Asteroids and comets are not only the most pristine remnants from the formation of our Solar System, but also pathways for distributing volatiles such as water - a basic element for life. Recent space missions, including Rosetta, Dawn and Hayabusa 2, to asteroids and comets have revealed the previously unknown diversity and complexity of these small bodies’ surfaces, by means of image and spectral data. In particular, ices exposed to the vacuum of space sublimate, forming distinct morphologic features by outgassing. The investigation of these features leads to a better understanding of the processes forming asteroidal and cometary surfaces, and enables us to constrain the distribution and abundance of volatiles in the Solar System.
We are a young academics group (Nachwuchsgruppe) focussing on volatile-related morphologies on asteroids and comets and the physics responsible for their formation. We use a comprehensive interdisciplinary approach, combining cartography, geology and physics, to investigate morphologic features related to volatiles on the surfaces of asteroids and comets, applying both experimental and numerical methods to remote sensing spacecraft data.
We conduct laboratory experiments to investigate the evolution of volatile-rich surfaces. To this end, we produce mixtures of micron-sized silicate dust and ice particles in the micrometre range and examine the material properties of these mixtures including their cohesion and porosity (Figure 1). These cometary analogues are shaped to a specific morphology such as cliffs or pits and are placed in a vacuum chamber, under radiative heating. The behaviour of the subliming surface is documented and compared with observations of real cometary surfaces. This allows us to draw conclusions about the evolution of cometary features as observed by spacecraft. This work is supported by our colleagues at the universities of Braunschweig and Bern, where we use the cometary simulation equipment.
Additionally, we conduct spectral experiments at the Planetary Spectroscopy Laboratory at our institute to investigate the spectral variation of materials interacting with volatiles.
Figure 1: Example of a tensile strength test of a compressed disc of a dust-ice mixture produced in the laboratory at the University of Braunschweig. By pushing a piston onto the disc a fracture propagates through the sample and finally breaks the disc in half (panels left to right). The force at which the fracture begins to develop enables the calculation of the tensile strength of the material. Credit: Haack et al., European Planetary Science Congress 2018, #921.
In parallel with laboratory experiments, we perform numerical simulations to reproduce observed morphologic features on comets and asteroids - for instance, landslides, cliff collapses, aeolian-like deposits (“wind tails”) and moats around large boulders, thermal fracturing of sintered ice-rich materials, and the outgassing effects of volatiles. In our simulations (discrete element method/DEM), the material is composed of a large number of small particles that are governed by contact forces and the relatively low gravity of small Solar System bodies (Figure 2). The model is based on LIGGGHTS - an open source DEM code that can be modified to include user-specific physics. By specifying the mechanical properties of the particles - calibrated by comparison with laboratory experiments - we seek to explain and predict macroscopic properties of the system.
Figure 2: DEM simulation of a boulder break-up. The boulder, composed of particles of different sizes connected by cohesion and sinter bonds, is impacting on a solid surface. Modelling parameters can be constrained by setting reasonable impact speeds and comparing the simulation results with morphologies and sizes of boulders observed on comet 67P. Credit: Kappel et al., European Planetary Science Congress 2018, #797.
Our remote sensing data analysis is based on images and spectral data (specifically in the visible wavelengths) from various planetary missions, including the Dawn, Rosetta and Hayabusa2 missions. Image processing can be used to produce shape models, together with colour and clear filter mosaics necessary for the study and analysis of surface morphologies, geologic mapping, georeferenced measurements and spectral analysis (Figure 3). Our group primarily focuses on data from asteroids Vesta and Ryugu, dwarf-planet Ceres and comet 67P/Churyumov-Gerasimenko.
Figure 3: Data processing steps illustrated by the example of Cornelia crater on asteroid Vesta. Credit for separate images: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA and Krohn et al., Planet. Space Sci. 103, 2014.
Various types of landslides have been identified on bodies in our Solar System, including on comets and asteroids. The investigation of mass wasting provides an opportunity to understand the gravitational, structural and material properties of planetary bodies, including the effect and presence of volatiles. Figure 4 shows examples of dry (left, Vesta) and volatile-rich (right, Ceres) landslides. The dislocated material has travelled downwards and slumped at the bottom of the slide. The lobate shape, with striations, seen on the Ceres landslide may be indicative of subsurface ice, not seen on the dry, granular Vesta landslide.
Figure 4: Material that moved down a steep crater wall in a granular manner on asteroid Vesta (left). Slumped material with flow margins and overprinting furrows in Ghanan crater on Ceres (right). The morphologic difference between the two landslides may be due to the existence of volatiles on Ceres but not on Vesta. Credit for separate images: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Some large boulders on comet 67P show morphologic features which resemble features on Earth or Mars that are typically associated with wind (Figure 5, left). However, as there is no substantial atmosphere on comet 67P, a particle stream mobilized by volatile outgassing and cometary activity may explain the formation of such features. Opposite to the incident direction of an assumed particle stream, there is often a triangular elevated surface in the shielded area behind the boulder. Depending on the particle properties and the boulder geometry, there is sometimes also a depression in front of the boulder. This feature appears like a partially-formed moat, possibly excavated by particles reflected by the stream-facing aspect of the boulder (Figure 5, right).
Figure 5: A boulder (~5m across, left) on comet 67P with an aeolian-like deposit (wind-tail) and moat. DEM simulation of a boulder sitting on a particle bed (right). A particle stream from the right (arrow) erodes the particle bed in front of the boulder whereas the boulder shields the area behind it and thus allows an accumulation of particles. Credit: left: ESA/Rosetta/Philae/ROLIS/DLR edited by D. Tirsch, right: Kappel et al., European Planetary Science Congress 2018, #797.
Temperature variations from diurnal and seasonal insolation cycles cause periodic expansion and contraction in the upper surface layer of a comet. Particles in this layer are typically bonded by sintering. The associated stresses can eventually lead to fracturing of the surface and the formation of crack patterns (Figure 6). The spatial scales of these patterns are characteristic to the mechanical properties of the surface material. In particular, we seek to study the bonding and geometric configuration of the particles that are affected by sintering and the water ice content.
Figure 6: Fractures on comet 67P (left) and top view of a bonded surface layer particle DEM simulation with bond network in white and broken bonds in black after several heating and cooling cycles. Credit: left: ESA/Rosetta/MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA, right: Kappel et al., European Planetary Science Congress 2018, #797.
Boulders and loose gravel are frequently observed on comet 67P/Churyumov-Gerasimenko. Collapsing cliffs are probably important sources of boulders, exposing fresh material to space (Figure 7). The loose material forms a gravitationally-accumulated deposit at the foot of the cliff. Recently emerged boulders often have a bright surface, reflecting their ice-rich composition. The size distribution of the gravel and boulders ranges from a few centimetres to tens of metres. Occasionally, isolated single boulders are found whose origin cannot be located. They may have been transported over long distances to their current position.
Figure 7: A collapsed cliff in the Khepry region on comet 67P. The bright areas are boulders with exposed water ice (left). A DEM simulation of a cliff collapse on 67P shows surviving bonds in the debris as short, coloured lines between the centres of translucent spheres (right). Credit: left: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA, right: Kappel et al., European Planetary Science Congress 2018, #797.
Pits and pitted terrains on asteroid Vesta are characterised by irregular, rimless, depressions and can often be found in ejecta blankets or impact craters. It has been suggested they form through degassing of volatile-bearing material heated by an impact. On asteroid Vesta, pitted terrains around the 60 km diameter crater Marcia are not only morphologically distinct, but also have unique photometric and spectral characteristics (Figure 8).
Figure 8: Pitted terrains on asteroid Vesta show higher reflectance (left) and stronger pyroxene absorption (green colour, right). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA and Michalik et al., European Planetary Science Congress 2018, #863.
Impact craters are one of the most prominent features on planetary surfaces and they give us the opportunity to peek into the composition and structure of the upper crust of a surface. During the impact process material is excavated from the impact site and deposited as ejecta on the surface. After crater formation, the cavity may relax over longer timescales due to the isostatic pressures generated by the crater. These processes may indicate the presence of volatiles via specific morphologic characteristics.
On volatile-rich surfaces, impact craters may exhibit ridge structures in their centres that can be described as elongated central peaks (Figure 9, left). The crust of the volatile-rich dwarf-planet Ceres possesses some of these central ridge craters, with an increased abundance towards the polar regions where temperatures are generally colder (Figure 9, right). Lengths, widths, and heights of the central ridges tend to correlate with diameter and depth of the impact craters. Observations suggest that the occurrence of central ridge craters is related to target material properties and the orientation of a central ridge is sensitive to the impact direction.
Figure 9: Crater Haulani (~34 km diameter) on dwarf-planet Ceres has a prominent east-west orientated central ridge (left). The density of craters with central ridges on Ceres is increased in the polar regions (right). Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA/PSI (left) and C. Jakob et al., European Planetary Science Congress 2018, #792 (right).
Various types of crater fracturing can be observed in some craters located in volatile-rich targets (Figure 10). These include floor fractures due to floor uplift of a low-density material and circumferential fractures around craters generated by the long-term relaxation of a subsurface low-viscosity layer. In cases such as the dwarf-planet Ceres the low density and low viscosity can best be explained by the presence of subsurface volatiles.
Figure 10: The 50 km diameter crater Azacca on dwarf-planet Ceres shows prominent floor fractures (a) as well as circumferential fractures beyond the rim (b). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
The craters on some volatile-rich planetary bodies are polygonal in shape, in contrast to the typically round craters usually observed on planetary surfaces. They exhibit straight rim sections which form an angular shape (Figure 11) and are commonly associated with fractures in the upper crust material. These pre-existing fractures may be related to stresses introduced by the deformation or desiccation of volatile-rich, subsurface, structures and may therefore indicate the presence of volatiles in the target body.
Figure 11: The 70 km diameter hexagonal crater Fejokoo on dwarf-planet Ceres possesses prominent straight rim sections. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Dr. Katharina Otto (group leader)
David Haack (PhD student)
Tanja Michalik (PhD student)
Rutu Parekh (DLR-DAAD PhD Student)
Dr. Manuel Sachse (postdoctoral researcher)
Dr. David Kappel
Dr. Nilda Oklay
The research project "The Physics of Volatile-Related Morphologies on Asteroids and Comets" is funded by the DLR Management Board Young Research Group Leader Programme and the Executive Board Member for Space Research and Technology. We gratefully acknowledge their financial support and endorsement.