jupiter's Icy Water-Moon

 In the colder, darker outer regions of our Solar System, a quartet of majestic giant planets circle our Sun. Of these gigantic, distant worlds, the banded behemoth, Jupiter, stands out in the crowd as by far the largest planet in our Sun's family. Jupiter, the "King of Planets", reigns in splendor from where it is situated beyond the terrestrial planet Mars, and the Main Asteroid Belt that separates the two very different sibling worlds. Jupiter is classified as a gas-giant that may--or may not--contain a small solid core well-hidden beneath its dense and heavy blanket of gas. This gigantic gaseous world is also orbited by an impressive retinue of mostly icy moons, four of which--Io, Europa, Ganymede, and Callisto--were discovered by Galileo in 1610, and were named the Galilean moons in his honor. Of the four moons, small, cracked, icy Europa stands out as a potentially habitable small moon-world, that is thought to have a sloshing, swirling subsurface ocean of life-sustaining liquid water beneath its cracked shell of ice. In November 2019, this possibility was further strengthened because planetary scientists received new evidence that this important ingredient for sustaining life as we know it may sometimes be shot out into space from enormous geysers pock-marking the frozen moon's mysterious surface. Life as we know it cannot exist without liquid water, and its presence indicates the possibility--though not the promise--that life exists on this distant moon-world.

Four decades ago, a traveling Voyager spacecraft obtained the first up close and personal images of Europa. These pictures revealed brownish cracks tearing through the moon's icy surface, making Europa look like a jumbo-sized egg with a cracked shell. Missions to the outer Solar System over the past forty years have since collected sufficient additional data about Europa to make it a high-priority target of investigation for NASA scientists searching for life beyond Earth.

What makes Europa so intriguing is the fascinating possibility that it may possess all of the ingredients necessary for the emergence and evolution of life. In November 2019, an international team of astronomers, led by NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, announced that they were able to confirm the presence of water in the plumes of Europa's geysers. They did this by directly measuring the water molecule itself. Up until their study, no one had been able to confirm the presence of water in these plumes by directly measuring the water molecule. The team measured water vapor by studying Europa through the W.M. Keck Observatory in Hawaii, one of the world's biggest telescopes.

Jupiter's Bewitching Moon

Europa, along with Jupiter's three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on January 8, 1610. The quartet of bewitching Jovian moons may also have been discovered independently by the German astronomer Simon Marius (1573-1625). The first reported observation of Io and Europa was made by Galileo on January 7, 1610. Galileo used a small refracting telescope--one of the first telescopes to be used for astronomical purposes--to make his discovery at the University of Padua. However, in that initial observation, Galileo was unable to distinguish Io and Europa as separate bodies because of the low magnification of his primitive telescope. For that reason, Io and Europa were recorded by Galileo as a single point of light. The next night, on January 8, 1610--the discovery date for Europa used by the International Astronomical Union (IAU)--Io and Europa were observed for the first time as separate moons during Galileo's observations of the Jovian system. Historically, this also marked the first time that a moon had been discovered in orbit around a planet other than Earth. Before Galileo's discovery, Earth's Moon was the Moon--the only Moon known to exist.

Europa is the smallest of the quartet of Galilean moons, and it is the sixth-closest moon to its parent-planet out of all the 79 known moons of Jupiter. It is also the sixth-largest moon in our Solar System, and it is only slightly smaller than Earth's large Moon. Europa is primarily composed of silicate rock, and its crust is made up of water-ice. It probably also has an iron-nickel core, as well as a very tenuous atmosphere that is composed mainly of oxygen. Also, this mysterious icy moon's surface is slashed with streaks and cracks. However, this frozen surface is scarred by very few craters. This suggests that Europa's icy shell is young, because smooth crusts indicate recent resurfacing that has erased previous cratering impacts. In addition to telescopes on Earth, Europa has been observed by a succession of space-probe flybys, the first of which occurred back in the early 1970s.

Indeed, Europa sports the smoothest surface of any known solid body in our Solar System. The apparent youth and smoothness of its icy surface suggests that a water ocean sloshes beneath it, which could possibly host extraterrestrial life-forms. The most widely accepted model proposes that heat resulting from tidal flexing causes the ocean to remain in a liquid form. This tidal flexing also drives ice movement that is similar to plate tectonics, and this could result in important life-sustaining chemicals from the surface being transferred into the ocean below. Sea salt from a subsurface ocean may be coating some geological features on Europa. This suggests that the ocean is, indeed, interacting with the sea floor. That observation may prove to be an important factor in determining whether Europa could be habitable. Furthermore, the Hubble Space Telescope (HST) spotted water vapor plumes similar to those seen on Enceladus of Saturn, which are believed to be the result of erupting cryogeysers (icy geysers). In May 2018, astronomers provided supporting evidence of water plume activity on Europa, that was based on an updated analysis of data obtained from the Galileo space probe, which circled Jupiter from 1995 to 2004. Similar plume activity could help astronomers search for signs of life swimming in the subsurface Europan ocean without having to actually land on the distant icy moon.

The Galileo mission, launched in 1989, currently provides most of the data on Europa. No spacecraft has yet landed on this distant icy mystery-moon, although there are several proposed future missions. The European Space Agency's (ESA's) Jupiter Icy Moon Explorer (JUICE) is a mission to Ganymede scheduled to launch in 2022, and it will include two flybys of Europa. NASA's upcoming Europa Clipper is scheduled to launch in 2025.

Europa circles its parent-planet in just a little more than three and a half days, with an orbital radius of approximately 670,000 kilometers. Like its three other Galilean sibling moons, Europa is tidally locked to its parent-planet, with one hemisphere constantly facing Jupiter, while the other is always turned away. Because of this tidal locking, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead in the Europan sky.