PLANETARY MUSINGS

Entries in "Earth"

Graduate Study in Earth and Planetary Science

The Department of Geological Sciences at Case Western Reserve University is currently accepting applications from students interested in pursuing graduate studies leading to M.S. and Ph.D. degrees in the earth, environmental, and planetary sciences. The Department offers flexible, research-intensive programs for graduate students. Applications are accepted on a continuing basis, though students requesting financial support are strongly encouraged to apply by February 1, 2008. Online applications are available through the School of Graduate Studies.

There are several opportunities for students interested in pursuing research in planetary science, particularly in the areas of planetary geology and geophysics, high-pressure and temperature geochemistry, and meteorites working with a group of faculty that includes myself, Prof. Harvey, and Prof. Van Orman.

At present I am collaborating with students to (1) understand the nature of Mars' crust and lithosphere and tectonic activity and (2) the mechanisms responsible for driving Ganymede's magnetic field. (3) I am also looking for graduate students interested in working with me on analyzing data from the MESSENGER Mission to Mercury to understand both the internal and tectonic evolution of that planet Additional opportunities within these may be available depending upon interest. We are also in the process of focusing new study on large lunar impact basins.

I would welcome the opportunity to discuss opportunities for graduate study in planetary science and/or geophysics with interested students (my contact info is available on my webpage).

Unbalanced climate?

Hansen, J, L Nazarenko, R Ruedy, M Sato, J Willis, A Del Genio, D Koch, A Lacis, K Lo, S Menon, T Novakov, J Perlwitz, G Russell, GA Schmidt, N Tausnev, Earth's energy imbalance: confirmation and implications, Science, 308, 1341-1435, (2005), doi:10.1126/science.1110252.

I've been a little busy to post lately, but we discussed this interesting paper by Hansen et al recently in our journal club. Basically, these authors take a climate model that includes forcing by human-made greenhouse gases, compares it to some data for validation (in particular recent global ocean heat content change) and then analyzes and discusses apparent disequilibrium in the present climate system. A major conclusion is that without an future changes in the composition of the atmosphere (levels of greenhouse gases stay constant instead of continually increasing like expected) that global climate will warm by another 0.6C because the climate system lags behind the current chemistry of the atmosphere. Therefore it is reasonable to expect that we will see at least 0.6C increase in the future, and probably more. The author's also discern from their model that the Earth absorbs 0.85 W/m^2 more than it emits back to space, a situation that if continued long-term could drastically raise ocean temps.

What I am curious about is what would happen if all greenhouse gas emissions were zeroed out today... how long would it take for the atmospheric chemistry to requilibrate (a residence time / feedback problem)? That's an unrealistic exercise, but I think it would give a sense of the maximum that we could do. Certain things cannot be changed (e.g., surface area covered in ice), but the problem is interesting nonetheless. It has also been a while since a major volcanic eruption which can result in global cooling, so what is the long-term energy imbalance? Is there such a thing?

The shape of a plume

Farnetani, CG and H Samuel, Beyond the thermal plume paradigm, GRL, 32, L07311, doi:10.1029/2005GL022360, (2005).

The issue of mantle plumes (i.e., whether they exist) is a long-running problem that just keeps going back and forth on the teeter-totter. I've always thought that with as complex a planet as we have here that they yes or no approach was a bit limiting given that the fluid physics of thermal plumes is well-established... but I digress.

Farnetani and Samuel tackle the formation of plumes from a more general framework than is usually taken. The canonical view of plumes is that a mushroom-like head and thin conduit tail plume structure is created at a deep thermal boundary layer and traverses the mantle to the surface with modest entrainment of surrounding mantle. The head impacts the surface, creates a large igneous province and the conduit creates a following chain of volcanic centers as a plate moves over it. That's the broad framework, first-order and all. The Earth is more complex and this paper shows what happens if chemical buoyancy effects, mantle wind (caused by imposed plate motion at the surface), phase transitions, and heterogeneities are considered (if memory serves, many of these effects have been considered before, though maybe not all at once with the spatial resolution of this study). And in this more "relaxed" study of the relevant parameters, the authors have discovered that the plume head-tail structure is but one possible structure of a plume as it reaches the upper mantle. A plume head isn't even necessary, and a concentrically zoned plume tail isn't even necessary either. Basically, models of the Earth can be messy - consistent with the geochemical and seismological view.

Mantle plumes are just plain hard to avoid - there is heat coming out of the core, the boundary layer down at the core-mantle boundary is going to get unstable in some places every once in a while, and there are going to be plumes. The real questions are: what do they look like, how do they sample the mantle, and what happens when (if?) they reach the surface? These questions are still open. But this paper shows us that not every plume looks alike.

But... this view of plumes that a plume head is possible not necessary is interesting when extended to other planets. Now the "mantle wind" may not be as extensive on a planet without plate tectonics, but what is the driver behind the headless-plume? On Venus there are all the coronae and volcanic rises, many of which seem consistent with a thermal plume or diapir source (e.g., recent conceptual framework of Johnson and Richards, 2003). If even some of those, e.g. coroane, require head-like features, what does that mean for the mantle? Maybe it is just a question of scale... but the questions about mantles and their plumes seem to abound.

Lithospheric thermal structure

McKenzie, D, J Jackson, K Priestley, Thermal structure of oceanic and continental lithosphere, EPSL, 233, 337-349, (2005), http://dx.doi.org/10.1016/j.epsl.2005.02.005

McKenzie et al re-examine the problem of what controls the depth of earthquakes in the lithosphere (excluding subducted oceanic lithosphere) primarily by using a more sophisticated thermal model than has been typically used. Until Hofmeister's [1999] paper on the variation of thermal conductivity of mantle materials, no simple model for the variation of thermal conductivity with temperature and pressure really existed. However, with Hofmeister's work, follow-on papers by other workers, and pre-existing data used in the Hofmeister-framework, more realistic thermal models are possible, as was done in this paper. Though the results are certainly reasonable, I was a little surprised at the choice to drop the second term from equation (3) if its effect is about 5 degrees C given some of the other small observational variations that the paper discusses. However, I doubt any major conclusions would be changed.... The choice of a linear interpolation for inital crustal temperature was a surprise since it is pretty much emplaced at the melting temperature; though maybe the intial cooling is more rapid (and hence unresolved) by the time-steps used. Influence would only be near the ridge axis anyway. The inability to accurately reproduce heat flow through old oceanic plates is something that has apparently been known for some time, but is relatively new to me. I suppose there may be a change in the style of convection (small scale) beneath oceanic plates.

Temperature as the primary control on depth of earthquakes is not a surprising idea, but I wonder how much of it has to do with strain-rate?