Saturday, March 30, 2013

Journal Club


Every week I download scores of megabytes worth of the scientific literature to my computer desktop—papers that I have come across in a search, papers I think will be relevant to my research, papers in my field of study, papers that interest me because of their titles, papers that have cited mine. I don’t even come close to reading all of these papers, but I skim through many, and read a few.

I’m curious how people handle the massive amounts of information. I once asked advice from a senior colleague who told me “Read everything, and remember it all.” Yikes. He continued: “Failing that, read the important papers of the important people in our field.”

My group has a weekly “journal club” meeting in which everyone presents a five minute summary of a paper in the literature—preferably recent and project-relevant. The goals are to make sure we all keep up with literature of interest to our research, and to spend time analyzing how papers are written to notice what works and what doesn’t. To help with this I came up with a brief rubric for presenting papers:

1. Read a paper either from the very recent literature or an old classic
2. What is the take-home message?
3. Why did you choose this paper (i.e. Relevant to your research? Interesting title? High-impact journal?)
4.  Science: what was successful about this paper? What was unsuccessful?
5.  Figures: Was there a successful figure? What about it is successful? Any unsuccessful figures? Why/why not?
6.  Writing: what was successful about the way the paper was written? What was unsuccessful?

Please let me know about your experiences with journal club. Is it useful? What formats work? What doesn’t work? What are people’s techniques for structuring time in order to do a better job of keeping up with the literature?

Tuesday, March 26, 2013

Geophysics Problem of the Day: Mantle Couette Flow



Geophysics Problem of the day: one-dimensional flow with varying viscosity and shear stress

I still have vivid and occasionally lucid dreams even 25+ years after my college roommate and I stopped telling each other our dreams every morning. In a recent dream, I was trying to set up this problem on the blackboard, but my legs kept buckling out from under me.

Here’s the picture I was trying to draw (in my dream).
Couette Flow Cartoon
 
Here’s today’s Geophysics Problem of the Day:
Part 1. Remind yourself how to derive Couette Flow (1-dimensional flow between plates). For the first part, assume that the viscosity of the fluid is constant. Wikepedia is a good help on this. Derive formulas for velocity as a function of depth and shear stress as a function of depth.

Part 2. Using your derived relationships, determine velocity and shear stress as a function of depth for the Earth’s upper mantle, assuming that the lower mantle is fixed. Also calculate accumulated strain over a ~100 million year timescale as a function of depth.Use typical plate velocities (1-10 cm/yr) and mantle viscosities (~1019-1020 Pa s)

In real materials, viscosity is strongly dependent on temperature, with the following general relationship:
where THom is the “homologous” temperature: the ratio of the actual temperature to the melting point. Using the homologous temperature is important here, because is shows that for many solids the viscosity—resistance to flow—decreases as the melting temperature is approached. Below the lithosphere, the temperature increases with depth through the mantle. But likely the melting temperature also increases with depth. So calculating homologous temperature as a function of depth is easy to say, but hard to do.

Part 3. Rederive Couette-style flow using a depth-dependent viscosity, and use these relationships to calculate velocity, shear stress, and accumulated strain as a function of depth for Earth’s upper mantle and transition zone using a depth-dependent viscosity. Try a depth-dependent viscosity model such as Peltier, Science 1996.

Part 3. Rederive Couette-style flow using a depth-dependent viscosity, and use these relationships to calculate velocity, shear stress, and accumulated strain as a function of depth for Earth’s upper mantle and transition zone using a depth-dependent viscosity. Try a depth-dependent viscosity model such as Peltier, Science 1996. 

Mantle Couette Cartoon.

Part 4. Viscosity describes a material's ability to support a shear stress over a long timescale. During a solid-state phase transformation, the ability of a material to support a shear stress is expected to approach zero at equilibrium (=long time scales). In the Earth, the top and bottom of the transition zone correspond to major mineralogical phase transitions. What if these transformations significantly reduce the viscosity in thin shells at the top and bottom of the transition zones? To examine the potential effects of this, calculate the depth-dependent velocity, shear stress, and accumulated strain for a constant viscosity mantle with two sharp lower-viscosity zones, corresponding to the top and bottom of the transition zone, as shown in the picture at left. You might want to do this for different values of low-viscosities. How high does it need to be to avoid an effective "free-slip" boundary condition, thereby ruining the flow for everything underneath?

Please send me your solutions!

Note 1: If this problem is too boring, then try it in a spherical coordinate system, with the outside surface of the sphere forced to rotate with an angular velocity, and a fixed small interior volume.

Note 2: If you need additional information or values such as constants or material properties don’t be afraid to look them up and/or make them up.

Sunday, March 24, 2013

Planetary Outreach Part 2: Giants

Some more planetary insights from the top of my head to fill in a potential NASA gap in education and outreach*.

Random Thoughts on Jupiter
Jupiter the planet is the best high-pressure experiment on hydrogen, and Jupiter’s huge magnetic field tells us of the metallization of hydrogen at high pressures and—a prediction made by Wigner & Huntington in 1935 and so far not confirmed by experiment. Not for lack of trying. Many diamonds and a more than a few careers have perished in the attempt to demonstrate electrically conducting hydrogen at high pressures. I’ll put this on my list of blog-post ideas, because there is so much to write about the combination science/sociology of this field.

Jupiter could have several Earth-sized objects in the center, and we cannot tell with our current observations. Jupiter is actually quite simple. A very simple first-order polytrope defining the density-pressure behavior of Jupiter’s interior does a superb job of explaining the mass, volume, and observed moments of inertia. In the graduate “Build-A-Planet” course that I teach at UCLA, we find that the size of Jupiter is quite stable. Add more mass and the gravity shrinks it in. Take away some mass and it expands out a bit. Saturn is a different story.

Saturn 
 Saturn is an important second experiment on the high pressure behavior of hydrogen, and just like in the lab where occasionally additional data points just mess up the beautiful story, Saturn complicates matters. Saturn is a little less dense than Jupiter, but planetary formation intuition suggests that Saturn and Jupiter have similar interior compositions. Yet Saturn has some funky gravitational behavior, that suggests a more complicated interior model than is required for Jupiter. [For a deeper and better description see the paper of Helled et al., 2009 paper in Icarus http://dx.doi.org/10.1016/j.icarus.2008.10.005]

On the Difficulties of Lecturing About Uranus
The gas giant Uranus is perhaps the most difficult to handle in the context of a classroom. See? When I taught UCLA’s ESS9 Introduction to the Solar System course, each night I practiced my lectures for my ~6-7 yr old son. We’d nestle in his bed and I would talk through the slides, and he would offer helpful commentary mostly consisting of single word reactions: either “boring” or “cool.” When it came time for the Uranus lecture, he thought it was the funniest thing he had ever heard. Every time I said the word Uranus in any context he exploded. “Uranus is a gas-giant.” “Uranus has rings.” “There might be diamonds in the interior of Uranus.” “Scientists are studying hot gas emissions from Uranus.” My son would roll around on his bed laughing. 

Diamonds in Neptune and Uranus
Like the sunspot cycle, every decade or so a scientific article detailing the diamonds in Uranus (excuse me) goes viral, and the science-media is temporarily flooded with imaginations of Neptune and Uranus and now exoplanets as giant-diamond planets. My advice to the scientist(s) who are enjoying the media spotlight on your new paper entitled something like “High pressure-laboratory investigations of a solar-subtracted-chondritic composition and potential implications for giant carbon planet interiors”: enjoy the publicity when it happens, but don’t stare straight at the sun. My advice for the rest of us: also enjoy our colleagues’ publicity and public exposure for the usual subterranean high pressure research, but remember that like many planetary phenomena, these media events are cyclical.

Here are some diamond-planet highlights from the last few cycles.
From ABC news: Artist's conception of 55 Cancri e. Haven Giguere/Yale University


*Yesterday’s fact-check on this revealed two NASA tweets suggesting EPO cuts are not necessarily happening but there is additional reporting that suggests otherwise: http://nasawatch.com/archives/2013/03/nasa-cancels-al.html

Note: My research program is not supported by NASA currently, but I have had some NASA support in the past. My salary and research program is supported by a combination of the State of California, NSF, and DOE.

Saturday, March 23, 2013

Planetary Outreach Part 1: Terrestrials

It’s important that we unpaid, unregulated, and non-reviewed bloggers step in to help fill the gap create by NASA’s suspending its education and outreach program*. So, as a service, I attach some personal stories concerning planets, intended for science outreach and public edutainment.

How to remember the planets**:
My Very Eager Mother Just Served Us Nothing! No Peanuts, No Potatoes, No Pasta, Nothing!

A true story about Love and Mercury: The last time someone came into my office with questions about Mercury’s interior was this past Thursday. The student was interested in detailed information about elastic properties of a supposed iron-sulfide constituent of the interior and had worked out detailed models about how pressure affects shear elastic properties, and how temperature affects shear elastic properties, and models were piled on models and the goal was to have The Right Number. I strongly believe we don’t know The Right Number for an assumption piled onto an assumption piled onto assumption…all the way down… but that the student could make a pretty good guess. We know the shear modulus for iron sulfide at room pressure and temperature. Pressure increases the value, but not too steeply. Temperature decreases the value, and perhaps by a lot—especially because rigidity falls quickly as a material approaches its melting temperature. Therefore, I suggested that the student choose the ambient value of rigidity, and claim it as an upper bound. The student asked me could this  reasoning be found in a paper so there is something to cite?  As it turns out, it is hard to find common science sense documented explicitly in the scientific literature. I’m thinking of adding a section on horse-sense to my next papers to address this deficiency. Back to the favorite part of this week's Mercury story: The student is doing this project to calculate Love Numbers for Mercury. Please submit suggestions for student's Dissertation title in the comments section below, and I will forward.

My thoughts on Venus Part 1: I can plz haz Venus NASA mission?!?
My thoughts on Venus Part 2: Venus is my Earth Global Warming Endpoint Nightmare.

My Biggest–Earth Question: What makes a planet interesting? Interesting is defined as “Ability to support life”. My hypothesized answer: Large scale electrochemical disequilibrium.

Comments on My Paper About Mars That Did Not Make It Into the Actual Paper: I wrote a paper about the martian core based on high P,T equation of state measurements I had made at the synchrotron while 7 months pregnant. I think I might have been the first pregnant user at the Advanced Photon Source. They sent people in suits to come talk with me. I signed a lot of paperwork. I did the experiments, came home and had the baby, and then analyzed the data and wrote the whole paper while nursing the baby. This paper is among my top five cited papers. The baby is now 14. It is still not known whether Mars has its own Martian D-double-prime-layer of perovskite at the base of its mantle.


**with acknowledgement to SML
*It's not clear the NASA is actually cutting EPO. Here are are the results of a quick fact-check expedition to NASA's website

 











Saturday, March 2, 2013

February Field Trip #1: Interface Science


Dear fellow mineral physicists, mineralogists, planetary scientists, space physicists, atmospheric physicists, geophysicists, geochemists, cosmochemists, environmental geochemists, structural geologists, seismologists, tectonophysicists, etc,

To the outside world we are all geologists.

Geologists do field work. It was a geology field trip that turned me from materials science engineering to Earth science. But even though field observations is what first brought me to this field, I am a laboratory scientist by long training.

I have some training in electrochemistry as part of my metallurgy/materials science training and I have a masters’ degree in corrosion engineering. One of the first experiments I did as an assistant professor of mineral physics was to electroplate metals from a salt bath and measure the stable isotope fractionation.  I can still hear the voice of my at-the-time department chair pleading with me to stick with the research program he had hired me to do. “Please make it easy for us” he said.

The two field trips I went on this month were small conferences sponsored in different fields: electrochemistry and materials science. At both workshops I played “Pet Geologist”, showing some of the research my group does in the context of big questions. e.g. experiments at high pressures and temperatures combines with geophysical and geochemical observations to tell us about Earth structure and evolution.

The first conference was an electrochemistry workshop. I learned about batteries, photovoltaics, catalysis. I presented some of the results of a series of fun experiments examining isotope effects in electrochemical processes. I also showed pictures of the Earth—inside and out. It was great fun, and I loved having the opportunity to talk with students, faculty, and postdocs in a different field.

I think it is a good thing to spend a lot of time learning a field in depth. I also think it is good to shift and move and spend time in other fields—especially neighboring fields. I personally find the interfaces fascinating—both the metaphorical interfaces (between different fields) and the literal interfaces (between different phases).  

Sculpture at University of British Columbia. Photo credit: A. Kavner