Sky and Telescope - December 2015 - 48

OBSERVING

Exploring the Moon

Humorum in the Eyepiece
Here's a medium-size lunar basin that's packed with interesting
geologic features observable through your telescope.

ain

The dark maria are the most conspicuous features of
the Moon, but more important are the giant basins that
contain them. Each basin controls the geology of the area
around it. One easy-to-see example is the Humorum
basin, whose central depression is partially ﬁ lled by the
lava plains of Mare Humorum. This mid-size impact
basin preserves many structural features that one can
appreciate only by careful observation. So let's do so!
Younger, less-modiﬁed lunar basins (Orientale comes
to mind) are deﬁned by multiple concentric mountain
ranges. We assume that the Humorum basin had them
as well when it formed, but only a few fragments of the

Gassendi

Mersenius

Rupes
Liebig
Liebig

Loewy
MARE
HUMORUM

Hippalus
Rimae Hippalus

Palmieri

pes

Doppelmayer

Lee M

Vitello

Mare Humorum's lava ﬂows ﬁll a medium-size lunar basin that's 425 km across.
Little remains today of the multiple rings that once marked its rim.

December 2015 sky & telescope

NASA / LUNAR RECONNAISSANCE ORBITER

Kel

vin

Rimae
Doppelmayer

original rims survive. Can you see any? The basin's
best-seen rim segment appears as a short lumpy ridge
(once known as the Percy Mountains) that starts on the
west side of the big crater Gassendi, which intrudes into
Mare Humorum's northern boundary.
If you draw an imaginary circle through this range,
and then continue along other isolated mountains east
of the craters Mersenius and Liebig, you will ﬁnd that
the extrapolated circle passes along the straight Rupes
Kelvin scarp on the southwestern side of the mare. To
the northeast from there, the basin rim becomes harder
to trace, having more gaps than continuity. This circular
rim, so discontinuous that its deﬁnition depends on faith
as much as evidence, has a diameter of about 425 km.
A second basin ring, even easier to overlook, is
marked by a low mountain ridge parallel to the Percy
Mountains but about 150 km to their northwest. Individual local hills hint at this barely visible ring curving
around the western side of Mersenius, but then farther
south and east it mostly disappears. The moat between
the Percy Mountains rim and this larger ring is at lower
elevation than most of the terrain beyond, a fact made
noticeable by the occurrence of mare patches between
Liebig and Palmieri, and between Ramsden and Campanus. This is a small-scale version of the low spots of
Sinus Aestuum, Mare Vaporum, and Mare Frigoris that
occur beyond the Apennine rim of the Imbrium basin.
Looking around the shore of Mare Humorum, you'll
see that the basin ﬂoor has subsided toward the basin
center. The evidence? Note the large craters with rims
breached on their mare-facing sides. Examples are
Doppelmayer and its larger neighbor, Lee M, along the
southern extent of the mare, and Hippalus and Loewy
along the eastern shore. To the north, Gassendi's walls
also dip inward. Perhaps the basin ﬂoor had a sloped,
saucer shape after its formation, or perhaps the inward
tilt occurred as the basin ﬁ lled with lava. Whatever the
reason, these craters tilted inward and the ejecta around
them became buried under ﬂows that erupted later.

Cracking Under Pressure
The massive mare slab also sank under its own weight
and caused fracturing around its eastern edge, producing
three parallel arc-shaped rilles - the Rimae Hippalus.

Sky and Telescope - December 2015

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