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Session 2: DISCUSSION AFTER NORM SLEEPS TALK:

Alain Leger: What would you expect if the Earths diameter were twice as large as it is?

Norm Sleep: The convective heat flow scales very weakly with radius, maybe even more weakly if plate tectonics is included. However, the radioactivity that needs to be gotten out per unit area scales linearly with the radius, so a larger planet will stay hotter in the interior much longer than the Earth did. It will have a thinner crust, the crust will penetrate downward, so we would still expect tectonic activity. We should also expect to get the analog of continents to some extent because there would still be hydrous rocks to melt. The danger is that the larger planet will accrete too much air, and wont be able to get rid of its hydrogen. So it could end up with an ocean of H, even a few bars, so it will retain its reducing atmosphere very long and that doesnt help the Pale Blue Dot. If its too big, wed end up getting Neptune. The tectonic part of the story is subtle. If the planet cant get rid of the reducing gases early on, there would be problems. The mantle of the Earth is oxidized. To get the ferric iron in the mantle requires an ocean of water to be oxidized, to get the ferrous iron oxidized maybe 10 oceans.

Alain Leger: Where would you put the limit on radius?

Norm Sleep: I cant put a limit this way; it would be set astrophysicallyfrom accreting gas.

Alain Leger: By gas you mean hydrogen?

Norm Sleep: Solar nebula gas. Therell be a limit to the Blue Dot, where hydrogen does not escape. Say a temperate planet, but one where there is too much hydrogen.

Carol Stoker: I was surprised to hear you say that if the Earth had about 1/2 its present amount of water, it would mostly be buried?

Norm Sleep: Yes, it would mostly be buried. Wed still have lakes dynamically maintained. The tectonics would still be producing continents. Thered still be a hydrological cycle, but only small bodies of water would exist, photosynthesis would be difficult, and, though this might still be a Blue Dot, the conditions for life would be far less attractive.

Anonymous: Planetary density is an important parameter, especially for convection. So, even for a terrestrial planet, Earth, if the core were twice as large the convection would be much larger.

Norm Sleep: Right, if had a larger core, convection would be much stronger, greater dominance of plumes. But also there would be less radioactivity in the total planet. So youd have a situation more like Mercury, where the internal heat would get dissipated more quickly.

Same person : So, if you change a parameter like density, plumes would form all over the planet and it would be very difficult to maintain a crust for very long. Correct?

Norm Sleep: No, because the limit is the total amount of heat available. The vigor of convection only scales as gravity**1/3. So no real difference between the Moon and the Earth in this regard.

Same person: Another aspect of planetary convection would be that, as the core cools, its radius increases, by crystallization. When a critical radius is reached, the dynamo is shut down. Right?

Norm Sleep: At some stage, but when you do the scaling, you see to first order that that is independent of planetary radius.

Same person: But when you do that, you no longer have a B field to shield the atmosphere from being sputtered away. So, thats the end of life as we know it, right?

Norm Sleep: Well, Venus has survived without a magnetic field. So, my guess is: if the Earths magnetic field were done away with, the people who sell compasses would be out of business, as would the folks who work on the Aurora Borealis. But I dont think the Earth would rapidly lose its atmosphere.

Alan Boss: You mentioned that it didnt look as if Mars had plate tectonics, at least not for a long time. Could you comment on recent results from Mars Global Surveyor that show magnetic striping on the surface, which has been interpreted as evidence for plate tectonics? If you believe it, what does this tell us about the age that plate tectonics ceased on Mars?

Norm Sleep: This would on be the older part of the planets surface. So it would tell us that plate tectonics with a relatively thick crust, existed maybe 4 billion years ago. And this is the one process we know of that generates large linear magnetic stripes on planetary surfaces. The north-south boundary looks like an analog of the margin on the East Coast of the US that formed when the continent split up and a mid-oceanic ridge formed and spread out. So, in the status of what we understand about plate tectonics, this would be the kind of process. There may be other processes that we don't know about, but at least in the case of plate tectonics, we can do the analysis, make predictions, and see if it fits.

Pat Cassen: Carol asked earlier about the case where there wasnt as much water as the Earth currently has. What about the other extreme? What if there are no exposed land masses and the Earth is entirely covered with water?

Norm Sleep: The typical burial then would be burial on sea floors that would then get subducted. And the oxygen would react with basalt, the carbon would end up on the seafloor and get subducted, so youd get rid of both of them at the same time. Then it would be difficult to build up a large surface of oxygen that could, in turn, build up an atmosphere.

Anonymous : What do you think about the importance of continental growth? Because, according to our calculations, it would have a main influence on the habitable zone. If the area of the continents is small, say in the past, then its hard to get the CO2 out of the atmosphere. But if continents will grow larger in the future, say at their present rate, then CO2 will go quickly from the atmosphere to the solid Earth and the planet will cease to be habitable.

Norm Sleep: There is another major sink for CO2, thats the oceanic crust. Now this oceanic basalt will react with sea water that contains CO2 dissolved in it and will form carbonates. At the present time, this process doesnt occur strongly because the continents are getting the first crack at the CO2. But if the continents go away, then the ocean crust becomes hard to overwhelm. So if the continental cycle could be turned off completely, leaving just the oceanic crust to do the job, the CO2 in the atmosphere might increase by up to a factor of 10, but there would still be a buffer. The Urey type buffer would still operate. If there is ejecta on the early Earth, thats a very efficient sink of CO2 and we could end up with an ice-covered planet, early on. If we just have small impact bodies opening lanes in Antarctica, where the CO2 in the air is strongly out of equilibrium with the water, then the atmosphere and the ocean will stay in more or less dynamic equilibrium. So, for an ice covered planet, where this occurred globally, you would have to take this into account.