Galileo Probe Finds Surprises on the Surface of Ganymede
The spacecraft Galileo astounded astronomers with its first close-up pictures of the planet-sized Ganymede, Jupiter's largest moon and the largest in the entire solar system. The photographs reveal that its surface has many deep ridges and wrinkles and cracks and crevices that suggest the moon is being rocked by "dynamic" forces like those that cause earthquakes, move mountains and shift continents on Earth.
Photo. The Galileo Regio region shows close-up the dark material that covers roughly half of the moon's surface. A larger size is also available. Courtesy NASA/Jet Propulsion Laboratory.
Ganymede, larger than the planets Mercury and Pluto and nearly three-quarters the size of Mars, is covered largely by rocky matter and ice. The large craters on the surface also indicate severe bombardments of asteroids and comets. Many of the craters have since been covered over by what appears to be volcanic or tectonic activity, all of which further wrinkled and warped the surface.
Photos. Voyager 2 / Galileo. These two images of the same region of Ganymede compare the resolution returned by the Voyager 2 spacecraft and by the Galileo craft. Each pixel in the left frame is four-fifths of a mile across while each pixel in the right frame is just 243 feet. This means that the resolution of the Galileo picture, on the right, is 17 times better than that of the Voyager 2 photograph, which was taken when the craft passed by in 1979. While the Voyager captured the light-colored ridges and dark areas, there was little detail; the picture returned by Galileo, however, shows much more definition and many intricate ridges and ripples not seen from Voyager. A larger size is also available. Courtesy NASA/Jet Propulsion Laboratory.
The most startling discovery was captured not in a picture but instead by Galileo's magnetometer, which recorded a definite rise in magnetic force when the craft passed by the moon. Scientists say this indicates that Ganymede has a magnetic field, like Earth's but far weaker. This may mean that a hot, molten metallic core lies embedded below the chilly, frozen surface, generating the moon's magnetosphere (magnetic field).
One of the craft's scientific instruments detected a sharp increase in the density of charged particles, which make up Ganymede's "bubble-shaped" magnetosphere. Ganymede -- its "baby" magnetosphere lying within Jupiter's larger, stronger magnetic field -- is the only known satellite to have a magnetic field. It is yet another of the moon's many planet-like characteristics.
Galileo flew by the moon on June 27th, but its antenna was slow sending the data back to Earth, where pictures were constructed from the data and released on July 10th. Since its launch in 1989, the craft has experienced major technical malfunctions, especially when the main antenna refused to deploy. NASA's Jet Propulsion Laboratory was forced to change the craft's software so that some data could be compressed before being sent over the low-power back-up antenna. In September, when Galileo is slated to fly past Ganymede again, pictures and data from the first encounter will still be arriving on Earth. Even so, the craft is expected to successfully perform three-quarters of its planned activities near Jupiter and its four largest moons.
Photo. The Uruk Sulcus region shows the variety of features on Ganymede's surface: rough ridges, smooth spots, and crevices and creaters. A larger size is also available. Courtesy NASA/Jet Propulsion Laboratory.