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What Dawn did learn at Vesta?

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Outline

• Vesta and the Dawn mission

• Goals of the mission

• What Dawn did learn at Vesta?• Giant impact basins• Protoplanetary paradigm• Mineralogy and its diversity across Vesta• Potential internal structures

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Vesta and the Dawn mission

• Launch in september 2007.• Gravitational boost from Mars.• Dawn entered in Vestian orbit in July 2011.• Measurements in low (210 km) and high orbits (685 km)• Left in late 2012.

• 286x279x223 km• 2,36 au• Rotational period: 5,3 hours• Differentiated body (3400 kg/m3)

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Goals of Dawn mission

• Mapping of two biggest bodies in the asteroid belt.

• Get the confirmation that HED (Howardites, Eucrites and Diogenites) family of meteorites originate from Vesta as earth-based observations suggested.

• Spectrometry and gravity measurements.

• Look for the presence of moons.

On board instruments:

Framing cameras (FC), VIR spectrometer and GRaND (Gamma-ray and Neutron detector)

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Hubble Space Telescope

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Dawn spacecraft

7Marchi et al. 2012

Crater density (D>4km)

number of craters per 104 km2

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Morphology of Vesta

• Two giant impacts in the southern hemisphere: Veneneia (2.1 Ga) and Rheasilvia (1 Ga).

• Vesta troughs (trench).

• Low impact melts.

• Source of the Vestoids. Located between the v6 secular and 3:1 resonance with Jupiter + Yarkovsky effect.

• Does mineralogical observations for Vesta match the composition observed from Vestoids?

Schenk et al. 2012

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HED – Vestoids

• Eucrites: Crustal basalts (similar to terrestrial basalts)

• Diogenites: Ultramafic cumulates formed in the lower crust of the asteroid.

• Howardites: a mix of E & D. McCord et al. 1970

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VIR Spectrometer

• Pyroxene absorption band at 0.9 µm and 1.9 µm.

• Depth of the absorption band.

• Centers of the absorption band are shifted for diogenites/eucrites.

From De Sanctis et al. (2012)

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Pyroxene absorption bands

From Reddy et al. (2012)

0.75/0.92

DEM

0.98/0.92

12Clenet et al. 2014 (Nature)

13Clenet et al. 2014 (Nature)

Ocean-magma crystallization model

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• Veneneia excavated material down to 25 km.

• Then a second impact, Rheasilvia, ejected materials from down to 80 km below the surface.

• If the magma-ocean crystallization model is preferred, we should then observe olivine within the ejecta and the central mound.

• One explanation is that only materials ejected from the first impact reached Earth. But no olivine in the central mound.

Clenet et al. 2014 (Nature), SPH model from Jutzi et al. 2013

15Clenet et al. 2014 (Nature)

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GRaND instrument

Aelia crater 14oS

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Orbital and physical parameters

From Russell et al. (2012)

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Summary

• Confirmation of early telescopic observations and interpretations– global elemental composition and mineralogy strongly agrees that Vesta is the source of HED meteorites

• Surface composition is remarkably uniform (basaltic and pyroxene rich)

• Hydrated materials has been delivered by meteorite impacts on the surface of Vesta.

• Low impact melts, suggesting mainly low-velocity impact.

• Composition of Vesta presents a clear and fundamental example of a silicate body that underwent differentiation.

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Questions?

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What is the Yarkovsky Effect?