I said a while back that nobody’s going to Mars any time soon. Which is true. But that doesn’t mean Mars isn’t interesting! Mars is very interesting.


So today’s paper is about Mars.  Okay, it’s about a moon of Mars. 

TLDR: one of Mars’ moons may periodically tear itself apart, turn into a system of rings around the planet, and then put itself back together.


You may recall that Mars has two small moons, Deimos and Phobos.  Emphasis on small; they’re about 12 km and 20 km across, respectively.  They’re so small that their weak gravity doesn’t pull them into spheres.  They’re both irregular lumps, vaguely potato-shaped.



Now we have to take a step back and talk a little bit about the physics of moons.

You’ve probably heard of geosynchronous orbits. There’s a particular distance from the Earth — it’s about 40,000 kilometers — where a satellite will take exactly 24 hours to complete one orbit.  Mars rotates much like Earth, so there are geosynchronous (1) orbits around Mars too.

So an interesting fact about moons: if a moon orbits above geosynchronous orbit, it will tend to very slowly spiral outwards, raising its orbit and moving further away from its planet.  (2) (“Very slowly” here means over billions of years.)  Our own Moon is doing this, drifting away a few centimeters per year.  Contrariwise, if a moon orbits /below/ geosynchronous orbit, it will tend to spiral /inward/, gradually getting closer to its planet.

Furthermore: the speed with which a moon’s orbit changes depends on the distance from the planet.  So if a moon is drifting outwards, that drift will gradually become slower as it gets further away.  It will never stop entirely, but it will slow down so much that the moon’s orbit will be stable over astronomical time — billions or tens of billions of years. 

But if a moon is drifting inwards?  Then as its orbit gets lower, the inward drift will accelerate, lowering the orbit even faster.  It’s a positive feedback loop.  Which is not going to end well for the moon.

“Hm,” you may ask yourself, “so if close-in moons tend to spiral inwards towards the planet, faster and faster… there probably aren’t a lot of close-in moons?”  And that’s exactly right!  There are (at the moment) 467 known moons in the Solar System.  Only six of them are below their planet’s geosynchronous orbit.

So what happens as a moon spirals inward?  Does it crash into the planet? 

As it turns out, no.  When a moon gets too close to its planet, tidal forces begin to tear the moon apart.  The point where this happens is called the “Roche Limit“, and it’s not a fixed distance — it depends on a bunch of things like the size of the planet, size of the moon, density of the moon, and what the moon is made of.  But wherever it is, if a moon hits the Roche limit, well…


[don’t stand]

[don’t stand so]

[don’t stand so]

[close to me]

The moon gets torn to shreds, and the shreds form rings.  This is (we think) how planets get rings around them.  Current thinking is that Saturn’s rings, for instance, probably originated with a now-extinct moon with the excellent name of Chrysalis.

[and Saturn throws in that crazy hexagon at its north pole, just to flex]

Okay, so back to Phobos.  Phobos is orbiting about 2.7 Martian radii from the center of Mars.  The Roche limit for a solid object is about 1.6 radii.  It’s expected that Phobos will hit that limit in about 40 million years, give or take.  It will then be pulled apart and destroyed.  And Mars will get a lovely set of rings!

Which, okay, but…  the Solar System is about 4.5 billion years old.  Phobos is scheduled for destruction in 40 million years.  That’s less than one percent of the lifetime of the Solar System.  Isn’t it a bit of a coincidence that we should be seeing Phobos right now, just as it’s starting its death spiral?  

(It’s true that we’re seeing a couple of other moons doing this at Jupiter and Neptune.  But those are giant planets that have ridiculous numbers of moons — Jupiter has over 100.  And their gravitational fields are so large and strong that they regularly capture new moons from wandering asteroids and such.  So a moon in a decaying orbit around Jupiter is not exactly a surprise.)

But okay, so Mars will have rings one day.   Here’s a thing about rings: they don’t last.  Over geological time, they tend to widen, spreading inwards and outwards. (3)

Eventually, the innermost ring particles hit the planet’s atmosphere and either burn up or crash.  Meanwhile the outermost ring particles drift outwards until the ring is attenuated into nothing.  This process can be delayed or complicated by the presence of other moons — Saturn famously has a bunch of “shepherd moons” constraining its rings — but the  point here is, rings don’t last forever.


[well, they don’t]

So a while back someone had a crazy idea:  what if, after Phobos breaks up into a ring, some of the ring particles disperse outwards and drift far enough from the Roche limit to re-coalesce?  Their mutual gravity would be very weak, sure.  But over millions of years, maybe they could gradually recombine into a new moon!  One outside the Roche limit! 

The new moon would be smaller, of course — at least half of Phobos’ mass would be lost.  But while Phobos is pretty small for a moon, it’s still about ten trillion tons.  Cut Phobos in half and you’ve still got a moon.

Alas, the math didn’t quite work.  Phobos’ Roche limit was too low.  Most of its mass would fall onto Mars.  Not enough ring material would climb high enough to form a new moon.

And there the matter rested for a bit, until this latest paper.  Which asks the question: well, what if Phobos isn’t a solid object?  What if it’s a rubble pile?

See, in the last little while we’ve been sending probes to asteroids.  And while asteroids all look pretty solid from a distance, when you get closer? Turns out a lot of them aren’t solid at all.  They’re just big floating piles of rocks sand and gravel, very loosely held together by weak gravity.

[everybody looks a bit rougher in close-up]

You remember the DART mission a little while back?  It’s when NASA blasted the hell out of a small asteroid, because it was cool.  I mean, sorry, because for planetary defense and also science.


[we tried negotiating with the so-called “moderate” asteroids]

Well, that impact didn’t just hit the small asteroid.  It literally blew half of it off into space.  Because that little asteroid was actually a rubble pile.  So the DART impact was a bit more… impactful, than expected.


[pretty much this, yeah]

Which brings us back to today’s paper!  Because if asteroids can be rubble piles, why not small, asteroid-sized moons as well?  

And it turns out that if Phobos is a rubble pile, everything changes.  Because then the Roche Limit will be higher — further out from Mars.  Because it’s much easier to tear apart a rubble pile than a solid object, yes?  And if that’s the case, then Phobos will die sooner than we think, and the ring system that it produces will start higher, and will spread out further away from Mars.

And if that’s the case, then… suddenly the math works.  Enough ring material will be high enough to re-combine into a smaller moon well outside the Roche limit.  But that moon will still be sub-geosynchronous, so it will start spiraling inwards again.  And so, over tens to hundreds of millions of years, the cycle will repeat. 

It won’t be able to repeat forever, because Phoenix Phobos will be smaller every time.  Eventually there won’t be enough ring material to produce a moon.  But it could potentially continue for several more cycles.



And extending it backwards into the past… yeah.  Maybe Phobos used to be a lot bigger!  But maybe it’s been through several cycles already.  Spiral inwards, hit the Roche Limit, break up into rings… rings spread out, inner part falls onto Mars, outer part recombines into a new, smaller version of Phobos… this could have been going on for a while now.  And you’ll notice that this solves the “why are we seeing Phobos just as it’s dying” question.  It’s not actually dying!  Sometimes Mars has two moons; sometimes it has one moon, and a pretty ring system. 

If the dinosaurs had owned telescopes, they could have seen rings around Mars.  Whatever intelligence inhabits Earth in 50 million AD (4) may see rings around Mars. Us?  We just happen to be catching Mars and Phobos at this particular point in their cycle. 

But wait!  As a bonus… remember Deimos?  The other, more distant moon?  Well, if the rubble pile model is correct, then some ring material might eventually be captured by Deimos.  So while Phobos would get smaller with every cycle, Deimos would get a little bit bigger.  And also, Deimos should be covered in a thick layer of Phobos material.

Okay!  Cool theory. 

Is it true?

Well, we don’t know.  But we might know pretty soon.  

JAXA, the Japanese space agency, is planning to send a probe to Phobos.  It’s scheduled to launch in the next Mars launch window, which is in November-December 2026.  That would bring it to Mars orbit by September 2027, give or take.  JAXA has only a fraction of NASA’s budget, but they have a pretty good track record of successfully sending probes to do cool science in space.  Their Phobos probe will orbit Phobos, scan it with a bunch of instruments, and drop a rover onto the moon’s surface.  Then it will swing in close and take a bite out of Phobos’ surface for a sample return to Earth.  And then for an encore, on its way out the door, it will do a close flyby of Deimos as well.


[unironically, fingers crossed for this]

So — if all goes well — we’re going to learn much, much more about the moons of Mars.  And we could have an answer to the “rubble pile or solid” question in the next couple of years.

And if the sample return succeeds… well, we’d have some stuff from another world, which is astonishing enough by itself.  But not just any stuff.  It would probably look like a handful of sand and gravel.  But it might be sand and gravel that has spent the last couple of billion years cycling between being part of a moon, then part of a ring system around Mars, and then part of a moon again.  

And that’s all.

(1) don’t be that guy
(2) because reasons
(3) because reasons 
(4) probably raccoons.

A drawing that riffs on the latest news and happenings.

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Hovertext:
Later it turns out the duck was getting with a porcupine and had a litter of Echidnas.

Today's News:

What makes a college course popular or unpopular? I’ve long been interested in courses for non-science majors that satisfy “general education” requirements, their aim being to foster overall scientific literacy and to convey an understanding of topics that are important to society. I often teach such courses at the University of Oregon, for example a biophysics-for-non-scientists course and one on renewable energy. Last term I again taught The Physics of Energy and the Environment, a course for non-science-majors that I’ve written about before (for example, this).

Here’s the enrollment in Physics of Energy and the Environment for the past 15 years. (See Methods for how I constructed the plot.) The datapoints with the circles are the terms in which I taught the course.

You’ll notice that there are enormous fluctuations, with the number ranging from about 40 to 140. Last term had among the lowest numbers of students. I wondered why.

Here’s enrollment data for The Physics of Light and Color, usually a popular course. Last term was particularly low, less than 50 when it’s usually over 150.

Are there “general education” Physics courses with more students, and in which enrollment last term was high? Yes: Essentials of Physics. Note the scale, 300 students last term:

These were the three general education Physics courses offered in Winter 2026. Even before the term started, I was paying attention to the enrollment, tensely checking to see if my course would cross the 20-student threshold to avoid cancellation. Here’s the graph, starting a week after enrollment opened:

300, by the way, is the maximum allowed for Essentials of Physics. The ceilings for Energy and the Environment and Light and Color were 76 and 218 respectively, indicated by dashed lines above.

What if we look at all Physics general education courses for the past 15 years?

There’s a spaghetti of lines, but it’s clear that something is unusual in recent terms.

What sets the Essentials of Physics course apart? Why is it so popular? The content is “Physics 101” for non-science-majors, i.e. not a particular theme of social or humanistic interest.

While you’re formulating a guess, I’ll note that I’ve often heard great things about the Physics of Light, Color, and Vision course.

Though I’m biased, I’ll note that students also seem fond of Physics of Energy and the Environment. I’ve had enthusiastic students tell me, sometimes even years later, that they like the course. Plus, it has a lot of real-world relevance, and we like to think our students care about this.

From this past term’s student evaluations:

“The relevance of this course content can’t be overstated. This course clearly connects to real world examples and helps explain world phenomenons.”

and

“He [i.e. me] also is very good at including active learning in his lectures by making students think first before directly stating answers.” (The relevance of this will be clear in a moment.)

I’ve posted all the student evaluations here, so you can verify that I’m not cherry-picking a few cheerful kids from an otherwise angry mob.

I have yet to hear praise of Essentials of Physics, though I haven’t specifically investigated. (We don’t have access to other courses’ evaluations.)

Modalities and the Ethics of Instruction

As you’ve likely guessed, what’s different about Essentials of Physics in Winter 2026 (and Winter 2025), is that it’s an online, asynchronous course. This means that there’s no in-person interaction; lectures are recorded. Most importantly, Most importantly, students submit all work online. In principle there could be proctored in person exams at a testing center, but this doesn’t exist for this course, or for most UO online courses. The other two courses, Light … and Energy and the Environment, like nearly all of our other Physics courses, are in person.

The University of Oregon is a residential university that makes a point of stressing in its public relations “live” interactions, student experiences, topical courses, etc. University of Oregon students, therefore, are presumably not enrolling from far away, nor enrolling with the aim of taking classes in their pajamas. The interactions enabled by actually having a room full of students, especially incorporating active learning methods that stimulate student engagement and allow a back-and-forth of questions and answers, are effective ways to enhance learning. Plus, they’re fun.

Apparently all this does not diminish the appeal (or temptation?) to students of online courses.

Obviously, one can’t think about online courses in 2026 without thinking about artificial intelligence. (This has been true since at least 2024, but in 2024 one could perhaps be unaware of AI without being professionally negligent.) Even in high-level undergraduate classes, there is nothing one can assign that can’t be answered perfectly by AI; in a general education course, perfect AI-delivered answers are trivial to obtain. We are all seeing as one of the consequences the evaporation of correlation between homework scores and (in person) exam scores, the former being generally perfect and the latter increasingly bimodal with a large fraction showing stunningly low levels of understanding.

The concern is not simply academic dishonesty, though addressing this is essential to avoiding the devaluation of higher education. Perhaps more sadly, we’re seeing students use AI as a crutch for their understanding. It’s easy to ask any modern LLM to answer and then explain a homework question, read that explanation, and think this is a substitute for thinking about the question and constructing the solution oneself. The student, then, bypasses the actual process of learning, and without meaningful assessments (like quizzes or exams), the students delude themselves about their skills.

Is the immediate filling of the 300-student Essentials of Physics really a consequence of it being online? As an additional datapoint, note the Physics Behind the Internet in the graph above. Having hovered between about 20 and 100 students, it surged to 150 two years ago, and 300 this term. What’s new about Physics Behind the Internet? Two years ago it became an online asynchronous course (ceiling 154 students in 2024, 300 now).

It is possible, I should add, to create a meaningful, rigorous asynchronous online course. As noted above, one can have human-proctored exams, though UO doesn’t have the capacity to do this for large courses. One can schedule online video chats for presentation and assessment (oral exams or quizzes); one of my colleagues in Biology does this — it is effective. This won’t scale to classes larger than 20 or maybe 30; certainly not 300.

It seems obvious that online courses are pedagogical disasters. There are, as mentioned, ways to structure them well. (Doing so requires more work than an in person course, I think!) And, of course, there are motivated and self-aware students who will learn very well from such courses, as they would from other courses. However, for a 300-person general education course with no independent assessment or validation, there’s no way to take such courses seriously, or to be proud to offer them. We may as well just tell students to send a check in return for an “A”, and spare everyone 10 weeks of pretending. There would be considerable student demand for this, just as there is currently considerable demand for the online asynchronous courses.

At a faculty meeting, I asked our department to stop permitting online assessments, which would effectively stop our teaching online asynchronous courses. There was some agreement and some concern with details, but not enough enthusiasm to move forward. I lacked the energy to push the issue vigorously enough, especially because there’s a structural problem with “unilaterally” taking such a step:

The resources of a department, such as my Physics department, are tied to the number of students it teaches. (This connection doesn’t need to exist, but it’s understandable; even more than most public universities, the University of Oregon is dependent on student tuition, so an administrative insistence that departments carry their weight is understandable.) My analysis above suggests that our online courses are siphoning students from our other general-education courses, so canceling these courses would send students to these other courses, like Energy and the Environment, which I would argue would be an educational improvement. However, it would likely also send students to online courses in other departments. Should we hurt our own income, which helps us accomplish our many worthwhile goals, to uphold a general principle about educational validity? I’d argue yes, but I can see that this isn’t an obvious choice.

What we need to solve this dilemma is a university-wide policy about online education that is honest and forthright about what learning looks like in 2026, that considers actual teaching goals and student experiences, and that has teeth. So far, we lack such a policy. UO is not unique; this is a common problem.

On the plus side, my many conversations about AI and teaching with faculty at many institutions, and with students, show a universal agreement that online, un-proctored assessment is meaningless and that universities need to think clearly about what they’re doing. (Students, by the way, are some of the strongest voices against AI-enabled cheating and its facilitation by clueless professors and administrators.) At some point, this will have to translate into changes in how we run universities. The institutions that do this quickly and well may survive more easily than those that don’t.

Methods

Data on course enrollment over time at the University of Oregon isn’t readily available, at least for those of us without any administrative superpowers. However, all our course schedules are available online, so it’s possible to get a web page for every course offered by a given department (like Physics) in a given term, and save it as an HTML file. Reading this by eye is easy. Writing code to read the HTML is hard — the table structure isn’t simple. This is a completely uninteresting programming task and is, therefore, ideal for current AI tools! (Without this, I would not have bothered with this analysis.) I therefore downloaded the HTML files, asked Claude (Sonnet 4.6) to convert all the HTML files to more comprehensible CSVs, and then asked it to write code to extract information from the CSVs. I then read the code, made a few changes, and ran it. this works well.

I don’t use AI to write prose, and I’m witnessing the disastrous results of students offloading learning to AI, but writing routine and boring code is an ideal task for modern artificial intelligence. There’s a lot to think about with all these developments.

Today’s illustration…

I painted a whale to use in a public talk I gave in January. My wife noted that I’ve had two whale paintings on the blog before, in 2013!

— Raghuveer Parthasarathy, April 12, 2025