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, while 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 of 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 to those we see here on Earth. 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.”
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. Finding a new target isn’t easy though, because comets are small bodies on eccentric orbits that make navigation and rendezvous difficult. After some deliberation, 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) 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 myself 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 and pooffing off of the dust cover. 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, the expanding sublimating gas can easily loft particles significant distances (since gravity is so small), ballistically transporting most of the sediment across the nucleus. This global sediment 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 each other, we mapped the whole surface of 67P. What we 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, integrated over a full orbit, there is much 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! 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. What you are left with is a sediment-bare southern hemisphere, and a northern hemisphere buried in ice-rich sediment, itself ready to sublimate the next time the sun rises on the region. In terms familiar to a geomorphologist, the south is “detachment-limited” while the north is “transport-limited.”
How did 67Ps landscapes evolve?
Surprisingly, few changes of the consolidated material were observed by Rosetta, indicating that this process acts either 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. 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.
Recently, in our GRL paper, we provide a combined observation and theoretical study that we believe has solved some of 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 depressions. 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.
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. To that end, our new thermal model of 67P, that counts for the fine-scale topography, is revealing that there are huge variations in the received energy flux over very small length scales. We are finding that predicted “hot spots” correlate very well with where we find changes and the fluxes predicted by our model match precisely the fluxes measured by ROSINA! Ultimately, the final product we are working towards is a complete sediment budget for 67P that is coupled to a detailed thermal model 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)? Further, how do comet nuclei thermally evolve throughout their lifetime in the solar system, how do you retain the highly volatile elements Rosetta detected, and how deep do they source from? Our thermal model can answer this question quite easily, rapidly solving for the temperature evolution with depth.
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 6 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!
Where else can we use this new knowledge?
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 we could tackle with the tools we’ve built at 67P 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 in the Solar System!
- 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 sublimative 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 (i.e., an ice-covered ocean world), yet complex life arose during this most inhospitable time. Could prolonged sublimation of the global, low latitude ice sheet surfaces have created habitable niches?
Samuel Birch 