DAWN Asteroids: DAWN Asteroids: 1 Ceres and 4 Vesta1 Ceres and 4 Vesta

By Christina O. LeeBy Christina O. Lee

DAWN Mission GoalsDAWN Mission Goals

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

Targets: 1 Ceres & 4 VestaTargets: 1 Ceres & 4 Vesta

• Two largest protoplanets Two largest protoplanets residing between Mars and residing between Mars and Jupiter. Jupiter.

• Each followed a very Each followed a very different evolutionary path different evolutionary path constrained by diversity of constrained by diversity of processes that operated processes that operated during the very first few during the very first few Myrs of solar system Myrs of solar system evolutionevolution

Relative sizesRelative sizes

• Ceres is 933 km diameter, 870000 x 10Ceres is 933 km diameter, 870000 x 101515 kg kg• Vesta is 510 km diameter, 300000 x 10Vesta is 510 km diameter, 300000 x 101515 kg kg

Vesta Quick FactsVesta Quick Facts

• Named after the Roman Named after the Roman Goddess of Hearth Goddess of Hearth

• The only asteroid The only asteroid visible with the naked visible with the naked eyeeye

• 3rd largest asteroid 3rd largest asteroid (510 km diameter)(510 km diameter)

• Distance: 2.361 AUDistance: 2.361 AU• Rotation rate: 5.342 hrRotation rate: 5.342 hr• Evolved and dry, Evolved and dry,

differentiated bodydifferentiated body

Vesta – some propertiesVesta – some properties

• Have rocks more strongly magnetized than Mars (> 100s nT) Have rocks more strongly magnetized than Mars (> 100s nT) – alter ideas of how and when dynamos arisealter ideas of how and when dynamos arise

• Have basaltic surface compositionHave basaltic surface composition– possibly possessing an early magma ocean like our Moonpossibly possessing an early magma ocean like our Moon

• Experienced significant excavating eventsExperienced significant excavating events– indicated by huge crater near southern poleindicated by huge crater near southern pole

Vesta meteoriteVesta meteorite

• Radioisotope chronology (i.e. cosmic ray dating) from the howardite, Radioisotope chronology (i.e. cosmic ray dating) from the howardite, eucrite, and diogenite (HED) meteorites believed to be from Vesta eucrite, and diogenite (HED) meteorites believed to be from Vesta suggests it accreted in only 5-15 million years (Vesta has same spectral suggests it accreted in only 5-15 million years (Vesta has same spectral signature as pyroxene)signature as pyroxene)– similar evidence indicates that Mars continued to accrete for close to 30 Myrs similar evidence indicates that Mars continued to accrete for close to 30 Myrs

and Earth for 50 Myrsand Earth for 50 Myrs

• Dating also indicate that impacts have released meteoritic material at Dating also indicate that impacts have released meteoritic material at least 5x in the last 50 Myrs.least 5x in the last 50 Myrs.

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.

Ceres – Quick Facts Ceres – Quick Facts

• Named after the Named after the Roman Goddess of Roman Goddess of AgricultureAgriculture

• Largest asteroid (933 Largest asteroid (933 km diameter)km diameter)

• Distance: 2.769 AUDistance: 2.769 AU• Rotation rate: 9.075 hrRotation rate: 9.075 hr• Very primitive surface Very primitive surface

w/ water bearing w/ water bearing minerals!minerals!

Ceres – Some propertiesCeres – Some properties

• Have active hydrological processes Have active hydrological processes leading to seasonal polar caps of water leading to seasonal polar caps of water frostfrost

• Have thin permanent atmosphere of frostHave thin permanent atmosphere of frost

• Have dry clay surface Have dry clay surface – indicated by microwave studies) indicated by microwave studies)

No Ceres MeteoriteNo Ceres Meteorite

• ……But it is expected to be like Vesta and But it is expected to be like Vesta and formed ~ 10 Mya. formed ~ 10 Mya.

• Possibly the excavating events or Possibly the excavating events or dynamic that provided the HED meteorites dynamic that provided the HED meteorites did not occur, but it's possible that the did not occur, but it's possible that the reflectance spectrum of the surface is not reflectance spectrum of the surface is not indicative of Ceres' crustal rocksindicative of Ceres' crustal rocks– on Vesta, the basaltic dust layer reflects its on Vesta, the basaltic dust layer reflects its

crustal compositioncrustal composition

Why are Ceres and Vesta so different?Why are Ceres and Vesta so different?

It appears that a rather short additional radial It appears that a rather short additional radial separation allowed Ceres (further away) to separation allowed Ceres (further away) to accrete wet and stay cool while early heat accrete wet and stay cool while early heat sources in the accreting material melted Vesta sources in the accreting material melted Vesta (slightly closer)(slightly closer)

Go DAWN mission!Go DAWN mission!

Will determine internal structure, density Will determine internal structure, density and by measuring their mass, shape, and by measuring their mass, shape, volume and spin rate with both imagery, volume and spin rate with both imagery, laser altimetry and gravitylaser altimetry and gravity