Sky and Telescope - April 2015 - 55

Contributing editor Charles A. Wood (lpod.wikispaces.com) is
coauthor of the book 21st Century Atlas of the Moon.

Charles A. Wood

Imbrium. This inner face, called the Apennine Front, is
the top of a deep fault formed when the ﬂoor of the original
Imbrium basin subsided. The fault probably slices 30 to 40
km down into the crust, curving inward with depth.
Another giant impact-generated fault scarp is visible as
the inward face of the Rupes Altai, a mountain arc that
rims the Nectaris basin and lies about 200 km southwest
of Mare Nectaris itself. Most other nearside basins are too
old to have retained their rims' fault-faced scarps.

NASA / GSFC / ARIZONA STATE UNIV.

Terraces along the inner
walls of 85-km-wide Tycho
stand out dramatically in
this mosaic from the Lunar
Reconnaissance Orbiter
Camera. These mark where
slumping occurred along
localized faults soon after
the crater formed 108 million years ago.

Stealth Faulting
An unexpected class of lunar faults show up as the low,
sinuous ridges (dorsa) common in all maria. The most
famous of these, informally named Serpentine Ridge,
runs north-south for about 350 km along the eastern edge
of Mare Serenitatis.
For a long time these features were called "wrinkle
ridges," which described how we thought they formed.
As with some mountains on Earth, geologists thought
wrinkle ridges resulted from horizontal compression that
caused folding. On Earth, the compressional force is due
to collisions between plates, and the mountain ranges
thus created are a few hundred kilometers wide and 5 to
10 km high. But the Moon's wrinkle ridges are very diﬀerent, typically only about 5 to 20 km wide and just 100 to
300 m high. With slopes of only a few degrees, they can
be easily observed only with low angles of illumination.
The mare ridges in Mare Serenitatis imply that these
ridges formed due to a combination of folding and faulting.
Imagine going back in time to watch the evolution of a justformed impact basin, 500 km across. In time, lava ﬂows
erupt from the heavily fractured ﬂoor and ﬁll the basin to
a depth of 4 or 5 km in the middle, thinning to nothing at
the outer margins. All that added mass causes the basin's

interior to sag, squeezing the now-solid pile of lava into a
depression of smaller radius. This compression perhaps
creates some folding concentric with the basin margin, but
most of the volume reduction involves low-angle faults that
thrust adjacent layers of lava over one another. Few of these
faults actually breach the surface, but the ones at shallow
depth trigger compressional folding in the topmost layers
of lava - creating wrinkle ridges.
Curiously, LRO altimetry data show that the inwardfacing sides of many mare ridges are 50 to 200 m lower
than their outer sides. This agrees with evidence from
old Apollo ground-penetrating radar scans. Intriguingly,
concentric mare ridges also occur at about the same relative distance from a basin's center as do the inner rings of
basins not inundated with lava (such as Orientale). And
we see the same thing on a smaller scale where mare
ridges mark the rims of buried impact craters, such as the
one just south of Lambert in Mare Imbrium.
So mare ridges appear to be triggered by underlying
topography of one kind or another. Keep a lookout for
these and other faulty features when you're next eyeing
the lunar landscape through your telescope. ✦

The Moon * April 2015
Montes
Apenninus

Phases

24
21

FULL MOON

April 4, 12:06 UT

Distances

Librations
April 1, 13h UT

Apogee
252,284 miles

diam. 29′ 26″

Perigee
224,330 miles

April 17, 4h UT
diam. 33′ 6″

Apogee
251,706 miles

April 29, 4h UT
diam. 29′ 30″

LAST QUARTER

April 12, 3:44 UT

April 6

NEW MOON

April 18, 18:57 UT
FIRST QUARTER

April 25, 23:55 UT
Tycho

Krasnov (crater)
April 6
Rydberg (crater)
April 11
Vashakidze (crater)
April 21
Boss (crater)
April 24

For key dates, yellow dots indicate which part of the Moon's limb is
tipped the most toward Earth by libration under favorable illumination.
S&T: DENNIS DI CICCO

Sk yandTelescope.com April 2015

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Sky and Telescope - April 2015

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