Mike Brown's Planets: 2017

Update from Samantha on her paper that just came out today (see: https://arxiv.org/abs/1708.07922)

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In the months since I first posted about the potential hotspot on Europa associated with a potential plume on Europa, I’ve been refining our computer model and digging deeper into trying to understand what is going on. As you’ll remember from the last post, a potential plume spotted on Europa looked like it might be coming from a spot that the Galileo spacecraft had earlier shown was hotter at night than it should be. We discussed two potential explanations for this night time hot spot. The more exciting explanation was that the spot in question could be experiencing excess subsurface heat flow due to recent or ongoing geologic activity, as one might expect from an area with potential active plumes or geysers or volcanoes or whatever. The other possibility was that the spot may be hot at night due to its specific thermal properties, particularly its thermal inertia. A high thermal inertia could keep the location warm during the night, but it would also make the same spot harder to heat up during the day – think about how pavement stays warm after a hot day long after the sun has done down but is also cooler than it should be in the morning. A spot actively heated by geologic activity, in contrast, would maintain elevated temperatures throughout the day-night cycle.

With only the Galileo night time temperature measurements, there was no way to know which of these two scenarios was occurring. Luckily, we have recently obtained daytime temperature measurements using the new massive new ALMA telescope in Chile. Our daytime ALMA observations allow us to tell the difference between these two scenarios. We left you last time with the puzzling observation that the potential hot spot was actually a little colder in the ALMA daytime image than our model predicted. After extensive testing and refinement of the model, that finding remains true. Here is our updated data-model comparison.

The location of the proposed hot spot is indicated by the white circle and, relative to our model, it is cold during the day and hot at night. At first glance, this pattern seems more like a potential thermal inertia anomaly than an active hot spot. To look a bit closer, we modeled the location throughout the Europa day to better examine the day-night temperature profile and see what it would take to fit both the Galileo and ALMA temperatures. Below you can see our three modeled scenarios.

The green curve is our model’s predictions for the proposed hot spot. Like you saw in the first figure, it underestimates the nighttime temperature from Galileo on the left and overestimates the daytime temperature from ALMA on the right. To test the hypothesis of subsurface heating, we increased the heat flow in our model, which produced the red curve. In this case, the amount of extra heating needed to match the Galileo nighttime temperature created a daytime temperature that is much higher than we observe with ALMA. However, when we simply increased the model thermal inertia (with a small albedo adjustment within our uncertainties), we were able to fit both temperatures well. Sadly, this suggests that the potential hot spot associated with the potential plumes is most likely just a spot with a higher than average thermal inertia, making it especially good at retaining daytime heat into the night.

You might rightly be wondering why this one spot should have such a relatively high thermal inertia. The answer could be because of its proximity to Pwyll, the biggest, freshest crater on Europa. Pwyll Crater is just below and to the right of the proposed plume location and, interestingly, is even more anomalous. It is also cold during the day, and it is the big, obvious red anomaly on the night side. So, it is not just the proposed plume source that appears to have an elevated thermal inertia, but the entire Pwyll Crater region. This could be because material ejected during crater formation is blockier than the rest of the surface, so that it acts more like rock than sand. It’s also possible that the impact exposed purer water ice, allowing sunlight to penetrate deeper into the surface in this area. That sunlight would be stored as heat below the surface, which would be released slowly at night, mimicking the effects of a high thermal inertia. Really, we don’t know for sure what would cause the elevated thermal inertia, but it looks like the possibility of subsurface heating is unlikely.

So the purported hot spot is still unique, but not so hot. What does this mean for the plumes? Our observations do not specifically address the existence or nonexistence of the plumes. They do, however, suggest that the proposed detections are not associated with an active hot spot, which would have otherwise made the potential plume detections much more convincing. In the end, we still don’t know, but we are excited about what else the ALMA datasets might tell us about the surface.

Europa is hot right now. With the planning for the Europa Clipper mission underway and talk even of a lander, scientists are paying more attention to the little icy satellite than ever. Much of the recent excitement has been a discussion of the now-you-see-them-now-you-don't plumes that might be jetting material from the interior ocean. Such a possibility would be quite exciting indeed, as it would allow us to understand the interior conditions of Europa without having to do something crazy like dig a hole through the ice. But the plumes have been on the tantalizing edge of detection so far.

Today another tantalizing bit of evidence came out. One of the possible plumes was possibly seen again in the same place. The new detection, like all of the old ones, is still just on the verge of being believable. I personally remain skeptical, but I would call it skeptical but hopeful rather than skeptical and dismissive.

While I have no extra insight into the plumes, one of the really interesting things about the potentially-repeating plume on Europa is where it is seen. As mentioned today, old data from the Galileo spacecraft suggests that the possible source location of the possible plume is possibly a spot on Europa that is hotter than it should be. Hotter than it should be! This is very exciting, because it would both suggest that liquid water is probably close to the surface, but it is also something that I can actually address.

For the past year my graduate student Samantha Trumbo and I, along with our colleague Bryan Butler and NRAO have been mapping the surface temperature of Europa from the new ALMA millimeter telescope in Chile. So is there really a hot spot on Europa????????

The answer is complicated. To know if a spot on Europa is hotter than it should be we have to figure out how hot it should be. This requires a lot of work and is part of Samantha's Ph.D. thesis. We don't have all of the answers yet, but inspired by the news today she took a careful look at that spot on Europa. This is all still very preliminary work. We would normally wait to show results until our analyses were complete. But with the excitement of a potentially plume and hot spot we though we would give a quick example of what our data from ALMA show.

I'll let Samantha tell the rest of the story:

The recent Hubble Space Telescope (HST) observations of Europa show a recurring anomaly that may be consistent with a water vapor plume. The scientists have located the most likely surface source region for this feature and found that it coincides with a potential hotspot seen in temperature data from the Galileo orbiter. If this hotspot is real, this could be strong evidence that the HST anomaly is really due to geologic activity on Europa, rather than some less interesting cause.

We've observed Europa using the Atacama Large Millimeter/submillimeter Array (ALMA) and obtained complete temperature maps of the surface. These can also be used to look for hotspots that may be indicative of geologic activity or even plumes. However, the thermal maps are strongly influenced by the surface properties of Europa, particularly in how well the different surface materials absorb light, radiate heat, and resist changes in temperature. Therefore, in order to look for hotspots, we developed a computer model to try to account for these variables. The model calculates the absorption and re-radiation of sunlight, as well as the day-to-night cycle of heat flow into and out of Europa’s near-surface layers. This allows us to create model ALMA images that we can compare to our observations in search of anomalies.

One of our observations captures the inferred source region of the potential plume signature captured by HST. When we compare this observation to the results of our thermal model, the region does not stand out as anomalously warm. In fact, it is a little colder than our model predicts. Critically, though, we look at Europa during Europa's daytime (because we can't do otherwise). The Galileo spacecraft, however, could sit behind Europa and look at its night side, where the surface should have cooled. While most of the surface has cooled, that one spot hasn't cooled very much. The area near the purported plume source, which also includes the large Pwyll impact crater, stands out as much warmer on the night side than the model predicts.

The top row shows temperatures on Europa measured from ALMA compared to our expectations ("thermal model"). The circle at about the 4 o'clock location is the alleged hot spot. The far left shows the difference between the measurements and our expectations. As you can see, our computer model is not yet perfect, but it does a pretty good of predicting the temperature of Europa. The bottom row shows the same computer model but now comparing to the Galileo nightside data (note the change in temperature scale; its a lot colder at night). Again, the match is not too bad except that in one place the nightside temperature is ~15 degrees C hotter than it should be. That is a huge difference. In fact, the dayside and nightside temperature at that one spot are nearly the same. Why? Maybe there is hot material just beneath the surface. Maybe the material retains heat better, like hot pavement on a cool night. Further analysis should answer the question.

Something that is warm at night is definitely a hot spot, but why is it hot? It’s possible that spatial variations in the surface properties could cause localized areas to appear anomalous at night. Indeed, the Galileo team noted that the region in the vicinity of Europa’s large Pwyll crater was hotter than expected and suggested that a large thermal inertia may be to blame, although they did not rule out the possibility of subsurface heating. Thermal inertia is essentially the ability to resist temperature change. Sand, for instance, has a low thermal inertia; it gets very hot during the day, but quickly becomes cold at night. Rocks, on the other hand, have a much higher thermal inertia; they can remain warm well into the evening.

Just knowing that the spot is hot at night can't tell you why. But the good news is that the combination of daytime temperature and nighttime temperature will allow us to answer the question. We plan on refining our model to see if it is possible to explain this hotspot in the Galileo data with variations in thermal properties of the surface materials, but at this time we cannot rule out the intriguing possibility of subsurface or plume activity. We will also use our data to search for other potential hotspots, which may not have been visible during the HST observations. Stay tuned for more excitement.

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