Intoorbit
Long journey MESSENGER swung by Earth
and Venus before passing Mercury three times
and settling into orbit around the ;rst rock from
the sun (right).
August 2004
Launch from
Cape Canaveral,
Fla.
August 2005
Earth ;yby
October 2006
First
Venus ;yby
MESSENGER
pays a yearlong
visit to Mercury
By Ron Cowen
Hidden by the sun’s glare and ismissed as a heavily cratered rock no different than Earth’s moon, Mercury has for decades
been relegated to the back burner of planetary research. But studies of the planet
closest to the sun have now ignited.
On March 17, NASA’s MESSENGER
spacecraft became the first probe to enter
orbit around the metal-rich body (SN
Online: 3/17/11). MESSENGER — short
for Mercury Surface, Space Environment, Geochemistry and Ranging—
completes a lap every 12 hours, traveling
an oval path that swoops close to the planet’s north polar region and keeps a greater
distance from the southern pole.
A yearlong effort to understand the
planet’s volcanism, core, magnetic field
and other features got under way April 4,
when the craft’s suite of seven instruments began regularly beaming data to
Earth. With MESSENGER gleaning new
information, researchers will attempt
to solve a number of unanswered questions about Mercury, many brought to
light when the craft flew by the planet
in 2008 and 2009.
These earlier encounters with Mercury revealed that volcanic upheaval
played a major and perhaps dominant
role in sculpting the planet’s surface.
During MESSENGER’s yearlong mis-
sion, gamma-ray and X-ray detectors
will examine the composition of surface
rocks to look for signs that volatile gases
could in fact have driven the volcanism.
“We should get relatively unam-
biguous measurements of the atomic
composition of surface rocks and their
variation from place to place,” says Bill
McKinnon of Washington University
in St. Louis. “You can’t claim to know a
planet’s history if you don’t know what
the planet’s rocks are made of.”
Depending on what the craft finds, the-
orists may have to invoke nontraditional
volatile compounds as possible explo-
sive power sources or consider models
in which comets or asteroids delivered
volatiles after Mercury
had begun cooling.
MESSENGER is getting
a detailed look at
cratered regions near
Mercury’s two poles.
That finding was a surprise to scientists
who had thought that the solar system’s
tiniest planet wouldn’t have retained
enough heat to drive widespread volcanic activity.
The first hints of volcanism came in
the 1970s when Mariner 10, the only
other spacecraft to fly past Mercury, cap-
tured images of smooth plains between
heavily cratered regions. But it took
Explosive volcanism
on Mercury is a puzzle
because the planet’s hot
birth should have imme-
diately driven away compounds that
easily vaporize, says MESSENGER lead
scientist Sean Solomon of the Carnegie
Institution for Science in Washington,
D. C. Any remaining volatile compounds
ought to have exited the planet early
in its history, when scientists believe a
massive body collided with Mercury.
Hidden within
Studying surface rocks
may also offer clues
about Mercury’s unusu-
ally high density, another
of the planet’s strange
properties. This density,
first deduced in the early
1950s from the planet’s
gravitational influence
on other planets, sug-
gests that Mercury has an iron core that
accounts for some 60 percent of its mass,
says Solomon. In contrast, the metallic
cores of Earth, Mars and most likely
Venus make up only about 30 percent of
the mass of those planets.
Explanations vary, from a collision that stripped away lightweight
silicates in the planet’s outer shell to
an abundance of iron in the region of
the planet-forming disk from which
