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RESEARCH TOPICS IN ASTRONOMY AND ASTROPHYSICS
1.4.3 Terrestrial Planets
Our Solar System has four Terrestrial planets (named after the Earth): Mercury, Venus,
Earth and Mars (Figure 1.15). These planets have very different properties than the
Jovian planets. In comparison, they are relatively small, mostly rocky in composition, and
have relatively thin gaseous atmospheres.
Figure 1.15. Visible-light images of the Terrestrial planets (not to scale). From top left,
Mercury, Venus, Earth and Mars. Courtesy of NASA.
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Physical and orbital parameters of the terrestrial planets are summarized in Table 1.6.
Table 1.6. Properties of the terrestrial planets
Parameter
mass
Mercury
Venus
Earth
Mars
3.303 × 10 23 kg
4.869 × 10 24 kg
5.976 × 10 24 kg
6.421 × 10 23 kg
0.055 M ⊕
0.815 M ⊕
1.000 M ⊕
0.107 M ⊕
equatorial
radius
2439.7 km
0.383 R⊕
6051.8 km
0.949 R⊕
6378.14 km
1.000 R⊕
3397.2 km
0.533 R⊕
mean density
5420 kg m −3
5250 kg m −3
5515 kg m −3
3940 kg m −3
rotation period
obliquity
58.6462 days
0.00°
243.0187 days
177.36°
23.9345 hr
23.45°
24.6229 hr
25.19°
mean distance
from Sun
5.791 × 1010 m
0.3871 AU
87.969 days
1.082 × 1011 m
0.7233 AU
224.701 days
1.496 × 1011 m
1.000 AU
365.265 days
2.2794 × 1011 m
1.5237 AU
686.98 days
0.2056
0.0068
0.0167
0.0934
7.004°
3.394°
0.000°
1.850°
0.10
452 K
0.65
701 K
0.37
288 K
0.15
210 K
N2
77%
O2
21%
Other 2%
CO2
N2
Ar
orbital period
orbital
eccentricity
orbital
inclination
albedo
mean surface
temperature
atmospheric
composition
He
42%
Na
42%
O2
15%
Other 1%
CO2
N2
97%
4%
95%
3%
2%
Mercury is a small rocky planet orbiting closest to the Sun. The Sun’s gravitational field
raises a tidal bulge and the gravitational attraction between the bulge and the Sun creates
a torque that affects Mercury’s rotation speed. As a result, Mercury is locked in a 2:3
spin-orbit resonance. Because of Mercury’s small mass and high surface temperature, it
has lost almost all its atmosphere. It surface is much like that of the Moon, covered with
impact craters of all sizes.
Venus is comparable in size and mass to the Earth, but has a much denser atmosphere (96
times the surface pressure) that consists mainly of CO2. This is a greenhouse gas, which
traps infrared radiation that would normally cool the planet. Because of this, the surface
temperature on Venus is hot enough to melt lead! The surface of Venus is not visible
because of the constant cloud cover. Radar imaging reveals a rocky surface with high
mountains, volcanoes, lava flows, rift valleys, and a few large meteor craters. About 65%
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of the surface consists of upland plateaus and about 20% are low-lying flat areas called
planitiae. There are two major highland areas (terra) that rise as much as 10 km above the
plains. These cover about 10% of the surface. Venus shows no sign of plate tectonics.
Figure 1.16. Topographic map of Venus, reconstructed from Magellan radar imaging data
(NASA).
The Earth is an active planet with a modest atmosphere and a rocky surface. Water covers
71% of its area, with a mean depth of 4 km. The atmosphere consists mainly of N2 and
O2, the latter having been generated by plants through photosynthesis, after life arose.
The Earth’s continents and mountain ranges rise as high as 12 km above the ocean floor.
The surface is continually reworked by tectonic processes and erosion. There are a few
large impact craters, but none are older than several hundred million years.
Mars is about half the diameter of the Earth, and 50% further from the Sun. It has a thin
C02 atmosphere and a mean surface temperature of approximately –60C. The surface is
quite varied, with the highest mountains and largest volcanoes, and deepest valleys in the
solar system, impact craters. The surface is covered with a fine dust that is blown by the
wind. The reddish colour is due to the presence of iron oxide. There are two small polar
caps that consist mostly of frozen CO2.
There is now ample evidence that liquid water has been present on the surface of Mars.
This includes images of what appear to be river valleys, drainage features, flooding, and
geological evidence from the Mars Rovers. Mars may have been warmer, with a dense
atmosphere, in the past. Or, the water might have resulted from sudden heating of
subsurface ice deposits by volcanic activity or meteorite impact.
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Figure 1.17. Olympus Mons, a large shield volcano on Mars. This structure is
approximately 600 km in diameter and 25 km high (NASA).
Figure 1.18. Mosaic view of Mars, from Viking orbiter images, shows Valles Marineris, a
3000 km long, 30 km wide and 8 km deep rift valley, and several volcanoes (NASA).
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Figure 1.19. Interior structure of the terrestrial planets. Left to right, Mercury, Venus,
Earth, Mars. (Courtesy Calvin J. Hamilton)
The interiors of the terrestrial planets consist of a relatively thin crust covering a rocky
mantle surrounding a metallic core. In Mercury, the core dominates, containing 70% of
the mass of the planet and 50% of the interior volume. The interior structure of Venus is
thought to be similar to that of the Earth, although it is not known whether the core is
liquid or solid.
The Earth has a relatively thin crust, approximately 30 km thick, surrounding a rocky
mantle. The mantle consists mainly of iron, magnesium silicon and oxygen in the form of
silicate compounds. It is solid, but can deform slowly like plastic. The iron core has a
radius of about 3500 km. The outer portion of the core is liquid, containing about 10%
sulfur, and the inner portion is solid. In Mars, the crust is is an average of 50 km thick.
This is thicker than the Earth’s crust, due to the lower internal temperatures. The mantle
is believed to be partially molten but there is no evidence of plate tectonics. The core is
thought to be solid and consist mainly of iron sulfide.
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