Vesta

Minor planet (4) Vesta was first sighted through a telescope on March 29, 1807 by Heinrich Wilhelm Matthäus Olbers (1758-1840). Subsequent observations determined that the planetary body has an orbital period of 3.63 years, with a semi-major axis of around 2.36 AUs (slightly closer than Ceres's 2.77 AUs) and an eccentricity of 0.090 and an inclination of 7.14 degrees from the plane of the ecliptic (NASA factsheet). Its orbit places Vesta in the Main Asteriod Belt, but the object more closely resembles a small planet or Earth's Moon than another asteroid (more discussion).

Dawn Mission, UCLA, MPS, DLR,
IDA, PSI, CalTech, JPL, NASA




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Comparison of Vesta's size with
Mars, Mercury, Earth's Moon, and
Ceres (more).

Not quite round, Vesta's diameter is roughly 330 miles (530 kilometers) across, making it the second largest planetary body in the Main Asteroid Belt after Ceres. Both objects formed among the rocky and icy protoplanets beyond the Solar System's "ice line" now located around 2.7 AUs, but the early development of Jupiter apparently prevented such large protoplanets between the gas giant and planet Mars from agglomerating into even bigger planetary bodies, by sweeping many into pulverizing collisions as well as slinging them into the Sun or Oort Cloud, or even beyond Sol's gravitational reach altogether.

Dawn Mission, UCLA, MPS / DLR / IDA,
CalTech, JPL, NASA



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The second largest body in the Main Asteroid Belt,
Vesta appears to have a cratered surface like
Earth's Moon (more).

Unlike Ceres, which orbits just around or beyond the Solar System's ice line, Vesta's surface is not icy, like Ceres, Vesta appears to be an evolved body. In 1972, basaltic rocks (made of solidified lava) were detected on Vesta's surface, which indicated that the planetary body had melted early in its past. Vesta appears to have been large enough and hot enough to melt internally from decaying radioactive elements as well as impacts so that lighter elements floated upwards from denser material to form layers towards its surface. As a result, Vesta "differentiated" into a relatively dense metallic core (of approximately 136 miles or 220 kilometers across), lighter mantle, and crust, like the rocky inner planets, many large planetary satellite's like the Earth's Moon, and probably most, if not all, of the newly named "dwarf planets" like Ceres.

MNH, SI

Larger illustration.

Like Earth, Vesta is a differentiated
planetary body (more).

The most conspicuous feature on Vesta is a giant impact crater located around its south polar region, which is 310 miles (499 kilometers) across and nearly 12 miles (19 km) deep around a "bull's-eye" central peak rising 11 miles (or 18 km) above the exposed mantle rock of the crater floor -- that is characteristic of rock rebounding from an impact. This crater is so large that, if Earth had one proportionately as large as Vesta's, it would fill the Pacific Ocean. New high-resolution observations of Vesta by the DAWN mission reveal that the creatr is part of an even larger impact basin that was created by two massive overlapping impacts roughly around one and two billion years ago, which scooped out around one percent of the asteroid's estimated volume and blasted it into space (more from NASA's DAWN Mission and NEO Program; 1997 NASA / Hubble news release; Cornell University; Thomas et al, 1997; Kelley et al, 2003; and Jerry Coffrey, Universe Today, June 15 2009; and Russell et al, 2012; Jaumann et al, 2012; Marchi et al, 2012; and Schenk et al, 2012).

Thomas et al, 1997; Cornell; STScI; NASA


Larger image.


Vesta has a very large impact crater
located around its south pole (more).

Vesta's south polar basin was created by impacts roughly a billion years apart. The second impactor was 50- to 60-kilometer-wide (31- to 37-mile-wide), and it scooped out the younger crater, Rheasilvia, which is nearly 500 kilometers wide and 19 kilometers deep and is similar other large basins found on low-gravity icy bodies. While there was extensive ejecta deposits from the Rheasilvia impact, impact melt volume was low, and a spiral fracture pattern was created which is likely related to faulting during uplift and convergence of the basin floor. The Rheasilvia impact obliterated half of an older 400-kilometer-wide and 12-km-deep impact basin, named Veneneia (one of the first Vestal Virgins. Both basins are unexpectedly young, roughly 1 to 2 billion years, and their formation substantially reset Vestan geology and excavated enough diverse crust rocks to have created the Vestoids (asteroids with similar spectral characteristics that could be collisional fragments of Vesta) and the Howardite-Eucrite-Diogenite (HED) meteorites.

DAWN Mission, UCLA, MPS / DLR / IDA,
CalTech, JPL, NASA



Side image in false color.



Vesta's south polar basin is dominated by
two overlapping craters, where the younger
Rheasilvia impact obliterated most of the
slightly smaller Veneneia crater about a
billion years ago (more).

Over the past two billion years, some of Vesta's crustal debris rained down on Earth as HED meteorites, which account for about six percent of all those meteorites found on Earth. The signatures of pyroxene, an iron- and magnesium-rich mineral, in these meteorites have been found to match those of rocks observed on Vesta's surface. When viewed through a polarizing microscope, cross-sections of HED meteorites have minerals are seen in different colors under a polarizing microscope. The constrasting texture of different meteorites reveals that each crystallized at a different rate. (On the image shown below: the rock on the left comes from a meteorite named QUE 97053 that was found in Antarctica, which is made of basaltic eucrite; the middle rock comes from a cumulate eucrite found in Moore County, North Carolina; and the one on the right comes from a diogenite meteorite named GRA 98108 found in Antarctica). (De Sanctis et al, 2012; Reddy et al, 2012; and Lisa Grossman, New Scientist, May 11, 2012).

University of Tennessee



Larger polarized composite image.



NASA's observations of Vesta
have confirmed that "HED"
meteorites are fragments of
that planetary body (more).