Sky and Telescope - March 2018 - 49

this year and slips away before month's end. So don't bother
marking the calendar, just grab your binos and head outside
at the ﬁrst opportunity.
The closest planet to the Sun, Mercury always hews very
closely to the ecliptic, the path our star appears to follow
across the sky during the year. At this time of year, when
we're approaching the spring equinox, the ecliptic is tipped up
at a steep angle in relation to the western horizon. This sharp
slope creates ideal conditions for observing Mercury even
when its angular separation from the Sun isn't exceptionally large. On March 15th Mercury reaches a greatest eastern
elongation of 18° - a fair distance, but short of the maximum possible 28°.
Mercury shines brightest in the fortnight leading up to
greatest elongation: It's a lustrous -1.3 on the 1st and dimmer
-0.4 on the 15th. However, it's highest between the 12th and
15th, when it stands at 12° about 30 minutes after sunset.
Under clear, transparent skies, Mercury should be visible as
deep into the calendar as March 25th, but you've a much better chance at ﬁnding it on the earlier dates. If you're having
trouble spotting it, use Venus, beaming at magnitude -3.9, as
a guide. Mercury begins this apparition lower right of Venus
before climbing above and right of its brilliant neighbor. If
you can keep Mercury in sight after the 15th, you'll see it sink
toward the horizon once more.

p London-based astrophotographer Roger Hutchinson captured this
image of Mercury during the September 2017 apparition with his 11-inch
Schmidt-Cassegrain with a 685 nm IR pass filter.

The god of thieves doesn't look like much in binoculars,
but even moderately sized telescopes can reveal its phases.
On the 1st Mercury's phase is fat and gibbous, showing a
face 93% lit, but it rapidly thins to a slender crescent. On the
15th, the planet is only 47% lit, and by the time Mercury ﬂees
the night sky to head toward morning, it's only 10% lit. Track
Mercury's motion and phases for a few months and you'll
get a good sense of the innermost planet's quick, tight orbit
around the Sun.

CR ATER: N ASA / JOHNS HOPK INS UNIV ERSIT Y A PPLIED PH YSICS L A BOR ATORY / CA R NEGIE INSTIT U TION OF WASHINGTON

A Dark Story
Mercury might appear as a bright light in
your binoculars, but the planet turns out to
be much darker than it should be based on
its elemental composition. Mercury reflects
much less sunlight than does the Moon,
whose shine is amplified by iron-rich minerals. One explanation for Mercury's low
reflectance may be the presence of an unusual amount of carbon at its surface. But
how did this carbon find its way to Mercury's uppermost layers? In 2016, scientists from Johns Hopkins University Applied
Physics Laboratory, Carnegie Institution of
Washington, and Columbia University drew
on data returned from NASA's Messenger
spacecraft to offer a new explanation for its
presence. The carbon probably originated
in the graphite crust of an early magma
ocean. Fragments of the ancient layer
surfaced during comet impacts and mixed
with the overlying volcanic materials. The
end result of this cataclysmic process was
the darkening of Mercury's topmost layer.

p This enhanced color view of Mercury's Derain Crater highlights the different
types of rocks in the volcanic basin. The dark blue areas show the locations of
low-reflectance material that may have originated in an ancient magma ocean.
The red and orange areas represent brighter, newer regions of volcanic fill.

sk yandtelescope.com * M A RCH 2 018

http://www.skyandtelescope.com

Sky and Telescope - March 2018

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