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DAWN Asteroids: 1 Ceres and 4 Vesta
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  1. DAWN Asteroids: 1 Ceres and 4 Vesta By Christina O. Lee Astro 249

  2. DAWN Mission Goals • To characterize conditions and processes of the solar system's earliest epoch by investigating in detail the two largest protoplanets remaining intact since their formations. The growth of these bodies (and others in the belt) were presumably interrupted by the formation of Jupiter, whose gravitational forcing countered the accretionary process. Astro 249

  3. Targets: 1 Ceres & 4 Vesta • Two largest protoplanets residing between Mars and Jupiter. • Each followed a very different evolutionary path constrained by diversity of processes that operated during the very first few Myrs of solar system evolution Astro 249

  4. Relative sizes • Ceres is 933 km diameter, 870000 x 1015 kg • Vesta is 510 km diameter, 300000 x 1015 kg Astro 249

  5. Vesta Quick Facts • Named after the Roman Goddess of Hearth • The only asteroid visible with the naked eye • 3rd largest asteroid (510 km diameter) • Distance: 2.361 AU • Rotation rate: 5.342 hr • Evolved and dry, differentiated body Astro 249

  6. Vesta – some properties • Have rocks more strongly magnetized than Mars (> 100s nT) • alter ideas of how and when dynamos arise • Have basaltic surface composition • possibly possessing an early magma ocean like our Moon • Experienced significant excavating events • indicated by huge crater near southern pole Astro 249

  7. Vesta meteorite Almost entirely made of pyroxene, commonly found in lava flows on Earth Mineral grain structure also indicates it was once molten, and its oxygen isotopes are unlike oxygen isotopes found for all other rocks of the Earth and Moon. • Radioisotope chronology (i.e. cosmic ray dating) from the howardite, eucrite, and diogenite (HED) meteorites believed to be from Vesta suggests it accreted in only 5-15 million years (Vesta has same spectral signature as pyroxene) • similar evidence indicates that Mars continued to accrete for close to 30 Myrs and Earth for 50 Myrs • Dating also indicate that impacts have released meteoritic material at least 5x in the last 50 Myrs. Astro 249

  8. Ceres – Quick Facts • Named after the Roman Goddess of Agriculture • Largest asteroid (933 km diameter) • Distance: 2.769 AU • Rotation rate: 9.075 hr • Very primitive surface w/ water bearing minerals! Astro 249

  9. Ceres – Some properties • Have active hydrological processes leading to seasonal polar caps of water frost • Have thin permanent atmosphere of frost • Have dry clay surface • indicated by microwave studies) Astro 249

  10. No Ceres Meteorite • …But it is expected to be like Vesta and formed ~ 10 Mya. • Possibly the excavating events or dynamic that provided the HED meteorites did not occur, but it's possible that the reflectance spectrum of the surface is not indicative of Ceres' crustal rocks • on Vesta, the basaltic dust layer reflects its crustal composition Astro 249

  11. Why are Ceres and Vesta so different? It appears that a rather short additional radial separation allowed Ceres (further away) to accrete wet and stay cool while early heat sources in the accreting material melted Vesta (slightly closer) Astro 249

  12. Go DAWN mission! Will determine internal structure, density and by measuring their mass, shape, volume and spin rate with both imagery, laser altimetry and gravity Astro 249