Geoscience Reference
In-Depth Information
regions or the parts of high-latitude craters permanently in the
shadow). Such non-uniformity could lead to a non-uniform spatial exo-
spheric distribution.
3.1.
Variation due to Mercury's orbit
Mercury's eccentricity (0.2056) is the largest of the inner planets of the
solar system. The variation of the heliocentric distance from perihelion
(0.306 AU) to aphelion (0.466 AU) implies a variation of the average inten-
sity of the solar wind and photon fluxes by a factor 2.3. Therefore, solar
wind sputtering, photon-stimulated desorption or photon ionization will
change in eciency by such a factor in an average (also the brightness of
Mercury's exosphere as seen from the Earth). Variation of the solar UV flux
and of the solar wind flux on short-time scale is also an important cause of
variability in the exosphere (see as an example the increase of the UV flux
displayed Plate 6 of Ref. 7 and Sec. 3.2.1).
As a consequence, Mercury's surface temperature also changes signifi-
cantly from a maximum temperature around 650 K at perihelion down to
a maximum temperature around 550 K at aphelion.
30
Thermal desorption
and the average energy of the ejected particles by this latter mechanism
will also significantly change along Mercury's year.
Mercury rotates around the Sun in a 2-3 resonance between orbit
(87.97 Earth days) and sidereal rotation (58.6 Earth days), its diurnal period
being therefore of 176 Earth days.
31
This motion leads to an irregular appar-
ent motion of the Sun as seen from Mercury's surface, in particular, a slow-
ing down and subsequent reversal of the Sun's apparent motion during a
short period equivalent to few Earth days at perihelion. The Sun's apparent
motion at Mercury is faster at aphelion. This 2-3 resonance produces hot
and cold longitudes, which correspond to regions of Mercury's surface that
receive maxima and minima of solar flux along Mercury's year.
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It also
implies that Mercury's nightside surface enriched in volatiles (see Sec. 3.3.1
for further explanations), moves into the dayside with a much larger speed
at aphelion than at perihelion. As a consequence, the quantity of volatiles
available for ejection into Mercury's exosphere is larger at aphelion than at
perihelion, and therefore, Mercury's total exospheric content may be larger
at aphelion than at perihelion, a trend that seems to be confirmed by the
now extended set of observation of Mercury's sodium exosphere.
9
,
10
,
14
In
particular, Killen
et al.
10
have shown by compiling a large set of observa-
tions of Mercury's sodium exosphere that Mercury's sodium exosphere total
