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Our Understanding of Mercury's Surface Grows Clearer

Research led by PSI Senior Scientist Deborah Domingue Lorin offers new insights on Mercury’s surface materials. She is lead author on an Icarus paper whose results indicate that Mercury’s regolith is smoother on micrometer scales and has a narrower particle size distribution with a lower mea particle size than the lunar regolith. Regolith grain structures are different than those of regolith particles on either the Moon or those asteroids observed to date by spacecraft, and suggest that Mercury’s regolith contains a compositionally distinct component as compared to the Moon.
When images are acquired by spacecraft of a planetary surface, they are not all acquired at the same illumination and viewing geometries. Nor are they typically acquired under the standard illumination and viewing geometries as laboratory spectral measurements of minerals, to which they are often compared to derive surface composition. Photometric models are used to standardize images obtained at a variety of illumination and viewing geometries, thus facilitating the construction of image mosaics and maps in addition to the comparison with laboratory measurements. Application of these photometric models are also used to infer the physical properties of the upper surface, such as porosity and roughness. Two of the most commonly used models, those of Hapke and Kaasalainen-Shkuratov, were applied to the 8-color image data set acquired by the MESSENGER spacecraft of Mercury’s surface. This study examined which model would provide the better corrections for constructing the 8-color global mosaic. In addition the results were used to infer properties of Mercury’s regolith. 
Above, these mosaics of the Caloris Basin were constructed using the Hapke Basic model (top) and the KS3 model (bottom) to provide the photometric correction to standard illumination and viewing geometries of 30°, 0°, 30° in incidence, emission, and phase angles, respectively. Both mosaics are constructed from the 1000-nm filter images and stretched to the same dynamical range. 
March 14, 2016

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