Geoscience Reference
In-Depth Information
most debated and recent questions and the key information that may be
derived from ground-based observations, on Mercury's exosphere — magne-
tosphere — upper surface system. Such information is particularly needed
in preparation of the forthcoming Messenger and BepiColombo missions.
In Sec. 2, we give a brief overview of the known neutral and ion compo-
sitions of Mercury's exosphere. Section 3 describes some important sources
of variations of the exospheric abundance related to the coupling between
Mercury's exosphere, its upper surface, and its magnetosphere. Section 4
concludes this paper.
2. Exospheric Composition
The only few elements that Mariner-10 provided on Mercury's neutral exo-
sphere are:
(i) the measurements of H and He with surface density of 250 H/cm
3
and
6
10
3
He/cm
3
,
11
(ii) an identification of O column density with suggested value for its sur-
face density of 4
.
4
×
10
4
O/cm
3
,
12
,
13
(iii) an upper limit for the total content of the exosphere at the subso-
lar point: 10
7
×
neutral particles/cm
3
from the solar occultation UV
measurement.
12
Ground-based observations identified:
(i) Na atoms with zenith sub-solar column density of 10
11
Na/cm
2
,
4
,
14
(ii) K atoms with column density of 10
9
K/cm
2
,
4
,
15
,
16
(iii) Ca atoms with column density of 10
8
Ca/cm
2
.
17
Only upper limits are known for other elements from Mariner-10 solar
occultation UV experiment: at the surface density 1
.
4
10
7
H
2
/cm
3
,2
.
5
×
×
10
7
O
2
/cm
3
,2
.
3
10
7
N
2
/cm
3
,1
.
5
10
7
H
2
O/cm
3
,1
.
6
10
7
CO
2
/cm
3
and
×
×
×
10
7
Ar/cm
3
.
12
Hunten
et al.
11
suggested that densities of 10
4
H
2
/cm
3
,
1H
2
O/cm
3
,and1CH
4
/cm
3
are more realistic for these species (for H
2
O
and CH
4
in particular because of a short-lifetime against photoionization).
At the Moon, which is the best analog of Mercury's exosphere, O
2
has
never been measured, N
2
has been measured with 8
3
.
1
×
10
2
N
2
/cm
3
,CO
2
×
with 10
3
CO
2
/cm
3
10
4
to 10
5
Ar/cm
3
.
18
Upper imitsattheMoonforO,N,C,Fe,andS
below 10
3
particles/cm
3
,SiandAlbelow10
2
particles/cm
3
have also been
reported.
18
(the same density as for CO), and Ar with 4
×
