[Un article de The Conversation écrit par Cyril Mergny – Chercheur postdoctoral, Université Paris-Saclay – & Frédéric Schmidt – Professeur, Planétologie, Université Paris-Saclay]
But there are still many unknowns on ice on the surface of Europe. Two new studies lift the veil on an unexpected phenomenon.
Thanks to two new studies, one theoretical and the other resulting from observations of the James-Webb telescope, we today understand the ice surface of Europe. We have notably shown that the atomic structure of ice changes over the seasons, which can be seen in the light reflected by this moon, a bit like a lighthouse that would sparkle in the night.
These new knowledge will be useful for one day, to consider placing an landing on Europe, but also to better understand the geological processes that shape the surface – we still do not know, for example, to explain the origin of “stripes” that shape the surface of Europe.
In the coming years, we hope that the flicker of the “atomic lighthouse” of Europe can be really observed, in particular by the Europa Clipper of NASA as well as the JUICE mission of ESA.
Ice on earth and ice in space are different
On earth, water ice in its natural environment is in one form: a crystal structure, commonly known as “hexagonal ice”.
However, in space, as in Europe, it's another story: it is so cold that water ice can adopt more exotic forms with different properties.
Thus, the most widespread ice shape in the universe is the so -called “amorphous” ice.
It is a form of ice where the arrangement of water molecules has no large -scale order, unlike crystalline ice which has repetitive patterns.
An analogy on our human scale would be a display of oranges. In the crystalline case, the elements are all tidy, in the form of a periodic network. In the amorphous case, the elements are in bulk without any regular position.
Our daily life includes examples of amorphous or crystalline versions of the same material: for example, the daddy's beard contains a amorphous shape of sugar, while the usual kitchen sugar is crystalline.
In fact, we expect the external solar system to have ice mainly in an amorphous form, first because very low temperature (-170 ° C on Europe), the molecules do not have enough energy to organize properly; But also because the crystalline structure tends to break under the effect of particle bombings from the sun, deflected by the Jupiter magnetosphere, as if we sent a disruptive orange in a well -stored stall.
Previous spatial observations of the 1990s and then in the decade 2010 had shown that European ice is a mixture of amorphous and crystalline forms. But, so far, no model explained why.
A structure that changes with the seasons
For the first time, we quantified the competition between crystallization, due to temperature during the hottest hours of the day, and the amorphization induced by the bombardment on the surface of particles from the Jupiter magnetosphere.
We have thus shown that crystallinity is stratified on Europe: a very thin layer on the surface is amorphous, while the deep layer is crystaline.
Even more remarkable, the simulation revealed that the crystallinity of the ice on the surface could vary depending on the seasons! Although seasonal variations do not affect the amount of particles that bomb Europe, it is warmer in summer, which makes crystallization more effective and thus tips the scales in its favor. In summer, it is an average of 5 ° C hot in winter, which makes ice up to 35 % more crystalline than in winter in certain regions.
We concluded that if we observed Europe over the seasons through a spectroscope, this would give the impression that the surface “sparkles” over a period of twelve years (the duration of a year on Europe), like a lighthouse in the night.
How do we know the atomic structure of ice at a distance of 700 million kilometers?
Simultaneously in our study, NASA astronomers observed Europe with the powerful James-Webb telescope. Their study has just shown that the results of our simulations agree with their observations. Indeed, although the two approaches use radically different methods, they lead to the same conclusions.
Thanks to the James-Webb spectrometer, the researchers were able to estimate, remotely, the atomic structure of ice on the surface of Europe (on the first micrometer thick). To do this, they analyzed the light reflected by Europe in the infrared (slightly redest than what our eye can perceive) with the wavelength of 3.1 micrometers which reflects the crystallization of the water ice.
They thus established a crystalline card of the ice moon. By comparing their card observed with the one we have simulated, we see a very good agreement, which strengthens our confidence in these results.
On Europe, the surface is therefore dotted with regions with amorphous water ice and others with crystalline water ice, because the temperature varies according to the areas. Overall, the darkest regions absorb the sun's rays more, which warms them and, as on earth, temperatures are higher near the equator and lower near the poles.
However, the observational study using the James-Webb telescope captured a photo of Europe. For the moment, it cannot detect the sparkles in the infrared, because it would be necessary to observe the surface in several years to distinguish a change. These surface fluctuations are a novelty that we have discovered in our simulation study, and they remain confirmed by observations.
We hope that the juice and Europa clipper probes will soon observe these seasonal oscillations of light reflected by Europe in the infrared.
Our interest is now also focused on other frozen moons in Jupiter, where a cohabitation between amorphous ice and crystalline ice could exist, like on Ganymede and Callisto, but also on other bodies such as Entelade, in orbit around Saturn, or even on comets.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
