832 Encyclopedia of the Solar System
FIGURE 3 Lava flows in southwest Mare Imbrium on the
Moon. The source vents are off the image to the lower left and
the∼300 km long flows extend down a gentle slope toward the
center of the mare basin beyond the upper right edge of the
frame. (NASAApollophotograph.)
A second class of lunar volcanic features associated with
the edges of large basins is the sinuous rilles. These are me-
andering depressions, commonly hundreds of meters wide,
tens of meters deep, and tens of kilometers long, which oc-
cur almost entirely within the mare basalts. Some are dis-
continuous, giving the impression of an underground tube
that has been partly revealed by partial collapse of its roof,
and these are almost certainly the equivalent of lava tube
systems (lava flows whose top surface has completely solid-
ified) on Earth. Other sinuous rilles are continuous open
channels all along their length; these generally have origins
in source depressions two or three times wider than the rille
itself, and become narrower and shallower with increasing
downslope distance from the source. At least some of these
sinuous rilles appear to have been caused by long-duration
lava flows that were very turbulent (i.e., the hot interior was
being constantly mixed with the cooler top and bottom of
the flow). As a result the flows were able to heat up the pre-
existing surface until some of its minerals melted, allowing
material to be carried away and an eroded channel to form.
In contrast to the lava flows and lava channels, two types
of pyroclastic deposit are recognized on the Moon. There
are numerous regions called dark mantles, often roughly cir-
cular and up to at least 200 km in diameter, where the frag-
mental lunar surface regolith is less reflective than usual,
and spectroscopic evidence shows that it contains a com-
ponent of small volcanic particles in addition to the locally
derived rock fragments. The centers of these regions are
commonly near the edges of mare basins, suggesting that
the dark mantle deposits are produced by the same (or sim-
ilar) source vents as the lava flows. Chemical analyses of
theApollolava samples show that the Moon’s mantle is
totally devoid of common volatiles like water and carbon
dioxide due to its hot origin [seeTheMoon] and suggest
that the main gas released from mare lava vents was carbon
monoxide, produced in amounts up to a few hundred parts
per million by weight as a result of a chemical reaction be-
tween free carbon and metal oxides, mainly iron oxide, in
the magma as it neared the surface.
Several smaller, dark, fragmental deposits occur on the
floor of the old, 90 km diameter impact crater Alphonsus.
These patches, called dark haloes, extend for a few kilo-
meters from the rims of subdued craters that are centered
on, and elongated along, linear fault-bounded depressions
(called linear rilles) on the crater floor. It is inferred that
these are the sites of less energetic volcanic explosions.
Localized volcanic constructs such as shield volcanoes
and domes are generally rare on the Moon, though more
than 200 low, shieldlike features with diameters mainly in
the range 3–10 km are found in the Marius region within
Oceanus Procellarum, in northeast Mare Tranquillitatis,
and in the region between the craters Kepler and Coper-
nicus. Conspicuously absent are edifices with substantial
summit calderas. This implies that large, shallow magma
reservoirs are very rare, almost certainly a consequence of
the difficulty with which very dense magmas rising from the
mantle penetrate the low-density lunar crust. However, a
few collapse pits with diameters up to 3 km do occur, located
near the tops of domes or aligned along linear rilles.1.3 Mars
About 60% of the surface of Mars consists of an ancient
crust containing impact craters and basins. Spectroscopic
evidence from orbiting spacecraft suggests that it is com-
posed mainly of volcanic rocks. The other 40% of the
planet consists of relatively young, flat, lower lying, plains-
forming units that are a mixture of wind-blown sediments,
lava flows, and rock debris washed into the lowlands by
episodes of water release from beneath the surface. Com-
bining orbital observations with analyses made by the five
probes that have so far landed successfully on the surface