Hosting a sub-surface ocean, Jupiter’s moon Europa is one of the most compelling targets for planetary exploration in the Solar System. Probing the moon’s geology, sub-surface ocean, and the possibility for life are the foci of NASA’s upcoming Europa Clipper mission, set for launch in 2024.
One of the longest standing mysteries of Europa is the origin of its so-called hemispheric dichotomy. In short, as Europa revolves around Jupiter, it keeps one face always pointed toward the planet in a rotation state called “synchronous rotation”. (Earth’s own moon does the same thing.)
Ninety degrees to one side of the Jupiter-facing hemisphere is the leading hemisphere, the half of Europa that points in the direction of its orbital motion. The opposite side is the trailing hemisphere, which faces Europa’s past orbital location.
Perhaps counter-intuitively, Europa’s trailing hemisphere bears the fury of Jupiter’s magnetic field and radiation belts. Because Jupiter rotates so more quickly than Europa orbits (one rotation every 10 hours vs. 3.5 days for Europa’s orbit), charged solar wind particles trapped in the magnetic field are continually hurled at Europa’s trailing hemisphere. This constant sputtering chemically darkens the surface ice on the trailing hemisphere and likely explains the dichotomy.
This explanation makes one big assumption: that Europa has been tidally locked forever, or at least long enough that evidence for a different rotation state has been wiped away. Indeed, the rotation rate directly measured from Voyager and Galileo Missions images agrees completely is exactly what you’d expect for a tidally locked satellite.
But those constraints also allow for a tiny amount of non-synchronicity, perhaps allowing a reversal of the leading and trailing hemispheres every 6,000 years. That’s good because non-sychronous rotation seems to be required to explain one of the most dramatic geological formations on Europa.
Under (Tidal) Pressure
Geophysical scars mar Europa’s surface at almost all scales, from global to regional. The rifts and valleys that cross Europa, in many cases, arise from tidal stresses induced by Jupiter’s enormous gravity: as Europa circles Jupiter, the planet’s gravity stretches and compresses the satellite.
This tidal flexure can crack and rip Europa’s brittle icy shell, and the resulting rifts propagate across the surface in arcuate tracks that follow the maximum tidal stresses. In turn, the combination of Europa’s orbital motion and its rotation determine the path of maximum tidal stress across the surface.
Given that the tidal rifts form over the course of millennia, we can, in principle, use their shapes and orientations to infer Europa’s past rotation state. And analyses of the locations and orientations of cycloids implicate some non-synchronous rotation in Europa’s past.
In that case, then, we might expect the hemispheric dichotomy not to be so dichotomous — the darkening ought to extend a little ways across the boundary between the leading and trailing hemispheres. So does it?
Flummoxed by Fourier
The short answer seems to be “no”. A recent study by Burnett and Hayne of UC Boulder analyzed the contrast between the two hemispheres in ultra-violet and infrared wavelengths to enhance the contrast.
For their study, Burnett and Hayne used a technique called Fourier analysis. It turns that any oscillating pattern — electrical signals, photographs, ocean waves — can be broken down into lots of smaller oscillating signals, each with completely regular periods, as shown in the video below.
The benefit of breaking a complicated signal into lots of small regular ones is that you can then analyze each of them individually to learn about how the big picture signal arises.
In their study, Burnett and Hayne broke down Europa’s hemispheric dichotomy into two oscillating pieces: one piece that oscillated as if Europa’s rotation has always (or at least for a long time) been synchronous and another piece for any non-synchronous rotation. The figure below shows the result of their analysis.
They found that the Fourier term for non-synchronous rotation was much smaller than the term involving only synchronous rotation, meaning that any non-sychronous rotation must have been very small.
How small? Estimating the rate of non-synchronous rotation requires have some feature on Europa’s surface whose age is known. Thankfully, a cosmic collision provides one such feature.
Puzzled by Pwyll
Europa’s surface is geologically very young, probably less than 200 million years old. We can tell that because, unlike Earth’s moon, the surface of this moon is almost bereft of impact craters: the older a solid surface is geologically, the more asteroids and comets have left their marks on the surface.
If a planet or satellite experiences wide-spread volcanism or other geological activity, that can cover up any impact craters. Indeed, there is evidence that Europa’s sub-surface ocean periodically breaks through the icy crust, potentially erasing impact craters and renewing the surface.
However, Europa does host a few craters and few so spectacular as Pwyll, a 40-km icy splat mark smack in the middle of Europa’s trailing hemisphere. Pwyll’s bright impact rays contrast vividly against the splotchy trailing hemisphere. That means the sputtering process which darkens the rest of the hemisphere hasn’t completely obscured Pwyll yet.
In their study, Burnett and Hayne estimate that Pywll is about 1 million years old, a geological newborn, and therefore the darkening process must take at least 1 million years to darken a fresh surface on Europa.
Given how small the non-synchronous Fourier term is, this age estimate translates into a non-sychronous rotation period of almost 1 billion years. In other words, the two hemispheres can’t reverse more often than once every 500 million years — a very long time.
So how to reconcile this result with the apparent requirement for non-synchronous rotation from the cycloidal analysis? Burnett and Hayne can only shrug and say it’s a puzzle.
We’ll just have to send a probe to Europa to find out.
There were lots of great things about the movie “The Martian” (and a few inaccurate things), but one of the best things, for aeolian scientists like myself anyway, was the depiction of ubiquitous, enormous dust devils.
Mars loves to make dust devils. It’s relatively easy for sunlight to heat the atmosphere and get it churning, and thick dust deposits blanket enormous regions on Mars.
Dust devils on Mars help keep the atmosphere dusty, which warms the climate and helps drive weather. However, as surprising as it might be, we don’t totally understand how dust devils actually lift dust.
Sure it’s true that dust devils are windy, but when you actually plug the windspeeds measured in dust devils into the dust-lifting equations, the amount of dust they *should* lift can be much less than what they *do* lift. So some other mechanism besides just wind must help lift dust in devils.
One possibility is that dust devils act like vacuum cleaners and actually suck dust up off the martian surface. See, a lot of the dust sitting on the surface of Mars has been sitting there for a long time, not moving. As a result, the dusty surface can become vacuum-packed, trapping some gas in between the dust grains.
At the center of a dust devil is a small dip in the atmospheric pressure (created by the convecting air inside the dust devil). So when a dust devil skitters over the hermetically sealed dust surface, the trapped gas pressure can launch the dust into the air, where the devil can pick it up.
But this vacuum cleaner effect is still just a hypothesis, so to test this idea, the Experimental Astrophysics group at University of Duisburg-Essen, experts in astrophysical dust experiments, set up a test chamber to mimic the martian surface under a low-pressure (1% of Earth’s) martian atmosphere.
They created a thin layer of small dust grains on a membrane, with a pressure differential across the membrane, to see if a small pressure differential could really lift the dust grains up. The answer is yes!
Now whether this experiment accurately replicates conditions on Mars is not totally clear, but some of the measurements made by the soon-to-be-launched and recently named Mars 2020 rover Perseverance may help to test the idea.
In addition to collecting geological samples for later return to Earth, Perseverance will collect high-resolution imagery of dust and mineral grains on the surface of Mars. It will also continuously measure meteorological conditions, which we know from past missions can reveal the presence of dust devils. So in addition to telling us about the possibility of past life on Mars, Perseverance may also help us test whether there are dust devil vacuum cleaners on Mars.
However, scientific research articles are very unlike newspaper or magazine articles — they don’t usually employ a narrative structure, and they include confusing words and references. Consequently, it can be hard for people new to the field to read and understand them.
So in response to insightful requests from my students, here’s a short primer about how to read astronomical research articles. A lot of this information probably applies to all scientific articles, but there are also some aspects unique to astronomical articles. If you have suggestions to improve this post, don’t hesitate to contact me.
How to Access Scientific Articles
There are lots of services to find astronomical articles, but the vast majority of astronomers use NASA’s Astrophysics Data System service to find articles. That service provides links to articles hosted on official journals’ websites (which may require a subscription to access) and (if they are available) to free versions on the open-access pre-print server astro-ph (which is part of the arxiv.org service).
Most journals allow the article authors to send out free versions of the published articles to anyone who requests them. So if you can work up the nerve (and most astronomers are very nice people or at least eager to have others read their work), e-mail the authors directly to politely request a copy. (Here’s a good article about how to e-mail scientists.)
Journals are the official repositories for the final versions of scientific articles. If an article appears in such a journal, it’s (probably) been through a review process (see below) and meets some basic standards of quality.
That doesn’t mean the results of an article are correct, and it’s not unusual for results in one article to be contradicted by subsequent articles (even subsequent articles from the same scientists). But with a published article, you can have some confidence in the results and process.
Many journals nowadays are managed by private companies, which must turn a profit. Consequently, subscriptions to some journals are very expensive, which severely limits public access to scientific research that has been supported by public tax dollars. (See stories like this one.)
This is why the astro-ph archive is a big deal – you can usually get free access to an article. One caution, though: ANYONE can post articles to astro-ph, and astro-ph articles have NOT necessarily been reviewed for accuracy by anyone.
How Articles are Written
The process of publication is, in many ways, arcane, confusing, and backwards, and scientists in many (but maybe not all) fields are working to improve it.
In a nutshell, a professional astronomer will spend several months, sometimes years, on a research project – running computer code, collecting observations, conducting experiments, etc.
At some point in the course of the project, the scientists will decide they have a self-contained, compelling story (knowing when to cut off a project and publish is almost more art than science). Then they will (if they haven’t already started) draft a scientific manuscript.
That manuscript usually includes
Context and motivation for the project – What does recent, past work say about the problem? What questions remained unanswered?
Technical aspects of their approach – What physical approximations were used in the code, and where might they fail? How long was each astronomical observation?
Results from the project – What did the observations tell us about the planetary system?
Summary of the conclusions and plans or suggestions for future work – How do the new results relate to the previous work? What observations should we collect next?
Eventually, the authors are satisfied with (or at least resigned to) the draft manuscript, at which point they submit to a journal.
The journal sends the article out to the other scientists (called “referees”) with relevant expertise for a (hopefully but not always) objective assessment of the work. There is usually some back-and-forth between the referees and authors (who are often kept anonymous to one another), with suggestions for improvements. Eventually, a final manuscript is “accepted for publication” and printed and/or posted online.
Reading Research Articles
Scientific articles can seem a little bit like a Gordian knot – convoluted and indecipherable. But the best way to read a scientific paper is to chop it into pieces, not to read the article from beginning to end like a short story. I’ll use a recent article from my own group as an example.
The image above shows the article’s first page. Different journals have different formats, but most will have the same information on the first page:
Title of the article – (hopefully) tells you what the article is about
Author list and affiliations – Who wrote the article, how can you get in touch with them. Usually, the person listed first (the “first author”) was in charge of the project and is the person you should contact if you have questions.
Abstract – a short summary of article and main conclusions
The introduction – Background and context for the project
Throughout the article, you will see references to previous work – for this article, those references look like “(Knutson et al. 2007)”. That means an article written by a group (“et al.“) led by someone named Knutson in 2007. In “The Astrophysical Journal”, the complete reference information is given at the end of the paper.
When I read a paper, I usually read the abstract carefully to get a clear sense for the paper’s about and what the authors conclude. Then I read the introduction if it’s pretty short (about a page). Longer than that, and I usually skim the introduction.
Usually the next section you see will be the technical description, which will include lots of figures and equations. I skip this section on my first read-through. It’s easy to get lost in the details, and if you’re not familiar with the techniques, this section will be undecipherable.
Then comes the results section. I will usually read this section if it’s short, with “short” meaning again one or two pages.
Finally, comes the conclusion and discussion. Though this is the last section of the paper, it’s usually the second section I read (after the abstract and/or introduction).
The last page of the article will usually include an Acknowledgments sections in which the authors will thank anyone who contributed to the article but is not listed as an author (including the anonymous referees who reviewed the paper).
After that comes the references section, which provides the citation information for all the previous work referenced. Journals nowadays often use abbreviations that can be a little cryptic, so let’s look at the earlier example:
The figure above shows the reference information. We see the first three authors listed: Knutson, H.A, and then Charbonneau, D., and then Allen, L.E. The “et al.” means there were more contributing authors, but they are not listed to save space.
The “2007” means that was the year the article was published (but not necessarily the year the work was done), followed by “Natur”. In “The Astrophysical Journal”, “Natur” is short-hand for the journal “Nature“. Some journals don’t use such abbreviations, and others will have different abbreviations.
Finally, we see “447, 183”. These numbers usually refer to the volume and page number(s) in the journal where the article appears. Not all journals have volume or page numbers like this, so reference styles may vary.
All of this information is helpful if you want to find the referenced articles, which you can usually do through NASA ADS. In fact, usually all you need is the first author’s last name and the year the paper was published to find it, and ADS has a good guide about how to use the service to find articles.