Page 304 - NS-2 Textbook

ASTRONOMY 299

number
planetary diameters distance from Sun orbital period of
moons
•
Mercury 3000 miles 4/10 114(88 days) o
()
Venus 7526 miles 7110 315(225 days) o
(J
Earth 7918 miles 1 = 93,000,000 miles 1 =365 days
•
Mars 4200 miles 1-1/2 1-9110(687 days) 2

5 12 years 63

9-112 29 years 50
G)

Uranus 29,850 miles 19 84 years 27
o

Neptune 31,250 miles 30 165 years 13
•
Pluto (1430) miles 39 248 years

Table of data for the nine traditional planets (data current as of mid-2005). Distances from the Sun are given as multiples of Earth's mean
distance from the Sun (called an astronomical unit), and orbital periods are given in Earth days and years.

tell the difference. Unlike most planets, Venus spins Venus in 1991 and began an extended radar survey of the
clockwise, opposite to its orbit armmd the Sun. planet in strips 10 to 17 miles wide. This effort enabled
Photographed by Mariller 10 on its way to Mercury scientists to view details the size of a football field in
and Piolleer 12, as well as by cameras aboard the Galileo razor-sharp detail. Magellall showed that the surface was
spacecraft on its way to Jupiter in 1991, the clouds above full of enormous lava flows, lmexpected pancake-like
Venus race at more than 200 miles per hom from east to structures, and large impact craters up to 120 miles in di-
west. These clouds lie up to 40 miles above the huge, shal- ameter. Large areas of the surface} however, are relatively
low craters that have been detected on the stu·face. Above smooth, with very few slnall craters, indicating that the
the clouds, a haze extends another 15 miles. Findings sent surface is probably only about 400-800 million years old,
back so far do not reveal the composition of the inner relatively young on a geological time scale. The absence
cloud layers. The tops, however! seem to consist mainly of of small craters indicates that the planet has had a thick
fine sulfmic-acid droplets-a mist that is thought to be atmosphere for at least that long.
more corrosive than automobile-battery acid. Magellall's radar maps showed no signs of past major
While the missions to Venus described above were water bodies such as shorelines or ocean basins. Unlike
certainly important, all the data they accumulated was Earth, there is no evidence of plate tectonics (move-
dwarfed by the last u.s. space probe to visit the planet, ments of crustal mass), which may indicate it lacks an
the Magellall spacecraft. Launched in 1989, it reached asthenosphere between its crust and mantle. The distrib-