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)

The nine planets of the Solar System

Ref.: J. K. Beatty et al., The New Solar System (1999), Ch. 2.

eight

No Pluto!

http://starryskies.com/solar_system/planets.gif

Gas giants

Ice giants

Terrestrial planets

Solar Systemplanet types

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…

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

James Lovelock

1979 1988

• Earth’s climate is regulated by (and for?) the biota

Lynn Margulis (1938-2011)

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 asseen byMagellan

• Image made using synthetic aperture radar (SAR)

http://www.crystalinks.com/venus703.jpg

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

http://sos.noaa.gov/download/dataset_table.html

Earth topography

• Earth’s topography shows tectonic features such as midocean ridges and linear mountain chains

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 PATHFINDERTwin peaks view

• Today, the surface of Mars is a frozen desert

Courtesy of NASA

Nanedi Vallis(from Mars Global Surveyor)

~3 km

River channel

• 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

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

V

E

J

exoplanet.eu

Radial velocity (Doppler) method

http://www.eso.org/public/videos/eso1035g/

• The pull of the planet on its host star makes the star wobble back and forth in the observer’s line of sight

Kepler Mission

http://www.nmm.ac.uk/uploads/jpg/kepler.jpg

• This space-based telescope points at a patch of the Milky Way and monitors the brightness of ~150,000 stars, looking for transits of Earth- sized (and other) planets• 105 precision photometry can find Earths• Launched: March 7, 2009

Transit Method

Known extrasolar planets

• 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)

704

~ 2300 more “candidate” planets from Kepler mission !!

December 2011 data release

Candidate label

Candidate size (RE)

Number of candidates

Earth-size Rp < 1.25 207

Super-Earths 1.25 < Rp < 2.0

680

Neptune-size 2.0 < Rp < 6.0

1181

Jupiter-size 6.0 < Rp < 15 203

Very-large-size

15 < Rp < 22.4

55

TOTAL 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