Sky and Telescope - November 2018 - 26

Exploring Mercury

those faults even today. Can BepiColombo ﬁnd evidence
for geologically recent fault motion? Can it map different
amounts or types of crustal crumpling in rocks of different
ages to discover how Mercury shrank with time?

A Hot Planet's Ice
We know that the planet closest to the Sun has deposits of
nearly pure water ice at its poles. This seems outlandish, given
how strongly the Sun beats down on Mercury's surface (up
to some 10 times more intensely than at Earth). Even more
amazing is that we ﬁrst discovered that polar ice in 1991
using radio telescopes on Earth, which detected deposits near
the pole that looked bright in radar images (S&T: Jan. 1992,
p. 35). Messenger proved that the round radar features near
the north pole all lie inside deep impact craters whose ﬂoors
and north-facing slopes never see the Sun, and whose interiors plummet to about -200°C (-330°F).
Later in the mission, Messenger scientists commanded its
camera to shoot long-exposure images of those permanently
shadowed crater ﬂoors. Using light reﬂected off the sunlit
south-facing rims, the spacecraft was able to reveal the radarbright deposits. Unexpectedly, these images showed some of
them to be as black as coal, while others are bright. The interpretation: The cold, shadowed craters contain ice deposits
that are tens of meters thick at most, mantled with a varying
amount of dark, carbon-rich material.
How does this stuff get to Mercury's poles? Water and
most carbon-containing organic molecules are both highly
volatile at Mercury - once delivered by asteroids or comets,
they don't sit around on the hot surface; instead, they turn
into gas and ﬂoat off. Gravity might bring them back down,
but they can't remain stuck to the planet's hot surface. However, any volatile molecule ﬂoating in Mercury's thin atmosphere that happens to touch down on the incredibly cold
surface of a permanently shadowed ﬂoor can become trapped
there forever, unless it's disturbed by an impact.

THERE'S ICE IN THEM CRATERS Radar
observations made from Arecibo Observatory
reveal bright deposits (tinted yellow) in several
shadowed craters at Mercury's north pole, which
have been revealed by long-exposure Messenger
images. Scientists suspect that these deposits
consist of fairly pure water ice.

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

The Moon also has polar ice deposits, but they are patchy
and impure. So why are these two worlds' polar ices so different? Could it be that Mercury's ice looks cleaner because it's
fresher, delivered recently in a single impact event?
If that's the case, then the impact should have left a crater
we can see. You can tell which craters are relatively young on
Mercury (or the Moon, for that matter) by the presence of
bright rays. And there is no more impressive rayed crater in
the solar system than Mercury's Hokusai, a 114-km-wide scar
near 60°N with bright rays stretching across the face of the
planet, making Mercury look like a gray watermelon.
A team led by Carolyn Ernst (Johns Hopkins University Applied Physics Laboratory) has calculated what kind
of impacting body would create Hokusai. Observing its
horseshoe-shaped peak ring and the large volume of formerly molten rock that ﬁlls its ﬂoor, they calculated that a
reasonable-sized comet or asteroid impactor (25 km) traveling
a reasonable speed (less than 30 km per second) could have
produced Hokusai and easily delivered sufﬁcient water to
account for everything now at Mercury's north pole.
As for the south pole, recent Arecibo observations suggest
that radar-bright deposits there cover roughly double the area
that their northern counterparts do. Moreover, the southern
pole region is more heavily cratered. We don't know what
surprises might still hide there, but BepiColombo will get the
ﬁrst close views of this region.

Embedded in the Stellar Wind
Much of Messenger's mission focused on the environment
around the planet - particularly its magnetic ﬁelds and the
charged and neutral particles residing there. No other world
in the solar system has the intense relationship with the
Sun that Mercury does. At Venus and Earth, the solar wind's
interactions are primarily with their ionospheres, far above
the ground. But at times the solar wind can interact directly
with Mercury's rocky surface. One bizarre implication is that,

Sky and Telescope - November 2018

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