Asteroids and Comets

1. Asteroids and Comets Clues to the origin of the Solar System

2. Asteroids • Rocky and metallic objects that orbit the Sun too small to be planets • Main Asteroid Belt • Orbiting between Mars and Jupiter • Physics predict a planet should be in that orbit • Document the transition between terrestrial and gas rich planets • Evidence of a temperature gradient in the ancient solar nebula • Volatile poor inner belt • Volatile rich outer belt • Apollo or Near Earth Asteroids (NEA’s) • Smaller pop • Highly elliptical orbits • May cross Earths orbit • Est. >1000 over 1km in diameter • 50million tons of new material to Earth and its atmosphere per year

3. Asteroids (cont’d) • Main Asteroid Belt (the white cloud) • Hildas (orange "triangle" just inside the orbit of Jupiter) • Jovian Trojans (green) • Group leading Jupiter called the "Greeks“ • Group trailing called the "Trojans” • Est. btwn 1.1 and 1.9 million asteroids > than 1 km in diam

4. 4-Vesta, Ceres and our Moon Asteroids (cont’d) • Left-overs from the formation of the Solar System • Two theories of formation • Remains of planet destroyed by gravitational pull between Sun and Jupiter • Never accreted into planet for same reason • Interest • Material from early formation of Solar System • Those of igneous composition can be radiometrically dated • 4.6by • Knowledge comes from debris falling to the Earth • Show us composition of SS based on location of formation • Record of planetesimals, formation, differentiation, break up

5. Asteroids (cont’d) • Distinct histories based on age, composition, internal structure,and size of bodies from which they originated • Stony – (92.8%) - silicate composition • Chondrites – (85.7%) - carbonaceous and enstatite • Ordinary – metamorphosed chondrites • Carbonaceous – carbon & volitile-rich, undifferentiated : similar to primative nebular material from which the planets formed (4.6by) • Achondrites - (7.1%) – igneous textures, differentiated : examples of primative crusts surrounding a differentiating planetoid • Some as young as 1.3by • SNC’s (shergottites, Nakhlites, and chassignites) may be pieces of Martian crust • ~30 known Chondrules

6. Asteroids (cont’d) • Stony Iron – (1.5%) - silicate-metal mixture, differentiated • Quite rare • Some represent fragments of transition zone between metallic core and rocky mantle • Requires differentiation of larger asteroids (hundreds of km) • Iron (5%) • Fe and Ni composition (5.7%) • Easiest to identify • Widmanstatten figures • Evidence of originally molten material • Cooling history suggests these formed as core material of larger asteroids

7. Impact Craters • Record of early Solar System environment • Size and number indicative of age of surface • Mercury, Moon • Lack recent active geologic processes • Provide measuring stick for cratering throughout age of Solar System • Earth • 120 terrestrial impacts known • Previous covered by erosion or destroyed by plate tectonics • Meteor Crater (Berringer Crater) – Arizona • ~100m diameter • Destruction radius > 40km • Chicxulub Crater – Yucatan Peninsula Mexico • 170km diameter • 64.98my • K-T boundary • Extinction of >70% of life forms • Iridium

8. Impact CratersPeru, 2006

9. Iron meteorite found at Derrick Peak, Antarctica • Composed mostly of Fe and Ni • Probably a small piece from the core of a large asteroid that broke apart

10. Antarctic Discoveries • Many meteorites are found on the Antarctic ice sheet • Ice movement consolidates meteorites at terminus of glaciers • Easy to differentiate from surroundings • Martian, Lunar, Asteroids • Meteorites of composition compatible with nearby celestial bodies provide only samples we have (except Lunar rocks returned by Apollo missions)

11. Asteroids (NEA’s) • NASA Near Earth Objects (NEO) utilizes observations of various entities to identify, monitor and plot potentially dangerous objects • Asteroids that approach or cross Earth’s orbit are known as NEA’s • ~2000 known NEA’s • Potentially Hazardous Asteroids (PHA’s) • Predicted bodies that may impact at some future time

12. Asteroid Encounters • Galileo • Encountered Gaspra and Ida in flyby mode on way to Jupiter • NEAR (Eros) • In orbit for ~year • Landed on surface despite not a mission goal • Highest resolution images ever acquired • Deep Impact • Tempel Comet – designed to crash into comet

13. Comets Harbingers of ill omen

14. Comets • Most primordial material from formation of outer proto-solar nebula • If SS cooled from out to in then material would not have changed much over time • Direct condensation of solar nebula • Impactors during early SS • Provided volatiles to inner planets • Origin of organic molecules • Building blocks of life?

15. Comet Components • Composed of ices and dusts • Coma • Nucleus or compacted body of comet • Icy dirtball w/carbonaceous mat • 20% of surface is active • Tails • 2 tails • Visible larger • Volatiles being sublimated by solar power • Dust reflecting sunlight • Invisible (bluish) smaller • CO2, H2O • Begins around Mars orbit • Always points away from Sun • Can be >10’s of millions of km

16. Comets • Kuiper Belt • Close to Pluto • Short orbital periods • Orbits are in same plane as SS • Oort Cloud • No direct evidence for cloud • Highly elliptical orbits • Vary from SS plane • May extend 200,000 times 1AU (1/10th distance to nearest star)

17. CometShoemaker-Levy • Discovered in 1993 • Captured by Jupiter’s gravitational pull • Ripped into “string of pearls” • from July 16-22 collided with Jupiter in the first observed large impact within our SS • Bright explosive plumes easily visible with amateur telescopes • Larger than Earth • Light from Jupiter increased fifty-fold

18. Shoemaker-Levy • Image of Jupiter taken on the NASA Infrared Telescope Facility, Mauna Kea, Hawaii, at 08:54 on July 21, 1994 • Image taken using the IRTF's facility near infrared camera • Io, the closest of the jovian moons, can be seen crossing the planet (top right) • The Great Red Spot is visible in the lower left • At the collision latitudes, the impact due to Fragment Q is just setting on the west. Just to the east of it, the R Fragment impact site shows up very brightly. Another four impact sites form a chain of spots behind R

19. Shoemaker-Levy • The G impact site is visible as a complex pattern of circles seen in the lower left of the partial planet image. The small dark feature to the left of the pattern of circles is the impact site of fragment D. The dark, sharp ring at the site of the fragment G impact is 80% of the size of the Earth • Comet Shoemaker-Levy 9 broke up into 21 fragments during a close passage by Jupiter in July of 1992. Fragment G was one of the brightest and likely the largest of the 21 fragments • Scientists estimate that the combined energy from all of the impacts will approach the equivalent of 40 million megatons of TNT.

20. Shoemaker-Levy • Image of Jupiter with the Hubble Space Telescope Planetary Camera • Eight impact sites are visible. From left to right are the E/F complex (barely visible on the edge of the planet), the star-shaped H site, the impact sites for tiny N, Q1, small Q2, and R, and on the far right limb the D/G complex. The D/G complex also shows extended haze at the edge of the planet • The features are rapidly evolving on timescales of days The smallest features in this image are less than 200 kilometers across.

21. Eros • High resolution images of Eros as acquired by NEAR Shoemaker while in orbit around the asteroid

22. Tempel 1 • Deep Impact of Comet Tempel 1 • Flyby and impactor probe • Goal to acquire data on material from inside a comet by creating an artificial impact crater • Images of ejected material can be spectroscopically analyzed for composition

23. Stardust • Sampled Wild2 comet • Silica Aerogel – glass but @ 1% density • Tail material is “clumped” • Calcium Aluminum Inclusions • Some of oldest SS material found in asteroids • Need heat to form • Collectors • Sweep up material from inner and outer SS • Smaller grains = outer • Larger grains = inner

24. Extinctions • Used to coarsely divide geologic time • i.e. Eras • Large scale global die offs • Over 70% of species living at time • Marine and continental K-T boundary near Alberta, Canada

25. Extinctions • In last 550 my at least 5 major extinctions • Late Paleozoic continents tectonically assembled into "Pangea" • Spanned pole to pole • North to south barrier blocking ocean circulation and severely disrupting climate • Dire effect on global ecosystems • Largest mass extinction in all of Earth history • Over 90% of all known species of life disappeared

26. K-T BoundaryAsteroid • ~65mya • ~70% species die-off • Asteroid Theory • Luis & son Walter Alvarez postulated reason for K-T extinction • Iridium layer • Chicxulub Crater • Cenotes • Gravity anomoly • Tektites

27. Chicxulub Crater • The age of the crater & the Cretaceous–Tertiary (K–T boundary) coincide • Crater is more than 180 km (110 mi) in diameter • One of the largest confirmed impact structures on Earth • Bolide that formed the crater was at least 10 km (6 mi) in diameter • Discovered by Antonio Camargo and Glen Penfield • Geophysicists who had been looking for petroleum in the Yucatán during the late 1970s • Couldn’t release info initially • Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas • Supported theory of extinction by Alvarez’s

28. Cenotes

29. K-T Boundary - Volcanism • Deccan Traps, India • Layered basalts today are 2,000 m thick and cover 500,000 km² • Est. as large as 1.5 million km² • Approximately half the size of modern India • Eruption associated with a deep mantle plume • Caused Deccan Traps eruption • Volcanic gases contributed to an apparently massive global warming • Est. aver rise in temp of 8 °C (14 °F) in the last half million years before the impact at Chicxulu