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Skywatching

Mission to Europa

When the Voyager spacecraft sent back the first close-up images of Jupiter's moon Europa, we all had a shock.

It looked nothing like what we expected. Until then we thought that the moons of other planets would be just like ours, except maybe colder and icier. It did not turn out like that. Europa was one of the biggest exceptions. Instead of an airless, cratered surface floored with ancient lava flows.

We saw a body entirely covered by a smooth layer of ice, punctuated by cracks, some ancient, some more recent. Around some of the cracks are reddish and brownish patches.

We can start to understand Europa by considering Io, the closest moon to Jupiter. Io is by far the most volcanic body in the solar system; its surface is coloured by flows of sulphur-rich lava.

There are always eruptions taking place somewhere on Io, with a lot of material being ejected into space.

On Earth, volcanism is mainly driven by plate motions and subduction. Io has nothing like that. Instead, it is being continuously stretched and kneaded by Jupiter's intense gravity. This generates a huge amount of heat inside Io, driving the volcanism.

Europa is more distant from Jupiter, so the tidal stretching or kneading is less. However, it is enough to sustain a huge ocean under the outer icy layer, where the surface temperature never exceeds -200 C.

Europa's ocean is heated from the bottom, so it is constantly circulating due to convection, preventing it from freezing. On Earth, along the mid-Atlantic ridge and at other locations, jets of hot, mineral-rich water are coming through the seabed.

These are known as hydrothermal vents. As the hot water meets the cold seawater, some of the minerals precipitate out, forming tubular towers and clouds of dark powder, hence these vents often called black smokers.

Around them live communities of crustaceans, worms and other creatures. 

These communities are unique in that their survival owes nothing to the sun. They are sustained by heat and nourishment from within the Earth. Could there be life in Europa's oceans, perhaps concentrated around hydrothermal vents?

We know that life began on Earth about 3.5 billion years ago, almost certainly in the ocean, although it is not agreed yet exactly where. Some have suggested the deep ocean, others muddy shallows and rock pools, and others propose it got going around the hydrothermal vents.

However, the living creatures we see today colonizing the vents in our oceans are relatives of creatures that are found throughout the world. What we see around the vents today are colonists, not originals.

That does not rule out rudimentary life developing at the vents, and then moving off into the oceans, with their descendants returning "home" billions of years later.

Something like this could have happened on Europa and on other moons with ice-covered oceans, such as Saturn's moon Enceladus. The coloured stains on Europa's surface indicate the presence of organic chemicals in that hidden ocean — the building blocks of life.

We need to land a spacecraft on Europa, which would lower a hot probe that will melt its way down through the ice to the underlying ocean. In one proposal the probe would release a robot submarine that could head off to explore.

However, before designing this lander we have to know what we will be faced with on Europa, in particular how thick is the ice layer? If it is a few kilometres thick, getting through it to the ocean will be a challenge. If the ice is 50 km thick or more it will be a problem of a different order.

We will need to know the answer to this and other questions before designing a lander that will have the best chance of telling us what lurks in that remote, dark ocean.

A mission to do that has been announced. Launch is planned to take place in 2023.

  • Jupiter and Saturn lie low in the southern sky after dark.
  • The moon will reach First Quarter on the 5th.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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