Sky and Telescope - November 2018 - 24

Exploring Mercury

MMO
400 km to
12,000 km
Plasma Sheet

Solar
Wind

MPO
400 km to
1,500 km
Messenger
200 km to
15,200 km

Two space agencies are taking on the challenge together.
Their joint mission will deliver two spacecraft into orbit at
Mercury: Europe's Mercury Planetary Orbiter (MPO), and
Japan's Mercury Magnetospheric Orbiter (MMO, nicknamed
Mio). The pair will loop around Mercury in polar orbits, as
Messenger did, but with crucial differences that will give the
BepiColombo mission a much more thorough view.
Unlike Messenger's path, which took it close to the north
pole and far from the south one, MPO's orbit will be nearly
circular and close to the planet, giving its imagers and spectrometers a detailed look at all latitudes. MMO will follow a
more elongated orbit that takes it out farther than MPO to
sample more regions of the magnetic field. Both spacecraft
will always orbit in the same plane, enabling scientists to
coordinate observations.
The set of instruments that BepiColombo will bring to
bear at Mercury is broadly similar to Messenger's, only more
capable. MPO has color cameras, infrared spectrometers, a
laser altimeter, and particle detectors. MMO has a magnetometer, two instruments to study plasma in the magnetic
field, plus an exosphere imager and a dust monitor. What will
these spacecraft learn that Messenger could not?

Dark Surface, Light Elements
The most surprising discovery made by Messenger is Mercury's surface composition. It took the orbiter's gamma-ray
and neutron spectrometers until the end of the mission to
u CALORIS BASIN This mosaic of Mercury's signature, 1,500-kmwide impact structure combines enhanced-color and topographic data
to reveal differences in geologic features. Lava flows appear orange.
Several smaller, subsequent craters have punched through the surface
lava to expose what geologists term low-reflectance material (blue).
This material is likely part of the original basin floor (see close-up in
center). Based on the craters, the volcanic layer appears to be between
2½ and 3½ km thick.

24

N OV E M B E R 2 018 * SK Y & TELESCOPE

accumulate enough data for quality compositional maps.
The result: Mercury's crust is about 5% sulfur by mass. This
is roughly 100 times sulfur's abundance in Earth's crust
and was totally unexpected. The crust is also 1.4% carbon.
The abundance of these two elements directly contradicts
formerly favored ideas about how Mercury's bulk composition
could be so very metal-rich.
Sulfur and carbon are both relatively "volatile" elements
and were not expected to condense in the hottest, innermost
portion of the solar nebula as our solar system formed. Prior
to Messenger, some scientists had hypothesized that modernday Mercury is little more than the core of a preexisting
larger planet whose mantle was blasted off by a large oblique
impact, or perhaps that its lighter materials were blown away
by an early, hot Sun. But sulfur and carbon should have been
scarce in the aftermath of either scenario. Their abundance
confounded mission scientists.
Messenger did confirm ground-based studies that the surface contains little iron or titanium. These are the elements
that make the Moon's basalt-filled maria dark, and without
them, scientists had difficulty explaining why Mercury's surface is so dark. The unexpectedly high abundance of carbon
offers a clue: It might be that graphite (a crystalline form of
carbon), not iron, darkens Mercury, mixed among the silicate
rocks of its crust.
Mercury has patches of what's been dubbed low-reflectance
material that's even darker than the usual crust. It's generally exposed by impacts, which dig into the crust and exca-

ORBITS: LE A H TISCIONE / S&T; CA LORIS BASIN: N ASA /
JHU A PL / CA R NEGIE INSTIT U TION FOR SCIENCE

Magnetopause

Bow
Shock

t MESSENGER VS. BEPICOLOMBO BepiColombo's components will
follow polar orbits, as Messenger did, but they won't fly as far from the
planet and will be oriented differently to the planet's spin axis.



Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018

Contents
Sky and Telescope - November 2018 - Cover1
Sky and Telescope - November 2018 - Cover2
Sky and Telescope - November 2018 - 1
Sky and Telescope - November 2018 - Contents
Sky and Telescope - November 2018 - 3
Sky and Telescope - November 2018 - 4
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