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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