The two Martian moons Phobos and Deimos have been a mystery since their discovery in 1877. The y are very small. At 22 kilometers, the diameter of Phobos is 160 times smaller than the diameter of our moon. Deimos is even smaller with a diameter of only twelve kilometers. “Our moon is also beautifully spherical, while Phobos and Deimos are very irregularly shaped – like potatoes,” says Amirhossein Bagheri, doctoral student at the Institute for Geophysics at ETH Zurich. “ The Martian moons are more like an asteroid than a moon.”
The refore, it was suggested that Phobos and Deimos could be asteroids that were captured by Mars due to its gravity. “But that’s exactly where the problems began,” says Bagheri. Because captured objects would have to orbit the planet in an eccentric orbit, which could also have any inclination. But the orbits of the Martian moons are almost circular and run in the equatorial plane of Mars. This contradicts the trapped asteroid hypothesis. But how can one explain the current orbits of Phobos and Deimos? In order to solve this dynamic problem, the researchers used computer simulations.
Calculate back into the past
“ The idea was to trace the pathways and their changes back in time,” explains Amir Khan, Senior Scientist at the Physics Institute at the University of Zurich and at the Institute for Geophysics at ETH Zurich. It seemed that the paths of Phobos and Deimos had crossed in the past. “This means that the moons were very likely in the same place and must therefore have the same origin,” says Khan. The researchers conclude that at that time a larger celestial body orbited Mars. This original moon was probably hit by another chunk and thereby torn apart. “Phobos and Deimos are the remains of this lost moon,” says Bagheri, first author of the study, which has now been published in the journal Nature Astronomy.
What is easy to understand required extensive preparatory work. The first thing to do was to further develop the existing theory that describes the interaction between the moons and Mars. “What comes together in all of these celestial bodies are the tidal forces,” explains Khan. Energy is converted in the process. This so-called energy dissipation depends on the size and the interior of the celestial bodies and not least on their distance.
Insights into the interior of Mars and its moons
Mars is currently being explored by the NASA Insight mission, in which ETH Zurich is involved. A seismometer, the electronics of which were built at ETH, records marsquakes and meteorite impacts. “With it you can look inside the planet,” says Khan. “This data is used to limit the Mars model in our calculations and the dissipation occurring inside the red planet.”
Phobos and Deimos are known from pictures and measurements of other Mars probes that they are made of very porous material. At less than 2 grams per cubic centimeter, their density is significantly smaller than the mean density of the earth, which is 5.5 grams per cubic centimeter. “Inside Phobos there are many cavities in which there might be water ice,” suspects Khan. “ The re is a lot of energy dissipation due to the tides.”
With the help of this knowledge and the further developed theory of the effects of the tides, the researchers started hundreds of computer runs that tracked the moon’s orbits backwards in time until they reached the point of intersection – the birth of Phobos and Deimos. Depending on the simulation, this was in a period of 1 to 2.7 billion years in the past. “ The exact time depends on the physical properties of Phobos and Deimos,” explains Bagheri. A Japanese probe is scheduled to be sent to Phobos in 2025 and will bring rock samples back to Earth. The researchers hope that this will provide further details on the interior of the Martian moons, which will enable more precise calculations of their origin.
The calculations also show that the common ancestor of Phobos and Deimos was further away from Mars than Phobos is today. While the small Deimos stayed roughly where it was created, the larger Phobos is approaching Mars due to the tidal forces – a process that continues, the researchers explain. The computer simulations also show the future development of the lunar orbits. Accordingly, Deimos is slowly moving away from Mars, just as our moon is slowly moving away from the earth. Phobos, on the other hand, will fall to the red planet in less than 40 million years or be torn apart by gravitational forces as it approaches Mars.