This study aimed at assessing the changes in the shoreline and the area of Sandwip Island, Bangladesh using multi-temporal satellite image and geospatial techniques. The study results (1980-2014) indicate that the island has gained approximately 25 km 2 land areas and has lost about 64 km 2 of its original land area during this time. The net loss of the shore-length was approximately 3.1 km or 0.1 km/year and the net loss of 38.84 km 2 or 1.14 km 2 / year areas. The erosion process has been taken place along the western, southwestern , and southern banks of the island. In these areas, the landward movement and rate of the shoreline were higher with a highest value of the net shoreline movement (NSM) around 3.8 km and the end point rate (EPR) more than 100 m/year in some parts. The accretion, on the other hand, has been taken place along the northern and northeastern banks of the island at relatively slower rates with the NSM \2.8 km and the EPR \100 m/ year.

We analyze the uncertainty in grain size estimation of pure methane (CH4) and nitrogen saturated with methane (N2:CH4) ices, the most abundant volatile materials on trans-Neptunian objects (TNOs) and Kuiper Belt objects (KBOs). We compare the single scattering albedo, which determines the grain size estimation of outer solar system regolith, of these ices using the Mie scattering model and two other Hapke approximations (Hapke 1993) in radiative transfer scattering models (RTMs) at near-infrared (NIR) wavelengths (1–5 μm). The equivalent slab (Hapke slab) approximation model predicts results much closer to Mie scattering over the NIR wavelengths at a wide range of grain sizes. In contrast, even though the internal scattering model predicts an approximate particle diameter close to the Mie model for particles with a 10 μm radii, it exhibits higher discrepancies in the predicted estimation for larger grain sizes (e.g., 100 and 1000 μm radii). Owing to the Rayleigh effect on single-scattering properties, neither Hapke approximate models could predict an accurate grain size estimation for the small particles (radii ≤5 μm). We recommend that future studies should favor the Hapke slab approximation when employing RTMs for estimating grain sizes of the vast number of TNOs and KBOs in the outer solar system.

The Mars Global Digital Dune Database (MGD 3) contains information on Martian dune fields and prepared manually from the Thermal Emission Imaging System (THEMIS; 100 m/pixel) images. Although the MGD 3 outlines dune fields, it overlooks the recognition of smaller dune forms. This paper aims to identify individual dunes from a semiautomated object-based image analysis technique and characterize dune materials at Hargraves crater, Mars. MGD 3 would benefit to be updated for an improved understanding of the Martian surface and its atmospheric mechanisms at a local scale. An object-based image analysis technique was applied here to the Context Camera (CTX; 6 m/pixel) data set to extract dune data in a more efficient, reliable, and accurate fashion. This study is a test case in validating a remote sensing method that has wide applicability to the entire Martian surface resulting in an update to the dune database at a higher spatial resolution-providing a better understanding of surface and atmospheric behavior of Mars at the local scale. We also explored the wind flow and dune stability-presenting an insight into the dune modification mechanism-within the crater. The prevailing wind inside the crater flows to the west-northwest. The dunes are labeled as active (stability index of 2) and do not appear to have been influenced by subsurface water ice or volatiles. We emphasize that the technique used here has a wide prospect in temporal monitoring of dune sediment flux, dune migration or erosion rates, improving near-surface airflow modeling, and dune stability analysis. Plain Language Summary The Mars Global Digital Dune Database (MGD 3) contains morphologic information on Martian dune fields. MGD 3 has various science applications including atmospheric circulation modeling, planetary climate change, mechanical weathering processes, and future rover lander missions to Mars. However, the database was prepared manually through visual photointerpretation from low-resolution satellite images of 100 m/pixel and, consequently, overlooked the recognition of smaller dune forms. Thus, MGD 3 would benefit to be updated for an improved understanding of the Martian surface and its atmospheric mechanisms including near-surface wind patterns at a local scale as opposed to the global scale. We used a semiautomated object-based image analysis (OBIA) technique from higher-resolution images of~6 m/pixel-more than 10 times higher spatial resolution compared to the previous imagery. Using the technique, we extracted individual dunes at Hargraves crater in a more efficient, reliable, and accurate fashion. The validated and accurate result in this study indicates the applicability of the OBIA method for the entire surface of Mars. Thus, the application OBIA method will be a great improvement for futures studies on Martian (and terrestrial) dune fields including temporal monitoring with better estimates of sediment flux, dune migration or erosion rates, and improving near-surface airflow modeling.

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