Transcript all Solar System objects have the same composition

EART160
Planetary Sciences
Mikhail Kreslavsky
The Solar System
consists 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
1μm
– 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
“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”
Chondrites
Ca-Al-rich inclusion (CAI)
Such inclusions in chondrites
are 4.6 Ga old; the oldest material
in the Solar System
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 composition
as 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:
Does this make sense?
v
2r
2R
 2v
R  
 g
2
1/ 4
 3/ 4
 r

a
• 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
Very oblique impacts
• Strength regime
– Smaller craters
– Weaker gravity
– Stronger material
• Gravity regime
– Larger craters
– Stronger gravity
– Weaker material
Phases of impact process:
 Compression
 Excavation
 Compression
 Excavation
 Modification
• Simple craters
• Simple craters (smaller)
• Complex craters (larger)
• Basins (largest)
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
•Meteoroid – a rock in space
20 km
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)
Complex craters on Venus (radar image)
Eroded simple and complex craters on Ganymede
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 …