Image credit: NASA/JPL-Caltech/SETI Institute
Europa is possibly the most spoken about moon in our news today with sensational headlines and scientific evidence both pointing to the vast amount of water under its surface said to be able to harness life of some kind. As is the case of any good story there are twists and turns and one must start from the beginning.
Discovery and name
Galileo Galilei, born in Pisa in 1564, is known to many as the ‘father of science’. Galileo developed a telescope with three times the magnification of previous prototypes in 1609, eventually creating an even more effective model with thirty times magnification using refraction. In January 1610, and with his own telescope in hand, Galileo looked up into the sky. His focus was on Jupiter and three stars he believed he could see forming a line away from the gas giant. The next morning he looked up again but the stars had flipped, forming a line away from the planet on the other side. Four days of observation passed before a fourth ‘star’ appeared in line with the others. Another week went by and Galileo noticed the stars would travel around the planet in the same way, never moving out of turn. It suddenly clicked. Galileo altered his perception, now realising he was in fact looking at four moons that were in orbit around Jupiter.
This may sound reasonably straightforward in the 21st century but Galileo was dealing with conflicting religious stances in the 1600s. By discovering that there were planetary bodies orbiting Jupiter he was completely disproving the Ptolemaic system thought out by Greek philosopher Ptolemy and backed up by his peer Aristotle. The Ptolemaic system was widely accepted by the Catholic church and said that the Earth is the centre of the universe with all things orbiting it. Galileo did, however, provide stunning evidence for the opposing theory of the Copernican system. The Copernican system insists that the actual centre of our solar system is the Sun with the planets orbiting around it, common knowledge for civilisation in modern times.
There is some contention between who actually discovered the moons of Jupiter with the aforementioned Galileo publishing his observations before Simon Marius, a German astronomer, could supposedly do so. Regardless of who pipped who to the post the pair both have a long lasting legacy through said moons. The system of moons is now commonly referred to as the Galilean moons whilst the individual moons are referred to by their names given to them by Marius. Marius decided that Galileo’s method of numbering them would be far too complicated so opted for names from Greek mythology: Io, Callisto, Ganymede, and Europa; all associated with the Greek god of Zeus who in Roman mythology is known as Jupiter.
Europa is the smallest of the four Galilean moons but the most intriguing to humans. William Sparks, an astronomer at the Space Telescope Institute in Baltimore, describes Europa as “a moon of Jupiter that is about the same size as our moon, but it’s got more ocean on it than Planet Earth.” In fact its 1900 mile diameter makes it smaller than Earth’s moon but it is estimated by some to hold twice as much water as Earth.
With original estimates claiming Europa was as old as Jupiter, 4.5 billion years old, it is now claimed that, after closely studying Europa’s surface, the moon is a lot younger in the region of 20 to 180 million years old. This new found geological youth comes from the abundant lack of craters on Europa. This fact combined with the completely frozen ice surface makes Europa one of the smoothest planetary bodies in our solar system. Bernard Henin, in his 2018 book Exploring the Ocean Worlds of Our Solar System , portrays Europa’s surface wonderfully: “From orbit, Europa is a perfect white sphere. In fact, it is one of the smoothest surfaces of any known solid object in the solar system […] Standing on its surface, space travellers would observe a uniformly flat horizon everywhere they looked, like being on a giant snooker ball.”
However all on Europa is not as rosy as first seems. It takes Europa three and a half days to fully orbit Jupiter. Jupiter is the largest planet in our solar system and its gravitational lock to Europa sees the same half of the small moon constantly facing it. The orbit around Jupiter is elliptical thus the near side facing Jupiter feels the gravitational pull a lot more than the far side. This may sound slightly technical but the end result is that the vast amount of water and its tides are rising higher in certain parts creating cracks or fractures caused by huge gravitational differences.
The large gravitational forces at play also perhaps create the only sense of heat or warmth on Europa with surface temperatures stable at around -160°C. The movement of the oceans caused by said forces are described by Ian Sample of the Guardian as “tidal kneading” and act to create a thermal environment amid the icy depths of a vast ocean. Sample extrapolates, stating that “if life has gained what passes for a foothold on Europa, it may be huddled around hydrothermal vents at the bottom of the ocean in a world of frigid, perpetual night.”
Europa is similar to Earth in some regards, exciting astronomers, astrobiologists and geologists across the globe in equal measure. Europa has an iron core, a rocky mantle layer, and a salt water ocean. This sounds familiar right? As the years tick by the evidence stack gets higher. In 2014 scientists Simon Kattenhorn, of the University of Idaho, and Louise Prockter, of Johns Hopkins University, discovered that the Earth may not be alone in having a system of plate tectonics. The pair studied photos of Europa taken by the Galileo spacecraft and noticed that the landscape was sometimes prone to change. They deduced it may be huge ice slabs sliding over and under eachother near the centre of Europa. In 2016 another study, conducted by a research team led by Steve Vance, based at NASAs jet propulsion laboratory in California, brings up the suggestion that Europa produces ten times more oxygen than hydrogen which is similar to our planet. The theorem is based upon the relationship between Jupiter’s radiation, Europa’s vast ocean, and gravity.
The main difference between Europa and Earth is that Europa’s ocean lies below a sheet of ice said to be at least three kilometres thick by Elizabeth P. Turtle and Elisabetta Pierazzo of the UA lunar and planetary laboratory. The pair simulated impacts powerful enough to create the craters observed by passing spacecraft. Based on observations of impacts and crater peaks they came up with their estimations however Turtle does state that “what we’ve done is put a lower limit on the thickness of the ice.” Regardless of how thick the ices is, the water underneath is the key. Said water is suggested to be in contact with the core of Europa, potentially inducing a whole host of complex chemical reactions which could be creating or sustaining life forms of some kind. It is for this reason that many see Jupiter’s smallest moon as the most likely body to host extra terrestrial life.
Our knowledge of Europa has been expanding slowly throughout the decades. In 1973 Pioneer 10 flew past Europa at a significant distance away but noticed some variations in surface brightness, Pioneer 11 whizzed by a year later in 1974, 600,000 kilometres away, but also spotted surface variation. Later on in the decade, 1979 to be exact, both Voyager 1 and Voyager 2 passed by the Jupiter system. Voyager 1 shed light on gravitational pulls and interactions within the system whilst Voyager 2 confirmed to astronomers on Earth that Europa’s surface had stripes running across it, suggesting cracks or fissures. The next mission to launch was the Galileo Europa mission.
The Galileo was an unmanned spacecraft standing at twenty feet tall and weighing around 2500 kilograms. It was built to explore and excite, it did just that. Michael Meltzer, a respected space exploration writer, wrote in his 2009 NASA published book Mission to Jupiter: A History of the Galileo Project, “When Galileo lifted off from Kennedy space centre on 18 October 1989, it began an interplanetary voyage that took it to Venus, to two asteroids, back to Earth, and finally on to Jupiter. The crafts instruments studied Jupiter’s enormous magnetosphere and it’s belts of intense radiation. The spacecraft also sent off a planetary probe that accomplished the most difficult atmospheric entry ever attempted. After this, the craft spent several years visiting Jupiter’s moons and delving into their structures and properties.”
The Galileo orbiter was destroyed in 2003 after 35 orbits of Jupiter, during this time it provided scientists with unbelievable amounts of key data. One of the missions most important discoveries was that the magnetic field of Jupiter is disrupted by it’s moon Europa. Europa, according to the evidence, seems to be creating it’s own magnetic field beneath it’s surface. Due to the fact it’s surface is ice the straightforward and likely answer was a gargantuan ocean on Europa. This enthralled scientists and space enthusiasts alike and it seemed the quest for planetary bodies potentially habitable to life was on.
Looking back to go forward
A decade later and in 2012, 2014, and 2016, scientists utilising the Hubble space telescope observed pixelated imagery of what appears to be water plumes spreading out into space from Europa. Different research teams came up with similar evidence. Researchers stressed that the plumes were not yet confirmed but this year, in 2018, it appears they have been. In May 2018 a paper was published titled Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures: written by Xianzhe Jia, an associate professor in climate and space sciences at the University of Michigan, Margaret G. Kivelson, a senior scientist on the Galileo mission and now a professor of space physics at the University of California, Krishan Khurana, a geophysicist working at the University of California, and William Kurth, a professor of physics and astronomy at the University of Iowa.
The paper states that “signatures in some Hubble space telescope images have been associated with putative water plumes rising above Europa’s surface, providing support for the ocean theory. However, all telescopic detections reported were made at the limit of sensitivity of the data, thereby calling for a search for plume signatures in in-situ measurements. Here, we report in-situ evidence of a plume on Europa from the magnetic field and plasma wave observations acquired on Galileo’s closest encounter with the moon.”
Galileo’s closest encounter with Europa occurred on 16 December 1997 as the spacecraft whizzed by at a speed of over 2230 miles per hour and at an altitude of less than 250 miles. As the spacecraft passed Europa there was a very brief period of odd recordings. But as the dials whirred scientists in 1997 did not know what the anomalous data meant. Eleven years later, however, and they did. The magnetic field had massively altered and there was an increase in plasma density, scientists believe this is evidence of a water plume.
Xianzhe Jia, lead author of the study, created computer simulated water plumes that were 120 miles high and recorded the same readings as the Galileo spacecraft. The aforementioned Margaret Kivelson was astounded when speaking to NASA TV, “There were some strange signatures in the magnetic field that we had never really been able to account for. When he ran this simulation, it agreed just beautifully with the data that we had collected in 1997.”
In 2013, before said study was published, the US National Research Council’s Planetary Science Decadal review took place. In which, the organisation issued a ten year recommendation plan for NASA with the exploration of Europa ranking as the highest priority mission. Since then, with huge backing and new evidence, space agencies have been working hard to get missions on the go. Two missions stand out, JUICE and Europa Clipper.
JUICE, standing for Jupiter icy moons explorer, is expected to launch in the 2020’s and will observe Europa and several other moons. Once JUICE has arrived at Europa it will study organic compounds and other factors that may make the moon habitable. The mission, flying past Europa at 200 miles away, will also attempt to identify how thick the surface of the moon actually is. A closer, more explorative and in-depth mission of Europa, is the Europa Clipper mission also set to launch in the 2020’s. Europa Clipper will flyby Europa around 45 times at varying altitudes including just sixteen miles up. This low altitude will provide Europa Clipper with supreme potential and unmatched access to potential water plumes and the chemicals and compounds found within.
The spacecraft will include cameras to beam high resolution images back to Earth, radars to detail Europa’s layers and liquids, and a magnetometer to uncover more about the moon’s magnetic field. The Europa Clipper will also be scouting out locations for a potential lander mission in the future. Xianzhe Jia is excited his work helped contribute to the Europa cause and has high hopes for the future, “Given the evidence of plumes available so far, there is a good chance that those spacecraft may obtain direct measurements of plumes ejecting material from the subsurface ocean into space. Those observations will provide crucial information for us to access Europa’s potential for life.”
It is unclear whether the search for life will be successful but a study published in July 2018 ensures the search will be easier than first expected. In a paper titled Preservation of potential biosignatures in the shallow subsurface of Europa, Tom Nordheim, research scientist at JPL, Kevin P. Hand, astrobiologist and planetary scientist at JPL, and Chris Paranicas, a planetary scientist at JHUAPL, make the point that differing levels of radiation on Europa may have an effect on surface data collection.
In the paper’s abstract it is written, “Jupiter’s moon Europa, which is thought to possess a large liquid water ocean beneath it’s icy crust, is one of the most compelling targets in the search for life beyond Earth. It’s geologically young surface, along with a number of surface features, indicate that material from Europa’s interior may be emplaced on the surface. However, the surface is affected by the harsh radiation environment of Jupiter’s magnetosphere, which over time may lead to chemical alteration and destruction of biosignatures.” The research conducted by the team concluded that the difference between a lander digging in a relatively safe spot and a lander digging in ‘high-blast’ zones of radiation would be digging down an extra 10-20 centimetres. Whether organisms are still alive at such depths and under such force remains to be seen but Nordheim remains in high spirits: “Even in the harshest radiation zones on Europa, you really don’t have to do more than scratch beneath the surface to find material that isn’t heavily modified or damaged by radiation.”
Further information and links
Hear Kevin Hand speak about Europa and it’s significance.
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