Part 4

Smaller Objects:
! Pluto
• Not a planet anymore (dwarf planet)
• A large Kuiper belt object
• Only 2,274 km in diameter
• 39.5 AU from the Sun (average)
• Mostly ice, some rocky material
• Orbited by Charon, , nearly as large as Pluto
! Kuiper Belt
• A disk-shaped region of space from 30 to 50 AU distant from the Sun
• Contains millions of tiny ice particles, some larger cometary bodies
• Comets are thought to derive mainly from the Kuiper Belt
• Eris - a Kuiper object discovered in 2005, it is ~2,500 km in diameter (larger
than Pluto) at 96.7 AU from the Sun
• Likely to be tens of thousands of objects >100 km in size, still undiscovered
! Oort Cloud
• Inferred from solar system formation models, not directly observed
• Thought to be a cloud of cometary bodies surrounding the Sun in a broad sphere
• May extend out to nearly 0.5 a light year from the Sun, and contain as much
mass as Jupiter (in total)

Asteroids
! Asteroid Belt
• Region of space between Mars and Jupiter, populated with thousands of large
planetesimals, , rocks, dust
• Likely a failed planet, or region where no single large mass was able to coalesce,
due to tidal effects from Jupiter’s s gravity
• Most mass in Asteroid Belt is found in a few large planetoids:
• Ceres - largest asteroid, at 950 km diameter, spherical
• 4 Vesta - 530 km spheroid
• 2 Pallas - 550 km spheroid
• 10 Hygeia - ~450 km irregular shape
• Asteroid types - reflective of meteor types, because most meteorites come from
shattered asteroids
• Stony - silicate rock, varying amounts of metal, former asteroid crust/mantle
• Metallic - mostly Fe-Ni metal, former asteroid core
• Carbonaceous - primitive, unprocessed solar nebula material
• Silicates, organic compounds (amino acids, hydrocarbons, saccharides, , etc.)
• Icy - rare in Asteroid Belt, objects made mostly of ice
• Probably from outer solar system originally, captured comets or Kuiper objects

Asteroids
951 Gaspra
(~18 km)
Ceres (950 km)

Meteorites
! Largely, meteorites are residue from Solar System formation and condensation
! Comprise a wide range of compositions, which formed at various epochs in our
system’s s early history
! Chondrites
• Most common type of meteorite, date to 4.5 Ga
• Undifferentiated, congealed directly by condensation from solar nebula
• Relative concentrations of elements is close to solar (sans H, He)
! Achondrites
! Irons
• Partially differentiated meteorites
• May derive from planetoids that underwent partial chemical differentiation
• Differ in mineral composition
• Thought to derive from earliest phases of solar nebula condensation, or from cores of
differentiated planetoids that were subsequently shattered by impact
! Carbonaceous Chondrites
• Most primitive meteorites, fairly rare
• Contain abundant organic carbon, amino acids, hydrocarbons… retained from condensation
in early solar system evolution in outer system

Comets
! Formation residue form solar nebula, in the outer solar system
! Mostly ices (H 2 O, CH 4 , CO 2 , NH 3 , N 2 , other volatiles)
! Substantial organic content, some light silicate components likely
! Highly variable in size
• Range from meters to kilometers in diameter. Comets are essentially outer system analogs
of inner system primordial debris (e. g. asteroids, meteors)
! Elliptical orbits
• Some comets follow highly elliptical or hyperbolic orbits taking them from the Kuiper / Oort
regions to the inner solar system
• Majority are probably in stable orbits in their source regions, largely unseen
! Earth-crossing asteroids
• A few are probably desiccated remnants or cores of comets now in close solar orbit
! Organics
• Likely that Earth’s s surface volatiles are largely attributable to cometary impacts during the
Late Heavy Bombardment (4.2 - 3.8 Ga)
• Oceans, atmospheric CO , N 2 2
• All inner planets were likely enriched initially with volatiles from comets