Sky and Telescope - August 2016 - 52

OBSERVING

Exploring the Moon

Twin Basins with a Twist
Track down two big basins - one obvious and the other nearly invisible.
When looking through a telescope at the Moon, you see
the cumulative eﬀect of 4½ billion years of stuﬀ happening. Some big features from long ago - for example,
Montes Apenninus, the towering mountain chain created during the Imbrium basin's excavation 3.8 billion
years ago - are still clearly visible. But other features,
even sizeable ones, have vanished from view due to subsequent volcanism and impact erosion.
Fortunately, as I described in February's issue (p. 48),
precision mapping of the pull of lunar gravity by the
twin Gravity Recovery and Interior Laboratory (GRAIL)
spacecraft can reveal subsurface evidence of hidden
landforms. In particular, impact basins produce the
largest gravity anomalies on the Moon, typically with a
central gravity "high" or mascon (mass concentration),
surrounded by a ring of gravity "lows." These appear as
blues and reds, respectively, in the map below. The high
in a basin's center results from the rise of dense mantle
material under the basin and from dense mare lavas
that partially ﬁll its ﬂoor. The low corresponds to a thick
annulus of less-dense crustal material.
That's the pattern underlying the Orientale impact
basin, which barely peeks around the Moon's western
limb. Nearby are two smaller red-around-blue gravity
signatures of basins. The northern bull's eye is associated with the Grimaldi basin, whose 220-km-wide
mascon is about the same diameter as the distinct rim
that surrounds its central plain of dark lava.

300 km

August 2016 sky & telescope

Grimaldi

CrügerSirsalis
Orientale
Humorum
NASA / LRO / GRAIL

NASA's GRAIL
mission mapped
in detail where
the lunar gravity
ﬁeld is locally
stronger (blues)
or weaker (reds)
- and its global
map reveals the
distinct bull'seye signatures
that underlie
impact basins.

It's important to note that Grimaldi is not a crater but
rather a basin (an impact site having two or more rims).
Its large (460-km-wide) and more subtle outer rim is
diﬃcult to see in images, but its red coloration makes it
stand out in a gravity map. Technically, Grimaldi's lavacovered ﬂoor could have been named Mare Grimaldis.
Immediately to its south is another gravity bull's eye
of nearly the same size. It too must mark the site of a
basin - and yet there's no obvious evidence for one on
the surface. Lunar geologists call this hidden feature the
Crüger-Sirsalis basin (CSB), named for two craters that
it spans. Crüger, 46 km across, is the lava-ﬂoored crater
near the basin's center, and Sirsalis is the brighter of
two overlapping craters to the northeast. Based on their
high-low gravity signature, CSB's phantom rims have
diameters of about 240 and 400 km.
CSB's center is a patch of relatively ﬂat terrain with
few large craters. Usually the most visible parts of a
basin are its rims, but none exist for CSB. Their disappearance is probably related to the location of the basin
near the younger and larger Orientale basin. The latter's
ejecta swept across this area, knocking down and covering preexisting terrain with torrents of debris. Grimaldi
basin is about the same distance from Orientale, and yet
more of its rim structure remains. Perhaps CSB predates
Grimaldi, such that its surface features had already been
degraded when the Orientale impact occurred. Interestingly, the centers of CSB and Grimaldi are both 1 to 1½ km

Sky and Telescope - August 2016

Table of Contents for the Digital Edition of Sky and Telescope - August 2016