BSU Journal Club

This artist’s rendering shows NASA’s Europa mission spacecraft. From https://en.wikipedia.org/wiki/Europa_Clipper.

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.

While we wait for that mission, scientists keep chipping away at the Europan enigma using data from telescopic observations and from previous missions, particularly the Galileo mission. For example, Hubble observations suggest Europa has active geysers.

The Two Faces of Europa

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.

A map of Europa’s surface, with the leading hemisphere at the map’s center and the trailing hemisphere straddling the left and right edges.

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

Cycloids criss-crossing Europa’s surface.

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.

Propagation of tidal stress (arrows pointing outward from cycloidal track) across Europa’s 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

Europa’s trailing (darkened region at left) and leading hemispheres as observed in ultra-violet and infrared. From Burnett & Hayne (2020).

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.

Burnett and Hayne’s model fit (smooth, orange curve) to Europa’s hemispheric asymmetry (jagged, blue curve).

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 impact crater 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.

This animation demonstrates how deformation in the icy surface of Europa could transport subsurface ocean water to the moon’s surface. From https://www.nasa.gov/feature/jpl/europa-s-ocean-ascending.

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.

Mark Watney survery the desolate martian landscape.

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.

A map of dust deposits on Mars. From https://sci.esa.int/web/mars-express/-/51861-mars-dust-map.

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.

A dust devil on Mars. From https://en.wikipedia.org/wiki/Dust_devil.

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.

The vacuum cleaner effect illustrated. The sub-surface pressure p2 is greater than the pressure at the center of the dust devil p1. From Bila et al. (2020).

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.

The experimental set-up. The dust dispenser poured dust particles onto a fine mesh membrane, with ambient pressure p1 on one side, and higher pressure p2 on the other. From Bila et al. (2020).

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!

Dust grains of different sizes lofted by a pressure differential.

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.

Naming the Mars 2020 rover. From https://www.youtube.com/watch?v=Muj4s7BCiMI.

Professional astronomers spend a lot of time on reading other astronomers’ research to learn what’s going on in the field and to incorporate new, best results into their work.

Traditionally, that research is officially presented to the astronomical community in the form of a peer-reviewed article printed (or at least hosted) in a professional astronomical journal, such as “The Astrophysical Journal“, “The Astronomical Journal“, or the venerable “Monthly Notices of the Royal Astronomical Society“.

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.

First page of a recent research article

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.

Model/Data Analysis Section

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.

Results Section

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).

Last page of article

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:

Example reference

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.

https://arxiv.org/abs/1910.09523

https://en.wikipedia.org/wiki/Kelvin_wave

The birth of El Niño. This animation shows anomalies, or departures from normal, in Sea Surface Temperature (SST) over the past year. As spring became summer in 1997, a Kelvin Wave of warm water crossed the Pacific and accumulated off the coast of South America, shown here in red. From https://www.pbs.org/wgbh/nova/elnino/anatomy/sst.html.
Map of atmospheric temperature and changes in temperature from Komacek & Showman (2019).
Brightness map and changes in brightness from Komacek & Showman (2019).
Variations in the brightness of the hot Jupiter Kepler-76b. From Jackson et al. (2019).
Martian dunes west of the giant Hellas impact basin and observed by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter. From https://www.nasa.gov/multimedia/imagegallery/image_feature_1569.html.

Mars is a dry, dusty place, with globe-girdling sand seas, mile-wide dust devils, and frequent world-wide dust storms.

In spite of the ubiquity of dust on Mars, though, the physics of dust-lifting and transport remain mysterious. For instance, the seasonal appearance of dark streaks on slopes across the surface of Mars, called recurring slope lineae, were thought to result from flow of brine. Recently, though, we’ve found they are more likely granular flow, but what exactly drives their seasonality is unknown.

Warm-season features originally thought to be evidence of salty liquid water active on Mars today. More recent studies show they are very probably granular flow. From https://en.wikipedia.org/wiki/Seasonal_flows_on_warm_Martian_slopes.

Experiments conducted by the aeolian physics group at University of Duisburg-Essen has brought a little clarity to the mystery of martian dust.

One of the biggest challenges for experiments exploring martian dust transport is replicating Mars’ low gravity (40% of Earth’s) and air pressure (10% of Earth’s). Since gravity and pressure help determine how winds move dust, accurate experiments must somehow create winds in a low-gravity environment under near vacuum.

To make a little pocket of Mars on Earth, Maximilian Kruss and colleagues took a small vacuum chamber centrifuge onto a “vomit comet” and conducted parabolic flights to create short periods of microgravity.

Kruss and colleagues’ low-pressure, microgravity chamber. From Kruss et al. (2019).

They filled the chamber with martian-like dust grains and turned up the fan to figure out when the winds were strong enough to start blowing the dust. By imaging the grain bed and tracking the grains, they estimated this threshold wind velocity, which is key to understanding when and where Mars can blow dust around.

Video of grains blowing across a chamber bed. From https://www.uni-due.de/physik/agw/research.php.

Kruss and colleagues found, reassuringly, that theoretical models about dust transport were accurate. These results help us understand aeolian processes on a wide range of bodies, not only on Mars but any body with a low-pressure atmosphere.

Dunes (left) and wind streaks (right) on the surface of the comet 67P as seen by the Rosetta Mission. From https://sci.esa.int/web/rosetta/-/55308-ripples-and-wind-tails.

Indeed, even comets play host to aeolian processes. When the Rosetta Mission flew past comet 67P, it saw features on the comet’s surface that looked for all the world like wind streaks and dune fields.

Kruss and colleagues suggest dust transport may be important on some exoplanets, where gravities and atmospheric pressures span an even wider range than in our solar system. And so, these results, taken from a tiny vacuum chamber, may bear on processes on worlds across the whole galaxy.

Hot Jupiters are gas giant planets like Jupiter but orbiting so close to their host stars that they can be as hot as small stars. In fact, some suffer such strong irradiation that their atmospheres are being blasted off into space. These objects were among the first exoplanets detected, and their origins and fates remain unclear.

Being so hot, their atmospheres are also unlike any planet’s in our solar system, but we can be certain that, with temperatures hot enough to vaporize rock, the weather on these planets is highly dynamic. The video below shows what happens to one such planet as it gets blast-roasted by its host star – a giant thermal wave screams around the planet.

An animation showing meteorological chaos on HD80606b.

Weather forecasts on hot Jupiters would to have keep track of super-sonic winds, ruby rain, and warm fronts heated by magnetic fields. And a recent study using observations from the Hubble Space Telescope shows that these dynamic atmospheres remain mysterious.

Led by Jacob Arcangeli of the University of Amsterdam, the study used the Wide-Field Camera onboard Hubble to watch WASP-18 as the hot Jupiter circled its star.

As the planet swings revolves around its star, we see first the cool nightside of the planet and then the blistering dayside. By measuring the diurnal cycle of infrared light emitted from the two sides of the planet, called the phase curve, Arcangeli and colleagues were able to measure their respective temperatures and learn something about the planet’s weather.

The phase curve of WASP-18b measured by Hubble (shown in blue), along with model fits.

The figure above shows WASP-18b’s phase curve as blue points and a model fit to the data as a black line. By applying computer simulations for the planet’s weather, Arcangeli and colleagues estimated what they expected the phase curve to look like various assumptions about the planet’s composition and atmospheric dynamics.

A map of atmospheric temperatures (in degrees Kelvin) in WASP-18b’s atmosphere.

Using their models, they were able to draw a rough map of temperatures in WASP-18b’s atmosphere (shown above), like a weather map for the Earth. Like most hot Jupiters observed show far, the hottest place in WASP-18b’s atmosphere lies a little east of the point that receives the most sunlight, the substellar point. That’s because high windspeeds blow the strongly heated gas away toward the east, a little like the jet stream on Earth.

However, the winds inferred from the Hubble observations weren’t as strong as expected from the model, suggesting there is some form of drag or friction in WASP-18b’s atmosphere.

The likely culprit: the planet’s very own magnetic field. Gas in WASP-18b’s atmosphere is actually so hot, it can become somewhat plasma-fied, and plasma, consisting of hot charged particles, can interact with the hot Jupiter’s magnetic field.

This is totally unlike planets in our own solar system, where the atmospheres are comparatively cool and none of the gas has turned into plasma.

The upshot of this result is that we may be able to use observations of the meteorology on distant worlds to learn something about the planets’ magnetic fields, which originate deep inside the planet. So their weather may allow us to plumb the depths of these distant worlds.

Artist’s conception of Neptune’s newest moon Hippocamp.

In our research group meeting this week, we discussed the recent discovery of a new moon orbiting Neptune, named after Poseidon’s chimerical winged pega-fish Hippocamp.

Hippocamp is about 12 km across, so small and dim that it wasn’t seen when Voyager 2 flew past in 1989, back when the B-52s were heading down the Atlanta Highway. In fact, Showalter and colleagues had to use high-precision Hubble observations and a new data-processing approach to spot the little moon circling Neptune just interior to another moon Proteus.

This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune. From https://en.wikipedia.org/wiki/Hippocamp_(moon)#/media/File:S-2004_N1_Hubble_montage.jpg

Hippocamp orbits so close to Proteus that Showalter and colleagues suggest it may have originated from this larger moon in a massive collision. That same collision may have created Proteus’ enormous impact basin Pharos, and Showalter suggests that collision would have liberated debris, some of which later accreted interior to Proteus’ orbit to form Hippocamp.

If Hippocamp really did form from such an impact, it has probably experienced numerous disruptive collisions itself over its billion year history. Based on studies of the frequency of large cometary collisions out near Neptune’s orbit, Showalter and colleagues estimate that Hippocamp may have been disrupted and re-accreted about 9 times in the last 4 billion years.

Having risen from its own ashes so many times, Hippocamp may be less like a mythical sea-horse and more like a cynthian phoenix.

Artist’s conception of a hot Jupiter shedding mass.

The very first exoplanet discovered around a Sun-like star, 51 Peg b, was a shocker – it’s a giant planet like Jupiter made mostly of hydrogen and helium but 100 times closer to its sun than Jupiter is to ours and whizzes around its orbit every 4 days.

Indeed, when its discoverers Michel Mayor and Didier Queloz first spotted the telltale spectral wobble of a planet in a 4-day orbit, they didn’t believe their discovery. At the time, everyone knew (or thought they knew) that planets like Jupiter could only form very far away from their host star.

Worse, so close to its star, 51 Peg b’s was being super-heated, and Mayor and Queloz worried that such a hot gas giant might quickly lose its hot, bloated atmosphere. And in their discovery paper, they suggested that the giant planet we see today as 51 Peg b might have started out as a brown dwarfthat shed trillions and trillions of lbs.

Later studies showed those early concerns about atmospheric blow-off were overblown and planets as massive as 51 Peg b, even if they are as scorched, probably can’t lose more than a fraction of their original mass. Since then, hot Jupiters like 51 Peg b, while cosmically rare, have become a fairly common type of exoplanet discovery.

But that doesn’t mean these planets aren’t losing a lot of mass, and a recent study from David Sing and colleagues looks at one of the mass-losing-est planets we know of, WASP-107b

Artist’s conception of WASP-107b transiting its host star.
From https://en.wikipedia.org/wiki/WASP-107b.

Sing and colleagues collected transit observations in infrared wavelengths of the WASP-107 system using the venerable Hubble Space Telescope. By looking in the infrared, they could search for the spectral signals of different gases in WASP-107b’s atmosphere.

WASP-107b is an especially good target for atmospheric characterization because its host star is very bright (compared to other planet hosts) and the planet itself is very low density – it has a mass a tenth that of Jupiter’s but a radius almost as big, giving the planet a density comparable to wind-packed snow.

With such a low density, WASP-107b’s atmosphere is puffy and distended, which means that its atmospheric gases can easily imprint their spectral signatures on the light observed by Hubble, making them easy to detect.

And for the first time in any exoplanet, Sing and colleagues saw signs of helium gas in WASP-107b’s atmospheric spectrum. In fact, the helium signal they saw was so whopping big that it suggests WASP-107b’s atmosphere is actively escaping, at a rate of about 10,000 tons per second.

Escape of WASP-107b’s atmosphere. The planet is the small grey circle near bottom, the star is the yellow circle, and the escaping atmosphere is shown in blue. The black line is the planet’s orbit. From Sing et al. (2018).

Even with such a high escape rate, WASP-107b won’t fall apart anytime soon – Sing and colleagues estimate it would only lose about 4% of its mass in a billion years.

But as we continue to find more exoplanets, we should probably expect to find more even closer to their host stars with even puffier atmospheres, perhaps some on the verge of being gravitationally ripped apart. So as with 51 Peg b’s discovery, exoplanets are likely to keep challenging our preconceived notions about where planets can and cannot be.

On Cowboy Bebop’s Titan, troops advance across a fictitious dune field that turned out not to be so fictitious.

Arguably the best animé of all time, “Cowboy Bebop” is set in a not-too-distant future, when humans inhabit planets and moons across the solar system. In fact, one of the moon inhabited, Saturn’s moon Titan, features as the site of a violent, Desert Storm-like battle among sand dunes and scorpions.

“Cowboy Bebop” aired in Japan in 1998-1999, about a year after NASA launched the Cassini-Huygens mission to explore the Saturn system, including Titan. Shortly after arriving at Saturn, Cassini began collecting infrared observations of Titan, allowing scientists to peer through Titan’s hazy atmosphere. They found a surprisingly Earth-like world — violent but irregular storms (albeit of methane and ethane) and vast seas (primarily confined to the poles).

Scientists also found expansive dune fields girding the equator. Unlike terrestrial dunes, which are made mostly from silicate grains, these dunes were made (somehow) from carbon- and ice-rich particles. And an even more recent analysis of Cassini observations shows that Titan has something else in common with Earth: large dust storms.

In their study from late last year, Sébastien Rodriguez, an astronomer at the University Paris Diderot, and co-authors looked at maps of Titan collected by Cassini’s VIMS instrument and found that, over the dune regions, a large bright feature appeared and disappeared several times over the course of a few weeks.

Figure 1 from Rodriguez et al. (2018) showing the dust storm, indicated by the white arrow.

Now, just because there is a bright spot on Titan that changes with time does NOT mean it has to be a dust storm, but Rodriguez and colleagues go to great lengths to show that other explanations don’t fit.

For instance, previous observations of Titan found equatorial clouds that produced downpours of methane and ethane on the surface. Superficially, these clouds resembled Rodriguez’s putative dust storms.

Methane Rain Possible on Titan. From APOD.

But Rodriguez’s dust clouds can only be seen in the few wavelengths of infrared light that are known to penetrate Titan’s atmosphere all the way to the ground. Storm clouds can be seen even in wavelengths that don’t reach the ground since they ascend high into the atmosphere.

Rodriguez and colleagues are even able to estimate the size of the dust grains — the dust clouds are much easier to see at 5 micron-wavelengths than at the shorter wavelengths that also probe to Titan’s surface. That probably means the grains are about 5 microns.

If Titan’s dust really is that small, it’s much smaller than sand grains and even smaller than the dust we usually see on Earth. It turns out that the aerodynamic behavior of a wind-blown particle depends, among other things, on its size. For a planet with a given atmospheric density, winds are good at blowing particles of a specific size — too small and the particles stick together; too big and the wind can’t lift the particles.

Rodriguez and colleagues estimate that windspeeds of about 3+ meters per second (about 7 miles per hour) would required to loft 5-micron dust grains on Titan. That may seem small, but the winds measured by the Huygens probe during its descent onto Titan measured even weaker near-surface winds of less than 2 meters per second.

However, the study’s authors point that stronger winds probably accompany Titanian rain storms. If such a storm had just taken place before they spotted the dust cloud, that could easily explain how the dust was lofted.

Ultimately, answering the question of Titan’s dust storms will require visiting the world again. Fortunately, NASA is investigating sending an automated drone to fly the Titanian skies, the Dragonfly mission, back to Titan in 2025. Whether that mission flies or not will be decided later this year.

Artist’s conception of ‘Oumuamua.

The recent visit to our solar system by the interstellar object ‘Oumuamua has raised considerable controversy in the scientific community. Based on the fact that it is moving too fast to be trapped in an orbit around the Sun, ‘Oumuamua is the first object confidently identified as originating from outside our solar system. Its origin is unclear, and one likely possibility is that it is debris ejected from another planetary system. But Avi Loeb, chair of astronomy at Harvard, has proposed a more exciting but understandably controversial idea: ‘Oumuamua may be a probe from an alien civilization.

The idea’s not as crazy as it sounds — since interstellar distances take so long to cross (with current technologies, it’s at least 20 years to *our* nearest stellar neighbor), we think aliens are likely to explore using automated spacecraft rather than sending themselves.

And ‘Oumuamua did behave strangely during its short visit to our solar system. As it rounded the Sun, astronomers observed an anomalous acceleration inconsistent with the pull of the Sun’s gravity. For comets, such accelerations are common and attributed to jetting from vaporizing ice. But astronomers saw no evidence for such jetting from ‘Oumuamua. In addition, ‘Oumuamua has a funny shape, perhaps resembling a cigar, unusual but not totally impossible for a comet-like body.

Loeb has explained these anomalies by proposing that ‘Oumuamua is an alien solar sail, harnessing the radiation pressure from the Sun to navigate the cosmos. Such solar sails may be a low-cost, efficient means of plying the interstellar waters and have featured in recent technology demonstrations from the Planetary Society. If ‘Oumuamua were, indeed, a solar sail, that might explain both the anomalous acceleration and the unusual shape.

Fig. 1 from Bialy & Loeb (2019). The maximum distance a solar sail can travel L_max depends on the sail’s mass-to-area ratio, m/A, but Bialy and Loeb’s calculation suggest ‘Oumuamua could safely traverse the entire Milky Way.

One potential problem for is that, to work, a solar sail must be very light-weight and thin – the Planetary Society’s LightSail 2 spacecraft is a square almost six meters to a side, but weighing less than a bowling ball. It’s easy to imagine that such a cosmic tissue might not survive the rigors of interstellar travel. And so in a recent paper, Shmuel Bialy and Avi Loeb conduct a series of back-of-the-envelope calculations to argue that the interstellar rigors might not be so rigorous.

 

One of the biggest hazards for a solar sail would collisions with interstellar dust and gas – each collision could sap ‘Oumuamua’s momentum and vaporize its surface. However, Bialy and Loeb estimate that a solar-sail ‘Oumuamua could plausibly traverse tens or hundreds of kiloparsecs before such collisions would be a problem. That means ‘Oumuamua could cross the entire Milky Way before suffering a mission-ending number of collisions.

As compelling as their calculations are, my instinct (and that of most astronomers) is that ‘Oumuamua is something more mundane than an alien craft. But the conversation catalyzed by Loeb’s suggestions is probably healthy for the field of SETI, and certainly the public enthusiasm is encouraging – people love space and want desperately to find extraterrestrials.

The challenge is to keep these debates firmly scientific, to strike a balance between pushing the envelope and tearing the envelope to shreds. And the line between science and pseudo-science in the realm of alien life can be as tissue-thin as a solar sail.