Sky and Telescope - January 2018 - 53

SK E TCHES: SPLEND OUR OF THE HE AVENS (PHILLIPS, 1925), L A PL ANE TE MER CURY (ANTONIADI, 1934);
HOK USAI CR ATER: N ASA / JHU -A PL

ers had been deceived: the planet's true
rotation period is only 58.65 days -
exactly two-thirds the period of revolution and an example of the dynamic
process called spin-orbit coupling. The
"twilight zone" was gone forever, as was
the basis for believing in Mercurian
clouds. Any bright or dark patches seen
on this relentlessly Sun-baked little
world had to be rocks and soil.
Schröter's enormous mountain was
also relegated to myth - but the blunting he saw was no ﬁgment. As modern
hermophiles know, at times the southern cusp does appear markedly darker
than its northern counterpart. The
cusp's dimming is sometimes seen at
greatest eastern elongation, sometimes
at a western elongation, or it might not
be evident at all.
This pattern would not be the case if
either Schröter's 24-hour spin or Schiaparelli's 88-day value had been correct.
However, it does ﬁt the actual 58.65-day
rotation period. The spin-orbit coupling
means that Mercury completes exactly
1½ rotations each time it orbits the
Sun. This presents us with diametrically
opposed longitude regions at consecutive
eastern or western elongations, causing
different features to drift into view over
time. Depending on the alignment of
darker or lighter albedo features, a cusp
might appear blunted - or not.
Meanwhile, many observers over the
years have noted bright polar regions
- especially near the northern cusp.
Occasionally these caps were suspected
of being snow deposits - not so farfetched in the era when Mercury was
believed to have an atmosphere. In
recent years, data from NASA's Messenger orbiter (short for Mercury Surface,
Space Environment, Geochemistry,
and Ranging) have shown the kernel of
truth in these ﬁndings.
It turns out that light and dark
features are asymmetrically distributed
across Mercury's face. In the north is a
broad, bright belt of relatively smooth
volcanic plains, whose albedo, or reﬂectivity, averages about 0.20 - compared
to less than 0.10 for most of the rest
of the planet, which is dominated by
older, more rugged terrain. (The Moon's

Hokusai

p NASA's Messenger spacecraft revealed all of Mercury in high resolution. This view shows the
impressive pattern of bright rays radiating from Hokusai, a 95-km-wide crater.

surface looks just the opposite; its lavacovered maria appear much darker than
its ancient highlands.)
The brightness of Mercury's broad
northern belt is particularly enhanced
in an area of clustered rayed craters near
the classical albedo feature Solitudo Aphrodites (280°W, 30°N), and in an area,
once known as "Apollonia," centered on
the bright-rayed crater Hokusai (18°W,
58°N). The albedos of rayed craters on
Mercury are high, 0.36 to 0.40. Consequently, at the low resolution we see telescopically, these bright areas can create
the impression of a bright polar cap.
The terrain around the southern pole
is somewhat duskier than Mercury's
average surface, but Schröter's blunted

Left: During intensive observations in
1800-01, Johann Schröter found that the
southern cusp of Mercury (at bottom)
occasionally appeared blunted.
Right: A sketch of Mercury by Eugène M.
Antoniadi from the late 1920s.

southern horn must have been, in part,
an effect of contrast with the brighter
northern one. It's not so much the
southern horn is blunted as that the
northern one protrudes.
(Incidentally, we do now know
deﬁnitively, from ground-based radar
and Messenger observations, that water
ice lies inside certain perpetually shadowed craters near the poles - such as
Prokoﬁev and Kandinsky in the north,
and Chao Meng-Fu in the south. The
source of the ice, though still debated,
is probably from comets and meteoroids
containing hydrated minerals. It has
nothing to do, of course, with the "polar
caps" reported by visual observers.)
At this month's favorable morning
apparition of Mercury, visual observers
and CCD imagers are encouraged to
sleuth out the north "polar cap" - or,
equivalently, Schröter's blunted southern cusp. The planet will rise at least 1½
hours before the Sun from the last week
of December until about January 10th.
We now know a great deal about the
surface of this fascinating little world,
but the impressions of a morning's vigil
at the telescope still have a place: They
recall a more innocent and fanciful era,
when 11-mile-high mountains, clouds,
and even snowﬁelds seemed plausible -
and no surmise was too strange to lie
outside the realm of possibility.
¢ WILLIAM SHEEHAN is a psychiatrist
based in Flagstaff, Arizona, where he
dreams about the planets. His new book,
Mercury, will be published in 2018.
sk yandtelescope.com * JA N UA RY 2 018

http://www.skyandtelescope.com/

Sky and Telescope - January 2018

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