http://www.skyandtelescope.com.au 55associated 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 M^3 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 deep-seated 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
M^3 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.CHUCK WOOD explores hundreds of
lunar mysteries at www2.lpod.org.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 H 2 O
and/or OH, occurs along the southern
shore of Mare Humorum. This water-
infused 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 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.
NASA / LUNAR RECONNAISSANCE ORBITER The lack of water signatures
SWhen viewed at times near full Moon, dark expanses along the southwestern margin of Mare
Humorum mark the locations of water-enriched volcanic deposits.PuiseuxVitello50 kmDoppelmayerMARE HUMORUMRimae Doppelmayer