Deep Impact?

Since their discovery in 1877 the origin of Phobos and Deimos has been a mystery. While they look like small asteroids, their orbits are not compatible with gravitational capture. So how did they come to be? Thomas Ronnet tells us why his work on the composition of the moons gives vital clues…

Phobos and Deimos are two small moons orbiting around Mars. Their origin has been something of a mystery – until recently.

Due to their irregular shapes and low masses, the moons have long been associated with small asteroids that would have been gravitationally captured by the planet. Moreover, some observations of the surface of Phobos in the visible-near infrared range of the light spectrum seemed to support this theory. The features observed indeed resemble that of D-type asteroids (a type of asteroids found in the outer part of the asteroid belt). The theory of gravitational capture, however, suffers a major caveat – the orbit of Phobos and Deimos. While there exist examples of captured objects in the solar system, notably the outer small satellites of Jupiter, none of them have circular orbits aligned with the equatorial plane of the parent planet just as our Moon or Phobos and Deimos. In fact, the probability that even one object is captured with such an orbit is extremely small. It is thus practically impossible that Mars has gravitationally captured both Phobos and Deimos onto circular and co-planar orbits. So where do the satellites of Mars come from? Over the past few years, some scientists have proposed that Phobos and Deimos could have formed following a giant impact between Mars and another quite large object – similar to the event thought to have given birth to the Moon. This scenario is attractive because the debris that would be ejected from the collision would naturally orbit onto circular orbits in the equatorial plane of the planet. If Phobos and Deimos were born from such a disk of debris, it is only natural that their orbits are circular and in the same plane. However, the physical properties of the moons are not straightforwardly accounted for in the large impact theory, making the origin of the satellites a longstanding problem.

[caption id="attachment_55894" align="alignnone" width="620"] Artist's impressions of how Phobos and Deimos were created[/caption]

In our study¹, we inferred what the composition of the objects formed in a disk of debris around Mars would be and what spectral features they would exhibit. We could then compare our results to the observations of Phobos so far available. We used impactors with different compositions and considered different mixing ratios between material from the impactor and material from Mars' mantle inside the debris disk. We had to study several cases because of our current lack of knowledge regarding the nature of the impacting object and the proportion of material from each body that would be set in orbit around the planet. After such a violent event, the rocks that are ejected are heated to such high temperatures that they are mostly vaporised. The gas then cools down and solids condense again to form a disk of debris surrounded by an atmosphere of silicate vapour – solids from which bodies such as Phobos and Deimos would have formed. We thus inferred from the initial chemical composition of the disk what minerals would have crystallised within it. It is these minerals that possess characteristic spectral absorption bands we can compare with the observations of Phobos. We have found that Phobos and Deimos could not have formed similarly to the Moon, that is, in the inner parts of the disk of debris where the temperature and pressure are so high that the rocks are melted into a magma. If they did, Phobos and Deimos would have low internal porosity, incompatible with their very low densities that suggest these bodies are very porous, and their spectra would not resemble those observed. The only possibility is that Phobos and Deimos actually formed in the outer parts of the disk, where the pressure is very low. Under these conditions, the solids that eventually formed the moons would be very fine dust grains with sizes in the order of 1 micrometer or less. Such a fine grained structure would explain both the spectra of Phobos as well as the low densities of the two martian moons.

The theory of gravitational capture, however, suffers a major caveat – the orbit of Phobos and Deimos

This theory would also explain why the Martian system is very different from our own Earth-Moon system, merely because the mechanism of formation of the satellites was very different, although it all started with a giant impact. Interestingly, another study² of the martian impact used numerical simulations to show that a rather big body would have formed in the inner part of the disk, similarly to the Moon (we hear from the author of this study next week). This body would have catalysed the formation of smaller objects in the outer region of the disk because of gravitational interactions that induced resonances between the bodies. This bigger body would have impacted the surface of Mars a long time ago because of tidal interactions with the planets. This is in very good agreement with our findings, finally explaining how both the orbits and the spectral and physical characteristics of Phobos and Deimos are reconciled in a giant impact scenario.

The large impact therefore seems to be a coherent scenario for the origin of Phobos and Deimos. It could also explain why the Northern hemisphere of Mars is very different from the Southern and appears to be much younger. It could in fact be the huge impact basin resulting from the collision that gave birth to the moons. It is also interesting to note that the orbits of the moons are not the only argument against them being captured objects. In fact we have noted that the D-type asteroids are far from being the most abundant type, so how is this that Mars captured two of them and no others? Furthermore, because they are certainly made of water ice and dust, D-type asteroids the size of Phobos and Deimos would have densities much lower than those determined for the martian moons. This supports the theory that Phobos and Deimos have a different composition from the asteroids and actually contain no water ice, as would be the case if they formed from an impact generated debris disk. We will soon have a lot more information on the origin of Phobos and Deimos and, if the impact theory is proven to be right, more information on the nature of the impactor and how much material from Mars composes the moons. The Japanese space agency (JAXA) is indeed planning to launch a sample return mission on Phobos in 2022, called the Mars Moon Exploration mission (MMX). A similar mission is currently studied by the european agency (ESA) in collaboration with the Russian Agency (Roscosmos) and would be launched in 2024.

Next week find out how mathematical models were also used to explain how Phobos and Deimos came to be.

Thomas Ronnet is a PhD student at Aix-Marseille Université in France working on the formation of satellites across the solar system with special emphasis on the galilean satellites which orbit around Jupiter.

References: 1. T.Ronnet et al.,The Astrophysical Journal, 2016 2. P.Rosenblatt et al., Nature Geoscience, 2016.