Planets and Moons

All of the planets and moons in our solar system emit strongly in the infrared. This infrared emission is the heat from atmospheres and surfaces which peaks in the mid to far-infrared (15-100 microns - a micron is 1 millionth of a meter). Solar system objects also reflect infrared radiation from the Sun. This reflected radiation peaks in the near-infrared at about 0.5 microns. The study of the infrared radiation from solar system objects has given us much information about their composition. For planets and moons which have atmospheres, infrared studies can show us how the abundance and composition of atmospheric gases as well as how the temperature of the atmosphere vary with depth.

JUPITER and SATURN: Jupiter radiates about 1.6 times as much heat, in the form of infrared energy, as it receives from the Sun. This indicates that Jupiter has an internal source of energy - probably heat created by Jupiter's collapse when it was formed. By studying the infrared emission from Jupiter we can learn much about its cloud structure. Jupiter's belts (its dark horizontal bands) are brighter in the infrared than its zones (its bright horizontal bands) This indicates that the belts are regions of hotter gas. The temperature of Jupiter also increases towards its center, so the zones are at higher levels in Jupiter's atmosphere than the bands. Below (left) is an infrared image of Jupiter and one of its moons, Io (the bright spot on the image). Io has several active volcanoes and is heated by tidal forces caused by the gravitational pull of Jupiter and the other Galilean moons. This causes Io to glow brightly in the infrared. Like Jupiter, Saturn also radiates about twice as much radiation in the infrared as it receives from the Sun. Infrared measurements give us information about the composition and size of particles in Saturn's atmosphere as well as in its large ring system. The infrared image of Saturn below (right) shows the details of its cloud structure.

Phil Nicholson (Cornell), Mike Meyer (UMASS)
Guy Worthey (St. Ambrose University)


URANUS and NEPTUNE: Like Jupiter and Saturn, Uranus and Neptune are mostly gaseous planets, and only the upper layers of their atmosphere can be studied. Infrared studies of Uranus and Neptune can give us information about the composition and structure of their atmospheres. Infrared images also give us a more detailed view of their clouds and rings. In the infrared image of Uranus shown below (left), the different colors represent different altitudes in atmosphere. Because Uranus basically spins on its side, its polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The reason for this is still unknown. Also, unlike the other giant gas planets, Uranus does not radiate more energy than it receives from the Sun. Infrared observations of Uranus may help to solve these mysteries. Neptune does radiate about twice as much energy as it recieves from the Sun, indicating that it, like Jupiter and Saturn, has an internal heat source. Neptune has rapid winds and large storms. It's winds are the fastest in the solar system, reaching 2000 km/hour. Infrared studies of Neptune can tell us about the properties of individual cloud features, and give us information about the planet's atmospheric circulation. In the infrared image shown below (right), the bright region is a massive cloud which is about the size of Europe. Numerous smaller clouds can also be seen.

Infrared view of Uranus
E. Karkoschka et al. (Univ. of Arizona), NICMOS, HST, NASA

Infrared image of Neptune
Don Banfield (Cornell), JPL, NASA

TITAN and TRITON: In 1944, Gerard Kuiper discovered that Titan, the largest moon of Saturn, had an atmosphere. He detected the methane in Titan's atmosphere by studying its infrared emission. In 1994, astronomers using the Wide Field Planetary Camera on the Hubble Space Telescope made the first images of the surface of Titan. These images were made in the near-infrared since infrared radiation is able to penetrate the hazy atmosphere of Titan. Titan is larger than the planet Mercury and only a bit smaller than Mars. The infrared images of Titan's surface show a bright area which is a surface feature that is about 2,500 miles across (about the size of Australia). The left image below is a set of four near infrared pictures of Titan. Compare this to the visible light image of Titan on the right taken by Voyager 2. Triton, the largest moon of Neptune, also has an atmosphere. Triton's thin atmosphere is composed mainly of nitrogen with a small amounts of methane. This atmosphere most likely originates from Triton's volcanic activity which is driven by seasonal heating by the Sun. Triton, Io and Venus are the only bodies in the solar system besides Earth that are known to be volcanically active at the present time. Infrared observations of Triton can give us additonal information about the composition of its atmosphere and help us learn more about its volcanic activity.

Infrared (left) and visible (right) views of Titon
NASA and the NSSDC

The Spitzer Space Telescope will study the infrared emission from the outer planets and moons in our solar system. Although Spitzer is not designed to provide sharp images of the outer planets, such as Uranus and Neptune, it will be able to measure the temperature variations and chemical compositions within their atmospheres. Spitzer will also study the thermal properties of Pluto. The largest moons of the outer planets will also be studied with Spitzer. Moons which have an atmosphere, such as Titan (the largest moon of Saturn) and Triton (the largest moon of Neptune) will receive special attention. The Spitzer Space Telescope will be used to help scientists understand the composition and study large-scale changes in the atmospheres of these two moons.