Sky & Telescope - December 2018 - 53

LUN A R IM AG E: ROBERT REE V ES;
CREDIT
R A DA R: J. VIERINEN / UNIV ERSIT Y OF TRO MSØ / JRO

interpreting lunar radar images from
the Jicamarca Radio Observatory in
Peru. Impact-fractured terrain appears
bright in radar images due to multiple
reﬂections within the jumbled rock,
while coherent (solid) lava ﬂows don't
scatter radar energy as much and thus
appear dark. In the Jicamarca image,
Terra Laboris appears as dark as Mare
Frigoris. Apparently, the radar energy's
6-meter wavelength penetrated deeply
enough to detect mare material under
the Plato and Iridum ejecta. In fact, by
observing carefully near full Moon, you
can see that dark mare lavas from Mare
Frigoris embay some of this area.
The existence of lavas under Plato
helps explain several features, including its lava-ﬂooded ﬂoor, sinuous rilles
with vents near Plato that cross rugged
terrain nearby, and the rilles between
Maupertuis and Plato. These volcanic
manifestations were difﬁcult to explain
before the Vierinen team's ﬁndings.
The second piece of evidence comes
from a global map of mare ridges visible
in Lunar Reconnaissance Orbiter Camera (LROC) images compiled by Tyler
Thompson of Arizona State University
and colleagues. This data set, and many
others, is available in the Layers option
of the recently updated LROC QuickMap (quickmap.lroc.asu.edu). Since
many of Frigoris's ridges are parallel to
the mare's northern margin, compressional forces were squeezing the crust
from north and south. So Frigoris was
not formed by extension - the megaslide theory doesn't work.
The third bit of evidence comes
from a map of gravitational gradients
produced by the Gravity Recovery and
Interior Laboratory (GRAIL) spacecraft. Jeffrey Andrews-Hanna, then at
Colorado School of Mines, and many
colleagues unexpectedly discovered a
series of long, narrow gravity anomalies
- I call them "worms" - that border
a region of the lunar nearside that has
high concentrations of radioactive elements and contains most maria. One
of the best-deﬁned worms parallels the
northern shore of Mare Frigoris from
Atlas in the east all the way to Harding, considerably west of the ofﬁcial

boundary of Frigoris. (These
GRAIL data are also available on QuickMap and can
be overlain over imagery
that highlights the wrinkle
ridges, as seen on the bottom of the facing page.)
Andrews-Hanna and his
team noticed that nearly all
of the ridges that parallel the
northern shore of Frigoris
coincide with the worms.
They interpret the worms as
subsurface rifts and dikes
that once fed magma to the
surface and are now ﬁlled
with dense, solidiﬁed lava.
If so, Mare Frigoris exists because of
the underlying worms, the byproduct
of high heat concentrations, which still
has no good explanation.
So what does this new information tell us about Mare Frigoris? The
radar data show that Terra Laboris
and surrounding crater ejecta sits atop
mare lava. Does this mean that the
Frigoris ﬂows are more widespread than
we thought? Lavas in this area from
Imbrium and Frigoris are compositionally identical (iron-rich, titanium-poor)
basalts of similar age (3.4 billion years).
And the surface of Mare Imbrium slopes
downward to the north. So the Terra
Laboris lavas could have ﬂowed in from
either mare or might have erupted from
their own vents.
The western end of Mare Frigoris is
generally considered to be near Harpalus

Plato

p This deep-penetrating radar image shows
mare lava as blue. The area west of the crater
Plato (appearing as the greenish oval), previously known as Terra Laboris, is nearly invisible,
implying mare lavas underlie the bright material
we see at the telescope.

p Wrinkle ridges running parallel to the
northern border of Mare Frigoris are seen in
this image captured shortly following the third
quarter phase.

crater at about 45° W longitude. However, the continuation of the Frigoris
worm and its overlying mare ridges to
about 75° W encourages the speculation that Mare Frigoris, as deﬁned by
the lava sources that formed it, actually extends that far to the west. If so,
Sinus Roris is simply a patch of darker,
high-titanium lava that is part of Mare
Frigoris, whereas Frigoris and the suggested western extension are all lighter,
low-titanium mare lavas. This means
that the northern limit of Oceanus
Procellarum is near the crater Rümker.
Indeed, the lavas just east of Rümker are
rich in titanium and are less than 2 billion years old, like many of the Oceanus
Procellarum ﬂows farther south.
Frigoris is often the forgotten mare
because it lacks the concentric fractures, sinuous rilles, and magniﬁcent
craters common to other major maria.
But next time you observe it, look for
the subtle wrinkle ridges that mark its
buried worms, and the suggested extensions under Terra Laboris and west of
Sinus Roris.
¢ Contributing Editor CHUCK WOOD
now has an asteroid to worry about:
363115 Chuckwood (formerly 2001
FW224) is a 16th-magnitude dark and
icy mountain cruising around the Sun at
an average distance of 3.1 a.u.

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p e .*c o D
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http://quickmap.lroc.asu.edu http://www.skyandtelescope.com

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018