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was –180°C, the surface was more like a thick organic stew
than a solid or liquid.
Cassini has also discovered that Saturn’s icy moon Encel-
adus has a significant atmosphere. Before this discovery, Titan
was the only moon in the solar system known to have an atmo-
sphere. Enceladus is too small to hold onto an atmosphere, so it
must have a continuous source that may be volcanic, geysers, or
gases escaping from the interior.
Cassini also provided a test of Einstein’s general theory of
relativity (p. 198). According to this theory, a massive object
such as the Sun should cause spacetime to curve. This curve
means that it should take longer for anything to travel by the
Sun since a curved path is a greater distance. The general theory
of relativity was tested by analyzing radio waves that traveled
by the Sun on their way from Cassini to Earth. The researchers
found that the radio waves traveling by the Sun were indeed de-
layed, and the amount of delay agreed precisely with predictions
made from Einstein’s theory.
The Cassini-Huygens mission is a cooperative project of
NASA, the European Space Agency, and the Italian Space Agency.

URANUS AND NEPTUNE
Uranus and Neptune are two more giant planets that are far, far
away from Earth. Uranus revolves around the Sun at an average
distance of over 19 AU, taking about 84 years to circle the Sun
once. Neptune is an average 30 AU from the Sun and takes about
165 years for one complete orbit. Thus, Uranus is about twice as
far away from the Sun as Saturn, and Neptune is three times as far
away. To give you an idea of these tremendous dis tances, consider
that the time required for radio signals to travel from Uranus to
A
Earth is more than 2.5 hours! It would be most difficult to carry
on a conversation by radio with someone such a distance away.
Even farther away, a radio signal from a transmitter near Neptune
would require over 4 hours to reach Earth, which means 8 hours
would be required for two people to just say “Hello” to each other!
Uranus and Neptune are more similar to each other than Sat-
urn is to Jupiter (Figure 15.13). Both are the smallest of the giant
B
planets, with a diameter of about 50,000 km (about 30,000 mi),
FIGURE 15.11 (A) This image, made by the Hubble Space which is about one-third the size of Jupiter. Both planets are
Telescope, clearly shows the large impact site made by fragment G thought to have similar interior structures (Figure 15.14), which
of former Comet Shoemaker-Levy 9 when it collided with Jupiter. consist of water and water ice surrounding a rocky core and an at-
(B) This is a picture of Comet Shoemaker-Levy 9 after it broke into mosphere of hydrogen and helium. Because of their great distances
22 pieces, lined up in this row, then proceeded to plummet into
Jupiter during July 1994. The picture was made by the Hubble from the Sun, both have very low average surface temperatures.
Space Telescope.

15.2 SMALL BODIES OF
(meaning carbon-containing) molecules. Most of the clouds THE SOLAR SYSTEM
on Titan occur over the south pole, and scientists believe this
is where a cycle of methane rain, runoff with channel carving, Comets, asteroids, and meteorites are the leftovers from the for-
and liquid methane evaporation is most active. mation of the Sun and planets. Presently, the total mass of all
On January 14, 2005, Cassini delivered a detachable these leftovers in and around the solar system may account for
probe, called Huygens, to the moon Titan. The probe dropped a significant fraction of the mass of the solar system, perhaps as
by parachute, sampling the chemical composition of the at- much as two-thirds of the total mass. It must have been much
mosphere for the 2.5 hour descent and for 90 minutes af- greater in the past, however, as evidenced by the intense bom-
ter landing. It landed with more of a “splat” than a thud or bardment that took place on the Moon and other planets up to
splash, indicating that even though the surface temperature some 4 billion years ago.

388 CHAPTER 15 The Solar System 15-12