Phobos Photometric Properties From Mars Express HRSC Observations (Comet Capture?)

By Keith Cowing
Press Release
May 30, 2024
Filed under , , , ,
Phobos Photometric Properties From Mars Express HRSC Observations (Comet Capture?)
Examples of Phobos’ reflectance factor for areas observed at different phase angles. The I/F value has been corrected for the illumination conditions using the Lommel-Seeliger law. — astro-ph.EP

This study aims to analyze Phobos’ photometric properties using Mars Express mission observations to support the Martian Moons eXploration mission.

We analyzed resolved images of Phobos acquired between 2004 and 2022 by the HRSC and the SRC cameras on board the Mars Express spacecraft. We performed photometric analysis using the Hapke model for both integrated and disk-resolved data.

The Phobos phase function has 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.

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.

Images showing the flux increasing with a decreasing phase angle from images acquired during the orbit K076 (top panel) and M247 (bottom panel) with SRC. The flux is in DU s−1 ; no other normalization was applied here (the heliocentric distance was the same for a given orbit). — astro-ph.EP

SRC map showing the relative variation of the albedo compared to the average value (7.1%). Top: Relative albedo variation from the image 2019-11-17T03:27:01 acquired during K076 orbit. Bottom: Relative albedo variation from the image 2020-09-26T02:18:40 acquired during L155 orbit. — astro-ph.EP

Comparison of Phobos photometric properties with those of other Solar System bodies. Top: Phobos reflectance ratio (at α=0.3o over α=5o ) versus geometric albedo compared to the values observed for different classes of asteroids (following the Tholen & Barucci (1989) classification scheme), derived from Belskaya & Shevchenko (2000), and comet 67P (Fornasier et al. 2015). Bottom: Figure adapted from Verbiscer et al. (2022) showing the phase integral versus geometric albedo for dark Solar System bodies. The Phobos value derived in this paper from the green filter and using the Hapke modeling of resolved photometry is represented by a red star (uncertainties are within the star symbol). Literature data of Phobos (q=0.30) from Simonelli et al. (1998) are also reported. We refer to Verbiscer et al. (2022) (their Fig. 11) for the references of the individual phase integral and geometric albedo values of the objects. — astro-ph.EP

S. Fornasier, A. Wargnier, P.H. Hasselmann, D. Tirsch, K.-D. Matz, A. Doressoundiram, T. Gautier, M.A. Barucci

Comments: 30 pages, 16 figures, accepted for publication in Astron. & Astroph
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2403.12156 [astro-ph.EP] (or arXiv:2403.12156v1 [astro-ph.EP] for this version)
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Submission history
From: Sonia Fornasier
[v1] Mon, 18 Mar 2024 18:12:48 UTC (18,769 KB)

Astrobiology, Astrochemistry

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