Geology Reference
In-Depth Information
anttidalforces.Consequently,theybothhavedeep,encircling,ice-coveredoceans—places
targeted by NASA, in its ongoing search for life on other worlds.
Saturn, the next planet out from the Sun, is endowed with almost two dozen moons, not
tomentionagloriousringsystemdominatedbysmallbitsofbrilliantlyreflectivewaterice.
Most of Saturn's moons are relatively small, some captured asteroids and others formed
from Saturn's gassy leftovers; but its largest moon, Titan, is bigger than Mercury and en-
shrouded in a thick orange atmosphere. Thanks to the European Space Agency's Huygens
lander, which touched down on January 14, 2005, we have close-up views of Titan's dy-
namic surface. A branching network of rivers and streams feeds frigid lakes of liquid hy-
drocarbons; the dense, colorful, and turbulent atmosphere is laced with organic molecules.
Titan is yet another world worth exploring for signs of life.
The most distant gas giant planets, Uranus and Neptune, are no less well endowed with
interesting moons. Most show signs of water ice, organic molecules, and ongoing dynamic
activity.Neptune'sbigmoonTritonevenhasanitrogen-rich atmosphere. AndbothUranus
and Neptune have their own complex ring systems, though apparently they are composed
ofautomobile-size chunksofdarkcarbon-rich material, quite unlike theluminous particles
that constitute the icy rings of Saturn.
Rocky Worlds
Closer to home, gravity also held sway. With most of the hydrogen and helium blown
outward to the realm of the gas giants after ignition of the Sun, the inner Solar System
had much less mass to play with, and most of that consisted of hard rocks—the stuff of
chondrite and achondrite meteorites. Mercury, the smallest and driest rocky planet, formed
closest to the Sun. A hostile scorched world, this innermost planet appears to be dead and
battered: its billions of years of intensely cratered surface is preserved under an airless sky.
If you are ever asked to name objects in the Solar System where you'd bet against life,
Mercury should top your list.
Venus, the next planet out, is Earth's twin in size but radically different in habitability,
thanks in large measure to its orbit, almost thirty million miles closer to the Sun. It may
have had a modest store of water early in its history, and even a shallow ocean, but subjec-
ted to the Sun's heat and solar wind, most Venusian water appears to have boiled off, pre-
venting that world from being wet. Carbon dioxide, the dominant gas in the thick Venusian
atmosphere, sealed in the Sun's radiant energy and created a runaway greenhouse effect.
Today Venus's average surface temperatures exceed 900 degrees Fahrenheit—hot enough
to melt lead.
Mars, one stop out from Earth, is a lot smaller—only a tenth its mass—but it is in many
respectsthemostEarth-like.Likealltherockyplanets,Marshasametalcoreandasilicate
