Chapter 9 - The Terrestrial Planets and Their Satellites: Why ...
Chapter 9 - The Terrestrial Planets and Their Satellites:
Why are they Different?
? In addition to the Earth and Moon, the other terrestrial planets are Mercury,
Mars, and Venus.
? By comparing these disparate worlds one can gain insight into the processes
that made and modified their surfaces: volcanism, plate tectonics, erosion by
wind, water, and ice, and chemical alteration.
? One can examine how these mechanisms are determined or influenced by the
size of a planet, its distance from the Sun, and the presence of a significant
atmosphere, which itself is determined by the gravitational force at a planet¡¯s
surface.
Chapter Photo. Sunrise at Mercury. From nrl.navy.mil
Key Physical Concepts
? comparative planetology of the terrestrial planets,
? correlation of the characteristics of planets with their size and heliocentric
distance,
? evolution of the Martian atmosphere
I.
Introduction
We will study the three terrestrial planets, other than the Earth: Mercury, Venus, and
Mars.
II. Mercury
? Mercury is the closest planet to the Sun.
? small size, 2/5 the diameter of the Earth, and nearness to the much brighter
Sun, it is usually difficult to see without aid of a telescope.
? when it¡¯s separation from the Sun is at maximum it can be seen with the naked
eye close to the Sun either shortly after sunset or before sunrise.
? Mercury, named for the Roman god who acted as the messenger of the gods,
zips around the Sun with a period of 88 days, but rotates on its spin axis only
once each 59 days.
Table 9.1: Bulk Properties of Mercury
Property
Measurement
Mass
0.055 MEarth
Radius
2439 km
0.382 REarth
Average Density
5430 kg/m3
Average Distance from the Sun
0.387 AU
Orbital Period - Sidereal
88 Earth days
Orbital Eccentricity
0.206
Sidereal Rotational Period
58.7 Earth
days
? Mercury¡¯s cratered surface appears somewhat similar to Moon, with no lunarlike maria on Mercury, only broad plains between craters.
? If maria formed on Mercury as they did on the Moon, cratering must have
occurred after Mercury¡¯s maria formed, leaving no smooth maria but cratered
plains between the Mercurian highlands.
Figure 1. A comparison of the surfaces of Mercury, Venus, Earth, and Mars (to
scale). The images of Mercury, Earth and Mars are true-color images. The Venus
image is constructed from Magellan spacecraft radar imagery and shows
topographical features with shades of red (false-color).
? The most dramatic feature on the surface of Mercury is the Caloris Basin
(Figure 2), 1300 kilometers in diameter, surrounded by a 2 kilometer-wide
rim of mountains, which is in turn surrounded by plains.
? In the interior of the Caloris Basin are a few, widely spaced craters indicating
a relatively young, lightly cratered surface.
? The impact that formed the Caloris Basin also caused a wrinkled hilly region
directly on the opposite side of the planet covering one-half million square
kilometers.
Figure 2. A Mariner 10 spacecraft image of Mercury. NASA.
Figure 3. The surface of Mercury antipodal (opposite) to the Caloris Basin.
NASA.
? Mercury is covered by gently rolling plains and scarps, or cliffs
? which indicate a thin lithosphere that cracked as Mercury cooled and
contracted.
? This lack of geological activity suggests that Mercury is covered by a thick
crust.
? Scarps are thought to have formed by the solidifying and shrinking Mercurian
core and mantle,that rumpled the surface like an orange shrunken and
wrinkled in the hot Sun.
Figure 4. A scarp on Mercury¡¯s surface. A scarp, or cliff, 1 km high and 100 km
long cuts through several craters on the surface of Mercury. (NASA)
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