Phobos and Deimos might be D-type asteroids. But what does that mean?

There are two main hypothesis for how the moons of Mars formed. In the first, Phobos and Deimos coalesced from debris thrown into orbit during a giant impact with the red planet. In the second, the pair were asteroids that were captured by Mars’s gravity as they migrated inwards towards the Sun (watch: video on the formation of Phobos and Deimos).

The asteroid hypothesis stems from two pieces of evidence. The moons resemble asteroids in that their lumpy, non-spherical shape is similar to other small bodies that orbit the Sun in the asteroid belt, and more distant reservoirs such as the Jupiter Trojans, Centaurs and Kuiper belt. The second is that the spectrum of light reflected from the moons resembles that of D-type asteroids. But what are D-type asteroids and what would be the scientific importance of the MMX mission returning a sample from this asteroid class?

What are D-type asteroids? (And the other asteroid classes). (YouTube)

Viewed from the Earth, asteroids are usually point sources; bright localised pinpoints of light that indicate the presence of an object, but not its geometry. While variations in brightness as the object rotates can be used to model a shape, the result is not very precise. Prior to the Hayabusa2 spacecraft reaching the vicinity of asteroid Ryugu in June 2018, our best guess at the asteroid’s appearance was a semi-spherical blob. The surface of Ryugu was such a mystery, that the mission team held a space art contest to recruit guesses as to the asteroid’s appearance. The suggestions were varied and exotic, but not even the mission team predicted the heavily bouldered surface on the diamond-shaped asteroid.

While the appearance of these tiny worlds cannot be deciphered from Earth, differences in the wavelengths of the reflected light from their surface is enough to categorise asteroids into different classes.

The reflected light from an asteroid has two main properties that determine the asteroid class. The first is the total amount of light reflected from the surface. This is known as the “albedo”. Phobos and Deimos have very low albedos, meaning that the moons are extremely dark and reflect very little light.

S-type asteroid Itokawa (left), C-type asteroid Ryugu (centre) and Phobos (credit: JAXA / U. Tokyo and collaborators / ESA /DLR / FU Berlin).

The second property is the amount of light that is reflected at different wavelengths. This is the asteroid’s “spectrum”, usually shown as a graph where the amount of reflected light is plotted at each wavelength.

If the reflected light increases at longer wavelengths, the asteroid is said to be “red”. The reverse, when the spectrum shows a decreasing line that indicates less light at longer wavelengths, marks a “blue” asteroid. These names indicate whether the reflected light is mainly at longer or shorter wavelengths, as red and blue are the longest and shortest wavelengths in optical light. But if you look at the asteroids, they would not appear red or blue as the majority of the reflected light is at infrared wavelengths.

Both the albedo and the shape of the spectrum are dependent on the asteroid’s composition, making these good tools to categorise asteroids types.

Phobos and Deimos have very red spectra, reflecting light strongest at long wavelengths. This combination of a low albedo and red spectrum also defines the D-type asteroids, which is why the two moons are suspect to be captured examples of this class.

Conversely, the asteroids Itokawa and Ryugu, that were the destinations of the Hayabusa and Hayabusa2 missions, are S-type and C-type asteroids respectively.

Schematic spectra of three different asteroid classes: S-type (like Itokawa), C-type (like Ryugu) and D-type (potentially like Phobos and Deimos).

S-type (or “stony” / “siliceous”) asteroids

This asteroid class is principally found in the asteroid belt, especially in the inner region of the belt that is nearest the Sun. They have moderate albedos between about 0.1 – 0.22, and a red spectra that increases approximately linearly from about 0.3 – 0.7 microns, before flattening. A dip (feature) in the spectrum at around 1 micron indicated absorption by silicates, which form the main component of S-type asteroids.

The sample returned by the Hayabusa mission from S-type asteroid Itokawa, revealed that S-type asteroids correspond to the ordinary chondrite meteorites discovered on the Earth’s surface. This is a common meteorite type that is poor in volatiles, such as water.

Unlike their C-type asteroid brethren, S-type asteroids are thought to have experienced strong heating since their formation. This altered their chemical composition and explains the lack of volatiles, which would have been depleted during such processes.

C-type (or “carbonaceous”) asteroids

These asteroids have low albedos usually between about 0.03 – 0.09. The destination of Hayabusa2, the C-type asteroid Ryugu, was particularly dark, with an albedo of about 0.04. This forced the Hayabusa2 mission team to adjust the settings of the onboard LIDAR so the poorly reflected beam could be detected to give an altitude reading. The spectrum of C-type asteroids is flatter and featureless compared with the S-type class.

C-type asteroids are the most common type of asteroid in the main asteroid belt, and their numbers increase towards the outer edge. This more distant location compared to the inner S-type asteroids means that meteorites from C-type asteroids are less commonly found on Earth.

Carbonaceous chondrite meteorites are thought to originate from C-type asteroids and are rich in carbon-based molecules, such as organics, and volatiles, such as hydrated minerals. C-type asteroids are believed to have formed further from the Sun than the Earth, beyond the ice line where water becomes solid ice. Since formation in the early Solar System, this asteroid class is thought to have undergone relatively little alteration. The Ryugu sample is therefore anticipated to reveal information about how organics and water were delivered to the young Earth from further out in the Solar System.

D-type asteroids

Like the C-type asteroids, D-type asteroids have a low albedo but possess a much steeper red slope to their spectrum than either S-type or C-type asteroids. The majority of D-type asteroids sit outside the main asteroid belt, throughout the realm of the gas giants. The redness of the D-asteroid spectrum is surpassed only by small bodies near the outer edge of the Solar System, towards and beyond Neptune.

Not a vast amount is known about D-type asteroids, but it is suspected that the Tagish Lake meteorite originated from a D-type asteroid. This asteroid class is thought to form further out than the C-type asteroids, past where carbon dioxide also freezes into a solid. These asteroids are therefore expected to contain even more volatiles than C-type asteroids, including water and carbon dioxide, and be rich in organics.

MMX spacecraft artist’s concept image during descending/landing on a Martian moon based on the spacecraft design in FY2019.

A main goal of the MMX mission is to determine the origin of the Martian moons through examination of their composition. If the moons consist of material different from that of Mars, the chances are good that the pair are captured D-type asteroids. Mars would therefore have snagged and preserved an example of material that is thought to have moved from the outer Solar System to the region of the terrestrial planets.

A sample of moon-asteroid would tell us about conditions in the early Solar System at the intriguing location where ices were forming. Combined with data from the C-type Ryugu, this would elucidate the mix of water and organic ingredients that was available in the terrestrial planet region.

The results from this could help uncover how planetary systems form and mix their solid material, and also the starting conditions for habitability versus what developed later on the planet.

In addition to the MMX and the two Hayabusa missions, NASA’s OSIRIS-REx is currently returning to Earth with a sample from a B-type asteroid, which is a similar class to C-type. NASA’s LUCY mission to the Trojan asteroids of Jupiter also plans to make close-up observations of both C-type and D-type asteroids in that vicinity. By examining these fragments of the start of our existence, we can build up a map of our planetary system began.