The Rings of Mars: Awaiting Discovery?

Phobos and Deimos have long been suspected of creating rings of dusty debris around Mars. Soter [1] first predicted that such rings should exist and numerous theoretical studies have been published since then. Several attempts have been made to directly detect the rings but none have been successful. Additionally, observations from the plasma instruments onboard the Phobos-2 spacecraft have been controversially attributed to Martian rings. We describe proposals to directly observe the rings using the Spitzer Space Telescope and indirectly detect them using data from the magnetometer (MAG/ER) onboard Mars Global Surveyor (MGS). Theoretical expectations for the rings: Phobos and Deimos are both small, irregularly shaped satellites that orbit relatively close to Mars. Deimos has an approximate radius of 6 km and has a 23,000 km wide orbit (Mars' radius is 3393 km). Phobos has a radius of 10 km and has a 9400 km wide orbit (or within two Mars radii of the Martian surface). Based on their sizes, shapes, and spectral compositions they are thought to be captured asteroids, although dynamical models indicate that such a capture scenario is extremely unlikely; there is currently no resolution to this apparent paradox [2]. The basic mechanism for the origin of the putative rings is the impact of the moons by high speed interplanetary material which then lofts dust into orbit around Mars. Dynamical forces then distribute the dust into circumplanetary rings or tori. Similar dusty rings have been found to be produced by small satellites at Jupiter, Saturn, Uranus, and Neptune (cf. [3] for a recent review). Krivov and Hamilton [4] give the most recent comprehensive description of the theoretical expectations for the rings. They find that the Deimos ring might be more accurately called a torus with a height of about 7000 km and a width of 50,000 km. The Phobos ring is expected to be much thinner (about 400 km) and only extend to a maximum of about 30,000 km. See Figure 1. Beyond their basic shape, the rings are expected to have various morphological asymmetries due to the physics of their production and maintenance [5]. The dynamical interplay between the precession of particles due to Mars' oblateness, the originating bodies orbital distance from Mars, and the effects of solar radiation are expected to cause the Phobos ring to offset toward the Sun by approximately one Mars radius and the Deimos torus to be offset toward the Sun Figure 1 Distribution of optical fluxes from the Phobos ring (top) and the Deimos torus (bottom) as seen from Earth in December 2007. Adapted from [6].