Spacecraft exploration of our solar system has revealed a great diversity of planetary surfaces. Throughout the solar system, we have discovered process both familiar to those here on Earth but also very new process that are typically not thought of as important in terrestrial geomorphology. From the arid, ancient landscapes of Mars, to Titan’s active rivers and climate, to the massive nitrogen glaciers of Pluto, each of these worlds provide us a unique opportunity to study how climate governs the long-term development of landscapes.
So how do I fit in? Broadly, my research seeks to understand the full suite of processes responsible for shaping planetary surfaces and climates, leveraging this broad spectrum of environmental, mechanical, and chemical differences that we find across our solar system. And I don’t want to just stop there. The ultimate aim is to then take what we learn from these various planetary worlds, and use some of that knowledge to better understand what’s happening here on Earth, either today or in the distant past!
Left: False color image of Ligeia Mare on Titan, a vast hydrocarbon sea; Top Middle: Sublimation-driven landslides on Saturn’s moon Helene; Top Right: Comet 67P as seen by Rosetta; Bottom Middle: The nitrogen glaciers in Pluto’s Sputnik Planitia; Bottom Right: A possible ancient lake bed in Gale Crater on Mars, as seen by Curiosity.
Much of my work to date has attempted to unravel the landforms on both Saturn’s largest moon Titan and the Jupiter Family Comet 67P/Churyumov-Gerasimenko (67P). As I can’t reasonably restrict myself to just two worlds, I also have projects that are looking at the surfaces of Mars, Triton, Europa, Pluto, Earth, and yet more comets; all under the same umbrella of trying understand the full suite of processes responsible for shaping planetary surfaces and climates. I like to think of each of these planetary landscapes as massive, natural experiments, which are freely offering us the chance to observe both the familiar and the alien. So, while Titan and 67P are each amazing worlds, I also want to explore as many unique environments as time (and resources of course) allows!
To answer the many questions I have on all these various worlds, I employ many of the quantitative techniques and methods common to terrestrial geomorphologists. Historically in the planetary sciences, this knowledge has frequently been transferred from Earth to Mars, and my aim is to finally apply these techniques across the rest of the solar system. These methods include developing simple analytical models, the use of numerical simulations and laboratory studies that allow us to experiment with many processes (both familiar and exotic) over a range of spatial and temporal scales and, of course, analyses of new spacecraft data through my participation in the operation and development of yet more missions (see here).
Titan – The Only other World with Active Weather
Titan is unique in the solar system in that carbon, water, and energy – typically taken as the requirements for life – are interacting on the surface today. Also remarkable is that, when we look across Titan’s surface, we see landforms that bear a striking resemblance to those we find on Earth.
If one were to land near Titan’s polar regions, they would find hundreds of small lakes and vast hydrocarbon seas. These smaller lakes (which we have termed “sharp-edged depressions”) look in part to be formed by a type of collapse mechanism, yet they also retain massive rims around their perimeters that make them unlike anything we have observed on the Earth. The larger seas, meanwhile, are truly impressive, larger in area than any of Earth’s large lakes! These seas also look like they have flooded pre-existing high-standing topography, evidencing that Titan’s climate is actively changing.
At the equator, you would find an entirely different planet. Nearly wrapping around the entire globe, Titan’s organic linear dunes are 100’s of meters tall, and embay nearly everything. You would also find alluvial fans draining off of Titan’s water ice mountains, and almost all of Titan’s impact craters, sparse as they are. These low latitudes then appear like a vast desert, presenting quite the dichotomy with the humid, hydrologically-rich poles. Titan is therefore unlike any other known world, one that remains the place to go for studying both Earth-like surface processes.
Cassini Synthetic Aperture Radar (SAR) images of Titan; Top Left: Small lakes near Titan’s north pole; Top Right: Titan’s two deltas along the shoreline of Ontario Lacus near the south pole; Bottom Left: Longitudinal dunes migrating around topographic highs near Titan’s equator; Bottom Right: The vid flumina river network draining into Ligeia Mare.
Why care about Titan?
What’s most exciting to me about Titan is that, outside Earth, Titan has the only other active planetary hydrologic cycle. Whereas Mars once was active, little has happened for billions of years, and much of the record has been lost. Not to denigrate Mars, but the place to study planetary hydrology is clearly Titan, it is active right now!
What makes Titan even more interesting is that, while everything “looks” familiar, everything about the Titan environment is completely different from Earth. Instead of water, Titan’s rain, rivers, and seas are liquid methane and ethane with dissolved atmospheric nitrogen, the volume of which is highly sensitive to temperature, pressure, and methane-ethane composition. These properties lead to dynamic and significant (up to 50%) variations in a river’s density and viscosity that can vary both across a drainage basin and where methane rivers interface with methane-ethane seas. Also, Titan’s solids are not silicates or carbonates, the properties of which are very familiar to anyone who has ever picked up a rock. Though we don’t know for sure (yet), Titan’s “rocks” are likely some combination of water ice (which makes up its lithosphere) and organic solids precipitated from the atmosphere itself, possibly forming entirely new minerals.
And perhaps most exciting of all is that, while Titan is far from the Sun and lacking as much energy as Earth to drive climatic processes, it is the sluggishness of Titan’s thick atmosphere that may have some truly fascinating implications! Imagine a typical large storm on Earth: it occurs roughly once per year, and lasts for about a day. If you go out to the landscape after such a storm, you would easily observe the effects of floods, strong winds, etc. Now imagine that such storms occur every 7 years (or twice per Titan summer if you live on Titan), but the storms last for months! The effect that such massive storms have on the landscape will be way different than Earth, challenging our understanding of basic concepts like the “bankfull” flood.
As the cherry on top, Titan also lacks the complex interplay of diverse physical and chemical processes – including active tectonics, diverse climate zones, vegetation, and humans. This means Titan is a rather simple environment, one that can isolate for us the basic fundamentals of how climate influences the evolution of landscapes. So, while Mars is really cool, I think that we need to continue to explore Titan too (or instead!), as it provides a truly unmatched opportunity to study the fundamentals of all the processes so familiar to us on Earth!
Where are do we go next with Titan?
While Cassini ended its mission in spectacular fashion in 2017, its datasets will be dissected for decades to come and there is still much to be learned. Laboratory experiments are also a terrific avenue to persue, as we can approximate the Titan environment using analog experiments. Finally, numerical experiments can be a useful tool for exploring how Titan’s climate and landscapes are coupled, though they should be used with caution given the dearth of understanding of Titan’s materials.
One question that has caught my attention lately is why, unlike many coastal rivers on Earth, most of Titan’s rivers do not terminate in deltas. Titan has liquid filled seas, we know it rains, we see kilometer-scale rivers intersect with the shorelines, yet we see only one river that has a delta at its terminus. Why?
Left: Titan’s two confirmed deltas along the shoreline of Ontario Lacus; Right: The intersection of Vid Flumina and Ligeia Mare, showing what is more typical on Titan, where the rivers just deposit into…nothing!
Possible solutions to the question of Titan’s missing deltas fall into two main categories: (1) deltas like those we see on Earth do not form (or rarely form) on Titan because of differences in materials, dynamics, and coastal conditions between the two worlds, or (2) there are many deltas on Titan, but the characteristics of the deltas and of Cassini datasets make them difficult to identify – Titan’s deltas may simply “look” different. Deciphering between these two end-members has captured much of my attention these days, and is the focus of my postdoc at MIT.
We have begun deciphering between these two end-members through a series of different theoretical, experimental and numerical investigations. To study the dynamics of sediment transport within a Titan river, we have developed analytic expressions based on the concept of hydraulic geometry. This new work, currently under review, has produced some fascinating results that have implications for rivers not only on Titan, but on Mars and Earth too! We also developed a nifty numerical model that simulates the appearance of deltas as Cassini would see them on Titan, which we are now using in our ongoing search through Cassini data for deltaic and coastal features. Once these studies are all done, we can couple our numerical model with the output of upcoming laboratory plume experiments. These experiments should be a lot of fun, and will only begin the investigations into river plume behaviors as we change variables like: the density contrast between the river plume and the ambient fluid, gravitational acceleration, basin slope, and river discharge. Are there new dynamics that may be at play on Titan that we haven’t had to think about, such as fingering instabilities when less viscous (methane) river enters a more viscous (methane-ethane) sea, or nitrogen bubbling? So, through all these studies, hopefully in the next couple years we can finally understand what happened to Titan’s deltas!
Beyond these missing deltas, Titan’s similarity to Earth poses many additional questions. For example, during the Pre-Cambrian period here on Earth, most of Earth’s rivers were thought to be braided. Only with the rise of vegetation did single-thread channels arise, as mud was able to form through a process called “flocculation,” adding cohesion to the banks of rivers. Titan has no vegetation, so does it have single thread rivers? If so, what analogous process creates “mud” out Titan’s materials? If not, that means Titan’s rivers are reflective of the rivers that evolved Earth’s landscapes for the first 90% of its life! Deposits from these rivers on Earth are extremely rare, yet they may be sitting there on Titan today, waiting for us to study them!
Another big question is whether Titan’s atmosphere has been around for billions of years, or whether it is very young, having been outgassed only in the last few million years. We can use Titan’s landscapes to answer this question! If the atmosphere is then old, then there must be an additional thermostat, outside the carbonate-silicate cycle that regulates Earth’s climate, that can stabilize planetary climates!
As we learn more about Titan’s climate, we can also use its landscapes to understand what the impacts of these months-long, years apart storms are. The famous Woman & Miller study in the 1950’s gave rise to this concept of a “bankfull” discharge, where they postulated that there was a sweet spot between events large enough to shape landscapes, but frequent enough to modify. On Earth, this ended up being the 1-2 year flood. What is the relevant magnitude and frequency of storms on Titan, and what are the implications of this?
And of course, there are so many other questions we will be asking over the next couple years and decades. But Cassini has its limitations, and laboratory and numerical experiments can only be so useful. More data is always needed!
Fortunately, a new mission is also on the near horizon! Dragonfly, a mobile (aerial) lander, was selected by the New Frontiers 4 program in 2019, and is now under development at the Applied Physics Laboratory (APL). Nominally, it will land at the Selk impact crater near Titan’s equator in the mid-2030’s. Once safely on the surface, it will begin its hunt for prebiotic chemical processes that may be precursors to life, by searching for locations where impact melt (i.e. liquid water) was in contact with organics. Fortunately, that isn’t all Dragonfly can do! Making multiple flights, Dragonfly, will also measure the surface properties of Titan’s dunes and impact ejecta, and make measurements of any rivers and fluvial deposits it encounters along the way. With an in situ, well-equipped platform, our understanding of Titan surface science will take enormous strides forward!
Oh what could be! Left, a 25-meter resolution image that covers ~0.002% of Titan’s surface by the Huygens lander; Right: The exact same region as on the left, except as seen by Cassini at ~1-kilometer resolution. Imagine what an orbiter with 25-meter resolution images would see over the rest of Titan!
And hopefully it doesn’t end with just Dragonfly! A terrific opportunity exists, in the coming decades, to go back to Titan a second time. Simply, we are in a similar state to what Mars surface science was in the 1970’s after the Mariner missions, where surface features <1-kilometer in size were not resolved and topography was generally absent. Just as the Mars Global Surveyor changed our understanding of Mars, global topographic data and a factor of ~30 increase in image resolution offered by an orbiter would radically change our understanding of the entire Titan system. Yet, unlike Mars, I’ll remind you that Titan is active today! And so we could watch its dunes evolve, see how beaches around its seas migrate during these large storms, and look for plumes as they enter the seas. Only at the Earth can we watch such processes, and so I cannot wait for the day when we can do the same with Earth’s most similar planetary sibling!
Comets – Landscape Evolution sans Gravity
Why are comets so interesting to planetary scientists? Mostly it boils down to the fact that they are the oldest objects in our solar system, typifying the remnant materials from which all other larger planets and moons were constructed. They were, essentially, the bits left out of the planet formation process, instead getting scattered across the outer solar system. Once in those cold outer reaches of the solar system, far too distant for the Sun to modify their surfaces, these objects sit dormant for much of their lives. Every once-in-a-while though, one of these objects gets kicked into the inner solar system, where the pull of Jupiter’s gravity shapes their orbits (hence why we call them Jupiter Family Comets). Now far closer to the Sun, ices can sublimate on their surfaces, interacting with the “dust” (or non-ice) in very fascinating ways, giving rise to the brilliant comae we see from Earth! If we could grab a chunk of one of these close in comets, we would, in essence, be bringing back an ancient piece of our solar system, a window or sorts into the very earliest era of our solar system.
Why are comets interesting to me, specifically? Well, with no atmosphere, almost no gravity (~100,000x less than Earth), and a material yield stress similar to a powdery snow, at first it’s hard to imagine any physical mechanisms that can effectively erode a comet’s surface into recognizable landforms other than just a large, flat plain. And yet, when we look at comets, we see very dramatic topography, with landscapes that look, without context, rather familiar. I then like to think of comets as end-members in our solar system, where we have landscape evolution, except with gravity nearly “turned off.”
This is real Rosetta image of 67P, with terrains that look strikingly familiar. The relief here is a few hundred meters, and we are witnessing the effects of sublimation weathering that can produce a transportable sediment.
A Small Body Revolution
While the Rosetta mission initially targeted comet 46P/Wirtanen, a delay of the launch in 2003 meant that the Rosetta mission had to pick a new target. Comets are small bodies on eccentric orbits that make navigation and rendezvous difficult, and so the Rosetta project picked 67P instead. Fortuitously, this target change provided us with unprecedented observations of perhaps the ideal comet to study the early formation of the solar system.
With its rendezvous of 67P in the summer of 2014, Rosetta’s dataset finally provided access to the spatial scales where the processes relevant to comet surface evolution could be directly observed! These observations have allowed for detailed analyses of morphologies down to the meter scale across the entire nucleus and provided a long (2+ years) temporal data set with which to search for changes on the surface. The work of Rosetta (and of course all those involved to make it happen) have allowed me and my group to apply the same principles of detailed geomorphic analyses that we have for Titan, to a comet.
So what did we find at 67P? The dominant landscape-shaping process is obviously the sublimation of near surface ices. Yet the result is not simply a gradual lowering of surface topography. Instead, we see two distinct landscapes: “rough” and “smooth” terrains. The rough terrains can be thought of as the “bedrock” of the nucleus, and are consolidated enough to retain significant relief. Sublimation of ices then acts as a weathering mechanism, that produces unconsolidated, transportable sediment. While some of this weathered debris sits at the base of large cliffs, most of the sediment is transported ballistically across the nucleus, as the expanding sublimating gas can easily loft particles significant distances (since gravity is so small). This global transport process creates the smooth terrains, which are vast fallback deposits of centimeter-to-decimeter sized, ice-rich particles.
To understand how these landscapes related to eachother, we mapped the whole surface of 67P. What w found was a significant dichotomy: smooth terrains blanket large portions of the 67P’s northern hemisphere, but are very sparse in the south. We now think that this dichotomy is a result of seasonal differences in where energy from the Sun is deposited – a climate-like process on an airless, tiny world! The story is as follows. At perihelion, 67P’s southern hemisphere is fully illuminated (i.e., it is southern summer when the comet is closest to the Sun), while the north is in polar winter. This means that there is more weathering in the south, which liberates ice-rich sediment from the surface. These particles then follow ballistic trajectories to the colder, gravitational lows in the northern hemisphere. Throughout their journey, ice within these particles can continue to sublimate, though most of the ice is actually retained! So what you are left with is a sediment-bare southern hemisphere, and a northern hemisphere buried in ice-rich sediment, itself ready to sublimate. In terms familiar to a geomorphologist, the south is “detachment-limited” while the north is “transport-limited.” Finally, the Sun never rises again on the northern hemisphere until the comet is too far from the Sun, and so these smooth terrains sit dormant until 67P comes around for its next orbit…
A technical schematic of how we think 67P is evolving. On the left you can see two 3D renderings of our maps, with the smooth terrains in greens and yellow, clearly blanketing the north. The rough terrains, in various blues, cover the south, and only outcrop in the north. On the right, is a cartoon of 67P’s orbit relative to Earth, Mars and Jupiter, with notes of particular times of interest. Due both to 67P’s eccentricity and obliquity, we get seasons on the comet, with different regions active at different times with different magnitudes!
How did 67Ps landscapes evolve?
Surprisingly, few changes of the consolidated material were observed by Rosetta, indicating that this process acts on multi-orbit timescales or at a different time in a comet’s life (perhaps when they are briefly Centaurs, who knows!). Instead, many rapid changes were detected in the smooth terrains while 67P was approaching perihelion. These changes include both morphological changes like the formation of small depressions and “honeycombs,” and spectroscopic changes in the color of the nucleus. Together, they suggest that all those ices retained during the sediment’s northward journey, during the previous perihelion passage, finally were able to sublimate. One way to think of this is that 67P finally “puffed off” a layer of sediment that it received nearly 6 years prior. Yet to date, no satisfactory explanation has been offered to describe this seasonal mass loss, and so our understanding of the most basic processes capable of modifying cometary surfaces remains limited.
Time sequence of Hapi’s smooth plains: (a) On February 28, 2015 the smooth plains are uniform and featureless. Depressions (yellow arrows in all panels) began to appear in this region in March 2015 (b/c), and then coalesced into larger features over a 2+ month period (d/e). Images 12 months later (after perihelion; panel f) show a similar surface prior to depression formation (a), with featureless smooth terrains. Underlying boulders, however, have been excavated (cyan arrows) as a result of the changes in panels b-e.
Recently, our new paper in GRL provides a combined observation and theoretical study that we believe has solved how 67P’s smooth terrains evolve. In that paper, we show that material is lost from Hapi’s smooth terrains (in the comet’s neck region) not by a gradual diffusive process, but by localized bursts of sublimation within migrating depression scarps. For just a short time (~2 months), these scarps grow and migrate across the smooth terrains, removing a meter-thick layer of smooth terrains from the nucleus entirely! In fact, our combined observations and model tie back to numerous past observations made by the Rosetta mission, and provide a critical missing link as to how 67P’s smooth terrains seasonally erode and how comets ultimately lose mass.
What else can we do at 67P?
Hapi is just one small part of 67P, and it is reasonable to question whether what we found in our GRL paper is a universal process. To understand the relevance and context of Hapi’s changes, we require a database of all the changes, with maps of when and where they occur. Fortunately, we just received 3 years of NASA funding through the Rosetta Data Analysis Program to answer this very question!
To that end, our group currently has a few papers that are nearing completion. One focuses on the Imhotep region, the largest deposit of smooth terrains on 67P. This region also straddles the equator, meaning it is never in polar winter, and thus, can always be active. We are finding that this region is much more dynamic than Hapi, both receiving and removing smooth terrain material simultaneously. Fortunately, we have a nifty tool that can measure the erosion and deposition of materials, using large boulders as stream gauges of sorts. With our topographic tools, we can watch the Imhotep basin “fill up” and “drain” throughout 67P’s orbit, making precise maps of the volume of transported sediment. We are also seeing the same scarps we did in Hapi, yet they are bigger and faster in Imhotep. But, most confusing of all, is how localized the erosion and deposition is, with some parts of the basin accumulating and others losing mass.
Left: Some line profiles over a single region of Imhotep that we use to map the depth of erosion/deposition. The top image shows the pre-perihelion surface, and the bottom shows the same region after perihelion, with deposition clearly burying many boulders; Middle: Our actual topographic measurements, suggesting that ~1-meter of material was deposited in this specific region; Bottom Right: Imhotep mapped on the nucleus, for context; Top Right: a map of all such measurements across Imhotep so far. What is immediately obvious is how patchy everything is! The region in the middle and left panels is labeled `A’ in this panel
There is also the rest of 67P to look at! Unsurprisingly, we are finding yet more ways in which 67P’s smooth terrains are changing, again at different times than either Hapi or Imhotep. This all suggests that these smooth terrains are very sensitive to the local temperature conditions. The final product we are working towards is a complete sediment budget for 67P, something we hope to share very soon!
Of course, there is a lot more to think about with 67P. For one, we have sediment falling down onto the surface, which will naturally “shake up” the granular material. What are the effects of thousands of small impacts on the smooth terrains? Can the smooth terrains convect such that larger sediment is brought to the surface? Is there a stratigraphy that governs how material is shed from the smooth terrains?
On top of this, we have little idea as to how the ice is encapsulated into the sediment itself. We know that the sediment is some mix of ice and dust, but significant arguments remain regarding both the precise ratio, but also the structure of the ice. Are the particles well-mixed ice and dust (essentially a cometary conglomerate) reflecting how ice and dust were first incorporated into comets before the planets even formed? Or has the ice migrated and separated over time, with the ice now binding smaller pieces dust together as a sort of glue (a sort of cementing agent like a cometary sandstone)?
On top of understanding how the comet’s landscapes changed, my other aim is to make progress towards understanding how you actually lift a particle off the surface, something that remains an outstanding question. Depending on how the sediment sorts itself, how the ice is bounded within the sediment, and how temperature is distributed across the surface and near subsurface, you can get a range of outcomes. One possibility is that you can quench activity if ice-poor material is armoring the surface. Alternatively, the sediments themselves may act as “mini-comets,” launching off the nucleus when the correct temperature conditions are met! We are now attacking this problem both from the perspective of theory and chemical modeling, but also with the aid of numerical simulations. Hopefully we can soon shed some more light on how comets lose mass!
Finally, while Rosetta has provided a rich dataset that is allowing us to begin to understand cometary surface evolution, samples returned from a comet will again revolutionize our understanding of both comet surface evolution, and the solar system more generally. Because of this, a comet sample return mission is highlighted as a high priority in the recent Planetary Science Decadal Survey, and one will fly in the coming decades. Due to significant engineering challenges and the limited budget allocated to these missions, however, comet sample return missions will be restricted to sampling the top few centimeters of the current-day surface. For this very reason, my research group has focused intensely on understanding the the origin and the physical/chemical processes responsible for evolving 67P’s surface. With such knowledge in hand, we could properly decipher what any acquired sample could teach us about the formation and evolution of the solar system. One such terrific sample return concept is the CAESAR mission! Though it was not selected as part of the New Frontiers 4 call, despite satisfying all the Decadal Survey goals and much more, it is being re-proposed for the next New Frontiers opportunity. If ultimately selected, CAESAR would massively advance our understanding of the evolution of cometary nuclei, and its return of a sample would provide a ground truth for over 50+ years of research dedicated to understanding the formation of our solar system!
How can we use this new knowledge elsewhere?
Why stop at just 67P when there are so many other worlds where the sublimation of ices control the evolution of its landscapes? Not only that, but the largest of these also have planetary scale atmospheres, also sourced by these surficial ices. There are far too many science questions to list in a reasonable amount of space, but some such questions include:
- How does sublimation aid/quench cryovolcanic activity on moons like Europa and Enceladus, where localized, endogenic hotspots should drive activity?
- Are Triton’s landscapes and plumes driven by the sublimation of surface ices? If so, Triton’s plumes would be far-and-away the most energetic sublimative process!
- How do the swiss-cheese terrains on Mars form? Are Mercury’s hollows formed in a similar manner? What about the large pits in Pluto highlands? Are such pits the default landform?
- How does sublimation aid in the runout length and mobility of landslides on icy worlds like Helene or Ceres?
- What about other comets, Kuiper Belt Objects, and interstellar objects, are there other means through which activity can be quenched or enhanced?
- And most interestingly, did sublimation play a role during the snowball Earth? Curiously, Earth was more like Europa than anything we are familiar with today, yet complex life arose during this most inhospitable time. Could prolonged sublimation of the global, low latitude ice sheet surfaces have created habitable niches?
Top, from left to right: Hollows on Mercury, swiss-cheese terrain on the polar caps of Mars, and Triton’s Southern Hemisphere terrains and dark plume deposits. Bottom, from left to right: Comet 67P’s Hapi region, the edge of Pluto’s Sputnik Planitia, and Saturn’s moon Helene; Right: Cryoconite holes on top of a glacier in Antarctica, image taken from Hoffman et al. (2017). In all these images, the wonderful ways in which the sublimation of volatiles shape a landscape are obvious, despite their dispersion across the entire solar system!
These questions above are just the tip of the proverbial iceberg, ones that I would love to collaborate on with anyone who may have read this far (and remains interested!). But to finish, I think that the era of studying sublimation-dominated world has just be begun, and I’m very excited to see what more we will discover. With so many mission concepts either accepted and in development for flight, or in competition now that would go to such worlds, we are in for an exciting next few decades of exploration!