CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 491
Summary
▶ Venus is Earth’s twin in size but is slightly closer to the sun.
▶ Venus rotates so slowly that solar heat at the subsolar point (p. 466)
produces strong atmospheric circulation that circles the planet in only
four days.
▶ The atmosphere of Venus is 95 times thicker than Earth’s and
composed almost entirely of carbon dioxide.
▶ Venus is heated by a runaway greenhouse effect. The temperature at
the surface of Venus is about 470°C (880°F), hotter than Mercury even
though it is farther from the sun.
▶ The surface of Venus is so hot that compounds have cooked out of the
crust to form traces of sulfuric, hydrochloric, and hydrofl uoric acids in
the atmosphere. The very high clouds on Venus are composed of small
droplets of sulfuric acid and sulfur crystals.
▶ Although it is perpetually hidden below thick clouds, the surface of
Venus can be studied by radar mapping, which reveals higher uplands
and low rolling plains. Volcanoes, lava fl ows and channels, and impact
craters are detectable. Volcanism is apparently common on Venus, and
much of the surface is solidifi ed lava fl ows. Volcanoes are probably
still active on Venus.
▶ Radar maps can measure altitude, roughness, and in some cases,
composition of the surface.
▶ Landers have analyzed the surface rock and found it to be similar to
basalts on Earth.
▶ Composite volcanoes (p. 472) on Earth have steep slopes and are
associated with subduction zones. Shield volcanoes (p. 472) have
shallow slopes and are associated with hot spots. The volcanoes on
Venus are shield volcanoes.
▶ Plate motion across a hot spot produces chains of volcanic peaks.
The large size of the volcanic peaks on Venus, and the fact that they
are shield volcanoes, shows that the crust is not made of moving
plates.
▶ Coronae (p. 471) form where rising currents of magma push the
crust up and then withdraw, forming circular faults with associated
volcanoes and lava fl ows.
▶ Because Venus formed closer to the sun than Earth, it was initially
warmer, and whatever oceans it may originally have had were not able
to persist and remove carbon dioxide from the atmosphere. The accu-
mulating carbon dioxide produced an intense greenhouse effect, made
the planet very hot, and evaporated the remaining water in a runaway
greenhouse effect.
▶ (^) The surface of Venus appears to be about half a billion years old.
Planetary scientists suspect that the entire planet was resurfaced by
an outpouring of lava.
▶ (^) Venus has no detectable magnetic fi eld, so its core does not support
the dynamo effect, which is a puzzle.
▶ (^) Venus rotates retrograde (backward). This may have been caused by an
off-center impact by a very large planetesimal as the planet was forming
or by solar tides raised in its thick atmosphere.
▶ (^) Mars is smaller than Earth but larger than the moon. It has lost the
lower-mass atoms from its atmosphere because of its low escape
velocity.
▶ (^) The atmosphere on Mars is very low-density, consisting mostly of
carbon dioxide, and the pressure at the surface is too low to prevent
water from boiling away.
▶ Mars has no magnetic fi eld to protect it from the solar wind, and some
of its atmosphere probably has been blasted away over its history by
the pressure of the solar wind.
▶ Although 19th-century astronomers thought they saw networks of
canals on Mars, images from spacecraft show that Mars is a dry, desert
world on which liquid water does not currently exist.
▶ The southern hemisphere of Mars is old and heavily cratered, but the
northern lowlands are smooth and mostly free of craters.
▶ Images from spacecraft orbiting Mars reveal outfl ow channels
(p. 483) that appear to have been cut by massive fl oods and valley
networks (p. 483) that resemble dry riverbeds. Crater counts show
that these features are in very old terrain.
▶ The smooth lowlands of Mars’s northern hemisphere may have once
contained a liquid-water ocean. Some outfl ow channels lead into the
lowlands, and features resembling shorelines have been found.
▶ Evidence suggests that whatever water Mars retains is now frozen in
the crust as permafrost and as large deposits under the polar caps.
Where it seeps out, it can cut gullies and form similar fl ow features,
but rivers, lakes, and oceans cannot now exist on Mars because of the
low atmospheric pressure.
▶ There is evidence that Mars once had a molten core that generated a
magnetic fi eld, but it has no detectable magnetic fi eld now. Planetary
scientists hypothesize that most of the core has solidifi ed, and the
molten outer layer of the core is too small to generate a magnetic
fi eld.
▶ (^) The Noachian period (p. 487) extended from the formation of the
crust to the end of heavy cratering about 3.7 billion years ago. The
valley networks formed during this period, which suggests that the
atmosphere was denser then, and water fell as rain or snow.
▶ (^) The Hesperian period (p. 488) began as cratering declined and
massive lava fl ows resurfaced some regions. The climate was colder
and the atmosphere thinner, with water frozen in the crust. Massive
fl oods and outfl ow channels seem to have been produced by sudden
melting of subsurface ice.
▶ (^) The Amazonian period (p. 488) extended from about 3 billion years
ago to the present and is marked by continued low-rate cratering
and erosion by wind, and by small amounts of water seeping from
subsurface ice.
▶ (^) Volcanism has been important throughout the history of Mars, and
the Tharsis rise is a huge volcanic uplift. Volcanoes were active in the
Noachian period and probably still erupt on Mars.
▶ (^) Olympus Mons is a very large volcano, but it has not sunk into the
crust, and that shows that the crust of Mars is now quite thick.
▶ (^) The development of the Tharsis rise may have altered the rotation of
Mars and changed its climate.
▶ (^) Mars has captured two asteroids into orbit as moons. Phobos and
Deimos are small, irregularly shaped, and cratered. Both are tidally
locked to Mars.
Review Questions
- Why might you expect Venus and Earth conditions to be similar?
- What evidence can you cite that Venus and Mars once had more water
than at present? Where did that water come from? Where did it go? - What features would you look for in high-resolution radar maps of
Venus to search for plate tectonics?