CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 465
Th ere wasn’t a breath in that land
of death...
— ROBERT SERVICE, “THE CREMATION OF SAM McGEE”T
he temperature on Mars on a hot summer day at
noon might feel pretty pleasant at around 20°C (68°F).
But without a spacesuit, you could live only about
30 seconds because the air is mostly carbon dioxide with almost
no oxygen and is deadly dry. Even more important, the air pres-
sure is less than 1 percent that at the surface of Earth, so your
exposed body fl uids such as tears and saliva would boil if you
stepped outside your spaceship unprotected.
Not even a spacesuit would save you on Venus. Th e air pres-
sure there is 90 times Earth’s, and the air is almost entirely carbon
dioxide, with traces of various acids. Worse yet, the surface is hot
enough to melt lead.
Venus and Mars resemble Earth in some ways, so why are
they such unfriendly places to visit? Comparative planetology
will give you some clues.
Venus
An astronomer once became annoyed when someone referred
to Venus as a planet gone wrong. “No,” she argued, “Venus is
probably a fairly normal planet. It is Earth that is peculiar. Th e
universe probably contains more planets like Venus than like
Earth.” To understand how unusual Earth is, you need only to
compare it with Venus.
In many ways, Venus is a twin of Earth, and you might expect
surface conditions on the two planets to be quite similar. Venus
and Earth are almost exactly the same size and mass, with Venus
having 95 percent the diameter of Earth (Celestial Profi le 5).
Venus and Earth have similar average densities, and they formed
in the same part of the solar nebula—Venus’s orbit is the closest to
Earth’s of all the planets, so Venus’s overall composition is similar
to Earth’s (see Chapter 19). Also, planets the size of Earth and
Venus cool slowly, so you might expect Venus, like Earth, to have
a molten metallic core and an active crust with plate tectonics.
Unfortunately, the surface of Venus is perpetually hidden by
thick clouds that completely envelop the planet, preventing us
from easily observing conditions there. From the time of Galileo
until the early 1960s, astronomers could only speculate about
Earth’s twin. Some science fi ction writers imagined that Venus
was a steamy swamp planet inhabited by strange creatures, while
others imagined that it was completely covered by an ocean or by
a planetwide dusty desert.
Starting in the 1960s, astronomers have used microwave
blackbody emission from Venus to measure its surface tempera-
ture, and radar to penetrate the clouds, image the surface, and
measure the planet’s rotation. At least 23 spacecraft have fl own
22-1
Venus is only 5 percent smaller than Earth, but its atmosphere
is perpetually cloudy, and its surface is hot enough to melt
lead. It may have a hot core about the size of Earth’s.Celestial Profi le 5: Venus
Motion:
Average distance from the sun 0.723 AU (1.08 108 km)
Eccentricity of orbit 0.007
Inclination of orbit to ecliptic 3.4°
Average orbital velocity 35.0 km/s
Orbital period 0.6152 y (224.7days)
Period of rotation 243.0 days (retrograde)
Inclination of equator to orbit 177°Characteristics:
Equatorial diameter 1.21 104 km (0.949 D⊕)
Mass 4.87 1024 kg (0.815 M⊕)
Average density 5.24 g/cm^3 (4.2 g/cm^3 uncompressed)
Surface gravity 0.90 Earth gravity
Escape velocity 10.3 km/s (0.92 V⊕)
Surface temperature 470°C (880°F)
Albedo (cloud tops) 0.76
Oblateness 0Personality Point:
Venus is named for the Roman goddess of love, perhaps because the
planet often shines so beautifully in the evening or dawn sky. In
contrast, the ancient Maya identifi ed Venus as their war god Kukulkan and
sacrifi ced human victims to the planet when it rose in the dawn sky.