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conditions could change the magnetospheric structure
39
and induce signif-
icant changes in the recycling of magnetospheric ions inducing correlated
variation in the neutral exosphere. Using different neutral exospheric and
magnetospheric models, Killen
et al.
10
found that between 45 and 65% of
the photoions reimpact the surface, most on the dayside and underlined the
significant role of the electric field of convection in the global dawn to dusk
balance of the recycled ion.
3.3.
Upper surface-exosphere relations
3.3.1.
Migration of the volatiles
A particle ejected from Mercury's dayside not on an escaping trajectory
(that is with less than
0.1 eV/amu when ejected) will randomly hop and
become temporally absorbed (as example for most of the sodium atoms) or
partially energetically accommodated in Mercury's upper surface at each
hop (as example for most of the hydrogen and helium atoms).
40
Due to
the very high temperature of Mercury's dayside surface (more than 400 K
during most of the day, see Fig. 2), the period during which such a particle
is absorbed in Mercury's upper surface is shorter than few Earth minutes
or few Mercury's seconds (values valid in the case of a sodium atom using
recent laboratory measurements
41
and including porosity effect
9
). There-
fore, the time needed for such a particle to encounter a cold surface where
it can be trapped for a long period (with respect to Mercury's day) will be
much shorter than Mercury's hour. These cold regions are essentially late
evening, early morning or high-latitude regions. This migration of volatiles
into cold regions should lead to a global larger density of trapped volatiles
on the nightside than on the dayside.
9
An immediate consequence is a sig-
nificant morning/evening asymmetries of Mercury's exosphere because of
the release of trapped nightside volatiles at morning. Such morning/evening
asymmetry in the case of Mercury's sodium exosphere has been discussed
for a long-time up to its recent and unambiguous observation.
42
Another putative consequence of this preferential migration of ejected
volatiles into cold surfaces could be the enrichment of Mercury's surface at
high-latitude early morning. Indeed as illustrated in Fig. 2, in the morn-
ing, equatorial regions get hotter earlier by few degrees in longitude than
high-latitude regions. As an example, 60
◦
latitude regions reach equatorial
surface temperature few tens of Earth hours later. Such a period is roughly
the time for a thermally desorbed particle (with less than 0.06 eV when
ejected) to move by a distance of the order of Mercury radius. Such a time
∼
