Welcome to GeoUnion, the graduate student body of the Department of Earth, Environmental and Planetary Sciences. GeoUnion strives to supplement the overall graduate student experience at Rice and DEEPS. GeoUnion represents DEEPS in the overall Rice grad student community, acts as a liaison between students and faculty and organizes a number of intra- and inter-departmental events throughout the academic year.
The effect of a strong pressure bump in the Sun’s natal disk: Terrestrial planet formation via planetesimal accretion rather than pebble accretion
André Izidoro, Bertram Bitsch, Rajdeep Dasgupta
Mass-independent isotopic anomalies of carbonaceous and noncarbonaceous meteorites show a clear dichotomy suggesting an efficient separation of the inner and outer solar system. Observations show that ring-like structures in the distribution of millimeter-sized pebbles in protoplanetary disks are common. These structures are often associated with drifting pebbles being trapped by local pressure maxima in the gas disk. Similar structures may also have existed in the Sun’s natal disk, which could naturally explain the meteorite/planetary isotopic dichotomy. Here, we test the effects of a strong pressure bump in the outer disk (e.g., ~5 au) on the formation of the inner solar system. We model dust coagulation and evolution, planetesimal formation, as well as embryo growth via planetesimal and pebble accretion. Our results show that terrestrial embryos formed via planetesimal accretion rather than pebble accretion. In our model, the radial drift of pebbles fosters planetesimal formation. However, once a pressure bump forms, pebbles in the inner disk are lost via drift before they can be efficiently accreted by embryos growing at 1 au. Embryos inside ~0.5–1.0 au grow relatively faster and can accrete pebbles more efficiently. However, these same embryos grow to larger masses so they should migrate inwards substantially, which is inconsistent with the current solar system. Therefore, terrestrial planets most likely accreted from giant impacts of Moon to roughly Mars-mass planetary embryos formed around 1.0 au. Finally, our simulations produce a steep radial mass distribution of planetesimals in the terrestrial region, which is qualitatively aligned with formation models suggesting that the asteroid belt was born low mass.
Izidoro, A., Bitsch, B. & Dasgupta, R. (2021). The effect of a strong pressure bump in the Sun’s natal disk: Terrestrial planet formation via planetesimal accretion rather than pebble accretion. The Astrophysical Journal 915, 62. doi:10.3847/1538-4357/abfe0b
Laurence Y. Yeung, Lee T. Murray, Asmita Banerjee, Xin Tie, Yuzhen Yan, Elliot L. Atlas, Sue M. Schuaffler, and Kristie A. Boering
Tropospheric 18O18O is an emerging proxy for past tropospheric ozone and free-tropospheric temperatures. The basis of these applications is the idea that isotope-exchange reactions in the atmosphere drive 18O18O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the 18O18O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the 18O18O proxy, we developed a scheme to simulate atmospheric 18O18O abundances (quantified as ∆36 values) online within the GEOS-Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O2 isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36 values are too high elsewhere. Investigations using a comprehensive model of the O-O2-O3 isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36 values: the anomalously efficient titration of 18O18O to form ozone. Incorporating these effects into the online ∆36 calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of 18O18O in O2, the modeled change in the mean tropospheric ∆36 value since 1850 C.E. is only slightly altered; it is still quantitatively consistent with the ice-core ∆36 record, implying that the tropospheric ozone burden increased less than ∼40% over the twentieth century.
Oxygen in the air is constantly being broken apart and remade. Its constituent atoms are shuffled around by light-induced chemical reactions, which cause changes in the number of heavy oxygen atoms that are bound together. The number of these heavy-atom “clumps” is sensitive to air temperatures and the presence of air pollution; hence, their variations are being used to understand past high-altitude temperatures and atmospheric chemistry. This study incorporates oxygen clumping into an atmospheric chemistry model and compares the results to measurements of oxygen clumping in the atmosphere. We find that the model can explain all the modern-day measurements (from the surface to 33 km altitude), but only if the broader fates of oxygen atoms―i.e., their incorporation into other molecules beyond O2―are considered. Simulations of the preindustrial atmosphere are also generally consistent with snapshots of the ancient atmosphere obtained from O2 trapped in ice cores. The developments described herein will thus enable models to simulate heavy oxygen-atom clumping in past cold and warm climates and enable simulated high-altitude atmospheric changes to be evaluated directly against ice-core snapshots of the ancient atmosphere.
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