Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons

Abstract

International audiencePhobos and Deimos, the two small satellites of Mars, are thought either to be asteroids captured by the planet or to have formed in a disc of debris surrounding Mars following a giant impact. Both scenarios, however, have been unable to account for the current Mars system. Here we use numerical simulations to suggest that Phobos and Deimos accreted from the outer portion of a debris disc formed after a giant impact on Mars. Larger moons are formed from the denser inner disc and migrate outwards due to gravitational interactions with it. The resulting orbital resonances spread outwards and gather the dispersed debris, facilitating accretion into two satellites of sizes similar to Phobos and Deimos. The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. Our results clarify why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor

A dynamical context for the origin of Phobos and Deimos

“…This can potentially be mitigated depending on where the mass the deposited relative to the Roche limit (Rosenblatt & Charnoz 2012) or if there is substantial evolution in the Moon system that leaves only a fraction of the original mass visible today (e.g. Rosenblatt et al 2016;Hesselbrock & Minton 2016). Our results suggest that such evolution can be the consequence of an extended series of external perturbations.…”

“…If we postulate that the last giant impact experienced by each TMA produces a disk of debris that assembles to form a Moon (Hartman & Davis 1975;Cameron & Ward 1976;Craddock 2011;Rosenblatt & Charnoz 2012;Citron, Gendra & Ida 2015;Canup & Salmon 2016;Rosenblatt et al 2016;Hyodo et al 2017), then we can examine the degree to which the continued perturbations resulting from the close passages of other bodies will disturb such a disk. This leads to the hypothesis that the much smaller mass of the Martian moons is due to the dynamical erosion of the original reservoir.…”

“…Rosenblatt & Charnoz 2012) could yield small enough grains to match the observations, but such extended disks are not favoured by the simulations of impacts (Citron et al 2015, Hyodo et al 2017). This has led to revised scenarios in which the remaining satellites are formed from the outer edge of the original disk and were shepherded outwards by larger moons that subsequently migrated inwards due to tides and were disrupted (Rosenblatt et al 2016;Hesselbrock & Minton 2016).…”

Formation of Massive Rocky Exomoons by Giant Impact

“…Moreover, there is no analytic scaling relationship to predict the outcome of an impact between two planet-scale objects in the parameter space conducive to forming large satellites. Recent work shows that the creation of large Ms/M pl is not assured in a giant impact: depending on impact geometry and speed, giant impacts can also yield systems with much smaller Ms/M pl , such as Phobos and Deimos at Mars (Citron et al 2015;Rosenblatt et al 2016) and the small ice-rich moons of the Kuiper Belt Object Haumea (Leinhardt et al 2010).…”

Temporary Capture of Asteroids by an Eccentric Planet

“…Mars has two satellites: Phobos and Deimos. Two major theories of the origin of these satellites are (1) in situ formation through accretion of an impact-generated debris by a large impact inferred from the Borealis basin (e.g., Citron et al 2015;Rosenblatt et al 2016) and (2) capture of asteroids (e.g., Burns 1978). While the large impact model may explain the circular, noninclined orbits of Phobos and Deimos, which is not easily explained by the capture origin, the surface characteristics of the satellites are similar to those of primitive asteroids.…”