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.
As Europa travels slightly closer to Jupiter, Jupiter’s gravitational pull grows even more powerful. This causes Europa to elongate towards its parent-planet. As Europa moves slightly farther from Jupiter, Jupiter’s gravitational attraction grows weaker. This weakening causes Europa to relax back into a more spherical shape, which creates tides in its subsurface ocean. The orbital eccentricity of Europa is constantly being pumped by its mean-motion resonance with its sibling moon Io. Because of this, the tidal flexing stretches and then squeezes Europa’s interior, creating a source of heat. This heat source is what possibly allows its subsurface ocean to stay in its liquid phase while also driving subsurface geological processes. The ultimate source of this energy is Jupiter’s rotation which is tapped into by Io by way of tides it raises on Jupiter. It is then transferred to Europa and Ganymede by the orbital resonance.
Jupiter’s Water Moon
By validating the presence of water vapor on Europa, scientists can gain a better comprehension of what is going on in the interior of this mysterious icy moon. By detecting water vapor, astronomers can become more confident that a sloshing subsurface ocean of liquid water does swirl beneath Europa’s cracked crust. This ocean may potentially be twice as large as Earth’s, and it would be situated very deep beneath this moon’s miles-thick cracked icy shell.
Another suggestion as a possible source of water for the Europan plumes has been proposed by other planetary scientists. This possible alternative source could be in the form of shallow reservoirs of melted water ice located not very far beneath Europa’s surface. It has also been proposed that Jupiter’s powerful radiation field is tearing water particles away from the frigid moon’s thick shell of ice. However, more recent investigations have weakened this possibility.
“Essential chemical elements (carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur) and sources of energy, two of three requirements for life, are found all over the Solar System. But the third–liquid water–is somewhat hard to find beyond Earth. While scientists have not yet detected liquid water directly, we’ve found the next best thing: water in vapor form,” explained Dr. Lucas Paganini in a November 18, 2019 Keck Observatory Press Release. Dr. Paganini is a NASA planetary scientists who led the water detection investigation.
In their study, published in the November 18, 2019 issue of the journal Nature Astronomy, Dr. Paganini and his team reported that they have detected enough water being hurled out from Europa (5,202 pounds per second) to fill an Olympic-size swimming pool within mere minutes. However, the astronomers also found that the water appears infrequently–at least in quantities that are sufficient to be detected from Earth, according to Dr. Paganini.
“For me, the interesting thing about this work is not only the first direct detection of water above Europa, but also the lack thereof within the limits of our detection method,” he added.
Dr. Paganini and his team spotted the faint signal of water vapor only once during 17 nights of observations conducted between 2016 and 2017. Observing the moon from Keck Observatory, the planetary scientists detected water molecules at Europa’s leading hemisphere. The leading hemisphere is the side of a moon that’s always facing in the direction of its orbit around its parent-planet. In this way, Europa is like Earth’s own Moon. This is because both moons are gravitationally locked to their planets, which results in the leading hemisphere always facing in the direction of the orbit, while the trailing hemisphere is always turned in the opposite direction.
The team of planetary scientists used Keck Observatory’s Near-Infrared Spectrograph (NIRSPEC) to conduct their observations of Europa. NIRSPEC measures the chemical composition of planetary atmospheres through the infrared light they absorb or emit. Molecules–such as water molecules–emit specific tattle-tale frequencies of infrared light as they dance with solar radiation.
There were other intriguing discoveries made about Europa before this recent water vapor detection. The first of these tantalizing findings came courtesy of NASA’s Galileo spacecraft. Galileo successfully measured disturbances in Jupiter’s magnetic field near Europa while the spacecraft was in orbit around the gigantic planet between 1995 and 2003. The measurements indicated to planetary scientists that an electrically conductive fluid was causing the observed magnetic perturbations, and that the most likely fluid was a salty ocean swirling beneath Europa’s cracked shell of ice. When the scientists analyzed the magnetic perturbations in greater detail in 2018, they found evidence of possible water vapor plumes.
Back in 2013, planetary scientists announced that they had used NASA’s HST to spot the chemical elements hydrogen (H) and oxygen (O), which are components of water (H2O). The astronomers noted that they had discovered the hydrogen and oxygen in plume-like configurations in Europa’s atmosphere. A few years later, a different team of scientists also used HST to collect still more evidence of potential plume eruptions. This group of planetary scientists obtained images of finger-like projections, that showed up in silhouette, as the icy moon passed in front of its banded parent-planet.
“This first direct identification of water vapor on Europa is a critical confirmation of our original detections of atomic species, and it highlights the apparent sparsity of large plumes on this icy world,” Dr. Lorenz Roth noted in the November 18, 2019 Keck Observatory Press Release. Dr. Roth is an astronomer and physicist from KTH Royal Institute of Technology in Stockholm, Sweden, who led the 2013 HST investigation and was a co-author of this more recent study.
Dr. Roth’s study, along with these earlier findings, have only measured the components of water existing on the Europan icy surface shell. This means that spotting the presence of water vapor on other distant worlds still presents a challenge. Current spaecraft have only limited capabilities to detect its presence, and planetary scientists using ground-based telescopes to search for water in deep space have to take into account the distorting effect of water existing in our own planet’s atmosphere. In order to minimize the effect of Earth’s atmospheric water, Dr. Paganini’s team used complex computer and mathematical modeling in order to simulate the conditions of our planet’s atmosphere. The scientists did this so they could differentiate Earth’s atmospheric water from Europa’s in the data obtained by the NIRSPEC.
“We performed diligent safety checks to remove possible contaminants in ground-based observations. But, eventually, we’ll have to get closer to Europa to see what’s really going on,” noted Dr. Avi Mandell in the November 18, 2019 Keck Observatory Press Release. Dr. Mandell is a Goddard Space Flight Center planetary scientist on Dr. Paganini’s team.
Astronomers will soon be able to get close enough to Europa to solve some of the still-lingering mysteries concerning the outer and inner workings of this potentially habitable little moon-world. The upcoming Europa Clipper mission, scheduled to launch in the mid-2020s, will shed still more light on this fascinating small icy world. Scientists have been investigating Europa for almost fifty years. These studies began with a modest photo of a weird world that appeared similar in appearance to a big egg with a cracked shell.
When the Clipper orbiter reaches Europa, after its long journey through interplanetary space, it will conduct a detailed survey of the Europan surface, deep interior, tenuous atmosphere, subsurface sloshing ocean of liquid water, and possibly even smaller active surface vents. The Clipper will snap pictures of any plumes that exist on Europa’s mysterious surface and obtain samples of the molecules it discovers in the thin atmosphere with its mass spectrometers. It will also search for a promising site from which a future Europa lander could collect samples.
These future investigations should solve still more of the tantalizing mysteries of this icy little moon-world–especially whether or not Europa is a possible habitat for life as we know it, swimming around in the secretive distant dark waters of its hidden ocean.