Composition of Surface Materials on the Moons of Mars

doi:10.1016/j.pss.2014.02.008

Planetary and Space Science

Volume 102, 1 November 2014, Pages 144-151

https://doi.org/10.1016/j.pss.2014.02.008 Get rights and content

Highlights

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The composition of Phobos constrains its origin.
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Two distinctly different materials are seen across the surface of Phobos.
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The unique regolith of Phobos and Deimos dominate observations.
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The composition of Phobos and Deimos remains ambiguous.

Abstract

The two small asteroid-like bodies orbiting Mars, Phobos and Deimos, are low albedo and exhibit similar visible to near-infrared spectra. Determining the origin of these moons is closely tied to determining their composition. From available spectroscopic data Phobos exhibits two distinct types of materials across its surface, and data from both Mars Express and Mars Reconnaissance Orbiter have provided additional details about the properties of these materials and their spatial relation to one another. Although no prominent diagnostic absorptions have been detected, systematic weak features are seen in some data. An extensive regolith is observed to have developed on both moons with characteristics that may be unique due to their special environment in Mars orbit. Understanding the character and evolution of the regolith of Phobos and Deimos is central to interpreting the moons׳ physical and optical properties. The cumulative data available for compositional analyses across the surface of Phobos and Deimos, however, remain incomplete in scope and character and ambiguous in interpretation. Consequently the composition of the moons of Mars remains uncertain.

Section snippets

Introduction and background

The composition of Phobos and Deimos is one of the most fundamental properties of these small moons of interest to both science and exploration. Knowledge of their composition would significantly constrain their origin: Are they composed of material derived from or near Mars? Or do they represent primitive material from other parts of the solar system that has been captured in the Mars environment? Such knowledge would also resolve at least some issues about whether these bodies can provide

Two types of material on Phobos

Examples of recent spectral data for Phobos acquired by Mars Express and MRO are shown in Fig. 2, Fig. 3. The orbit of MRO is within the orbit of Phobos and measurements are thus limited to the Mars-facing side of Phobos. However, since the orbit of Mars Express can extend beyond the orbit of Phobos, it has the capability to acquire data for other views. Examples of extended coverage by HRSC on Mars Express are shown in Fig. 4, Fig. 5.

Visible to near-infrared spectra acquired with imaging

Regolith development and space weathering issues in the Mars environment

Unlike other small bodies found in the asteroid belt or Near-Earth-Objects, Phobos and Deimos are bound in orbits around a terrestrial planet, Mars. Phobos׳ orbital period is only 7 h 39 min, so the moon rises and sets more than two times during a single Mars day. Phobos is in a phase locked orbit and keeps the same face toward Mars, which is only ~6000 km away. As is evident from the color data summarized above, the surface properties of Phobos are not uniform and this may or may not be related

Age and stratigraphy of Phobos

Evaluating and dating the surface of solar system bodies through analysis of the density of superposed impact craters has substantially evolved over the last few decades (e.g., Neukum et al., 2001). Results from modern spacecraft data for Phobos (Schmedemann et al., 2014) suggest that not only is the surface of Phobos ancient (~4.3–3.7 Ga), but that the large crater Stickney itself is no younger than ~3.5 Ga. If these dates are a lower bound to how long Phobos has been a moon of Mars, they also

Current understanding and need

Given the observed spatial relationships, it is difficult to formulate a single model through which the red unit is largely a depositional unit or is derived from the blue unit by some form of space weathering. A preferred model for Phobos that is consistent with observational data now available involves the existence of inherent compositional heterogeneity on the scale of 100 m׳s to several km as originally proposed by Murchie et al. (1991) and recently supported by Basilevsky et al. (2014).

Exploration of Phobos and Deimos

Although the scientific investigation of Phobos and Deimos has certainly benefited from the group of spacecraft in orbit studying Mars, there are enormous gaps in our information about these satellites. In planning future exploration of the enigmatic moons of Mars, there are two key recommendations that are most relevant for understanding the composition and evolution of these small bodies:

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A global assessment of composition is needed to identify and evaluate the character and geologic context

Acknowledgements

We appreciate the support of the International Space Science Institute for valuable discussions among authors of different chapters during early phases of manuscript preparation. We thank Shi Xian, Ken Ramsley, V. Patsyn, and Marita Wählisch for providing figure data from their ongoing research. CMP was supported through the NASA Lunar Science Institute and Solar System Exploration Research Virtual Institute during these analyses.

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Space weathering can also modify the optical properties (e.g., slope, albedo, band intensities) of dark airless bodies, like Phobos and Deimos, depending on its mineralogical composition, suggested by laboratory reproducing experiments and spectral measurements (e.g., Kaluna et al., 2016; Hiroi et al., 2020; Matsuoka et al., 2020). Phobos and Deimos are darker than even most space weathered lunar materials (Pieters et al., 2014). Space weathering effects (e.g., solar wind sputtering vs. impact vaporization/deposition by micrometeoroids) can contribute to spectral variation in Phobos and Deimos, and the degree to which its processes operate varies from an airless body to another in the solar system.
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