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Representation of the planet Mars and its entourage of Trojans circling the Lagrange points L4 and L5. The dashed curve traces the orbit of the planet. At L5, asteroid 101429 is represented by the blue dot, asteroid Eureka and its family are represented by red and amber respectively. Credit: Armagh Observatory
An international team of planetary scientists led by astronomers at AOP has found an asteroid behind Mars with a composition very similar to that of the Moon. The asteroid could be an ancient piece of debris, dating back to the gigantic impacts that formed the Moon and the other rocky planets in our solar system like Mars and Earth. The research, which was published in the journal Icarus, it also has implications for finding such primordial objects associated with our own planet.
Trojans are a class of asteroid that follows planets in their orbits like a flock of sheep might follow a shepherd, trapped in 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 leftover material from the formation and early evolution of the solar system. Several thousand such Trojans exist throughout the giant planet’s orbit. Jupiter. Closer to the Sun, astronomers have so far discovered only a handful of Trojans from Mars, Earth’s neighboring planet.
Reflectance spectrum of asteroid 101429 from data obtained at the Very Large Telescope of the European Southern Observatory in Chile and the NASA Infrared Telescope Facility in Hawaii. This is plotted against a spectrum of an area around Littrow Crater near the Apollo 17 landing site on the Moon obtained from the Mauna Kea Observatory in Hawaii. Credit: Armagh Observatory
Where could such an unusual object have 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 may 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 littered with debris and collisions were commonplace. Large asteroids, which we call planetesimals, were constantly hitting the Moon and the other planets. A fragment from such a collision could have reached the orbit of Mars while the planet was still forming and trapped in its Trojan clouds. “
A third scenario, and perhaps more likely, 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 shell 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 on its way to the planet’s L5 Lagrangian point. ” In fact, a Mars origin was proposed a few years ago for the Trojan brothers of 101429, a group of Trojans known collectively as the Eureka family (Figure 1). These asteroids also have an unusual composition but, while 101429 is rich in pyroxene, these Eureka family asteroids are mostly olivine, a mineral found deep within the planetary mantle.
101429 and his brothers also have something to teach us about how to find ground Trojans, if they exist at all. Previous work by the team had shown that solar radiation causes debris, in the form of chunks the size of a rock or city, from these asteroids to slowly seep out of the Trojan clouds on Mars. If terrestrial Trojans are like those on Mars, the same mechanism would act as a source of small near-Earth asteroids that would stand out for their unusual composition.
Finding these objects could be a job for the Vera C. Rubin Observatory, set to begin the most ambitious study of the solar system to date. Rubin is expected to discover about ten times more asteroids than are currently known and, together with the GAIA satellite already surveying the sky from the L2 Earth-Sun Lagrange point, it may offer us the best near-term prospects for tracking the debris from the earth. Fellow Trojans.
Reference: “Composition and origin of the L5 Trojan asteroids on Mars: knowledge of spectroscopy” by Apostolos A. Christou,
Galin Borisov. Aldo Dell’Oro. Alberto Cellino and Maxime Devogèle, August 1, 2020, Icarus.
DOI: 10.1016 / j.icarus.2020.113994
arXiv: 2010.10947
Acknowledgments: The UK Science and Technology Facilities Council, Northern Ireland Department for Communities and Communities provided funding and facilities to support this work. THAT, the European Organization for Astronomical Research in the Southern Hemisphere.
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