CHAPTER 8
The Terrestrial Planets
CHAPTER OUTLINE
8-1 Mercury
Mercury as Seen from Earth
1. The least seen planet by people on Earth. Because of its proximity to the Sun it can be seen low
on the horizon just before sunrise in the east or just after sunset in the west.
2. Mercury exhibits phases like Venus.
3. Features on Mercury are hard to discern from Earth because Mercury is small and, since it is
seen near the horizon, its light passes through more of the Earth’s atmosphere than if it were
overhead.
Mercury via Mariner — Comparison with the Moon
1. Mariner 10 flew by Mercury in 1974 (and subsequently twice more), returning a total of 4,000
photographs for the three fly-bys.
2. Mercury appears similar to our Moon; both are covered with many impact craters. Mercury’s
craters are less prominent and its surface has less extensive ray patterns; the planet’s surface
gravity is twice that of the Moon so loose material does not stack as steeply and doesn’t travel as
far.
3. Mercury lacks the larger maria seen on the Moon. Because Mercury cooled more slowly,
meteorites were able to penetrate its crust over a longer period, which allowed lava to flow out
and obliterate older craters. This resulted in the smooth plains seen between the evenly spread
craters.
4. More scarps—long cliffs in a line—are found on Mercury than on the Moon. Most are believed
to have formed from the shrinking of Mercury’s crust after cooling.
5. A large “bulls-eye” impact crater called Caloris Basin (the size of Texas) is visible in
Mariner’s images.
6. Mariner confirmed that Mercury has negligible atmosphere.
Structural Characteristics
1. Mercury’s density is slightly smaller than Earth’s while its surface gravity is only 38% that of
Earth’s. Since surface rocks on Mercury look similar to Earth rocks, Mercury must have a very
large iron core, perhaps 65%–70% of its total mass.
2. Astronomers speculate that early on a collision with a large asteroid blasted away most of
Mercury’s rocky mantle.
3. Mariner 10 did detect a magnetic field on Mercury, but it is 1% as strong as Earth’s. Mercury’s
magnetic field suggests that part of its metallic core must be molten in order for the dynamo effect
to operate. Recent measurements of heat loss indicate that its core is not molten, but the question
is open.
4. The dynamo effect explains the generation of magnetic fields as a consequence of circulating
electric charges, such as in an electric generator or in molten magnetic material within a planet’s
core.
Mercury’s Motions
1. Compared to the other planets, Mercury circles the Sun in less time (88 days), moves faster in
its orbit (48 km/s) and (except for dwarf planet Pluto) has the most eccentric orbit (varying
between 46 and 70 million km from the Sun).
2. Radar observations show that Mercury rotates once very 58.65 Earth days (precisely 2/3 of its
orbital period of 87.97 Earth days). This sidereal day is quite different from Mercury’s solar day,
which is 176 Earth days long. Thus a Mercurian day lasts two Mercurian years.
3. The coupling between Mercury’s rotation revolution periods is probably due to the unbalanced
nature of its mass. The leading hypothesis is that the impact object that created the Caloris Basin
was very dense, and its presence under Mercury’s surface has caused the planet to be lopsided.
4. High temperatures on Mercury can reach 450°C (842°F), well above the melting point of lead
(330°C or 626°F). On the night-side of Mercury, temperatures can fall to 150°C (238°F).
5. Radar observations show a high albedo—fraction of incident sunlight that an object reflects—
for Mercury’s polar regions, suggesting the presence of ice. Yet ice should evaporate over the
ages. The presence of ice for so long remains a mystery.
8-2 Venus
Structural Characteristics
1. Like Mercury, Venus is visible only in the evening sky after sunset or in the morning sky
before sunrise.
2. Venus is Earth’s sister planet with a bit smaller diameter (95% of Earth’s), smaller mass (82%)
and smaller density (95%).
3. The surface gravity of Venus (91% of Earth’s) and the similarities between the two planets’
surface rocks suggest than Venus has a dense interior with probably a metallic core.
4. Venus does not seem to have an intrinsic magnetic field. It is possible that its field is now in
the process of reversing.
Venus’ Motions
1. Its orbit is almost circular with a period of 225 days; orbital speed is a nearly constant 35 km/s
(78,300 mi/hr).
2. Venus’ surface is shrouded by heavy clouds. Since 1961 we have been bouncing radar signals
off its surface to learn about its rotation rate and surface features.
3. Venus’ sidereal rotation period is 243 days, its revolution period is 225 days, and these
combine to produce a solar day that is 117 Earth days long.
4. Venus’ axis is tilted 177°; because the angle is greater than 90°, Venus’ direction of rotation is
backward compared to most other directions of rotation and revolution in the solar system.
5. The definition of a planet’s North Pole is based on the right-hand rule: grab the planet with
your right hand so that your fingers point in the direction of the planet’s rotation; your thumb then
points to its north pole.
The Surface of Venus
1. Since 1962, many spacecraft from the U.S. and Russia have visited Venus. The former Soviet
Union landed 11 spacecraft, some of which produced close-up photos of the surface.
2. Photos of sharp-edged rocks confirm that winds at the surface are fairly calm.
3. Orbiting probes Pioneer Venus 1 (1978) and Magellan (1990) have produced detailed radar
maps of Venus’s surface. About two-thirds of Venus’s surface is covered with rolling hills.
Highlands occupy <10% of the surface, with lower-lying areas making up the rest.
4. Venus has about 1,000 craters that are larger than a few kilometers in diameter. Venus has no
craters older than ~800 million years; average surface age is estimated at 500 million years, about
twice as old as Earth’s.
5. Venus shows past evidence of volcanic and tectonic activity: mountains, large lava flows,
volcanoes. There is no evidence of current volcanic activity.
6. The presence of volcanism suggests that Venus has a molten interior. However, there is no
evidence to suggest the presence of plate tectonics.
Advancing the Model: Our Changing View of Venus
1. Venus is most brilliant when its elongation is 39, which occurs about 36 days before or after
its new phase.
2. It was during Venus’ solar transits in 1761 and 1769 that parallax measurements allowed us to
measure the actual distance between Earth and Venus and thus the actual value of the A.U.
The Atmosphere of Venus
1. Venus’ atmosphere is composed of 96% carbon dioxide (CO2), 3.5% nitrogen (N2), and small
amounts of water (H2O), sulfuric (H2SO4) and hydrochloric acid (HCl). Venus is inhospitable.
2. The upper atmosphere is very windy; wind speeds reach 350 km/hr (218 mi/hr). The wind
speed decreases to almost zero as one descends toward Venus’s surface.
3. The atmospheric pressure on Venus’ surface is about 90 times that found at the Earth’s surface.
4. Surface temperature of Venus has been measured at about 464°C (867°F).
5. Venus’ clouds form a layer between altitudes of 50 and 70 km. A haze layer extends from the
cloud layer down to 30 km. From there to the ground the atmosphere is quite clear.
A Hypothesis Explaining Venus / Earth Differences
1. Venus’ high surface temperature (due to it being closer to the Sun) did not allow water to
condense out of its atmosphere into oceans that could absorb CO2 (as happened on Earth). High
in Venus’s atmosphere, the Sun’s UV light broke down water molecules into hydrogen (which
escaped the planet) and oxygen (which combined with other elements).
2. CO2 remained in the atmosphere where it trapped the outgoing infrared radiation from the
planet’s surface, leading to higher temperatures. As Venus became hotter, more CO2 was baked
out of the surface rocks. This created a runaway greenhouse effect.
3. Finally, equilibrium was reached between the amount of radiant energy leaving the planet and
that striking it.
4. Earth’s temperature is elevated about 35C (63F) by the natural greenhouse effect.
5. For the past 150 years humans have been injecting CO2 and other greenhouse-causing
chemicals into the atmosphere at an ever increasing rate. Some studies indicate that doubling the
CO2 in the atmosphere could increase Earth’s average temperature between 2.8°C (5.0°F) and
5.2°C (9.4°F).
6. Plant growth naturally decreases atmospheric CO2. Unfortunately, we are destroying tropical
forests at the rate of 1 acre per second.
7. So much solar radiation is reflected from Venus’ clouds that if Venus had no greenhouse
effect, its atmosphere would be cooler than Earth’s.
8-3 Mars
Mars as Seen from Earth
1. Mars is the only planet with surface features that can be seen from Earth. Its surface visibility
varies greatly, depending on its distance from Earth and the presence (or absence) of dust storms.
2. Mars is best seen at opposition—the configuration of a planet when it is opposite the Sun in
our sky. Opposition of Mars occurs every 2.2 years.
3. Because Mars’ orbit is somewhat eccentric, the Earth-Mars distance can vary between 55
million km and 100 million km.
4. The tilt of Mars’ axis is similar to Earth’s.
5. Dark markings were observed as early as 1660, and Mars’ rotation rate was determined from
their motion. Changes in the dark areas on Mars led to speculation that there is vegetation on the
planet that changes color in response to seasonal growth.
6. Two polar caps can be seen on Mars; a polar cap is seen diminishing in size as that pole faces
the Sun and then grows again when it faces away from the Sun.
Structural Characteristics
1. Mars’ diameter (53% of Earth’s) is between that of Mercury and Venus. Its mass is only 11%
that of Earth’s, while its density is about 71% that of Earth’s.
2. Mars rotates almost as fast as Earth but has no magnetic field. Its surface rocks are rich in
silicon and iron.
3. From all the above data we conclude that a significant portion of Mars’ iron is probably
distributed throughout the planet.
4. Observations from the Mars Global Surveyor (MGS) suggest that Mars’ solid inner core (about
half the size of the planet) is surrounded by a liquid outer core, and that the core has a significant
fraction of a lighter element such as sulfur.
5. Mars does not show any evidence for an intrinsic global magnetic field even though it probably
had at some point an interior dynamo like Earth’s. MGS has detected a few magnetized regions in
the planet’s crust. Observations by Mars Express show that the solar wind penetrates deeply into
the Martian atmosphere.
Mars’ Motions
1. Mars orbits the Sun at an average distance of about 1.5 AU (about 228 million km). Its orbit is
more eccentric than Earth’s, so Mars’s distance from the Sun varies from 210 million km to 250
million km.
2. Mars takes 1.88 Earth years to complete its orbit around the Sun. Its sidereal period is 24h37m
while its day is 24h40m long, very similar to that of Earth.
3. Mars’s equator is tilted 25.2° with respect to its orbital plane, close to Earth’s 23.4°. As a
result, Mars has seasons.
4. Mars is closer to the Sun during summer in its southern hemisphere (just like Earth). However,
because of Mar’s eccentric orbit, its southern hemisphere exhibits greater seasonal shifts in
temperature than does the northern hemisphere.
Early Speculations on Life on Mars
1. In 1887 Schiaparelli’s drawing of channels or canali on Mars was misinterpreted by the public
to mean canals dug by a race of intelligent beings.
2. Lowell, who opened his observatory in Flagstaff, AZ, in 1894, reported he saw many canals.
Other astronomers could not confirm his findings.
Invasion and Its Results
1. The Mariner spacecraft of the late 1960s and early 1970s returned images that ended the
speculation about canals. Instead, the planet was seen to be covered by deserts, canyons,
volcanoes and craters.
2. Mars’ atmosphere is extremely thin, with a surface pressure about 1/160 of Earth’s. This
explains why the craters have not been worn down by the major dust storms that are common on
Mars.
3. The many missions to Mars since Pathfinder in 1997 show our immense interest in
understanding the planet and have provided us with a wealth of information about it.
Tools of Astronomy: Viking’s Search for Life
1. One of the primary purposes of the Viking mission (the landers touched down in 1976) was to
look for signs of life on Mars. All tests performed failed to find any such signs.
2. The combination of UV light, dry conditions, atmospheric oxygen, and mineral grain surfaces
produce oxidants, which can destroy organic molecules.
3. Life might still exist beneath the planet’s surface on Mars or in places protected from oxidants.
The Surface of Mars
1. The largest volcano is Olympus Mons, whose height of 24 km (15 mi) is twice that of Earth’s
largest mountain.
2. Mars and Venus have volcanoes larger than Earth because they lack tectonic plates. Formed
over a hot spot of lava that wells up from within a planet, a volcano can grow to enormous size if
the crust does not move off the hot spot. As Earth’s crust moves across a given hot spot, the
volcanic action shifts along the surface before the volcano can become as large as Olympus
Mons.
3. Venus lacks tectonic plates because its crust is probably too thin to be strong enough to remain
in large pieces. Mars lacks tectonic plates because it cooled quickly, leaving its crust too thick and
strong to be broken into pieces.
4. Valles Marineris is an enormous canyon on Mars that stretches nearly 4,800 km (3,000 mi).
5. Photographs taken by the Pathfinder spacecraft (which landed on Mars in 1994) supported the
idea that Mars had once a great amount of water on its surface. Sojourner (Pathfinder’s roving
robot) investigated the composition of some of the rocks around the landing site. This mission
showed that Mars is more similar to Earth than previously thought.
6. The Viking mission confirmed that Mars has dry riverbeds (similar to arroyos of the U.S. desert
southwest). It is tempting to conclude that at one time there was liquid water flowing on Mars.
7. Mars’ polar caps consist of a water ice base covered during the winter by dry ice (frozen CO2).
Additional water seems to be hidden in permafrost below the Martian surface. We don’t know
exactly how much water is on Mars.
8. An alternative hypothesis for the observed riverbeds is that they formed during brief and
cataclysmic periods when ice was melted by meteorite impacts or volcanic heating.
9. Data from Mars Express and Mars Global Surveyor show that liquid water seems to have
played an important role in shaping the planet’s surface. But other factors (volcanism, tectonic
shifts, ice, winds, etc.) have also contributed.
10. About 34 meteorites found on Earth have been identified as having come from Mars. In them,
some scientists find clues for the existence of water and life on Mars; others dispute these claims.
Surface Conditions and the Case for Water on Mars
1. 3D maps made by the MGS since 1999 show a hemispheric dichotomy. The northern
hemisphere consists of young plains, low in elevation, while the southern hemisphere is mainly
ancient cratered highlands. Also, the crust thins progressively from the south pole toward the
north pole, from an average of 45 miles to 22 miles in thickness.
2. This difference between the two hemispheres could be the result of subcrustal erosion by a
large-scale convection cell or one large impact (or multiple impacts) in the lowlands.
3. Differences also exist between the polar regions. The part of the polar cap that survives the
summer is mostly water ice in the north and dry ice in the south. This supports the idea that a
large ocean once covered the northern lowlands.
4. MGS and Mars Odyssey observations show that the total amount of CO2 on Mars is a small
fraction of that found on Earth and Venus. This finding contradicts the long-held view that the
three planets were formed with similar total amounts of CO2.
5. The presence of features on Mars that look like gullies formed by flowing water suggests that
there may be current sources of liquid water at or near the planet’s surface. However, no
explanation to date for these features has been universally accepted.
6. Layers of sedimentary rock discovered on Mars suggest a history that either included warm,
wet climates that lasted for millions of years; however, the abundant presence of Mars’ surface of
the mineral olivine (which weathers easily in water) suggests a cold and dry Mars through its
geologic history.
7. It is possible that Mars is cold and dry for hundreds of millions of years and then becomes
warmer and wetter during brief episodes triggered by internal planetary heat and lasting
thousands of years.
8. The observed lack of carbonates on the planet’s surface is a discouraging finding about the
possibility of life on Mars. Even though MGS found traces of carbonates in Martian dust, it
probably came from the interaction between dust and tiny amounts of water in the atmosphere,
not from marine deposits.
9. Observations by Mars Odyssey based on the detection of hydrogen on Mars’ surface suggest
that there is enough water (in the form of ice) just a few feet under the surface to cover the planet
ankle deep. It is not clear yet how the water got into the soil and rocks under the surface.
Atmospheric Conditions
1. Near the Martian equator, noontime temperatures reach as high as 30°C (86°F). At night the
temperature drops to −135°C (−210°F). Even though Mars’s atmosphere is 95% CO2, there is
simply too little atmosphere to significantly moderate the temperature.
2. Mars’ escape velocity is 5 km/s, less than 1/2 of Earth’s.
3. Though Mars is colder than Earth, its lower escape velocity and lack of ozone layer has
allowed water vapor (which is broken up by UV radiation) as well as methane and ammonia to
escape.
4. As the water vapor was broken up by UV, the hydrogen escaped and the oxygen reacted with
the iron-rich crust to form rust—giving Mars its characteristic red color.
5. Mars’ seasonal color changes are caused by spring winds stripping the lighter-colored dust
grains from the darker underlying surface. The global dust storms on Mars provide us with a realtime laboratory for observing global warming.
6. Though the Viking landers found no direct evidence of life on Mars, the possibility has not
been completely ruled out.
The Moons of Mars
1. Mars has two small moons: Phobos (17  14 12 mi) and Deimos (10  9  7.5 mi). Both
moons are small and irregularly shaped, with dark surfaces, similar to many asteroids.
2. Phobos has a density of 1.9 that of water compared to Deimos’ 2.1 and Earth’s 5.52.
3. Both moons could be captured asteroids.
4. Phobos orbits Mars in 0.3 days; Deimos’s period is 1.3 days. Both orbit very close to Mars.
5. Though Mars is the most oblate of the terrestrial planets, it is still very nearly spherical. The
terrestrial planets are massive enough for their gravity to have formed them into almost perfect
spheres. Phobos and Deimos, however, are too small for gravity to have formed them into
spheres; like them, most asteroids are also nonspherical in shape.
8-4 Why Explore?
1. Seeking knowledge is one of the things that separates humans from other creatures.
2. Studying the features and atmospheres of other planets tells us much about Earth and the
possible futures that await us.