1. John Curchin, USGS, Denver
Space Rocks !
John Curchin, USGS, Denver

2. Questions to be Considered
What are asteroids and how are they classified (Astronomy)?
Are they a threat to Earth (Geology)?
Do we already have samples (Meteoritics)?
The answers to all three have origins with the ‘state of science’ in 1804.

3. Astronomy in 1804 (and 2004) 1 Ceres 3 Juno
Uranus is discovered in 1781 by the English musician, William Herschel using a home-built telescope
The first 3 asteroids Ceres, Pallas, and Juno are discovered between 1801 and1804
‘Bode’s Law’ holds up; nature seems to be deterministic and predictable
1 Ceres
3 Juno

4. Asteroid Belt as viewed from Above
Over 100,000 objects greater than 10 km. now identified in the Main Belt
Total mass less than 1% of moon’s mass
Over 100 NEAs greater than 1 km. across are being tracked; probably part of a population of about 2000
Kirkwood gap (and others) occur in the belt where there are orbital resonances with Jupiter
Asteroids classified by ‘spectral group

5. How to Classify Asteroids
Glass (or a fine mist of water droplets) separates lignt into separate wavelengths due to ‘differential refraction’
Eyes are sensitive to brightness variations (rod cells) and 3 colors (R, G, B cone cells)

6. Spectral Identification of Minerals

7. S Asteroids (‘silicaceous’)
951 Gaspra 433 Eros (true color) Ida (and Dactyl)
19 x 12 x 11 km x 13 x13 km 58 x 23 km (1km)
Galileo flyby, NEAR orbit/landing Galileo flyby, 1993
Grooves, curved near-Earth asteroid, member of Koronis
depressions, ridges space weathering family, first ID of
(Phobos-like) effects documented asteroid ‘moons’

8. C Asteroids (‘carbonaceous’)
253 Mathilde; 66 x 48 x 46 km, visited by NEAR Shoemaker
Surface as dark as charcoal; typical outer belt asteroid

9. Comets Comet Borrelly, visited by Deep Space 1, 1999
8 x 3 x 3 km (bowling pin)
Variety of surface terrains, albedos (craters?)
Comet Wild 2, visited by Stardust in January, 2004
5.5 x 4 x 3.3 km (hamburger)
Craters may be due to impact or outflow jets of gases; indicate cohesive strength of nucleus

10. Comet Shoemaker-Levy 9 fragments impact Jupiter, July 16-22, 1994
‘Bull’s eye’ on Jupiter larger than Earth; first evidence of water in the jovian atmospher

11. What is the Asteroid Threat ? ‘Can’ they strike Earth and how often?
Controversial until late 20th century; few NEAs were known, spectral matches between asteroids and meteorites were poor, and no known mechanism could account for their delivery from the asteroid belt
Recognition of ‘chaos’, extreme sensitivity to initial conditions, as fundamental to most natural processes, especially for orbital dynamics (Comet SL 9, 1994)
Collisional (orbital) and radiation (space weathering, Yarkovsky effect) processes become important to objects in asteroid belt over billions of years
Combination of processes provides a ‘conveyer belt’ of (reddened) material to Earth orbit
Must look to geology for ‘ground truth’ – what is the evidence for impact, size-frequency distribution of impacting bodies?

12. Geology in 1804
“Theory of the Earth” by James Hutton, establishes geology as a science, with the its primary doctrine of uniformitarianism (explained by Lyell)
Application of this doctrine to the stratigraphy and structure of terrestrial rocks suggests an ancient Earth
Georges Cuvier, a French paleontologist, recognizes that fossils are ancient life forms, these forms change through time, and that most fossils are of forms now extinct

13. Full Moon (telescope view) with lighter highlands and darker basalt plains, filling multi-ringed basins
Apollo 16 view of Descartes Highlands, with impact craters at all scales

14. Meteor Crater Owned by Barringer family since 1903; 1.2 km
Formed ~50,000 years ago from 50m impactor
Origin established by Gene Shoemaker in 1950s
Associated with Canyon Diablo meteorite field

15. Wolfe Creek ~1/2 mile across; 300,000 years old, W. Australia
Also associated with many small iron meteorites

16. Simple vs. Complex Craters
Simple bowl structure
Diameter is times diameter of impacting object
All less than 1-2 miles across on Earth
Complex structure with central peak, peak ring, or multiple rings
Melt sheet generated and thick breccia lens
Terraced, collapsed walls; about 10x impactor diameter

17. Clearwater Lakes 14 and 20 miles wide; 290 million years old
Located near Hudson Bay, Quebec
Submerged central peak in smaller lake

18. Manicouagan, Ontario 60+ miles across; including annular melt sheet
Approx. 212 million years old
Extensive shock features in crystalline rocks

19. Chixulub, Yucatan penninsula, Mexico
Gravity map of buried structure
180 miles across; 65 millions years old
Identified in early 1990s with seismic data, after 10 year ‘search’

20. Other Impact-related Features
a) Shatter cones
b) Planar deform-ation featrures
c) Vitrified (and high pressure) mineral phases
d) Impact melt lens

21. A tektite from Czechoslovakia
Tektite buttons
Moldavite
A tektite from Czechoslovakia

22. Tunguska, Siberia, June 30, 1908
Black and white photos taken during field expedition in 1927; color photo taken in 1990

23. Jackson Hole Fireball, August 10, 1972

24. Potentially Hazardous Asteroid Threat Size-frequency diagram for impacting objects
~100 tons of meteroritic dust falls each day
50 m impactor once per 1000 yr (local effects)
500 m impactor once per million years (regional effects)
5 km. impactor once per 100 million years (global effects)

25. Meteoritics in 1804
Ernst Chladni, a German physicist, proposes an extraterrestrial origin for meteorites in 1794
Numerous witnessed meteorite falls occur in the 1790s, especially at Siena, Italy in 1794 and at Wold Cottage, England, in 1795
Chemical analysis on many ‘fallen stones’ during , establishes their chemical similarity to each other, and distinctive differences from terrestrial rocks

26. Hoba Iron 3m x 2m x 1m; 60+ tons Found 1920, Namibia
No crater, classified ataxite

27. Gibeon Iron 3000+ gm full slice
Distinctive Widmanstatten pattern of intergrown iron-nickel alloys
Found Namibia, 1836
Strewn field with over 50 tons of ‘irons’
Available on E-bay for $

28. Ordinary Chondrites (S Asteroids?)

29. Stereoscope adapted for Polarized Light Viewing
Thin sections are wafer thin slices of rock (.03 mm) glued to a standard glass slide
For geologic purposes, standard (‘biologic’) microscopes are adapted with two polarizers and a rotating stage
The unique optical properties of different mineral crystals affect polarized light differently

30. Chondrites in Thin Section
Tuxtuac, Mexico; fall Lost Creek, Kansas
classified LL classified H3.8
‘barred’ olivine chondrule radial pyroxene (~ 1 mm diameter) chondrule

31. Allende (C asteroid?) Fell in Mexico, Feb, 1969
Carbonaceous, subclass of the stony chondrites
Primitive composition (solar, minus lightest elements)
Contains abundant chondrules and CAIs, calcium-aluminum inclusions, dated at billion years old

32. Glorietta Mountain New Mexico Pallasite (full slice)
Stony-iron meteorite
Olivine suspended in an iron matrix
Etched iron shows Widmanstatten pattern
Olivines with very uniform composition
Likely source: core-mantle boundary region of a once differentiated and since-shattered asteroid

33. Howardites, Eucrites and Diogenites
‘Achondrites’ – meteorites without chondrules; from differentiated objects that have melted inside
Eucrites similar to terresrial basalts
Diogenites, of almost pure pyroxene, resemble terrestrial ‘cumulates’
Howardites are breccias of other two
Spectral similarities with V asteroid class

34. Three Views of Vesta Hubble image, model and color-shaded topography
Largest member of V class of asteroids (vestoids)
Spectral variations consistent with HEDs

35. Differentiated Worlds
Terrestrial basalt, Mt. Holyoke flow, Connecticut
Martian basalt, zagami meteorite
Vestan basalt
Lunar low Ti basalt

36. But how do we know?!
Oxygen isotope ratios distinguish among solar system materials chemically; Earth and Moon plot together
Planetary processes ‘smear’ O isotopes along a trend within one world; different initial ratios for each world

37. What were the processes and products in the early Solar System (Meteoritics, 2004)
Impact features on all planetary surfaces; planets formed by accretion of planetesimals from a turbulent solar nebula
Much mixing of components; completed in 5-10 million years
‘Residual’ debris forms asteroid belt; Kuiper belt, Oort cloud

38. Star-forming region in Large Magellenic Cloud, Hubble, 2003

39. Cassini approaching Saturn March 27, 2004

40. Closing in on Phoebe
Phoebe is an outer moon of Satrurn, 220 km. in diameter, and a retrograde orbit
Top 3 images taken between June 4th and 7th
Discovered in 1898, it has an albedo of 6% and a density of 1.6 gm/cc.
June 10th image shows craters, peaks and bright-
ness variations

41. Phoebe
High resolution mosaic taken at closest approach on June 11, 2004
Contrast is highly ‘stretched’ in this image to show icy areas (bright streaks on crater walls)
Craters visible at all scales; ancient surface
Probably a remnant from an early, icy outer population of planetes-
imals now in the Kuiper Belt beyond Neptune

42. Phoebe Mineral Maps Images taken at visible and infrared wavelengths
Red, green and blue are assigned to different IR wavelengths representing different materials
Composite image shows mineral distribution of ferrous (+2) iron, water ice and unidentified ‘dirt’ component

43. Titan in Natural Colors
Atmosphere thicker than Earth’s; composed of nitrogen and methane
Reactions with sun-
light in the upper atmosphere generate a rich organic smog
Conditions at surface (low temp.; high pressure) suggest possible lakes and/or oceans of complex hydrocarbons at surface
May be similar to conditions on early Earth; Huygen’s probe to enters Titan’s atmosphere Jan. 14, 2005

44. Titan at Different Wavelengths
‘Pictures’ of Titan taken at three different wavelengths (2 of which actually ‘saw’ the surface)
Brightness variations in each image are scaled to either red, green or blue
RBG composite yields ‘surface composition’ map

45. Rings of Saturn Visible rings 99%+ water ice particles
A ring: ice mountains
Cassini division: ice cubes
B ring: ice boulders
C ring: snowflakes

46. Saturn’s Rings at Different Wavelengths
Image taken above rings with transmitted light at closest approach June 25
IR reflectance shows thickness; ice concentrated in outer A ring
Cassini division shows both ice and the ‘dirt’ signature seen at Phoebe

47. Saturn’s Rings in Ultraviolet Light
Cassini Division and entire A ring; 15,000 km wide
A ring increasingly icy to outside; Encke gap‘dirty’?
C ring B ring transition
Trend from ‘dirty’ outer C ring on left to ‘icier’ B ring

48. Target Earth