Devon Parkos

NOx production and rainout from Chicxulub impact ejecta reentry

The Chicxulub impact 66.0 Ma ago initiated the second biggest extinction in the Phanerozoic Eon. The cause of the concurrent oceanic nitrogen isotopic anomaly, however, remains elusive. The Chicxulub impactor struck the Yucatan peninsula, ejecting 2 × 1015 kg of molten and vaporized rock that reentered globally as approximately 1023 microscopic spherules. Here we report that modern techniques indicate that this ejecta generates 1.5 × 1014 moles of NOx, which is enough to cause the observed nitrogen enrichment of the basal layer. Additionally, reentry-based NO production would explain the anomalously heavy isotopic composition of the observed nitrogen. We include N, O, N2, O2, and NO species in simulations of nonequilibrium chemically reacting flow around a reentering spherule. We then determine the net production of NO from all the spherules and use turbulence models to determine how quickly this yield diffuses through the atmosphere. Upon reaching the stratosphere and troposphere, cloud moisture absorbs the NOx and forms nitric acid. We model this process and determine the acidity of the resulting precipitation, which peaks about 1 year after the impact. The precipitation ultimately reaches the upper ocean, where we assume that the well-mixed surface layer is 100 m deep. We then model the naturally occurring carbonate/bicarbonate buffer and determine the net pH. We find that insufficient NOx reaches the ocean to directly cause the observed end-Cretaceous oceanic extinction via acidification and buffer removal. However, the resulting nitrates are sufficient to explain the concurrent nitrogen isotopic anomaly and facilitate an end-Cretaceous algae bloom.

Climatic effects of the Chicxulub impact ejecta

Examining the short and long term effects of the Chicxulub impact is critical for understanding how life developed on Earth. While the aftermath of the initial impact would have produced harmful levels of radiation sufficient for eradicating a large portion of terrestrial life, this process does not explain the concurrent marine extinction. Following the primary impact, a large quantity of smaller spherules would de-orbit and re-enter the earths atmosphere, dispersed nearly uniformly across the planet. This secondary wave of debris would re-enter at high velocities, altering the chemical composition of the atmosphere. Furthermore, the combined surface area for the spherules would be much larger than for the original asteroid, resulting in considerably more potential reactions. For this reason, a new method was developed for predicting the total amount of toxic species produced by the spherule re-entry phase of the Chicxulub event. Using non-equilibrium properties obtained from direct simulation Monte Carlo (DSMC) methods coupled with spherule trajectory integration, the most likely cause of the observed marine extinction was determined.

Near-Contact Gas Damping and Dynamic Response of High-g MEMS Accelerometer Beams

This paper introduces and experimentally validates a new model for near-contact gas damping of microbeams. The model is formulated based on numerical simulations of rarefied gas dynamics using the Boltzmann Ellipsoidal Statistical Bhatnagar-Gross-Krook (ES-BGK) equation. The result is compared with existing models by simulating the motion of beams under high-g acceleration. To experimentally validate the damping models, single crystal silicon MEMS g-switches with cantilever microbeams of various lengths were utilized. The experimental measurements of beam dynamics under peak accelerations of approximately 50,000 g and acceleration ramp rates from 600 to 3,000 g/μs are compared with simulations. Additionally, the damping coefficients are extracted from existing vibrational mode data, and the resulting values are compared to the various models. The new near-contact model was found to predict contact and release times within a root-mean-square deviation from experiment below 9 and 7 for contact and release events, respectively. The damping values for the vibrational modes away from contact were predicted within 33% error, showing a more consistent predictive capability than provided by earlier models.

Near-contact damping model and dynamic response of μ-beams under high-g loads

This paper presents the first near-contact aerodynamic damping model based on rarefied flow modeling for use in dynamic simulations of large-displacement motion and contacting behavior of microbeams. The damping model is constructed based on high-fidelity simulations of rarefied gas flow around microbeams based on the Boltzmann kinetic equation with the Ellipsoidal Statistical Bhatnagar-Gross-Krook (ES-BGK) collision relaxation model. The predictions using the new model and previously published models are compared with experimentally measured responses of silicon microbeams under a high-g dynamic load. The new model is validated by measuring the near-contact behavior of silicon microbeams under loads up to 52,500 g and with ramping rates up to 2,750 g/µs. The model and experiments were found to be in close agreement with a maximum variation of less than 13.1%.

HCN production from impact ejecta on the early Earth

Major impact events have drastically altered the evolution of life on Earth. The reentry of ejecta formed from these events can trigger widespread chemical changes to the atmosphere on a global scale. This mechanism was proposed as a source of HCN during the Late Heavy Bombardment (LHB), 4.1 to 3.8 billion years ago. Significant concentrations of HCN in surface water could directly lead to adenine formation, a precursor for RNA. This work uses the Direct Simulation Monte Carlo (DSMC) method to examine the production of CN and HCN due to the reentry of impact ejecta. We use the Statistical Modeling in Low-Density Environment (SMILE) code, which utilizes the Total Collisional Energy (TCE) model for reactions. The collisions are described by the Variable Soft Sphere (VSS) and Larsen-Borgnakke (LB) models. We compare this nonequilibrium production to equilibrium concentrations from bulk atmospheric heating. The equilibrium HCN yield for a 1023 J impact is 7.0×104 moles, corresponding to a 2.5×1014 molecules p...

HCN production via Impact Ejecta Reentry during the Late Heavy Bombardment

Major impact events have shaped the Earth as we know it. The Late Heavy Bombardment is of particular interest because it immediately precedes the first evidence of life. The reentry of impact ejecta creates numerous chemical byproducts, including biotic precursors such as HCN. This work examines the production of HCN during the Late Heavy Bombardment in more detail. We stochastically simulate the range of impacts on the early Earth, and use models developed from existing studies to predict the corresponding ejecta properties. Using multi-phase flow methods and finite rate equilibrium chemistry, we then find the HCN production due to the resulting atmospheric heating. We use DSMC to develop a correction factor to account for increased yields due to thermochemical nonequilibrium. We then model 1D atmospheric turbulent diffusion to find the time-accurate transport of HCN to lower altitudes and ultimately surface water. Existing works estimate the necessary HCN molarity threshold to promote polymerization is 0.01 M. For a mixing depth of 100 m, we find that the Late Heavy Bombardment will produce at least 1 impact event above this threshold with probability 24.1% for an oxidized atmosphere and 56.3% for a partially reduced atmosphere. For a mixing depth of 10 m, the probability is 79.5% for an oxidized atmosphere and 96.9% for a partially reduced atmosphere. Therefore, LHB impact ejecta is likely an HCN source sufficient for polymerization in shallow bodies of water, particularly if the atmosphere were in a partially reduced state.