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, along with all other landforms arrayed across these surfaces have a wide spread of topographic forms and characteristics. As we continue to explore 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.
In fact, since William Davis’ concept of “Geographical Cycles” in the late 19th Century, 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 and numerical simulations. 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 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 a geologically active world with a methane-based hydrologic cycle that mimics the water-based hydrologic cycle here on Earth. The various exchange processes within this hydrologic cycle result in large hydrocarbon polar seas, vast equatorial solid-hydrocarbon sand seas that nearly envelop the whole moon, and a thick N2-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.
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 are now 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. The margins of the seas are also peculiar, particularly where they intersect with channels, as sedimentary deposits are never seen. Why this is remains a mystery, but our work on alluvial fans (see our paper here in Icarus) and our geomorphologic mapping of the polar regions (see our paper here in Icarus) has provided a dataset that has the potential to help us understand the mechanisms that generate, transport and deposit sediment on Titan.
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. 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 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.
The opportunity also exists in the coming decades, at least in part, as missions are now being developed to go back to Titan. Essentially, we are in a similar state that Mars surface science was 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 similarly change our understanding of the entire Titan system. A mission to the surface of Titan, though unable to survey the entire moon, would also provide invaluable data as to the properties of Titan’s surface, including the erosional resistance, and composition(s). Right now, the Dragonfly mission, a mobile (aerial) lander, is under development and would measure these exact surface properties at Titan’s equatorial latitudes, thus allowing us to understand this small piece of the Titan system in much greater detail.
Despite these fundamental gaps in our knowledge, the surface of Titan, despite the differences in physical conditions, still presents a familiar array of landforms that have been sculpted by rivers, dotted with polar lakes and seas, and covered with vast, dry equatorial sand seas. Future exploration of Titan by spacecraft should continue to advance our understanding of the evolution of Titan’s surface. Most exciting to me, these missions offer the opportunity to apply the knowledge gained of Titan’s hydrologic cycle to those of other planets and moons more generally.
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 erosion causes cliff collapse, mass wasting and 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!
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 and Oort cloud look like. Other than the upcoming New Horizons observation 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. 67P can therefore be used not only as a benchmark for understanding cometary geology and the processes that act to modify their surfaces, but as a window into the processes that modify the surface of the primitive worlds of the Kuiper Belt, bodies that are much more difficult to observe.
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. Instead, outgassing occurs in the form of collimated jets that act to weaken the underlying bedrock, causing failure and generating a transportable regolith. A significant fraction of the regolith ejected from 67P during these jet events does not escape the nucleus, but instead falls back to the surface to create smooth terrains. Due to 67P’s orbit and obliquity, it also has extreme seasons that transports this fallback material from one hemisphere to the other. As it is southern summer at perihelion, cliffs at these latitudes are bevelled and any regolith is instead transported toward the colder northern hemisphere. This process is inferred to deposit ~2 meters of loose debris in gravitational lows every perihelion passage, locally modifying, or creating, deposits of smooth terrain in the northern hemisphere. Even though the nucleus is just 4 kilometers in size, this global sedimentary cycle creates a stark dichotomy of surface morphologies between the two hemispheres that are just now beginning to be understood.
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. Currently, the CAESAR mission has made it through to Phase A of the New Frontiers 4 call, and would satisfy all the Decadal Survey goals and more. CAESAR would advance our understanding of the evolution of cometary nuclei tremendously, and its return of a sample on November 20th, 2038 (at 11:14 am EST) would provide a ground truth for over 40 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.