February 14, 2014

The wildly complicated surface of Ganymede, the largest moon in the solar system, appears in colorful detail in a new map published by the United States Geological Survey. The map is based on images taken by NASA’s Voyager 1 and Voyager 2 spacecraft, which flew by Jupiter and its moons in 1979, and the Galileo probe, which orbited Jupiter from 1995 to 2003. The map is the first global geologic map of Ganymede, Jupiter’s seventh moon, or any other icy moon, for that matter.

Ganymede, also Jupiter’s largest moon, is bigger than the planet Mercury and almost as large as Mars. Ganymede is one of four Jovian moons discovered by the Italian scientist Galileo Galilei in 1610. From observations using Earth-based telescopes and orbiting spacecraft, scientists have long known that Ganymede’s icy surface is made up of almost equal amounts of dark and light material. A thin ocean of salty water may lay about 105 miles (170 kilometers) below the surface. The presence of water makes Ganymede and the other large moons of Jupiter—along with Mars—prime candidates in the search for extraterrestrial life.

The false colors in a new map of Ganymede reveal geologic features created during different times in the moon's history. (USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech)

The new false-color map, which was created by a team of scientists headed by Geoffrey Collins of Wheaton College in Massachusetts, provides the first solid evidence for three major geological periods in the moon’s history. During the oldest period, represented in brown, asteroids and comets bombarded the surface, leaving many impact craters. Forces released by the biggest impacts also created large cracks in the moon’s crust. Lighter colors on the map represent two somewhat younger periods. During one of these, changes in the temperature of the moon caused the surface to expand and crack. The new terrain (land) created by these tectonic forces was marked with parallel sets of ridges and valleys. During the third period, geologic activity declined. As the crust settled, new ridges and valleys appeared.

Additional articles in World Book:

• Callisto
• Europa
• Io
• Satellite (Satellites of the gas giants)
• Probing the Planets (a Special Report)
• The Thirty-Year Journey of Voyagers 1 & 2 (a Special Report)

September 10, 2012

Bearing greetings from Earthlings now more than 11 billion miles (18 billion kilometers) away, the Voyager 1 spacecraft continued its epic trek to the edge of the solar system on September 5, 35 years after its launch into space. NASA had launched a companion craft, Voyager 2, on August 20, also in 1977. Together, the two probes have lasted longer and traveled farther than any other space probes in history. Voyager 2 is now about 9 billion miles (15 billion kilometers) from the sun, going south, the opposite direction from Voyager 1.

The space probe Voyager 2 was launched on Aug. 20, 1977. Its path through the solar system is shown in red. Voyager 2 flew past and photographed Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. (World Book illustration by Ken Tiessen, Koralik Associates)

Initially, the two spacecraft were given a five-year mission to explore the gas giants Jupiter and Saturn. The data they sent back to Earth have answered many questions about the planets’ atmospheres, interiors, rings, magnetospheres (surrounding zones of strong magnetic fields), and satellite systems. With this part of the Voyager mission completed, engineers at NASA’s Jet Propulsion Laboratory in Pasadena, California, in 1981 decided to push the two craft farther into space. Voyager 2 became the first–and so far–only spacecraft to visit Uranus and Neptune. They found evidence of geologic activity on two previously known moons—volcanoes on Jupiter’s moon Io and icy geysers on Neptune’s moon Triton.

Since 1989, the two probes have been exploring the heliosphere, a vast, tear-drop-shaped region of space that contains the solar system, the sun’s magnetic field, and the solar wind (electrically charged particles given off by the sun). In late 2004, Voyager 1 crossed a shock wave called the termination shock, becoming the first craft to reach the region of space that lies just inside the heliopause (the final boundary of the solar system). The crossing occurred at a distance of about 8.7 billion miles (14 billion kilometers) from the sun. In 2007, Voyager 2 crossed the shock in a different area and at a distance of about 7.8 billion miles (12.6 billion kilometers) from the sun. By detecting the shock at different distances from the sun, the two craft confirmed scientists’ belief that the solar system is not perfectly round.

The two Voyager spacecraft explore the outer edges of the heliosphere, a vast, tear-drop-shaped region of space that contains the solar system, in an artist’s illuustration. (NASA/JPL-Caltech)

Scientists are not sure when Voyager 1 and 2 will leave the heliosphere and go where no spacecraft has gone before–interstellar space. Some signals from Voyager 1 suggest that the craft’s escape from the solar system may occur in the near future. Regardless, the two spacecraft will have enough electrical power to continue collecting data and communicating it back to Earth through 2020, and possibly through 2025.

Additional World Book articles:

• Space exploration
• The Thirty-Year Journey of Voyagers 1 & 2 (A Special Report)
• Astronomy (1979) (A Back in Time article)
• Astronomy (1989) (A Back in Time article)
• Space Exploration (1977) (A Back in Time article)
• Space Exploration (1979) (A Back in Time article)