Transcript Powerpoint slides - Earth & Planetary Sciences

EART164: PLANETARY
ATMOSPHERES
Francis Nimmo
F.Nimmo EART164 Spring 11
Course Overview
• How do we know about the gas envelopes of
planetary bodies? Their structure, dynamics,
composition and evolution.
• Techniques to answer these questions
– Remote sensing (mostly)
– In situ sampling
– Modelling
• Case studies – examples from this Solar System (and
exoplanets)
F.Nimmo EART164 Spring 11
Course Outline
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Week 1 – Introduction, overview, basics
Week 2 – Energy balance, temperature
Week 3 – Composition and chemistry
Week 4 – Clouds and dust
Week 5 – Radiative Transfer; Midterm
Week 6 – Dynamics 1
Week 7 – Dynamics 2
Week 8 – Exoplanets
Week 9 – Climate change & Evolution
Week 10 –Recap; Final
F.Nimmo EART164 Spring 11
Logistics
• Website:
http://www.es.ucsc.edu/~fnimmo/eart164
• Set text –F.W. Taylor, Planetary Atmospheres (2010)
• Another good reference (higher level) is Lissauer &
DePater, Planetary Sciences 2nd ed. (2010), Chs. 3&4
• Prerequisites – some knowledge of calculus expected
• Grading – based on weekly homeworks (~40%),
midterm (~20%), final (~40%).
• Homeworks due by 5pm on Monday (10% penalty per
day)
• Location/Timing –MWF 2:00-3:10 in E&MS D236
• Office hours – MWF 3:15-4:15 (A219 E&MS) or by
appointment (email: [email protected])
• Questions? - Yes please!
F.Nimmo EART164 Spring 11
Expectations
• I’m going to assume some knowledge of calculus
• Homework typically consists of 3 questions
• If it’s taking you more than 1 hour per question on
average, you’ve got a problem – come and see me
• Midterm/finals consist of short (compulsory) and long
(pick from a list) questions
• Showing up and asking questions are usually routes to
a good grade
• Plagiarism – see website for policy.
F.Nimmo EART164 Spring 11
This Week
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Introductory stuff
Overview/Highlights (Taylor Ch. 1)
How do planets form? (Taylor Ch. 2)
Where do atmospheres come from?
What observational constraints do we have on
atmospheric properties? (Taylor Ch. 3)
• Introduction to atmospheric structure
F.Nimmo EART164 Spring 11
Three classes of planetary bodies
“Rock”
1 ME
300 GPa
~6000 K
GJ876d
“Rock”+ice
~0.1 ME
~10 GPa
~1500 K
Ice + H,He
~15 ME
800 GPa
~8000 K
Other solar systems will
certainly contain planets
very different from ours
(super-Earths, miniJupiters, iron planets . . .)
Mainly H,He
~300 ME
7000 GPa
~20,000 K
HD149026b
F.Nimmo EART164 Spring 11
Useful Data
Venus
Earth
Solar constant (Wm-2)
2620
Obliquity (o)
Titan
Jupiter
Saturn
Uranus
Neptune
1380 594
15.6
50.5
14.9
3.7
1.5
177
23.4
24.0
(27)
3.1
26.7
98
28.3
Orbital period (years)
0.62
1
1.88
(29.4)
11.9
29.4
84
165
Rotation period (hours)
5832
24
24.6
383
9.9
10.7
17.2
16.1
Bond albedo A
0.76
0.4
0.15
0.3
0.34
0.34
0.3
0.29
Molecular wt. m (g/mol)
43
29
43
29
2.2
2.1
2.6
2.6
Tsurface or T1bar (K)
730
288
220
95
165
134
76
72
Surface pressure (bar)
92
1
.007
1.47
n/a
n/a
n/a
n/a
g (ms-2)
8.9
9.8
3.7
1.35
24.2
10.0
8.8
11.1
Teq (K)
229
245
217
83
113
84
60
48
Scale height H (km)
15
8.5
12
23
27
60
28
20
Radius (km)
6052
6370 3390 2575
71,500
60,300 25,000 24,800
Mass (1024 kg)
4.87
5.97
1900
568
Data mostly from Taylor, Appendix A
Mars
0.64
0.13
87
102
F.Nimmo EART164 Spring 11
Units!
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SI in general but
1 bar = 105 Pa
g/cc vs. kg/m3
Per mol vs. per kg
F.Nimmo EART164 Spring 11
Overview/Highlights
F.Nimmo EART164 Spring 11
Venus
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Thick CO2 atmosphere
Hot (“runaway greenhouse”)
Cloud-covered
Lost a lot of water
Slow rotator (retrograde), not tilted
Fast winds (“superrotation”)
Sulphur cycle (active volcanism)
• Pioneer Venus, Venera & Vega probes (USSR),
Magellan, Venus Express (ESA)
F.Nimmo EART164 Spring 11
Earth
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Mostly N2,O2
Moderate greenhouse
Hydrological cycle & oceans
Weathering buffer
Moderate rotator
Tilted (seasons)
Hadley cell
Milankovitch cycles
Biological activity
F.Nimmo EART164 Spring 11
Mars
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Thin CO2 atmosphere
Dust and polar caps important
Massive climate change
Moderate rotator
Tilted (seasons)
Global dust storms
Orbital forcing important (Milankovitch cycles)
• Mars Odyssey, Mars Express (ESA), Mars Exploration
Rovers, Mars Science Laboratory, MAVEN
F.Nimmo EART164 Spring 11
Jupiter & Saturn
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Thick H/He atmospheres
~10 Earth mass rock/ice cores
Internal energy sources
Rapid rotators
Saturn is tilted
Banded winds + storms
Multiple cloud layers
• Voyagers, Cassini, Galileo, Juno (we hope)
F.Nimmo EART164 Spring 11
Uranus & Neptune
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Thin (relatively) H/He atmos.
Massive rock/ice cores
Rapid rotators
Banded winds + storms
Multiple cloud layers
Uranus is tilted (seasons)
Poorly understood
• Voyagers
F.Nimmo EART164 Spring 11
Titan
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Moderate N2 atmosphere
“Hydrological” cycle (methane)
Subsurface replenishment
Moderate rotator
Saturn tilted (seasons)
Local clouds and storms
Large atmospheric loss?
• Voyager, Cassini/Huygens
F.Nimmo EART164 Spring 11
Thin Atmospheres
F.Nimmo EART164 Spring 11
Exoplanets
Swain et al. 2008
F.Nimmo EART164 Spring 11
1. How do planets form?
F.Nimmo EART164 Spring 11
Solar System Formation - Overview
• Some event (e.g. supernova) triggers gravitational
collapse of a cloud (nebula) of dust and gas
• As the nebula collapses, it forms a spinning disk (due to
conservation of angular momentum)
• The collapse releases gravitational energy, which heats
the centre
• The central hot portion forms a star
• The outer, cooler particles suffer repeated collisions,
building planet-sized bodies from dust grains (accretion)
• Young stellar activity (T-Tauri phase) blows off any
remaining gas and leaves an embryonic solar system
• These argument suggest that the planets and the Sun
should all have (more or less) the same composition
F.Nimmo EART164 Spring 11
Sequence of events
• 1. Nebular disk
formation
• 2. Initial coagulation
(~10km, ~105 yrs)
• 3. Orderly growth (to
Moon size, ~106 yrs)
• 4. Runaway growth
(to Mars size, ~107
yrs), gas blowoff
• 5. Late-stage
collisions (~107-8 yrs)
F.Nimmo EART164 Spring 11
What is the nebular composition?
• Why do we care? It will control what the planets (and
their initial atmospheres) are made of!
• How do we know?
– Composition of the Sun (photosphere)
– Primitive meteorites (see below)
– (Remote sensing of other solar systems - not yet very
useful)
• An important result is that the solar photosphere
and the primitive meteorites give very similar
answers: this gives us confidence that our
estimates of nebular composition are correct
F.Nimmo EART164 Spring 11
1.4 million km
Solar photosphere
Note sunspots
(roughly Earth-size)
• Visible surface of the Sun
• Assumed to represent the
bulk solar composition (is
this a good assumption?)
• Compositions are obtained
by spectroscopy
• Only source of information
on the most volatile
elements (which are
depleted in meteorites):
H,C,N,O
F.Nimmo EART164 Spring 11
Primitive Meteorites
• Meteorites fall to Earth and can be analyzed
• Radiometric dating techniques suggest that they formed
during solar system formation (4.55 Gyr B.P.)
• Carbonaceous (CI) chondrites contain chondrules and
do not appear to have been significantly altered
• They are also rich in volatile
elements
• Compositions are very
similar to Comet Halley,
also assumed to be ancient,
unaltered and volatile-rich
1cm
chondrules
F.Nimmo EART164 Spring 11
Meteorites vs. Photosphere
• This plot shows the
striking similarity between
meteoritic and
photospheric compositions
• Note that volatiles (N,C,O)
are enriched in
photosphere relative to
meteorites
• We can use this
information to obtain a
best-guess nebular
composition
Basaltic Volcanism Terrestrial Planets, 1981
F.Nimmo EART164 Spring 11
Nebular Composition
• Based on solar photosphere and chondrite compositions,
we can come up with a best-guess at the nebular
composition (here relative to 106 Si atoms):
Element
H
He C
N
O
Ne
Mg Si
Log10 (No. 10.44 9.44 7.00
Atoms)
6.42 7.32 6.52 6.0
Condens.
Temp (K)
120
180
--
78
--
--
6.0
S
Ar
5.65
5.05 5.95
1340 1529 674
40
Fe
1337
• Blue are volatile, red are refractory
• We would expect planetary atmospheres to consist
primarily of H, He, C,N,O,Ne, Ar and their compounds
Data from Lodders and Fegley, Planetary Scientist’s Companion, CUP, 1998
This is for all elements with relative abundances > 105 atoms.
F.Nimmo EART164 Spring 11
Temperature and Condensation
Nebular conditions can be used to predict what components of
the solar nebula will be present as gases or solids:
Mid-plane
Photosphere
“Snow line”
“Snow line”
Earth Saturn
(~300K) (~50 K)
Temperature profiles in a young (T
Tauri) stellar nebula, D’Alessio et al.,
A.J. 1998
Condensation behaviour of most abundant elements
of solar nebula e.g. C is stable as CO above 1000K,
CH4 above 60K, and then condenses to CH4.6H2O.
From Lissauer and DePater, Planetary Sciences
F.Nimmo EART164 Spring 11
“Snow line”
• Beyond the “snow line” (~180 K), water ice
condenses
• Ice is ~10 times more abundant (by mass) than
rock in the solar nebula
• So it is much easier to build big planets beyond
the snow line
• Gas giants need a big solid core to start
accumulating H or He (see next slide)
• Close-in exoplanets almost certainly formed
beyond the snow line and then migrated
F.Nimmo EART164 Spring 11
Gas/ice giant formation
• Once a solid planet gets to ~10 Earth masses, its gravity is large
enough to trap H2 and He present in the local nebula
• J,S,U and N all have cores made of “high-Z” elements (rock+ice)
• J,S have thick H/He envelopes; U,N have thin H/He envelopes
• So the cores of J&S probably grew early enough to trap nebular
H/He before it dissipated. U&N were too slow. Why?
F.Nimmo EART164 Spring 11
Migration (hot Jupiters)
Gas disk (with
density waves)
planet
• If the gas disk is still
present, planets will migrate
inwards
• This migration can be very
rapid (~104-105 yrs)
• Migration stops where the
disk stops (e.g. due to stellar
magnetic fields)
• This is why there are so
many “hot Jupiters”
• But it apparently didn’t
happen in our solar
system
F.Nimmo EART164 Spring 11
Nice Model
Early in solar system
Ejected planetesimals (Oort cloud)
“Hot” population
J
S
U
Initial edge of
planetesimal
swarm
N
18 AU
Present day
J
30 AU
“Hot” population
Planetesimals transiently pushed
out by Neptune 2:1 resonance
S
U
48 AU
N
“Cold”
population
Neptune
3:2 Neptune 2:1 Neptune
stops at
resonance
original edge resonance
(Pluto)
See Gomes, Icarus 2003 and Levison & Morbidelli Nature 2003F.Nimmo EART164 Spring 11
Planet Formation - Summary
• Initial nebular composition is well-known
• Planetary volatile abudance depends (mostly)
on where the planet formed (temperature)
• Timing of planet growth relative to nebular
blowoff also important
• The planets may have moved during or after
the formation phase
F.Nimmo EART164 Spring 11
2.Where do atmospheres come
from?
F.Nimmo EART164 Spring 11
Where do atmospheres come from?
• Primary – directly accreted from nebula
• Secondary – outgassed from planet
• Tertiary – derived from comets, asteroids and/or solar
wind
• We’ll discuss more later in the quarter. Examples:
– Does Earth’s hydrosphere come from comets or asteroids?
(D/H ratio)
– How much outgassing has there been on Earth, Venus,
Mars, Titan? (40Ar)
– Did the gas giants acquire a solar composition? (C/H, H/He)
F.Nimmo EART164 Spring 11
Where do atmospheres go to?
• Again, we’ll discuss more later, but there are
several processes which can remove atmospheres
• Loss to space
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Thermal processes (Jeans escape)
Hydrodynamic escape
Sputtering & photodissociation
Impacts
• Loss to surface/interior
– Chemical reactions (e.g. carbonate formation)
– “Ingassing” (e.g. plate tectonics)
– Freeze-out (Mars, Pluto)
F.Nimmo EART164 Spring 11
3. Observational constraints
(see Taylor ch.3)
F.Nimmo EART164 Spring 11
Radiometry (Spectroscopy)
• “Near” infra-red: 0.7-5 mm, reflected sunlight
• “Thermal” IR: 5-1000 mm, emission from atmosphere
• Absorption/emission tells us what species are present,
and where in the atmosphere they are
• Background spectrum (~black body) tells us about
temperature structure of atmosphere
F.Nimmo EART164 Spring 11
Radiometry (cont’d)
• We can see to different depths
within an atmosphere by using
different wavelengths
• By looking at emission from the
limb, we can probe the vertical
temperature and pressure structure
F.Nimmo EART164 Spring 11
Occultations
observer
observer
• Atmospheric absorption
of light/radio waves
provides information on
composition, pressure
and temperature
• Good for probing thin atmospheres (e.g. Pluto, Enceladus)
Hansen et
al. 2006
F.Nimmo EART164 Spring 11
In situ sampling
• Galileo probe (Jupiter)
• Huygens (Titan)
• Venera/Vega
probes/balloons
• Viking landers
• Cassini INMS
LeBreton et al. Nature 2005
• Very useful! Ground truth for pressure, wind,
temperature etc. Sensitive to trace gases (GCMS).
• Generally limited duration (e.g. Venus)
• Point measurement – what happens if you land in
an anomalous region? (Galileo probe) F.Nimmo EART164 Spring 11
4. Atmospheric structure
F.Nimmo EART164 Spring 11
Typical structure
z
stratosphere
tropopause
troposphere
T
Temperature structure of
stratosphere in reality can
be more complicated
because of
photochemistry (e.g.
ozone)
Lower atmosphere
consists of a thick part
(troposphere) where
convection dominates,
and a thinner part above
(stratosphere) where
radiation dominates
F.Nimmo EART164 Spring 11
Ideal Gas Equation
RT
P
m
P=pressure, =density, R=gas constant,
T=temperature (in K), m=molar mass (in kg)
What is density of air at Earth’s surface?
What is the column mass of Earth’s atmosphere? (kg/m2)
m
m
Venus (CO2)
0.04
Jupiter (H,He)
0.0022
Earth (N2,O2)
0.03
Saturn (H,He)
0.0021
Mars (CO2)
0.04
Uranus (H,He)
0.0026
Titan (N2)
0.03
Neptune (H,He)
0.0026
F.Nimmo EART164 Spring 11
Atmospheric Structure (1)
• Atmosphere is hydrostatic:
RT
• Gas law gives us:
P
dP
dz
  ( z) g ( z)
m
• Combining these two (and neglecting latent heat):
dP
gm
 P
dz
RT
Here R is the gas constant, m is the mass of one mole, and
RT/gm is the pressure scale height of the (isothermal)
atmosphere (~10 km) which tells you how rapidly pressure
decreases with height
e.g. what is the pressure at the top of Mt Everest?
Most scale heights are in the range 10-30 km
F.Nimmo EART164 Spring 11
Exobase and mean free path
• The exobase is the place where the mean free path of
molecules exceeds scale height. This is where
molecules can start to escape efficiently (if travelling
fast enough)
• You can think of the exobase as the effective “top” of
the atmosphere
• For planets with thin atmospheres, the exobase may
be at the surface!
mmol
l
2
p rmol
prmol2
l
What’s the mean free path
at the surface of the Earth?
rmol is typically 1 Angstrom=10-10 m
F.Nimmo EART164 Spring 11
Key concepts
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Snow line
Migration
Troposphere/stratosphere
Primary/secondary/tertiary atmosphere
Emission/absorption
Occultation
First homework
Scale height
due next
Hydrostatic equilibrium
Monday!
Exobase
Mean free path
F.Nimmo EART164 Spring 11
End of lecture
F.Nimmo EART164 Spring 11
An Artist’s Impression
The young Sun
gas/dust
nebula
solid planetesimals
F.Nimmo EART164 Spring 11
Observations (1)
• Early stages of solar system formation can be imaged directly – dust
disks have large surface area, radiate effectively in the infra-red
• Unfortunately, once planets form, the IR signal disappears, so until
very recently we couldn’t detect planets (now we know of ~400)
• Timescale of clearing of nebula (~1-10 Myr) is known because young
stellar ages are easy to determine from mass/luminosity relationship.
Thick disk
This is a Hubble image of a young solar
system. You can see the vertical green
plasma jet which is guided by the star’s
magnetic field. The white zones are gas
and dust, being illuminated from inside by
the young star. The dark central zone is
where the dust is so optically thick that the
light is not being transmitted.
F.Nimmo EART164 Spring 11
Observations (2)
• We can use the presentday observed planetary
masses and
compositions to
reconstruct how much
mass was there initially
– the minimum mass
solar nebula
• This gives us a constraint on the initial nebula conditions e.g.
how rapidly did its density fall off with distance?
• The picture gets more complicated if the planets have moved . . .
• The observed change in planetary compositions with distance
gives us another clue – silicates and iron close to the Sun,
volatile elements more common further out
F.Nimmo EART164 Spring 11
Cartoon of Nebular Processes
Disk cools by radiation
Polar jets
Hot,
high 
Dust grains
Infalling
material
Nebula disk
(dust/gas)
Cold,
low 
Stellar magnetic field
(sweeps innermost disk clear,
reduces stellar spin rate)
• Scale height increases radially (why?)
• Temperatures decrease radially – consequence of lower
irradiation, and lower surface density and optical depth
leading to more efficient cooling
F.Nimmo EART164 Spring 11
Planetary Compositions
• Which elements actually condense will depend on the
local nebular conditions (temperature)
• E.g. volatile species will only be stable beyond a “snow
line”. This is why the inner planets are rock-rich and the
outer planets gas- and ice-rich
• The compounds formed from the elements will be
determined by temperature (see next slide)
• The rates at which reactions occur are also governed by
temperature. In the outer solar system, reaction rates
may be so slow that the equilibrium condensation
compounds are not produced
• The mass of a planet determines the mass and
composition of its atmosphere
F.Nimmo EART164 Spring 11