Mars Encounters cause fresh surfaces on some near-Earth asteroids

All airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1My) timescales. The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying they have been recently been exposed. Previous work found that Earth encounters were the dominant freshening mechanism and could be responsible for all near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type sample of by a factor of three. We present the orbital distributions of 64 Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces. Our sample reveals two important results: i) the relatively steady fraction of Q-types with increasing semi-major axis and ii) the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection Distances (MOID) that do not have orbit solutions that cross Earth. Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened. The high Earth-MOID asteroids represent 10% of the Q-type population and all are in Amor orbits. While surface refreshing could also be caused by Main Belt collisions or mass shedding from YORP spinup, all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces.Comment: Accepted for publication in Icarus -- 14 pages, 8 figures, 1 table, 2 appendice

Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_{1-x}Co_x)_2As_2 Single Crystals using Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopic studies of $Ba(Fe_{1-x}Co_x)_2As_2$ (x = 0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition $T_c$. The two-gap nature and the slightly doping- and energy-dependent quasiparticle scattering interferences near the wave-vectors $(\pm \pi, 0)$ and $(0, \pm \pi)$ are consistent with sign-changing $s$-wave superconductivity. The excess zero-bias conductance and the large gap-to-$T_c$ ratios suggest dominant unitary impurity scattering.Comment: 4 pages, 4 figures. Paper accepted for publication in Physical Review Letters. Contact author: Nai-Chang Yeh ([email protected]

Adventures in Invariant Theory

We provide an introduction to enumerating and constructing invariants of group representations via character methods. The problem is contextualised via two case studies arising from our recent work: entanglement measures, for characterising the structure of state spaces for composite quantum systems; and Markov invariants, a robust alternative to parameter-estimation intensive methods of statistical inference in molecular phylogenetics.Comment: 12 pp, includes supplementary discussion of example

Spectral and Spin Measurement of Two Small and Fast-Rotating Near-Earth Asteroids

In May 2012 two asteroids made near-miss "grazing" passes at distances of a few Earth-radii: 2012 KP24 passed at nine Earth-radii and 2012 KT42 at only three Earth-radii. The latter passed inside the orbital distance of geosynchronous satellites. From spectral and imaging measurements using NASA's 3-m Infrared Telescope Facility (IRTF), we deduce taxonomic, rotational, and physical properties. Their spectral characteristics are somewhat atypical among near-Earth asteroids: C-complex for 2012 KP24 and B-type for 2012 KT42, from which we interpret the albedos of both asteroids to be between 0.10 and 0.15 and effective diameters of 20+-2 and 6+-1 meters, respectively. Among B-type asteroids, the spectrum of 2012 KT42 is most similar to 3200 Phaethon and 4015 Wilson-Harrington. Not only are these among the smallest asteroids spectrally measured, we also find they are among the fastest-spinning: 2012 KP24 completes a rotation in 2.5008+-0.0006 minutes and 2012 KT42 rotates in 3.634+-0.001 minutes.Comment: 4 pages, 3 figures, accepted for publication in Icaru

Studies of microwave scattering and canopy architecture for boreal forests

This is an annual report on the project titled 'Study of Microwave Scattering and Canopy Architecture for Boreal Forests.' The objectives of our work are to study the interaction of microwave signals with vegetation components and to determine the radar's ability to provide accurate estimates of biophysical parameters such as biomass. Our research is aimed at refining the current microwave models and using these improvements to facilitate more accurate interpretations of SAR (synthetic aperture radar) imagery

Studies of microwave scattering and canopy architecture for boreal forests

Our primary objectives during the last year have been to develop a helicopter-borne radar system for measuring microwave backscatter from vegetation and to use this system to study the characteristics of backscatter from the boreal forest. Our research is aimed at refining the current microwave models and using these improvements for more accurate interpretation of SAR data. SAR data are very useful for monitoring the boreal forest region because of the microwave signal's ability to penetrate clouds and to see at night. Meeting these objectives involves several stages of development. The first stage is the design and implementation of a frequency-modulated continuous-wave (FM-CW) radar system with the capability of measuring backscatter at three frequencies and four polarizations at each frequency. These requirements necessitate a twelve-channel radar system. Using three frequencies is advantageous because it allows us to look at different parts of the canopy. For instance, the lower frequency signal penetrates deeper into the canopy and allows us to see the ground while the high frequency signal is scattered more by the leaves and needles and typically does not penetrate to the floor of the forest. We designed the radar starting with the antenna system. We then developed the intermediate frequency (IF) and radio frequency (RF) sections of the radar. Also, the need to collect data from twelve channels during each flight line presented a complex data acquisition problem that we solved by using a high-speed data acquisition board. After construction, the radar was tested at the lab. We performed extensive testing of the IF and RF systems of the radar during this time. Once we were satisfied with the operation of the radar it was shipped to Canada for use in the second intensive field campaign (IFC-2) from July 16 - August 8, 1994. During IFC-2, we collected backscatter data over the experimental sites in the southern study area (SSA). Additionally, we used a ground-based step-frequency radar to measure the reflection coefficient of the forest floor at the old jack pine (OJP) and young jack pine (YJP) sites. The ground-based radar data have been processed and a frew examples are included in this report. we are currently processing the helicopter-borne radar data

World Workshop on Oral Medicine VII : What participants perceive as important

Our objective was to identify which aspects of World Workshop on Oral Medicine (WWOM) participation were perceived as significant for participants\u2019 professional development. Online survey was sent to previous WWOM participants. Qualitative analysis of participants\u2019 responses to an open-ended question was performed. Fifty-two WWOM participants responded. Nearly three quarters of respondents (72.3%) felt that participation in the WWOM helped their career. A high percentage of respondents (67.3%) provided answers that fell under the domains of international collaboration, followed by personal academic benefits (48%). Overall, the results indicate that WWOM participation played an important role in individual's professional development. We identified aspects of WWOM involvement that the participants perceived as important. This information will be used for the development of an objective instrument for measuring impact of WWOM on participant's professional path

Mobile radio interferometric geodetic systems

Operation of the Astronomical Radio Interferometric Earth Surveying (ARIES) in a proof of concept mode is discussed. Accuracy demonstrations over a short baseline, a 180 km baseline, and a 380 km baseline are documented. Use of ARIES in the Sea Slope Experiment of the National Geodetic Survey to study the apparent differences between oceanographic and geodetic leveling determinations of the sea surface along the Pacific Coast is described. Intergration of the NAVSTAR Global Positioning System and a concept called SERIES (Satellite Emission Radio Interferometric Earth Surveying) is briefly reviewed

The role of chemical activation in the formation and loss of atmospheric carbonyl species

The Earth’s atmosphere is often compared to a low temperature combustion system in which solar energy drives radical oxidation of trace gases. The OH radical is the key daytime radical oxidant and reacts rapidly with the majority of the anthropogenic and biogenic volatile organic compounds (VOCs) released to the atmosphere. Over the past two decades, field campaigns in remote regions, characterised by high concentrations of hydrocarbons, such as isoprene, but relatively low concentrations of NOx (NO + NO2) have highlighted significant discrepancies between measured and modelled concentrations of OH; with modelled OH concentrations underestimating the measured daytime values by up to an order of magnitude. Consequently, a number of experimental and theoretical studies have sought novel OH generating reactions that are currently not implemented into atmospheric models. One such suggestion is that under low NOx conditions (sub 100 pptv), certain peroxy radical species, formed following the addition of O2 to radicals produced through OH initiated VOC oxidation, might undergo unimolecular dissociation reactions that regenerate OH. In this thesis, a number of OH initiated oxidation systems have been studied which produce radical intermediates that recycle OH in the presence of O2. These systems have been investigated experimentally by monitoring the OH directly using laser flash photolysis coupled with laser induced fluorescence (LFP – LIF). By monitoring the OH kinetics directly, it is possible to quantify the yield of OH recycled in the presence of O2 as a function of pressure, temperature, and O2 concentration from the ratio of rate coefficients measured in the presence and absence of O2; this OH cycling methodology was used extensively in the work presented here. The first experimental work presented in this thesis focused on the OH initiated oxidation of a series of alkynes (acetylene, propyne, and 2-butyne). These reactions proceed initially via OH addition across the alkyne triple bond, to generate an adduct that exists in two energetically distinct conformations. These adducts react rapidly with O2 to generate a bicarbonyl species and recycle OH, or an organic acid and acyl radical as first generation products; with product branching ratios dictated by the stereochemistry of the adduct at the point of reaction with O2. The nascent adduct forms following the OH + alkyne reaction with excess energy. It is widely accepted that at pressures relevant to the troposphere, any excess energy in reaction products is dissipated through inelastic collisions prior to the onset of secondary bimolecular chemistry. However, experimental and theoretical work presented here suggests that under atmospheric conditions, a significant fraction of the total product yield associated with the OH + alkyne/O2 systems, form before the internal quantum states of the adducts have fully relaxed. The product branching observed for the OH + alkyne/O2 system is said to be influenced by chemical activation, whereby the exothermicity of an initial reaction is utilised by the products to undergo secondary reactions not accessible to the thermalised products. Attention then turns to OH oxidation reactions that proceed via a hydrogen-atom abstraction channel. Abstraction reactions are often considered to deposit the majority of the available reaction exothermicity into the newly formed bond, particularly if the reaction involved has an early transition state. Experimental evidence presented here suggests that some atmospherically relevant carbonyl reactions, that are considered to proceed via direct hydrogen-atom abstraction, partition a significant fraction of the reaction exothermicity into the radical fragment. The OH + acetaldehyde, CH3CHO, reaction is considered an archetypal abstraction reaction. The acetyl, CH3CO, produced is known to react with O2 at low pressures to generate OH, with a unity yield at zero pressure. However, the pressure dependent OH yields observed for the OH + CH3CHO/O2 system suggest that ~15% of the CH3CO produced through the OH + CH3CHO reaction dissociates promptly to CH3 + CO. CH3CO fragmentation requires more than 50% of the total exothermicity of the OH + CH3CHO reaction to be channelled into the CH3CO. The second hydrogen-abstraction channel considered here is the OH + glyoxal, (HCO)2, reaction that results in production of the HC(O)CO radical. HC(O)CO chemistry is governed by a competition between unimolecular dissociation, and bimolecular association with O2. Recent calculations have suggested that the HC(O)CO + O2 reaction proceeds directly to OH + CO + CO2. This channel has been verified here through experiment, with OH yields associated with the OH + (HCO)2/O2 reaction quantified for the first time as a function of pressure (5 – 80 Torr), temperature (212 – 295 K), and O2 concentration. The OH yields increase with O2 concentration under all experimental conditions, as the bimolecular HC(O)CO + O2 reaction increasingly competes with unimolecular HC(O)CO decomposition, but converge on a limiting yield under high O2 conditions, suggesting that a fraction of the HC(O)CO produced following the OH + (HCO)2 reaction dissociates promptly to HCO + CO. In the final experimental section of this thesis a laser system was developed to detect HCO via LIF. Attempts were made to monitor both prompt and growth HCO signal following the Cl + (HCO)2 reaction, and quantify the rate of thermal HC(O)CO decomposition as a function of pressure at low temperatures (212 K). However, rapid HCO removal was observed at the low experimental temperatures required. Further experimental evidence suggested that HCO reacts rapidly with (HCO)2 and other aldehydes at 212 K. Quantitative studies focused on the reaction of HCO with formaldehyde, HCHO, and acetaldehyde, CH3CHO, with rate coefficients of (3.44 ± 0.15) and (1.24 ± 0.05) × 10-11 cm3 molecule-1 s-1 measured, respectively