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The moon of Jupiter, Europa, with its underground ocean, can have what it takes to favor extraterrestrial life. Many astronomers have sent a mission to this icy moon, but new research suggests it will be difficult to land there: its surface seems to be littered with icy peaks, the highest of which are five storeys high.
A new study published this week in Nature Geoscience suggests that Europa's surface is covered with high sharp-edged ice blades, called penitents (pronounced PEN-E-TENT-EES). If you're a geologist, it's a great discovery; But if you are an astrobiologist, this is a major problem. This is because Europa, like Saturn's moon, Enceladus, presents a vast underwater ocean that could serve as a potential habitat for extraterrestrial life. The presence of penitents, some of which reach 50 meters in the sky, could "constitute a danger for a future lander on Europa," the researchers write in this new study.
The penitents are composed of snow and ice and come from sublimation, a chemical process in which the ice is transformed directly into water vapor without first melting into a liquid. The remnants of this process are the penitents themselves, who always point to the midday sun. To form, these iced ears require intense and continuous sunlight as well as a cold, dry and calm air.
We have penitents here on Earth, but we only find them in high altitude areas near the tropics, like the Andes, and their size varies from about 3 to 16 feet. The conditions on Europa are markedly better for the penitents' training, as the authors of the new study, led by Daniel Hobley of Cardiff University, point out. These conditions include an ice-covered surface, ideal temperatures, and other environmental factors conducive to sublimation, as well as a constant angle of incoming solar radiation.
To date, the resolution of the photographs taken on the surface of Europa has not been fine enough to show the penitents, so they remain hypothetical. That said, the computer models managed by Hobley and his colleagues strongly support their presence. Using pre-existing observational data, the researchers calculated sublimation rates at various points on the moon's surface, allowing them to estimate both the size and distribution of the penitents. According to the models, the penitents of Europa can reach a height of about 15 meters, spaced about 7.5 meters. According to the study, ice peaks will probably be more frequent around the equatorial regions of Europa.
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If there is good news in all this, it is that a future mission to Europe could in any case avoid the equatorial regions. Research published last July suggests that life-hunting scientists will have a better chance of discovering potential biosignatures in young Europa ice and in low-radiation areas. This type of ice, less than 10 million years old, is expected to exist in mid-to-high latitudes, where the moon points to Jupiter. And as the new study shows, these areas are less likely to host these dangerous ice peaks.
Further observations are needed to prove definitively that Europa's surface is covered with penitents, but this could happen soon. The launch of NASA's Europa Clipper spacecraft is scheduled between 2022 and 2025. This orbital probe is bursting with instruments and will be less than 25 km from the Europa surface. It is interesting to note that the Europa Clipper mission could be the forerunner of a landing mission on the Jovian moon, in which a probe would pierce the icy surface and plunge into the dark ocean below.
The technical imperatives of such a mission disturb the spirit, and now, with the announcement that Europa's surface might contain vast fields of penitents, there is an additional complication. But as we often say, space is difficult. And if we continue to push the limits, it will only get worse.
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