The depth/diameter ratio for new meter-to decameter-scale Martian craters formed in the last 20 years averages 0.23, only slightly deeper than that expected for simple primary craters on rocky surfaces. Large variations in depth/diameter (d/D) between impact sites indicate that differences between the sites such as target material properties, impact velocity, angle, and physical state of the bolide(s) are important in determining the depth of small craters in the strength regime. On the Moon, the d/D of random fresh small craters with similar diameters averages only 0.10, indicating that either the majority of them are unrecognized secondaries or some proportion are degraded primaries. Older craters such as these may be shallower due to erosional infilling, which is probably not linear over time but more effective over recently disturbed and steeper surfaces, processes that are not yet acting on the new Martian craters. Brand new meter-to decameter-scale craters such as the Martian ones studied here are statistically easily distinguishable as primaries, but the origins of older craters of the same size, such as the lunar ones in this study, are ambiguous.

The properties of granular material have an important effect on surface landforms and processes. Recently, it has been suggested that material properties called dynamic and static angle of repose vary with gravitational acceleration, which would have a significant effect on many planetary surface processes such as crater collapse and gully formation. In order to test that hypothesis, we measured lee slopes of active aeolian sand dunes on Mars using the High Resolution Imaging Experiment (HiRISE) DTMs (Digital Terrain Model). We examined dune fields in Nili Patera, Herschel Crater, and Gale Crater. Our measurements showed that the dynamic angles of repose for the sands in these areas are 33–34° in the first region and 30–31° in the other two. These results fall within the 30° to 35° window for the dynamic angles of repose for terrestrial dunes with similar flow depths and grain properties and thus show that this angle does not significantly vary with decreasing gravity.

AU Mon is a long-period (11.113 d) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked Hα emission. We present previously unpublished optical spectra of AU Mon which were obtained over several years with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including Hα, Hβ, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R ⊙ , thickness of 5.2R ⊙ , density of 1.0 × 10 −13 g cm −3 , and maximum temperature of 14000 K, along with a gas stream at a temperature of ∼8000 K transferring ∼2.4 × 10 −9 M ⊙ yr −1 . We show Hα Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions from the stars and accretion structures. The tomograms provide independent support for the Shellspec modeling, while also illustrating that residual emission at sub-Keplerian velocities persists even after subtracting the disk and stream emission. Spectral variability in the Hα profile beyond that expected from either the orbital or the long-period cycle is present on both multi-week and multi-year timescales, and may reflect quasi-random changes in the mass transfer rate or the disk structure. Finally, a transient UV spectral absorption feature may be modeled as an occasional outflow launched from the vicinity of the disk-stream interaction region.

Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 13Impact melt flows are observed to emerge from the continuous and discontinuous ejecta 14 blanket of the 9 km lunar crater Pierazzo, from the crater rim to more than 40 km away 15 from the center of the crater. Our mapping and modeling results suggest that melt can be 16 incorporated into ejecta and emplaced ballistically. It also confirms the idea that impact 17 melt can travel beyond the continuous ejecta blanket.. Our analysis is based on the 18 identification of established melt morphology for these in-ejecta flows, and their 19 occurrence on 6 to 18 slopes -too shallow for dry granular flows beginning at rest. We 20 also compared the fractal dimension of the flow boundaries to established melt and granular 21 flows, providing more support for these flows being melt-rich instead of granular in origin. 22Ejected melt flows are noted within just 1.5% of the mapping area, suggesting that 23 the surface expression of impact melt in the extended ejecta around craters of this size is 24 rare. We hypothesize that a mix of solid and molten ejecta impacts the ground together and 25 continues to travel across the surrounding terrain at speeds high enough to maintain 26 turbulent mixing. This quickly quenches the melt present, preventing most coherent melt 27 pockets from settling out within the majority of the extended ejecta deposit, unless the melt 28 'pocket' is especially large. As most of the flows mapped in this work occur on crater-29 facing slopes, the development of defined melt flows within ejecta deposits might be 30 facilitated by influence of high crater-facing topography to stall or impede the ejecta flow 31 soon after it makes ground contact, preventing the continuation of turbulent mixing. These 32 surface expressions of melt within ejecta blankets suggest that melt rock masses can exist 33 within ejecta, creating a heterogeneous deposit. 34 stereo image data.