Geoscience Reference
In-Depth Information
There are only two published works providing some estimates of the
abundances of other non-identified elements.
19
,
20
Only a measurement of Mercury's electron density has been performed
by Mariner-10 radio occultation experiment.
21
These authors reported
an upper limit for the electron density of 10
3
electrons/cm
3
.Thisis
probably why, up to now, very few studies on the ion exosphere have
been published.
22
-
26
However, the ion exosphere has been widely quoted
to play.
(i) A crucial role for the neutral exosphere first of all as the product of its
ionization
11
but also as agent for sputtering of the surface.
27
Planetary
ions are not only at the origin of one mechanism producing the neu-
tral exosphere but also a significant source of enrichment of the upper
surface from which the neutral exosphere is formed.
15
,
28
(ii) Ions are also seen as a larger source of loss for Mercury's exosphere
through solar wind entrainment rather than direct neutral loss.
The ion exosphere is fully unknown but simple calculations based on
photo ionization of the neutral component can be done to infer what could
be the main ion species in Mercury's exosphere.
24
,
29
3. Sources of Variation of Mercury's Exosphere Content
The exospheric column and surface densities given in the previous section
should be also considered with regard to all the sources of temporal and
spatial variations of Mercury's exosphere that is as follows:
(i) Mercury's orbit around the Sun induces changes in the average solar
flux by a factor 2.3 but also an irregular apparent motion of the Sun
as seen from Mercury,
(ii) Short and long-term variations of the solar wind should significantly
change Mercury's magnetosphere, and as a consequence its exosphere,
through changes of solar wind sputtering rate. Coronal mass ejection or
flare events could compress Mercury's magnetosphere up to its surface
or open large regions to the solar wind. Magnetospheric recycling of
planetary ions could also significantly influence Mercury's exospheric
content and spatial distribution,
(iii) Mercury's upper surface content in volatiles is most probably non-
uniform due to the preferential migration of the volatiles into regions
less submitted to sputtering, desorption or vaporization (like nightside
