Chapter 9 - The Terrestrial Planets and Their Satellites ...

[Pages:33]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 Radius

0.055 MEarth 2439 km

Average Density

0.382 REarth 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)

At 5% of the mass of Earth, Mercury has no atmosphere except for hydrogen and helium boiled from Sun and subsequently impacting Mercury's surface and absorbed by surface rocks, and sodium and potassium that were ejected from surface rocks when they were hit by the solar wind.

Mercury has a density of 5430 kg/m3, close to Earth's (5520 kg/m3). Both Mercury and Earth have high density iron cores. The Earth's greater mass compresses it giving it a slightly higher density. Models of Mercury's interior predict a mantle approximately 600 kilometers

thick.

Figure 5. Scale drawing of the interiors of Mercury, Venus, Moon, and Mars. From

Mercury is located close enough to the Sun so that it is not expected to contain low-density volatile materials.

As a result, Mercury is the most iron-rich planet in the Solar System. Mercury's weak magnetic field (about 1% of the strength of the Earth's) and

high density indicate that it has an iron core. It has not been settled whether Mercury's iron core is molten or solid.

 Because of the extremely long day and closeness to the Sun, the surface temperature of Mercury varies wildly, from 700 K (800 degrees F) to 100 K (-280 degrees F).

This compares to a typical 11 K variation between night and day for a location on Earth.

In 1991 Caltech scientists used radar to determine that Mercury's north polar region is highly reflective.

Polar areas are deeply shadowed from sunlight and are therefore extremely cold.

It is presumed that the highly reflective polar cap is indicative of a water ice cap.

Even though the daytime surface of Mercury can be extremely hot, the north polar region is cold enough to support the existence of water ice.

III. Venus

Venus can be regarded as the Earth's twin, with nearly the same size, density, and mass as the Earth.

But it is nearly one-third closer to the Sun.

Venus appears to have twice the maximum angular separation from the Sun as Mercury.

Venus is second in brightness only to Sun and Mercury and can often be seen during the day.

Figure 6. An ultraviolet image of Venus taken by the Pioneer orbiter shows the global cloud patterns in the planet's upper atmosphere. Surface features are hidden from view by the opaque Venusian atmosphere. (NASA).

The Venusian surface is permanently overcast with clouds, so that the surface cannot be seen in visible light.

In the 1960s the Soviets and US sent probes to inspect Venus.

Soviet probes penetrated the atmosphere. In 1970 a Soviet probe survived the high surface temperature and corrosive atmosphere, measuring surface temp of 750 K (900 F) and pressure equivalent to a depth of nearly 1 kilometer under water.

Table 9.2: Bulk Properties of Venus

Property

Measurement

Mass Radius

0.815 MEarth 6051 km

Average Density

0.949 REarth 5250 kg/m3

Average Distance from the Sun 0.723 AU

Orbital Period - Sidereal

224.7 Earth

days

Orbital Eccentricity

0.007

Sidereal Rotational Period

243 Earth

days

retrograde

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