Space Rocks ! John Curchin, USGS, Denver. Questions to be Considered 1. What are asteroids and how are they classified (Astronomy)? 2. Are they a threat.

Space Rocks ! John Curchin, USGS, Denver

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.

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

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

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)

Spectral Identification of Minerals

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’

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

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

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

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?

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

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 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

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

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

Clearwater Lakes 14 and 20 miles wide; 290 million years oldLocated near Hudson Bay, QuebecSubmerged central peak in smaller lake

Manicouagan, Ontario

60+ miles across; including annular melt sheetApprox. 212 million years oldExtensive shock features in crystalline rocks

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’

Other Impact-related Featuresa) Shatter

cones

b) Planar deform-ation featrures

c) Vitrified (and high pressure) mineral phases

d) Impact melt lens

Tektite buttons

MoldaviteA 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 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)

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

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

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

Ordinary Chondrites (S Asteroids?)

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

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

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

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

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

Three Views

of Vesta

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

Differentiated WorldsTerrestrial basalt,

Mt. Holyoke flow, Connecticut

Martian basalt, zagami meteorite

Vestan basalt

Lunar low Ti basalt

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

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

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

Cassini approaching Saturn March 27, 2004

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

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

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

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

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

Rings of Saturn

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

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 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’?

Target Earth