UCLA

Precise clusters offer a new set of building blocks with unique properties that can be leveraged both individually and in materials in which their coupling can be controlled by choice of linker, dimensionality, and structure. Initial measurements in both of these worlds have been made. Isolated adsorbed or tethered clusters are probed with low-temperature scanning tunneling microscopy and spectroscopy. Even closely related elements behave differently on identical substrates. Surprising spectral variations are found for repeated measurements of single isolated, tethered clusters. In periodic solids, precise clusters joined by linkers can be measured experimentally and treated theoretically with excellent agreement, in part due to the relatively weak coupling of the clusters. This coupling can be controlled and exploited to produce materials with tailored properties. Some of the rules of thumb for predicting these properties are being developed through these initial studies and the limit to which they can be applied is being explored.

I will discuss recent insights that we have gained into planet formation and debris disks. In the first half of my talk, I will focus on the Kuiper belt, located at the outskirts of our planetary system, and the formation of debris disks. I will show how studying small km-sized Kuiper belt objects enables us to put our Kuiper belt into context of debris disks around other stars and I will explain how we can use the size distribution of small Kuiper belt objects and debris disks to gain insights into collisional cascades and the material properties of the objects themselves. In the second half, I will review dynamical models and geochemical constraints from the Earth, Moon and Mars and discuss their implications for the last stage of terrestrial planet formation.

The emerging picture of Mars’ first billion years includes diverse environments involving liquid water and chemical alteration. Clay, carbonate, chloride, and sulfate minerals have all been detected and mapped from orbit in coherent geologic units. When near-infrared spectroscopic detections of minerals from the orbiting CRISM imaging spectrometer are coupled with high-resolution images of morphology provided by orbiting cameras, distinctive aqueous, potentially habitable, environments can be identified, preserved in the geologic record. I will give a global overview of the most recent findings, delve into the details of transitions recorded in a few key stratigraphic sections, and discuss the hypothesis that the most widespread and long-lived aqueous environments on early Mars were in the subsurface.

The physical mechanism of intermediate-depth earthquakes is still under debate. In contrast to conditions in the crust and shallow lithosphere, at temperatures and pressures corresponding to depths >50 km one would expect rocks to yield by creep or flow and not by brittle failure, so there has to be a physical mechanism that allows for brittle or brittle-like failure for intermediate-depth earthquakes. Two such mechanisms have been proposed: dehydration embrittlement and thermal shear runaway. Earthquake nests represent a region with high earthquake concentration that is isolated from nearby activity. I will discuss general observations on the three famous intermediate-depth earthquake nests – Vrancea, Hindu-Kush and Bucaramanga. The emphasis will be on the Bucaramanga nest (Colombia) and how high-resolution seismological observations (tectonic setting, precise earthquake locations, focal mechanisms, stress drops, etc.) may provide key constraints on the mechanism responsible. Given the nature and characteristics of this nest, it can be thought as natural laboratory for understanding the physics of intermediate-depth earthquakes.

Knowing that extrasolar planetary systems are common, we would like to learn whether the Earth is normal or distinctive. Bulk Earth is 94% composed of O, Mg, Si and Fe and very deficient compared to the Sun in volatiles such as C and N. With our recent observations of white dwarf stars that have recently accreted tidally-disrupted minor planets that are about 300 km in diameter, we find a similar compositional pattern in extrasolar asteroids. While there must be individual exceptions, in aggregate, the studied extrasolar asteroids also are as “dry”; they probably formed interior to a snow line. Although the current sample is tiny, it appears that bulk Earth is compositionally normal for a rocky body. In the future, we may learn whether extrasolar planetesimals have undergone differentiation, a fundamentally important process in the history of our own planet.

Mapping the Amorphous-to-crystalline transitions in CaCO3 biominerals with 20-nm resolution One of the most fascinating aspects of calcite biominerals is their intricate and curved morphology, quite different from the rhombohedral crystal habit of geologic calcite. These morphologies, as well as space-filling and greater resistance to fracture, are achieved via amorphous precursor mineral phases (1). In this talk we will show that in sea urchin larval spicules two distinct phase transitions occur, 12 and 23 (2). Both transitions are regulated by inhibiting proteins, which introduce activation barriers between states otherwise spontaneously transforming because they are energetically downhill (3). 1. Y Politi, RA Metzler, M Abrecht, B Gilbert, FH Wilt, I Sagi, L Addadi, S Weiner, and PUPA Gilbert. Mechanism of transformation of amorphous calcium carbonate into calcite in the sea urchin larval spicule. Procs. Natl. Acad. Sci. USA 105, 17362-17366, 2008. 2. AV Radha, TZ Forbes, CE Killian, PUPA Gilbert, and A Navrotsky. Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate. Procs. Natl. Acad. Sci. USA 107, 16438–16443, 2010. 3. YUT Gong, CE Killian, IC Olson, NP Appathurai, RA Metzler, AL Amasino, FH Wilt, PUPA Gilbert. Phase Transitions in Sea Urchin Larval Spicules. Under review.

I will give an overview of the challenges associated with harnessing nuclear fusion as a terrestrial power source and the progress that has been made in research in this area. In particular, I will discuss turbulence in magnetically-confined plasmas and how transport associated with this turbulence limits the confinement achievable in current and planned experiments. I will present recent UCLA research that has helped advance our understanding of the basic physics of turbulence and turbulent transport in magnetized plasmas.

Study of the earthquake source brings about a set of fascinating interdisciplinary problems characterized by nonlinearity, a broad range of spatial and temporal scales, rare but catastrophic events, competing physical mechanisms, remote observations, inverse problems, non-uniqueness, and substantial societal significance. The ultimate challenge is to understand and quantify factors controlling the spatio-temporal behavior of active faults, including earthquake nucleation, seismic patterns, and the interaction of seismic and aseismic fault slip. My research aims to address this challenge by developing realistic physical models of earthquake source over several seismic cycles that rely on recent dramatic advances in observations, computational resources, and laboratory experiments. The goal is to use the models in conjunction with seismic, geodetic, and geological observations to constrain earthquake-source properties in terms of experimentally-derived constitutive laws, and then to study the potential set of future behaviors. Here, I will present two examples that illustrate this approach. In the first one, numerical modeling is used to establish the relation between variations in fault friction properties, the pattern of interseismic coupling (which characterizes the degree of fault locking between seismic events), the properties of earthquake sequences, and the observable characteristics of individual seismic events. The second example presents an innovative method for inferring fault friction properties based on comparison of numerical simulations and geodetic observations, which is applied to the central section of the North Anatolian fault (Turkey).