Geoscience Reference
In-Depth Information
3.3.2.
Topography effects
Another relation between upper surface and exosphere is related to the
presence of large craters and basins at Mercury's surface. In particular,
Sprague
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suggested relation between the presence in Mercury's morning
of Caloris basin, a crater with a diameter equal to half-Mercury's radius,
and an apparent increase in Mercury's potassium total content. In the same
way, Potter
et al.
36
observed a three times enhancement of Mercury's total
sodium content when Caloris basin was in Mercury's early morning. Such
type of large topographic anomalies at Mercury's surface as well as smaller
craters
14
could therefore, have an important role in supplying Mercury's
exosphere in volatiles.
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4. Conclusions
Up to now, only few elements of Mercury's exosphere have been identified,
most probably representing only few percents of the total content of the
exosphere. Moreover, the influences of Mercury's magnetosphere and of
Mercury's surface composition are key points to be solved in order to fully
understand the origin, dynamic, and composition of Mercury's exosphere.
Any description of Mercury's exosphere has then to consider the numer-
ous sources of variation which are:
(i) The changes along Mercury's orbit of the different mechanisms of pro-
duction, destruction, and recycling.
(ii) The role of the solar wind and photon flux and their short and long-
times variations.
(iii) The changes of Mercury's magnetosphere with respect to the inter-
planetary magnetic field and also possible encounters with solar event.
(iv) The role of large and small topographic structures at Mercury's
surface.
Very little has been said in this paper about the different mechanisms
of ejection from the surface thought to be at work at Mercury.
7
,
45
Actually,
the way each species is ejected from Mercury's upper surface defines their
initial energy and therefore, their spatial distribution. Spatial variations
also depend on the regions of Mercury's surface where such species are pro-
duced and the variations of the ejected flux intensity with respect to solar
conditions, to distance to the Sun and to position on Mercury's surface.
At the end, the solar radiation pressure has been suggested to model
the shape of Mercury's exosphere for some species.
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In particular, it has
