Discovering a surface made of ice

Figure 1 Image of Europa’s icy surface taken by NASA’s Voyager 2 probe. The surface is cracked, with the cracks filled with material thought to come from beneath the ice.Show descriptionA black and white photograph showing one segment of Europa. The surface is shades of grey and white, with black cracks in no particular pattern or arrangement.Evidence of a water-ice surface on Europa was first provided in 1953 via telescopic observations. The Pioneer 10 and Voyager spacecraft then confirmed this in the 1970s with the first images of the icy moon (Figure 1). These early images revealed reddish-brown cracks and ridges that criss-cross Europa’s surface. Recent missions, such as the Galileo probe (that orbited Jupiter between 1995 and 2003) and the Juno spacecraft (2016–present) provided initial evidence of a 60–150 km deep sub-surface ocean, beneath a 15–25 km icy crust.

A landscape of chaos and mystery

Closer inspection of the high-resolution images captured by Juno reveal complex surface features. One such feature is ‘chaos terrain’, large expanses of broken and fragmented ice. Chaos terrain is evidence that Europa may have a sub-surface liquid water ocean, which may rise through the icy crust, creating subsurface lakes that freeze and cause the overlying ice to fragment, float and resettle into new ‘chaotic’ positions (Figure 1). This ice bares resemblance to icebergs here on Earth.

The ice that creates these features is water ice, but by performing multiple surface flybys of Europa, Galileo and Juno revealed that the ice may also contain salt and even elements associated with the ingredients for life, such as carbon, sulfur, oxygen, hydrogen and nitrogen. These are likely to come from the ice interacting with the ocean below. Thanks to these missions and their discoveries, Europa is now a prime target for astrobiological research.

Interestingly, the moon has a notable lack of craters and mountains, suggesting the surface is quite young and is regularly changing, similar to Earth’s surface with its tectonic plates. Other ‘geological’ features include ridges (1–2 km wide) extending across Europa’s surface, evidence of sub-surface material forcing its way to the surface. There are also ‘puddles’ of smooth material burying parts of pre-existing ridges, implying liquid water or a slurry of water-ice flooding areas of the surface.

Figure 2 Chaos terrain on Europa’s surface. Images taken by the Galileo spacecraft.Show descriptionTwo photographs. On the right, a photograph of Europa appearing as a disk. The disk is blue-grey at the poles. In lower latitudes it is brown, with distinctive brown cracks. A square is drawn in the northern hemisphere and this area is expanded in the photograph on the left. This is coloured blue, with brown smudges and cracks. There are three very distinct cracks, two running parallel from south west to north east, and one running north-north-west to south-south-east. These are brown with blue-green lines in the middle.

While the origins of Europa’s icy surface landscape are puzzling, they represent an ice crust that is dynamic and changing.

Plumes on Europa?

On Saturn’s moon Enceladus, geyser-like plumes eject material into space from a sub-surface ocean. Using the Hubble Space Telescope, scientists identified the presence of hydrogen and oxygen, and finger-link projections in photographs that hinted at the presence of plumes. Using the Near Infra-Red Spectrograph (NIRSPEC) at the Keck Observatory in Hawaii, researchers then identified the signal for water vapour at Europa (Video 1). NIRSPEC detects the specific frequencies of infrared light that water molecules emit or absorb, and suggested the presence of water vapour coming from plumes at Europa. Future discoveries by the upcoming Europa Clipper mission may provide evidence to answer whether this water vapor originates from a sub-surface ocean, like at Enceladus.

Video 1 Detection of plumes at Europa

Scientists at AstrobiologyOU study Enceladus’ plumes and simulate their environment to understand if its sub-surface ocean is habitable or contains life. Now, a search is underway to detect plumes that demonstrate evidence of sub-surface oceans on other icy moons where life may exist. Europa is top of the list, and we can use what we’ve learned about Enceladus to inform our understanding of Europa.

All eyes on Europa: the Europa Clipper mission

NASA’s Europa Clipper spacecraft hopes to investigate how a potential sub-surface ocean, the moon’s geology and ice shell all interact, and if this creates habitable pockets for microbial life. Though Clipper’s current planned arrival at Europa is not until 2030, once there, the craft – over four years – will fly close to the moon’s surface multiple times to gather data. Clipper will produce maps and high-resolution images of Europa’s surface and use ice-penetrating radar to scan for sub-surface water. Onboard thermal instruments will also scan for warmer patches of ice, which are evidence of recent plume eruptions. Combined, all these experiments will detail Europa’s surface and interior bringing scientists one step closer to determining the icy moon’s habitability.