Astronomers find orbit of Mars hosts remains of ancient mini-planets

April 3, 2017
Left: The paths traced by the known Martian Trojans around L4 or L5 (crosses) relative to Mars (red disk) and the Sun (yellow disk). The dotted circle indicates the average Sun-Mars distance. Right: Enlargement of inset (dashed rectangle) showing the paths of the 8 L5 Trojans: 1998 VF31 (marked as "VF31" - blue), Eureka (red) and the 6 objects identified as family members (amber). The filled disks indicate the relative sizes of the asteroids. Eureka, the largest member, is about 2 km across. Credit: Apostolos Christou

The planet Mars shares its orbit with a handful of small asteroids, the so-called Trojans. Now an international team of astronomers using the Very Large Telescope in Chile have found that most of these objects share a common composition; they are likely the remains of a mini-planet that was destroyed by a collision long ago. The findings are reported in a paper to appear in Monthly Notices of the Royal Astronomical Society in April.

Trojan asteroids move in orbits with the same average distance from the Sun as a planet, trapped within gravitational "safe havens" 60 degrees in front of and behind the planet. The special significance of these locations was worked out by 18th century French Mathematician Joseph-Louis Lagrange. In his honour, they are now known as "Lagrange points"; the point leading the planet is L4; that trailing the planet is L5.

About 6000 such Trojans have been found at the orbit of Jupiter and about 10 at Neptune's. They are believed to date from the solar system's earliest times when the distribution of planets, asteroids and comets was very different than the one we observe today.

Mars is so far the only terrestrial planet known to have Trojan companions in stable orbits. The first Mars Trojan was discovered over 25 years ago at L5 and named "Eureka" in reference to the famous exclamation by Ancient Greek mathematician Archimedes. The present tally is nine, a factor of 600 fewer than Jupiter Trojans, but even this relatively puny sample shows interesting structure not seen elsewhere in the solar system.

Spectra of Eureka family asteroids (385250) 2001 DH47 (red) and (311999) 2007 NS2 (black) obtained with the X-SHOOTER spectrograph at the Very Large Telescope (VLT) in Chile. A spectrum of 5261 Eureka is shown in blue. The three spectra are very similar, indicating a common composition that is also rare among asteroids. Credit: Galin Borisov

For starters, all the Trojans, save one, are trailing Mars at its L5 Lagrange point (Figure 1, left panel). What's more, the orbits of all but one of the 8 L5 Trojans group up around Eureka itself (Figure 1, right panel). The cause for the uneven distribution of these objects has not yet been pinned down, though there are a couple of possibilities. In one scenario, a collision broke up a precursor asteroid at the L5 point, the fragments making up the group we observe today. Another possibility is that a process called rotational fission caused Eureka to spin up, eventually spawning off small pieces of itself in heliocentric orbit. Whatever the cause, the grouping strongly suggests that the asteroids in this "Eureka family" were once part of a single object or a progenitor body. Although the circumstantial evidence for this hypothesis is strong, the acid test is to find out whether the asteroids share a common composition or not. Fortunately, this can be done at the telescope by measuring the colour of sunlight reflected off the asteroid's surface, in other words obtaining its spectrum.

For this purpose, an international team of astronomers led by Apostolos Christou and Galin Borisov at the Armagh Observatory and Planetarium in Northern Ireland, UK, used the X-SHOOTER spectrograph mounted on "Kueyen", the Unit 2 telescope of the European Southern Observatory's Very Large Telescope facility in Chile in early 2016 to record the spectra of two asteroids that belong to the Eureka family, 311999 and 385250. Analysing the spectra, they found that both objects are "dead ringers" for Eureka (Figure 2), so confirming the genetic relationship between family asteroids. The finding also marks a significant "first" for asteroid studies; the spectra show that these asteroids are predominantly composed of olivine, a mineral that typically forms within much larger objects under conditions of high pressure and temperature. The implication is that these asteroids are likely relict mantle material from within mini-planets or "planetesimals" which, like the Earth, developed a crust, mantle and core through the process of differentiation but have long since been destroyed by collisions.

Christou points out that "Many other families exist in the Asteroid Belt between Mars and Jupiter, and even among Jupiter's Trojans, but none are made up of such olivine-dominated asteroids". This is related to the so-called missing-mantle problem: that is, if one adds up the mass of different minerals in the belt and particularly those thought to be pieces of broken up, differentiated asteroids, there is a deficit of mantle material compared to rocky crust and metallic core material.

Although the discovery of this olivine-dominated family does not provide a final solution to the missing mantle problem, it does show that mantle material was present near Mars early in the solar system's history. As Christou explains: "Our findings suggest that such material has participated in the formation of Mars and perhaps its planetary neighbour, our own Earth."

Explore further: NASA's OSIRIS-REx begins Earth-Trojan asteroid search

More information: G. Borisov et al. The olivine-dominated composition of the Eureka family of Mars Trojan asteroids, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stw3075

Related Stories

NASA's OSIRIS-REx begins Earth-Trojan asteroid search

February 10, 2017

A NASA spacecraft begins its search Thursday for an enigmatic class of near-Earth objects known as Earth-Trojan asteroids. OSIRIS-REx, currently on a two-year outbound journey to the asteroid Bennu, will spend almost two ...

An asteroid pile-up in the orbit of Mars

October 11, 2013

The orbit of the planet Mars is host to the remains of an ancient collision that created many of its Trojan asteroids, a new study has concluded. It paints a new picture of how these objects came to be and may even hold important ...

Searching for trojan asteroids, Earth's unseen companions

December 13, 2016

The University of Arizona-led OSIRIS-REx mission will take advantage of its two-year outbound cruise to the asteroid Bennu in order to conduct a search for elusive solar system objects. Between Feb. 9 and 20, the NASA spacecraft ...

Three new 'Trojan' asteroids found sharing Neptune's orbit

June 15, 2006

Three new objects locked into roughly the same orbit as Neptune--called "Trojan" asteroids--have been found by researchers from the Carnegie Institution's Department of Terrestrial Magnetism (DTM) and the Gemini Observatory ...

OSIRIS-REx asteroid search tests instruments, science team

March 24, 2017

During an almost two-week search, NASA's OSIRIS-REx mission team activated the spacecraft's MapCam imager and scanned part of the surrounding space for elusive Earth-Trojan asteroids—objects that scientists believe may ...

Fragmented asteroid pair develops twin comet-like tails

March 2, 2017

Asteroids on the main belt, situated between Mars and Jupiter, move around the sun in quasi circular orbits, so they do not undergo the temperature changes which, in comets, produce the characteristic tails. Nevertheless, ...

Recommended for you

Dawn mission extended at Ceres

October 20, 2017

NASA has authorized a second extension of the Dawn mission at Ceres, the largest object in the asteroid belt between Mars and Jupiter. During this extension, the spacecraft will descend to lower altitudes than ever before ...

5 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

HannesAlfven
1.4 / 5 (9) Apr 03, 2017
More suggestion that Mars itself has had a catastrophic past. It should not be simply assumed that this occurred a really long time ago. There's plenty of reason by now to suspect it was geologically recent.
Shootist
5 / 5 (6) Apr 03, 2017
More suggestion that Mars itself has had a catastrophic past. It should not be simply assumed that this occurred a really long time ago. There's plenty of reason by now to suspect it was geologically recent.


no there isn't. Crater Retention Age is enough to prove the relative ages of major topographical features on Mars while comparing those features with the known ages, from zircom interpretation, of lunar features
jonesdave
5 / 5 (7) Apr 03, 2017
More suggestion that Mars itself has had a catastrophic past. It should not be simply assumed that this occurred a really long time ago. There's plenty of reason by now to suspect it was geologically recent.


no there isn't. Crater Retention Age is enough to prove the relative ages of major topographical features on Mars while comparing those features with the known ages, from zircom interpretation, of lunar features


Pointless arguing, mate. The guy is scientifically illiterate. Believes in Velikovskian woo. You get my drift? :)
Shootist
not rated yet Apr 04, 2017
I do. speaking of Velikovshy, et. al.

http://www.jerryp...vsky.htm
jonesdave
5 / 5 (4) Apr 04, 2017
I do. speaking of Velikovshy, et. al.

http://www.jerryp...vsky.htm


No offence, but haven't got time to read all that. Suffice to say, as somebody put it elsewhere, (paraphrasing) "Velikovsky wasn't just your common or garden moron; he was a world class moron."
That's all you need to know.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.