Day 2/5, Session 1: Oceans

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Good morning, this is Frederic taking over from Nawal. We have two review talks and two contributed talks this morning.

Colin Goldblatt, from University of Victoria, kicked off the session with a sweeping review of early Earth climate. Here are a few nuggets I could catch from this great talk:

  • we usually associate the making of an atmosphere only with volcanic outgassing, but there is more: impact outgassing, and “ingassing” (re-integration of gas in the crust and mantle)
  • on Earth, all volatiles except water and noble gases (that leaves CO2, methane, nitrogen, sulphur, etc…) are under “geo-bio control”. They are regulated both by geological and biological cycles.
  • under heavy biogenic production of oxygen, the atmosphere has two stable states, a low-oxygen state (<1 ppm), and a high oxygen state (>1%). It can jump from one to the other. This implies that a planet can have widespread photosynthesis humming along for billion of years without producing detectable O2 in the atmosphere.
  • Nitrogen on Earth is an “ingassed” volatile. It is gradually incorporated in the lithosphere, mainly by fixation in rocks of biogenic ammonia.
  • about the “faint sun paradox”, Goldblatt thinks CO2 is still a good candidate, and recent debates about this are overstated.
  • The habitable zone is a two-dimensional region, dependent not only in distance from the star but on the amount of greenhouse gases.

Finally a direct quote:

“has the notion of habitability come to the end of its useful scientific life?”

Yes   (I think)

Next comes Daniel Koll from Chicago University. He presents a circulation simulation for an ocean planet (Earth entirely covered by oceans), varying the amount of heat carried by ocean currents.

The main result is that the atmosphere compensates for the heat that the oceans carry, so that the total heat transport remains nearly constant.

David Ferreira, standing in for John Marshall at MIT, also presents simulations for an ocean planet, using the MIT GCM. They examined two specific cases: a pole-on planet, and a tidally locked planet. The details are fascinating – although unfortunately it would take too long to go into them here. In summary, in the high-obliquity case, the oceans store the heat during the 6-month summer and release it to warm the 6-month dark winter. In the tidally locked case, the ocean currents become dominant over the atmospheric circulation to warm the night side.

Robin Wordsworth of Chicago University presents the result of a GCM applied to early Mars and to the exoplanet GJ 581 d, that receives Mars-like levels of star light, but is probably tidally locked.

In his last slides Robin touches upon the interesting issue of H2 as a greenhouse gas in the early Earth (before it was lost to space). A small fraction of H2 (<1%) in a heavy nitrogen atmosphere could produce enough greenhouse effect to compensate the early Sun faintness. Similarly, H2 could make super-Earths habitable  for a transient period early on.

That’s all for this morning.

Feature image: NASA

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.