Sky and Telescope - August 2015 - 10

News Notes

MISSIONS I Messenger Ends

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ZOOM: NASA / JHU-APL / CARNEGIE INST. OF WASHINGTON; GLOBE: S&T / NASA / JHU-APL / CARNEGIE INST. OF WASHINGTON

After orbiting Mercury for four years,
NASA's Messenger spacecraft crashed
into the planet on April 30th at 19:26
Universal Time. By then the orbiter had
been operating on borrowed time for
months, its fuel tanks nearly empty after
a decade of interplanetary maneuvering.
Ultimately Mercury's gravity - coupled
with the Sun's perturbing pull - forced
the inevitable collision.
The mood in the mission control center at Johns Hopkins University's Applied
Physics Laboratory was both celebratory
and somber as team members watched
the craft's last transmissions arrive after
4,105 orbits around Mercury. In its ﬁnal

moments, Messenger (a contraction for
Mercury Surface, Space Environment,
Geochemistry, and Ranging) skimmed
over the planet's unseen side before striking an unnamed ridge located at 54.5°°
north, 210.1° east.
Launched on August 3, 2004, Messenger ﬁrst became acquainted with Mercury
during three close ﬂybys in 2008 and
2009. When the spacecraft ﬁnally settled
around the planet for keeps on March 18,
2011 (UT), it assumed an elliptical orbit
that ranged in altitude from 15,000 km
(9,300 miles) to as close as 200 km (120
miles) every 12 hours.
The nominal mission was to last only

IRON PLANET I Why Is Mercury So Dark?
One of the Messenger mission's most
unexpected results is that the rocks and
dust on Mercury's surface contain very
little iron. It's so surprising because this
planet has a huge, iron-dominated core
that takes up 85% of its diameter and more
than half of its volume (S&T: Apr. 2012,
p. 26). So geochemists expected that the
planet's surface would contain lots of iron.
This ﬁnding, curious in itself, has a
bearing on another Mercurian mystery.
The planet's surface is very dark, even
more so than the Moon's, reﬂecting
only about 7% of the sunlight striking
it. Researchers have long known that
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August 2015 sky & telescope

the Moon's surface darkens over time
because tiny meteorites pepper the lunar
dust, momentarily ﬂash-melting ironbearing silicate minerals and creating
submicroscopic bits of metallic iron.
These iron particles are what make the
lunar landscape appear dark. But given
Mercury's iron-poor surface, some other
process must be involved.
In the March 31st Nature Geoscience,
Megan Bruck Syal (Lawrence Livermore
National Laboratory) and others oﬀer a
possible cause: a veneer of carbon-rich
organic compounds derived from comets.
Syal and colleagues ﬁrst estimated that

Messenger impacted Mercury's northern hemisphere (top, outline approximates zoom) on
April 30th. The crater at upper left, Janacek, is
48 km (30 miles) across. Color indicates terrain
elevation, with the tallest regions in red.

a year, but with the spacecraft healthy
NASA managers opted to continue the
mission and, in March 2014, to lower the
periapse (close point) of each orbit to less
than 50 km. These extensions, and particularly the spacecraft's movement closer
in, paid big dividends in terms of surface
photography and geochemical assays (see
below for one example).
■ J. KELLY BEATTY

the infall of comet stuﬀ during the past
200 million years could have infused the
top layer of Mercurian dirt with 3% to
6% carbon. Then they conducted impact
simulations at the NASA Ames Vertical
Gun Range to conﬁrm that comet-borne
carbon would actually stick around, in the
form of tiny particle clusters.
The resulting surface layer would have
a bland spectrum, exactly what Messenger found. "We show that carbon acts like
a stealth darkening agent," explains team
member Peter Schultz (Brown University). "From the standpoint of spectral
analysis, it's like an invisible paint" that
has been building up for billions of years.
■ J. KELLY BEATTY

Sky and Telescope - August 2015

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