WWW.ASTRONOMY.COM 47of the air we breathe today comes from the atmospheric building
blocks of early eruptions. The other terrestrial worlds — Mercury,
Venus, the Moon, and Mars — formed in much the same way as
Earth. Venus and Mars have extensive volcanic structures, and
some of them may be dormant or still active today. The plains of
our cloud-covered inner neighbor host hundreds of thousands of
shield volcanoes — a type that features broad, shallow slopes and
forms when relatively f luid lava f lows for a long time. While most
of them are less than 12 miles (20 kilometers) across, more than
150 span 60 miles (100 km) or more.
Mars hosts the solar system’s largest known volcano. Olympus
Mons has a summit that rises 16 miles (25 km) above the sur-
rounding plains, three times taller than Mount Everest. The shield
volcano’s base stretches 380 miles (610 km) and would cover the
state of Arizona. Olympus Mons has a lot of company, too. It
stands near the edge of a massive rise called the Tharsis Bulge,
a dome built by many gigantic volcanoes. Tharsis rises 6 miles
(10 km) above the martian plains and spreads some 2,500 miles
(4,000 km) across. Dozens of volcanic structures dot the region,
ranging from tiny cinder cones to volcanoes in Olympus Mons’
class. On the opposite side of Mars, some of the planet’s youngest
lava f lows blanket the volcanic Elysium province.
Beyond the terrestrial planets, volcanoes often take on an alien
character. Some of this arises from different initial conditions —
many outer worlds formed from a nearly equal mix of rock and
ice. And often, forces that don’t drive earthly eruptions, such as
tidal heating, power these distant volcanoes.
The solar system’s most volcanically active world is Jupiter’s
moon Io. Its plumes rocket 300 miles (500 km) into the airless sky
before raining down in a hail of frozen sulfur. Although the inten-
sity of Io’s eruptions dwarfs what we see on Earth, the moon’s lava
lakes find a cousin at the center of Mount Erebus’ summit.
Molten rock, or magma, is not the only recipe for volcanic
eruptions in the outer solar system. On some moons, strange
brews of frigid gases escape the surface. On others, “magmas”
of exotic chemistry — superchilled water mixed with ammonia,
methanol, and other concoctions — power eruptions. Scientists
call these alien eruptions cryovolcanism, and they have left their
mark on many moons. While conventional volcanism may exist
on the seaf loor of Jupiter’s ocean moon, Europa, recent Hubble
Space Telescope observations indicate that this satellite could be
erupting water sporadically even today.
The pyramidal tents of our Fang Glacier encampment appear tiny in front
of Mount Erebus, the world’s southernmost active volcano. EVAN MILLER
The solar system’s largest volcano, Mars’ Olympus Mons,
rises some 16 miles (25 km) above the martian plains. NASA/JPLAbove: Water
vapor fountains
gush more
than 60 miles
(100 km) into
the skies above
Enceladus.
NASA/JPL/SSI
Upper Left:
Enceladus’
geysers erupt
from vents that
line the tiger-
stripe features
near the moon’s
south pole.
NASA/JPL/SSI
Left: Doom
Mons, a possible
cryovolcano
on Saturn’s
moon Titan,
rises 0.9 mile
(1.5 km) above
the nearby
plains. NASA/
JPL-CALTECH/ASI/USGS/
UNIVERSITY OF ARIZONAVOLCANIC ACTIVITY BEYOND EARTH