EART160 Planetary Sciences

Meteorites, Asteroids, and Minor Bodies

Meteorite-Asteroid Connections From Kring (2006), Unlocking the solar system’s past, Astronomy, August, pp

Asteroid Belt

Formation/evolution Mass: ~5% of Moon’s mass Previously believed to be an exploded or disrupted planet. Now believed to closer to “failed planet.” Gravitational perturbations by Jupiter prevented final accretion of planetesimals and promoted large orbital changes, and ejections. – Initial mass of belt may have been times greater. – Cleared within millions of years – Ceres (500 km radius) is the largest body left.

Historical aside: Titus Bode “Law” (year 1715) Believed to be a coincidence, more of a “rule” than a “law.” Asteroid belt approximately takes the position of a predicted planet (Ceres discovered 1801) Neptune doesn’t work (1846). Pluto? Not the best rule or law! a =  2 n n = -inf, 0, 1, 2… (k=2 n )

Kirkwood Gaps – evidence of Jupiter’s effect Destabilizing mean motion resonances with Jupiter deplete zones of semimajor axis.

Lagrangian Points & Trojans Definition: Points where the gravity of two large bodies provide a centripetal acceleration that permits a third body to remain stationary in a rotating reference frame. Trojans (and Greek camp) – Dynamical group: occupy Sun-Jupiter L4 and L5. L4 and L5 are stable like the bottom of a valley (attractor) L1-L3 points are stable like a ball on a hill (or ridge)

Hildas – dynamical group 3:2 Mean motion resonance with Jupiter Smaller semi-major axis than the Trojans Moderate eccentricities Triangular distribution

Large asteroids Now a dwarf planet

Ceres (more later) Hubble image, contrast enhanced

Dwarf Planet Definition International Astronomical Union (IAU): A celestial body orbiting a star that is massive enough to be spherical as a result of its own gravity, but has not cleared its neighboring region of planetesimals and is not a satellite. – Hydrostatic equilibrium Problems with this definition? Still debated

Dwarf Planets Name Region of Solar System Orbital radius (AU) Orbital period (years) Mean orbital speed (km/s) Inclination to ecliptic (°) Eccentricity Equatorial Diameter (km) Ceres Asteroid belt  3.2 PlutoKuiper belt  10 HaumeaKuiper belt MakemakeKuiper belt Eris Scattered disc

Asteroid Classifications Spectral categories C-group: carbonaceous, ~75%, albedo < 0.1 S-type: siliceous composition (stony), ~17% M-group: metallic, but diverse interpretations Other, less common types exist Generally, people think of C-group, S-group, and M-group corresponding to meteorites of carbonaceous, siliceous, and metallic composition.

Near Earth Objects (asteroids) More easily accessible by spacecraft. Impact the inner planets Perihelion < 1.3 AU – but could have high e There are about 7000 documented NEOs, almost all asteroids (NEAs). The largest is ~32 km in diameter (1036 Ganymed).

NEA examples 433 Eros, 34 x 11 x 11 km Second largest NEO

Why go to an asteroid? Can’t you just study meteorites?

Just One Reason: Space Weathering Vapor is produced by impacts of solar wind and micrometeorites. Iron particles condense out of the vapor, with nanometer length scales This reddens and darkens the surface Fresh materials appear bright, and old materials are darker and have weaker spectral bands. This makes it difficult to determine what an asteroid (or the Moon) is really made of.

Other reasons What do they look like? – How did they form?

NEAR mission Gamma ray spectrometer Measure spectral properties Study regolith processes One goal was to link the asteroid (S-type) to meteorites on the ground – Didn’t really work out, but still learned a lot. Launched in 1996, arrived in 2000

NEAR at Eros Shape model Geology highlights: regolith exists, ponding and albedo changes

Itokawa Goal: return a sample, study a much smaller size asteroid.

Itokawa Highlights: regolith sorting, color contrasts. Sample returned! (?)

Rosetta Mission ESA Rosetta Spacecraft – Land on a comet in 2014

Lutetia July 2010 Rosetta flyby at 3000 km. 120 km length Mass and density?

Doppler shift

Deep Space Station meter antenna in Madrid, Spain

Lutetia July 2010 Density of 3.4 g/cm 3. – Greater than stony meteorites. – Partially differentiated?

Dawn – Main belt mission Energetically difficult, uses ion propulsion. Targets two very different bodies: – 1 Ceres (largest asteroid) – carbonaceous – 4 Vesta (third largest asteroid) – basaltic (melted, differentiated)

Ceres Hubble image Mysterious bright spot. An intact, surviving protoplanet! 1/3 rd the asteroid belt’s mass. Density: 2.1 g/ccm Equatorial radius: 487 km Albedo: 0.09 Carbonaceous (C-type) Hydrated minerals. Possibly partially differentiated. ??????

Vesta Hubble image Much less spherical. Mean radius: 265 km Albedo: 0.43 Density: 3.42 grams/ccm ! Unique V-type (Vestoid) Much drier than Ceres. Differentiated, likely formed a core, basaltic eruptions. Magma ocean? Why so hot? Why so different? What does its shape tell you?  Only asteroid definitively linked to meteorites (HED meteorites).

Dawn at Vesta

Asteroid spin rates

Meteorites

“Meteorite belts” Antarctica Deserts in NW Africa – Fall vs. find

Meteorite classifications

Chondrules Small spherules. Rapidly heated and cooled grains found in chondrite meteorites. Made of mostly olivine and pyroxene. – Can make up a large (>50%) fraction of meteorite mass. – Some of the earliest solid material in the SS. Formation mechanism not understood – Shock processes in the nebular gas? – Droplets from impacts? Millimeter scale bar

Meteorite Classifications Chondrites (ordinary) – 80% of all meteorites. Not melted, but more processed than CCs. Represent terrestrial planet materials. Carbonaceous chondrites – 5% of meteorites. Carbon and water rich. Not significantly heated. Close to solar nebular composition. Achondrites: no chondrules, igneous processes and heating. – 8% of all meteorites. – Mostly HED’s – Lunar and martian meteorites Irons: rich in iron, large crystal sizes in the metal means long cooling times deep in a planetesimal core.

HED meteorites & Vesta

Why does Vesta yield such nice spectra? Vesta + fresh eurcrite

Why does Vesta yield such nice spectra? Vesta + fresh eurcrite Magnetic shielding of solar wind?

Key concepts Asteroid belt, Kirkwood gaps Lagrangian points Near earth objects Space weathering Some geologic observations of asteroids Vesta vs. Ceres Chondrules and chondrite meteorites HED meteorites and Vesta