Inevitability of Plate Tectonics on Super-Earths

The recent discovery of super-Earths (masses less or equal to 10 earth-masses) has initiated a discussion about conditions for habitable worlds. Among these is the mode of convection, which influences a planet's thermal evolution and surface conditions. On Earth, plate tectonics has been proposed as a necessary condition for life. Here we show, that super-Earths will also have plate tectonics. We demonstrate that as planetary mass increases, the shear stress available to overcome resistance to plate motion increases while the plate thickness decreases, thereby enhancing plate weakness. These effects contribute favorably to the subduction of the lithosphere, an essential component of plate tectonics. Moreover, uncertainties in achieving plate tectonics in the one earth-mass regime disappear as mass increases: super-Earths, even if dry, will exhibit plate tectonic behaviour.Comment: 13 pages, 2 figures and 1 table; in press in ApJ

Rheology of the Mantle

There have been two main avenues of investigation of the rheology of the mantle: theoretical and laboratory studies of the mechanical behavior of solids and direct estimates obtained by comparing geologic observations with model calculations in which rheological parameters are the principal variables. Studies of large-scale mantle flow, such as convection, might also put some constraints on mantle rheology, although at the present better knowledge of the rheology is needed to specify suitable models for investigation. The main advances of the last 4 years in our understanding of mantle rheology have been in the realization that the lower mantle may be less resistant to creep than was previously believed and in the realization and preliminary investigation of the complexities of the convection problem

Dynamics of submergence and uplift of a sedimentary basin underlain by a phase-change boundary

A procedure is given for calculating the evolution of a sedimentary basin underlain by a phase-change boundary that is in isostatic equilibrium. The equation of motion of the water depth w(t) as a function of the water depth and as a function of the sedimentation rate ds/dt is Q_w(w) dw/dt + Q_s ds/dt + (1 - ρ_1/ρ_2) dl/dt = 0 where l(t) is the lag of the phase boundary behind the transient equilibrium position. Numerically precise integrals of this equation are given for different time regimes by use of certain approximations for the temperature field that determine dl/dt. Solutions are demonstrated that are attenuated, explosive, or near periodic, depending on the physical parameters used. The results clearly show the possibility of depositing very thick sedimentary sections and the existence of self-sustained oscillations (above and below sea level) for a sedimentary basin of this type

Dynamics of the motion of a phase change boundary to changes in pressure

Because of the significance of both shallow and deep phase changes to geophysical problems, the dynamical response of a phase change to pressure loading was investigated. It was found that the characteristic behavior of the system may be analyzed in terms of simple parameters of the system by using analytic expressions that apply for the initial part and the final part of the motion of the phase boundary. These expressions are obtained from approximations based on generalizations of Neumann's solution for melting at a constant temperature or from simple physical approximations based on the over-all geometry of the model. The range of applicability of the approximations can be obtained from the approximations themselves. The analytic results compare very favorably with exact numerical solutions. The distribution of heat sources and convective heat transport are shown to be generally of minor importance on the motion of the phase boundary; the effect of convective heat transport can be estimated from the analytic approximation. The important parameters are the latent heat of the phase change and the difference in slope between the Clapeyron curve and the temperature distribution in the earth. In addition, the long-term motion depends primarily on the over-all geometry of the model and the boundary condition at depth. The analytic results indicate the time at which thermal blanketing by sediments becomes important and the effect of the rate of sedimentation on the response of the system; they also define slow and fast sedimentation and secular equilibrium. The effect of isostasy in conjunction with a shallow phase change is shown to be of major importance, and for certain cases the sediment thickness that can accumulate in a sedimentary basin may depend only on the sedimentation rate and not the initial depth of the basin. The analytic results permit a more physical discussion of the problem, since the functional dependence of the solution on the parameters may be seen. In addition, important results for a variety of models can be obtained by relatively simple calculations, without resorting to separate numerical solutions for each model considered

O'Connell Receives 2000 Inge Lehmann Medal

Richard J. O'Connell was awarded the Inge Lehmann Medal at the AGU Fall Meeting Honors Ceremony, which was held on December 17, 2000, in San Francisco, California. The medal recognizes outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth's mantle and core

Perfluoroarene-based peptide macrocycles that inhibit the Nrf2/Keap1 interaction

The Nrf2/Keap1 interaction is a target in the development of new therapeutic agents, where inhibition of the interaction activates Nrf2 and leads to the generation of downstream anti-inflammatory effects. Peptides that mimic the β-turn in the Keap1 active site and are constrained by a disulfide bridge have high affinity for Keap1 but no intracellular activity. The introduction of a perfluoroalkyl- bridging group to constrain the peptides, coupled with glutamic acid to proline replacement leads to a new peptide with a Ki of 6.1 nM for the Nrf2/Keap1 binding interaction, although this does not translate into intracellular activity

The Interior Dynamics of Water Planets

The ever-expanding catalog of detected super-Earths calls for theoretical studies of their properties in the case of a substantial water layer. This work considers such water planets with a range of masses and water mass fractions (2 to 5 M_Earth, 0.02% to 50% H2 O). First, we model the thermal and dynamical structure of the near-surface for icy and oceanic surfaces, finding separate regimes where the planet is expected to maintain a subsurface liquid ocean and where it is expected to exhibit ice tectonics. Newly discovered exoplanets may be placed into one of these regimes given estimates of surface temperature, heat flux, and gravity. Second, we construct a parameterized convection model for the underlying ice mantle of higher ice phases, finding that materials released from the silicate iron core should traverse the ice mantle on the timescale of 0.1 to 100 megayears. We present the dependence of the overturn times of the ice mantle and the planetary radius on total mass and water mass fraction. Finally, we discuss the implications of these internal processes on atmospheric observables.Comment: 9 page 4 figure

Gurnis, McComas receive Macelwane Medals

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95077/1/eost9813.pd

Work-Poor Households: the Welfare Implications of Changing Household Employment Patterns. ESRI Policy Series No. 52. March 2004

In the following report we consider how the employment situation of working-age Irish households has changed over a period of remarkable economic growth between 1994 and 2000. High levels of household joblessness became a matter of public concern in Ireland and elsewhere during the late 1980s and early 1990s. The concentration of unemployment and non-employment within households meant that many of the unemployed did not have access to the protection and support afforded by living with someone in employment. Therefore, household joblessness has serious implications for the financial situation of households and consequently for the psychological well-being of their members. It also has important implications for the scale of public support necessary to prevent poverty. For a given level of unemployment, a concentration of joblessness within households will require greater financing because there is no other source of household income

Novel Materials Containing Single-Wall Carbon Nanotubes Wrapped in Polymer Molecules

In this design, single-wall carbon nanotubes (SWNTs) have been coated in polymer molecules to create a new type of material that has low electrical conductivity, but still contains individual nanotubes, and small ropes of individual nanotubes, which are themselves good electrical conductors and serve as small conducting rods immersed in an electrically insulating matrix. The polymer is attached through weak chemical forces that are primarily non-covalent in nature, caused primarily through polarization rather than the sharing of valence electrons. Therefore, the electronic structure of the SWNT involved is substantially the same as that of free, individual (and small ropes of) SWNT. Their high conductivity makes the individual nanotubes extremely electrically polarizable, and materials containing these individual, highly polarizable molecules exhibit novel electrical properties including a high dielectric constant