Devon M. Burr

Mercury hosts widespread smooth plains that are concentrated in the Caloris impact basin, in an annulus surrounding the Caloris basin, and in the adjacent northern smooth plains. The origins of these smooth plains are uncertain, although prior work suggests these plains in the northwestern Caloris annulus might reflect volcanic activity, impact ejecta, or a combination of the two. Deciphering the timing and mode of emplacement of these plains would provide a critical constraint on regional late-stage volcanism or impact effects. In this work, the region northwest of Caloris was investigated using geomorphological and color-based mapping, crater counting techniques, and spectral analyses with the goal of placing constraints on the source of the observed units and identifying the primary emplacement mechanism. Mapping and spectral analyses confirm previous findings of two distinct, yet intermingled, units within these plains, each with similar crater count model ages that postdate the formation of the Caloris impact basin. Mapping, spectra analysis, ages, and the identification of potential flow pathways are more consistent with a predominantly volcanic origin for the smooth plains materials, although these data do not rule out contributions from impact ejecta or impact melt. We propose several hypothetical scenarios, including post-emplacement modification by near-surface volatiles, to explain these observations and clarify the emplacement mechanism for these specific smooth plains regions. Further observations from the BepiColombo mission should provide data to potentially address the outstanding questions from this work.

Grain sizes of Martian sand dunes are critical sedimentological data on sand provenance and transport pathways. Thermal inertia values are used to characterize the grain sizes of dune sand. Most early characterizations involved single dune fields. Recent work based on global data sets has provided more wide‐spread dune sand locations, though these data sets include the non‐sandy interdune areas. To provide a more accurate grain size characterization, we leverage a global thermal inertia data set, a global dune database and a global imaging mosaic to develop a freeware‐based methodology for deriving grain sizes. This methodology involves delineation of sand‐only areas within dune fields and collection of thermal inertia values from those areas. We consider a unimodal histogram of values with a mode <∼350 thermal inertia units (J m⁻² K⁻¹ s^−1/2) to imply an effective exclusion of non‐sand surfaces. Application of this methodology to dune fields for which thermal inertia values have been previous derived shows our results fall within the envelope of those values. We apply our methodology to tropical dune fields on Mars for which Dust Cover Index data imply dust‐free surfaces. Conversion of these thermal inertia values to sand grain sizes yields a range of sand classifications of fine sand to granules. Comparison of sand size classifications with geographic location shows grain size ranges that are distinctive by location, consistent with local sourcing. This work points toward geographically diverse sand formation mechanisms yielding diverse grain sizes, while providing a freeware‐based and thus widely accessible method for expanding the derivation of these critical data.

Aeolian sand transport on Mars is active today and was likely so throughout its history. Widespread dune motion is theorized to comminute sand to sub-sand sizes, a process also implied by lab experiments. In view of this sand destruction, discovering the source(s) and origin(s) of Martian sand provides critical information for understanding Martian sediment cycling.Local sand sources have been discovered and considered to be consistent with the long-standing hypothesis for Martian sand as volcaniclastic in origin. A local source of Martian sand has recently been inferred in the western Medusae Fossae Formation (wMFF). Given the pyroclastic origin of the vast MFF, the new discovery of sand generation from that deposit substantiates a volcaniclastic origin of Martian sand.However, the wMFF is limited in extent and unlikely to constitute an origin for the globally distributed dune fields on Mars. Continued exploration for sand origins is needed to explain this widespread distribution.We examined the five global geological units interpreted as volcaniclastic, which yielded limited evidence of sand sourcing outside the wMFF. In these five units, sand sourcing was detected in visible-wavelength data in the Hesperian and Amazonian transitional units that comprise the central and eastern MFF and in the Noachian units of Arabia Terra. Investigation to characterize sand production from these units is revealing a variety of sand source outcrops.Tracing sand deposits back to their sources is another approach for determining sand origins, as was used in determining the source – and thereby the origin – of sand in and from the wMFF. Determining sources for the widespread sand on Mars requires determining sand survivability: how far could sand travel from their sources before being destroyed by comminution to sub sand sizes? Simulation of aeolian transport on Mars has shown different sand mineralogies comminuting at different rates, suggesting that the bulk mineralogy of a sediment may change with increased transport distance. Building on that previous experimental work, we are undertaking comminution of 14 different Mars-analog sands to more fully characterize the mineralogical and physical effects on sand of aeolian transport. The results will support using dune sand compositions and distances from possible source outcrops to test if these outcrops sourced the sand.Thermal inertia is used to characterize Martian sand, e.g., to estimate grain sizes. Available dune field mapping facilitates investigation into dune sand thermal inertia values, thereby providing data, e.g., on sand particle sizes and induration states. As available mapping incorporates non-sand substrate, we are remapping dune fields to include only visible sand and using the distributions of thermal inertia values to assess if non-sand substrate is still included in our mapping. Having completed remapping of tropical dune fields, we are beginning analysis of their thermal inertia values. The results will reveal any trends relative to geography, underlying geologic unit, elevation, and other factors.These three investigations – into the sources and origins, effects of transport, and thermal inertia values of Martian sand – will support improved understanding of Martian aeolian sand cycling, one of the most active geologic agents on Mars.

As detailed in a recent journal publication by the authors of this abstract, women have made significant contributions in the fluvial, aeolian, and (cryo)volcanic subdisciplines of planetary geomorphology, despite undeserved challenges to their participation. Some women—in particular, women of color—are highlighted in this work to show a part of these foundational contributions. As the latter half of the 20th century was a revolutionary time for terrestrial geomorphology and the inception of the discipline of planetary geomorphology, we focused our research into these contributions on women scientists who were working during this time. We also focused on women working in our scientific subdisciplines so that we could provide proper context for their work. These contributions have occurred both as discoveries in terrestrial geomorphology leading to follow-on discoveries in planetary geomorphology and through serving as educators and role models. With women increasingly achieving positions of influence both in the geo- and planetary sciences as in American society, this research allows us to celebrate these contributions of women and particularly women of color while looking forward to a more complete record of their past contributions and greater future achievements.

(1) Methods, Figures S1–S3, and captions for additional supplemental information, (2) Animation S1, (3) the ArcGIS geodatabase from this work, (4) CRISM browse products, and (5) the ArcGIS database with the locations of grain size derivations.

(1) Methods, Figures S1–S3, and captions for additional supplemental information, (2) Animation S1, (3) the ArcGIS geodatabase from this work, (4) CRISM browse products, and (5) the ArcGIS database with the locations of grain size derivations.

(1) Methods, Figures S1–S3, and captions for additional supplemental information, (2) Animation S1, (3) the ArcGIS geodatabase from this work, (4) CRISM browse products, and (5) the ArcGIS database with the locations of grain size derivations.