Title: Lecture 9: Planetary Atmospheres
1Lecture 9 Planetary Atmospheres
Earths atmosphere seen from space
- Claire Max
- May 10, 2007
- Astro 18 Planets and Planetary Systems
- UC Santa Cruz
2Planet hunter Michael Brown to speak at UCSC
Thurs, May 24
- "Pluto, Eris, and the Dwarf Planets of the Solar
System - 730 pm in the Humanities Lecture Hall
- Planet hunter Michael Brown will discuss his
discoveries at the outer edge of the solar system - Brown, a professor of planetary astronomy at
Caltech, has been scanning the skies for planets
beyond Pluto for the past 7 years. - In 2005 his team discovered Eris, the largest
object found in the solar system in the past 150
years and the first new candidate for planethood
to be discovered since Pluto.
3Practicalities Projects
- Presentations what do we expect?
- Time about 20 minutes per group
- Each person in group should speak about their own
"questions" - Format Your choice.
- PowerPoint
- Speak from written notes and hold up figures
- Make a poster and describe it to class
- In past years one group has done a dramatic
presentation. Harder to do and still convey
enough information.
4Projects, continued
- Written report on your projects what do we
expect? - Each group hand in your written contributions
together as one paper - Cover page listing overall title, all group
members (with email addresses) - Table of contents listing overall title and then
topics that each person will write about - 5 pages (or more if you want) from each person on
"questions" each person is addressing
5Written project reports, continued
- Each person's five pages
- Introduction describing your "questions" and how
they relate to the overall topic of your group - Then describe your "questions" in more detail
- Then give a logical discussion of what you found
out, including what your sources were for each
potential "answer" you present - use numbered references in text, referring to
numbered bibliography at end of your section - Summarize your overall conclusions describe what
new questions your investigation has brought out
6Planetary atmospheres Outline
- What is an atmosphere? What is its structure?
- Temperature of a planet, neglecting effects of
atmosphere (no-greenhouse temperatures) - Generic atmospheric structure
- Global climate change
- Earth
- Venus
- Mars
Please remind me to take a break at 245 pm!
7The Main Points
- Planetary atmospheres as a balancing act
- Gravity vs. thermal motions of air molecules
- Heating by Sun vs. heat radiated back into space
- Weather as a way to equalize pressures at
different places on Earths surface - Atmospheres of terrestrial planets are very
different now from the way they were born - Formation volcanoes, comets
- Destruction escape, incorporation into rocks,
oceans - Huge changes over a billion years or less
- Prospect of human-induced global warming on Earth
needs to be taken seriously
8The thin blue line
- Earth diameter
- 12,000 km
- Top of troposphere
- 12 km
- Thickness of atmosphere divided by Earth diameter
1 / 1000
9Role of atmospheric pressure in "holding up" the
atmosphere
10In an atmosphere in equilibrium, pressure
balances gravity
volume
11Implications profile of pressure and density
with altitude
- Pressure, density fall off exponentially with
altitude - Higher temperature T ? larger scale height
h0 - Stronger gravity g ? shorter scale height h0
12How big is pressure scale height?
- h0 kT / mg
- height at which pressure has fallen by 1/e
0.368 - Earth h0 8 km
- the thin blue line
- Venus h0 15 km
- (g a bit lower, T higher)
- Mars h0 16 km
- (both g and T lower)
13Equilibrium atmospheric temperature
14Temperature of a planet balance solar heating
against cooling
No-greenhouse temperature
- albedo fraction of sunlight that is reflected
by a surface
15No-greenhouse temperatures
- Conclusion for Venus and Earth, at least,
something else is going on! (not just radiation
into space)
16Lights Effects on the Atmosphere
- Ionization Removal of an electron
- Dissociation Destruction of a molecule
- Scattering Change in photons direction
- Absorption Photons energy is absorbed
17How do different energy photons interact with
atmosphere?
18The greenhouse effect
19Greenhouse gases
- carbon dioxide CO2
- water vapor H20
- methane CH4
- others too (NO2, ....)
20Generic atmospheric structure
21Temperature structure of Earths atmosphere
22Space shuttle view of top of troposphere
23Compare Earth, Venus, Mars
24Ozone and the Stratosphere
- Ultraviolet light can break up O2 molecules,
allowing ozone (O3) to form - Without plants to release O2, there would be no
ozone in stratosphere to absorb UV light
25Role of ozone for Earths atmosphere
- Ultra-violet light from Sun dissociates oxygen
molecules O2 to produce O in stratosphere - O combines with O2 to form O3 (ozone)
- Ozone in stratosphere absorbs harmful ultraviolet
light from Sun, providing land-based life with a
protective shield - Manmade aerosols (chlorofluorocarbons, CFCs)
inhibit ozone formation - Result Ozone hole
26Ozone hole over antarctica
27Antarctic ozone hole is worst in late southern
winter
28Ozone hole questions
- Why is ozone hole much deeper and larger in the
antarctic than in the arctic? - Why is it so tightly correlated with the south
polar vortex?
29History of atmospheres on Venus, Earth, Mars
- Huge changes took place over the 4.6 billion
years since planets formed! - Early atmospheres didnt resemble current ones at
all - Question why are atmospheres of Venus, Earth,
Mars so different?
30Sources of atmospheric gases
comets bring water, carbon componds
31Kilauea volcano outgassing
32Sinks of atmospheric gases
33Thermal Escape of atmospheric gases
34Components of atmospheres on Venus, Earth, Mars
- Why are they so different?
- Were they always this different from each other?
35The three atmospheres of EarthFirst Atmosphere
- First Atmosphere Primordial elements
- Composition - Probably H2, He
- Today these gases are relatively rare on Earth
compared to other places in the universe. - Were probably lost to space early in Earth's
history because - Earth's gravity is not strong enough to hold
lightest gases - Earth still did not have a differentiated core
(solid inner/liquid outer core) which creates
Earth's magnetic field (magnetosphere Van Allen
Belt) which deflects solar wind. Protects any
atmosphere. - Once the core differentiated, gases could be
retained
36Second atmosphere produced by volcanic
outgassing
- Gases similar to those from modern volcanoes
(H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3
(ammonia) and CH4 (methane) - No free oxygen O2 (O2 not found in volcanic
gases) - Ocean Formation - As the Earth cooled, H2O
produced by outgassing could exist as liquid - Evidence - pillow basalts, deep marine sediments
37Third atmosphere Free oxygen came late
- Today, atmosphere is 21 free oxygen. How did
oxygen reach this level? - Oxygen Production
- Photochemical dissociation - breakup of water
molecules by ultraviolet light - Produced O2 levels 1-2 current levels
- At these levels O3 (Ozone) can form to shield
Earth surface from UV - Photosynthesis CO2 H2O sunlight organic
compounds O2 - produced by cyanobacteria, and
eventually higher plants - supplied the rest of
O2 to atmosphere. - Oxygen Consumers
- Chemical Weathering - through oxidation of
surface materials (early consumer) - Respiration (much later)
- Burning of Fossil Fuels (much, much later)
- Once rocks at the surface were sufficiently
oxidized, more oxygen could remain free in the
atmosphere
38- Source Kasting, Scientific American
39Cyanobacteria and stromatolites
40Evidence from rocks Earths oxygen increased
with time
- Iron (Fe) i s extremely reactive with oxygen. If
we look at the oxidation state of Fe in the rock
record, we can infer a great deal about
atmospheric evolution. - Archean - In sediments, find occurrence of
minerals that only form in non-oxidizing
environments Pyrite (Fools gold FeS2),
Uraninite (UO2). These minerals are easily
dissolved out of rocks under todays atmospheric
conditions. - So things must have been different back then.
- Red beds (continental siliciclastic deposits) are
never found in rocks older than 2.3 billion years
but are common during Phanerozoic time. Red
because of the highly oxidized mineral hematite
(Fe2O3). - Conclusion - amount of O2 in the atmosphere has
increased with time.
41Biological evidence
- Chemical building blocks of life could not have
formed in the presence of atmospheric oxygen.
Chemical reactions that yield amino acids are
inhibited by presence of very small amounts of
oxygen. - Oxygen prevents growth of the most primitive
living bacteria such as photosynthetic bacteria,
methane-producing bacteria and bacteria that
derive energy from fermentation. - Conclusion - Since today's most primitive life
forms are anaerobic, the first forms of cellular
life probably had similar metabolisms. - Today these anaerobic life forms are restricted
to anoxic (low oxygen) habitats such as swamps,
ponds, and lagoons.
42Earth hydrological cycle
43Did Earth get its water from comets?
- Potential source of the Earth's ocean water is
comet-like balls of ice. - Measure about 9 m (30 ft) in diameter.
- Enter atmosphere at rate of about 20/second.
- At the observed rate of occurrence, Earth would
receive 0.0025 mm of water per year. - Four billion years of such bombardment would give
enough water to fill the oceans to their present
volume. - Possible problems with isotope ratios. Under
active research.
44What factors can cause long-term climate change?
45Solar Brightening
- Sun very gradually grows brighter with time,
increasing the amount of sunlight warming planets
46Changes in Axis Tilt
- Greater tilt makes more extreme seasons, while
smaller tilt keeps polar regions colder
47Changes in Reflectivity
- Higher reflectivity tends to cool a planet, while
lower reflectivity leads to warming
48Changes in Greenhouse Gases
- Increase in greenhouse gases leads to warming,
while a decrease leads to cooling
49Global Warming on Earth
- Global temperatures have tracked CO2
concentration for last 500,000 years - Antarctic air bubbles indicate current CO2
concentration is highest in at least 500,000
years
50Global Warming on Earth
- Most of CO2 increase has happened in last 50
years!
51Intergovernmental Panel on Climate Change
- IPCC - series of important reports
- International scientific consensus
- Website http//www.ipcc.ch/
52Direct Observations of Recent Climate Change
IPCC Report 2007
Global mean temperature Global average sea
level Northern hemisphere Snow cover
53Global mean surface temperatures have increased
IPCC Report 2007
54Sea Levels have risen
IPCC Report 2007
55Glaciers and frozen ground are receding
IPCC Report 2007
Area of seasonally frozen ground in NH has
decreased by 7 from 1901 to 2002
Increased Glacier retreat since the early 1990s
56IPCC Report 2007
Proportion of heavy rainfalls increasing in most
land areas
Regions of disproportionate changes in heavy
(95th) and very heavy (99th) precipitation
57Human and Natural Drivers of Climate Change
- CO2, CH4, N2O Concentrations
- - far exceed pre-industrial values
- - increased markedly since 1750
- due to human activities
Relatively little variation before the industrial
era
IPCC Report 2007
58IPCC Report 2007
Model Predictions Surface warming following
doubling of CO2 concentrations
Best estimate 3C likely 2-4.5C very
unlikely less than 1.5C higher values not
ruled out
59CO2 concentrations, temp., sea level continue to
rise long after CO2 emissions are reduced
IPCC Report 2007
60Constant emissions of CO2 do not lead to
stabilization of atmospheric concentrations
IPCC Report 2007
61Developing countries are the most vulnerable to
climate change
IPCC Report 2007
- Impacts are worse
- Already more flood and drought prone
- Larger share of the economy is in climate
sensitive sectors - Lower capacity to adapt
- Lack of financial, institutional and
technological capacity - Climate change is likely to impact
disproportionately upon the poorest countries - .. and the poorest people within countries
- Net economic effects expected to be negative in
most developing countries
62Venus Climate
63Present-day tectonics very different on Earth and
Venus
64Venus tectonics, contd
- No evidence for plate tectonics on Venus
- No mid-ocean rifts
- No subduction trenches
- Volcanos spread evenly across surface instead of
at plate boundaries, as on Earth. - Lithosphere not broken into plates probably
because heat at surface slightly softens the
lithosphere.
65No carbon-silicate cycle on Venus
66Another graphic of Earths carbonate cycle
67- Source Kasting, Scientific American
68Resurfacing on Venus
- Venus has far fewer impact craters than Moon
Mercury, but more than Earth. - The atmosphere protects it from smaller impacts
- Geologic activity (volcanic resurfacing) has
erased much of the evidence - Surface age is only about a billion years.
- Rather uniform age implies that Venus was
"resurfaced" by lava flows during a recent,
relatively short period - This differs profoundly from Earth's crustal
history. What is it telling us? - Could Venus' present crust only have formed that
recently? - Could there have been a growing crust before 1
billion years ago that "turned over" as heat
built up underneath, to lead to a new era of
major lava flows? - Why?
69There was once liquid water on Mars
- Geomorphological evidence (lots of it)
- Shape of ocean basins
70Why did Mars climate change?
71Climate Change on Mars
- Mars has not had widespread surface water for 3
billion years - Greenhouse effect probably kept surface warmer
before that - Somehow Mars lost most of its atmosphere
72Climate Change on Mars
One possible scenario
- Magnetic field may have preserved early Martian
atmosphere - Solar wind may have stripped atmosphere after
field decreased because of interior cooling
73History of Mars atmospherea more complex
scenario
- Shortly after Mars formed, its surface
temperature was equal to its blackbody
temperature (around -55 C). - As volcanoes dumped CO2 and H2O vapor into
atmosphere, greenhouse effect increased
temperature above 0 C (freezing) so liquid water
could exist. - Liquid water was present, so rocks could
efficiently remove CO2 from atmosphere. - Two competing effects determined amount of CO2 in
atmosphere volcanoes adding CO2, and rocks
absorbing CO2. Result moderate level of CO2 . - Greenhouse effect could keep surface T gt 0 C, as
long as volcanoes kept erupting. - Eventually Mars' core cooled and solidified (Mars
is small). Volcanic activity subsided. Magnetic
field went away, solar wind particles eroded
atmosphere. - Once rate of eruptions tapered off, CO2 in the
atmosphere started to fall. - As the atmosphere thinned out, the greenhouse
effect weakened. Eventually the average surface
temperature dropped, and surface water froze.
74Mars vs. Venus key issues
- Balancing act between injection and removal of
CO2 from atmosphere - Role of liquid water in sequestering CO2
- Venus too hot
- Mars too cold
- Earth just right
- What is evidence for these scenarios? How could
you test them?
75New space missions to Venus
- Venus Express (European)
- Orbiting Venus now
- Atmosphere, greenhouse gases, plasma environment
- Planet-C (Japan)
- Launch in 2010
- Atmosphere, volcanic activity, lightning
76New space missions to Mars
- Mars Reconnaissance Orbiter (NASA)
- At Mars now
- Atmosphere and climate are two of many goals
- Mars Climate Sounder producing global weather maps
77The Main Points
- Planetary atmospheres as a balancing act
- Gravity vs. thermal motions of air molecules
- Heating by Sun vs. heat radiated back into space
- Weather as a way to equalize pressures at
different places on Earths surface - Atmospheres of terrestrial planets are very
different now from the way they were born - Formation volcanoes, comets
- Destruction escape, incorporation into rocks,
oceans - Huge changes over a billion years or less
- Prospect of human-induced global warming on Earth
needs to be taken seriously