Hubble captures rare tri-moon conjunction of Galilean moons

Chuck Bednar for redOrbit.com – Your Universe Online

New images captured by the Hubble Space Telescope depict a rare event known as a tri-moon conjunction, in which three of Jupiter’s largest moons are passing across the banded face of the gas-giant at the same time, NASA officials announced on Thursday.

In the photographs, Europa, Callisto, and Io are shown passing in front of Jupiter on January 24, starting with Callisto and Io appearing above the planet’s cloud tops, the US space agency noted. At this point, while the shadows of all three planets can be seen, Europa itself is not visible.

Near the end of the event, which occurs approximately 42 minutes later, Europa finally shows up in the lower left of one of the image. In this picture, Callisto is above Europa and to its right, and the faster-moving Io is nearing the eastern edge of Jupiter. Io’s shadow is no longer visible in the image, while Europa’s shadow is towards the left side and Callisto’s is on the right.

The three moons depicted in the image are three of the four so-called Galilean moons, named in honor of the man who first discovered them, 17th-century scientist Galileo Galilei. The fourth of the Galilean moons, Ganymede, is missing from the photos. It was outside of Hubble’s field of view and too far away from Jupiter to be part of the conjunction, according to NASA.

The Galilean moons complete orbits around Jupiter with durations ranging from 2 days to 17 days, and can usually be seen transiting the face of Jupiter and casting shadows onto its cloud tops. However, seeing three of the moons transiting the planet’s face at the same time is rare, officials at the agency said. Such an event only takes place once or twice a decade.

In the images, which were taken with Hubble’s Wide Field Camera 3 in visible light, each of the moons are depicted as having distinctive colors. Callisto’s ancient cratered surface is brownish, while Europa’s smooth and ice-covered surface appears to be yellowish-while and Io’s volcanic and sulfur-dioxide-rich surface has an orange-colored hue, NASA researchers explained.

Space.com reports that the entire event lasted approximately 42 minutes, and is so rare because of the different speeds travelled by each of the Galilean moons. Io completes one lap around its host planet every 1.8 days, while Europa takes 3.6 days to orbit Jupiter, Ganymede (the largest natural satellite in the solar system) takes 7.2 days and Callisto requires 16.7 days.

Europa, which is 1,900 miles (3,100 kilometers) wide, is the smallest Galilean moon, the website added. However, beneath its icy shell lies an ocean of liquid water which many scientists believe could harbor chemical reactions that could be conducive to supporting extraterrestrial life.

When NASA revealed its budget for fiscal year 2016 earlier this week, it revealed that it had obtained funding for a potential future mission to Europa. That mission would consist of a new spacecraft, called the Europa Clipper, that would orbit Jupiter and make an estimated 45 flybys of the moon’s surface the span of three years.

Europa’s sub-surface ocean holds three times as much water as the oceans here on Earth, and many experts believe that it organic life could be found within its 62-mile (100 km) deep waters. However, Kevin Hand, JPL’s Deputy Chief Scientist for Solar System Exploration said that the mission will not search directly for life. Rather, it is designed to “understand habitability.”

Hand noted that a mission to actually find life on the Jovian moon would require a mission to its surface, and such an endeavor is currently beyond NASA’s technological capability. Reports indicate that the  Europa Clipper spacecraft could be ready to launch within the next decade, and once it is ready, the Space Launch System (SLS) could carry it to the moon in under three years.

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