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TABLE 15.1

Properties of the planets
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Average distance from the Sun:
6
in 10 km 58 108 150 228 778 1,400 3,000 4,497
in AU 0.38 0.72 1.0 1.5 5.2 9.5 19.2 30.1
Inclination to ecliptic 7° 3.4° 0° 1.9° 1.3° 2.5° 0.8° 1.8°
Revolution period 0.24 0.62 1.00 1.88 11.86 29.46 84.01 164.8
(Earth years)
Rotation period 59 days –243 days* 23 h 24 h 9 h 10 h –17 h* 16 h
(Earth days, h, 56 min 37 min 50 min 39 min 14 min 6.7 min
min, and s) 4 s 23 s 30 s
Mass (Earth = 1) 0.05 0.82 1.00 0.11 317.9 95.2 14.6 17.2
Equatorial dimensions:
diameter in km 4,880 12,104 12,756 6,787 142,984 120,536 57,118 49,528
in Earth radius = 1 0.38 0.95 1.00 0.53 11 9 4 4
3
Density (g/cm ) 5.43 5.25 5.52 3.95 1.33 0.69 1.29 1.64
Atmosphere None CO 2 N 2 , O 2 CO 2 H 2 , He H 2 , He H 2 , He, CH 4 H 2 , He, CH 4
(major compounds)
Solar energy received 13.4 3.8 2.0 0.86 0.08 0.02 0.006 0.002
2
(cal/cm /s)
*Negative means spin is opposite to motion in orbit.

gases for an atmosphere. The lack of an atmosphere to even by lava in the past, some time after most of the impact craters
the heat gains from the long days and heat losses from the long were formed (Figure 15.4).
nights results in some very large temperature differences. The A spacecraft named MESSENGER is now on its way to
temperature of the surface of Mercury ranges from above the investigate the planet Mercury. Its name is an acronym for
melting point of lead on the sunny side to below the tempera- mercury surface, space environment, geochemistry, and rang-
ture of liquid oxygen on the dark side. ing. It was launched August 3, 2004, on a 7.9 billion km (about
Mercury has been visited by Mariner 10, which flew by 4.9 billion mi) trip designed to slow the spacecraft as it falls toward
three times in 1973 and 1974. The photographs transmitted by the Sun. Overall, it looped by Earth in August 2005, then twice
Mariner 10 revealed that the surface of Mercury is covered with by Venus in October 2006 and June 2007, and then three times
craters and very much resembles the surface of Earth’s Moon. by Mercury in January 2008, October 2008, and September 2009.
There are large craters, small craters, super imposed craters, and Eventually, it will be slow enough to be captured by the planet
craters with lighter colored rays coming from them just like the Mercury as it flies by in March 2011. It will then become the first
craters on the Moon. Also as on Earth’s Moon, there are hills spacecraft to orbit Mercury. Mercury has a high surface tem-
and smooth areas with light and dark colors that were covered perature, and the spacecraft instruments will be protected from
high radiant energy from Mercury and the Sun by a sunshade of
heat-resistant ceramic fabric. There is a NASA mission page at
www.nasa.gov/mission_pages/messenger/main/index.html.
Mercury has no natural satellites, or moons, it has a weak
Mercury (showing phases)
magnetic field, and it has an average density more similar to
that of Venus or Earth than to that of the Moon. The presence
Sun
of the magnetic field and the relatively high density for such a
small body must mean that Mercury probably has a relatively
large core of iron with at least part of the core molten. Because
Earth of its high density, it is thought that Mercury lost much of its less
dense, outer layer of rock materials during its formation.

FIGURE 15.3 Mercury is close to the Sun and is visible only VENUS
briefly before or after sunrise or sunset, showing phases. Mercury
actually appears much smaller and is in an orbit that is not tilted Venus is the brilliant evening and morning “star” that appears
as much as shown in this figure. near sunrise or sunset, sometimes shining so brightly that you

380 CHAPTER 15 The Solar System 15-4