Sky and Telescope - December 2015 - 49

Charles A. Wood

Visit Contributing Editor Charles A. Wood's Lunar Picture of the Day website at www2.lpod.org.

Rup

es L
ieb

Mersenius D

Liebig

NASA / LUNAR RECONNAISSANCE ORBITER

Other basins have such extension cracks, but few are
as large and dramatic as these. Also look for a family
of three or four mare ridges along the eastern interior
of the mare that are best seen with sunrise illumination (when the Moon's phase is waxing gibbous). These
formed as the basin's center subsided, which forced the
solidiﬁed mare ﬂows to ﬁt into a smaller volume. The
kilometer-thick slab accommodated this squeezing by
fracturing multiple times. Some parts of the mare slid
up over adjacent parts, creating the mare ridges.
Use high magniﬁcation to observe that the western
edge of the mare, just inside the Percy Mountains, is
sharply bounded by a fault that separates the smooth
lava from older rugged terrain to the west. Farther south,
along the mare's western margin and inward from the
39-km-wide crater Liebig, the fault boundary becomes
detectable again - at least near local sunrise or sunset,
when the scarp's higher western face is starkly illuminated or shadowed, respectively. Rupes Liebig truly is a
fault, as dramatically illustrated by an older, 10-km-wide
crater just east of Liebig that's been sliced in half. Its
eastern side dropped down and became smothered by
mare lavas, making its eastern rim completely invisible.
Just to its south, look for a younger, 8-km crater gouged
out right on the fault scarp. Topographic data from the
Lunar Reconnaissance Orbiter (LRO) show that the eastern rim of this crater is 500 m lower than the western
rim. That's not at all obvious telescopically.
When you observe Humorum near full Moon,
note that the area around Rimae Doppelmayer in the
southwestern corner is much darker than most of Mare
Humorum. Spectral studies conﬁrm that this dark
material consists of pyroclastic glass beads that must

Liebig F

Rimae
Doppelmayer

25 km

This close-up of western Mare Humorum shows how the long fracture Rupes
Liebig has sliced through an unnamed crater (arrow). Its right (eastern) half
dropped down and was later covered with lava ﬂows.

have been tossed out of a volcanic vent. But no vents
are visible, even in high-resolution LRO images, so
the explosive activity that laid down the dark deposits
probably came from eruptions along the rille's 300-km
length. In fact, the rille runs down the middle of a wide
ridge, 7 to 8 km across, that rises up to 100 m above the
surrounding mare. This is probably the pile of pyroclastic material that accumulated around the long eruption
ﬁssure. Sadly, we're about 3½ billion years too late to
witness the ﬁreworks! ✦

The Moon * December 2015

21
24

Phases

Distances

LAST QUARTER

Apogee

December 5, 15h UT

251,531 miles

diam. 29′ 46″

December 11, 10:29 UT

Perigee

December 23, 20h UT

FIRST QUARTER

228,924 miles

diam. 32′ 18″

NASA / LRO

December 3, 7:40 UT
NEW MOON

December 18, 15:14 UT
FULL MOON

December 25, 11:11 UT
MARE
HUMORUM

December 7

For key dates, yellow dots indicate which part of the Moon's
limb is tipped the most toward
Earth by libration under favorable illumination.

Librations
Bailly (crater)

December 7

Byrd (crater)

December 21

Mare Humboldtianum

December 24

Sk yandTelescope.com December 2015

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Sky and Telescope - December 2015

Table of Contents for the Digital Edition of Sky and Telescope - December 2015