Presentation Information
[PPS07-P03]Surface Gravity and Slope Environment of Phobos Considering Physical Libration
*Hiroshi Kikuchi1, Ryodo Hemmi2, Yuta Shimizu3, Naoyuki Hirata4, Tomohiro Takemura3, Makito Kobayashi3, Sho Sasaki5, Koji Wada6, Haruhisa Tabata2, Naru Hirata7, Hideaki Miyamoto3 (1.Gakushuin university, 2.JAXA, 3.University of Tokyo, 4.Kobe University, 5.University of Osaka, 6.Chiba Institute of Technology, 7.University of Aizu)
Keywords:
Phobos,Surface Gravity Field,Dynamic Slope
Within the MMX mission, the Surface Science and Geology Science Strategy Team (SSGSST), led by Miyamoto, oversees investigations of Phobos’ surface environment. As part of SSGSST, our subgroup evaluates the surface gravity field and slope environment of Phobos, supporting landing-site safety and surface-process assessments. The effective surface gravity of Phobos—accounting for Martian tidal and centrifugal forces—varies periodically over its orbit [1]. In addition, Phobos undergoes physical libration driven by the Martian gravity gradient, causing attitude oscillations that can locally perturb the direction of the effective gravity vector and, consequently, the dynamic slope. This effect introduces an important source of uncertainty in assessing landing-site safety and the stability of surface materials.
To address this, we developed a computational pipeline to quantify time-series variations of the surface gravity vector and dynamic slope on a global scale using an attitude model that includes physical libration. Using the latest Ernst (v003) shape model [1] together with SPICE kernels, we computed the temporal evolution of the gravity vector and dynamic slope over ~12 million surface polygons throughout one Phobos orbital period (~7.6 h), referenced to an epoch of 1 January 2029. We also developed a methodology for high-resolution surface gravity calculations on local digital elevation models (DEMs) tailored to landing operations. For localized, high-resolution DEMs, achieving high-precision gravity estimates requires methodological refinements beyond global polygon-based computations, including dedicated treatments of boundary conditions and resolution mismatches.
Our analysis shows that, relative to conventional libration-free models, the global mean range of dynamic-slope variation over one orbital period remains largely unchanged; however, localized differences in dynamic slope can reach up to ~5° in regions where topography and libration phase are strongly coupled. As a result, some areas previously classified as “safe” (e.g., dynamic slope < 10°) under conventional models may warrant re-evaluation when libration effects are included. Finally, a preliminary application of our local-DEM gravity framework to asteroid Bennu yielded trends consistent with previously published gravity and slope assessments. We will present the computational methodology and quantitative evaluations of how physical libration affects localized gravity and slope environments.
References:
[1] Ernst, C. M., et al. (2023), High-resolution shape models of Phobos and Deimos from stereophotoclinometry, Earth, Planets and Space, 75, 103.
[2] Ballouz, R.-L., et al. (2019), Surface refreshing of Martian moon Phobos by orbital eccentricity-driven grain motion, Nature Geoscience, 12(4), 229–234.
To address this, we developed a computational pipeline to quantify time-series variations of the surface gravity vector and dynamic slope on a global scale using an attitude model that includes physical libration. Using the latest Ernst (v003) shape model [1] together with SPICE kernels, we computed the temporal evolution of the gravity vector and dynamic slope over ~12 million surface polygons throughout one Phobos orbital period (~7.6 h), referenced to an epoch of 1 January 2029. We also developed a methodology for high-resolution surface gravity calculations on local digital elevation models (DEMs) tailored to landing operations. For localized, high-resolution DEMs, achieving high-precision gravity estimates requires methodological refinements beyond global polygon-based computations, including dedicated treatments of boundary conditions and resolution mismatches.
Our analysis shows that, relative to conventional libration-free models, the global mean range of dynamic-slope variation over one orbital period remains largely unchanged; however, localized differences in dynamic slope can reach up to ~5° in regions where topography and libration phase are strongly coupled. As a result, some areas previously classified as “safe” (e.g., dynamic slope < 10°) under conventional models may warrant re-evaluation when libration effects are included. Finally, a preliminary application of our local-DEM gravity framework to asteroid Bennu yielded trends consistent with previously published gravity and slope assessments. We will present the computational methodology and quantitative evaluations of how physical libration affects localized gravity and slope environments.
References:
[1] Ernst, C. M., et al. (2023), High-resolution shape models of Phobos and Deimos from stereophotoclinometry, Earth, Planets and Space, 75, 103.
[2] Ballouz, R.-L., et al. (2019), Surface refreshing of Martian moon Phobos by orbital eccentricity-driven grain motion, Nature Geoscience, 12(4), 229–234.