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