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Simultaneous Double Transits of Phobos and Deimos as Seen from the Martian Surface: A Millennium Catalogue

Samuel Cody

Abstract

I present the first systematic catalogue of simultaneous solar transits of both Phobos and Deimos as observed from the surface of Mars. Using the JPL mar099 ephemeris (Brozovic et al. 2025) and SPICE toolkit, I searched the millennium 1600-2600 CE for epochs at which both Martian moons project onto the solar disc at the same instant for at least one surface location. I identify 8565 grazing double transits, 49 partial-overlap double transits (both moons simultaneously on the solar disc, with at least one partially cut off by the limb), and 17 full double transits in which both moons lie wholly within the solar disc at the same moment. All events cluster tightly around the Martian equinoxes (Ls ~ 0 deg or 180 deg) and within +/- 9 deg of the equator, reflecting the near-equatorial orbital inclinations of both moons. I derive a hard theoretical latitude limit of +/- 13.1 deg beyond which simultaneous double transits are geometrically impossible. The next observable partial double transit, excluding less prominent grazing events, is predicted for 2034 April 17. The next full double transit, with both moons wholly inside the solar disc, with a gap between each silhouette and the solar limb, occurs on 2118 November 20. The geometries for both these events were confirmed with JPL Horizons. I provide uncertainty estimates based on the Brozovic et al. covariance model, with predicted position errors growing from ~1 km for near-term events to ~600 km at the catalogue boundaries, and note that the JAXA MMX mission (~2031) will dramatically reduce uncertainties for all post-2030 predictions.

Simultaneous Double Transits of Phobos and Deimos as Seen from the Martian Surface: A Millennium Catalogue

Abstract

I present the first systematic catalogue of simultaneous solar transits of both Phobos and Deimos as observed from the surface of Mars. Using the JPL mar099 ephemeris (Brozovic et al. 2025) and SPICE toolkit, I searched the millennium 1600-2600 CE for epochs at which both Martian moons project onto the solar disc at the same instant for at least one surface location. I identify 8565 grazing double transits, 49 partial-overlap double transits (both moons simultaneously on the solar disc, with at least one partially cut off by the limb), and 17 full double transits in which both moons lie wholly within the solar disc at the same moment. All events cluster tightly around the Martian equinoxes (Ls ~ 0 deg or 180 deg) and within +/- 9 deg of the equator, reflecting the near-equatorial orbital inclinations of both moons. I derive a hard theoretical latitude limit of +/- 13.1 deg beyond which simultaneous double transits are geometrically impossible. The next observable partial double transit, excluding less prominent grazing events, is predicted for 2034 April 17. The next full double transit, with both moons wholly inside the solar disc, with a gap between each silhouette and the solar limb, occurs on 2118 November 20. The geometries for both these events were confirmed with JPL Horizons. I provide uncertainty estimates based on the Brozovic et al. covariance model, with predicted position errors growing from ~1 km for near-term events to ~600 km at the catalogue boundaries, and note that the JAXA MMX mission (~2031) will dramatically reduce uncertainties for all post-2030 predictions.
Paper Structure (30 sections, 6 equations, 2 figures)

This paper contains 30 sections, 6 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Geometry and Definitions
  3. Single Transit Geometry
  4. Observer Parallax
  5. Theoretical Latitude Limit
  6. Search Methodology
  7. Ephemeris
  8. Search Algorithm
  9. Phase 1: Conjunction Timing.
  10. Phase 2: Shadow Geometry.
  11. Phase 3: Overlap Classification.
  12. Validation with JPL Horizons
  13. Results
  14. Census
  15. Seasonal Clustering
  16. ...and 15 more sections

Figures (2)

  • Figure 1: Distribution of simultaneous double transits in areocentric solar longitude $L_s$. All three overlap classes---grazing (E), partial (P), and full (F)---cluster tightly around the Martian equinoxes ($L_s \approx 0^\circ$ and $180^\circ$), reflecting the near-equatorial orbital planes of both moons. Each histogram is peak-normalised.
  • Figure 2: Geographic distribution of predicted simultaneous double transits of Phobos and Deimos on the Martian surface, 1800--2200 CE, shown on an MGS TES Lambert Albedo basemap in Simple Cylindrical projection. Ellipses indicate $2\sigma$ position uncertainties propagated from the Brozović et al. (2025) covariance model (Equations \ref{['eq:sigma_lon']}--\ref{['eq:sigma_lat']}): events near the 2019 data arc have sub-km errors (nearly circular markers), while those at the catalogue boundaries show strongly elongated east--west ellipses reflecting the dominant along-track (longitude) uncertainty. Solid ellipses denote full-overlap (F) events; dashed ellipses denote partial-overlap (P). Coral-red and blue mark future events; amber marks past events. Green triangles show the locations of all successful Mars landers. The dashed cyan band marks the $\pm 13.1^\circ$ theoretical latitude limit. See Appendix Tables \ref{['tab:fc_fo_full']} and \ref{['tab:po_full']} for the full catalogues.