space rocks ! john curchin, usgs, denver. questions to be considered 1. what are asteroids and how...
TRANSCRIPT
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Space Rocks ! John Curchin, USGS, Denver
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Questions to be Considered
1. What are asteroids and how are they classified (Astronomy)?
2. Are they a threat to Earth (Geology)?
3. Do we already have samples (Meteoritics)?
The answers to all three have origins with the ‘state of science’ in 1804.
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Astronomy in 1804 (and 2004) 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
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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
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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)
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Spectral Identification of Minerals
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S Asteroids (‘silicaceous’)
951 Gaspra 433 Eros (true color) Ida (and Dactyl)
19 x 12 x 11 km 33 x 13 x13 km 58 x 23 km (1km) Galileo flyby, 199 NEAR orbit/landingGalileo flyby, 1993 Grooves, curved near-Earth asteroid, member of Koronis
depressions, ridges space weathering family, first ID of
(Phobos-like) effects documented asteroid ‘moons’
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C Asteroids (‘carbonaceous’) 253 Mathilde; 66 x 48 x 46 km, visited by NEAR Shoemaker Surface as dark as charcoal; typical outer belt asteroid
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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
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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
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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?
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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
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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
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Meteor CraterOwned by Barringer family since 1903; 1.2 kmFormed ~50,000 years ago from 50m impactorOrigin established by Gene Shoemaker in 1950sAssociated with Canyon Diablo meteorite field
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Wolfe Creek ~1/2 mile across; 300,000 years old, W. AustraliaAlso associated with many small iron meteorites
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Simple bowl structure Diameter is 15-20
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
Simple vs. Complex Craters
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Clearwater Lakes 14 and 20 miles wide; 290 million years oldLocated near Hudson Bay, QuebecSubmerged central peak in smaller lake
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Manicouagan, Ontario
60+ miles across; including annular melt sheetApprox. 212 million years oldExtensive shock features in crystalline rocks
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Chixulub, Yucatan penninsula, Mexico
Gravity map of buried structure180 miles across; 65 millions years oldIdentified in early 1990s with seismic data, after 10 year ‘search’
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Other Impact-related Featuresa) Shatter
cones
b) Planar deform-ation featrures
c) Vitrified (and high pressure) mineral phases
d) Impact melt lens
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Tektite buttons
MoldaviteA tektite from
Czechoslovakia
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Tunguska, Siberia, June 30, 1908
Black and white photos taken during field expedition in 1927; color photo
taken in 1990
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Jackson Hole Fireball, August 10, 1972
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Potentially Hazardous Asteroid ThreatSize-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)
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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 1802-1803,
establishes their chemical similarity to each other, and distinctive differences from terrestrial rocks
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Hoba Iron 3m x 2m x 1m; 60+ tons Found 1920, Namibia No crater, classified ataxite
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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
$1995.00
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Ordinary Chondrites (S Asteroids?)
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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
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Chondrites in Thin Section
Tuxtuac, Mexico; fall 1975 Lost Creek, Kansas classified LL5 classified H3.8 ‘barred’ olivine chondrule radial pyroxene
(~ 1 mm diameter) chondrule
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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 4.567 billion years old
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Glorietta Mountain New MexicoPallasite (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
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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
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Three Views
of Vesta
Hubble image, model and color-shaded topography Largest member of V class of asteroids (vestoids) Spectral variations consistent with HEDs
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Differentiated WorldsTerrestrial basalt,
Mt. Holyoke flow, Connecticut
Martian basalt, zagami meteorite
Vestan basalt
Lunar low Ti basalt
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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
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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
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Star-forming region in Large Magellenic Cloud, Hubble, 2003
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Cassini approaching Saturn March 27, 2004
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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
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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
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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
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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
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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
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Rings of Saturn
Visible rings 99%+ water ice particles
A ring: ice mountains
Cassini division: ice cubes
B ring: ice boulders
C ring: snowflakes
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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
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Saturn’s Rings in Ultraviolet Light C ring B ring transition Trend from ‘dirty’ outer C
ring on left to ‘icier’ B ring
• Cassini Division and entire A ring; 15,000 km wide
• A ring increasingly icy to outside; Encke gap‘dirty’?
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Target Earth