De-mystifying Martian Clouds

 Two of the lab's PhD students have just published analysis on how Martian clouds interact with sunlight in companion papers over in the Planetary Science Journal! The results represent work on thousands of images of the sky taken by the Curiosity Rover over more than ten (Earth) years and describe the thickness of the clouds we saw and give information on the crystals that make up those clouds. How can you use pictures of clouds to figure out what they are like on the scale of less than a thousandth of a cm? Read on to learn more!

by Alex Innanen & Conor Hayes

Here at PVL, we’re a big fan of Martian clouds. For over eleven (Earth) years, we’ve tasked our favourite PVL-er, the Mars Science Laboratory (MSL) Curiosity rover, with staring up at the sky for a handful of minutes every few sols to capture the clouds drifting over its home in Gale Crater. Recently, we had two papers accepted that discuss some of these cloud observations. Our papers are very similar – both look at how light interacts with water-ice clouds during the same time of year (the Aphelion Cloud Belt, or ACB, season) using the same cameras (Curiosity’s Navigation Cameras, or Navcams). Between our papers we have 33 figures and over 15 000 words. Reading through that much material can be a daunting proposition! So, presented below for the cloud-curious is a brief and hopefully engaging summary.

Over the years, clouds have made frequent appearances on this blog, but as a quick refresher, yes, there are clouds on Mars! Some are made of dust, some are made of carbon dioxide, and some are made of water-ice. The water-ice ones are the ones we’re interested in, particularly those that form as part of the ACB. Every year, when Mars approaches its furthest point from the sun, it sees an increase in water-ice cloud formation around the equator. Gale Crater, where Curiosity lives, is just five degrees south of the equator, so it sees the southern edge of this belt of clouds. This is really great for those of us on the environmental science team – we get an opportunity every year to study these clouds and look for patterns in their behaviour from year-to-year. 

ACB clouds above Gale crater tend to be fairly tenuous – think of those wispy cirrus clouds you might see on Earth (shown above). Like cirrus clouds, they’re made of tiny crystals of water ice. (On Earth we don’t tend to specify what kind of ice, but on Mars the atmosphere can be cold enough for both water and carbon dioxide to freeze, so it’s helpful to differentiate between the two.) These crystals form when water vapour in the atmosphere condenses on some kind of nucleus – usually dust particles.

On Earth, atmospheric water vapour tends to freeze into a certain set of shapes depending on the specific conditions present when the ice crystals are forming.  These shapes have been catalogued thanks to the fact that we can actually fly into and directly sample our clouds taking close-up pictures of the ice crystals. While it may be possible that the ice crystals in Martian clouds have similar shapes to those in terrestrial clouds, we unfortunately cannot (yet) directly image them like we can here on Earth. Instead, we have to rely on some physics tricks. One of these tricks is looking at how light interacts with the clouds. These light interactions can produce a myriad of cool optical effects, but more importantly can give us information about the size and shape of these particles. This information is captured by what’s called a ‘phase function’. The phase function is a mathematical description of how much light is scattered by some particle at different angles from 0 to 180°, visually represented by a curve. The exact shape of that curve depends on the shape and size of the ice crystals in the clouds, so we can attempt to determine the nature of martian ice crystals by comparing our phase function measurements to those that have been made for ice crystals on Earth.

To see how light is scattered by the clouds across different angles, former PVL member Brittney Cooper came up with the phase function sky survey: Curiosity takes a series of 9 small cloud movies looking in different directions all around the rover, like you can see in the gif below.

From this we get information about how sunlight is scattered by the clouds at different locations around the rover and put together an average phase function for the clouds we observe throughout the cloudy season at Gale Crater. We’ve been doing this observation through four Mars years so far, which means that we can compare different Mars years to see if there’s any change in the phase function. The ACB is a very stable feature – it doesn’t change much from year to year. Likewise, the average phase function at Gale doesn’t change much either. Nor does it show much difference between morning and afternoon observations. So, what does that tell us? Mostly it’s just that – the phase function doesn’t change much from year to year, or from morning to afternoon. But this could also suggest that the water-ice crystals that make up the clouds aren’t changing.

Which brings us back to the question of what those crystals look like. This problem has been tackled a number of ways in the past, often by comparing an observed phase function with one for a known ice crystal shape. Brittney took this approach, as did Alex in their Master’s work. However, we found that none of the modeled ice crystal shapes fit our curves very well. This could be for a couple reasons – the particles Brittney looked at were much bigger than the water-ice particles that we tend to see on Mars. It’s also likely that the water-ice crystals are not forming the same shapes they do on earth.

In the phase function paper, instead of forging along directly comparing our curves to known water-ice crystal shapes, we took a slightly different approach. It turns out you can make a simple approximation of a phase function mathematically using what’s called a Henyey-Greenstein (or HG) function. There are two values that go into making an HG function – the creatively named ‘b’ and ‘c’.  Helpfully for our purposes, the b and c values also give information about the particle shape. If we look at the b and c values we see in the Gale Crater phase functions, they’re close to b and c values for rough, irregularly shaped particles – not those relatively simpler geometries we see in Earth clouds. It’s not as exciting as actually having a picture of what a martian water-ice crystal looks like, but it is still a solid starting point.

The phase function is important not just because it gives us information about the shapes and sizes of the ice crystals in the clouds, but also because it is a critical input into various models. These include Martian global climate models (GCMs), which must include the effects of clouds on the amount of light that is transmitted through the atmosphere. It is also important for the topic of our second paper: the opacity of the ACB.

A cloud’s opacity basically describes how thick it is. An opacity (or “tau”) of zero means that there are no clouds and all light passes through.  As tau increases, more and more light is blocked, either through absorption or reflection into new directions (also called elastic scattering). In theory, tau can be arbitrarily high, but at a certain point so much light is blocked that it can barely be measured. During the Mars Year 34 global dust storm in 2018, Curiosity measured a tau as high as 8.5, meaning that about 99.98% of the Sun’s light was blocked by atmospheric dust (hence why the solar-powered Opportunity rover did not survive the storm).

We use a fairly simple model to determine the opacity of water-ice clouds. The math is not particularly exciting, but in essence it takes the amount of sunlight reaching Mars at the top of its atmosphere and determines how much “stuff” there has to be between the top of the atmosphere and the ground to explain the amount of light that Curiosity measures. The phase function is important here because it tells us how much of that light is being indirectly scattered towards the rover by the clouds.

The opacity of ACB clouds has been the topic of a number of PVL papers before this one, most recently by former member Jake Kloos in 2018. That paper covered the first two Mars Years of measurements. When we began writing this paper, we had just passed five Mars Years at Gale, so we were very much due for an update! We had initially hoped that this would be a fairly straightforward paper to put together. Because our model had already been well-established in our previous papers, we thought it would just be a matter of running the new data through the old model. Unfortunately, once we did so, the results pretty obviously made no sense. As previously noted, the ACB doesn’t change much from one year to the next, so we’d expect that the opacities would stay pretty much the same from year-to-year. Instead, the opacities output by our model for the new data were all over the place! They were neither consistent with each other nor with the old data, so we had to go hunting for a reason why.

It didn’t take much digging to find the cause. When we plotted the opacities as a function of each measurement’s distance from the Sun on the sky, there was a sharp increase as we got closer to the Sun. There’s no physical reason why clouds near the Sun should be thicker than those elsewhere, so our model was clearly breaking down in this area. The culprit, as it turned out, was the phase function. All of our previous opacity papers had assumed that the phase function was flat, taking on a single value of 1/15 at all angles. The results from the phase function sky survey have shown that this is very much not the case near the Sun, where the value of the phase function rapidly increases.

By assuming that the cloud opacities shouldn’t change very much over the ACB season, we were able to derive another phase function for ACB clouds, one that is reasonably similar to the one found using the phase function sky survey (which is good since we're all looking at the same clouds using the same cameras on the same rover!). After adding this new phase function into our opacity model, we were finally able to take a proper look at how ACB opacities have changed over five Mars Years.

In short, much like the phase function itself, they don’t really change at all, which makes sense given the consistency of the ACB between years. Notably, these new results invalidated one of the findings of Jake’s 2018 paper: that ACB clouds in the morning tend to be thicker than those in the afternoon. Although thicker clouds do appear more frequently in the morning in our new data, it doesn’t seem that this is the case generally. In fact, we found that observations in the morning tend to be taken closer to the Sun than those in the afternoon, which was artificially increasing their opacity values when using a flat phase function. Why didn't Jake include this in his paper? Without access to as much data as we have now, he simply didn't know that martian clouds behaved this way! (no one did) Therefore, while it can feel a little awkward calling out a former labmate’s paper as incomplete, science ultimately moves forward through incremental methodological improvements.

Just for fun, we also compared our opacity measurements with those taken by two cameras orbiting Mars: the MARs Colour Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO), and the Emirates Exploration Imager (EXI) onboard the Emirates Mars Mission (EMM) Hope probe. Our methods did feel a little cyclical (assume the opacities don’t change much to derive a phase function, then use that phase function to conclude that the opacities don’t change much), so if we can match our ground-based measurements with those taken from orbit, we can have more confidence in our results.

Happily, the agreement between the MSL and MARCI/EXI measurements ended up being excellent, matching almost exactly with a few differences that can generally be explained by regional dust storm events that aren’t accounted for in the orbital data’s models. Thus, we can confidently say that our results reflect reality and probably aren’t a consequence of any assumptions that we made.

And don't forget to check out the papers themselves, available open-access at

Hayes et al. (2024)
Five Mars Years of Cloud Observations at Gale Crater: Opacities, Variability, and Ice Crystal Habits
&
Innanen et al. (2024)
Three Years of ACB Phase Function Observations from the Mars Science Laboratory: Interannual and Diurnal Variability and Constraints on Ice Crystal Habit

The Crunch

There comes a point when working on any large project when you can run into roadblocks or motivation can flag. This is almost guaranteed with something as long and as challenging as a PhD. Indeed, statistics suggest that in Canada about a quarter of science and engineering PhD students do not complete their degrees within 9 years (as of 2013). Sometimes, the greatest challenge can arise just before the end in "The Crunch" to finish, as Dr. Kevin Axelrod, our new Postdoctoral Fellow attests in this week's very personal post below. But if you find yourself in this situation, don't loose hope!  As the saying goes, it's often darkest just before the dawn.

(Photo above courtesy of Dr. Axelrod: "The view from the roof of the main building of the Desert Research Institute.  I spent a lot of time up here over five years, all four seasons.  It’s that nice.")

by Dr. Kevin Axelrod

So, it’s been a pretty crazy 12 months.  In January of 2023 (one calendar year before this blog is being posted), I was lying on the couch for two straight weeks in my shared house in Reno, Nevada, recovering from leg surgery, high on hydrocodone, and needing my housemates to get food from the kitchen for me (thanks, Heather and Brie).  Not appearing in the lab at the Desert Research Institute for two full weeks, I still had not completed the experimentation for my second publication of my Ph.D. research at the University of Nevada at Reno.  I still did not have a set date for when I would defend my dissertation and graduate from school, and quite frankly I did not yet know where my life was going.  And, believe it or not, I had never heard of York University.  

I had spent the last year and a half worrying about where my research was headed and how it was going to help me take the next step in life after graduation (if I even graduated).  At this point, I was supposed to be in “the crunch” - the last year of a Ph.D. tenure in which a student is supposed to devote their life, body, mind, spirit, overall being, consciousness, life-force, qi, etc. to their research and nothing else.  Instead, for two weeks, I watched Clarkson’s Farm on Amazon Prime (not sponsored, by the way) while eating chocolate pudding.  Not exactly the demeanor of someone who had spent the last 4.5 years of their life in graduate school and was now supposed to be in the crunch.  Of course, I could not walk and thus could not come into lab to work on my experiments, and I struggled to write anything because most of the time, I could not even sit up.  I felt stuck – I was seriously questioning whether I could graduate in August of 2023, which was a date delayed from a previous goal of May 2023, which was a date delayed from my original goal of December 2022 that I laid out in my prospectus defense.  

This was just 12 months ago.  And now, I am writing a blog for the Planetary Volatiles Laboratory, supervised by Dr. John Moores, at YorkU in Ontario.  Back in January, I would not have guessed that I would be here now. 

So, this blog is not about how cool my Ph.D. research is, a summary of an important meeting or event, or a case study of a planetary atmosphere.  This blog is about Ph.D. students in “the crunch”, who are anxious, unsure of their future, feeling consistently unprepared or inadequate, and always being very busy while still feeling like they get nothing done.    

Hopefully, that is not the case for most Ph.D. students who read this.  Hopefully, most Ph.D. students are constantly ecstatic about their research, enjoying all the once-in-a-lifetime experiences that they had dreamed about since childhood when they first watched Bill Nye the Science Guy or Mythbusters.  That was not me, however, and I know I am not the only one.  I had been working on this one singular project (bioaerosol chemistry, and more specifically pollen chemistry) for 4.5 years, and though it came with a lot of intrigue and enjoyment, I had also made many mistakes, suffered setbacks, and was disappointed with what I viewed to be a low level of progress. As a result, I was feeling very stressed and burned out – I just wanted to finally complete it and move onto new things.

After I got to the point where I could walk again, I returned to the lab with a new motivation - to get my life together.  And that involved two tasks: finishing my research on the volatility of bioaerosol constituents in the atmosphere, and also looking past my Ph.D. and finding a place where I could continue my passion for scientific research on a new project which would allow me to expand my knowledge further.  And I ended up finding such an opportunity with the PVL via a flyer that Dr. Moores posted on the American Geophysical Union website’s career listings.  

Upon my first interview with Dr. Moores, I knew right away that I wanted to join the lab – I was completely overwhelmed when he extended the offer to join.  I accepted.  It would be an exciting change of pace - a new project on the development of a functioning methane spectrometer for the Martian atmosphere (and so far, it has been a very exciting change of pace).  But, in March 2023 when I first interviewed, in the back (and front) of my head was a lingering doubt – would I actually be able to finish my Ph.D. research in time to move to Toronto and start research at YorkU in September 2023?

One thing was for certain – the pressure was on like never before.  Pressure not just to produce manuscripts, but to start a new chapter in life.  To self-improve, if you will.  In my opinion, that was the subject of my dissertation writing, even though self-improvement is never mentioned in it.  

And, for the most part, that pressure was good for me.  It made me more focused and motivated towards my bioaerosol research.  And as my leg improved, so did the state of my dissertation.  By the end of March, I completed the experimentation for my second publication and was busy writing the manuscript for it, while simultaneously taking care of in-lab work for my third research chapter in my dissertation.  By May, I had finished the writing of the publication and was wrapping up the in-lab research.  And by July 10, I was holding my dissertation defense.

Granted, the defense was far from perfect (almost nothing ever is in academia).  The night before was my most disturbed night of “sleep” ever. The morning of, I woke up at 4:30 AM and was instantly wide awake – something that had only happened one other time in my life, which was the morning of my prospectus defense two years earlier.  I held off on coffee that morning because it would have had no effect.  My jitteriness was already at a maximum due to the nervous energy surging through me. 
I was in a state of extreme anxiety.  But, I took solace in the fact that I had given the past year, “the crunch”, my best effort – motivated by my desire to make it to my postdoctoral fellowship.  And if my best effort was not enough, then oh well.    

The defense was an absolute fever dream – I don’t even remember most of it.  But it went well, and after two and a half hours I walked out of the presentation room with the blessings of my committee.  After living in Reno for five years, I was finally going to start a new chapter in life.  Provided, of course, that I take care of a few other things before I left, such as updating some of my writing and attempting to gather some results via a secondary analysis of some of my aerosol samples because one of my previous experiments failed.

But before any of that, I had another immediate task: attending my first in-person conference as a graduate student (no thanks to you, COVID), at the International Conference for Carbonaceous Particles in the Atmosphere (ICCPA) in Berkeley, California.  After my defense, my next task was to drive for four hours (on two hours of sleep) to California.  Though I was driving at night and did not arrive at the conference hotel until 2AM, it was one of the most euphoric drives of my life.  

The next day, I finally got to enjoy an in-person conference, as a reward for passing the defense.  It was a great time – I presented a poster on my research, sat in on an absurd number of exciting platform presentation sessions, met several new people and research groups, and certainly did not skimp on the catered wine.  By all estimates, it was one of the most enjoyable excursions of my time as a graduate student.

And one month later, I stuffed all my belongings into my sedan and left Reno, driving them back to my parents’ house before jumping onto a plane two weeks later.  

I will miss Reno.  I will miss the incredible natural landscapes around Lake Tahoe.  I will miss the excitement that I had back when I first moved there in 2018 as a grad student, realizing that I was about to take part in cutting-edge research for the first time.  And I will also miss a lot of the time I spent in lab over those 5 years.  I am forever grateful that I had a great advisor, a great program director, and great co-researchers and classmates, without all of whom I would not have graduated.  I will forever cherish the research topics that I was able to take part in while at the Desert Research Institute.  But there were certainly things that I will not miss: the many times that I made mistakes in my experimentation, the many re-do’s that needed to be done, the eternal frustration of trial and error, followed by finally obtaining a set of results that I thought were interesting enough to be published (and then writing about them for several months), only to have the manuscript murdered by some very truculent reviewers.  This cycle of frustration made it feel like I was stagnating – that I was not moving forward in research or in life.  It made bioaerosol research, a topic that I intrinsically enjoy, into something that stressed me out.  It’s the part of the scientific method that they do not show on Mythbusters.

So, to any current Ph.D. student who feels the same way right now, I would say: try to think about what you want to do after your graduation, even though it can be difficult to think about.  A visualization of your “next chapter” will get you over the hump.  Scientific research has both excitement and disappointment.  A Ph.D. may sometimes seem like it has more disappointment than excitement.  But after completion, you will feel just like the Mythbusters right after they blow something up: total ecstasy.  And that feeling will fuel my motivation for further research here at YorkU - hopefully I can keep it going for a while.