Habitable Planet, Chapters 1-2



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Habitable Planet, Chapters 1-2

  • Habitable Planet, Chapters 1-2

  • Chambers, EPSL (2004)

  • Grad students (ABIOL 574) only

    • Tsiganis et al., Nature (2005)
    • Gomes et al., Nature (2005)
    • Walsh et al., Nature (2011)






Earth is by far the most interesting planet because it harbors life, including us

  • 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



Strong dipole moment has several useful consequences

  • 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…





Question: Why is Earth’s climate stable?

  • Question: Why is Earth’s climate stable?

  • Possible answers:

  • Stabilizing, largely abiotic feedback processes (e.g., carbonate-silicate cycle)

  • Stabilizing biotic feedback processes (the Gaia hypothesis)

  • Because we were lucky (the Rare Earth hypothesis)





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.

  • 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.



Earth is “lucky” in a number of respects

  • 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?



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…

  • 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…



UV image (false color) from the Galileo spacecraft

  • 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)









Mars is small

  • 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







MSL will assess whether Mars ever was, or is still today, an environment able to support microbial life.

  • 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



318 Earth masses (M)

  • 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)



2nd-most of the 4 Galilean moons

  • 2nd-most of the 4 Galilean moons

  • Tidally heated  liquid beneath its icy shell

  • How thick is the ice?

    • And what lies beneath it?


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.

  • 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.



The ringed planet

  • The ringed planet

  • Rings appear to be dynamically unstable

    • How and when did they form?
  • Has an interesting moon…



Largest moon in the Solar System

  • Largest moon in the Solar System

  • Atmosphere is 15 times as dense as Earth’s

  • Haze is thought to form from photolysis (and charged particle irradiation) of CH4







Picture (from the Cassini spacecraft) published Dec. 23, 2009

  • Picture (from the Cassini spacecraft) published Dec. 23, 2009

  • This is the first direct evidence for (filled) liquid lakes on Titan



Our Solar System has only 8 planets, so our knowledge of planets in general has been limited up until fairly recently

  • 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…









Few, if any, of these planets are very interesting, however, from an astrobiological standpoint

  • 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)





600 l.y. distant

  • 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)




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