Post on 10-Jun-2020
F.Nimmo EART164 Spring 11
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 • 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: fnimmo@es.ucsc.edu)
• 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
• 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
“Rock”+ice
~0.1 ME
~10 GPa
~1500 K
Ice + H,He
~15 ME
800 GPa
~8000 K
Mainly H,He
~300 ME
7000 GPa
~20,000 K
HD149026b
Other solar systems will
certainly contain planets
very different from ours
(super-Earths, mini-
Jupiters, iron planets . . .)
GJ876d
F.Nimmo EART164 Spring 11
Venus Earth Mars Titan Jupiter Saturn Uranus Neptune
Solar constant (Wm-2) 2620 1380 594 15.6 50.5 14.9 3.7 1.5
Obliquity (o) 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 0.64 0.13 1900 568 87 102
Useful Data
Data mostly from Taylor, Appendix A
F.Nimmo EART164 Spring 11
• SI in general but
• 1 bar = 105 Pa
• g/cc vs. kg/m3
• Per mol vs. per kg
Units!
F.Nimmo EART164 Spring 11
Overview/Highlights
F.Nimmo EART164 Spring 11
• 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)
Venus
F.Nimmo EART164 Spring 11
Earth • 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 • 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 • 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 • 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 • 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
Solar photosphere • 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
Note sunspots
(roughly Earth-size)
1.4
mil
lion k
m
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
1cm chondrules
• They are also rich in volatile
elements
• Compositions are very
similar to Comet Halley,
also assumed to be ancient,
unaltered and volatile-rich
F.Nimmo EART164 Spring 11
Meteorites vs. Photosphere
Basaltic Volcanism Terrestrial Planets, 1981
• 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
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):
Data from Lodders and Fegley, Planetary Scientist’s Companion, CUP, 1998
This is for all elements with relative abundances > 105 atoms.
Element H He C N O Ne Mg Si S Ar Fe
Log10 (No.
Atoms)
10.44 9.44 7.00 6.42 7.32 6.52 6.0 6.0 5.65 5.05 5.95
Condens.
Temp (K)
180 -- 78 120 -- -- 1340 1529 674 40 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
F.Nimmo EART164 Spring 11
Temperature and Condensation
Temperature profiles in a young (T
Tauri) stellar nebula, D’Alessio et al.,
A.J. 1998
Nebular conditions can be used to predict what components of
the solar nebula will be present as gases or solids:
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
Mid-plane
Photosphere
Earth
(~300K)
Saturn
(~50 K)
“Snow line” “Snow line”
F.Nimmo EART164 Spring 11
• 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
“Snow line”
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
• 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
Migration (hot Jupiters)
planet Gas disk (with
density waves)
F.Nimmo EART164 Spring 11
Nice Model
Initial edge of
planetesimal
swarm
30 AU
Ejected planetesimals (Oort cloud)
48 AU 18 AU
J S U N
“Hot” population
Early in solar system
2:1 Neptune
resonance
J S U N
Neptune
stops at
original edge
Planetesimals transiently pushed
out by Neptune 2:1 resonance
“Hot” population
“Cold”
population
See Gomes, Icarus 2003 and Levison & Morbidelli Nature 2003
3:2 Neptune
resonance
(Pluto)
Present day
F.Nimmo EART164 Spring 11
• 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
Planet Formation - Summary
F.Nimmo EART164 Spring 11
2.Where do atmospheres come
from?
F.Nimmo EART164 Spring 11
• 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)
Where do atmospheres come from?
F.Nimmo EART164 Spring 11
• Again, we’ll discuss more later, but there are
several processes which can remove atmospheres
• Loss to space
– 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)
Where do atmospheres go to?
F.Nimmo EART164 Spring 11
3. Observational constraints
(see Taylor ch.3)
F.Nimmo EART164 Spring 11
• “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
Radiometry (Spectroscopy)
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
• Atmospheric absorption
of light/radio waves
provides information on
composition, pressure
and temperature
Occultations
observer
observer
• Good for probing thin atmospheres (e.g. Pluto, Enceladus)
Hansen et
al. 2006
F.Nimmo EART164 Spring 11
In situ sampling
LeBreton et al. Nature 2005
• Galileo probe (Jupiter)
• Huygens (Titan)
• Venera/Vega
probes/balloons
• Viking landers
• Cassini INMS
• 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
Temperature structure of
stratosphere in reality can
be more complicated
because of
photochemistry (e.g.
ozone)
Typical structure
T
z
troposphere
stratosphere
tropopause
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
m
RTP
What is density of air at Earth’s surface?
What is the column mass of Earth’s atmosphere? (kg/m2)
P=pressure, =density, R=gas constant,
T=temperature (in K), m=molar mass (in kg)
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:
• Gas law gives us:
• Combining these two (and neglecting latent heat):
)()( zgzdzdP
RT
gP
dz
dP m
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
m
RTP
F.Nimmo EART164 Spring 11
• 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!
Exobase and mean free path
l prmol
2
2 mol
mol
r
m
pl
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
• Snow line
• Migration
• Troposphere/stratosphere
• Primary/secondary/tertiary atmosphere
• Emission/absorption
• Occultation
• Scale height
• Hydrostatic equilibrium
• Exobase
• Mean free path
Key concepts
First homework
due next
Monday!
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.
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.
Thick disk
F.Nimmo EART164 Spring 11
Observations (2)
• We can use the present-
day 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
• Scale height increases radially (why?)
• Temperatures decrease radially – consequence of lower
irradiation, and lower surface density and optical depth
leading to more efficient cooling
Polar jets
Stellar magnetic field
(sweeps innermost disk clear,
reduces stellar spin rate)
Disk cools by radiation
Dust grains Infalling
material
Nebula disk
(dust/gas)
Hot,
high
Cold,
low
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