; b) 0.4-0.5 $\mu$m spectral slope; c) visible radiance
wavelength peak; d) 0.5-0.8 $\mu$m spectral slope. Each point corresponds to
the average value of the spectral indicators as computed on an annulus defined
by a tangent altitude between 1 and 2.5 km from the nucleus’ surface on a
single observation. For comparison, the temporal changes occurring on the
nucleus are shown in Fig. 31. From _Filacchione et al._ (2020). Figure 31:
Time-series of 67P/CG nucleus colors evolution as measured through the 0.5–0.8
$\mu$m spectral slope on 12 areas of the surface. MTPs are Rosetta’s Medium
Term Planning Phases starting from MTP5 (July 2014) and ending in MTP28 (May
2016). Each MTP is one month-long. Orbital passage at perihelion is during
MTP19 (marked by the vertical dashed line on each plot). The region name and
position (lon, lat) of the 12 test areas are indicated in each plot. For
comparison, the temporal changes occurring on the coma are shown in Fig. 30.
From _Filacchione et al._ (2020).

When comparing spectral properties derived from two years of continuous VIRTIS
observations of the dust dispersed in 67P/CG coma and nucleus surface (Fig.
30), two opposite seasonal colour cycles become evident through its perihelion
passage (_Filacchione et al._ , 2020): when far from the Sun the nucleus
appears redder than the dust particles in the bluish coma, which are dominated
by water ice grains of about 100 $\mu$m in size; moving on the inbound orbit
towards perihelion passage the nucleus shows a progressive exposure of water
ice-rich and pristine materials; as a consequence of this process the nucleus
reaches the maximum blueing at perihelion (Fig. 31); concurrently, the large
quantities of dust lifted in the coma become progressively redder orbiting
closer to the Sun due to an enrichment of organic matter and amorphous carbon-
rich submicron grains (Fig. 30); the high temperatures reached by the grains
cause the rapid sublimation of any residual ice within them, a mechanism
through which the grains are rapidly dehydrated once lifted off from the
nucleus surface (_Bockelée-Morvan et al._ , 2019). This process develops in
reverse order after the perihelion passage (plus about one month of delay)
along the outbound orbit when the activity settles and the redeposition of
dehydrated dust particles on the nucleus make it red-colored again while water
ice particles start again to populate the coma. The overall cycle of colors
developing on the nucleus and coma of 67P/CG along its orbit is summarised in
Fig. 32.

Figure 32: A synthesis of the color cycling occurring on the nucleus surface
and inner coma of 67P/CG. From ESA’s Science and Technology Blog,
https://sci.esa.int/s/8kaR0bw.

The trends observed by VIRTIS were also confirmed by OSIRIS observations:
_Fornasier et al._ (2016) monitored the color evolution of the nucleus from
3.6 AU to perihelion (1.24 AU) and beyond by measuring changes in the spectral
slope in the 535-882 nm range (Fig. 27). They found that the whole nucleus
became relatively bluer near perihelion, when the increasing level of activity
provides a progressive exposure of water ice. This color evolution is
accompanied by a change in the phase reddening effect, that decreases at
perihelion, as reported in section 2.1.

###  4.3 Local activity-driven composition evolution

The Rosetta spacecraft has been orbiting comet 67P/CG for more than 2 years,
providing the unique opportunity to study the surface evolution over time. It
has been therefore possible for the very first time not only to identify a
number of morphological changes in several regions of the nucleus, but also to
follow the composition variations produced by the cometary activity and by the
diurnal and seasonal cycles of water (see section 4.2). A number of jets and
outbursts have been identified and investigated in the literature: the very
first outburst was caught by OSIRIS on April 2014, when the comet was far from
the Sun ($\sim$ 4 AU) and unresolved (_Tubiana et al._ , 2015), followed
shortly after by the detection of water vapor with the MIRO instrument
(_Gulkis et al._ , 2015); Hapi was the most active region during the first
resolved observations (_Pajola et al._ , 2019), and characterized by a
brighter than average and relatively blue colors (_Fornasier et al._ , 2015),
indicating a local enrichment of water ice which was in fact detected by
VIRTIS (_Capaccioni et al._ , 2015); but it was during the cometary summer
close to the perihelion passage that a number of outbursts and jets were
individually identified and their source position on the nucleus retrieved
through geometrical tracing (_Vincent et al._ , 2016a; _Knollenberg et al._ ,
2016), or direct imaging (_Vincent et al._ , 2016b; _Shi et al._ , 2016;
_Pajola et al._ , 2017a; _Fornasier et al._ , 2019b); finally, during the
Rosetta extended mission from January to September 2016, some plumes/faint
jets were observed in the equatorial and southern regions in the high-
resolution images (_Fornasier et al._ , 2017, 2019a; _Agarwal et al._ , 2017;
_Deshapriya et al._ , 2018; _Hasselmann et al._ , 2019).

Thanks to the direct identification of activity sources on the nucleus, a
number of regions were carefully investigated to look for morphological and
composition changes produced by the activity. The main processes originating
composition variations on the nucleus can be categorized as the following:

Figure 33: Examples of composition changes associated with activity events.
Panel A: The Anhur region showing the two larger water-ice rich patches
(left), the new scarp formed between August and December 2015 nearby the
patches (center), and the corresponding RGB map from images acquired on 7 May
2016, UT 04:15, showing the new scarp and the bluer and water ice-rich
material at its feet (from _Fornasier et al._ (2017)). Panel B: example of jet
locations (left side) and some associated activity events in the Anhur region,
including the perihelion outburst which took place on 12 August 2015 (from
_Fornasier et al._ (2019b)). The right side of panel B shows a RGB composite
image (from the images acquired with filters centered at 882, 649, and 480 nm)
of the Anhur region from observations acquired on 25 June 2016 at UT01h37 at a
resolution of 0.35 m/px. Several exposures of water ice are also detected,
notably at the base of a new scarp, and frost observed within the canyon
structure (Figure adapted from _Fornasier et al._ (2019a)). Panel C: the plume
and associated new cavity formation with exposure of water ice identified in
the Imhotep region by _Agarwal et al._ (2017). Panel D: the outburst and cliff
collapse in Aswan site with the exposure of the fresh water ice rich layer at
the wall of the cliff (from _Pajola et al._ (2017a)).

  1. 1.

Dust removal and redeposition by volatiles sublimation: the surface dust is
mobilized by means of the sublimation of volatile species (mainly CO2
_Gundlach et al._ (2020) and H2O). The cometary activity progressively removes
dust and exposes pristine material including water ice able to survive on the
surface until sublimation occurs. This is causing the composition changes
observed on the overall nucleus, or on extended areas of it, where the
seasonal and diurnal cycle of water (as detailed in section 4.2) are
manifested, as well as the spectral phase reddening effect variations (see
section 2.1).

The transfer and redeposition of the dust are strongly related to the
sublimation of volatiles and thus to the activity level, lift pressure and
local topography (_Thomas et al._ , 2015; _Keller et al._ , 2017; _Hu et al._
, 2017; _Pajola et al._ , 2017b). Because the southern hemisphere experiences
strong thermal effects being intensely illuminated during, and close to, the
perihelion passage, it is highly eroded. By modeling only water ice
sublimation _Keller et al._ (2017) has found that dust ejection occurs from
the south and a significant fraction of it is redeposited in the northern
regions. _Lai et al._ (2016) estimated a dust loss of about 0.4 m in depth per
orbit in Hapi, and up to 1.8 m in Wosret, where the highest water production
rate was measured by MIRO during the perihelion passage (_Marshall et al._ ,
2017). Concurrent OSIRIS measurements during the inbound orbit show an erosion
of about 1.7$\pm$0.2 m in the Hapi region which is nearly balanced by a
1.4$\pm$0.8 m fallout layer from the southern hemisphere during perihelion
(_Cambianica et al._ , 2020).

The first consequence of dust mobilization is the seasonal variation of
colors, with the nucleus becoming bluer approaching perihelion because of the
exposure of underlying layers enriched in water ice (Fig. 27).

Secondly, the morphological landscape changed locally: a local removal of
$\sim$ 4 m dust coating was observed in part of Imhotep, with the exhumation
of boulders and roundish features (_El-Maarry et al._ , 2017), and up to
$\sim$14 m in a cavity of Khonsu region (_Hasselmann et al._ , 2019) and
within the canyon-like structure in Anhur (_Fornasier et al._ , 2019a). Both
these southern hemisphere regions originated several jets, and in particular,
the canyon in Anhur was the source of the so-called perihelion outburst, one
of the strongest activity events reported for comet 67P/CG. Moreover, several
ice exposures were observed in the Anhur and Khonsu regions (_Deshapriya et
al._ , 2018; _Fornasier et al._ , 2019a; _Hasselmann et al._ , 2019), and
water frost was repeatedly observed within the Anhur canyon (see panel B in
Fig. 33).

Finally, the dust that fell back onto the nucleus is preserving some water
ice, as suggested by _Keller et al._ (2017) and observed by _Fornasier et al._
(2019a). In fact, these authors noticed an asymmetry in the presence of
frost/water ice on the 67P/CG comet by comparing pre- and post-perihelion
images acquired at similar spatial resolutions and at similar heliocentric
distances. While during the inbound orbits water frost/ice was observed only
in Hapi, in the outbound orbits it was observed in several regions, including
Seth, Anhur, Khonsu, Imhotep, and, in a minor amount, in Khonsu (_Lucchetti et
al._ , 2017; _Fornasier et al._ , 2019a, 2021; _Hasselmann et al._ , 2019;
_Deshapriya et al._ , 2016, 2018; _Oklay et al._ , 2017). All these pieces of
evidence proved that the falling back dust still preserves some ice.

  2. 2.

local surface deflation: this mechanism is associated with the sublimation of
subsurface reservoirs of volatiles, resulting in a local surface erosion. An
example of manifestation of this mechanism has occurred in the large smooth
central area of the Imhotep region with the appearance of two roundish
depressions with sizes increasing in time up to several hundred meters and
depth of 5$\pm$2 m when the comet was approaching perihelion (_Groussin et
al._ , 2015b). These authors invoked exothermic processes such as the
crystallization of water ice and/or the clathrate destabilization to explain
the expansion rate of these features, which was higher than 18 cm hour-1. Tiny
bright spots, as well as relatively bluer colors/spectral slopes near the
rims/edges of these roundish features, were observed, indicating a local
enrichment in volatiles.

  3. 3.

cliffs/overhangs collapse: they may be triggered by episodic and explosive
activity events, producing the mechanical disruption of old terrains
(_Groussin et al._ , 2015a; _Pommerol et al._ , 2015). A notable example is
the July 2015 outburst associated with the cliff’s collapse in the Aswan site
in the Seth region (_Pajola et al._ , 2017a). Following the cliff’s collapse,
an inner layer enriched in fresh and bright icy material (with albedo beyond
40$\%$) has been exposed. This layer has survived for several months: few
examples of this kind of event are shown in panels A and D in Fig. 33. Several
jets were identified by _Fornasier et al._ (2019b) directly on the nucleus and
they were found arising below or close to cliffs or scarps. Often, the walls
or the debris fields of cliffs and overhangs are spectrally bluer and brighter
indicating the local enrichment of water ice, from the freshly exposed
underneath layers (_Pommerol et al._ , 2015; _Filacchione et al._ , 2016b;
_Vincent et al._ , 2016b; _Oklay et al._ , 2017; _Fornasier et al._ , 2017,
2019a).

  4. 4.

local activity: outburst and jets arise from cliffs, pits, bright spots,
fractures, or from cavities that cast shadows and favor the recondensation of
volatiles showing periodic activity. These activity sources are often observed
to be brighter and relatively bluer in colors/spectral slope, showing a local
enrichment in volatiles (_Vincent et al._ , 2016b; _Agarwal et al._ , 2017;
_Fornasier et al._ , 2017, 2019a, 2019b; _Hasselmann et al._ , 2019;
_Deshapriya et al._ , 2018; _Hoang et al._ , 2020). Examples are reported in
panels B and C of Fig. 33. The investigation of activity events from _Vincent
et al._ (2016a) and _Fornasier et al._ (2019b) shows that cometary activity is
triggered by the local illumination conditions and it is stable between
several nucleus rotation. _Fornasier et al._ (2019b) identified the source
areas of more than 200 jets and found that the activity events are not
correlated with the nucleus morphology, originating both from consolidated and
smooth terrains. Interestingly, _Fornasier et al._ (2019b) found that some
faint jets may have an extremely short duration time, less than a couple of
minutes, so it may be difficult to catch this kind of events that are probably
common in comets.

Table 4: Summary of the observed behavior and physical properties of active areas located on the small and big lobe of comet 67P/CG from _Fornasier et al._ (2021). These properties are derived from the investigation of southern hemisphere regions submitted to similar high heating at perihelion. | Small lobe (Wosret) | Big lobe | References
---|---|---|---
incoming solar flux |  $\sim$ 550 W m-2 |  $\sim$ 550 W m-2 (Anhur and Khonsu) | _Marshall et al._ (2017)
morphology | Consolidated material that appears | Consolidated material with significant | _Thomas et al._ (2018)
| highly fractured with occasional pits | intermediate scale roughness (Anhur) |
exposed water ice |  in a few and small bright spots ($\sim m^{2}$) | in many bright spots and | _Fornasier et al._ (2021)
|  |  some in big patches (1500 m2) | _Fornasier et al._ (2016, 2019a)
relatively blue area | in very spatially limited area, | in extended area, periodic | _Fornasier et al._ (2016, 2017)
enriched in frost/ice | periodic, not frequently observed | often observed | _Fornasier et al._ (2019a, 2021)
|  |  | _Hasselmann et al._ (2019)
average |  the largest in 67P/CG: 4.7$\pm$1.5 m | 2.2-3.2 m (Seth, Imhothep, | _Fornasier et al._ (2021)
goosebump |  | Anubis, | _Sierks et al._ (2015)
diameter |  | Atum) | _Davidsson et al._ (2016)
level of activity | very high | very high | _Fornasier et al._ (2021, 2019b)
|  |  | _Hasselmann et al._ (2019)
surface morphology | few and not so important: | many and important : |  _Fornasier et al._ (2021),
changes | a relatively small cavity, | local dust removal up to 14m in depth, | _Fornasier et al._ (2017, 2019a)
|  local dust removal ($\sim$ 1m depth) | new relatively big scarps and cavities, | _Hasselmann et al._ (2019)
| revealing a cluster of outcrops | big vanishing structures, boulders |
|  | displacements and fragmentation |
surface mass loss |  $\sim$ 1.2$\times 10^{6}$ kg |  $>50\times$106 kg in Anhur | _Fornasier et al._ (2016, 2019a)
|  |  $\sim 2\times$108 kg in Khonsu | _Hasselmann et al._ (2019)
  5. 5.

composition heterogeneities: local color and composition heterogeneities have
been identified in different regions of the 67P/CG nucleus and several of them
are associated with the local exposure of volatiles. A notable example is
Anhur which, as discussed in section 3.1, showed a first exposure of CO2 ice
(_Filacchione et al._ , 2016c), followed by the exposure of two large water
ice rich patches (_Fornasier et al._ , 2016) on the same area. These authors
deduced composition heterogeneities in the subsurface composition on a scale
of tenths of meters in the Anhur region. The sublimation of the surface and
subsurface volatile reservoir here observed is at the origin of the fragile
terrain where a new scarp formed as shown in Fig. 33 (panel A), just nearby
the two bright patches location.

Also in comet 9P/Tempel 1 some composition inhomogeneities linked to the
detection of dirty water-ice rich material have been reported by _Sunshine et
al._ (2006). Moreover, the comparison of Tempel 1 images from Deep Impact and
the Stardust flybys show morphological changes (_Veverka et al._ , 2013),
interpreted as the progressive sublimation and depletion of volatiles.

Comparing the physical and composition properties of two southern hemisphere
regions, Anhur and Wosret, subject to the same strong thermal heating during
the cometary summer but located in the large and small lobes, respectively,
_Fornasier et al._ (2021) have noticed a number of differences in the physical
and mechanical properties among the two lobes, although no appreciable
differences were reported in the literature on the global surface composition
(_Capaccioni et al._ , 2015) and on the $D/H$ ratio (_Schroeder et al._ ,
2019). Wosret, in the small lobe, shows larger goosebumps features, very few
morphological changes compared to Anhur and Khonsu regions, despite they all
experienced the same high insolation, and less frequent and smaller frost and
water ice enriched areas. Considering that Wosret is highly eroded and thus
exposes the inner layers of the small lobe, _Fornasier et al._ (2021) deduced
that the small lobe has a lower volatile content, at least in its top layers,
than the big one, and different mechanical properties, as already noticed by
_El-Maarry et al._ (2016).

A comparison of the observed surface properties on the active regions of the
small and big lobes of 67P/CG (Table 4) allows to appreciate how the latter is
more eroded and processed by solar heating during the perihelion passages.

###  4.4 From surface evolution to the internal structure of cometary nuclei

Even though the blueing evolution hints a global water ice enrichment towards
perihelion passage (see section 4.2), the water ice is observed in a patchy
distribution across the nucleus. As we have discussed in the previous section
3.1.1, high spatial resolution observations of 67P/CG have revealed a rich
phenomenology of localized water-ice rich spots, ranging in size from less
than one meter up to tens of meters. These spots appear distributed across the
nucleus surface and are recognizable thanks to their intrinsic high
reflectance and blue-colored visible spectra which distinguish them from the
average dark and red-colored terrain (we refer to such spots as Blue Patches,
or BPs in brief).

Figure 34: The progressive blueing of 67P/CG surface orbiting towards
perihelion is caused by the progressive exposure to sunlight of Water-Ice-
Enriched Blocks (WEBs). In this model the comet nucleus is made by two
populations of pebbles composed of refractories (organic matter and minerals)
and CO2 ice with, respectively: 1) high abundance of H2O ice (composing the
WEBs); and 2) low abundance of H2O ice (H2O-ice poor pebbles). The composition
of the H2O-ice poor pebbles corresponds to the material observed during the
occasional exposure of CO2 ice (_Filacchione et al._ , 2016c), making most of
the nucleus. The CO2 ice is stable at depths $>0.1$m where is localized its
condensation front (_Gundlach et al._ , 2020). When approaching perihelion,
the sublimation rate of CO2 steadily increases causing the erosion of the
surface through the ejection of chunks and exposing the sub-surface WEBs. Once
exposed, the WEBs rapidly lose their CO2 content and appear as blue patches
(BPs). After this, the solar flux activates the sublimation of the H2O ice
which erodes the BPs, while removing sub-cm sized dust from their surface and
preventing the formation of a dry crust (_Fulle et al._ , 2020). The BPs can
survive on the surface until their water-ice fraction is gone. In the
meanwhile, their presence is the origin of the observed surface blueing. While
WEBs evolve along this path, H2O-ice poor pebbles experience dehydration
rather than erosion due to their intrinsic lower abundance of water ice
(_Fulle et al._ , 2020), whereas the formation of a crust is hampered by the
concurrent CO2 driven erosion. The same water flux is released by the BPs and
the dehydrating surface units as both contain some water ice and are assumed
to share the same temperature (_Fulle et al._ , 2020). The resulting blueing
trend when 67P/CG approaches perihelion is shown along the right axis. Adapted
from _Ciarniello et al._ (2022), where further details can be found.

A similar property is also in agreement with the dust ejection rates reported
by _Fulle et al._ (2016b) derived by GIADA at perihelion (_Fulle et al._ ,
2020).

Starting from these observational evidences, _Ciarniello et al._ (2022) have
explained the color changes occurring on 67P surface in the framework of the
water-driven activity model of _Fulle et al._ (2020) for a nucleus made of cm-
sized pebbles (_Blum et al._ , 2017), by defining a new model of the comet
internal structure. According to the proposed scenario (Fig. 34), the
perihelic blueing of 67P is interpreted as an increase of the areal fraction
covered in BPs. In this respect, the BPs represent the exposed counterparts of
a population of Water-Ice-Enriched Blocks (WEBs, in the following), formed of
pebbles with high content of water ice, immerse in a matrix of drier pebbles
(see details in Fig. 34 and caption). The WEBs are exposed as a consequence of
the surface erosion through chunk ejection induced by CO2 ice sublimation
(_Gundlach et al._ , 2020), and are eroded by H2O ice sublimation ejecting
sub-cm dust (_Fulle et al._ , 2020) when directly reached by sunlight. The
observed color evolution of the nucleus is the result of these two competing
mechanisms. By matching the temporal variation of the surface color while
taking into account the constraints on the surface erosion by CO2-driven chunk
emission (_Fulle et al._ , 2019; _Cambianica et al._ , 2020; _Fulle et al._ ,
2020) and the modeled water-driven erosion rates 1.57 m2 and, _Ciarniello et
al._ (2022) infer a dominant size for the WEBs of 0.5-1 m, with a volumetric
abundance in the nucleus from 9.5 to 5.5$\%$. The presence of sub-metre WEBs
in the nucleus of 67P/CG is in agreement with CONSERT radar observations,
which found a homogeneous internal structure down to a few-metres scale
(_Ciarletti et al._ , 2017).

A uniform distribution of WEBs in the interior of 67P/CG nucleus allows to
explain both the observed seasonal spectral changes and the measured dust
ejection rates, making the macroscopic compositional dishomogeneities observed
in comets, e.g. BPs, compatible with the homogeneous structure at small scale
(the cm-sized pebbles), and with the sublimation processes occurring at
microscopic (sub-pebble) scales. If this is the case, WEBs must be of
primordial origin: in fact, during the orbital evolution water ice can
recondensate only within a very shallow layer (_De Sanctis et al._ , 2015;
_Fornasier et al._ , 2016) but not in the deep internal parts. With similar
characteristics, WEBs are imposing strict constraints on comets’ formation
scenarios. The detection of crystalline water ice in Proto-Planetary Disks
(PPDs) by _Min et al._ (2016) would be compatible with the formation of water
ice-rich pebbles from local ice recondensation, likely stickier than the water
poor ones, and possibly favouring the accretion into WEBs during the
gravitational collapse originated by the streaming instabilities in PPDs
(_Blum et al._ , 2017).

##  5 Conclusions

Comets’ nuclei are repository of primordial materials syntethized in the ISM
and then processed in the protosolar disk. As such, they are cold time-
capsules able to preserve primordial matter until nowadays and offer us the
possibility, through their exploration, to understand the conditions occurring
during the formation of our Solar System. To reach this purpose, the Rosetta
mission has implemented an innovative approach in which measurements performed
thorough a wide range of techniques were aiming to infer 67P/CG composition
from the orbiter and from the Philae lander at different heliocentric
distances. In this respect, Rosetta mission has been a game-changer in
cometary science.

So far, the analysis of Rosetta data discussed in this chapter in the context
of previous cometary space missions, gives us some important clues about the
nature of cometary matter which is compatible with a _macromolecular
assemblage in which ices, organic matter, salts and minerals_ are bound
together. While the volatile species have been measured and characterized as
ices on the nucleus and as gases in the coma by multiple instruments, the
determination of the non-volatile fraction is much more complicated to
achieve. At now it is still unclear how to reconcile the elemental composition
of gaseous species and dust grains achieved by in situ instruments (ROSINA,
COSIMA, PTOLEMY, COSAC) with the optical properties of the nucleus at VIS-IR
wavelengths (VIRTIS, OSIRIS). The complex organic matter, showing both
aromatic and aliphatic bonds, appears well-mixed with minerals, as silicates,
Fe-sulfides, carbon, and ammoniated salts. The mixing of all these materials
results in the low albedo and red color of the dust covering the whole
nucleus. When far from the Sun, the majority of the surface appears very
dehydrated, with a water ice residual of no more than 1 $\%$.

The composition of the non-icy materials allows to back-trace the origins of
comets: the nature of the refractories, including organic aromatic and
aliphatic matter and silicates, shares similarities with pre-solar matter. The
stronger intensity of the CH3 asymmetric stretch (3.38 $\mu$m) with respect to
the CH2 (3.42 $\mu$m) resembles the properties observed in the ISM and IOM in
primitive carbonaceous condrites. Also the 2.8 $\mu$m OH band in hydroxylated-
magnesium silicates indicates a possible genetic link between cometary
refractories and ISM, where amorphous silicates are irradiated by keV-MeV
energy protons. Moreover, Rosetta’s results allow to explain the apparent
nitrogen depletion observed in cometary comae being it sequestered in semi-
volatile ammoniated salts. In this respect, comets shows analogies with other
asteroids and outer solar system objects observed at VIS-IR wavelengths,
suggesting an evolutionary link among all these objects. These evidences show
that to understand comets’ composition it will be necessary to improve
statistics (so far only 6 comets have been explored by space missions) and
comparisons with asteroids, outer solar system objects and ISM data.

The Rosetta mission faced multiple (and unexpected) challenges in exploring
67P/CG: the very low albedo, the complex shape and morphology of nucleus, the
evolution of the solar heating, the changing activity responsible of the dust
and gas environment in the coma were factors playing against a straightforward
determination of the nucleus’ composition. Moreover, the un-nominal landing of
Philae further complicated the mission planning and the scientific
interpretation of the results. Similar difficulties will remain in place for
any future cometary mission relying on remote sensing and in-situ payloads. In
order to overcome them we need a paradigm shift in cometary exploration by
implementing a more ambitious exploration approach, such as by adopting a
cryogenic sample return, in which a sample containing original cometary
material, keeping unaltered the ices and refractory parts, it is returned to
Earth to conduct composition analyses with multiple analitical techniques
(_Bockelée-Morvan et al._ , 2021). A similar sample shall be the "Holy Grail"
for the cosmochemistry community allowing to shed light on many unresolved
questions: apart determining cometary material composition and solar or pre-
solar origin, it would permit to understand cometary formation mechanisms,
internal structure, dust-to-ice ratio and presence of prebiotic molecules.

Acknowledgments.
The authors gratefully thank dr. Sébastien Besse (ESA-ESAC, Madrid, Spain) and
dr. Javier Licandro (Instituto de Astrofísica de Canarias, Spain) for their
thorough reviews which helped to improve the quality of the manuscript. G.F,
M.C., and A.R. acknowledge support from INAF-IAPS and ASI, Italian Space
Agency. S.F. acknowledges the support from LESIA, the France Agence Nationale
de la Recherche (programme Classy, ANR-17-CE31-0004), and from the Institut
Universitaire de France. This research has made use of NASA’s Astrophysics
Data System (NASA-ADS).

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