J/A+A/686/A203       Phobos photometric properties            (Fornasier+, 2024)

Phobos photometric properties from Mars Express HRSC observations. Fornasier S., Wargnier A., Hasselmann P.H., Tirsch D., Matz K.-D., Doressoundiram A., Gautier T., Barucci M.A. <Astron. Astrophys. 686, A203 (2024)> =2024A&A...686A.203F 2024A&A...686A.203F (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Planets ; Photometry ; Optical ; Infrared Keywords: satellites: individual: Phobos - methods: data analysis - methods: observational - techniques: photometric Abstract: This study aims to analyze Phobos' photometric properties using Mars Express mission observations to support the Martian Moons exploration mission (MMX) devoted to the investigation of the Martian system and to the return of Phobos samples. We analyzed resolved images of Phobos acquired between 2004 and 2022 by the High Resolution Stereo Camera (HRSC) on board the Mars Express spacecraft at a resolution ranging from ∼30m/px to 330m/px. We used data acquired with the blue, green, red, and IR filters of HRSC and the panchromatic data of the Super Resolution Channel (SRC). The SRC data are unique because they cover small phase angles (0.2-10°), permitting the investigation of the Phobos opposition effect. We simulated illumination and geometric conditions for the different observations using the Marx Express and the camera spice kernels provided by the HRSC team. We performed photometric analysis using the Hapke model for both integrated and disk-resolved data. The Phobos phase function is characterized by a strong opposition effect due to shadow hiding, with an amplitude and a half-width of the opposition surge of 2.28±0.03 and 0.0573±0.0001, respectively. Overall, the surface of Phobos is dark, with a geometric albedo of 6.8% in the green filter and backscattering. Its single-scattering albedo (SSA) value (7.2% in the green filter) is much higher than what has been found for primitive asteroids and cometary nuclei and is close to the values reported in the literature for Ceres. We also found a surface porosity of 87%, indicating the presence of a thick dust mantle or of fractal aggregates on the top surface. The SSA maps revealed high reflectance variability, with the blue unit area in the northeast Stickney rim being up to 65% brighter than average, while the Stickney floor is among the darkest regions, with reflectance 10 to 20% lower than average. Photometric modeling of the regions of interest selected in the red and blue units indicates that red unit terrains have a stronger opposition effect and a smaller SSA value than the blue ones, but they have similar porosity and backscattering properties. The HRSC data provide a unique investigation of the Phobos phase function and opposition surge, which is valuable information for the MMX observational planning. The Phobos opposition surge, surface porosity, phase integral, and spectral slope are very similar to the values observed for the comet 67P and for Jupiter family comets in general. Based on these similarities, we formulate a hypothesis that the Mars satellites might be the results of a binary or bilobated comet captured by Mars. Description: We investigated Phobos' photometric properties using the Mars Express HRSC data. The Mars Express mission was launched in 2003 by the European Space Agency, and it is principally devoted to the study of the Martian surface and atmosphere, especially to the observation of traces of water on the planet's surface. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file tableb1.dat 33 107 Observing conditions for the blue filter images used in our analysis tableb2.dat 33 117 Observing conditions for the green filter images used in our analysis tableb3.dat 33 94 Observing conditions for the red filter images used in our analysis tableb4.dat 33 110 Observing conditions for the IR filter images used in our analysis tableb5.dat 41 299 Observing conditions for the SRC camera images used in our analysis of the Phobos opposition surge -------------------------------------------------------------------------------- Byte-by-byte Description of file: tableb[1234].dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 23 A23 "datime" Time Time of the observations 24 A1 --- n_Time [*] Note on Time (1) 26- 29 F4.1 deg Angle Phase angle 31- 33 I3 m/pix Res Spatial resolution -------------------------------------------------------------------------------- Note (1): Images in bold and indicated by an asterisk were utilized to generate the single scattering albedo map in the blue filter shown in Fig. A.3 -------------------------------------------------------------------------------- Byte-by-byte Description of file: tableb5.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 23 A23 "datime" Time Time of the observations 25- 29 F5.2 deg Angle Phase angle 31- 35 F5.3 au Dhelio Heliocentric distance 37- 41 F5.2 m/pix Res Spatial resolution -------------------------------------------------------------------------------- Acknowledgements: Sonia Fornasier, sonia.fornasier(at)obspm.fr
(End) Patricia Vannier [CDS] 12-Mar-2024
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