Europa’s significant discovery

Europa is Jupiter’s fourth largest moon, and with a radius of 1562 km it is only slightly smaller than our own Moon (radius 1737 km). It was first detected as a ‘bright star’ next to Jupiter by Italian astronomer Galileo Galilei in 1610 along with Jupiter’s three other largest moons (Io, Ganymede and Callisto).

Despite using a rudimentary telescope to spot them, his discovery of these ‘Galilean’ moons fundamentally changed our understanding of the Solar System. Since Greek times, it had been assumed that the Earth was the centre of the Solar System, with other objects orbiting around it. Galileo’s observation, that these moons were orbiting Jupiter and not Earth, contradicted that long-held view and instead supported Copernicus’ proposal that the Earth and other bodies moved round the Sun (heliocentricity, Figure 1). This proposal was contrary to western religious viewpoints at the time and put him in conflict with the Catholic church. Nevertheless, as telescopes improved and further observations produced irrefutable scientific evidence, heliocentricity became the prevailing worldview. Europa had made its first of several contributions to understanding our Solar System.

Figure 1 Two views of planetary motion: (a) the Earth-centred view of the ancient Greeks; (b) the Copernican (heliocentric) system, in which the planets move in collections of circles around the Sun, as supported by Europa’s discovery.Show descriptionThe figure contains two photographs. A is photograph of a coloured sketch that has the Earth at the centre. Around this are rings at different inclinations, that indicate different planetary orbits. B is a photograph of the page of a black and white book showing a diagram. This has the Sun at the centre surrounded by concentric circles that indicate planetary orbits.

An icy, lifeless body?

In the 1950s, astronomers using ground-based telescopes found evidence that Europa’s surface was made of water ice. At that time, it was assumed that Europa was geologically inactive, i.e. there were no processes operating on or beneath the ice, so this icy surface was expected to be covered in impact craters from bombardment over billions of years.

However, in the 1970s, when NASA’s Pioneer 10 and 11 and Voyager 1 and 2 flyby missions began sending images back to Earth, there was very little evidence of impacts. Instead, Pioneer 10 showed the ice was largely smooth and flat, with dark red and light patches, and Voyager 1 showed a surface covered in intersecting dark red lines (now known as lineae) (Figure 2).

Figure 2 Photograph returned by the Voyager 2 probe of Europa’s surface, showing dark red lines.Show descriptionThe photograph shows approximately half of Europa’s northern hemisphere, with the black background of space on the left-hand side. Europa’s surface appears brown, with darker brown streaks and diffuse brown patches.

From these observations, scientists theorised that the ice was likely to be relatively young, and unlikely to be as cold as expected. This would allow the ice to flow or flex, like water ice does on Earth, and so the moon could even be relatively warm underneath the surface. The red lines were also proposed to be cracks in the ice that might have been filled by material that upwelled from beneath. Europa was not the lifeless moon suggested just two decades earlier – and this raised the possibility that it may even have liquid water.

Galileo’s legacy

A decade after Voyager 2’s observations, NASA’s Galileo mission – named in honour of Galileo Galilei – launched to explore the Jovian (Jupiter) system further. The mission captured the most close-up and high-resolution images of Europa’s surface at that time (6 m per pixel compared to nearly 2 km per pixel from Voyager) that revealed the presence of chaos terrain (where ridges, fractures and other features intersect without any defined pattern), lineae and lenticulae (circular pits and domes).

Video 1 Animation created from close-up images of Europa’s Conamara chaos terrain.                   Source: NASA/Lunar and Planetary Institute/Paul Schenk

Density measurements made by Galileo also showed that Europa had a solid, dense interior, probably made of a metallic core surrounded by rock, and a thin atmosphere containing oxygen and ozone.

However, one of the mission’s most significant findings was that the moon also had a magnetic field. This meant that Europa had to contain electrically conductive material. Ice is not conductive, but the measurements matched closely with those expected from a subsurface layer of salty water. With this evidence came huge excitement about the possibility of an Earth-like ocean beneath the ice, since the presence of liquid water satisfies one of the key requirements in the search for life.

With that in mind, at the end of the mission in 2003, NASA programmed the spacecraft to descend into Jupiter’s atmosphere to be destroyed, rather than leaving it in orbit where there was a possibility that it might accidentally collide with Europa’s surface. This decision was critical for the future exploration of Europa, to preserve its environment should life exist there. These planetary protection considerations are at the heart of future missions to Europa and other bodies with the potential to harbour life.

Is Europa habitable?

Figure 3 James Webb Space Telescope image of Europa taken using its NIRCam (near infra-red camera).Show descriptionThe photograph shows Europa as a bright blue disk on the black background of space. The image is pixelated but the moon is darker blue in the northern hemisphere and the very south of the southern hemisphere. Elsewhere it is white or light blue. A feint red colour can be seen around the edge of the disk.Enthusiasm for further exploration of Europa was heightened in the 2010s, when NASA’s Juno mission improved on Galileo’s photographs of the moon and identified that oxygen was produced at the surface by reactions with the space environment. The Hubble Space Telescope also detected diffuse plumes made of water vapour emanating from the moon’s surface, suggesting the possibility of hydrothermal activity on the ocean floor. On Earth, reactions between rock and water can create energy that’s used by microbial life.

More recently, the James Webb Space Telescope detected carbon dioxide on the moon’s ice surface, within a region of chaos terrain. This carbon dioxide is proposed to have originated from the ocean beneath. Carbon is one of the bioessential elements for life, making astrobiologists even more excited to learn more about Europa.

In the coming years, data returned from ESA’s JUICE and NASA’s Europa Clipper missions will be able to shed some light on whether or not Europa might be habitable for life. Until then, space scientists, including those in AstrobiologyOU, use computer modelling or create Europa-like environments in the laboratory to understand its environment and potential for life.