Download Lecture 2: Origin of atmospheres (mainly rocky planets)

Lecture 2: Origin of
atmospheres (mainly rocky
planets)
David Catling
[email protected]
Outline
2.0 Atmospheres & origin of solar system
2.1 Where did volatile substances come from?
– H2O, carbon, nitrogen, and sulfur.
2.2 Geochemical indicators of origin
- e.g. deuterium/hydrogen ratio in water
2.3 Earth’s earliest atmosphere: the first 1
billion years.
Part 2.0:
Setting the stage: the
origin of the solar system
Solar System
Formation
Orion Nebula
nebula = a cloud of dust and gas
The
Nebular
Model
5 light years
The Trapezium,
@ the center
Of the Orion
Molecular Cloud
Young stars:
brightest a few
million years old
Solar Nebula: Quick Overview
Star formation occurs from gravitational collapse of (part of) a
molecular cloud, creating a rotating circumstellar disk.
A=model where cm to meter-size
objects clump quickly (accrete) to
planetary embryos (103-104 km)
B=standard runaway growth up
to embryos.
A-B converge: oligarchic growth:
each region of the disk contains a
planetary embryo, along with
numerous, small planetesimals
(0.1-10 km)
Fig: from S. Raymond
Giants: cores >10-15 Earth
masses to capture H2 and He
106s-107 yrs: nebula gas cleared
by a violent young solar wind.
‘Secondary’ origin of Earth/Venus/Martian atmospheres?
1)Noble gases (
Relative abundances
) severely depleted
•) negligible gas from solar nebula
(e.g., very little neon)
Earth/Sun
•) atmospheres derive from solids
H2O
C
- hydrated minerals
- hydrocarbons
(+carbonates?)
2) Theory:
•) pre-main sequence T-Tauri
phase ~107 yr blows away any
accreted primary atmosphere
+ hydrodynamic escape
r
•) planet formation from
protoplanets (103-104 km size)
Outgassing, degassing and ingassing
[Qu.) What is a volatile?
Old Ideas:
relatively low melting or boiling points, so that they are
present as liquids or gases in a planet’s hydrosphere or
atmosphere.
Outgassing by volcanoes: William Rubey, USGS (1955) and
subsequent workers presumed that the atmosphere and oceans were
derived this way after the Earth formed
Current Ideas:
Impact degassing:
Once Earth reaches ~1/3 present mass, volatiles in accelerated
bolides get vaporized. Atmosphere starts to form as the planet forms,
then ocean condenses.
Ingassing:
On Earth, at least, some volatiles have returned to the interior, e.g.,
carbon.
Part 2.1:
Where did volatiles come
from?
Volatile Delivery: Why we care for Astrobiology
• Key volatiles
– H2O, carbon, nitrogen, and sulfur.
– These (plus phosphorus) are also the so-called
“SPONCH” elements from which life is made.
If impacts ->atmospheres, meteorites must provide clues
Types of meteorite
(Source: Bunch & Wittke, nau.edu)
Basic types of meteorite
1) Irons – predominantly iron.
2) Stony – predominantly silicates.
(a)
>90% chondrites, i.e., contains chondrules (globules of silicate
minerals, up to a few mm size, interpreted as rapidly cooled silicate
melt formed by condensation of melted dust in the nebula).
•
Ordinary chondrites, 5-15% Fe-Ni
•
Carbonaceous chondrites, C-rich
(from Beatty et
al., The New Solar
System, Ch. 26)
(b) achondrites do not contain chondrules and formed from igneous
rocks of their parents (e.g. Martian meteorites)
3) Stony-iron – silicate and iron mixture.
Volatiles, meteorites and oceans
• Carbonaceous chondrites
- Up to 20 wt.% H2O
– ~3.5 wt% organic C, 0.3 wt%N
– Nitrogen and noble gases are trapped within the
organic carbon matrix
- Carbonaceous chondrite Earth:
6 x1024 kg (x 0.15) = 9 x1023 kg ~ 600 oceans
• Ordinary chondrites (97% of all chondrites in
collection)
– ~0.1 wt% H2O, ~0.03 wt% N, ~0.1 wt% C
- Ordinary chondrite Earth:
6 x1024 kg (x0.001) = 6 x 1021 kg ~ 4 oceans
=>
A few planetesimals with carbonaceous chondrite composition suffice
The “condensation model” (John Lewis, 1970s)
Materials that condense
from the nebula as it
cools (assumed 10-4
bar )
Shaded rows are
chemical groups of
substances.
Lewis proposed how this
explains why rocky closer
to the Sun have silicates
and Fe-Ni alloy.
The nebula became cool
enough at ~5 AU to allow
H2O to condense out as
water ice; hence this
distance is called the ice
line (or snow line).
Inner and Outer Nebula
Increase of 2-4 in the amount of
solids due to freeze out of water
More modern idea of icy body scattering
– “C-type” asteroids (>75% of known) between 2-4 AU
– dark, carbon-rich like carbonaceous chondrites
●
SUN
1 AU
H2O ice JUPITER
1-10 m.y.
10-100 m.y.
Orbital
resonances
disperse
asteroids
>100 m.y.
For details: various papers by J. Chambers, A. Morbidelli, S. Raymond
Increasingly elliptical orbits
Planet Formation Simulation
circular
orbit
•
Simulation by Sean Raymond
0.1%
water
1%
water
10%
water
Leftover debris: 2 belts (asteroid belt, Kuiper belt) +
halo of Oort Cloud. Studying the “debris” gives
evidence for the formation of the solar system
Part 2.2:
Geochemical indicators
of origin
Hydrogen
Deuterium
Source of oceans: What does D/H tell us?
Mainly outer asteroids, modified by smaller cometary component
Earth:
Ocean D/H is 1/2 that in long period comet H2O (Halley, Hyakutake,
Hale-Bopp, 2002T7 and Tuttle)
Dynamics unfavorable for comet impacts (about 1 in 3000 hit)
(e.g. Morbidelli et al. (2000), MPS)
103P/Hartley 2 – a so-called “Jupiter-family” comet from the Kuiper Belt - has similar
D/H ratio as seawater, but 15N/14N is ~2 times higher.
Mars:
Lower SNC D/H cluster around 2 x Earth => cometary veneer
Venus:
Bulk from asteroids, probably. Noble gases suggest a cometary
component (Ne, Ar: 20, 70 times more abundant than on Earth;
Ar/Kr ~solar) (e.g. Owen & Bar-Nun (1995) Icarus)
Deuterium/hydrogen
(D/H) ratio
Vertical gray lines
shows the Sun, Earth
and comets.
TOP DOWN:
Histograms show
values for
carbonaceous
chondrites, ordinary
chondrites, and
micrometeorites and
Interplanetary Dust
Particles (IDPs).
From Marty & Yokuchi
(2006).
Volatile depletion
Earth, Mars, Venus:
Depleted in
geochemical volatiles
compared to Sun but
Mars is less depleted.
Mars formed farther
out in a cooler part of
the solar nebula. (CI=
carbonaceous
chondrite)
Abundance of
atmophiles differs
greatly for Venus,
Earth and Mars:
divergent fates of their
atmospheres.
Venus lost of water
through a runaway
greenhouse effect.
Mars lost air to space.
Clue to Venus volcanic history in argon?
Argon-40 is produced from the radioactive decay of
40K with a half-life of 1.25 b.y., and so should
gradually accumulate in an atmosphere if outgassing
is efficient.
Venus’ atmosphere has only ~¼ of the 40Ar found in
the Earth’s atmosphere.
A possible (surprising?) explanation is that Venus is
not as efficiently outgassed, so its volcanism must
have been relatively quiescent for most of its history,
whereas Earth’s volcanism was continuous as a
consequence of plate tectonics.
Part 2.3:
Earth’s earliest
atmosphere: the first
billion years
The earliest times in the Solar System
Timescales after the formation of the Solar System, which occurred
when the first solids, calcium-aluminum inclusions (CAIs) of
chondrite meteorites, condensed in the solar disk, at 4.567 Ga
impact degassing atmosphere
Zahnle et al. 1988
Early steam
atmosphere
Pressure and
temperature during
impact degassing during
accretion of the Earth.
Degassing > escape to
space when growing
planet is ~0.5 Earth
radius.
Then, the atmosphere is
opaque to the thermal
IR in a runaway
greenhouse and the
surface melts.
The steam atmosphere
collapses when the
planet nearly reaches
the current Earth radius.
Evolution of Earth’s nasty early atmosphere
Zahnle et al., 2010
•
0-103 yr: atmosphere of 2500 K rock vapor
•
103 to ~2 m.yr.: magma ocean, runaway steam greenhouse
•
2 m.y. to ~108 yr: Solid surface, salty ocean, atmosphere of
100-200 bar CO2, gradually removed by reacting with the crust
•
~108 to ~109 yr: the Hadean atmosphere– poorly known
Ideas about evolution the early atmosphere
a. Earth’s planetary embryo
may have captured some
primitive atmosphere, i.e.,
solar composition. A magma
ocean produced by impacts
should have allowed some of
atmosphere to dissolve in
ingassing.
b. Magma ocean froze.
Collapse of the steam
atmosphere + outgassing
produced an ocean and
atmosphere. Small return C,
N and S to the mantle.
Possible “late veneer” of
volatiles added during Late
Heavy Bombardment
c. Then a balance between
outgassing and ingassing.
Late Heavy Bombardment (LHB)
• Big lunar craters imply
that about a hundred
~100-km bodies and
thousands of 10-km
bodies pelted the early
Earth
• Lunar basin ages derived
from radiometric dating of
lunar samples cluster around
3.9 Ga, but there is debate
Effect on the early atmosphere
about this.and life:
Rare impactors larger than ~500 km vaporized the
entire ocean.
Statistically, 1-4 such impactors should have hit the
Impact bombardment connection to astrobiology
“Tree of Life” from ribosomal
RNA
Source:
D. C. Catling (2013)
Astrobiology: A Very Short
Introduction, Oxford Univ.
Press.
Your “great, great, great…(lots of greats)” grandmother was a
thermophile.
Did the last big impact prune the “tree of life”?
Was there a “Miller-Urey”
atmosphere?
(a reducing atmosphere
that naturally formed
prebiotic organic
molecules?)
Miller-Urey Experiment (1953)
Harold Urey grad
student Stanley
Miller performed a
famous experiment
at U. Chicago in
which he
synthesized
possible prebiotic
compounds (amino
acids) from early
atmospheric gases
The assumption
was a
hydrogen-rich
mixture.
Redox of volcanic gases: before and after core formation
The oxidation state of volcanic gases (e.g., H2/H2O) is
governed by the oxidation state of the upper mantle.
(a) During core(a)
formation
(b) After (b)
core formation
Core formation: fast ~10-50 m.y., so subsequent atmospheres
should have been weakly reducing.
Some uncertainty persists due to the lack of definitive
evidence.
“Strongly reducing” versus “weakly reducing”
atmospheres:
Examples
Volcanic gases
EARTH: Typical volcanic gas from a subaerial volcano is:
H2O ~ 80-90% by volume
CO2 ~6-12%
CO < 0.4%
H2 ~0.6-1.5
Note: Hydrothermal (seafloor, crust) emissions are at a lower temperature and
more H2-rich
MARS: Analysis in Mars meteorites of partitioning of redox sensitive elements
(e.g., Eu3+/Eu2+) provides estimates of mantle redox (Herd, 2003, Meteoritics
Planet. Sci. 38, 1793-1805) => Mars’ upper mantle may be more reducing than
Earth’s.
This would mean that volcanic gases on Mars could be more hydrogen-rich than
above.
Page 35
Atmospheric maintenance: Recycling of volatiles
On geologic timescales: The recycling of C, S and water by
plate tectonics maintains the habitability of the Earth.
More in the next lecture on climate evolution…
Summary
2.0 Atmospheres & origin of solar system
Volatiles on rocky planets were mostly acquired as
solids from regions in the Solar System through
accretion over ~108 yr
2.1 Where did volatile substances come from to make atmosphere
+ ocean?
– Models & data suggest a large component of
volatiles from asteroids, i.e., H2O from beyond the
“ice line”.
2.2 Geochemical indicators of origin
- Lack of atmophiles and different atmophile patterns
on the rocky planets (compared to solar composition)
indicates very different fates of atmospheres of
Venus, Earth and Mars.
2.4 Earth’s earliest atmosphere: the first 1 billion