The Terrestrial Roster

The terrestrial planets are Mercury, Venus, Earth, and Mars. Except for Earth, all are named after Roman gods. Mercury, the winged-foot messenger of the gods, is an apt name for the planet closest to the sun; its sidereal period is a mere 88 Earth days, and its average orbital speed (30 miles per second or 48 km/s) is the fastest of all the planets. Mercury orbits the sun in less than a college semester, or about four times for each Earth orbit.

Venus, named for the Roman goddess of love and fertility, is (to observers on Earth) the brightest of the planets, and, even to the naked eye, quite beautiful to behold. Its atmosphere, we shall see, is not so loving. The planet is completely enveloped by carbon dioxide and thick clouds that consist mostly of sulfuric acid.

The name of the bloody Roman war god, Mars, suits the orange-red face of our nearest planetary neighbor—the planet that has most intrigued observers and that seems, at first glance, the least alien of all our fellow travelers around the sun.

We looked at some vital statistics of the planets in Chapter 12, "Solar System Family Snapshot." Here are some more numbers, specifically for the terrestrial planets. Notice that the presence of an atmosphere (on Venus and Earth) causes there to be much less variation in surface temperature.

Surface

Radius in

Gravity

Rotation

Surface

Mass in

Miles

(Relative

Period

Temperature

Planet

Kilograms

(and km)

to Earth)

in Solar Days

in K

Mercury

3.3 x 1023

1,488 (2,400)

0.4

59

100-700

Venus

4.9 x 1024

3,782 (6,100)

0.9

-243*

730

Earth

6.0 x 1024

3,968 (6,400)

1.0

1.0

290

Mars

6.4 x 1023

2,108 (3,400)

0.4

1.0

100-250

*The reason for the minus sign is that the rotation of Venus is retrograde; that is, the planet rotates on its axis in the opposite direction from the other planets. Viewed from above the North Pole of the earth, all of the planets except Venus rotate counterclockwise. That means that on Venus, the sun would rise (if you could see it through the thick cloud cover) in the west.

*The reason for the minus sign is that the rotation of Venus is retrograde; that is, the planet rotates on its axis in the opposite direction from the other planets. Viewed from above the North Pole of the earth, all of the planets except Venus rotate counterclockwise. That means that on Venus, the sun would rise (if you could see it through the thick cloud cover) in the west.

If you recall, when we discussed the formation of the solar system, we mentioned a few observational facts that "constrained" our models of formation. A few rules of planetary motions are immediately apparent. All four terrestrial planets orbit the sun in the same direction. All except Venus rotate on their axes in the same direction as they orbit the sun. The orbital paths of the inner four planets are nearly circular. And the planets all orbit the sun in roughly the same plane.

But the solar system is a dynamic and real system, not a theoretical construct, and there are interesting exceptions to these rules. The exceptions can give us insight into the formation of the solar system.

Close Encounter

Most of us in the United States are accustomed to the Fahrenheit temperature scale. The rest of the world uses the Celsius (Centigrade) scale. Astronomers, like most scientists, measure temperature on the Kelvin scale. Throughout this book, we have expressed distance in the units familiar to most of our readers: miles (with kilometers or meters given parenthetically). For mass we give all values in kilograms. The Kelvin scale for temperature is conventional and very useful in astronomy; let us explain.

The Fahrenheit scale is really quite arbitrary, because its zero point is based on the temperature at which alcohol freezes. What's fundamental about that? Worse, it puts at peculiar points the benchmarks that most of us do care about. For example, water freezes at 32° F at atmospheric pressure and boils at 212° F. The Celsius scale is somewhat less arbitrary, because it is laid out such that water freezes at 0° C and boils at 100° C at atmospheric pressure. But the atmospheric pressure of the earth is by no means a universal quantity, so astronomers and others looked for more fundamental benchmarks.

The Kelvin scale is least arbitrary of all. It forces us to ask a fundamental question: What is heat?

The atoms and molecules in any matter are in constant random motion, which represents thermal energy. As long as there is atomic or molecular motion, there is heat (even in objects that, to the human senses, feel very cold). We know of no matter in the universe whose atoms and molecules are entirely motionless, but, in theory, such an absolute zero point does exist. The Kelvin scale begins at that theoretical absolute zero, the point at which there is no atomic or molecular motion. On the Fahrenheit scale, that temperature is -459 degrees. On the Celsius scale, it is -273 degrees. On the Kelvin scale, it is merely 0 degrees. Thus, in the Kelvin scale there are no negative numbers, because absolute zero is—well—absolute. After all, atoms and molecules cannot get any more motionless than motionless, and being absolutely still would cause the constituents of atoms to violate some fundamental physical principles.

Note that water freezes at 273 K and boils at 373 K. If you want to convert Kelvin temperatures to Celsius temperatures, subtract 273 (to be precise, 273.15) from the Kelvin temperature. If you really want to, you can then convert the Celsius temperature to Fahrenheit by multiplying the Celsius reading by 9/5, then adding 32.

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