The debris could have come from giant impacts that ravaged the moon and other planets in the early years of the solar system.
Mars is – like some other planets in our solar system – chased by a number of so-called Trojans (see box). Researchers have now turned in a new study on the composition of those Trojans. And to their great surprise, they found out that the composition of one of these asteroids in the orbit of Mars is suspiciously similar to that of our moon.
What is a Trojan?
A planetoid that has the same orbit as a planet is also called a Trojan. Such a planetoid actually chases a planet in the same way that a flock of sheep chases a shepherd. Trojans are in stable positions, also known as Lagrangian points. Scientists are very interested in Trojans, especially because they represent remnants of the formation and early evolution of our solar system. Jupiter is the most rich in Trojans and has several thousand of them. A handful of Trojans have been discovered on Mars so far.
To find out the composition of the Trojans of Mars, the researchers used the X-shooter: the most sensitive spectrograph in the world equipped with ESO’s Very Large Telescope. In this way, the researchers hoped to determine whether the asteroids are made of material resembling rocky planets such as Earth, or whether they are carbon and water-rich material that is characteristic of the region beyond Jupiter, in the outer regions of our solar system.
Terrestrial
One of the Trojans that the research team studied in depth was asteroid (101429) 1998 VF3. The researchers combined new measurements with data previously obtained by NASA’s Infrared Telescope Facility in Hawaii to classify the object. But to their surprise, they found that the spectrum did not correspond to any particular type of meteorite or asteroid, but to our closest neighbor: the moon. “Many of the spectra we know for asteroids are very similar to those of the moon, but if you look closely, there are still important differences,” explains researcher Galin Borisov. “Consider, for example, the shape and depth of broad spectral absorption at some wavelengths. But the spectrum of this particular asteroid is very similar to certain parts of the moon where rock is exposed and exposed, such as in the interior of craters. ”
Image of the planet Mars and its retinue of Trojans orbiting the L4 and L5 Lagrangian points. The dotted line follows the orbit of the planet. On L5, Trojan 101429 is represented by the blue dot. The red dot is the asteroid Eureka. Image: Armagh Observatory and Planetarium
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The big question is, of course, where such an unusual object comes from. The researchers propose several options. First, it could be that 101429 is a fairly normal asteroid that owes its moon-like appearance to exposure to solar radiation; a process that we also call space processing. Space weathering is a bit like erosion on Earth. However, the erosion in space is not caused by wind and water, as on earth, but by cosmic radiation and radiation from the sun, for example.
Mars
A second possibility is that the striking asteroid is from Mars itself. “The shape of the spectrum tells us it is rich in pyroxene; a mineral found in the outermost layer or crust of planets, ”notes Christou. Mars – like the moon and the earth – was also ravaged by impacts in its early history. And one of them was responsible for the gigantic Borealis basin, which covers about 40 percent of the red planet. Such a colossal impact could easily have loosened rock which was then launched into space, sending the debris into a stable position and becoming a Trojan of Mars.
Debris of the Moon
But what could also be – and this is perhaps the most interesting scenario – is that 101429 is so similar to the moon, because it actually comes from the moon. And that’s not even such a crazy thought. “The early solar system was very different from today,” explains researcher Apostolos Christou. “The space between the newly formed planets was full of debris and collisions were the order of the day. Large asteroids constantly hit the moon and other planets. A shard from such a collision could then have entered Mars orbit while the planet was still forming. ”
Earthly Trojans
Future research will have to show which of the scenarios mentioned is correct. In any case, the study does provide us with new insights about the Trojans that are not even crazy far from Earth. The research therefore has important implications for finding such primal objects that haunt our own planet. To date, there is only one Trojan from Earth discovered; a 300-meter object that bears the name 2010 TK7. Finding terrestrial Trojans, however, is a major challenge, especially because they – if they exist – are close to the sun. And it is simply very difficult to point a telescope in the direction of the sun without blinding the telescope.
Finding Earth Trojans, however, could be added to the Vera C. Rubin Observatory’s to-do list. This observatory is currently under construction in the Chilean Andes and will be fully operational from 2022. Rubin is expected to reveal about ten times as many asteroids as is currently known. And together with the GAIA satellite, these two powerful telescopes may be able to reveal debris from Earth Trojans in the short term.
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