Pale Blue Dot ( PDFDrive.com ) (1)

kilometers thick. In our time, billions of years later, the surface of such a world may be quiet,
inactive, with no hint of current vulcanism. Or there may be—as on Earth—a few small-scale but
active reminders of an epoch when the entire surface was flooded with liquid rock.
In the early days of planetary geology, ground-based telescopic observations were all the data we
had. A fervent debate had been running for half a century on whether the craters of the Moon were due
to impacts or volcanos. A few low mounds with summit calderas were found—almost certainly lunar
volcanos. But the big craters—bowl or pan-shaped and sitting on the flat ground and not the tops of
mountains—were a different story. Some geologists saw in them similarities with certain highly
eroded volcanos on Earth. Others did not. The best counter-argument was that we know there are
asteroids and comets that fly past the Moon; they must hit it sometimes; and the collisions must make
craters. Over the history of the Moon a large number of such craters should have been punched out.
So if the craters we see are not due to impacts, where then are the impact craters? We now know
from direct laboratory examination of lunar craters that they are almost entirely of impact origin. But
4 billion years ago this little world, nearly dead today, was bubbling and churning away, driven by
primeval vulcanism from sources of internal heat now long gone.
In November 1971, NASA's Mariner 9 spacecraft arrived at Mars to find the planet completely
obscured by a global dust storm. Almost the only features to be seen were four circular spots rising
out of the reddish murk. But there was something peculiar about them: They had holes in their tops.
As the storm
cleared, we were able to see unmistakably that we had been viewing four huge volcanic mountains
penetrating through the dust cloud, each capped by a great summit caldera.
After the storm dissipated, the true scale of these volcanos became clear. The largest—
appropriately named Olympus Mons, or Mt. Olympus, after the home of the Greek gods—is more than
25 kilometers (roughly 15 miles) high, dwarfing not only the largest volcano on Earth but also the
largest mountain of any sort, Mt. Everest, which stands 9 kilometers above the Tibetan plateau. There
are some 20 large volcanos on Mars, but none so massive as Olympus Mons, which has a volume
about 100 times that of the largest volcano on Earth, Mauna Loa in Hawaii.
By counting the accumulated impact craters (made by small impacting asteroids, and readily
distinguished from summit calderas) on the flanks of the volcanos, estimates of their ages can be
derived. Some Martian volcanos turn out to be a few billion years old, although none dates back to
the very origin of Mars, about 4.5 billion years ago. Some, including Olympus Mons, are
comparatively new—perhaps only a few hundred million years old. It is clear that enormous volcanic
explosions occurred early in Martian history, perhaps providing an atmosphere much denser than the
one Mars holds today. What would the place have looked like if we had visited it then?
Some volcanic flows on Mars (for example, in Cerberus) formed as recently as 200 million years
ago. It is, I suppose, even possible—although there is no evidence either way—that Olympus Mons,
the largest volcano we know about for certain in the Solar System, will be active again.
Volcanologists, a patient sort, would doubtless welcome the event.
In 1990-93 the Magellan spacecraft returned surprising radar data about the landforms of Venus.