EART 160: Planetary Science

Itokawa

Enhydra lutris

Image copyright Fred Hsu

Image courtesyISAS/JAXA

Last Time

• Paper Discussion– Thommes et al. (1999)

• Origin of the Moon

• Planetary Migration– Uranus and Neptune– Extrasolar Planets: “Hot Jupiters”

Today

• Paper Discussions– Asphaug et al. (2006)

• Meteorites– Age of the Solar System– Types of Meteorites

• Asteroids– Main Belt– ECAs, Trojans, Centaurs

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

Earth-Crossing Asteroids

How old is the solar system?• We date the solar system using the decay of long-lived

radioactive nuclides e.g. 238U-206Pb (4.47 Gyr), 235U-207Pb (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.

Radioactive DatingN = N0 e-t = N0 e-t/

N – number of radionuclides nowN0 – number we started with at time 0t – time since start– Average lifetime of radionuclide

t1/2 = ln(2) Half-life (time for half the material to decay)

© 1996 Frank Steiger; permission granted for retransmission.

Short Lived Radioactive Isotopes

• Some meteorites once contained live 26Al, which has a half-life of only 0.7 Myr. So these meteorites must have formed within a few Myr of 26Al production (in the supernova).

• So the solar system itself is also 4550 Myr old• Decay of 26Al 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 26Al distributed uniformly?

Types of MeteoritesType Description Abundance

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%

Stony 95%Achondrites No metal or chondrites, similar to basalts

Crustal source?9%

Chondrites Contain glassy chondrules, not remelted

Ancient Planetesimals86%

Carbonaceous Low-T (< 500 K)

Volatiles, organics5%

Ordinary Higher-T

Little volatiles, C81%

Famous MeteoritesALH 84001 Martian MeteoriteOnce thought to have ET life

Images Courtesy NASA

WillametteIron Meteorite 15.5 tons

AllendeCarbonaceous Chondrite

How do you know it’s a meteorite?

• Look somewhere without rocks!

• Antarctica!

• 23000+ of meteorites found in Antarctica

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

• 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

Asteroids

Gaspra

Mathilde

Ida

Dactyl

Eros

NEAR Shoemaker: Flyby 1997Orbiter/Lander 2000-2001

Galileo: Flybys 19911993

52 km

18 km

33 km

54 km

Rubble Pile Monolithic?

Itokawa

Hayabusa Flyby/”Landing” 2005Sample Return 2010

Ceres

Dawn: Flybys 2011-2015

Vesta

535 km

950 km

530 km (mean)

Sizes

The Moon

1 Ceres

2 Pallas4 Vesta

3 Juno 5 Astraea

6 Hebe

7 Iris

8 Flora

9 Metis

10 Hygeia

MAB = 4% M

MCeres = 1/3 MAB

Number is order of discovery, not size

Distribution

• Main Belt• Trojans / Greeks• Hildas• Earth-Crossing

Asteroids

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

Orbital ParametersSemimajor Axes e vs. a

i vs. aImage courtesy NASA

Asteroids classified into groups based on a, e, i

Trojans

• Share Jupiter’s orbit– Lead and Trail Jupiter at 60º– L4 and L5 Lagrange points

• Also have Trojansat Neptume

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

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

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

Next Time

• Paper Discussion– Mars Core and Magnetism, Stevenson (2001)

• Planetary Surfaces– Composition– Impacts

• We’ll do Interiors after we do surfaces

• 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.