A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Europa is hot?

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


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.


  1. "The Solar System is Mostly Dead"

    Sometimes discoveries are so hyped that folks get their hopes up (about potential geothermal activity in this case), only to have their hopes dashed when confirmation of actual circumstances reveals that the hot spot is only "ordinary" geological rock/mineral ejecta from Europa's subsurface that retains heat from the Sun, then gradually releases it into cold space at night; wow...such a let down.

    Such findings point to the prospect that MOST worlds within the Solar System are geologically dead (inactive) and many of the ones with geological activity (such as Io) have their geological processes caused by external factors such as tidal forces from a primary world tugging on it.

    Dead worlds are much less interesting than living ones and the places (within our Solar System) where self-generated geological activities occur is very limited; Mercury (RIP), Venus (active vulcanism, but not much else), Mars (RIP), even Ceres is probably dead. Earth however is uber active (vulcanism, plate tectonics, radioactive decay of isotopes in the mantle and crust with primordial heat left over from the formation of the planet. Earth also has a dense atmosphere/hydrosphere to put frosting on the cake).

    Only the Saturnian planet-like moon Titan has a good variety of self-generated "living activities" such as cryovolcanos and a dense atmosphere that produces weather, including a hydrological cycle (it rains on Titan, now that is interesting).

    The problem is that everything within the Solar System is so painfully boring when compared to Earth. The more we uncover the truth, the more disappointing it appears...the Solar System is mostly dead.


    Guess this dashes my hopes on opening a hot springs resort on

  2. That's one of the things that jumps out when you look at the charts of exoplanet discoveries... at least, the ones we've made *so far*. Earth is maybe not entirely unique, but it IS =WEIRD=. Rocky planets of its size are less common than larger ones (thus the solar system is odd for it being the biggest - potentially the asteroid and kuiper belts speak of some ancient catastrophe that befell a larger rocky planet, and might even have involved terra/luna, phobos/deimos, neptune/titan and indeed the plutonian and haumean systems if we consider a one-armed-bandit "jackpot" cascade with something massive barrelling its way through the system hitting several coincidentally-conjunctive bodies one after the other)... and rocky planets of its size, orbiting at approximately 1AU around a star of this size and spectrum, are down in a corner at the end of several convergent long tails, with the vast majority of detected exoplanets being way off in a "main sequence"-looking band elsewhere through the chart.

    Which is maybe how it's ended up so active in the first place - relatively small with an unusually large, close satellite for anything that isn't a definite binary or a gas/ice giant, in an unusually *distant* orbit (keeping the worst of the solar radiation and wind from stripping the atmosphere or having other life-hostile effects) that means the opposing tidal forces of star and satellite can very nearly cancel out at some times and reinforce nearly twofold at others, causing a lot of internal heating and shifting-around... after having been smashed into by a pretty large object at some point also, with theoretically that aforementioned large satellite actually being composed of the remnants of an originally much thicker crust (which would have otherwise muted those other effects) that's been stripped away to leave a relatively thin layer over a more regularly exposed mantle and a more dominant magnetic core... and possibly even a lot of the water not coming so much from icy comets but actually originally being internal, and managing to escape into the atmosphere (and from there, fall onto the remaining crust) after that collision...

    Yeah, it's possible that Earth is VERY strange, in cosmic terms. Entirely possible that we could find another somewhere, but such a combination of starting conditions for both intense geological activity and, in a roundabout way, for life, could turn out to be highly reliant on a good number of separate low-probability events which all have to happen, possibly in a certain order, to produce such a fertile cradle. And how do you even scan exosystems for such characteristics, without a revolutionarily much more powerful telescope that can directly image a lot more of them (perhaps not enough to see more than single coarse pixels, but enough to get much more accurate spectroscopy at least) than we can at present. We might even be able to find a fair number that would, either directly or with some minor terraforming efforts, be able to sustain a human (or more likely a fairly pan-terran, as we'd have to take a lot of other species with us) colony... but the number with any extant life of their own could be disappointingly low, due to lacking geologic turnover or other preventative issues.

  3. Ugh... stupid blogger and its tiny 4kb post limits...

    And taking it further, there's the question of whether the asteroid and kuiper belts, being maybe the remnants of one or two (or more?) massive rocky worlds that went all Roche-y for some reason and formed what are essentially fairly broad rings around Sol, have helped protect us from other extrasystemic mayhem like gamma bursts, close transiting neighbour stars or rogue planets, bombardments of great numbers of far extrasolar comets and such, simply by presenting an absorbent barrier to anything coming in reasonably close to the ecliptic (with disruptors or impactors that are significantly out of it presenting a lower collision/mass disruption risk anyway), soaking up the hazards either through gravitic or straight up physical-collision means. So after the initial calamities some billions of years ago, the inner solar system with its four, relatively small and wide spaced rocky worlds, was kept unusually safe from further high-energy interference and could act as a creche for at least three different what-if scenarios (ie Venus, Earth, Mars).

    What I'm saying is, I guess, that the Fermi paradox may well point to certain missing, or vastly mis-estimated terms in the Drake equation, and that there's quite a few more barriers that need to be overcome to produce intelligent life than were initially postulated, and a lot of them are remarkably physical rather than biological or social. Like, add in the questions "does the system have planets in a strange place", "does at least one of them have an oversized moon formed largely of its own outer shell", "does that planet also have an unusually large iron core that produces a strongly protective magnetosphere", "has the system undergone events in its accretion stage or since then but still sometime in the distant past that has led to the formation of protective dust or debris rings around the parent star, between the candidate planet and extrastellar space?"... Sort in in-between "is the star of the right type" and "does it have planets formed out of a disc of suitable material", and "has it been seeded with suitable precursor substances to kickstart the formation of primordial life" and "is the life that's evolved both clever enough and curious enough to be able, and to want to, find out what's going on outside of its own atmosphere?".

  4. third & final chunk. thank heavens for Notepad.

    (On top of which, we also need to add a term that's common to more than a few sci-fi / sci-fantasy works... "are both the planetary atmosphere, and the region between the planet and extrastellar space, clear enough that the system's main star(s), any moons, other planets and most importantly other stars beyond the system clearly visible at least some of the time?". If they don't have anything to provide the context of their world existing in a larger universe or even a star system at all, because the atmosphere is cloudy or the system has dusty nebulae obscuring the galaxy and the rest - or even if, as a fundamental barrier, civilisation develops underwater, or underground, for whatever reason, then they might well think they live inside a fairly hermetic reality and never quite get the spark of inspiration that there could be something beyond, at least not until otherwise somewhat more advanced than we are now, or some significant event shakes things up. Which, in a lot of those stories, is actually the arrival of human explorers penetrating the Yavin-like permagloom and introducing the natives to the concept that actually, no, they don't live on the *inside* of a a ball...

    See, even small, seemingly negative and boring relevations can lead to long chains of intellectual discovery and realisation. Even if said realisations are, in the end, well... not so much boring, as just disappointing and a way of the universe ramping the difficulty level up several notches in response to our having found something of a cheat code (IE the vastly improved ability over the last quarter century, between better telescopes and better processing techniques and hardware, to scan the oppressive void for any other possible voices, or at least places they might live...)

    Just gotta keep on fishing.

  5. Hey Mike, i'm reading your book to my ten year old son, last night i read the part about you almost quitting, and antonin talking you out of it- and about not being normal and being an astronomer- fynn said that you can't quit because you've made so many important discoveries, and that if you did we'd have to go out there and talk some sense into you, to make sure you keep on searching the skies... you've touched a ten year olds heart, thank you.