Hubble has observed other places that may offer clues to our planet's past. One is found in the atmosphere of Saturn's largest moon, Titan. Titan is the second largest moon in the solar system, only slightly smaller than Jupiter's moon Ganymede. In size, it actually can be regarded as a small planet, since it is larger than two other worlds -- Mercury and Pluto.
Titan is a special world in another respect; it shares more in common with Earth than any other solar body. This comparison stems from Titan's dense atmosphere, an atmosphere 60% more dense than Earth's. Like Earth, nitrogen dominates the atmosphere, which includes secondary gases such as methane and argon. These gases are believed to have been present in Earth's early atmosphere and attracted the attention of many astronomers who thought Titan's atmosphere was evolving in a way similar to the primitive atmosphere of our planet.
Photo. Planet Earth. Courtesy Goddard Space Flight Center, NASA.
Rudolf Hanel of the Goddard Space Flight Center believes "...on Titan, we may have a snapshot of the atmospheric evolution that took place on Earth eons ago." Don't expect to find life on Titan; the similarities with Earth end there. Hanel is quick to point out the two are separated by one major difference. Titan's average temperature is a chilly -288 degrees F. He acknowledges that "It is so cold on Titan...nobody can imagine any life having formed there."
Due to Titan's dense atmosphere, the two Voyager spacecraft were unable to gather any information about the surface. Photographs taken as the probes flew by Titan revealed a cloud cover so thick it was opaque to their cameras. Puzzled at a surface seemingly hidden from view, scientists attempted to develop a method to peel away the atmosphere and catch a glimpse of the elusive landscape.
A team of planetary scientists lead by Peter Smith of the University of Arizona discovered a way to accomplish this with Hubble. They used the Wide/Field Planetary Camera 2 at a wavelength of 9400 angstroms to take more than 50 pictures of Titan. At this wavelength, Titan's atmosphere became transparent 40 degrees north and south of the equator. Scientists processed the images to remove variations and leave only constants, such as the surface.
Photo. Titan's surface was finally picked up by the Hubble Space Telescope. Courtesy STSCI, Peter Smith/University of Arizona, NASA. Hubble's four images of the moon's surface were put together to make this GIF animation by The Online Planetarium Show. If your browser is not displaying the animated, spinning image of the moon correctly, we also have the four still images of Titan.
The final map resulted in several discoveries. Two geological features were obvious, one called the Bright Feature and the other known as the Large Dark Area. The Bright Feature is about as large as Australia and could be a high continent surrounded by a darker hydrocarbon ocean.
Photos. The Bright Feature and The Large Dark Area. Both courtesy Space Telescope Science Institute, Peter Smith, University of Arizona, NASA.
Any questions that remain after the new map is evaluated should be answered in the near future. In two years, the Cassini spacecraft will be launched and will deploy a probe which is scheduled to land on Titan in 2004.
