By: Alyssa Rhoden
The fall meeting of the American Geophysical Union (AGU) is a monster of a conference. Held annually in San Francisco, the meeting draws more than 20,000 attendees and holds over 500 sessions every day. With so much activity, one would expect these sessions to cover anything and everything geoscience. Sadly, however, there hasn’t been a Europa-specific session at AGU in years.
We, at Destination: Europa, thought it was high time to showcase our favorite moon as well as the novel and noteworthy science being cleverly wrangled out of our limited data. We proposed an AGU session on Europa and received 32 abstracts, enough for both an oral and poster session.
Reading through the abstracts, the breadth of Europa science being conducted, all over the world, blew us away. Although it was difficult to select only 11 abstracts for our oral session, we did our best to make it the most diverse and exciting Europa session on record.
Cracking Europa.
This medium resolution image, with high-resolution inset, shows numerous overlapping fractures, bounded by ridges, chaos features (small, bottom right and larger, top left) and several pits an uplifts (top left). Although much of Europa’s geology can be explained by tidal stress and heating, due to Europa’s eccentric orbit, an additional mechanism is needed to explain pits and uplifts, a subset of fractures, and balance new surface area added at dilational bands. New work, presented at AGU, will help scientists solve these puzzles and better understand this icy, ocean moon.
Several researchers presented work in which they mapped and measured geologic features on Europa’s surface in order to understand its resurfacing history. Europa’s geology is made up of two broad classes of features; in global imagery of Europa, they appear as lines and spots. The lines are thought to be fractures and include hairline cracks, barely visible on the surface, prominent pairs of parallel ridges, and all stages in between. Regional and high-resolution images have revealed that spots encompass a wide variety of features. Chaos terrains are regions of broken-up surface material, perhaps formed when the outer portion of the shell collapsed into a shallow lake below, and then refroze. Pits and uplifts have also been identified, which have proved challenging for scientists to explain.
A new survey of pits, uplifts, and small chaos features, presented by Kelsi Singer, who recently received her PhD from Washington University in St. Louis, has revealed some interesting characteristics. Despite a thorough search, Kelsi was not able to identify any features smaller than 2.3 km across. Also, the heights of features – above the surrounding terrain for uplifts and below for pits – correlated strongly with their widths. Scientists can now use these characteristics to test models of the formation of pits, uplifts, and small chaos regions.
The most tantalizing result from these geologic studies was the identification of bands that correlate with a loss of surface material over time, which was presented by Simon Kattenhorn from the University of Idaho. In a process analogous to terrestrial plate tectonics, it appears that large portions of Europa’s surface have been subducted into the warmer ice below. If confirmed through continued analysis, Kattenhorn and his coauthor, Louise Prockter, may have solved a long-standing issue with balancing Europa’s budget – surface area budget, that is.
In many regions of Europa, scientists have identified dilational bands – fractures that were pulled apart over time, with new material rising up to fill in the gap. Similar to spreading centers on the Earth, these dilational bands represent an increase in Europa’s surface area that must be balanced by a loss of area elsewhere. Europa scientists have been looking for evidence of convergence, or subduction, for decades. A few convergence bands have been identified, but they are likely too sparse to independently offset the observed dilation. Another speaker in our session, undergraduate student Cansu Culha, from UC – Berkeley, identified both contraction and dilation along ridges. However, the contraction was, again, only a small fraction of what would be needed to balance the observed dilation. Adding in the bands identified by Kattenhorn and Prockter, however, may do the trick.
The mechanics of Europa’s plate tectonics are not yet understood. For example, we don’t know whether plate motion is driven by the opening of fractures or by the subduction process itself. Actually, the same questions are being debated for Earth! Identification of plate tectonics on Europa thus offers us an opportunity for comparative planetology – studies that compare processes across multiple solar system bodies. Unfortunately, the sparseness of data at Europa makes widespread identification of these so-called subsumption bands unlikely. Rather, it will take a new mission to gather enough data to identify Europa’s plates and determine the extent to which plate tectonics plays a role in Europa’s surface history.
Eat me, Earthlings.
Another important implication of a plate tectonics style process operating on Europa is that it would mix material from the surface and subsurface, a key element for habitability. Mixing within the ice shell may also be driven by small-scale melting and freezing at the ice-water boundary, leading to compositional differences in the the ice that can drive inter-ice plumes. As Divya Allu Peddinti, a graduate student at Arizona State University, explained, evidence of mixing may even be preserved on Europa’s surface, detectable by future compositional analysis of the ice.
To give us a better understanding of whether Europa can support life, Tori Hoehler, an exobiologist at NASA Ames, described the role of energy. As he explained, the method by which energy is delivered is as important for sustaining life as the total amount of energy. Some mechanisms do not supply energy consistently enough, or in large enough quantities, to maintain organisms. To better understand Europa, from an astrobiological perspective, we need more laboratory investigations and models of the energy exchange processes that may be at work in Europa’s ice shell and ocean and at the boundary between the ocean and the rocky interior.
Destination: Europa!
Given that our goal is to support a new mission to Europa, we were very excited to learn what different instruments could teach us. Nick Schmerr, from the University of Maryland at College Park, investigated the potential results that seismometers could deliver if they were ever deployed on Europa’s icy surface. As Europa moves through its eccentric orbit, the ocean responds to the changing gravitational pull of Jupiter. Cracks in Europa’s ice shell correlate well with predicted patterns of stress caused by the ocean moving underneath. Whereas terrestrial earthquakes are mainly driven by the motion of crustal plates, tides should drive strike-slip motion along Europa’s faults, producing cryoquakes. Seismometers on Europa could detect the shaking induced by fault motions over time. Then, seismological techniques developed for Earth could help us probe Europa’s interior structure, further refine predictions of tidal stress, and identify small pockets of water within the ice shell.
This image, captured using the Hubble Space Telescope, is interpreted as a plume emanating from Europa’s south pole. The emission was clear in this image, taken when Europa was at its farthest point from Jupiter (called apocenter) but did not appear in images taken at other times in Europa’s orbit. The eruption timing is reminiscent of Enceladus’ plumes, which change in intensity throughout its orbit, and suggest tidal control of Europa’s eruptions. (Image credit: Retherford, et al., 2013, Science)
We also had an update from Victoria Siegel, of Stone Aerospace, on Project Valkyrie: a cryobot that they hope to someday deploy on Europa. Unlike past concept studies we’ve heard about, Stone Aerospace actually has a working prototype, which they plan to test in Antarctica this summer. For the tests, energy from lasers, sent through a fiber optic cable, will power the bot. The engineers at Stone Aerospace pioneered this technology because nuclear power, the source envisioned for use at Europa, cannot be used in Antarctica. Once deployed, the crybot will melt its way through the ice, filtering melt water through sensors in order to catch any traces of life. After penetrating through the ice layers, it will slowly execute a u-turn, and return to the surface. Our session, which ran from 4-6pm on the last day of the meeting, culminated with a video of the Valkyrie cryobot descending into a large block of ice at the Stone Aerospace testing facility. It was pretty amazing footage; much of the audience stayed late just to watch Valkyrie in action.
An eruption of enthusiasm.
The Destination: Europa session was a smashing success, but it wasn’t the only session in which Europa results were presented. And no recap would be complete without mention of the astonishing discovery of plumes emanating from Europa’s south pole. Based on analysis of images from the Hubble Space Telescope (HST), scientists identified an anomalous oxygen emission in Europa’s UV spectrum, which is best explained by two plumes, each one rising about 200-km above Europa’s surface. Interestingly, that is about the same height as the plumes observed on Enceladus. Because Europa is much larger, and has much higher gravity, Europa’s plume material must be moving much faster than on Enceladus.
Whether Europa is currently active is certainly one of the most fundamental questions unanswered by our previous, data-limited analyses. Although the plumes have only been detected once, and additional observations will be needed to confirm the results, the implications are mind-blowing. Not only would these observations prove that Europa is currently active, the detection of plumes means that the surface-subsurface exchanges so critical to life do occur and that liquid water is present at shallow enough depths to be tapped by fractures. With this evidence in hand, we could simply not justify any further delay in exploring this active, watery world, which may be the best extraterrestrial habitat in our Solar System. We need a new mission to Europa, and we hope all of you will help us get there by sharing your enthusiasm with your friends, colleagues, and most importantly, your elected officials. Together, we can make Europa our next planetary destination!
Note: Over the next few days, we’ll be chatting with the scientists who discovered Europa’s plumes. Check our blog for all the details!
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Want to support a future mission to Europa? Learn how to contact Congress.
Clipper mission details can be found on our Mission page and on JPL’s Clipper site.
For additional coverage of Europa science at AGU, check out the articles on The Planetary Society’s blog and NASA’s Europa blog.
