Download Atmospheres: Composition

Atmospheres: Composition
AST111, Lecture 6a
This natural color image shows Titan's upper atmosphere -- where methane
molecules are being broken apart by solar ultraviolet light and the byproducts
combine to form compounds like ethane and acetylene. The haze preferentially
scatters blue and ultraviolet wavelengths of light, making its complex layered
structure more easily visible at the shorter visible wavelengths. A movie sequence
of images, shows movement of the haze layers over the course of a few hours (see
PIA06223).
Lower down in the atmosphere, the haze turns into a globe-enshrouding smog of
complex organic molecules. This thick, orange-colored haze absorbs visible
sunlight, allowing only perhaps 10 percent of the light to reach the surface. The
thick haze is also inefficient at holding in and then re-radiating infrared (thermal)
energy back down to the surface. Reverse greenhouse! Titan has a thicker
atmosphere than Earth, but the thick global haze causes the greenhouse effect there
to be somewhat weaker than it is on Earth. Images taken with the Cassini spacecraft
wide-angle camera using red, green and blue. The images were obtained at a
distance of approximately 9,500 kilometers (5,900 miles) from Titan on March 31,
2005.
This lecture
❑Observed molecules in different atmospheres
❑Phases of molecules
❑Condensation and the Clausius-Clapeyron
equation
❑Oxidizing vs Reducing atmospheres
❑ Using heavy noble gases as tracers of initial
composition
❑More on greenhouse effect
Earth, Venus and Mars
• Earth’s atmosphere is primarily Nitrogen (78% by
volume) and Oxygen (21%) but also contains CO2,
H20, Ar.
• Mars and Venus are dominated by CO2(98%)
• Ozone has been identified both on Mars and Earth
• Spectra from the planets varies with latitude.
• All three planets have deep CO2 absorption bands,
much of the atmospheric opacity is from CO2
• On Earth much of the atmospheric opacity is from
water.
Titan
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Titan is often discussed with
terrestrial planets because its
atmosphere is thick and contains
many of the same molecules
Titan is dominated by N2 which
shows up in the UV spectrum as
strong line emissions.
There is also some CH4, CO2,
CO and many hydrocarbons
Unidentified molecular haze, Keck adaptive optics image of Titan, Saturn's largest moon,
possibly produced by
obtained in infra-red light. The bright crescent at the
southern rim (bottom of image) is due to scattering of
photoelectric processes
sunlight by hazes in Titan's atmosphere. Titan's surface
consists of highlands or continents made of ice. Some of
the dark regions seen in the northern hemisphere are lakes
or seas made of liquid hydrocarbons.
Titan’s Haze
Cassini July 29, 2004
Titan is encircled in purple stratospheric haze. This image
shows two thin haze layers. The outer haze layer is detached
and appears to float high in the atmosphere. The image was
taken using a spectral filter sensitive to wavelengths of
ultraviolet light centered at 338 nanometers. The image has
been falsely colored: The globe of Titan retains the pale
orange hue our eyes usually see, and both the main
atmospheric haze and the thin detached layer have been
brightened and given a purple color. The best observations
of the layer are made in ultraviolet light because the small
haze particles scatter short wavelengths more efficiently
than longer visible or infrared wavelengths. The haze is at
altitudes above 400 kilometers, where ultraviolet light
breaks down methane and nitrogen molecules. The products
are believed to react to form more complex organic
molecules containing carbon, hydrogen and nitrogen that
combine to form the very small particles seen as haze.
Titan's Mottled Surface
PIA20016 From NASA's
Cassini spacecraft, acquired
during the mission's "T-114"
flyby on Nov. 13, 2015. The
visual and infrared mapping
spectrometer (VIMS) made
these observations, blue
represents wavelengths at 1.3
microns, green 2.0 microns,
and red 5.0 microns. A view
at visible wavelengths would
show only Titan's hazy
atmosphere. Near-infrared
wavelengths penetrate the
haze and reveal the moon's
surface.
Planets and Satellites with Tenuous
Atmospheres
• All objects contain atmospheres of some kind, however it
could be so tenuous that is very difficult to detect
• Continuous bombardment of energetic particles from the
solar wind, micro-meteorites and magnetosphere kicks up
atoms and molecules from the surface.
• Sputtering forms a corona which can sometimes be seen in
resonant emission lines
• For small bodies, the acceleration from gravity is small so
the tenuous atmospheres can have large scale heights
Mercury and the Moon
• Mercury’s atmosphere was first detected from
Space with Mariner 10’s airglow spectrometer.
Later on ground based observations in Sodium
have also detected it.
• It consists of atomic species Na, K, Mg, O
• A few thousand per cm3.
• H, He are captured from the solar wind.
• Apollo spacecraft detected a similar atmosphere on
the Moon with mass and UV spectrometers
• The Moon’s atmosphere is denser in the day time.
Lunar Exosphere
Images of the lunar sodium exosphere during total lunar eclipses. (Mendillo
et al., Observational Test for the Solar Wind Sputtering Origin of the
Moon's Extended Sodium Atmosphere, Icarus, 137, 12-23, 1999.)
Mercury’s tenuous atmosphere
Spectroscopic
measurements of sodium in
Mercury's atmosphere are
shown above. High
abundance of Na is red,
low abundance blue. The
dotted line around the
planet represents the
"seeing disk".
Spectrograph slit
measurements have been
interpolated to portray an
"image". From Anne
Sprague’s website.
Pluto and Triton
• Both extremely cold 40-60K
• However, surface temperatures are high enough to
partly sublime some ices N2, CH4(methane), CO2
• The amount of vapor on these objects can be
estimated from the equations for vapor pressure
equilibrium.
• N2 is believed to be the dominant molecule in
Pluto’s atmosphere
• We knew Pluto has an atmosphere because of
stellar occultations.
Io
• Io’s atmosphere consists
primarily of sulfur
dioxide! There is also
SO2 ice on the surface.
• Other gases SO, Na, K,
O.
• Cl inferred from its
presence in the plasma
torus.
Io’s plasma torus
Four ultraviolet observations of the Io torus
spanning 1 October to 14 November, 2000,
during the Cassini Jupiter flyby. The torus
is a donut of glowing gas orbiting Jupiter,
composed mostly of sulfur and oxygen gases
from Io's volcanic eruptions. Because the
atoms giving off the light are trapped by
Jupiter's tilted magnetic field, the torus
wobbles back and forth during the course of
a Jupiter day.
Europa and other icy satellites
• Europa is covered by water ice.
• Has an oxygen rich
atmosphere.
• Sputtering knocks H2O apart.
Hydrogen is more likely to
escape. This leaves oxygen
behind.
• A tenuous Oxygen atmosphere
has also been detected on
Ganymede
Atmospheres of Giant planets
Mostly composed of Molecular hydrogen H2
At UV wavelengths opacity provided by Rayleigh
scattering, you can only see the upper atmosphere
• Optical and IR light is reflected from clouds
• Opacity at longer wavelengths provided by
molecular hydrogen excitation and by scattering
from cloud particles
• Ammonia absorbs in the radio
Thermal infrared spectra reveal presence of H2O,
CH4 (methane), NH3 (ammonia) and H2S
(hydrogen Sulphide) on Jupiter.
However mixing ratios measured by Galileo probe
are difficult to account for and the abundances are
less than expected
Noble gasses such as Ar, Kr, Xe are 2.5 times solar!
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Saturn, Neptune and Uranus
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CH4 and NH3 are detected on Saturn but
only CH4 on Uranus and Neptune
Heavy elements in general are more
abundant on the more distant planets than
Jupiter.
There is evidence that the amount of gas
incorporated into each planet depends on
radial distance from the Sun.
The balance of CO vs CH4 and N2 vs NH3
depends on temperature and pressure
Even in Jupiter the upper atmosphere
contains CO rather than CH4
Convection causes methane to rise and be
heated, where it is converted to CO.
Convection also accounts for the
production of clouds at different heights.
As the pressure drops, water clouds,
followed by ammonia sulfide NH4SH
followed by ammonia clouds.
URANUS
https://en.wikipedia.org/wiki/Atmosphere_of_Uranus#/media/
File:Tropospheric_profile_Uranus_new.svg
Primitive Atmospheres
• Young stars are observed to be surrounded by gas
clouds
• Young stars probably collapse from gas clouds.
• The primitive solar nebula probably had
abundances (composition) similar to the Sun.
• However, how/when gas depletion occurred is
complex. Grain growth during driving of
photoevaporative winds?
Inert Gases as Tracers of Early
Conditions
• Inert gases such as Ne are not produced by
radioactive decay and are too heavy to escape
thermally.
• The amount of Ne probably hasn’t changed much.
• However Ne/H, Ne/O, Ne/N abundance ratios on
Earth, Mars and Venus are much smaller than Solar
abundances.
• Either primitive atmospheres were removed, swept
away, or primitive atmospheres were initially very
small.
Secondary Atmospheres
• Volcanos outgas, particularly in CO2 and H2O.
• Over a long period of time, much of our atmosphere could
have been made from volcanic outgasing.
• Venus, Mars and the Earth all have evidence for volcanoes.
• Iron rusts, removing oxygen. This could have caused a
hydrogen rich atmosphere at early times.
• There is little evidence for an oxygen rich atmosphere for
1/3 of the Earth’s lifetime.
• Oxygen is produced by disassociation of water by UV light,
and by photosynthesis.
Phases of molecules depend upon
pressure and temperature
Liquid water
Evolution of surface temperatures
of Venus, Earth and Mars
Cloud formation
High: Temperature low
Saturation partial
pressure of water is low,
Atmosphere cannot hold
much water gas
At this height the partial
pressure of water is
equal to the saturation
pressure. Water
condenses out of the air.
Low: Temperature high
Saturation Partial pressure
of water is high.
Atmosphere can hold lots
of water gas
Saturation Vapor Pressure
The saturation partial vapor pressure at temperature T is
given by the Clausius-Clapeyron equation.
P = CL e
Ls /(Rgas T )
Rgas the gas constant 8.314 𝗑107 erg K-1 mole-1
CL a constant — units pressure (bar)
Ls the latent heat units erg/mole
When the partial pressure of a molecule exceeds the
saturation vapor pressure, the molecule condenses out of the
atmosphere. A cloud forms.
At the base of the cloud the partial pressure is equal to the
saturation vapor pressure given by the Clausius Clapeyron
equation.
Cloud heights
• Consider gas with a constant fraction of a particular
molecule (e.g., water). Pressure and temperature decreases
with height. Partial pressure is the contribution to the total
pressure from water molecules. P = (Σnspecies)kT
• The saturation vapor pressure is the maximum partial
vapor pressure that this molecule can have. At some height
the pressure is low enough that the partial pressure of the
molecular is equal to the saturation pressure and molecules
condense / clouds form.
• The height where clouds form can be also be used to
estimate the partial pressure or abundance of particular
molecules.
Cloud base example
P = CL e
Ls /(Rgas T )
The base of a methane (CH4 ) cloud in Uranus’s atmosphere is
at a pressure level of 1.25 bars and a temperature of T=80K.
The saturation vapor pressure of methane is given by the
Clausius-Clapeyron equation with
CL = 4.658 𝗑 104 bar and Ls =9.71 𝗑 1010 erg mole-1
Rgas
the gas constant 8.314 𝗑107 erg K-1 mole-1
A. Estimate the partial pressure of CH4 at the cloud base.
B. Estimate the volume mixing ratio of methane at the cloud
base.
Cloud base example(continued)
The base of a methane (CH4 ) cloud in Uranus’s atmosphere is at a pressure
level of 1.25 bars and a temperature of T=80K. The saturation vapor pressure
is given by the Clausius-Clapeyron equation with
CL = 4.658 𝗑 104 bar and Ls =9.71 𝗑 1010 erg mole-1
Rgas the gas constant 8.314 𝗑107 erg K-1 mole-1
A. Estimate the partial pressure of CH4 at the cloud base.
The partial pressure is equal to the saturation vapor pressure at the
cloud base because there methane has started to condense. The
saturation vapor pressure is that given by the Clausius-Clapeyron
equation.
= 0.021 bar
The ratio of the partial pressure to the total pressure is 0.021bar/
1.25 bar = 0.017
Cloud base example (continued)
B. Estimate the volume mixing ratio of methane at the cloud base.
The volume mixing ratio is the fraction of molecules compared to total in a
given volume
The ratio of the partial pressure to the total pressure is
0.021bar/1.25 bar = 0.017
The idea gas law is P=nkT
So the partial pressure divided by the total pressure gives the
fraction of the number of molecules that are methane.
The volume mixing ratio of methane is 0.017
Formation of Clouds
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Earth primarily has water clouds
Giant planets have clouds of NH3, H2S, CH4
Mars has CO2 clouds
Venus has H2SO4 clouds
How do clouds modify the radiative
energy balance?
• Clouds can be highly reflective. Why Venus
has a very high albedo.
• Clouds block outgoing IR radiation, adding
to greenhouse effect
• Latent heat of condensation changes thermal
structure of updrafts
• As clouds condense, energy is released and
this affects the lapse rate (dT/dz).
Oxidizing vs Reducing Atmospheres
CH 4 + H 2O
2NH 3 + 2H 2O
H 2S + 2H 2O
CO + H 2O
CH 4
↔
↔
↔
↔
↔
CO + 3H 2
N 2 + 3H 2
SO 2 + 3H 2
CO 2 + H 2
C + 2H 2
Giant planets
Terrestrial planets
High Pressure
Low Pressure
Atmospheric escape
Carbon Dioxide on Earth
• Volcanic outgasing brings CO2 into the
atmosphere.
• Carbon dioxide is removed from the Earth’s
atmosphere by chemical reaction between CO2
dissolved in water with silicates. Urey weathering
reaction.
• The weathering rate is higher when the temperature
is higher. The more CO2 removed, the cooler the
planet
stability.
Carbon dioxide on Mars
• Lack of plate tectonics means Mars is no longer producing
CO2 and what was in the atmosphere could be condensed
onto the poles, or absorbed as Carbon into the rock.
Without much water, there is not much weathering, CO2 is
not added to the atmosphere.
• There is evidence of previous vulcanism so Mars must have
had copious CO2 and water. .
• Mars could have had a denser atmosphere and more water
in the past.
• Loss of water could be caused by impacts or large obliquity
or large eccentricity of orbit.
Carbon dioxide on Venus
• Venus is extremely dry.
• Oceans slowly evaporated and water
disassociated slowly (moist greenhouse)
(however current evaporation rate is estimated
to be insignificant).
• Or runaway greenhouse: All water turns to
steam, high convection and water is
disassociated at high altitude.
• Once all the water on the surface of Venus is
gone, then CO2 cannot be removed by
weathering.
The greenhouse effect
✓
L
16⇡ d2
◆ 14
278K
Teq ⇠
=p
d(in AU)
The terrestrial planets emit
most of their light at infrared
wavelengths.
❑ They would all be brightest
near a wavelength of 10
µm.
❑ Solar heating arrives
mostly at visible
wavelengths, where the
atmosphere is transparent.
Created for Global Warming Art by Robert
A. Rohde
The greenhouse effect (continued)
❑ Infrared light is absorbed
very strongly by molecules
in the atmosphere, notably
by water and CO2.
❑ Light can only escape
directly to outer space
through “windows”, of
which the most important
lie at wavelengths 8-13,
4.4-5, 3-4.2, 2-2.5, 1.5-1.8,
and 1-1.4 µm.
The greenhouse effect (continued)
❑ Hotter blackbodies shine
more at shorter
wavelengths, so if not
enough light escapes at
3-5 and 8-13 µm, the
surface heats up until
enough of the emission
leaks out in the shorterwavelength windows.
❑ This effect warms all three
of the atmosphere-bearing
planetary surfaces.
The greenhouse effect (continued)
If there’s liquid water on the surface, the greenhouse effect can
be self-stabilizing, as water droplets form clouds that reflect
sunlight. (CO2 forms neither droplets nor clouds.)
If temperature rises, → more water evaporates into atmosphere → more clouds form → albedo increases → less sunlight reaches surface → temperature drops.
And vice versa.
But on Venus, sunlight and the greenhouse effect was sufficient to
evaporate all of the water, leaving no liquid bodies on the surface.
The greenhouse effect (continued)
❑ Liquid water dissolves carbon dioxide, both from the atmosphere
and from rocks, creating carbonic acid:
—
H2CO3 (in solution H3O+ + HCO3 )
❑ From there the carbon can be incorporated in carbonate minerals
that can form readily in liquid water.
• These days, this is done most readily on Earth by oceandwelling organisms, creating CaCO3
❑ Thus if there is a lot of liquid water, carbon from CO2 will
eventually be locked up in carbonate minerals, rather than allowed
to be present in the atmosphere.
• This is the case, for example, on Earth.
• On Venus, though, the lack of liquid water let the CO2 remain in
the atmosphere.
The greenhouse effect (continued)
❑ Under solar ultraviolet illumination, water molecules high
in the atmosphere dissociate readily, producing atomic
hydrogen and oxygen.
• Oxygen goes on to react with other molecules;
hydrogen does not.
❑ Hydrogen is too light to be retained by Venus’s gravity, so
it escapes, relatively quickly.
• Soon there’s no more water, or possibilities for making
any more water! A dead world.
• And all the carbon and oxygen winds up in CO2, the
atmosphere pressurizes, and the greenhouse effect
cranks up to 735 K. A sterilized world.
Titan’s Haze
Inverse Greenhouse effect!
Review
• Primary vs secondary atmospheres,
Oxidizing vs Reducing atmospheres
• The composition of the atmospheres of
different planets
• Tenuous atmospheres
• The formation of clouds, cloud base height
• Phase diagrams
• The Clausius-Clapeyron equation