1. EART 160: Planetary Science Itokawa Enhydra lutris Image copyright Fred Hsu Image courtesy ISAS/JAXA

2. Last Time Paper Discussion –Thommes et al. (1999) Origin of the Moon Planetary Migration –Uranus and Neptune –Extrasolar Planets: “Hot Jupiters”

3. Today Paper Discussions –Asphaug et al. (2006) Meteorites –Age of the Solar System –Types of Meteorites Asteroids –Main Belt –ECAs, Trojans, Centaurs

4. Meteorites Extraterrestrial matter that falls on Earth –VERY late-stage accretion Why do we care? Pristine samples from the early solar system This is how we know what the early solar system was made of! Vital source of vitamins and minerals

5. Earth-Crossing Asteroids

6. How old is the solar system? We date the solar system using the decay of long-lived radioactive nuclides e.g. 238 U- 206 Pb (4.47 Gyr), 235 U- 207 Pb (0.70 Gyr) These nuclides were formed during the supernova which supplied the elements making up the original nebula Meteorite isochron (from Albarede, Geochemistry: An Introduction) The oldest objects are certain meteorites, which have an age of 4550 Myr B.P.

7. Radioactive Dating N = N 0 e - t = N 0 e -t/  N – number of radionuclides now N 0 – number we started with at time 0 t – time since start  – Average lifetime of radionuclide t 1/2 =  ln(2)Half-life (time for half the material to decay) © 1996 Frank Steiger; permission granted for retransmission.

8. Short Lived Radioactive Isotopes Some meteorites once contained live 26 Al, which has a half-life of only 0.7 Myr. So these meteorites must have formed within a few Myr of 26 Al production (in the supernova). So the solar system itself is also 4550 Myr old Decay of 26 Al releases a LOT of energy, could have melted, differentiated early-forming planetesimals CAI (Ca-Al inclusions) formation– oldest recorded event in SS. Early SS timing relative to this. Was 26 Al distributed uniformly?

9. Types of Meteorites TypeDescriptionAbundance Iron Fe, Ni. Low temperature inclusions Formed deep within differentiated planetary body Ni content tells us about parent body 4% Stony-Iron Mix of metal and rock. Intermediate depth 1% Stony95% Achondrites No metal or chondrites, similar to basalts Crustal source? 9% Chondrites Contain glassy chondrules, not remelted Ancient Planetesimals 86% Carbonaceous Low-T (< 500 K) Volatiles, organics 5% Ordinary Higher-T Little volatiles, C 81%

10. Famous Meteorites ALH 84001Martian Meteorite Once thought to have ET life Images Courtesy NASA Willamette Iron Meteorite15.5 tons Allende Carbonaceous Chondrite

11. How do you know it’s a meteorite? Look somewhere without rocks! Antarctica! 23000+ of meteorites found in Antarctica

12. Source We can use the meteorite composition to track it back to a type of asteroid Classify asteroids into types based on spectrum –C-type: carbonaceous –S-type:silicic –M-type:metallic NOT M-class planets

13. Sometimes we can determine the parent asteroid –V-types from Vesta –Pallasites DO NOT come from 2 Pallas Large asteroid broke up 160 Mya –298 Baptisma largest surviving fragment –Tycho Crater on the Moon –K-T Impact –Bottke et al. (2007) Nature 449, 48-53 Photo by Oliver Schwarzbach

14. Asteroids Gaspra Mathilde Ida Dactyl Eros NEAR Shoemaker: Flyby 1997 Orbiter/Lander 2000-2001 Galileo: Flybys 1991 1993 52 km 18 km 33 km 54 km

15. Rubble Pile Monolithic? Itokawa Hayabusa Flyby/”Landing” 2005 Sample Return 2010 Ceres Dawn: Flybys 2011-2015 Vesta 535 km 950 km 530 km (mean)

16. Sizes The Moon 1 Ceres 2 Pallas 4 Vesta 3 Juno 5 Astraea 6 Hebe 7 Iris 8 Flora 9 Metis 10 Hygeia M AB = 4% M  M Ceres = 1/3 M AB Number is order of discovery, not size

17. Distribution Main Belt Trojans / Greeks Hildas Earth-Crossing Asteroids

18. Main Belt 93% of all numbered asteroids Over 200,000 known 2.06 AU – 3.27 AU Perturbations from Jupiter disrupted formation of planet, most material ejected Jupiter maintains edges of main belt –4:1, 2:1 resonances w/ Jupiter –Kirkwood Gaps at 3:1, 5:2, 7:3

19. Orbital Parameters Semimajor Axes e vs. a i vs. a Image courtesy NASA Asteroids classified into groups based on a, e, i

20. Trojans Share Jupiter’s orbit –Lead and Trail Jupiter at 60º –L4 and L5 Lagrange points Also have Trojans at Neptume

21. Earth-Crossing Asteroids Have orbits that cross the Earth’s Pose a risk for impact Near misses –18 Mar 2004: 30 m asteroid passed 26,400 km away Spacewatch –NASA program to detect ECAs –Detect 90% of 1-km+ objects

22. What do we do? Send Bruce Willis to blow it up Send Robert Duvall to blow it up Send the Planet Express Crew to blow it up Launch a second ball of garbage to deflect it

23. Strategies Destroy it! –Can turn one falling object into many! Delay it! –Slow it down until Earth gets out of the way. Die Nuclear explosion – KABOOM! Collision – knock it off course Paint or dust it – radiation pressure, Yarkovsky effect Spacecraft Propulsion, Solar Sail

24. Next Time Paper Discussion –Mars Core and Magnetism, Stevenson (2001) Planetary Surfaces –Composition –Impacts We’ll do Interiors after we do surfaces

25. Find another isotope of the same element as the daughter that is never a result of radioactive decay (call that isotope ``B'' for below). Isotopes of a given element have the same chemical properties, so a radioactive rock will incorporate the NONradioactively derived proportions of the two isotopes in the same proportion as any nonradioactive rock. Measure the ratio of isotopes A and B in a nonradioactive rock. This ratio, R, will be the primitive (initial) proportion of the two isotopes. Multiply the amount of the non-daughter isotope (isotope B) in the radioactive rock by the ratio of the previous step: (isotope B) × R = initial amount of daughter isotope A that was not the result of decay. Subtract the initial amount of daughter isotope A from the rock sample to get the amount of daughter isotope A that IS due to radioactive decay. That number is also the amount of parent that has decayed (remember the rule #parent + #daughter = constant). Now you can determine the age as you did before. 26Al is a radioactive isotope that decays into 26Mg, a stable isotope, with a half-life of 0.73 million years. Although this is so short that all of it has decayed billions of years ago, its presence at the beginning of the solar system has been conclusively established by the discovery of excesses of its daughter isotope 26Mg in the most primitive solar system objects. If these objects containing 26Al at the time of their formation remained relatively undisturbed (i.e., did not experience high temperatures), the decay product 26Mg was frozen in and today provides a record of the original 26Al. The ratio of 26Mg excess measured now relative to the amount of the stable isotope 27Al yields the original 26Al/27Al ratio.