The debate about the formation of the Red Planet's satellites continues. Computer modelling has refuted last year's suggestion of a progenitor satellite for Deimos and Phobos.
Mars has only two satellites, Deimos and Phobos. Their origins raise many questions for astronomers. According to one hypothesis, once upon a time they were one body. And now computer modelling has shown that, in this case, the two satellites would have collided long ago to form a new system of satellites. Scientists have posted the results of their analysis to the public domain, and a scientific paper will be published in The Planetary Science Journal.
Deimos and Phobos are very different from the Moon we're used to. They are dozens of times smaller and far from spherical. Phobos, closer to the planet, is 22.7 kilometres in diameter, while Deimos is 12.6 kilometres. By comparison, the moon is 3,474.8 kilometres in diameter.
They are so irregular in shape that historically astronomers thought they were large asteroids of spectral class D, coming from the Kuiper Belt, a belt of small bodies beyond Neptune's orbit, where Pluto also flies. This assumption was partly confirmed by the similarity of the spectra of the satellites and class D astroids.
Later on, the Giant Impact Hypothesis emerged, similar to the generally accepted hypothesis of the origin of the Moon. It is believed that our satellite was formed billions of years ago from debris resulting from a collision between the Earth and a massive object. Apparently, in the case of Mars, the debris did not fuse into a single body but formed two satellites. Last year, it was suggested that that collision did form one primary satellite, which later disintegrated into Deimos and Phobos.
The new progenitor satellite hypothesis is called the "ring-moon recycling hypothesis". The authors suggested that between one and 2.8 billion years ago, the progenitor satellite collapsed due to tidal forces or a collision. The debris formed a ring from which Deimos and Phobos later formed. Analysis of their motion showed that the satellites' orbits overlapped in the past.
Other astronomers have criticised the study. Firstly, the intersection of the orbits was only obtained with specific tidal interaction parameters. Secondly, in such a short period of debris, the ring would not have had time to disappear completely. Among the critics was Ryuki Hyodo, a researcher from the Japan Aerospace Exploration Agency (JAXA).
To test the hypothesis, Hyodo and his colleagues took the assumption that Deimos and Phobos were indeed one body and then used calculations and computer simulations to simulate the evolution of the Mars satellite system.
Their calculations and models disproved the progenitor satellite hypothesis. If Deimos and Phobos were debris from the same body, they would have collided again within 100,000 years, forming a ring around the Red Planet that would still be seen today.
These results have added fuel to the debate about the origins of the Martian satellites. However, many of the world's agencies are planning space missions to Mars satellites. NASA, JAXA and the European Space Agency, among others, are working on the MMX (Martian Moons eXploration) mission, which will bring samples from Phobos to Earth. Analysis of such a sample will help resolve some questions about the formation of these satellites.