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Representation of the planet Mars and its entourage of Trojans circling Lagrange’s points L4 and L5. The dashed curve traces the orbit of the planet. In L5, the asteroid 101429 is represented by the blue dot, the asteroid Eureka and its family are represented in red and amber respectively. Credit: Armagh Observatory
An international team of planetary scientists led by AOP astronomers has found an asteroid trailing behind Mars with a composition very similar to that of the Moon. The asteroid could be an ancient fragment of debris, dating back to the gigantic impacts that formed the Moon and other rocky planets in our solar system such as Mars and Earth. The research, which was published in the journal Icarus, it also has implications for the search for such primordial objects associated with our planet.
Trojans are a class of asteroids that follow planets in their orbits like a flock of sheep might follow a shepherd, trapped inside gravitational “safe havens” at 60 degrees in front of and behind the planet (Figure 1). They are of great interest to scientists as they represent the remaining material from the formation and early evolution of the solar system. Several thousand of these Trojans exist along the giant planet’s orbit Jupiter. Closer to the Sun, astronomers have so far discovered only a handful of Trojans on Mars, the planet next to Earth.
Reflectance spectrum of asteroid 101429 from data obtained by the European Southern Observatory Very Large Telescope in Chile and NASA Infrared Telescope Facility in Hawaii. This is plotted on a spectrum of an area around Littrow Crater near the Apollo 17 moon landing site obtained from the Mauna Kea Observatory in Hawaii. Credit: Armagh Observatory
Where could such an unusual object come from? One possibility is that 101429 is just another asteroid, similar perhaps to ordinary chondrite meteorites, that acquired its lunar appearance through eons of exposure to solar radiation, a process called space weathering.
Alternatively, the asteroid could resemble the Moon because it comes from the Moon. Dr Apostolos Christou, AOP astronomer and lead author of the article explains: “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 the order of the day. Large asteroids – which we call planetesimals – were constantly hitting the Moon and other planets. A fragment of such a collision could have reached the orbit of Mars when the planet was still forming and was trapped in its clouds of Troy. “
A third, and perhaps more likely scenario, is that the object came from Mars itself. As Dr. Christou points out, “The shape of the 101429 spectrum tells us that it is rich in pyroxene, a mineral found in the outer layer or crust of planet-sized bodies. Mars, like the Moon and Earth, was hit by impacts early in its history, one of which was responsible for the gigantic Borealis Basin, a crater as wide as the planet itself. Such a colossal impact could easily have sent 101429 en route to the planet’s Lagrangian point L5. “Indeed, a Mars origin was proposed a few years ago for the Trojan brothers of 101429, a group of Trojans collectively known as the Eureka family. (Figure 1) These asteroids also have an unusual composition but, while 101429 is rich in pyroxenes, these Eureka family asteroids are primarily olivine, a mineral found deep within a planetary mantle.
101429 and his brothers also have something to teach us about how to find Earth’s Trojans, if they exist. Previous work by the team had shown that solar radiation causes debris, in the form of blocks the size of a boulder or an urban block, from these asteroids to slowly escape from Mars’ Trojan clouds. If the terrestrial Trojans were something similar to that of Mars, the same mechanism would act as a source of small asteroids near the Earth that will be distinguished by their unusual composition.
Finding these objects could turn out to be a job for the Vera C. Rubin Observatory, ready to begin the most ambitious investigation of the solar system to date. Rubin is expected to discover about ten times more asteroids than currently known and, together with the GAIA satellite already detecting the sky from Lagrange’s point L2 Earth-Sun, could offer us the best short-term prospects for tracking down Earth’s debris. Trojan companions.
Reference: “Composition and origin of the Trojan asteroids L5 of Mars: insights from spectroscopy” by Apostolos A. Christou,
Galin Borisov. Aldo Dell’Oro. Alberto Cellino and Maxime Devogèle, 1 August 2020, Icarus.
DOI: 10.1016 / j.icarus.2020.113994
arXiv: 2010.10947
Acknowledgments: Funds and facilities to support this work were provided by the United Kingdom Council for Science and Technology Facilities, the Northern Ireland Department of Communities and THAT, the European Organization for Astronomical Research in the Southern Hemisphere.
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