Mars may not be Earth’s ‘brother’. Japan launches mission to Phobos

A group of physicists from the University of Copenhagen has combined the chemistry of the two bodies with models of their formation and evolution, and used a great deal of computing time to reconstruct the formation of the two planets.

The result was quite clear: 73 per cent of Mars’ mass is due to collisions between objects similar in size to asteroids. In the case of Earth, however, around 75 per cent is thought to have come from small rocks and dust, which clumped together to form two protoplanets that subsequently collided with one another. Quite a difference.

This alternative explanation is suggested primarily by the amounts of sodium, potassium, lithium and zinc detected; these are moderately volatile elements, which are less abundant on Earth and Mars than in meteorites that preserve traces of the Solar System’s primordial material. The two processes preserve these elements in different ways: violent impacts generate heat and can therefore cause them to volatilise and disperse into space; a more gradual growth process, fuelled by pebbles, would instead favour their preservation.

The hypothesis put forward by the Danish researchers also concerns the origin of Earth’s water, an issue which, it seems, has not yet been fully clarified. If some of the smaller stones and pebbles had arrived from the outer regions of the Solar System, which are rich in ice, they could have contributed to the abundant water found on our planet.

Here, however, Jupiter throws a spanner in the works. Its enormous gravity, which acts on the mass of the smaller bodies in the solar system like a magnet, may have siphoned off material from the region where Mars was forming, meaning it would have been smaller, but above all less massive: just one-tenth the size of Earth; at the same time, it may have intercepted many of the icy rocks coming from the outer solar system. To put it more simply, if I have a large magnet on a wide table and I throw a large number of iron marbles at random, many will be attracted to it and will not pass beyond the magnet.

Once again, therefore, a buzzword is being used for the new hypothesis currently under investigation: hybrid. This would seem to be the best adjective to describe the new framework for the formation of these two planets: major impacts and the accumulation of small rocks – not as alternatives, but as possible competing processes for the formation of the planets.

Meanwhile, a probe is travelling directly towards Mars, but with a different destination: Phobos, the moon closest to the planet of its two moons. The Japanese MMX mission, Martian Moons Exploration, launched from Tanegashima aboard an H3 rocket. The aim is to land on the small moon, which has an average diameter of 22 kilometres, collect rocks and dust, and return them to Earth by 2031. The aim is to shed light on the origins of this small Martian moon – likely an asteroid captured by Mars despite its weak gravitational pull – and to clarify its relationship with the formation of the planet itself.

Mars has been identified for centuries with the god of war, due to its reddish colour, which is visible even to the naked eye, and it is fitting that its two moons are Phobos and Deimos, Fear and Terror – emotions that are always felt when there is war. Even today.

Phobos and Deimos may have been captured by Mars’s gravity, but their nearly circular orbits, at the planet’s equator, also suggest they formed following a massive impact. Something quite large may have struck Mars, and the debris may have coalesced to form the two small moons. Samples taken from Phobos could help to distinguish between these hypotheses.

It is due to arrive in August 2027. The MMX probe will follow an orbit similar to that of Phobos, observing its surface to identify the most suitable landing sites. Between late 2028 and early 2029, it will deploy, from a distance of around 40 metres, Idefix, the rover, a Franco-German project, a name undoubtedly familiar to Asterix fans: it is the name of Asterix’s own dog.

Phobos has low gravity – one thousandth of Earth’s – and will therefore exert little or no pull on the robot, which will touch down at a speed of around three kilometres per hour. It is to be expected that Idefix will bounce around quite a bit before coming to a halt, which is why it is designed to right itself should it tip over. Once it has come to a halt and is in the correct orientation, Idefix will explore the terrain using its instruments and cameras, moving at a speed of just 3.6 metres per hour, for just over three months.

In 2030, the probe will land on Phobos and remain there for just two and a half hours to collect at least ten grams of material. After flying past Deimos, it will begin its return journey to Earth. In 2031, once it has reached the correct distance from our planet, it will release a capsule containing the precious material, which will descend gently, thanks to a parachute, into Australia.

Led by JAXA, the mission also involves NASA and Europe: the United States is providing an instrument to analyse minerals and one of the sample collection devices, whilst the European Space Agency is supplying the communications equipment.

And the haul might hold a surprise: fragments of Mars, hurled into space by asteroid impacts and subsequently deposited on Phobos. The probe could thus provide us, alongside the history of an enigmatic moon, with tiny remnants of the Red Planet. Given that the mission will cost at least 500 million euros, it’s easy to work out the cost per gramme of the dust – literally scraped from Phobos – but it’s well worth it.