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.

Sea Salt (part 2)

[don’t miss part 1] 

One of the biggest problems with trying to figure out what is on the surface of Europa was that the spectrograph on the Galileo spacecraft didn’t have a very fine view of the reflected light coming off the surface. The analogy I used in Part 1 was that Galileo was looking at fingerprints where you could only discern the rough pattern and not the individual ridges. You couldn’t use those fingerprints to know for sure who had smudged your crystal, though you might be able to rule out some people and you might become more suspicious of others.

There are two main reasons that the views from Galileo were not as fine as we would like. First Galileo was old when it arrived at Jupiter. Serious work began on the spacecraft in 1977, and with typical delays and atypical space shuttle accidents, it was finally launched, via a space shuttle, in 1989. Even the trip to Jupiter took longer than initially planned -- the shuttle accident spawned new rules which required the use of a less powerful rocket to launch Galileo from the orbiting shuttle -- so Galileo could not go directly to Jupiter but instead had to get gravity sligshots off of Venus and Earth before finally heading towards Jupiter and arriving in 1995, nearly twenty years after construction began. It was old on the first day it took data at Jupiter. (It was intentionally crashed into Jupiter in 2002 to prevent, among other things, an accidental crash into Europa, which would clearly disturb the whales).  Not surprisingly, the old technology was not as good as current technology in seeing precise spectral fingerprints.

The second reason that the spectrograph on Galileo didn’t see as fine detail as one might like is that, being on a spacecraft, it was extremely tightly constrained in how big and heavy it could be. Even in 1977 the technology existed for finer detail, but it didn’t exist in a form that you could package into the small size needed to head to Jupiter.

Today, the technology for finer detail is readily available. But, sadly, we don’t have anything new headed to Jupiter any time soon. What to do? Head to the Keck telescope.

The Keck telescope is no longer particularly new, either (it is celebrating its 20th birthday this month!), but telescopes, unlike [most] spacecraft, get new instruments all the time. And, even better, the instruments can be the size of a room. Keck has a relatively new (and extremely large) spectrograph – called OSIRIS – which has a ~40 times finer view of the spectrum than the spectrograph on Galileo did. There is, however, one serious disadvantage to the Keck telescope: no one has yet figured out a way to fly it to Jupiter. Again: what to do? Luckily, astronomers (and , earlier, spooks) invented an answer here. Turn on your adaptive optics.

Telescopes on the surface of the Earth are limited in the details they can see in space by the smearing caused by the turbulence of the Earth’s atmosphere. A star no longer looks like a single point of light in the sky, but instead looks like a big smeary point of light in the sky. More importantly, Europa doesn’t look like a little world with interesting features, but instead looks like, well, a big smeary point of light in the sky. Adaptive optics changes everything. I had what I think of as a very nice description of just how adaptive optics works in my book (that would be How I Killed Pluto andWhy It Had It Coming, in case you missed it) which I will post next week for people interested in the details, but, for now, think of it as a magical device which makes the Earth’s atmosphere go away and allows the telescope a pristine view of the sky (if you’re a spook, you are excited by this, because it allows very pristine view of enemy satellites as they zip across the sky). A pristine view of the sky with no atmosphere in the way is still not as good as flying to Jupiter, but, from a monster telescope like the Keck telescope, it’s enough to get a nice regional map of Europa.

So we flew off to Hawaii to spend four nights at the Keck telescope taking spectra of Europa. Four nights is just enough time to watch Europa as it circles Jupiter and presents all of its different faces to us. Four nights should give us a better spectral view of Europa than we have ever had before.  Four nights could – fingers crossed – answer the questions about the composition of the surface of Europa that remained elusive even after up close scrutiny from the Galileo spectrograph.

Europa, from the Keck telescope and from the Galileo spacecraft, as processed by Ted Stryk. The brighter parts of the Keck image shows where there is less water ice. The Galileo image is of a slightly different face than the Keck image; the non-water ice spot that shows up so clearly in the Keck image is the paleish reddish yellowish terrain in the bottom left quadrant of the Voyager image. The composition of that material has been a mystery since the time of Voyager.

How does Europa look from the Keck telescope when you turn on the adaptive optics system? Here’s a good example. A picture of Europa from the Keck telescope, showing where there is a lot of water ice (blacker) vs. where there is stuff that has water but is definitely not water ice (redder), compared to a Voyager picture of Europa. Ok, Ok, the Voyager picture is definitely better, but  guess what? We now have a really good spectrum at ever point in the Keck picture of Europa. Pictures are nice and you can learn a lot by studying them, but specra. Spectra! That’s where we really learn what is going on. So what is going on?

[continue on to the very long part 3]


  1. Great blog. NASA actually does have *one* thing headed to Jupiter right now although of course I don't think Juno has any Europa flybys planned and its only spectrograph doesn't cover the wavelength range you need.

    1. No, the Juno mission will not investigate Europa at all nor, any other Jovian moon at all (save for a few photos taken at a distance).

      The mission to avoid sending probes to ocean moons continues apace.

  2. Has there ever been a concerted effort to spot active (as in over a few years) terrain changes on Europa as the result of geologic activity that might melt through the ice.

    If it is possible to spot, I'd think fresh ice would be a starting point, since the eruption would involve water or slush.

    While this is probably obvious, I have never run across any NASA info detailing any such effort.

  3. Juno has no Europa flybys planned. In fact, she's going to stay far away from all the moons. She's a Jupiter mission, not a moon mission.

    Doug M.

  4. Hi, Mike, very interesting as usual. Recently I have been delving into hard ultraviolet, but I'm not trying to look through the atmosphere, I'm trying to look AT it. I'll be needing a deliberately smeared image, i.e. turning a wide-angle view into a narrow beam for better spectroscopy. So I'm eating up anything that has to do with spectroscopy.

    Thanks, Mike Emmert