The surfaces of the outer solar system offer the opportunity to study erosional processes and landscape evolution under drastically different physical, chemical, atmospheric, and even gravitational environments than on the Earth. The mountains, valleys, and hillslopes and all other landforms arrayed across these surfaces have a wide spread of topographic forms and characteristics. As we continue to explore this portion of the solar system, the variety of these landforms, and the environments we find them in, appear to be ever-increasing. These complex topographic configurations ultimately reflect the interrelationships between three dominant factors: the structure/composition of the materials, the processes acting upon them, and the time over which those processes have acted.

It is the study of the interrelationships between time, form, and process that remains the fundamental concept in the field of geomorphology. This basic approach ultimately means that a landscape’s topographic and morphologic forms record information as to the physical processes that produced it. As in all natural environments, there are often variations in the local lithology, topography, climate, and even gravity across a multitude of spatial and temporal scales that can complicate this seemingly simple “form-to-process” ideal. This can be overcome, however, by carefully describing a given landscape, constantly keeping in mind the interrelationships and basic physical processes. By breaking a landscape down into its constituent parts, a seemingly simple organization of landforms is revealed. That nature often reveals these simple patterns reflects the fact that often just a few dominant physical processes are imparting enough of a physical stress upon the surface to modify it. When unraveled, these processes can then be quantified with known geomorphic transport laws (see here for a nice discussion on this concept).

This is the common theme of my research, where much of my work has attempted to unravel the landforms on both Saturn’s largest moon Titan and the Jupiter Family Comet 67P/Churyumov-Gerasimenko (67P) using a combination of both spacecraft observations, theory, numerical simulations, and laboratory experiments. The surfaces of Titan and 67P are being actively eroded to form new morphologies today, and at rates quick enough to make their surfaces among the youngest we observe in the solar system. Spacecraft observations permit us a first-order understanding of these processes. My participation in operating and developing missions that provide such observations, is thus a critical part of my research (see here). Yet, while often familiar, these processes are also sufficiently different that they reveal a variety of new mechanisms that are capable of modifying a planetary surface. Thus, numerical simulations and laboratory studies together allow us to experiment with many processes (both familiar and exotic) over a range of spatial and temporal scales, to dig deeper and, hopefully, understand how these surfaces evolve.

Titan Geomorphology and Surface Processes

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. Complex organic compounds are synthesized in Titan’s dense atmosphere and are subsequently transported and modified across the globe by the only known active extraterrestrial hydrologic cycle. Though we still have a very limited understanding of the dynamics and timescales of this cycle, it has nonetheless produced landforms that bear a striking resemblance to those we find on Earth. Titan is therefore unlike any other known world, one that remains a singular destination for studying both Earth-like surface processes, and the history of chemical processing of our own planet that led to the development of life.

The various exchange processes within Titan’s hydrologic cycle result in large hydrocarbon polar seas, vast equatorial solid-hydrocarbon sand seas that nearly envelop the whole moon, and a thick nitrogen-dominated atmosphere that results in both methane rain storms, and a continuous supply to the surface of complex solid organic sediments. These materials are then transported across the surface in channels and by winds during large storms.

Cassini Synthetic Aperture Radar (SAR) images of Titan; a: Southern shoreline of Ligeia Mare at Titan’s north pole, with a dendritic canyon network (Vid Flumina) draining into the sea; b: Meandering channel networks in the Xanadu region that extend for 100’s of kilometers; c: A network of braided channels that deposit into a large bajada by Minerva Crater, near Titan’s equator; d: Longitudinal dunes migrating around topographic highs in the Belet dune field at Titan’s equator.

Perhaps the most fascinating features on Titan are located around the polar regions, where flybys of the Cassini spacecraft have revealed hundreds of small lakes and vast seas. Meanwhile, the south polar region shows a markedly different environment with only a few filled lakes, and large empty seas (see our paper describing these surfaces here in Icarus). The small lakes (which we have termed “sharp-edged depressions”) are embedded within large sedimentary deposits, and look in part to be formed by a type of collapse mechanism (see a nice review on this concept here). A closer look at these lakes, however, reveals that they are not like anything we have observed on the Earth (see our review of this problem here).

The larger seas, meanwhile, look as if they have flooded pre-existing high-standing topography, and have forced the channels draining into them to respond accordingly (see our paper here for a detailed discussion as to the morphology of Titan’s poles). The margins of the seas are also peculiar, particularly where they intersect with channels, as sedimentary deposits (deltas) are almost never seen. 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, and decoding Titan’s record of tectonics, erosion and climate, requires a combined theoretical, laboratory and numerical investigation of the processes that occur when rivers enter Titan’s lakes and seas and a re-evaluation of Cassini data for evidence of the resulting landforms. We are currently undertaking this large project, one that we hope will provide the framework for delta dynamics on Titan, and pave the way for a more comprehensive understanding of Titan’s hydrology!

Titan’s active hydrological cycle thus makes it a testing ground where we can push the fundamentals of fluvial geomorphology under extreme conditions. Though the chemical composition of the surface and fluids, as well as the environmental conditions, are vastly different from the Earth’s, the familiarity of Titan’s landforms allow us to apply the principles of terrestrial geomorphology and hydrology to gain a first order understanding of how the surface has evolved. However, a direct application of terrestrial principles, while useful, is not entirely accurate. Gravity is one seventh of that on Earth, and there are two different fluids (methane and ethane) of differing densities that are simultaneously able to flow and mobilize sediment on surfaces that are both soluble (e.g., acetylene) and insoluble (e.g., water ice) under Titan conditions. Additionally, Titan’s atmosphere has a large thermal inertia compared to the Earth, is a slow rotator (16 Earth days), and the energy input to drive atmospheric dynamics at 10 A.U. is far less than received at the top of our atmosphere. This results in precipitation discharges and frequencies that are larger than Earth’s, but which occur over an order-of-magnitude less frequently. How these fundamental differences are expressed on Titan’s surface is the focus of much of my work. To further model Titan’s landscapes we have to understand and quantify key questions, such as:

1. How is sediment generated, transported and deposited?

2. What are the pathways for fluids across (and below) the surface?

3. What are the timescale(s) over which the surface evolves?

4. What is the origin of the sharp-edged lake depressions?

5. Why are there so few sedimentary deposits around the seas?

Answers to these questions can, in part, be revealed by numerical simulations and laboratory experiments that we are and will be undertaking over the next few years. While Cassini’s dataset completely changed our understanding of Titan and allowed for these questions to even be posed, it remains limited in resolution (~1-kilometer), and coverage (~47% of the surface). For this reason, we are currently developing landscape evolution simulations to supplement our many observations. Over the next year, their application will allow us to experiment with a multitude of processes, testing many unknown unknowns to push our understanding a little further. 

As missions are now being developed, the opportunity also exists, in the coming decades, to go back to Titan. Simply, we are in a similar state that Mars surface science was in the 1970’s after the Mariner missions, where surface features <1-kilometer in size were not resolved. Just as the Mars Global Surveyor changed our understanding of Mars, the factor of ~20 increase in resolution offered by an orbiter would radically change our understanding of the entire Titan system. A mission to the surface of Titan, though unable to survey the entire moon, would similarly provide invaluable data as to the properties of Titan’s surface, including the erosional resistance, composition(s), and a myriad of other surface properties. Excitingly, the Dragonfly mission, a mobile (aerial) lander, was just selected by the New Frontiers 4 program, and is now under development at the Applied Physics Laboratory (APL). Nominally, it will land on Titan in 2034, where it will measure these exact surface properties at Titan’s equatorial latitudes. Making multiple flights, Dragonfly will also explore a variety of locations on Titan, greatly expanding our knowledge of Titan’s surface and atmosphere and its history of organic processing.

Most exciting to me, Titan also offers the best location in our solar system to study the fundamentals of Earth-like surface processes. It lacks as complex of an interplay of diverse physical and chemical processes – including active tectonics, variable bedrock lithologies, diverse climate zones, vegetation, and even humans – and therefore serves as a natural laboratory for studying “Earth-like” surface processes! Future exploration of Titan by spacecraft will also continue to advance our understanding of the evolution of Titan’s unique, complex surface.

Cometary Geomorphology and Surface Processes

Compared to Titan, Comet 67P displays much more exotic landforms that are unique to a small body. With almost no gravity or atmosphere, and a material yield stress similar to a powdery snow, it is hard to imagine any physical stresses that can effectively erode a comet’s surface into recognizable landforms other than just a large, flat plain. And yet, we have observed explosive jet erosion that is driven by solar insolation, and watched this process produce dynamic changes of the surface throughout its perihelion passage. This sublimation-driven weathering of the nucleus results in a hemispheric sediment transport cycle (see a nice description of this process here), that has collectively shaped this small surface into a surprisingly complex collection of landforms (see our MNRAS paper here for a description of these landforms).

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 mission selected 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 has provided access to the spatial scales where the processes relevant to small body evolution can be directly observed! These observations have allowed for detailed analyses of morphologies down to the meter scale across the entire nucleus and have provided a long (2+ years) temporal data set with which to search for changes on the surface, important observations that have given us the opportunity to watch how a comet surface erodes. The work of Rosetta (and of course all those involved to make it happen) have allowed us to apply the same principles of detailed geomorphic analyses that we have for Titan, to a small body.

The nucleus of 67P was revealed to have an organic-rich, very dark and highly dehydrated surface, with little water ice within the top few centimeters. However, the total ice abundance that comprised the nucleus ranges from 20 wt%, up to even 50 wt% in select locations. The variation in this value, however, implies that there is clearly a dynamic process that is altering this ratio. Further, the upper few centimeters of the surface have an extremely low thermal inertia, indicating that there are large differences between subsurface and surface temperatures and that significant volumes of volatiles (primarily water ice) may hide just below the surface. Indeed, variegations in the color of the surface throughout perihelion, where “blue” regions of the surface indicate volatile-rich exposures are exposed as the nucleus erodes, hint at this very possibility. Together, these observations are consistent with the view that 67P has retained a significant fraction of the volatiles since its formation in the protoplanetary disk 4.5 billion years ago. Perhaps then, we were lucky in 2003 that Rosetta missed it’s launch date, and we fortuitously rendezvoused with this primitive small body instead! 

Visible images from Rosetta’s OSIRIS camera of characteristic terrain features on Comet 67P; a: Beveled topography of the southern hemisphere, with few vertical cliffs or smooth terrain deposits; b: Aeolian-like ripple structures in the smooth terrains of the Hapi (neck) region. The ripples have a characteristic spacing of ~12 meters; c: An outburst from the surface driven by sublimation of near-surface volatiles; d: Rugged topography of the northern hemisphere, with boulders accumulating as mass wasting deposits at the base of vertical cliffs; e: Smooth terrains that drape the underlying rugged topography in the northern hemisphere; f: A 100-meter deep pit in the Seth region, likely the result of a collapse-type mechanism. The origin of the texture on the pit wall remains unknown.

These chemical clues are all important because it means that 67P’s surface, only recently having experienced the heat of the inner solar system where water ice can sublimate, may reflect what the surfaces of other icy small bodies of the Kuiper Belt (KBO) and Oort cloud look like. Other than New Horizons’s observations of 2014 MU69, within my lifetime, we will likely never observe the surfaces of another one of these primitive bodies again, especially with the same level of detail provided by Rosetta. Comparing 67P to MU69, it is immediately clear that KBOs lack the topographic features that dominate 67P’s surface. When and how 67P’s cliffs and taluses form therefore remains an outstanding question. Perhaps with a future rendezvous with a distant Centaur (features thought to be intermediaries between KBOs and JFCs) we can solve this problem! 

While the processes that act to modify the surface of a comet are primarily sublimation erosion, the result is not simply a gradual lowering of surface topography. Early Rosetta observations led to hypotheses that sublimation-driven erosion of the consolidated nucleus (i.e., the cliffs) is a principal erosional process. The remnants of this process are the smooth terrains: vast deposits of fallback material of centimeter/decimeter-sized particles that blanket large portions of the 67P’s northern hemisphere, but are very sparse in the south. The formation of these smooth terrains is relatively well understood, where particles liberated by the intense insolation at perihelion of the southern hemisphere follow ballistic trajectories to cold, gravitational lows in the northern hemisphere. Most of this mass transport occurs seasonally during perihelion and is primarily driven by 67P’s obliquity.

The ubiquity of smooth terrains on other observed comet nuclei suggests that they play a fundamental role in shaping the surfaces of comets. Comet 103P/Hartley 2 has a distinctive “waist” of smooth terrain that has been demonstrated to be the terminus for the deposition of airfalling particles ejected from the tip of the comet’s nucleus. Similarly, 9P/Tempel 1 has large regions of smooth terrain material that retreated in between the Deep Impact and Stardust/NeXT flybys. Finally, though imaged with relatively poorer resolution, 19P/Borrelly has a similar waist to Hartley 2, suggesting a similar formation. Like on 67P, observations of these nuclei led to hypotheses that the erosion of consolidated material dominated the mass loss on comets, and that the smooth terrains were principally lag material unable to escape during the ejection process.

Yet few changes of the consolidated material were observed during the Rosetta mission, 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?). Instead, many rapid changes were detected in what were previously thought to be the comparably inactive smooth terrains. These changes occurred in the the months leading up to perihelion, and include both morphological changes like depressions and “honeycombs,” and spectroscopic changes in the color of the nucleus. Together, these changes suggest the nucleus removed an overlaying layer of dust prior to receiving its newest layer at perihelion. 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 not by a gradual diffusive process, but by localized bursts of sublimation within migrating depression scarps. For just a short time, these scarps grow and migrate across the smooth terrains, removing an entire 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.

Yet Hapi is just one small part of 67P, and it is reasonable to question whether this a universal process. To understand relevance and context of Hapi’s changes, requires a database of all the changes, with maps of when and where they occur. Fortunately, we just received 3 years of NASA funding to answer this very question, one we hope to share with the community very soon!

While Rosetta has provided a rich dataset that allows 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. This material is sourced from across the comet’s surface and has been altered to an unknown degree. Understanding the origin and the physical/chemical processes responsible for the evolution of this surface is therefore critical to being able to decipher what the acquired sample can teach us about the formation and evolution of the solar system, and is the primary focus of my research on 67P. Though the CAESAR mission was not selected as part of the New Frontiers 4 call, despite satisfying all the Decadal Survey goals and more, it is being re-proposed for the next New Frontiers opportunity. If ultimately selected, CAESAR would advance our understanding of the evolution of cometary nuclei tremendously, and its return of a sample would provide a ground truth for over 50 years research efforts dedicated to understanding the formation of our solar system!

It truly is an exciting time to be exploring the outer solar system. Titan and 67P are exciting worlds with a diverse set of landforms that have been revealed just recently by the Cassini and Rosetta spacecraft. Further study of these icy worlds should continue to yield insights as to the processes that can modify a planetary surface under a wide range of physical and chemical conditions. As these two worlds also feature prominently in NASA’s Decadal Survey, their continued exploration by future spacecraft will continue to teach us more about our own solar neighborhood.