[Illustration: Mariner 9 imagery of Olympus Mons Volcano on Mars
compared to the eight principal Hawaiian Islands at the same scale
(Mariner 9 Image Mosaic, NASA/JPL).]
Volcanoes and volcanism are not restricted to the planet Earth. Manned
and unmanned planetary explorations, beginning in the late 1960’s, have
furnished graphic evidence of past volcanism and its products on the
Moon, Mars, Venus and other planetary bodies. Many pounds of volcanic
rocks were collected by astronauts during the various Apollo lunar
landing missions. Only a small fraction of these samples have been
subjected to exhaustive study by scientists. The bulk of the material is
stored under controlled-environment conditions at NASA’s Lunar Receiving
Laboratory in Houston, Tex., for future study by scientists.
From the 1976-1979 Viking mission, scientists have been able to study
the volcanoes on Mars, and their studies are very revealing when
compared with those of volcanoes on Earth. For example, Martian and
Hawaiian volcanoes closely resemble each other in form. Both are shield
volcanoes, have gently sloping flanks, large multiple collapse pits at
their centers, and appear to be built of fluid lavas that have left
numerous flow features on their flanks. The most obvious difference
between the two is size. The Martian shields are enormous. They can grow
to over 17 miles in height and more than 350 miles across, in contrast
to a maximum height of about 6 miles and width of 74 miles for the
Hawaiian shields.
Voyager-2 spacecraft images taken of Io, a moon of Jupiter, captured
volcanoes in the actual process of eruption. The volcanic plumes shown
on the image rise some 60 to 100 miles above the surface of the moon.
Thus, active volcanism is taking place, at present, on at least one
planetary body in addition to our Earth.
[Illustration: Spacecraft image, made in July 1979, shows volcanic plume
rising some 60 to 100 miles above the surface of Io, a moon of Jupiter
(Voyager 2 photo, NASA).]