Astronomy 340 - PowerPoint Presentation

Slide1

Astronomy 340Fall 2007

4 October

2007

Class

#10Slide2

Review

Venus resurfacing event(s) 300-500

Myr

ago

Just how do you determine the age of a planetary surface?

Impacts

MarsSlide3

Martian Crustal Magnetization

Working model

Collection of strips 200 km wide, 30 km deep

Variation in polarization every few 100 km

3-5 reversals every 10

6

years (like seafloor spreading on Earth)

Some evidence for plate tectonics…but crust is rigid



Earth’s crust appears to be the only one that participates in convectionSlide4

Formation of Impact Craters

Impactor unperturbed by atmosphere

Impact velocity ~ escape velocity (11 km s

-1

)

 tens of meters in diameter

Impact velocity > speed of sound in rocks

 impact forms a shock

Pressures ~100 times stress levels of rock

 impact vaporizes rocks

Shock velocity ~10 km s

-1

 much faster than local sound speed so shock imparts kinetic energy into vaporized rockSlide5

Contact/Compression

Projectile stops 1-2 diameters into surface

 kinetic energy goes into shock wave  tremendous pressures  P ~ (1/2)

ρ

0

v

2

Peak shock pressures ~1000 kbar; pressure of vaporization ~600 kbar

Shock loses energy

Radial dilution (1/r

2

)

Heating/deformation of surface layer

Velocity drops to local sound speed – seismic wave transmitted through surface

Can get melting at impact point

Shock wave reflected back through projectile and it also gets vaporized

Total time ~ few secondsSlide6

Excavation

Shock wave imparts kinetic energy into vaporized debris

 excavation of both projectile and impact zone (defined as radius at which shock

velocty

~ sound speed (meters per second)

Timescale is just a dynamical/crossing time (t = (D/g)

1/2

Crater size? D goes as E

1/3

 empirically, it looks like ~ 10

times

diameter of projectile (but see equation 5.26b).

Can get secondary craters from debris blown out by initial impact

Large impacts



multiring

basins (Mars, Mercury, Moon)Slide7

Craters

35m

2m

4yr

Small Earthquake

1km

50m

1600yr

Barringer Meteor Crater

7km

350m

51,000yr

9.6 mag earthquake

10km

500m

10

5

yr

Sweden

200km

10km

150 Myr

Largest craters/KT impactorSlide8

Crater Density

See Figure 5.31 in your book

 number of craters km

-2

vs diameter

Saturation equilibrium – so many craters you just can’t tell….

Much of the lunar surface

Almost all of Mercury

Only Martian uplands

Venus, Earth not even close  note cut-off on Venus’ distribution

Calibrate with lunar surface rocks

10

7

times more small craters (100m) as there are large craters (500-1000 km)Slide9

Mercury South PoleSlide10

Lavinia Planum Impact Craters

Note ejecta surrounding craterSlide11
Slide12

“It’s the size of Texas, Mr. President” - from yet another bad movie

Comets – small,rocky/icy things

 10s of km

Asteroids – small, rocky things

 a few to 10s of km  the largest is the size of Texas (1000 km)

100-300 NEAs known

Close encounters….

Tunguska River in Siberia  30-50m meteroid exploded above ground  flattened huge swath of forestSlide13

You make the catastrophe…

Need high velocity

max velocity ~ 70 km s

-1

(combine Earth’s orbital velocity plus solar system escape velocity)

Earth-asteroid encounters

 25 km s

-1

Eart-comet encounters  60 km s

-1

Make it big….

E ~ mv

2

 something 1000 km would wipe out the entire western hemisphere, but let’s be realistic and go for ~10m (10

21

J) or ~1 km (10

23

J)

One impact imparts more energy in a few seconds than the Earth releases in a year via volcanism etc.Slide14

Surface Composition

Reflection

spectroscopy

. (remember

radiative

transfer!)

What is the surface made of? Rocks, mostly igneous

Minerals = solid chemical compounds with specific atomic structureSlide15

Common Minerals

Silicates

Si is produced via He-burning in stellar interiors, released via SNe.

O is produced in massive and intermediate mass stars

Si, O bind easily

 SiO

4

, SiO

3

bind with lots of other things (Mg, Al, Fe) and form a solid at high temperature

SiO

2

= quartz

(Fe,Mg)

2

SiO

4

= olivine (most common)

CaAl

2

Si

2

O

8

= feldspar

 60% of surface rocks on Earth

Various Oxides

Fe

2

O

3

= hematite

 generally formed from a reaction between Fe, O, and H

2

O  has been found in Martian samplesSlide16

Common Minerals

Silicon

3

rd

most abundant element (after O, Fe)

Cosmically as abundant as Fe, Mg

Less abundant than C,N,O

Chemically between metals and non-metals

Can survive as solid in interstellar/circumstellar environmentSlide17

Common Minerals

Silicon

3

rd

most abundant element (after O, Fe)

Cosmically as abundant as Fe, Mg

Less abundant than C,N,O

Chemically between metals and non-metals

Can survive as solid in interstellar/

circumstellar

environment

Silicates

“

lithophiles

” = silicates and things that tend to attach themselves to silicates

 low density minerals, reside in the crustSlide18

Common Minerals

Silicon

3

rd

most abundant element (after O, Fe)

Cosmically as abundant as Fe, Mg

Less abundant than C,N,O

Chemically between metals and non-metals

Can survive as solid in interstellar/

circumstellar

environment

Silicates

“

lithophiles

” = silicates and things that tend to attach themselves to silicates

 low density minerals, reside in the crust

Igneous rocks

40-75% SiO

2

O:Si ratio is high at high temperature crystallization and you get more olivine; low at low T and you get more quartzSlide19

Common Minerals

Silicon

3

rd

most abundant element (after O, Fe)

Cosmically as abundant as Fe, Mg

Less abundant than C,N,O

Chemically between metals and non-metals

Can survive as solid in interstellar/

circumstellar

environment

Silicates

“

lithophiles

” = silicates and things that tend to attach themselves to silicates

 low density minerals, reside in the crust

Igneous rocks

40-75% SiO

2

O:Si ratio is high at high temperature crystallization and you get more olivine; low at low T and you get more quartz

Differentiation

 absence of “

siderophiles

” in crust is evidence of differentiationSlide20

Tectonics  What Separates the Earth from Others

Convection

 means of transporting heat  driven by internal heat (radioactive decay?)

Crustal plates are cold upper lid on convective cells  “subsolidus” convection in mantle (3000 km thick)

Consequences

Volcanic activity, mountain chains

Mid-ocean ridges

Continental drift, earthquakesSlide21

Tectonics

1

st

evidence

 mapping magnetic field in Indian Ocean floor  detection of distinct linear features interpreted as “sea-floor spreading”

Puzzle-piece like nature of continents

Youngest rocks near mid-ocean ridgesSlide22

Earth Topographic MapSlide23

Dating: Radionuclide Chronometry

Processes (

Most Important Cases

)

40

K



40

Ar

t

½

= 1.4 x 10

9

yrs

87

Rb 

87

Sr

t

½

= 6 x10

10

yrs

238

U 

206

Pb

t

½

= 5 x 10

9

yrs

Lunar Results

Oldest Highland Anorthosite

t

solid.

= 4.2 x 10

9

yrs

Youngest Mare Basalts

t

solid.

= 3.1 x 10

9

yrs

Terrestrial ResultsSlide24

Radioactive Decay

Consider a number density, n, of atoms

Which decays at an average rate,

l

The solution to which is:

Now n = ½ n

0

at time t =

t

1/2

so

Or,

And finally: