ptys/astr 206asteroids/meteorites 4/17/07 asteroids and meteorites

PTYS/ASTR 206 Asteroids/Meteorites4/17/07

Asteroids and Meteorites


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Semantics• Asteroids

– Small rocky bodies in orbit about the Sun

• Comets– Small bodies that orbit the

Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail

• Meteoroids– Small asteroids

• Meteorites– the debris collected on Earth

• Meteors– A brief flash of light (i.e. a

shooting star)


1. How were asteroids discovered?

2. What is their origin?3. What do they look like?4. What are near-Earth

asteroids?5. What happens when an

asteroid intersects Earth?

Asteroids (starlike)

Ceres, the largest asteroid in the asteroid belt


Recall Bode’s Law

• As we discussed previously, Bode’s law is mathematical representation of the semi-major axis of each planet

• Not a scientific “law” in the usual sense

• Start with the simple sequence

0 3 6 12 24 48 96 192 384

• Then Add 4

4 7 10 16 28 52 100 196 388

• Then divide by 10

0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.2 38.8


Bodes Law prediction vs. actual(semi-major axis in AU)

OBJECT PREDICTED ACTUAL

Mercury 0.4 0.39

Venus 0.7 0.72

Earth 1.0 1.0

Mars 1.6 1.52

??? 2.8 ???

Jupiter 5.2 5.2

Saturn 10 9.5

Uranus 19.6 19.2

Neptune 38.8 30.1

Pluto 78 39.6


Discovery of Ceres

• Giuseppe Piazzi searched for the missing planet

• Found Ceres


Discovery of Ceres

• Ceres disappeared behind the Sun and then reappeared – just like a planet

• Ceres is about 2.8 AU from the Sun – just as predicted by Bode’s law

• However, Ceres looked like a star must be a very small planet !– It turns out that Ceres is

smaller than Pluto, but, by far, the largest of the asteroids in the asteroid belt


• Soon after Ceres was discovered, other small “planets” were found– All about 2.8 AU from

the Sun

• New technique (ca. 1800’s): – Invented by Max

Wolfe– long-exposure

photography– Look for asteroid trails

on photos


• Soon after Ceres was discovered, other small “planets” were found– All about 2.8 AU from

the Sun

• New technique (ca. 1800’s): – Invented by Max

Wolfe– long-exposure

photography– Look for asteroid trails

on photosDid the Missing Planet Even Exist ?


Discovery of the Asteroid Belt

• Astronomers were finding many asteroids with the new technique– Wolfe alone discovered

over 200

• Modern Technique– CCD images at various

times, have a computer look for differences

• Have most likely found all objects greater than 1km across

Year # known

1800 0

1844 4

1890 287

1900 452

1979 2125

1988 4044

2000 >10,000


Searching for Asteroids and Comets

IMAGE 1 IMAGE 2

IMAGE 1 – IMAGE 2

Difference between 2 images highlights differences that may be asteroids/comets !


Asteroid Names

• Many asteroids are discovered by amateurs

• The process can take decades– After discovery, it is reported to the Smithsonian

Astrophysical Observatory Minor Planet Center– Given a temporary name (1980 JE)– If the object is found at the same location at 4

successive oppositions, the discover gives it a name and it is assigned a number

• 1 Ceres• 2 Pallas• 3834 Zappafrank






Asteroids



Origin of Asteroids

• There is no “missing planet”

• Asteroids are not the remains of a long-destroyed large planet (not much combined mass)– The combined mass of all asteroids

in the asteroid belt is less than the mass of our Moon

• Asteroids are relics of planetesimalsthat failed to accrete into a full-sized planet– effect of Jupiter


Jupiter’s effect on the Asteroid Belt

• Numerical simulations indicate that the existence of Jupiter makes it unlikely that a planet would have formed in the asteroid belt

• If Jupiter were not there, it is likely that a fifth terrestrial planet would have formed

• Jupiter also depleted the asteroid belt removing any that got close to Jupiter (and were flung out into the solar system)

• Note: Asteroid belt doesn’t look like movie asteroid belts, it is mostly empty space


Kirkwood Gaps

• Even today, gravitational perturbations by Jupiter deplete certain orbits within the asteroid belt

• The resulting gaps, called Kirkwood gaps, occur at simple fractions of Jupiter’s orbital period

• Similar to gaps in planetary rings (but keep in mind eccentricity!)


Trojan Asteroids• Located at the Lagrange

points of Jupiter– Stable orbits, same

period as Jupiter– Named for Trojan war

characters– About 50 Trojan’s are

known, many more are suspected

• Earth has no Trojans– However we have

spacecraft at Earth’s Lagrange points (ACE/SOHO/Genesis and others)






Asteroids



Asteroids Physical Structure Size and Shape

• All are smaller than 1000 km – Ceres is the largest with a

diameter of 934 km (more than 2 times smaller than Pluto)

• Only the largest are round– Uniform brightness

• Most are irregular shaped– Elongated asteroids vary in

brightness as seen from Earth as they rotate and present varying amount of cross-sectional area

• Some have moons

Gaspra


Physical Structure: Composition

• Some are differentiated– Iron cores ?

• Density of some are consistent with them being made of rocky silicates

• Dark (low albedos)– Carbon rich ?

• Others have very low densities indicating that they are probably rubble piles Ida and its moon Dactyl


Some Asteroids are “Rubble Piles”

• A survey of asteroid rotation rates show that many rotate quite slowly– If an asteroid is a collection

of smaller chunks (i.e. a rubble pile), it would not be able to rotate very quickly without flying apart

• Spherical (or really small!) asteroids rotate more rapidly– They are solid


Asteroid Collisions

• Some asteroids have huge craters, like 253 Mathilde shown here

• Being a rubble pile, it more-easily absorbs energy from the collision and remains intact






Asteroids



Near-Earth Asteroids

• Asteroids that are within the orbit of Mars are called Near-Earth Objects (NEOs)– 2500 are currently known

• Fairly recently, it was thought that Asteroid 2004 MN4 had a 1 in 60 chance of hitting Earth in 2029– An impact has now been ruled

out– It is about a 400-meter sized

asteroid (would cause a lot of damage)

• The larger asteroid 1950 DA (1-km) has a 1:300 chance of striking Earth in 2880

Image by Jim Scotti of LPL’s Spacewatch program


Possibility of an Impact• Calculating asteroid trajectories,

precisely, can be tricky

– Need a detailed mapping of the Sun’s gravitational field

– Need a better understanding of the characteristics of the asteroid (rotation, orbit, shape, etc.)

– Yarkovsky effect• Diurnal and seasonal variations

in the thermal radiation emitted by the asteroid can act as a miniature propulsion system Radar image of 1950 DA


NEO’s and Impacts with Earth

• 30-meter sized asteroids come close to Earth about every 2 years– They strike Earth every

6000 years or so

• A recent close call– There was an explosion

over the Mediterranean on June 6, 2002 (a 10-meter sized asteroid)

• Have probably found all objects larger than 1km, so probably no global effects without advance warning






Asteroids



Collision Course

• “Burns” up in atmosphere (outer layers vaporize, very little heating of rock occurs)

• If large enough it slows down to terminal velocity, then falls to Earth as a meteorite

• If larger still, will strike the surface with a tremendous amount of kinetic energy creating an explosion– This is what created

Berringer Meteor Crater, and flung ejecta out to a distance of 1-2 km. Events like this occur ~6000 yrs


Meteor


Collision Course






“Impact” with surface


Ejecta Blanket


Collision Course






Meteorites


• All meteorites were broken off from larger objects and have fallen to Earth from space, but not all are from asteroids. Some are from the Moon and Mars (but not many). No Earth meteorites have been recognized.

• Falls are observed to land, finds are just found lying on ground

• 3 main categories– Stony

• Chondrites (contain chondrules)• Achondrite

– Stony-Iron– Iron

• Many other sub-classes of meteorites

Meteorites


Stony• Most abundant meteorite falls,

but only about 10% of collection– Hard to distinguish from Earth

rocks, erode quickly in Earth’s atmosphere

• Fusion Crust can make identification easier

• Some of them are the oldest rocks ever found (4.56 Ga)– This is one way we get the

age of the solar system

• One class has almost an identical composition to that of the Sun (outside of H and He)– Building blocks of the planets


Stony-Iron and Iron

• Stony-Iron – rarest in both fall and in collections

• Iron – not very abundant fall, most abundant in collection– Easy to find– Made of metal (uncommon

on Earth’s surface)• Only source of metal to

humans before 2000BC

– Take longer to erode


Meteorite/Asteroid Connection

• Undifferentiated asteroids– Chondrites

• Differentiated asteroids– Achondrites (mantle)– Stony-Iron (mantle-

core boundary)– Iron (core)


Widmanstätten pattern in Iron Meteorites

• Provides a conclusive test that the material is indeed meteoritic

• The pattern is produced as the iron-nickel material is cooled slowly (millions of years) – like the core of an asteroid parent body that remains molten for a long time after its formation


Chondrites• most abundant type of stony meteorite

• very primitive chemistry– not been modified by melting

or differentiation of a parent body

– Part of bodies that accreted right at the time the solar system formed

– Most contain chondrules (small spherical-shaped glassy-like objects embedded within the meteorite)

• They can also contain some of the material that existed prior to the formation of the Sun


Carbonaceous chondrites

• A small class of chondritic meteorites that contain high levels of water and organic compounds

• The presence of volatile elements and water indicate that the object was not heated significantly– Material straight from the

original solar-nebula

ptys/astr 206asteroids/meteorites 4/17/07 asteroids and meteorites

Documents

ceres slide

asteroid belt slide

discovery of ceres ceres

planet ceres

photos slide

section2 slide

asteroids starlike ceres

shooting star slide