EART160 Planetary Sciences

Meteorites, Asteroids, and Minor Bodies

Meteorite-Asteroid Connections

From Kring (2006), Unlocking the solar system’s past, Astronomy, August, pp. 33-37.

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 100-1000 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 = 0.4 + 0.3 2n

n = -inf, 0, 1, 2…

(k=2n)

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 ofSolar System

Orbitalradius (AU)

Orbital period(years)

Mean orbitalspeed (km/s)

Inclination to ecliptic (°) Eccentricity

Equatorial Diameter

(km)

Ceres Asteroid belt 2.77 4.60 17.882 10.59 0.080 974.63.2

Pluto Kuiper belt 39.48 248.09 4.666 17.14 0.249 230610

Haumea Kuiper belt 43.34 285.4 4.484 28.19 0.189 1150 +250 -100

Makemake Kuiper belt 45.79 309.9 4.419 28.96 0.159 1500 +400 -200

Eris Scattered disc 67.67 557 3.436 44.19 0.442 2340

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 kmSecond 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 63

70 meter antenna in Madrid, Spain

Lutetia July 2010• Density of 3.4 g/cm3.

– 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!1/3rd the asteroid belt’s mass.Density: 2.1 g/ccmEquatorial radius: 487 kmAlbedo: 0.09Carbonaceous (C-type)Hydrated minerals.Possibly partially differentiated.

??????

Vesta

Hubble image

Much less spherical.

Mean radius: 265 kmAlbedo: 0.43Density: 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