Phobos and Deimos polarimetric observations interpreted through analogue measurements

Introduction:

The Martian Moon eXploration (MMX) Japanese mission is scheduled to be launched in 2026 to explore the martian moons system. The mission will orbit and land on Phobos, observe Deimos and retrieve and return more than 10 grams of regolith from the surface of Phobos back to Earth by 2031 [1, 2]. The scientific goals of the mission are to better understand the formation and evolution of the martian moons, to decipher their origin, and observe the surface and atmosphere of Mars from a different vantage point.

The origin of Phobos and Deimos, whether as asteroids by capture, or fragments of Mars by impacts, remains debated in the community (see [3] and references therein). The peculiar dynamics of their orbits seems consistent with an origin from impacts. However, their infrared spectra are most consistent with primitive asteroids with a low albedo, red slope and few if any mineralogical absorption features [4].

Method:

Polarimetric observations are a useful tool to classify and decipher the evolution of planetary surfaces [5] and has been especially used to study the properties of asteroid families [6]. Few observations in polarization have been made in the 1970s of Phobos and Deimos by the Mariner 9 space mission and ground-based observations [7, 8]. At large phase angles from 74 to 81 degrees, those observations were done in the visible at 570 nm, while low phase angle observations below about 30 degrees were done in the ultraviolet at 233 nm. We can directly compare those measured values to the ones observed for different types of asteroids from the ground in the U and V astronomical filters [9, 10] as shown in Figure 1.

Then the PROGRA2 experiment is a polarimetric goniometer that studied the light-scattering properties of dust particles of various size distributions under Earth’s gravity either deposited or lifted by an air-draught, as well as levitated under microgravity conditions in dedicated flight campaigns. The experiment was used to study the polarimetric behavior of various astronomical analogues including crushed meteorite samples that were levitated or deposited [11]. These meteorites included: Allegan, an H5 ordinary chondrite analogue to S-type asteroids, Allende, a CV3 primitive carbonaceous chondrite analogue to C-, L- or K-type asteroids (crushed in two different grain sizes: fine sieved <50microns and large sieved <500microns), and Orgueil, a CI1 carbonaceous chondrite possibly linked to C-, B-, X- or D-type asteroids, although terrestrially weathered. Additional analogues of primitive asteroids surfaces were also generated using mixtures of olivine and iron sulfide (FeS) in submicrometre-size powders  [12] The phase functions of Phobos and Deimos can be compared with those of deposited meteorites as shown in Figure 2.

 

Figure 1. Comparison of the polarization measurements of the surface of Phobos and Deimos with the observations from primitive asteroids (types C, D, P) and the S-type asteroids. Top: data of the asteroids taken in the V filter, showing the specific phase curve of Ryugu C-type asteroid. Bottom: zoom on the negative branch of the phase curves. Asteroidal data taken in the U filter. The Phobos and Deimos polarization data in the UV is most consistent with the primitive types asteroids phase curves (red).

 

 

 

 

 

Figure 2. Comparison of the polarization observations of Phobos and Deimos with deposited crushed meteorite samples (Allegan, Allende and Orgueil) [11] and hyperfine olivine and FeS mixtures (20% and 30% olivine volume fraction) [12]. Top: full phase curve comparison. Bottom: zoom on the negative branch between 0 and 40 degrees phase angles. (in the legend, for a given color the solid line and the dashed line correspond to the first and the second name respectively)

Discussion:

Comparison of the phase curves with the ones observed for asteroids clearly confirms the link with C-type and primitive asteroids. At large phase angles the Phobos and Deimos data points fall on the C-type asteroidal trend. Lower than the Ryugu phase curve whose high polarization was interpreted to be due to the presence submillimeter-sized large grains on its surface layer [10]. At low phase angles also, the data points in the UV are most consistent with the trend of C-type asteroids, with a lower Pmin value than S-type asteroids.

Comparison to the meteorite samples and deposited olivine and FeS mixtures is most consistent with the deposited Allende large grains sizes sample (sieved to <500 microns) for both the small and large phase angles. While other samples appear very close such as Orgueil or the Olivine and FeS mixtures, their negative branch characteristics (P_min and P₀ values) are inconsistent with the ones observed for Deimos.

Though the experimental curves for Allende have been measured in the red (630 nm) the color trend is expected to be small. Similar maximum in polarization (i.e. P_max) values were obtained experimentally for dark particles deposited or in levitation showing that multiple scattering is negligible [11].

These results are then consistent with large particle grains (mm in size) of primitive meteorites such as carbonaceous chondrites constituting the regolith of both Phobos and Deimos.

Conclusions:

The polarization phase curves of Phobos and Deimos are consistent with primitive asteroids of types C, D and P, just like their infrared spectra. Comparison with crushed deposited meteorite samples and analogues is indicative of the presence of relatively large grains (mm sized) of primitive materials on the surface the martian moons.

References:

[1] Kuramoto K. et al. EPS 74.1 (2022): 12.

[2] Kawakatsu Y. et al. Acta Astronautica 202 (2023): 715-728.

[3] Kuramoto K. AREPS 52 (2024).

[4] Fraeman A. et al. Icarus 229 (2014): 196-205.

[5] Kolokolova, Hough, Levasseur-Regourd eds. "Polarimetry of stars and planetary systems." (2015) CUP.

[6] Cellino A. et al. MNRAS 455.2 (2016): 2091-2100.

[7] Noland M. et al. Icarus 20.4 (1973): 490-502.

[8] Zellner B. AJ, Vol. 77, p. 183 (1972) 77 (1972): 183.

[9] Lupishko D. Ed. (2022). Asteroid Polarimetric Database V2.0. urn:nasa:pds:asteroid_polarimetric_database::2.0. NASA PDS; doi: 10.26033/hyf9-4762.

[10] Kuroda D. et al. APh. Let. 911.2 (2021): L24.

[11] Hadamcik E. et al. MNRAS 520.2 (2023): 1963-1974.

[12] Sultana R. et al. Icarus 395 (2023): 115492.