Geoscience Reference
In-Depth Information
above the upper atmosphere near he equator and the two (ascending and
descending) nodal points to be located outside the main rings.
An advantage of the equatorial orbits is that Titan and the icy satellites
could be targeted. Microwave sounding of Titan's atmosphere must be very
interesting. A microwave spectrometer experiment was in fact proposed
for the Cassini mission but was not accepted. With such an instrument,
the global wind field and meteorology of Titan would be studied in detail.
Furthermore, the emission and expansion of water and other vapor gas from
the hot south pole of Enceladus can also be probed. It would probably not
come as a surprise if one day the cometary-like Enceladus is to become the
center-piece of the next mission to the Saturnian system. In passing, we
note that the Rosetta spacecraft to comet 67P/Churyumov-Gerasimenko
carries a microwave instrument (MIRO) onboard just for the very purpose
of detailed investigation of the gas and dust emission from active regions of
the cometary surface. From this point of view, a microwave spectrometer
will be an important instrument for future investigations of Enceladus and
other Saturnian icy satellites.
2.3.
The Phoebe rover or penetrators
On the way in towards Saturn before Saturn Orbit Insertion (SOI), the
camera system on the Cassini spacecraft obtained spectacular images of
the outer icy satellite, Phoebe (see Fig. 4) which is the largest member of
the outer Saturnian satellites in irregular orbits. Because of its retrograde
motion — with orbital inclination
i
175
◦
— Phoebe is generally believed
to be of capture origin, i.e., not a part of the family of regular icy satellites
like Titan, Rhea, Dione, Tethys, and Enceladus. The fundamental ques-
tion is where has it come from. One possibility is that Phoebe might have
been a Kuiper belt object in transneptunian orbit before being trapped
into the Saturnian system. From a dynamical point of view, capture from
the residual population of planetoids in the vicinity of the Saturnian accre-
tion zone would have been more probable. Either way, Phoebe represents
the primordial building blocks of the outer planets holding the key to the
solar system formation. Detailed analysis of the surface samples might pro-
vide very interesting information on the origin and evolution of this unique
object. Such measurements should be done in as wide an area as possible
by a mobile vehicle. Could the airbag hard-landing techniques for Martian
rovers be adapted for a Phoebe Rover? If possible, we should consider the
possibility of ejecting a daughter vehicle carrying the Rover towards Phoebe
∼
