On Earth, we believe that the first organisms formed at the bottom of our ocean along with vents that provided them with warm and nutrient-rich environments. This specific collection of layers, a warm bedrock lying beneath a liquid water ocean, was able to provide conditions that were just right for life to form here. In the search for life within our solar system, we are looking for objects with similar internal structures. One of the best candidates for potentially harboring life is Jupiter’s moon Europa, a body which scientists hypothesize also contains this type of layering. It is thought that underneath the surface of this icy moon lies a saltwater ocean that is kept from being frozen in the cold environment of space due to a warm rocky core heating it from the center. Astronomers at NMSU and JPL’s Ice Spectroscopy Laboratory have been working to understand the processes responsible for Europa’s massive ocean.
On Earth, if we want to study the conditions at this boundary we could simply send a probe directly to that location, however, Europa is not only extremely far away but it is also extremely expensive to launch space probes (for example, the Mars Perseverance mission cost $2.7 billion!). Additionally, Europa’s surface is believed to be completely made of ice so even if we land a probe on the surface, being able to reach the ocean will be very difficult. For this reason, scientists have to find more creative ways of studying this object. One way to do this is by observing the composition of its surface and using computer modeling to back out what processes might be occurring beneath it.
There are two ‘phases’ that water ice is found in, crystalline and amorphous. The word ‘phase’ used here is distinct from the states of matter that most of us are familiar with. Here, ‘phase’ refers to how the individual molecules are organized within the solid ice. Water ice with a crystalline structure has molecules packed into an ordered arrangement, whereas water ice in the amorphous phase has a less orderly structure. Amorphous water ice is formed when liquid water is flash frozen and the individual molecules do not have enough time to settle into their preferred crystalline structure, so the structure is random. When water freezes at a slower rate, there is enough time for the molecules to form into the ordered structure. Based on the surface temperature of Europa, scientists suggest that the water ice there should mostly be in the crystalline structure. There are, however, a number of other processes that can affect the “phase” of the water ice on Europa’s surface such as particle bombardment, thermal relaxation, and possible cryovolcanic activity. Determining how much of the water ice is in each phase will provide information about the specific processes that are occurring on Europa’s surface and therefore, what might be occurring below the surface within the ocean and/or at the seafloor.
Astronomy graduate student Jodi Berdis recently published work that revealed a discrepancy in our understanding of the processes occurring on Europa. “The crazy thing about water ice is that there are actually quite a few processes that influence [which phase it’s in],” says Berdis. Different types of cryovolcanic activity can result in different water ice phases. Berdis used observations of Europa taken at Apache Point Observatory’s 3.5m telescope in combination with laboratory data from the Ice Spectroscopy Lab at NASA’s Jet Propulsion Laboratory (JPL) to measure the percentage of the Europan surface that is in the crystalline phase. This measurement was then compared to predictions for the percentage of crystalline water ice made by modeling the effects of particle bombardment and thermal relaxation. What Berdis found was that observations indicate the surface contains more amorphous water ice than crystalline, but that the model indicates the surface contains more crystalline water ice than amorphous. The discrepancy between these values reveals that cryovolcanic activity, which was not included in the modeling of Europa’s surface, may play an important role in reducing the crystallinity of Europa’s surface.
The next step for Berdis is to investigate this discrepancy further by diving into higher resolution data of Europa’s surface that was taken by the Galileo flyby mission. This improved data will allow Berdis to search for signatures of cryovolcanic activity on smaller spatial scales in order to determine its potential contribution. Berdis also plans to continue using routine ground-based observations of Europa “to monitor any significant changes [in the crystallinity] that may take place over time.”
Berdis’ article can be found here: https://www.sciencedirect.com/science/article/abs/pii/S0019103519303999?via%3Dihub or here: https://arxiv.org/pdf/2002.04132.pdf
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