Home science The moon's 'long-lost twin' may have been found

The moon's 'long-lost twin' may have been found

The moon's 'long-lost twin' may have been found

Scientists from the Armagh Observatory and Planetarium in Northern Ireland have found that the asteroid has very similar composition to that of the Moon.

Space debris which follow planets in their orbits are known as ‘trojans’. They become trapped within the gravitational pull of the planets, up to 60 degrees in front and behind them.

It is vastly easier to find trojans of other planets than our own because any trojans that are following Earth’s orbits are so close to the Sun, which makes them harder to spot using telescopes.

The trojan that is following the Red Planet is asteroid (101429) 1998 VF31. In order to detect its composition, scientists use a spectrograph – an instrument that measures wavelengths in light – to see how the surface of the asteroid reflects colours.

The scientists compared the reflections of the light spectrum with those of other bodies in the solar system, and although they found the spectrum did not match particularly well with other mereorites or asteroids, it did with Earth’s Moon.

Many of the spectra we have for asteroids are not very different from the Moon but when you look closely there are important differences, for example the shape and depth of broad spectral absorptions at wavelengths of 1 and 2 microns,” explains Dr Galin Borisov, a Postdoctoral research assistant at Armagh Observatory and Planetarium.

It is speculated that the asteroid acquired these characteristics through exposure to solar radiation, which is known as ‘space weathering’. However, it is possible that it was dislodged from the Moon itself.

“The early solar system was very different from the place we see today. The space between the newly-formed planets was full of debris and collisions were commonplace. Large asteroids – we call these planetesimals – were constantly hitting the Moon and the other planets.” Said Dr Apostolos Christou, an astronomer who published a paper on the asteroid.

A shard from such a collision could have reached the orbit of Mars when the planet was still forming and was trapped in its Trojan clouds.”

While that is a possible scenario, it could be that the asteroid came from Mars itself. From the shape of the light spectrum, scientists can infer that the asteroid is rich in pyroxene, which is found in the outer layer of planet-sized bodies.

“Mars, like the Moon and the Earth, was pummelled by impacts early in its history, one of these was responsible for the gigantic Borealis basin, a crater as wide as the planet itself”, Dr Christou says, and it could be that impact that dislodged the trojan.

Such an origin has already been hypothesised in 2017 for another cluster of trojans called the Eureka family; however, in contrast to the pyroxene composition of this asteroid, that the Eureka family is mostly comprised of olivine which is found in a planet’s mantle rather than its crust.

It would allow researchers to find entirely new breakthroughs in pieces of lunar rock that are nearly 50 years old, and is so niche that many geologists have not heard of it, according to a researcher at the University of Chicago.



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