Sky & Telescope - December 2019 - 53

Lavoisier H, just to the east of
Lavoisier is 29 km across and has a
400-m-high domed ﬂoor cut by fractures and rilles. The center of the crater
ﬂoor is higher than the rim facing Oceanus Procellarum.
The last concentric crater in the
group is Lavoisier B, a relatively
normal crater at ﬁrst glance - at 25
km it's almost a larger morphological
twin of 16-km-wide Bessel - but its
western side is wreathed in concentric
rilles. Located right between the two
main craters, Lavoisier B is distinctly
visible because its eastern rim is bright
when the lunar phase is waning - even
appearing as a complete bright ring at
high Sun illumination.
This group of ﬂoor-fractured craters
might seem exceptional, but such craters
occur frequently along the western
edge of Oceanus Procellarum, from
Damoiseau at 4°S to Repsold at 51°N.
Elsewhere on the Moon, ﬂoor-fractured
craters almost always occur near impactbasin margins, where deep crustal
fractures provide conduits for magma to
rise and push crater ﬂoors up, creating
concentric ridges and rilles, and often
leaking onto the surface as lava ﬂows
and pyroclastic deposits.

This image captured August 6,
2010, just a few days before new
Moon, hints at the rich features
that await patient observers and
imagers with moderately largeaperture telescopes.

Oceanus Procellarum itself does not
ﬁll a recognized impact basin. However, in 1981 Ewen Whitaker proposed
a controversial basin boundary deﬁned
by Procellarum's semi-circular western
shoreline (S&T: Sep. 2005, p. 63). This
putative basin would be huge - 3,200
km wide - underlying Oceanus Procellarum, Mare Imbrium, Mare Nubium,
and Mare Serenitatis. Its existence has
been neither convincingly conﬁrmed nor
refuted, though most lunar scientists
don't support it. However, the occur-

q Most craters surrounding Lavoisier display fractured, mare-flooded floors.

von Braun

LRO OV ERHE A D: N ASA / GSFC / A RIZON A STATE UNIV ERSIT Y;
CREDIT
LIMB PHOTO: Y URI GORYACHKO

Lavoisier

rence of so many ﬂoor-fractured craters
along Procellarum's shoreline adds
another morphologic piece of evidence
consistent with a long arc of deep
fractures, as would be expected under a
basin rim. Additionally, recently discovered long gravity highs (S&T: Dec. 2018,
p. 52) coincide with Whitaker's proposed
Procellarum edges, and the thoriumrich, radioactive Procellarum KREEP Terrane (S&T: Oct. 2018, p. 52), the source
of most lunar volcanism, lies within it.
For the terrestrial observer, the
Lavoisier group is hard to study even
though it is theoretically visible every
lunation from full to almost new
Moon. Yet only the dark annular ring
of Lavoisier and the dark inner ﬂoor
of Lavoisier E appear to be visible from
Earth during average libration angles.
Perhaps a lunar-imaging pro with
a large scope and 5° to 8° of favorable libration can begin to reveal this
neglected area of the Moon.
Oh, the answer to the question at
the beginning of this article is "no." In
addition to Lavoisier, the astronomer
Jean Sylvain Bailly also lost his head
during the French Revolution's Reign of
Terror and is honored with a large crater
bearing his name on the southeastern
limb. At least they have an eternal
home on the Moon.
¢ Contributing Editor CHUCK WOOD
often explores the lunar limb hunting for
overlooked treasures.

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Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019