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Investigating 1064-nm Albedo along Mercury’s Hot and Cold Poles


Deutsch Ariel


NASA Ames Research Center / USRA


Auteur(s) supplémentaire(s)Gregory A. Neumann, David Trang, Noam Izenberg, Brett W. Denevi
Institution(s) supplémentaire(s)NASA GSFC, U. Hawaii, JHU/APL, JHU/APL


Because of its eccentric orbit and 3:2 spin-orbit resonance, Mercury has a surface thermal environment that varies not only with latitude, but also with longitude. “Hot-pole” longitudes, centered at 0°E and 180°E, reach maximum temperatures as much as 130 K warmer than “cold-pole” longitudes (90°E and 270°E) due to enhanced solar insolation [1]. Here we investigate whether these longitudinal variations in surface temperature may result in differential space weathering, leading to variations in the 1064-nm normal albedo of the surface.

Space weathering can influence the regolith’s optical surface in many ways, including lowering albedo [e.g., 4]. We analyze how the 1064-nm albedo varies along hot- and cold-pole longitudes using zero-phase angle MESSENGER Mercury Laser Altimeter (MLA) data. We present a new empirical correction for the MLA data, accounting for (1) drift and the natural degradation of the laser, (2) obliquity to mitigate bias associated with longitude, since a more oblique geometry was required to obtain ranges at the equatorial hot-pole longitudes, and (3) the receiver response in the case of high emission angle and extended range, both of which enlarge the laser footprint and cause the return pulse amplitude to decrease. The third of these corrections was applied more strongly to oblique data as the mission was extended and the laser strength and beam quality declined. The result is a photometrically near-uniform dataset independent of solar illumination geometry.

Previous analyses revealed possible longitude-dependent space-weathering effects on Mercury, including a decrease in optical maturity at the cold poles [2]. Visible and Infrared Spectrograph (VIRS) data are consistent with an enhancement of microphase Fe at equatorial latitudes (<60°) relative to poleward latitudes, and an enhancement of microphase Fe at hot-pole longitudes relative to cold-pole longitudes [3]. These trends may be consistent with Ostwald ripening, a process where nanophase opaque particles grow to produce larger (microphase) particles at high temperatures, which has been predicted to occur at Mercury’s equatorial latitudes and hot-pole longitudes [4]. Nanophase opaque particles are a product of space weathering and result in lower albedo [e.g., 4].

Our initial results indicate the 1064-nm albedo of hot poles is lower than that of cold poles at low-mid latitudes. Ongoing work includes analyzing albedo variations with respect to geologic unit and surface age, and comparing results with Mercury Dual Imaging System (MDIS) reflectance parameters. Preliminary results present an interesting test for the BepiColombo Laser Altimeter (BELA), which will measure albedo at the same wavelength as MLA and will provide more complete observations of equatorial regions.

[1] A.R. Vasavada et al. (1999) Icarus 141, 179–193. [2] J.T. Wilson et al. (2019) JGRP 124, 721–733. [3] D. Trang et al. (2017) Icarus 293, 206–217. [4] S.K. Noble & C.M. Pieters (2003) SSR 37, 31–35.

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