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Amanda Doyle looks at the evidence for life amid Europa's apparent plumes and examines the missions that could deliver proof
Europa is the smallest of Jupiter's Galilean moons, its strange icy surface riddled with fractures. But it is what lies beneath the surface that is of most interest to scientists: in September 2016, Hubble captured images of finger-like projections coming from Europa's limb.
One possible explanation for these findings is plumes of water vapour bursting out from Europa, a theory that is supported by earlier Hubble observations in 2012, when its spectroscope identified water vapour in the moon's south polar region. Both observations are strong evidence that Europa boasts a subsurface liquid ocean, making the moon one of the best places in the Solar System to search for alien life.
The first evidence for Europa's subsurface ocean came from the Galileo spacecraft. Jupiter's immense gravity causes a tidal bulge to be raised on Europa, and this tidal heating is sufficient to cause some of the ice to melt below the surface. Magnetometer readings from Galileo showed that Europa has an induced magnetic field, which can only occur if there is a medium in which a current can travel, such as salty water.
Water in the aurora
In 2013, a team led by Lorenz Roth from the Southwest Research Institute in Texas announced the detection of what appeared to be a plume rising into space in data. They used the spectrograph on Hubble to determine that an ultraviolet auroral glow from Europa's south pole — observed in 2012 — was possibly caused by water molecules being broken apart by Jupiter's powerful magnetic field.
The latest observation was announced in September 2016, when further evidence for these plumes was revealed. Initially searching for a tenuous atmosphere surrounding Europa by viewing the moon transiting in front of Jupiter, William Sparks from the Space Telescope Science Institute was surprised to find traces of a water plume.
"By an interesting coincidence, Roth and the team announced their discovery of evidence for plumes using [Hubble] STIS spectroscopy within a couple of weeks of our transit program beginning," said Sparks. "Our approach is independent, but that changed the landscape and people started looking right away for plumes."
Sparks' latest observation of the plumes adds weight to evidence for an active water cycle on Europa, but is this an environment that could support life? There are three key ingredients for life as we know it, and water is only one of them. The correct 'biogenic elements' need to be present in order to provide the building blocks for life and an energy source is also considered essential.
If life on Europa were to avail itself of photosynthesis as an energy source, it would have to be situated near the surface, where ice is potentially thin enough for sunlight to filter through. However, living near the surface has its pitfalls, as radiation from Jupiter would likely exterminate anything unprotected by thick ice.
As on Earth, so on Europa?
In the pitch-black depths of Earth's oceans, hydrothermal vents spew out enough hot material for ecosystems to thrive despite never seeing sunlight. If similar vents were to exist on Europa they could provide a safe haven for life. It is unknown how the tidal heating occurs on the icy moon but if the heating were to penetrate to the core, then the flow of heat up from the ocean floor could create vents. However, if the heating is restricted to the upper layers of ice, then venting would not occur.
It may seem far-fetched that life could exist in freezing conditions on Europa, but we know that there is microbial life on Earth that is extremely resilient to such hostile environments. Microorganisms are known to survive in Antarctic ice by producing their own antifreeze, and lakes situated far below the ice also have microbial life.
If some form of life exists on Europa, then could it be detected? "If the biomass in the plumes were high enough, it may be possible to find biosignatures," Sparks explains. "A more likely approach — and plumes are very relevant — is that presumably most of the plume material gets deposited back on to the surface. Along with all the other places on Europa where material appears to have seeped out onto the surface, that would certainly be a place you'd want to look."
The best way to explore Europa and the tantalising possibility of life would be to send a lander with a powerful drill, which would ultimately drop a probe into the ocean below. Such an ambitious project is still many decades away, but there are missions planned which will take the first steps in revealing the moon's secrets.
ESA's Jupiter Icy Moons Explorer (JUICE) is not a life-finding mission, but it does have the ability to finally confirm the existence of the subsurface ocean on Europa, as well as measure the thickness of the ice shell. JUICE will also explore the chemistry of the moon to ascertain if it has the right chemical soup needed for life. Meanwhile, NASA is planning the Europa Multiple-Flyby Mission, also known as the Europa Clipper. While the goals of this mission and JUICE are similar, the NASA mission will spend more time focused on Europa. There is also the possibility that this mission will include a small lander.
Both missions will further our knowledge of the surface of Europa and what lies beneath, thus paving the way for a mission dedicated to finding life.
The technology for drilling through Europa's thick ice crust has been tested
Drilling through the Antarctic ice to the lakes below is an excellent testbed for studying the type of life that might exist beneath Europa's surface, as such organisms are cut off from the atmosphere and from sunlight. These Antarctic lakes are kept in liquid form due to the immense pressure from the ice above.
There have been numerous drilling expeditions to such subglacial lakes, and the first successful breach of the overlying ice occurred in February 2012 when a Russian team reached the waters of Lake Vostok, some 4km below the ice. DNA analysis of the surrounding ice has shown that microbial life likely exists in the lake, although this has yet to be confirmed from the lake water itself.
Not all expeditions are so lucky. On Christmas Day in 2012, a UK-led project to explore Lake Ellsworth failed when they were unable to drill through the 3km of ice above the lake.
In 2013, an American team had more success when they broke through 800m of ice to reach Lake Whillans. There is also an extensive network of streams, and the entire area covers around 60 square kilometres. Analysis of the lake water revealed nearly 4,000 species of microbes.
Several planned missions will further our knowledge of whether the icy Jovian moon's environment is right for life
James Webb Space Telescope
The James Webb Space Telescope (JWST), due to launch in 2018, will have the capability of confirming the existence of plumes of water emanating from Europa. It will also be able to observe Europa in regions of infrared light that are invisible to Hubble. If water plumes do indeed exist, JWST will detect the water signatures in the infrared. These observations are impossible from Earth, as the water vapour in the atmosphere blocks the view. However, as the plumes appear to be intermittent, it may be difficult to time JWST observations just right in order to detect the plumes.
Europa Multiple-Flyby Mission
NASA's mission to Europa was approved in 2015 and a suite of nine scientific instruments has been announced for the orbiter. These include high-resolution cameras, spectrometers, an ice-penetrating radar and a magnetometer. The latter will be used to determine the depth and salinity of the ocean by measuring the direction and strength of Europa's magnetic field. Thermal mapping of the surface will also reveal any recent eruptions of warmer water from below the ice. The mission is due to launch in the 2020s, and will perform 45 flybys of Europa over three years, with the orbits varying from a height of 25km to 2,700km.
Jupiter Icy Moons Explorer
ESA's Jupiter Icy Moons Explorer (JUICE) is due to launch in 2022 and reach the Jovian system in 2030, spending three and a half years studying the moons of Jupiter. It will perform two flybys of Europa before moving on to Callisto and then eventually settling into an orbit around Ganymede, the main focus of the mission. JUICE will study surface features on Europa to ascertain how they formed. By thoroughly analysing the Jovian system — including Jupiter itself — JUICE will shed light on planet formation and the conditions needed for life to emerge on icy moons.
About the Writer
Amanda Doyle is a postdoctoral researcher at the University of Warwick and editor of the SPA's quarterly magazine.
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