IV.

Jupiter Encounter

On July 13, 1995, five months before Galileo’s arrival at Jupiter, the atmospheric probe was released from the spacecraft to fly on its own toward the giant planet. The probe entered the Jovian atmosphere on December 7, 1995, at 2:04 pm, Pacific Daylight Time (PDT), and began its fiery descent at a speed of more than 160,000 km/h (100,000 mph), deploying its 2.5-m (8-ft) parachute. Two minutes later the craft dropped its protective heat shields so that the probe could collect data.

The probe radioed atmospheric data to the orbiter for relay to Earth. Scientists were surprised when preliminary probe data indicated that Jupiter had much less water than expected, but further research indicated that the probe entered Jupiter’s atmosphere in a particularly dry spot. Toward the end of its descent, the probe detected winds of up to 530 km/h (330 mph) with intense turbulence, suggesting that Jupiter’s winds are driven by heat escaping from the planet’s interior. This differs from planets like Earth, Venus, and Mars, whose winds are driven by solar energy. The probe found less helium, neon, carbon, oxygen, and sulfur than expected. And it discovered that lightning occurs on Jupiter only about one-tenth as often as on Earth. As expected, the probe encountered no solid objects or surfaces during its entire 600-km (373-mi) plunge. After 57 minutes, the extreme temperature and pressure of Jupiter’s atmosphere destroyed the probe.

The Galileo orbiter continued to transmit data to Earth while orbiting Jupiter and making repeated passes of Jupiter’s large moons Ganymede, Europa, Io, and Callisto. Galileo has revealed that the three moons Ganymede, Europa, and Io have fairly strong magnetic fields, which means that the moons probably have cores of liquid metal. Molten metal cores provide heat, which could make the moons hospitable to some forms of life (see Exobiology). Galileo’s photographs of Europa and Ganymede suggest that these moons may have vast oceans of liquid water underneath their frozen surfaces.