EART160 Planetary Sciences

Mikhail Kreslavsky

The Solar Systemconsists of:

• Stars: – The Sun

• Planetary bodies regular shape (~sphere)

layered internal structure

own thermal history

• Small bodies irregular shape too small for own

thermal history

OR fragments of planetary

bodies

• Interplanetary medium– Dust– Particles– Fields

Materials of the Solar System

EART160 Planetary Sciences

Methods of study of materials(a very general classification)

• Remote sensing mostly with electromagnetic waves

mostly spectroscopy– For atmospheres:

• Extremely high sensitivity (qualitative detection of species)• Moderately accurate (quantitative measurements)

– For solid surfaces• Moderately sensitive and rather ambiguous (though widely used)

• Laboratory studies of samples• …Miraculous… …fantastic… …astonishing…

• Contact methods out of laboratory (e.g., robotic labs on other planets)

• Very limited so far, but improving…

Solar System materials accessible in laboratory• Terrestrial materials

– abundant – mostly from the upper crust, but also some lower crust and mantle – almost no old materials

• Lunar samples– 3 Luna missions, 6 Apollo missions; incl. old material; ~385 kg

• Asteroid samples– A few ~10 micron particles from Itokawa (Hayabusa mission)

• Comet samples– Dozens of ~10 micron particles (Stardust mission)

• Interplanetary / interstellar dust decelerated in the upper atmosphere of the Earth and gathered in stratosphere– contains (~10%) the only lab-accessible non-Solar-System material

• Meteorites:– Samples of original very old non-planetary material (“chondrites”)– Samples of shallow (“achondrites”) and deep (“irons”) interior

materials of destroyed planetary bodies– Samples of lunar upper crust– Samples of martian upper crust

Solar System materials accessible in laboratory• Terrestrial materials

– abundant – mostly from the upper crust, but also some lower crust and mantle – almost no old materials

• Lunar samples– 3 Luna missions, 6 Apollo missions; incl. old material; ~385 kg

• Asteroid samples– A few ~10 micron particles from Itokawa (Hayabusa mission)

• Interplanetary / interstellar dust decelerated in the upper atmosphere of the Earth and gathered in stratosphere– contains (~10%) the only lab-accessible non-Solar-System material

• Meteorites:– Samples of original very old non-planetary material (“chondrites”)– Samples of shallow (“achondrites”) and deep (“irons”) interior

materials of destroyed planetary bodies– Samples of martian upper crust– Samples of lunar upper crust

1μm

“Iron”

“Rock”

Meteorites:• Chondrites:

– Condensed from gas phase– Contain chondrules– Have never been into

planetary bodies– Aqueous alteration of some of them

• Achondrites:– Solidified from melts– Often were disintegrated

and re-aggregated – material of planetary bodies– “rocks” and “irons”

Ca-Al-rich inclusion (CAI)Such inclusions in chondrites are 4.6 Ga old; the oldest materialin the Solar System

Chondrites

The main message:• In some sense, all Solar System objects have

the same composition…– More accurately, ratios of abundances of

• rare earth elements, • stable non-radiogenic isotopes of refractory elements

are the same with high accuracy

All Solar System bodies have been formed from (almost) the same well-mixed material

• Extra-solar material (dust particles) have different composition (ratios of REE, isotopes, etc.)

Chondrites have the same compositionas the Sun (except volatiles).This is THE composition of the Solar System.

All compositional variations of planetary materials are due to differentiation of this primordial material

What word “metal” means for…• Astrophysicists:

– All elements except H or He (and sometimes Li, Be, B)

• Chemists / geochemists:– 80% of elements except H and the upper right corner of the periodic

table

• Physicists:– Specific type of condensed matter, mostly (but not only and not

always) crystalline phases of those 80% of elements

• Geophysicists, planetologists (in some context)

– Material of some meteorites and of cores of the Earth and planetary bodies

What word “rocks” means for…• Normal people:

– Stones, boulders, etc.

• Geologists / geochemists:– (types) of naturally occurring aggregates of solid-state phases

(e.g., basalts, granites, etc.)

• Geophysicists, planetologists (in some context)

– Silicate material of planetary bodies

“metals” “rocks” “ices” – major types of solid planetary materials

• Geochemists, planetologists (in other context)

– Type of meteorites (other than “irons”)

• Meteor – a phenomenon in upper atmosphere (“shooting star”)

• Meteoroid – small body in space (~ 1 cm – 100 m)

• Meteorite – meteoroid softly decelerated in the atmosphere and safely landed on a planet

• Meteoritics studies meteorites

• Meteorology studies weather

Planetary Surfaces

EART160 Planetary Sciences

• How many planets in the Solar System have surfaces?

Resurfacing:

• Endogenic– Volcanism– Tectonics

• Exogenic– Meteoritic impacts and space weathering– Mass wasting– Action of atmosphere and hydrosphere

Planetary Surfaces – impact processes

EART160 Planetary Sciences

Hypervelocity impacts:

• Impact velocity >> speed of sound in the target and in the projectile– Minimal impact velocity ~ escape velocity– How to estimate a typical / maximal impact velocity?

• First approach: quick release of much energy in little volume (explosion)– Crater size depends on impact energy only

• mv2, not m nor v nor impact angle

– Crater morphology depends on crater size only– This first approach is valid approximately– This first approach fails for very oblique impacts

Crater Sizes

• A good rule of thumb is that an impactor will create a crater roughly 10 times the size (depends on velocity)

• We can come up with a rough argument based on energy for how big the transient crater should be:

2r

v

2R

4/3

4/122r

g

vR

• E.g. on Earth an impactor of 0.1 (1) km radius and velocity of 10 km/s will make a crater of radius 2 (12) km

• For really small craters, the strength of the material which is being impacted becomes important

Does this make sense?

Very oblique impacts

• Strength regime– Smaller craters– Weaker gravity– Stronger material

Compression Excavation

• Simple craters

• Gravity regime– Larger craters– Stronger gravity– Weaker material

Compression Excavation Modification

• Simple craters (smaller)

• Complex craters (larger)

• Basins (largest)

Phases of impact process:

Post-impact modification

Microcrater

10 micron diameter

Compression

Excavation Formation of

transient cavity

Modification

• Gravity regime: Modification

• Simple craters (smaller)

• Complex craters (larger)

• Basins (largest)

Morphology of complex craters

• Ejecta• Rim• Walls• Terraces• Floor• Central peak / peak ring

The Moon, Crater Euler, D = 28 km

Morphology transition

• Mimas

• 130 km diameter complex crater

5 km

Craters on Mars

20 km

•Meteoroid – a rock in space

Craters on Mars (thermal IR image)

20 km

Craters on Mars (thermal IR image)

100 km

Impact basin on Mars

200 km

Old impact basin, old and young craters on Mars (thermal IR image)

Com

plex

cra

ters

on

Ven

us (

rada

r im

age)

Ero

ded

sim

ple

and

com

plex

cra

ters

on

Gan

ymed

e

Small impacts on atmosphereless bodies:

• Formation of planetary regolith• Geochemical effects• Mixing• Specific surface structure

Impact craters…

• Tools for study of relative and absolute ages• Probes for shallow subsurface• Probes for past atmosphere

… A useful thing …