Sky and Telescope - February 2018 - 53

50 km
MARE HUMORUM

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in broad pyroclastic deposits (involving
gas-driven eruptions) - and the biggest,
containing up to 500 ppm water, covers
the northern half of the Aristarchus
Plateau. Frothy ﬁre fountains erupted
from the "Cobra Head" volcanic cone
when Vallis Schröteri (Schröter's Valley)
formed. A blanket of fallout probably
surrounded that vent, but ejecta from
the subsequent impact that formed
Aristarchus crater covered volcanic ash
along the plateau's southern half.
The water-infused deposits also end
abruptly along the uplifted plateau's
edge, because later ﬂows that ﬂooded the
surrounding Oceanus Procellarum covered up the terrain lower down. Visually,
at times near full Moon, the southern area of the Aristarchus Plateau is
bright because of the crater's ejecta. The
pyroclastic accumulation to the north is
slightly darker, and that's where water is
adsorbed or bonded with ash deposits.
You'll ﬁnd another large pyroclastic deposit marked by a dark mantle
around the Sulpicius Gallus rilles on the
southwestern shore of Mare Serenitatis.
Viewing at high magniﬁcation, you can
see that the short rilles cut into older,
slightly higher lavas. The water-infused
ash that erupted from the rille drapes
across part of the adjacent highlands
but stops abruptly at the lower edge
of the old ﬂows. The mare lavas ﬁlling
most of Serenitatis are younger and
cover the older pyroclastic layer.
A third large lunar pyroclastic unit,
also boasting up to 500 ppm of H2O
and/or OH, occurs along the southern
shore of Mare Humorum. This waterinfused deposit occurs within the ruined
crater between Doppelmayer and Vitello,
and to the west along the Rimae Doppelmayer, which was the vent for that part
of the pyroclastic eruptions. These areas
likewise appear dark under a high Sun.
Smaller pyroclastic deposits, including those at Apollo 15 and Apollo 17
sites and around the small dark-halo
craters on the ﬂoor of Alphonsus, contain more than 250 ppm of water.
It makes geochemical sense that
water would be associated with pyroclastic eruptions. These very powerful
events are driven by expanding bubbles

Doppelmayer

Vitello

p When viewed at times near full Moon, dark expanses along the southwestern margin of Mare
Humorum mark the locations of water-enriched volcanic deposits.

of gas within rising columns of magma
that literally explode upon reaching the
surface. When this occurs, the magma
is torn to shreds and widely dispersed.
However, as Li and Milliken's maps
of infrared spectra show, water-bearing
deposits are also associated with a few
much tamer volcanic ﬂows. These tend
to involve silicic lavas (those rich in
silicon), which are so sluggishly viscous
that their volatiles cannot easily escape.
Silicic lavas typically ﬂow slowly and are
relatively short, with steep edges.
One observable example is the silicic
dome near the crater Mairan. First ﬁnd
Mairan T, a small, shield-shaped cone
with calderas on top at the edge of Sinus
Roris, 18° due north of Aristarchus.
Then look for a pudgy mound to its east.
That's where the water shows up.
The nearby Gruithuisen domes, a bit
farther south, have classic silicic dome
morphology yet are not water-rich. (In
fact, Li and Milliken found that four
other silicic volcanic formations likewise
lack water's telltale infrared absorption,
so perhaps the Mairan dome is an oddity.) Instead, look just northeast of the
domes for a ﬂat, mare-like surface. This
doesn't seem to be a pyroclastic deposit,
yet it does contain water.

The lack of water signatures associated with the maria themselves implies
either that the sources of these massive
ﬂows didn't contain much water or that
somehow it escaped to space before it
could be adsorbed.
The new M3 maps indicate that
impact craters typically do not bring
water-bearing deposits to the surface -
with one notable exception. The central
peak of 61-km-wide Bullialdus contains
up to 250 ppm water, yet none occurs
anywhere else within the crater or
nearby. A central peak represents rock
brought up from 5 to 10 km below the
crater ﬂoor, and in this case those deepseated rocks must have contained water.
So why only at Bullialdus and not
in other large craters that excavated
similar depths? That is a mystery not yet
solved - and we didn't even know it was
a mystery until Li and Milliken gave the
M3 data a closer look. But that's how
lunar science works: More data bring
more mysteries and ultimately improve
our understanding of how the Moon
operated billions of years ago.
¢ Contributing Editor CHUCK WOOD
explores hundreds of lunar mysteries at
www2.lpod.org.

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Sky and Telescope - February 2018

Table of Contents for the Digital Edition of Sky and Telescope - February 2018