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Looking Inside Vesta

The near-Earth asteroid 1999 AT10 is telling us a greal deal about an object much further away, the main belt asteroid Vesta. And that, in turn, is giving us new information about planet formation more than 4.5 billion years ago, when our Solar System was forming. The recently published paper, which precedes the August arrival of the Dawn spacecraft at Vesta, draws on infrared studies of 1999 TA10 by researchers from the Max Planck Institute for Solar System Research and the University of North Dakota, using the Infrared Telescope Facility on Mauna Kea.

The recent research compares the infrared radiation from the near-Earth asteroid to the spectral signature of Vesta. The latter, 525 kilometers in diameter, is quite an interesting object. It’s associated with a type of meteorite called HEDs — Howardite-Eucrite-Diogenite — which are thought to have originated in Vesta’s crust, traveling to Earth as the result of a huge impact that has left a crater on the asteroid’s southern hemisphere. Some of these meteorites (and about 5 percent of all falls are HEDs) have rock similar to what is found in Vesta’s mantle, but 1999 TA10 is the first near-Earth ‘Vestoid’ to have been found that matches Vesta’s mantle composition.

A mission backgrounder published in Acta Astronautica (reference below) notes the significance of the HED meteorites to the overall Dawn mission:

Comparison of reflectance spectra suggests that achondritic meteorites known as HEDs (for their principal constituents of howardite, eucrite, and diogenite, all of which are magmatic rocks that form at high temperatures) are fragments of Vesta. Confirmation of this important possibility would lend great weight to the use of laboratory studies of HED meteorites to contribute to the development of a description of Vesta’s structure and evolution. Discovering the geological context for the HED meteorites is, thus, a key objective for Dawn.

Image: On its southern side the asteroid Vesta shows a huge crater. This picture shows the asteroid in an image taken by the Hubble Space Telescope (top, left), as a reconstruction based on theoretical calculations (top, right), and as a topological map (bottom). Image: Ben Zellner (Georgia Southern University) / Peter Thomas (Cornell University) / NASA

The researchers believe, then, that the near-Earth object comes not from the outer rocky crust of Vesta but from deeper layers inside. With Dawn on the way, it should be possible to determine the thickness of Vesta’s crust and study its internal structure. The work has implications for planet formation, for Vesta has a differentiated inner structure, with a crust of cooled lava over a rocky mantle and an iron/nickel core. The similarity of this structure to terrestrial planets like Earth and Mars leads to the supposition that Vesta is a protoplanet, a still surviving relic of the ancient age when our system was beginning to come together.

The infrared studies found calcium-rich wollastonite and iron-rich ferrosillite, a fact whose significance is explained by Andreas Nathues (Max Planck Institute for Solar System Research), who notes that 1999 AT10 has a much lower concentration of iron than other Vestoids:

‘‘These materials can be found in Vesta’s mantle and crust. However, the ratio is decisive. This all points to 1999 TA10 having originated from the interior of Vesta.”

The crater at Vesta’s south pole is believed to be about 25 kilometers deep, which the new work suggests would be the maximum thickness of the outer crust. Understanding the thickness of the respective layers of Vesta’s geology should help us calculate the materials from which the protoplanet was formed, and by implication tell us about the kind of materials present when the entire Solar System emerged, and their relative ratios. Dawn’s visit to the remnant protoplanet will allow us to consider our hypotheses at length, for the spacecraft is scheduled to orbit Vesta for an entire year before moving on to Ceres.

The paper is Reddy et al., “First fragment of Asteroid 4 Vesta’s mantle detected,” in press at Icarus, published online on December 5, 2010 (abstract). For an overview of the Dawn mission, see Rayman et al., “Dawn: A mission in development for exploration of main belt asteroids Vesta and Ceres,” Acta Astronautica 58 (2006), pp. 605-616 (full text).

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  • Carl Keller January 7, 2011, 14:02

    Asimov wrote it was a peculiarity that Ceres, the largest of the minor planets in the main belt, was first to be discovered, and not Vesta, which has the highest magnitude viewed from Earth.

  • kurt9 January 7, 2011, 17:52

    I was not aware that any of the asteroids had a layered internal structure, like a planet. I assumed they were nothing more than giant space rocks of undifferentiated structure.

  • Paul Gilster January 7, 2011, 18:58

    Yes, Vesta is quite an interesting place. Here’s a bit more, this from the Dawn mission Web pages:

    “Dawn focuses on two of the first bodies formed in the solar system, the surviving protoplanets, Ceres and Vesta. Radioisotope chronology from the howardite, eucrite, and diogenite (HED) meteorites believed to be from Vesta suggests it accreted in only 5-15 million years. Similar evidence indicates that Mars continued to accrete for close to 30 million and Earth for 50 million years. The early cessation of accretion in the asteroid belt was presumably due to the formation of Jupiter whose gravitational forcing countered the accretionary process, and today is causing the disruption of the bodies that did accrete. Although we do not have similar meteorite evidence directly linked to Ceres, it too is expected to have formed in the first approximately 10 million years. In addition the asteroid belt may have been scoured by comets, scattered by the formation of the remaining gas giants. Today only some of the largest asteroids remain relatively undisrupted. The most massive of these are Ceres and Vesta, two most complementary minor planets. The former has a very primitive surface, water-bearing minerals, and possibly a very weak atmosphere and frost. The latter is a dry, differentiated body whose surface has been resurfaced by basaltic lava flows possibly possessing an early magma ocean like the Moon. Most importantly Vesta has experienced significant excavating events, most notably indicated by the huge crater near its southern pole. Cosmic ray exposure dating of HEDs indicates that impacts have released meteoritic material at least five times in the last 50 million years. Meteorites from these impacts have been used to piece together a most probable scenario for Vesta’s thermal evolution.”

  • andy January 7, 2011, 19:27

    Yes definitely Vesta is a worthwhile target for a space mission, especially given the combination of a differentiated structure and the large crater giving a view into the interior. I’m actually somewhat more excited by the prospects for the Vesta encounter than I am for Ceres, but I am sure both objects will be fascinating.

  • Tobias Holbrook January 13, 2011, 17:30

    Could Vesta, potentially, have reasonably high internal temperatures (by human standards), still, or will it have cooled completely? If the crust that formed insulated the hot rock beneath… temperatures on the order of a few hundred celcius could be found? If so, it would put geothermal in the running as a potential power source for a colony…