2.6 How can we find out more about Europa?

There are currently no scheduled missions to Jupiter's moons, since NASA's Jupiter Icy Moons Orbiter (JIMO) was cancelled in 2005, but Europa remains a high priority target for both NASA and ESA, so a mission with simlar objectives to JIMO seems likely by about 2020. On arrival at Jupiter, JIMO would have gone into orbit first round Callisto, then Ganymede and finally Europa.

The main objectives of JIMO at Europa would have been to:

1. Determine the presence or absence of a subsurface ocean.

2. Characterise the three-dimensional distribution of any subsurface liquid water and its overlying ice layers.

3. Understand the formation of surface features, including sites of recent or current activity, and identify candidate sites for future lander missions.

The answer we gave to Question 13 covers all we expected you to come up with, but there are other techniques that are also likely to be useful. Possible instruments would include an imaging system with spectroscopic capability, a laser altimeter, and an ice-penetrating radar. The laser altimeter would map Europa's topography, and in particular it would determine the height of Europa's tidal bulges. The bulges should be only about 1 m high if the ice is solid throughout, but about 30 m high if there is 10 km of ice floating on water, so altimetry is a neat way of addressing the presence or absence of a subsurface ocean. The radar would be directed directly downwards with the intention of recording echoes from the ice-water interface. Unless the ice is particularly salty, which would tend to attenuate the signal, the radar should detect the ice-water interface wherever it lies at less than about 10 km depth, as is likely to be the case in young chaos areas (see Section 2.4). This, plus any further visual clues to ice thinness or recent activity from the imaging system, would be the main means of selecting landing sites for future lander missions.

An artist's impression of a Europa-orbiting mission in action is shown in Figure 28. The current ambition for a mission (in the distant future) is to have a miniature robotic submarine (a 'hydrobot') capable of exploring the ocean to seek for signs of life. In order to deploy this, a way has to be found to make an access hole in the ice, which presumably must be done either by mechanical drilling or by using heat to melt a borehole. Even after landing on the thinnest ice, the technological challenges of making such a hole would be severe.

Figure 28 A Europa orbiter in action (from a mission that has now been cancelled). The blue beam illuminating the surface is a schematic indication of the ice-penetrating radar beam, which is intended to map the ice thickness with a depth resolution of about 100 m. (NASA)

There is also another problem, which is the planetary protection issue of how to prevent contamination of Europa's biosphere with organisms inadvertently carried from Earth. It would be foolish to send a sophisticated suite of instruments to Europa unless we could be as certain as possible that any signs of biological activity were not attributable to microbes carried to Europa by the same mission or any previous spacecraft. Contamination of Europa's biosphere (or the accidental establishment of a biosphere where none had previously existed) would undermine any conclusions about the independent origin and evolution of life that could otherwise be drawn following the discovery and study of Europan life. Most investigators would recognise an ethical duty to safeguard the integrity of future studies of Europa's biosphere and to protect against potential harm to any Europan organisms. This duty is codified in legal form by the 1967 United Nations' Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies.

Very few terrestrial microbes would survive a journey to Europa, and of these only a tiny proportion would be likely to be able to feed and reproduce on Europa or in its ocean. However, just one viable organism delivered to the right (or wrong!) place that was then able to feed and multiply would do incalculable harm. With this in mind, a report on preventing biological contamination of Europa published in 2000 by the US National Academy of Sciences recommends that the bioload of any Europa-bound mission should be minimised by using levels of cleanliness during assembly and subsequent sterilisation that are at least as stringent as those currently agreed for Mars missions.

Illuminating lessons about preparing to penetrate through a thick ice cover into a body of water may be learned from the case of Lake Vostok. This is a large lake that has been trapped beneath the Antarctic ice for possibly several million years, and is suspected of housing a sealed-in ecosystem. Exploration of Lake Vostok and the proper implementation of anti-contamination protocols are widely held to be realistic rehearsals for exploration of Europa's ocean, as discussed in Box 8.

Box 8: Lake Vostok - an ice conundrum

In 1974, Russian scientists began drilling deep into the ice at their Vostok research base, situated at the geomagnetic south pole in Antarctica. Samples of ice and the gases and other trace materials trapped within it provide a valuable and continuous record of climate changes and large volcanic eruptions during the past 400 000 years. Incidentally, viable micro-organisms were found entombed within the ancient ice too. It was not until 1994, by which time the borehole had reached a depth of about 3 km, that seismic and other studies revealed that the ice overlies the largest subglacial lake in the world, covering about 2 × 10⁵ km², which is the same area as Lake Ontario. This is known as Lake Vostok (Figure 29).

Figure 29 Satellite radar image showing the ice surface of part of Antarctica. Lake Vostok is the elongated flat area near the centre. Image is about 600 m across. (NASA)

In places the water depth reaches about 1 km. The oldest ice overlying the water is less than a million years old, but the ice sheet as a whole is slowly flowing across the lake, so the lake itself may have been sealed off from the surface for as long as 14 million years. The lake is suspected of supporting its own ecosystem, subsisting either by a meagre rain of organic matter at places where the overlying ice melts or by chemical energy at suspected hot springs.

These realisations united scientists from many nations in plans to bore through the base of the ice in order to sample the lake water and deploy a probe into the lake. One method suggested to keep the hole sealed and to prevent contamination was that the hole should be drilled to within a few metres of the roof of the lake. A cylindrical probe would then be lowered to the base of the hole that could sterilise itself while waiting for the hole above to freeze over, and then melt its way down into the lake. It would pay out a tether behind itself as it travelled, which would act as a communications link to the surface (Figure 30).

Figure 30 Artist's impression of a probe released into Lake Vostok from the base of the borehole. (© Rob Wood/Wood Ronsaville Harlin, Inc.)

However, two serious objections emerged that put at least a temporary halt to these schemes. First, the self-sterilisation techniques for the probe were untested. Secondly, when the Russians had begun drilling back in the 1970s, they were anxious to stop the hole freezing shut behind the drill bit so they pumped a mixture of aviation fuel and antifreeze (Freon) into the hole. There is now 60 tonnes of this toxic chemical mix in the hole, and no one can be sure that none of this will leak into the lake if the hole is continued. Pressure from a coalition of environmental groups caused drilling to stop in 2001 (Figure 31), and plans for any kind of penetration into the lake were put on hold for maybe a decade.

Figure 31 Schematic cross-section through Lake Vostok and the overlying ice (not to scale). The borehole stops less than 100 m before the roof of the lake. It has penetrated the full thickness of the ancient ice cap, and terminates within ice that has frozen more recently onto the roof.