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Register a new userMigrating Scarps as a Significant Driver for Cometary Surface Evolution
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Rosetta observations of 67P/Churyumov-Gerasimenko (67P) reveal that most changes occur in the fallback-generated smooth terrains, vast deposits of granular material blanketing the comets northern hemisphere. These changes express themselves both morphologically and spectrally across the nucleus, yet we lack a model that describes their formation and evolution. Here we present a self-consistent model that thoroughly explains the activity and mass loss from Hapis smooth terrains. Our model predicts the removal of dust via re-radiated solar insolation localized within depression scarps that are substantially more ice-rich than previously expected. We couple our model with numerous Rosetta observations to thoroughly capture the seasonal erosion of Hapis smooth terrains, where local scarp retreat gradually removes the uppermost dusty mantle. As sublimation-regolith interactions occur on rocky planets, comets, icy moons and KBOs, our coupled model and observations provide a foundation for future understanding of the myriad of sublimation-carved worlds.
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We present a statistical analysis of the distribution of large scale topographic features on comet 67P/Churyumov-Gerasimenko. We observe that the cumulative cliff height distribution across the surface follows a power law with a slope equal to -1.69 +- 0.02. When this distribution is studied independently for each region, we find a good correlation between the slope of the power law and the orbital erosion rate of the surface. For instance, the northern hemisphere topography is dominated by structures on the 100~m scale while the southern hemisphere topography, illuminated at perihelion, is dominated by 10~m scale terrain features. Our study suggest that the current size of a cliff is controlled not only by material cohesion but by the dominant erosional process in each region. This observation can be generalized to other comets, where we argue that primitive nuclei are characterized by the presence of large cliffs with a cumulative height power index equal to or above -1.5, while older, eroded cometary surfaces have a power index equal to or below -2.3. In effect, our model shows that a measure of the topography provides a quantitative assessment of a comets erosional history, i.e. its evolutionary age.
Among the most spectacular variable stars are the Luminous Blue Variables (LBVs), which can show three types of variability. The LBV phase of evolution is poorly understood, and the driving mechanisms for the variability are not known. The most common type of variability, the S Dor instability, occurs on timescales of tens of years. During an S Dor outburst, the visual magnitude of the star increases, while the bolometric magnitude stays approximately constant. In this work, we investigate pulsation as a possible trigger for the S Dor type outbursts. We calculate the pulsations of envelope models using a nonlinear hydrodynamics code including a time-dependent convection treatment. We initialize the pulsation in the hydrodynamic model based on linear non-adiabatic calculations. Pulsation properties for a full grid of models from 20 to 85 M$_{odot}$ were calculated, and in this paper we focus on the few models that show either long-period pulsations or outburst-like behaviour, with photospheric radial velocities reaching 70-80 km/s. At the present time, our models cannot follow mass loss, so once the outburst event begins, our simulations are terminated. Our results show that pulsations alone are not able to drive enough surface expansion to eject the outer layers. However, the outbursts and long-period pulsations discussed here produce large variations in effective temperature and luminosity, which are expected to produce large variations in the radiatively driven mass-loss rates.
To date, at least three comets -- 2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd) -- have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H2O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H2O of unity, and one model reaches a CO/H2O ratio > 10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H2O ice. In light of this, a population of CO ice enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own Solar System and in extrasolar systems, than previously expected.
Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and may allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the primary electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. This strongly suggests that electronic band structure is an important consideration in the design of future SME alloys.
The gas-driven dust activity of comets is still an unresolved question in cometary science. In the past, it was believed that comets are dirty snowballs and that the dust is ejected when the ice retreats. However, thanks to the various space missions to comets, it has become evident that comets have a much higher dust-to-ice ratio than previously thought and that most of the dust mass is ejected in large particles. Here we report on new comet-simulation experiments dedicated to the study of the ejection of dust aggregates caused by the sublimation of solid water ice. We find that dust ejection exactly occurs when the pressure of the water vapor above the ice surface exceeds the tensile strength plus the gravitational load of the covering dust layer. Furthermore, we observed the ejection of clusters of dust aggregates, whose sizes increase with increasing thickness of the ice-covering dust-aggregate layer. In addition, the trajectories of the ejected aggregates suggest that most of the aggregates obtained a non-vanishing initial velocity from the ejection event.
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