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METEO 466
Planetary Atmospheres
Reading for this week
1. Habitable Planet, Chapters 1-2
2. Chambers, EPSL (2004)
– Available from class website on Angel
3. Grad students (ABIOL 574) only
– Tsiganis et al., Nature (2005)
– Gomes et al., Nature (2005)
– Walsh et al., Nature (2011)
eight
The nine planets of the Solar
System
No Pluto!
Ref.: J. K. Beatty et al., The New Solar System (1999), Ch. 2.
Terrestrial planets
Solar System
planet types
Ice giants
Gas giants
http://starryskies.com/solar_system/planets.gif
Our home planet, Earth
• Earth is by far the
most interesting
planet because it
harbors life,
including us
• Planets with liquid
water on or beneath
their surfaces are
possible homes for
carbon-based life
Special properties of H2O
• Strong dipole moment has
several useful consequences
– Good solvent for polar
molecules
– Hydrogen bonding in DNA
– High heat capacity  helps
moderate climate on planets
(like Earth) with large
oceans
– The solid is less dense than
the liquid, i.e., ice floats!
• All of this leads to the
concept of the habitable
zone around stars…
(Note: Redraw in B&W)
The (liquid water) habitable zone
http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/
Question: Why is Earth’s climate stable?
Possible answers:
1. Stabilizing, largely abiotic feedback
processes (e.g., carbonate-silicate cycle)
2. Stabilizing biotic feedback processes (the
Gaia hypothesis)
3. Because we were lucky (the Rare Earth
hypothesis)
The Gaia Hypothesis
• Earth’s climate is regulated
by (and for?) the biota
James Lovelock
Lynn Margulis
(1938-2011)
1979
1988
Gaia—The Greek goddess
• Gaia is Mother Earth. She is
from whom everything comes,
but she is not quite a divinity,
because she is Earth. She
bore the Titans as well as
monsters like the hundred
armed men, and some of the
Cyclopes - others were sons of
Poseidon. She was the
daughter of Chaos, and the
mother of all creatures
(according to some). She was
the first and the last, and
wanted all of her children, no
matter what. She was primarily
spoken of as a Mother of other
Gods, rather than having her
own myths.
http://www.paleothea.com/Majors.html
The Rare Earth hypothesis
• Earth is “lucky” in a
number of respects
• Life itself may be
commonplace, but
complex life, i.e.,
animal life, is rare in
the universe
• Do we believe this?
Copernicus/Springer-Verlag
(2000)
• We will look at these questions.
However, we’ll also do a tour of the
Solar System on the way to see what
our neighboring planets, and particularly
their atmospheres and climates, are
like…
Venus
• UV image (false color)
from the Galileo
spacecraft
• Planet is nearly
featureless in the visible
• 93-bar, CO2-rich
atmosphere
• Surface temperature:
730 K
• Practically no water
• Very high D/H ratio
(~150 times Earth’s
value)
Image courtesy of NASA
Venus as
seen by
Magellan
• Image made using
synthetic aperture
radar (SAR)
http://www.crystalinks.com/venus703.jpg
Earth
topography
• Earth’s topography shows
tectonic features such as
midocean ridges
http://www.kidsgeo.com/geography-for-kids/0012-is-the-earth-round.php
Earth
topography
• Earth’s topography shows
tectonic features such as
midocean ridges and
linear mountain chains
http://sos.noaa.gov/download/dataset_table.html
Mars from HST
(Hubble Space Telescope)
• Mars is small
– ½ Earth’s radius
– 1/10th Earth’s mass
• Thin CO2-rich
atmosphere (~6-8
mbar)
• Mean surface
temperature: 218 K
(55oC)
• Polar caps of frozen
H2O and CO2
From: NASA Planetary Photojournal
MARS PATHFINDER
Twin peaks view
• Today, the surface of Mars is a frozen desert
River channel
Nanedi Vallis
(from Mars Global Surveyor)
• But there are lots of fluvial features
on the heavily cratered southern
highlands  Mars was wet early in
its history, and it may have been warm,
as well
Courtesy of NASA
~3 km
Mars Science Laboratory
(MSL, Nov 26 2011)
• MSL will assess
whether Mars ever was,
or is still today, an
environment able to
support microbial life.
• Other rovers found
evidence for highly
acidic water, which may
have been present as
the warm, wet period
ended
Courtesy of Joy Crisp
Jupiter (with Io and Europa)
• 318 Earth masses (M)
• 0.001 solar masses
(M)
– Smallest star is 0.08 M,
or 80 MJup
• Nearly solar in
composition
– Dominantly H and He
• Long-lived dynamical
features (The Great
Red Spot)
Europa (from Galileo)
• 2nd-most of the 4
Galilean moons
• Tidally heated 
liquid beneath its icy
shell
• How thick is the ice?
– And what lies
beneath it?
Courtesy: NASA Planetary
Photojournal
New evidence for liquid water on
Europa (Nov 16, 2011)
• Galileo probe has
discovered what
appears to be a body of
liquid water the volume
of the North American
Great Lakes locked
inside the icy shell of
Jupiter’s moon Europa.
Saturn (from Voyager 1)
• The ringed planet
• Rings appear to be
dynamically
unstable
– How and when did
they form?
• Has an interesting
moon…
Titan (from Voyager 2)
• Largest moon in the
Solar System
• Atmosphere is 15 times
as dense as Earth’s
– 1.5 bar surface pressure
– 93 K surface temperature
• Haze is thought to form
from photolysis (and
charged particle
irradiation) of CH4
Lakes on Titan?
• Image taken by the Huygens Probe, launched from the
Cassini spacecraft (January, 2005)
Titan shoreline?
Titan specular reflection (glint)
• Picture (from the
Cassini spacecraft)
published Dec. 23,
2009
• This is the first direct
evidence for (filled)
liquid lakes on Titan
http://ideafestival.typepad.com/my_weblog/2009/12/
specular-reflection-titan-lakes-.html
• Our Solar System has only 8 planets, so
our knowledge of planets in general has
been limited up until fairly recently
but
• Astronomers are now finding planets
around other stars…
Currently known exoplanets
exoplanet.eu
J
E
V
Radial velocity (Doppler) method
• The pull of the planet on its host star makes the star wobble
back and forth in the observer’s line of sight
http://www.eso.org/public/videos/eso1035g/
Transit Method
Kepler Mission
• This space-based telescope
points at a patch of the
Milky Way and monitors the
brightness of ~150,000 stars,
looking for transits of Earthsized (and other) planets
• 105 precision photometry
 can find Earths
• Launched: March 7, 2009
http://www.nmm.ac.uk/uploads/jpg/kepler.jpg
Known extrasolar planets
704
• Few, if any, of these planets
are very interesting, however,
from an astrobiological
standpoint
– Gliese 581g (the “Goldilocks
planet”) is probably not real
Howard et al.(2010)
~ 2300 more “candidate” planets from Kepler mission !!
December 2011 data release
Candidate
label
Earth-size
Super-Earths
Candidate size
(RE)
Rp < 1.25
1.25 < Rp < 2.0
Neptune-size
2.0 < Rp < 6.0
Jupiter-size
6.0 < Rp < 15
Very-large-size 15 < Rp < 22.4
TOTAL
Number of
candidates
207
680
1181
203
55
2326
• 48 of these planets are within their star’s habitable zone
Kepler-22b
• 600 l.y. distant
• 2.4 RE
• 290-day orbit, late G
star
• Not sure whether
this is a rocky planet
or a Neptune
(RNeptune = 3.9 RE)
http://www.nasa.gov/mission_pages/
kepler/news/kepscicon-briefing.html