Mercury 135
lide with the terrestrial planets during their final stages of
growth. The final growth and giant impacts occur within the
first 50 million years of solar system history. Such large im-
pacts may have resulted in certain unusual characteristics
of the terrestrial planets, such as the slow retrograde rota-
tion of Venus, the origin of the Moon, the martian crustal
dichotomy, and Mercury’s large iron core.
In computer simulations where proto-Mercury was 2.25
times the present mass of Mercury with an uncompressed
density of about 4000 kg/m^3 , nearly central collisions of
large projectiles with iron cores impacting at 20 km/s, or
noncentral collisions at 35 km/s resulted in a large silicate
loss and little iron loss (Fig. 22). In the former case, al-
though a large portion of Mercury’s iron core is lost, an
equally large part of the impactor’s iron core is retained re-
sulting in about the original core size. At Mercury’spresent
distance from the Sun, the ejected material reaccretes back
onto Mercury if the fragment sizes of the ejected material
are greater than a few centimeters. However, if the ejected
material is in the vapor phase or fine-grained (≤1 cm), then
it will be drawn into the Sun by thePoynting–Robertson
effectin a time shorter than the expected collision time
with Mercury (about 10^6 years). The proportion of fine-
grained to large-grained material ejected from such an im-
pact is uncertain. Therefore, it is not known if a large impact
at Mercury’s present distance could exclude enough man-
tle material to account for its large iron core. However,
the disruption event need not have occurred at Mercury’s
present distance from the Sun. It could have occurred at
a much greater distance (e.g.,>0.8 AU; Fig. 20). In this
case the ejected mantle material would be mostly swept
up by the larger terrestrial planets, particularly Earth and
Venus.
8. TheMessengerMission
Mercury is the least known of all the terrestrial planets, but it
is probably the only planet that holds the key to understand-
ing details of the origin and evolution of all these bodies.
Because only half of the planet has been imaged at relatively
low resolution, and because of the poor characterization of
its magnetic field and almost complete ignorance of its sil-
icate composition and variation across the surface, there is
little hope of deciding between competing hypotheses of
its origin and evolution until more detailed information is
obtained. Fortunately, help in on the way.
The spacecraftMESSENGERis now on its way to orbit
Mercury. This mission is one of NASA’s Discovery series of
planetary exploration missions.MESSENGERis managed
by the Applied Physics Laboratory of Johns Hopkins Uni-
versity in Maryland, and the Carnegie Institution of Wash-
ington, D.C.
On August 3, 2004, theMESSENGERspacecraft was
launched from Cape Canaveral, Florida, to explore Mercury
for the first time in over 30 years (Fig. 23). After the Earth
flyby that took place in August 2005, it will make two flybys
of Venus (October 2006 and June 2007) and three flybys of
Mercury (January and October 2008, and September 2009)
before it is inserted into Mercury orbit in March 2011. It will
take 7 years to put the spacecraft in orbit around Mercury
because the spacecraft must make six planetary encounters
to slow it enough to put it in orbit with a conventional retro-
rocket. A direct flight to Mercury would get the spacecraft
there in about 4 months, just likeMariner 10.However,
it would be traveling at such a high speed at Mercury en-
counter that it would take the equivalent of a launch rocket
to put it in orbit. That is the reasonMariner 10could not
be captured into orbit around Mercury.
There are seven main objectives of the mission, all
of which are important to understanding the origin and
evolution of Mercury and the inner planets. One is to deter-
mine the nature of the polar deposits including their com-
position. Another objective is to determine the properties
of Mercury’s core including its diameter and the thickness
of its outer fluid core. This is accomplished by accurately
measuring Mercury’s libration amplitude from the laser al-
timeter and radio science experiments. A third objective is
to determine variations in the structure of the lithosphere
and whether or not convection is currently taking place.
A fourth objective is to determine the nature of the mag-
netic field and to confirm whether it is a dipole. There are
several instruments to study the chemical and mineralog-
ical composition of the crust that should place constraints
on Mercury’s origin and, we hope, help us decide among
the three competing hypotheses. Also these data will be
extremely useful to help us decipher Mercury’s geology.
The geologic evolution of Mercury will be addressed by
the dual camera system that will image the entire surface
at high resolution and at a variety of wavelengths. Finally,
the exosphere will be studied to determine its composition
and how it interacts with the magnetosphere and
surface.
There are eight science experiments on board the space-
craft (Fig. 24). They are (1) a dual imaging system, (2) a
gamma-ray and neutron spectrometer, (3) a magnetome-
ter, (4) a laser altimeter, (5) atmospheric (0.155–0.6μm)
and surface (0.3–1.45μm) spectrometers, (6) an energetic
particle and plasma spectrometer, (7) an X-ray spectrom-
eter, and (8) a radio science experiment that uses the
telecommunication system. These instruments will be used
to accomplish the objectives discussed previously. They are
listed in Table 2 together with the measurements they will
make.
MESSENGERwill be placed in an elliptical orbit with
a 200-km periapse altitude located at about 60◦N latitude
(Fig. 25). The orbit has a 12-hour period when data will
be collected and read out. The spacecraft will also col-
lect valuable data on its three flybys of Mercury prior
to orbit insertion.MESSENGERshould provide the data