S
ETTING OUT to explore the Moon
with my telescope, I often gravitate
toward fairly obscure targets and
those that are sometimes even a little diffi-
cult to view. Although it’s not terribly well
known, Mare Australe is actually one of the
most interesting features on the lunar sur-
face. Most of this “southern sea” lies out of
sight, beyond the Moon’s southeastern limb
(remembering that east and west on the
Moon are the opposite of sky directions).
However, when libration tips the southeast-
ern limb Earthward, you can make out a
little more than one-third of the total mare.
Unique among the lunar seas, Mare Aus-
trale isn’t a single expanse of lava; rather, it’s
a collection of irregular patches and some
200 lava-filled craters. Together, these bits
and pieces cover a circular region roughly
900 kilometres in diameter. The formation
is also unusual for what it lacks—there’s no
sign of the expected mountainous rings or
evidence of Australe ejecta scarring the sur-
rounding area.So what happened here? Australe’s origin
appears to follow the typical basin-forma-
tion sequence. First, a tremendously pow-
erful impact excavated an enormous cavity.
Next, ongoing bombardment by asteroids,
comets and meteorites produced numerous
large craters (and many more smaller ones)
on the basin’s floor. Finally, on at least four
occasions, lava welled up from below to fill
the craters within Australe and flood the
gaps between them. Why, then, does the
resulting mare look so different? One pos-
sibility is that Australe is extremely ancient.
If so, it means there was plenty of time for
a steady rain of powerful impacts to erode
away most of the basin’s defining features.
Unlike relatively young basins (such as
nearby Nectaris), in which lavas fill a single
large, bowl-shaped depression, the lavas that
rose from beneath Australe oozed out onto
a badly battered landscape.
To begin a tour of Australe, first locate
the magnificent 188-kilometre-wide crater
Petavius, situated at the south end of MareFecunditatis. From Petavius, scan toward
the limb and slightly southward until
you reach a bunch of dark grey circular
patches. The biggest of these is Lyot, a cra -
ter that spans 132 kilometres and dis plays
a smooth, marelike floor. Despite its size,
Lyot is almost lost in its surroundings. More
distinctive is Oken, located just north of
Lyot. Although it’s less than half the size of
its neighbour, Oken has a clearly defined
rim measuring 73 kilometres across. But
instead of hopping from crater to crater, the
best way to appreciate Australe is to simply
crank up the magnification and leisurely
sweep along the limb, taking in the various
puddles and patches that define the western
shore of this most distinctive formation.
To view Mare Australe, circle Septem -
ber 2 to 6 and October 1 to 5 on your cal-
endar. On these evenings, the region is both
favourably librated and illuminated.✦Gary Seronik is this magazine’s editor and a
lifelong lunaphile.ON THE MOON by GARY SERONIK
SAILING AN ELUSIVE LUNAR SEA
Timing is everything for viewing limb-hugging Mare Australe
16 SKYNEWS •SEPTEMBER/OCTOBER 2019
A SOUTHERN SEAMare Australe is a fascinating formation that can
be viewed when libration tips the Moon’s southeastern limb Earthward.
Unlike most lunar maria, Australe consists of numerous small patches
instead of a single large expanse. PHOTO BY GARY SERONIK. INSET IMAGE COURTESY
NASA'S SCIENTIFIC VISUALIZATION STUDIO
80MARE
AUSTRALE90° E 100OkenLyotOVERHEAD VIEWThis orbital view shows Mare Australe in its full
glory. During favourable librations, the area to the left of the yellow line
(90° east) is accessible to observers on Earth. PHOTO COURTESY LROOkenLyot11 030° S405060