Quadrant 31:
— Images and text provided by Howard Eskildsen
Mare Serenitatis: The serene sea
Mare Serenitatis, the sea of serenity, lies
peacefully on the face of the Moon as it has for
billions of years, truly serene with nearly nothing
happening since last layers of lava covered its
floor. See the paltry few craters on its interior?
That is nearly all that visibly happened to it in
more than 3 billion years; quite serene, almost
to the point of boredom. Perhaps that is why so
many minuscule craters in this area have names,
whereas they would have been ignored in the
Southern Highlands where craters abound.
But it wasn’t always that way; once it
likely was as rugged as the Moon’s Southern
Highlands, but a powerful impact around four
billion years ago annihilated all pre-existing
craters and left the huge basin now known as
Mare Serenitatis. Originally, it probably had
multiple rings that survived for a time but were
then erased by subsequent basin-forming impacts
or covered with lava flows. Its only clear ring
remnant is the Montes Haemus, which likely
rivaled the Apennine Mountains in the distant
past, but now sits scoured and partly buried in
Apennine tailings.
A close look at the Montes Haemus shows rounded,
rutted mounds northwest of Menelaus, with less evidence
of devastation to the east of Menelaus since that part
was farther from the Imbrium impact. On Earth, such
degradation and wearing down of massive ranges took eons
of glaciation and erosion, but on the Moon the violence of
the moment took but minutes to hours as blasted material
seethed over the terrain and then later settled while the
Moon continued shaking with seismic tremors for days
afterwards.
The impacts that created the basins also left deep cracks
that traveled downward and provided conduits for lava
to rise to the surface and spread across the basin interior.
Gradually the solidified basalt accumulated to the point
where its weight caused the center to sink relative to the
surrounding areas. This subsidence created cracks around
the margins of the mare, known as arcuate rilles. A few
of these rilles are visible on the image; Rimae Sulpicius
Gallus, Rimae Menelaus and Rimae Plinius are examples.
More lava arose over eons, adding further basalt layers
over the floor of Serenitatis, covering most but not all of
the earlier layers. Note the dark area around Promontorium
Archerusia and Rimae Plinius; these are remnants of the
oldest visible basalts in the basin and are high in titanium.
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They are thought to be 3.7 to 3.8 billion years old.
Subsequent flows settled to the sunken center of
the basin and as it solidified, the basin sank further
causing compressive stress on the interior of the
mare. Small ridges, known as dorsa or wrinkle
ridges, are overthrust faults that formed to relieve
the compression stresses. The youngest lava layer is
believed to be about three billion years old, which
means the volcanic activity persisted for 700-800
million years. That is longer than it took complex
life to evolve on Earth from simple life forms!
Since that time very litt le else has happened that
can be seen through a telescope. A few craters such
as Menelaus and Bessel appeared in addition to a
handful of otherwise insignificant craters. Bessel
is notable primarily for the ray named after it even
though it had nothing to do with the origination
of the ray. Some have surmised that it is a streak
ejected from Tycho more than 2,100 kilometers
to the southwest, while others believe it may have
originated from some fresh crater to the northeast.
Another one-time source of confusion was the
crater Linné, a diminutive dot on the upper portion of the
image. It was once thought to have disappeared and been
replaced by a brilliant white patch. While it caused quite a
Sky ’ s
Up
stir, as any such discovery today would, it turned out to be
a result of varying illumination of a tiny, young crater at the
limit of Earth-bound telescopic visibility.
So, Mare Serenitatis serenely awaits repeated discovery by
Sky ’ s
Up
telescopic tourists. It’s peaceful now, but it was not always
so serene. Its face has gazed sedately towards Earth for
eons before our kind came into existence on Earth, and will
continue, perhaps, after we are all long gone.
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