1. EART160 Planetary Sciences

2. Meteorites, Asteroids, and Minor Bodies

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

4. Asteroid Belt

5. 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.

6. Historical aside: Titus Bode “Law” (year 1715)
a =  2n
n = -inf, 0, 1, 2…
(k=2n)
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!

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

8. 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)

9. Hildas – dynamical group
3:2 Mean motion resonance with Jupiter
Smaller semi-major axis than the Trojans
Moderate eccentricities
Triangular distribution,
avoid Jupiter
(aphelia opposite Jupiter)

10. Large asteroids
Now a
dwarf planet

11. Ceres (more later)
Hubble image, contrast enhanced

12. 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

13. 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
2.77
4.60
17.882
10.59
0.080
974.63.2
Pluto
Kuiper belt
39.48
248.09
4.666
17.14
0.249
230610
Haumea
43.34
285.4
4.484
28.19
0.189
Makemake
45.79
309.9
4.419
28.96
0.159
Eris
Scattered disc
67.67
557
3.436
44.19
0.442
2340

14. 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.

15. 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).

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

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

18. Just One Reason: Space Weathering
Vapor is produced by solar wind and micrometeorites.
Iron particles condense from vapor, with nanometer length scales
This reddens and darkens the surface
Fresh materials are bright, but 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.
Samples collected with proper context could help resolve this issue

19. Other reasons What do they look like? How did they form?
Interior structure?
Evolution?

20. OSIRIS-Rex
Launch 2016, return sample in 2023 from the 500-meter-asteroid Bennu
Radar image of Bennu

21. 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

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

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

24. Itokawa
Highlights: regolith sorting, color contrasts. Microscopic sample returned!

25. Hayabusa 2 Launched December 3, 2014.
Copper projectile with surface explosives!
Similar goals

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

27. Trajectory

28. Flyby Doppler Shift with Deep Space Network (station 63)
70 meter antenna in Madrid, Spain

29. vo = relative flyby velocity
Lutetia July 2010
Density of 3.4 g/cm3.
Greater than stony meteorites.
Partially differentiated?
vo = relative flyby velocity
d = flyby distance
fx = 8.4 GHz
Planned data dropout
Final frequency shift.

30. 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)

31. Ceres Hubble image Mysterious bright spot.
An intact, surviving protoplanet!
1/3rd 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.
??????

32. Vesta Much less spherical. Hubble image 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).

33. Dawn at Vesta

34. Dawn at Vesta

36. Ceres
Image from May 5, 2015

37. Ceres Bright Spot(s) Occator crater 92 km diameter Origin?
Carbonates is one idea
Ice?

38. Asteroid spin rates

39. Meteorites

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

41. Meteorite classifications

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

43. 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

44. 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.

45. HED meteorites & Vesta

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

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

48. 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

49. Photo of Earth from Hayabusa-2
Taken from 3,000,000 km on Nov. 27, 2015.
Landing in Dec on 1-km-asteroid Ryugu

50. Rosetta Mission
ESA Rosetta Spacecraft
Landed on a comet in 2014