Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: Asteroids smaller than about 100 meters are observed to rotate very fast, with periods often much shorter than the critical limit of 2.2 h. Some of these super-fast rotators can also achieve a very large semi-major axis drift induced by the Yarkovsky effect, that in turn, is determined by internal and surface physical properties. We consider the small super-fast rotating near-Earth asteroid 2016 GE1. This object rotates in just 34 seconds, and a large Yarkovsky effect has been determined from astrometry. Here we aim to constrain the thermal inertia of the surface of this extreme object. We used a recently developed statistical method to determine the thermal properties of near-Earth asteroids. The method is based on the comparison between the observed and the modelled Yarkovsky effect, and the thermal conductivity (inertia) is determined by a Monte Carlo approach. Parameters of the Yarkovsky effect model are either fixed if their uncertainty is negligible, modelled with a Gaussian distribution of the errors if they are measured, or deduced from general properties of the population of near-Earth asteroids when they are unknown. Using a well-established orbit determination procedure, we determined the Yarkovsky effect on 2016 GE1, and verified a significant semi-major axis drift rate. Using a statistical method, we showed that this semi-major axis drift rate could be explained only by low thermal inertia values below 100 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$: namely, 90\% of the probability density function of the model outcomes is contained at values smaller than 100 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$. We propose two possible interpretations for the extremely low values: a high porosity or a cracked surface, or a thin layer of fine regolith on the surface. Though this seems unexpected in either case, it opens up the possibility of a subclass of low thermal inertia, super-fast rotating asteroids.

Abstract: After performing a reassessment of the known dynamical asteroid families in the inner main belt, we report a newly discovered ancient asteroid family with an estimated age of $4.3\pm1.7$ billion years. Additionally, we report the most comprehensive list of planetesimals, which are asteroids that survived since the planet forming days of the solar system.

Abstract: We utilize the well-established properties of the solar gravitational lens (SGL) to consider realistic observational scenarios. Actual exoplanets, which may be the target of an SGL observational campaign, are not stationary. Their appearance changes as a result of their diurnal rotation and varying illumination due to their orbital motion around their host star. The nature of the SGL is such that imaging with one telescope is accomplished with a cadence of one pixel at a time, with substantial per-pixel integration times. Therefore, capturing a single snapshot of the target planet with a realistically-sized telescope is not possible. Instead, the planetary surface must be reconstructed by inverting the combined effect of the SGL's point-spread function and temporal changes induced by the planetary dynamics. Using the Earth as a stand-in, we demonstrate practical feasibility of this approach, by simulating a dynamical system and then recovering topographic images of acceptable quality. The dynamics-induced temporal variability of the exoplanet represents an added challenge, but even in the presence of such dynamics, use of the SGL for exoplanet imaging remains feasible.

Abstract: Direct imaging of exoplanets will allow us to directly observe the planet in reflected light. Such a scenario may eventually allow for the possibility to scan the planetary surface for the presence of artificial structures made by alien civilizations. Detectability of planetary scale structures, called megastructures, has been previously explored. In this work, we show that it is possible to detect structures of much smaller scale on exoplanetary surfaces by searching for the specular reflection of host starlight from the corresponding structures. As the planet rotates, these reflections can manifest as an optical transient riding atop the rotational light curve of the planet. Due to the directional nature of specular reflection, the reflected signal is very strong, and it is comparable to the planetary flux for surfaces covering only few ppm (parts per million) of the total planet surface area. By tracking the planet around its orbit, it should be possible to scan the planetary surface for any such structures covering a size larger than a few ppm of planetary surface. The proposed method will aid in the search for extra-terrestrial intelligence in the era of direct imaging of exoplanets.

Abstract: Saturn possesses a dynamically rich system containing numerous moons and impressive rings. Whether the rings of Saturn are much younger than the planet itself has been a long-open question; more recently a young age has been proposed for some moons. Recent detection of the fast orbital evolution of Rhea and Titan strongly suggest a highly frequency-dependent tidal response of Saturn, possibly through excitation of inertial waves within the planet's convective envelope. Here we show that the resonance locking to inertial waves cannot explain the dynamical structure of the Saturnian system or the current tidal heating of Enceladus. On the other hand, both the observation and our modelling results indicate that the system is not consistent with evolution under equilibrium tides. We propose that the system's architecture can best be explained by relatively high "background" tidal response coupled with discrete resonant modes. In this view, only Titan may be in a true long-term resonance lock with a tidal mode of Saturn. Rhea is most likely currently experiencing a transient period of fast tidal evolution as it passes through a mode, rather than being locked to it. Assuming that Enceladus went through a temporary period of fast tidal evolution, we can reproduce its present resonance with Dione and satisfy other dynamical constraints. Additionally, we conclude that the long-term tidal response of Saturn to Tethys must be weaker than expected from frequency-independent tides, as already found by observations.

Abstract: We present five morphological and kinematic criteria to aid in asserting the binary nature of a protoplanetary disc, based on 3D hydrodynamical simulations of circumbinary discs post-processed with Monte Carlo radiative transfer. We find that circumbinary discs may be identified by i) a central cavity, ii) spiral arms both in and outside of their central cavities, iii) non-localised perturbations in their iso-velocity curves, iv) asymmetry between the lines of maximum speed of the blue and red-shifted wings and v) asymmetry between the area of the blue and red-shifted wings. We provide quantitative metrics for the last two criteria that can be used, in conjunction with the morphological criteria, to signal whether a protoplanetary disc is likely to be a circumbinary disc.