Forty years ago, a Voyager spacecraft achieved the first close-up images of Europa, one of Jupiter’s 79 moons, which revealed brown cracks cutting off the moon’s icy surface, giving it the appearance of a venous eyeball. Subsequent missions have gathered enough additional information about Europa to make it a priority research target in NASA’s search for life.
Why is this moon so special? Because it has all the necessary ingredients for life, a promising extraterrestrial nursery.
Planetary scientists suspected that Europa, Jupiter’s largest moon, could harbor a vast ocean of liquid water beneath its thick icy crust. We merge this ocean with an energy source (such as hydrothermal vents) and some chemical elements, and we find a decent chance that the moon can withstand basic life forms.
Now, an international team of astronomers has announced the direct detection of water vapor in Europa’s atmosphere for the first time in history. As detailed in the study by the magazine Nature Astronomy, this method of detection is strong evidence that liquid water exists below the surface of Europa.
"This does not necessarily mean that water vapor comes from an ocean," stated the NASA planetary scientist Lucas Paganini. "But it seems that this detection is connected to liquid water below the surface".
The NASA team discovered that Europa releases enough water vapor to fill an Olympic swimming pool in a matter of minutes. Whether it sounds strange or not, it’s been enough to be detected from Earth.
Where does this water come from?
Of the 17 observations at the W. M. Keck Observatory in Hawaii, which uses a spectrograph to detect the chemical compositions of the atmospheres of other planets by scanning infrared light, whether released or absorbed, Scientists only detected water vapor in one. Molecules like water emit specific frequencies of infrared light when they interact with solar radiation.
"We suggest that the degassing of water vapour on Europa be conducted at lower levels than previously estimated, with localized and rare events of stronger activity," stated the authors.
Due to the fact that they only saw water vapour once, scientists believed that the water came from a plume, and where there are plumes, there’s a good chance there’s water in liquid form. The next step is to determine what the plumes are producing and whether they are indicative of large amounts of liquid water in Europa. A plume can come from degassing liquid water in the depths of Europa’s surface or from friction caused by ice, among other things. However, to solve the mystery, we’ll have to send some robots to see for ourselves. "If we want to get more knowledge about this ocean world, we really need to get closer," stated Paganini.
What is to come
NASA’s next Europa Clipper mission will take a much closer look at the surface of the icy moon. The mission will be launched in 2025 and will be the largest probe ever created, Clipper will be equipped with a set of instruments designed to look below Europa’s ice sheet (cameras, spectrometers, radars) that will also determine the composition of the moon and provide us with images never seen before. If Clipper finds evidence of life during the scheduled 45 overflights, it could finally justify a landing on this huge moon.
Reference: A measurement of water vapour amid a largely quiescent environment on Europa. L. Paganini, G. L. Villanueva, L. Roth, A. M. Mandell, T. A. Hurford, K. D. Retherford & M. J. Mumma. Nature Astronomy (2019) DOI: doi:10.1038/s41550-019-0933-6