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Space Rocks

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Space Rocks !
John Curchin, USGS, Denver
Questions to be Considered
1.
2.
3.
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.
Astronomy in 1804 (and 2004)
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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
Asteroid Belt as viewed from Above
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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
How to Classify Asteroids
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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)
Spectral Identification of Minerals
S Asteroids (�silicaceous’)
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951 Gaspra
19 x 12 x 11 km
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depressions, ridges
(Phobos-like)
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433 Eros (true color)
33 x 13 x13 km
NEAR orbit/landing
near-Earth asteroid,
space weathering
effects documented
Ida (and Dactyl)
58 x 23 km (1km)
Galileo flyby, 1993
member of Koronis
family, first ID of
asteroid �moons’
C Asteroids (�carbonaceous’)
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253 Mathilde; 66 x 48 x 46 km, visited by NEAR Shoemaker
Surface as dark as charcoal; typical outer belt asteroid
Comets
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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
Comet Shoemaker-Levy 9 fragments
impact Jupiter, July 16-22, 1994
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�Bull’s eye’
on Jupiter
larger than
Earth; first
evidence of
water in the
jovian
atmospher
What is the Asteroid Threat ?
�Can’ they strike Earth and how often?
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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?
Geology in 1804
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“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
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
Wolfe Creek
~1/2 mile across; 300,000 years old, W. Australia
Also associated with many small iron meteorites
Simple vs. Complex Craters
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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
Clearwater Lakes
14 and 20 miles wide; 290 million years old
Located near Hudson Bay, Quebec
Submerged central peak in smaller lake
Manicouagan,
Ontario
60+ miles across; including annular melt sheet
Approx. 212 million years old
Extensive shock features in crystalline rocks
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’
Other Impact-related Features
a) Shatter
cones
b) Planar
deformation
featrures
c) Vitrified
(and high
pressure)
mineral
phases
d) Impact
melt lens
Tektite buttons
Moldavite
A tektite from
Czechoslovakia
Tunguska, Siberia, June 30, 1908
Black and white
photos taken during
field expedition in
1927; color photo
taken in 1990
Jackson Hole Fireball, August 10, 1972
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)
Meteoritics in 1804
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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
Hoba Iron
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3m x 2m x 1m; 60+ tons
Found 1920, Namibia
No crater, classified ataxite
Gibeon Iron
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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
Ordinary Chondrites (S Asteroids?)
Stereoscope adapted for Polarized Light Viewing
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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
Chondrites in Thin Section
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Tuxtuac, Mexico; fall 1975
classified LL5
�barred’ olivine chondrule
(~ 1 mm diameter)
Lost Creek, Kansas
classified H3.8
radial pyroxene
chondrule
Allende (C asteroid?)
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Fell in Mexico, Feb, 1969
Carbonaceous, subclass of the stony chondrites
Primitive composition (solar, minus lightest elements)
Contains abundant chondrules and CAIs, calciumaluminum inclusions, dated at 4.567 billion years old
Glorietta Mountain
New Mexico
Pallasite (full slice)
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Stony-iron meteorite
Olivine suspended
in an iron matrix
Etched iron shows
Widmanstatten
pattern
Olivines with very
uniform composition
Likely source: coremantle boundary
region of a once
differentiated and
since-shattered
asteroid
Howardites, Eucrites and Diogenites
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�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
Three Views
of Vesta
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Hubble image, model and color-shaded topography
Largest member of V class of asteroids (vestoids)
Spectral variations consistent with HEDs
Differentiated Worlds
Terrestrial basalt,
Mt. Holyoke flow,
Connecticut
Martian basalt,
zagami meteorite
Vestan basalt
Lunar low Ti basalt
But how do we know?!
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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
What were the processes and products in
the early Solar System (Meteoritics, 2004)
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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
Star-forming region in Large
Magellenic Cloud, Hubble, 2003
Cassini approaching Saturn
March 27, 2004
Closing in on Phoebe
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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 brightness variations
Phoebe
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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 planetesimals now in the Kuiper
Belt beyond Neptune
Phoebe Mineral Maps
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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
Titan in Natural Colors
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Atmosphere thicker than
Earth’s; composed of
nitrogen and methane
Reactions with sunlight 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
Titan at Different Wavelengths
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�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
Rings of Saturn
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Visible
rings 99%+
water ice
particles
A ring: ice
mountains
Cassini
division:
ice cubes
B ring: ice
boulders
C ring:
snowflakes
Saturn’s Rings at Different Wavelengths
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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
Saturn’s Rings in Ultraviolet Light
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C ring B ring transition
Trend from �dirty’ outer C
ring on left to �icier’ B ring
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Cassini Division and entire
A ring; 15,000 km wide
A ring increasingly icy to
outside; Encke gap�dirty’?
Target Earth
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