Hubble Captures Rare Transit of Moons Across Jupiter

Hubble Captures Rare Transit of Moons Across Jupiter

Hubble Telescope Images of Jupiter Capture Unusual View of Three Moons Crossing Its Face: Europa, Callisto and Io

By Gautam Naik in the Wall Street Journal

New images from the Hubble space telescope show three of Jupiter’s largest moons moving across the face of the planet at the same time, a rare occurrence that happens only once or twice a decade.

The moons of Jupiter are often seen moving across Jupiter’s banded face and casting shadows on the layers of clouds below. But on Jan. 23, the Hubble telescope snagged the unusual view of three in the same frame. The moons in the photographs are Europa, Callisto and Io.

Jupiter and its moons have long been a source of fascination for astronomers. The planet has 50 moons, though many are small orbs. But one of them, Ganymede, is bigger than the planet Mercury and the largest moon in the solar system.

In 1610, Galileo Galilei observed Jupiter’s four largest moons through a homemade telescope. His realization that the moons weren’t stars but planetary bodies in orbit around Jupiter provided evidence to support the Copernican system, revealing that not every planetary body revolved around the Earth. His findings challenged the Catholic Church’s beliefs and led to Galileo being put on trial for heresy and imprisoned.

Now astronomers are once again setting their sights on the planet. The National Aeronautics and Space Administration is asking for $30 million in its fiscal year 2016 budget request to plan a mission to Jupiter’s moon Europa, a planetary body that many astrobiologists believe could have a “habitable zone” and therefore the potential to harbor some form of life.

Europa’s surface is mostly water ice, and there also appears to be a vast ocean of water or slushy ice beneath it. In all, the moon could have twice the amount of water as Earth. Scientists note that because life has been found in some of Earth’s extreme environments—near undersea volcanoes, for example—similar harsh locations on Europa may also harbor life.

NASA is asking for $30 million in its fiscal year 2016 budget request to plan a mission to Jupiter's moon Europa which it calls one of the most likely places to find current life beyond Earth. WSJ's Monika Auger reports.