Jupiter and its moons

1.9 Europa

David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.

Copyright © David Rothery

For all Io's majesty, its neighbour Europa excites the greatest scientific interest. Europa is a transitional world, with a density almost in the terrestrial planet league but an exterior that is icy down to a depth of about 100 km. It is not known whether the ice is solid throughout, or whether its lower part is liquid, which raises the fascinating possibility of a global ocean sandwiched between the solid ice and the underlying rock. Gravity data from Galileo show that, like Io, Europa has a dense, presumably iron-rich core (about 620 km in radius) below its rocky mantle. Europa has its own magnetic field, but it is not clear whether this is generated by convection within a liquid core or within a salty ocean beneath the ice.

Europa has a highly reflective surface with an albedo of about 0.7, and it has been known since the 1950s from spectroscopic studies that its composition is essentially that of clean water-ice. More detailed recent observations by Galileo and the Hubble Space Telescope reveal some regions where the ice appears to be salty, and also the presence of molecular oxygen (O2) and ozone (O3). The oxygen and ozone are thought to result from breakdown of water molecules in the ice because of exposure to solar ultraviolet radiation and charged particles. The hydrogen so liberated would escape rapidly to space, which has been observed on Ganymede though not on Europa. It is not known whether the oxygen and ozone detected on Europa constitute an extremely tenuous atmosphere or are mainly trapped within the ice.

Figure 9.13: Voyager 2 view of part of Europa[.] Bright plains cover most of this view, with mottled terrain occupying the lower right[.]
NASA

Europa's surface is relatively smooth and much younger than that of other icy satellites, to judge from the paucity of impact craters. This demonstrates that Europa experiences a significant amount of tidal heating, though less than Io. Images at Voyager resolution, such as Figure 9.13, show bright plains criss-crossed by a complex pattern of cracks filled by darker ice. There are several places where the pattern of these bright plains becomes blotchy, and these were dubbed 'mottled terrain' by the Voyager investigators.

The high resolution images sent back by Galileo show that the bright plains are amazingly complex in detail (Figure 9.14), being composed of a pattern of straight or slightly curved ridges, each usually bearing a central groove. The appearance of these parts of Europa has been described as resembling the surface of a ball of string; an apt description but not much help in trying to decipher how the surface was created. Each grooved ridge could represent a fissure that was the site of an eruptive episode, when some kind of icy lava was erupted.

Figure 9.14: 15-km-wide Galileo view of part of Europa, showing the level of complexity revealed in Europa's bright plains by Galileo's high resolution[.] The relatively smooth area in the lower third of this view corresponds to a dark crack at Voyager resolution, whereas the complexly rigid surface elsewhere looks bright and featureless on images like Figure 9.13[.] The large ridge cutting across the lower right is 300 m high[.] There is a 300-m-diameter impact crater near the centre and several smaller ones, so although young by Solar System standards this surface is likely to be at least several million years old[.]
NASA

It may seem strange to read of 'icy lava', but Solar System ice shares many important properties with the silicate rock that melts to produce lava on the terrestrial planets. Unless the ice is absolutely pure water, these properties include:

  • existing in the solid state as intergrown crystals of differing compositions

  • rock-like strength and rigidity under prevailing surface conditions contrasted with the ability to flow slowly and act as an asthenosphere at depth

  • the capacity to partially melt, yielding melts different in composition to the starting material.

Planetary scientists often use the term cryovolcanism to denote icy rather than silicate volcanism. Although by far the most abundant component in Europa's ice is water, it is likely to be contaminated by various salts (such as sulfates, carbonates and chlorides of magnesium, sodium and potassium) resulting from chemical reactions between water and the underlying rock. The spectroscopic data for Europa are most consistent with the salt-rich areas of surface being rich in hydrated sulfates of magnesium or carbonates of sodium, but could also indicate the presence of frozen sulfuric acid.

Contaminants such as these could make any melt liberated from the ice behave in a much more viscous (i.e. less runny) manner than pure water. If erupted as a liquid this type of lava would not necessarily spread very far before congealing, especially if confined by a chilled skin of the sort likely to form upon exposure to the vacuum of space in the cold outer Solar System. Contaminants also allow the ice to begin to melt at a much lower temperature than pure water-ice: salts can depress the melting temperature by a few degrees and sulfuric acid by as much as 55 degrees.

Maybe, then, Europa's ridges are simply highly viscous cryovolcanic flows fed from their central fissures. Alternatively, the cryovolcanic lava may not have flowed across the ground at all: it could have been flung up from the fissure in semi-molten clods by mild explosive activity, like a 'fire fountain' from a volcanic rift on Earth, and fallen back to coalesce as a rampart on either side of the fissure.

Irrespective of refinements such as this, it seems inescapable that each fissuring event must represent the opening of an extensional fracture in the crust. This cannot happen across an entire planetary body unless the globe is expanding, which seems highly unlikely. Therefore there must be some regions on Europa where surface has been destroyed at a rate sufficent to match the crustal extension elsewhere. Likely candidates for this on Europa are regions described as 'chaos', and part of one of Europa's chaos regions is shown in Plate 9.

[Click 'view document' to open Plate 9. Galileo image of a 60 km wide region of Europa known as Conamara Chaos. The surface of the former bright plains has been broken into rafts or ice floes that drifted apart before the intervening slush or water re-froze. Bright patches on the left are splashes of recent ejecta from a 26 km diameter impact crater 1000 km to the south.]

View document

Here typical-looking bright plains crust has been broken into rafts that have drifted apart, maybe because an underlying liquid ocean broke through to the surface. The areas intervening between rafts are a jumbled mess reminiscent of re-frozen sea-ice on Earth. Some rafts can be fitted back together, but it is apparent that many pieces of the 'jig-saw puzzle' are missing. Perhaps these missing pieces have sunk or been dragged down beneath the surface.

Regions like Plate 9 appeared as mottled terrain on Voyager images, but so did the region shown in Figure 9.15. Here, the surface of what was formerly normal looking bright terrain has been forced up into a number of domes up to 15 km across. Presumably this is because of the rise of pods of molten or semifluid low density material (described geologically as 'diapirs') toward the surface. In some cases the upwelling pod has actually ruptured the surface, to form a small chaos region bearing raftlets of surviving crust.

Figure 9.15: 60-km-wide Galileo view of part of Europa[.] This region, mapped as mottled terrain on Voyager images, is revealed to be conventional bright plains with 'ball of string' texture disrupted by a number of domes, some of which have pierced the former surface[.]
NASA

Although we understand all too little about the processes that have shaped Europa's surface, it is clear that it has had a complicated history. We cannot tell for sure how old each region of surface is, but there are abundant signs that there is, or has been, a liquid zone below the surface ice. A salty ocean below several km of ice is not necessarily a hostile environment for life, and indeed life down there could be much richer and complex than anything that is likely to have survived on Mars. In the depths of the Earth's oceans there are whole living communities that are independent of photosynthetic plants (requiring sunlight to live) and depend instead on bacteria-like microbes that make a living from the chemical energy supplied by springs of hot water ('hydrothermal vents') on the ocean floor.

Given that Europa is tidally heated, we can imagine zones where water is drawn down into the rocky mantle, becomes heated, dissolves chemicals out of the rock, and emerges at hydrothermal vents surrounded by life. This may sound far-fetched, but hydrothermal vents are now mooted as the most likely venue for life to have begun on Earth, so if it could happen here then why not on Europa too? It is the possibility of a life-bearing ocean below the ice that is the main driver for plans for the future exploration of Europa. [More information on the possibility for life on Europa can be found in the NASA video 'Europa: Cool Destination for Life?'.]

The proposed Europa Orbiter mission (Table 9.1) has primary goals of verifying the existence of an ocean below the ice, and identifying places where the ice is thin enough for future landing missions to release robotic submarines ('hydrobots') under the ice so that they can go hunting for hot springs and their attendant life. It will achieve this by gravity studies, by using an altimeter to determine the height of the tide raised on Europa by Jupiter (only 1 m if the ice is solid throughout, but about 30 m for 10 km of ice overlying a global ocean), and by using ice-penetrating radar to map ice thickness. High resolution conventional images will identify sites of recent eruptions. [The Europa Orbiter mission has since been cancelled by NASA.]

Insights into how best to go about the future exploration of Europa's ocean will probably be gained by study of Lake Vostok, a lake of 10,000 square km beneath 4 km of ice that was discovered in Antarctica in 1996. Its waters may have been isolated from the surface for as long as 30 million years, and may host a unique ecosystem. The development of technology to drill through the ice and explore Lake Vostok, and also the adoption of protocols to avoid biological contamination of this special environment should both provide valuable experience.