Geoscience Reference
In-Depth Information
projects are considered to be the masterpieces in space and planetary sci-
ence. While the Huygens probe provided basic information on the compo-
sition and origin of Titan's atmosphere, the scientific measurements by the
Galileo probe has shown that the noble gases and carbon, nitrogen, and
sulfur are enriched in the Jovian atmosphere by a factor of about two to
four in comparison to the solar abundances. Such near-uniform chemical
enrichment has been interpreted in terms of the accretion of cold icy plan-
etesimals by Jupiter.
1
-
3
On the other hand, Gautier
et al.
4
advocated the
role of the selective gas trapping effect of the clathrate hydrate icy grains in
the solar nebula. Hersant
et al.
5
have further suggested that the clathrate
hydrate model could explain the non-uniform chemical enrichments in S
(
5-9) in the Saturnian atmosphere. Other key
measurements include the isotopic ratios of
14
N/
15
Nand
16
O/
18
O. If the
Saturnian
14
N/
15
N ratio is the same as the Jovian value, it would mean that
N
2
was the dominant nitrogen reservoir in the solar nebula. This shows that
in situ
measurements of the noble gas and isotopic ratios in Saturn's upper
atmosphere will be crucial in answering how the outer planets were formed.
From an engineering point of view, these scientific goals can probably be
achieved by sending the atmospheric probe to regions with pressure no more
than 20 bars thus making the SAPPORO probe design relatively simpler
than that of the Galileo Probe.
∼
12
.
5), N (
∼
2), and C (
∼
2.2.
The microwave sounder
The Cassini mission is entering its second year of scientific operations. It
is very likely that the mission will be extended beyond 2008 after the end
of the prime mission. What are the important questions still left to be
answered after such long-term monitoring study is dicult to assess at this
point. However, if we follow the pattern of the Jovian exploration that
began with the flyby observations of the Pioneers 10 and 11 spacecraft,
then the Voyagers 1 and 2 spacecraft followed by the Galileo Orbiter, we
would probably consider the new JUNO Project for a polar orbiter mission
a most logical next step. The JUNO Orbiter will carry out particles-and-
fields measurements and microwave sounding observations on polar orbits
as illustrated in Fig. 3. Such orbital configurations will provide precious
information on the polar Jovian magnetosphere and ionosphere so far miss-
ing from the Galileo mission. Of course, the polar trajectories of the Cassini
Orbiter at the end of the primary mission were cleverly designed to study
the polar regions of the Saturnian magnetosphere. But what remains to be
