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ItemMultiple generations of grain aggregation in different environments preceded solar system body formation(Proceedings of the National Academy of Sciences of the United States of America, 2018-06-11)The solar system formed from interstellar dust and gas in a molecular cloud. Astronomical observations show that typical interstellar dust consists of amorphous (a-) silicate and organic carbon. Bona fide physical samples for laboratory studies would yield unprecedented insight about solar system formation, but they were largely destroyed. The most likely repositories of surviving presolar dust are the least altered extraterrestrial materials, interplanetary dust particles (IDPs) with probable cometary origins. Cometary IDPs contain abundant submicron a-silicate grains called GEMS, believed to be carbon-free. Some have detectable isotopically anomalous a-silicate components from other stars, proving they are preserved dust inherited from the interstellar medium. However, it is debated whether the majority of GEMS predate the solar system or formed in the solar nebula by condensation of high-temperature (>1300K) gas. Here, we map IDP compositions with single nanometer-scale resolution and find that GEMS contain organic carbon. Mapping reveals two generations of grain aggregation, the key process in growth from dust grains to planetesimals, mediated by carbon. GEMS grains, some with a-silicate subgrains mantled by organic carbon, comprise the earliest generation of aggregates. These aggregates (and other grains) are encapsulated in lower density organic carbon matrix, indicating a second generation of aggregation. Since this organic carbon thermally decomposes above ~450K, GEMS cannot have accreted in the hot solar nebula and formed, instead, in the cold presolar molecular cloud and/or outer protoplanetary disk. We suggest that GEMS are consistent with surviving interstellar dust, condensed in situ, and cycled through multiple molecular clouds.
ItemSupporting data CNO prior to corrections( 2018-06-06)Table of data of carbon, nitrogen and oxygen elemental compositions prior to corrections. Supporting data for journal article published in Proceedings of the National Academy of Sciences in June 2018, titled "Multiple generations of grain aggregation in different environments preceded solar system body formation".
ItemThe Indonesian seas and their role in the coupled ocean–climate system(Nature Geoscience, 2014-06-22)The Indonesian seas represent the only pathway that connects different ocean basins in the tropics, and therefore play a pivotal role in the coupled ocean and climate system. Here, water flows from the Pacific to the Indian Ocean through a series of narrow straits. The throughflow is characterized by strong velocities at water depths of about 100 m, with more minor contributions from surface flow than previously thought. A synthesis of observational data and model simulations indicates that the temperature, salinity and velocity depth profiles of the Indonesian throughflow are determined by intense vertical mixing within the Indonesian seas. This mixing results in the net upwelling of thermocline water in the Indonesian seas, which in turn lowers sea surface temperatures in this region by about 0.5 °C, with implications for precipitation and air–sea heat flux. Moreover, the depth and velocity of the core of the Indonesian throughflow has varied with the El Niño/Southern Oscillation and Indian Ocean Dipole on interannual to decadal timescales. Specifically, the throughflow slows and shoals during El Niño events. Changes in the Indonesian throughflow alter surface and subsurface heat content and sea level in the Indian Ocean between 10 and 15° S. We conclude that inter-ocean exchange through the Indonesian seas serves as a feedback modulating the regional precipitation and wind patterns.
ItemA 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors(Nature Publishing Group, 2013-11-14)Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave, but owing to lack of observations this is uncertain. Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (6100) kilotons of trinitrotoluene (TNT, where 1 kiloton ofTNT54.18531012 joules).Weshowthat a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates near the threshold impactor size are too high.Weperformed a global survey of airbursts of a kiloton ormore (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques. This suggests a non-equilibrium(if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.