Hunting for signs of life
TAKE a look to the northwestern sky as the twilight fades, and that is where you will currently find Jupiter. It looks like a very bright yellowish-white star, but it is, of course, the largest of the sun’s planets.
Each time I look at Jupiter these days, my thoughts turn to not only the Juno spacecraft in orbit around the planet, but also to Europa, one of Jupiter’s 69 known moons. That’s because Europa is one of the key places in the solar system in our search for extraterrestrial life, and plans are being put together to send spacecraft there for that very reason.
Most of Jupiter’s moons are so small and faint they are beyond the range of anything but large observatory telescopes or spacecraft flying past. However, four of them have been known since the early seventeenth century: Io, Europa, Ganymede and Callisto. All four are visible as points of light near Jupiter, even using very small telescopes or a good pair of binoculars. Europa, with a diameter of 3122km, is the smallest of the four, but is nevertheless much larger than the next one down in size: Amalthea, which is only 168km across.
Our excitement about Europa arises from the deduction that it has an ocean underneath the ice layer. This information comes from the data returned by the two Voyager spacecraft in 1979, and later, by the first spacecraft to orbit Jupiter, Galileo.
The Voyagers revealed to us that the icy surface has been in motion — the evidence being cracks in the ice — and Galileo observations told us that there is an electrically conductive fluid beneath the ice.
In addition, some watery plumes have been seen erupting through the surface — a phenomenon that has also been observed on Saturn’s moon Enceladus.
That was enough to make scientists around the world take notice. A further study of Europa — its surface, its ice layer, and its interior — became a high priority.
The call for dedicated spacecraft to study Europa has been strong. A very important consideration is that Europa has almost overtaken Mars as a possible source of life elsewhere in the solar system.
Now, there are some significant plans.
At one time there was a suggestion that a Europa lander could drill through the ice to the subsurface ocean, but it seems that the ice may be several kilometres thick, and that more or less put an end to that plan.
Since 2011, NASA has on its drawing board the Europa Clipper, destined for launch in the 2020s. Its mission is to study the moon in detail, especially using radar at different frequencies to penetrate to different depths in the ice layer and return important information. NASA has also considered a possible later craft to land on the surface.
But in April this year, there was a suggestion that I applaud very loudly: a joint mission operated by both NASA and the European Space Agency (ESA), aptly called the Joint Europa Mission (the JEM).
The proposed mission was detailed in a multiply-authored paper, entitled Joint Europa Mission (JEM): A multiscale study of Europa to characterise its habitability and search for life, presented in April this year at the European Geosciences Union Assembly held in Vienna, Austria.
The suggestion is that the two organisations co-operate to send both an orbiter and lander to Europa to carry out a thorough survey of the environment around Europa, the subsurface ocean, and the surface itself.
Already, the ESA has in its plans the Jupiter Icy Moon Explorer (JUICE), which has a planned launch in 2022. It is primarily intended to investigate Ganymede, Jupiter’s largest moon (and the largest moon in the solar system). A combination of NASA’s and the ESA’s plans would be a great idea, especially because of the increasing interest in Europa.