476 PART 4^ |^ THE SOLAR SYSTEM
In comparison, Earth avoided a runaway greenhouse eff ect
because it was farther from the sun and always cooler than Venus.
Consequently, it could form and preserve liquid-water oceans,
which absorbed the CO 2 and left an atmosphere of nitrogen that
was relatively transparent in some parts of the infrared. As you
learned earlier, if all of the carbon in Earth’s sediments was put
back into the atmosphere as CO 2 , our air would be as dense as
that of Venus, and Earth would suff er from a tremendous green-
house eff ect. Recall also from Chapter 20 how the use of fossil
fuels and the destruction of forests are increasing the CO 2 con-
centration in our atmosphere and warming the planet. Venus
warns us of what a greenhouse eff ect can do.
Fully 70 percent of the heat from Earth’s interior fl ows out-
ward through volcanism along midocean ridges. But Venus lacks
crustal rifts, and even its numerous volcanoes cannot carry much
heat out of the interior. Rather, Venus seems to get rid of its
interior heat through large currents of hot magma that rise
beneath the crust. Coronae, lava fl ows, and volcanism occur
above such currents. Th e surface rock on Venus is the same kind
of dark-gray basalt found in ocean crust on Earth.
True plate tectonics is not important on Venus. For one
thing, the crust is very dry and is consequently about 12 percent
less dense than Earth’s crust. Th is low-density crust is more buoy-
ant than Earth’s crust and resists being pushed into the interior.
Also, the crust is so hot that it is halfway to its melting point.
Such hot rock is not very stiff , so it cannot form the rigid plates
typical of plate tectonics on Earth.
Th ere is no sign of plate tectonics on Venus, but there is
evidence that convection currents below the crust are deforming
the crust to create coronae and push up mountains such as
Maxwell. Detailed measurements of the strength of gravity over
Venus’s mountains show that some must be held up not by deep
roots like mountains on Earth but by rising currents of magma.
Other mountains, like those around Ishtar Terra, appear to be
folded mountains caused by limited horizontal motions in the
crust, driven perhaps by convection currents in the mantle.
Th e small number of craters on the surface of Venus hints
that the entire crust has been replaced within the last half-billion
years or so. Th is may have occurred in a planetwide overturning
as the old crust broke up and sank and lava fl ows created a new
crust. Th is could happen periodically on Venus, or the planet
may have had geological processes more like Earth’s until a single
resurfacing geologically recently. In either case, unearthly Venus
may eventually reveal more about how our own world works.
Mars
Mercury and the moon are small. Venus and Earth are the
largest of the Terrestrial planets. Mars has an intermediate size. It
is twice the diameter of the moon but only a little more than half
of Earth’s diameter (Celestial Profi le 6). Mars’s small size has
allowed it to cool faster than Earth, and much of its atmosphere
has leaked away. Its present carbon-dioxide atmosphere is a bit less
than 1 percent as dense as Earth’s.No Canals on Mars
Long before the space age, the planet Mars was a mysterious
landscape in the public mind. In the century following Galileo’s
fi rst astronomical use of the telescope, astronomers discovered
dark markings on Mars as well as bright polar caps. Timing the
motions of the markings, they concluded that a Martian day was
about 24 hours 40 minutes long. Its axis is tipped 25.2° to its
orbit, almost exactly the same as Earth’s 23.4° tilt, so Mars has
seasons with about the same winter/summer contrast as Earth.
Mars’s year is about 1.88 Earth years long. Th ese similarities
with Earth encouraged the belief that Mars might be
inhabited.
In 1858, the Jesuit astronomer Angelo Secchi referred to a
region on Mars as Atlantic Canale. Th is is the fi rst use of the
Italian word canale (channel) to refer to a feature on Mars. Th en,
in the late summer of 1877, the Italian astronomer Giovanni
Schiaparelli, using a telescope only 8.75 in. in diameter, thought
he glimpsed fi ne, straight lines on Mars. He too used the Italian
word canali (plural) for these lines, and the word was translated
into English not as “channel,” a narrow body of water that is a
natural geological feature, but as “canal,” an artifi cially dug chan-
nel. Th us the “canals of Mars” were born. Many astronomers22-2
SCIENTIFIC ARGUMENT
What evidence can you point to that Venus does not have plate
tectonics?
Sometimes a scientifi c argument can be helpful by eliminating a
possibility. On Earth, plate tectonics is identifi able by the world-
wide network of faults, subduction zones, volcanism, and foldedmountain chains that outline the plates. Although some of these
features are visible on Venus, they do not occur in a planetwide
network of plate boundaries. Volcanism is widespread, but folded
mountain ranges occur in only a few places, such as near Lakshmi
Planum and Maxwell Montes, and, unlike on Earth, they do not
make up long mountain chains. Also, the large size of the shield
volcanoes on Venus shows that the crust is not moving over the hot
spots in the way the Pacifi c seafl oor is moving over the Hawaiian
hot spot.
At fi rst glance, you might think that Earth and Venus should
be as similar as siblings, but comparative planetology reveals that
they are more like cousins. You can blame the thick atmosphere
of Venus for altering its geology, but that calls for a new scien-
tifi c argument: Why isn’t Earth’s atmosphere similar to that of
Venus?