Constraints on the perturbed mutual motion in Didymos due to impact-induced deformation of its primary after the DART impact

Binary near-Earth asteroid (65803) Didymos is the target of the proposed NASA Double Asteroid Redirection Test (DART), part of the Asteroid Impact & Deflection Assessment (AIDA) mission concept. In this mission, the DART spacecraft is planned to impact the secondary body of Didymos, perturbing mutual dynamics of the system. The primary body is currently rotating at a spin period close to the spin barrier of asteroids, and materials ejected from the secondary due to the DART impact are likely to reach the primary. These conditions may cause the primary to reshape, due to landslides, or internal deformation, changing the permanent gravity field. Here, we propose that if shape deformation of the primary occurs, the mutual orbit of the system would be perturbed due to a change in the gravity field. We use a numerical simulation technique based on the full two-body problem to investigate the shape effect on the mutual dynamics in Didymos after the DART impact. The results show that under constant volume, shape deformation induces strong perturbation in the mutual motion. We find that the deformation process always causes the orbital period of the system to become shorter. If surface layers with a thickness greater than ~0.4 m on the poles of the primary move down to the equatorial region due to the DART impact, a change in the orbital period of the system and in the spin period of the primary will be detected by ground-based measurement.Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRA

Rapid submarine melting driven by subglacial discharge, LeConte Glacier, Alaska

We show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers.We show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers. This buoyant discharge draws in warm seawater, entraining it in a turbulent upwelling flow along the submarine face that melts glacier ice. To capture the effects of subglacial discharge on submarine melting, we conducted 4 days of hydrographic transects during late summer 2012 at LeConte Glacier, Alaska. A major rainstorm allowed us to document the influence of large changes in subglacial discharge. We found strong submarine melt fluxes that increased from 9.1 ± 1.0 to 16.8 ± 1.3 m d1 (ice face equivalent frontal ablation) as a result of the rainstorm. With projected continued global warming and increased glacial runoff, our results highlight the direct impact that increases in subglacial discharge will have on tidewater outlet systems. These effects must be considered when modeling glacier response to future warming and increased runoff.This work was funded by a grant from the Gordon and Betty Moore Foundation grant GBMF2627 to M.T. and M.F. Additional support for J.M.A. was provided by NSF grant ANT0944193. The manuscript was greatly improved by comments from two anonymous re- viewers. We thank Captain Scott Hursey for vessel support and safely navi- gating us through icebergs. J. Elliot provided the orthorectified World View image in Figure 1c.Ye

Dynamic Limits on Planar Libration-Orbit Coupling Around an Oblate Primary

This paper explores the dynamic properties of the planar system of an ellipsoidal satellite in an equatorial orbit about an oblate primary. In particular, we investigate the conditions for which the satellite is bound in librational motion or when the satellite will circulate with respect to the primary. We find the existence of stable equilibrium points about which the satellite can librate, and explore both the linearized and non-linear dynamics around these points. Absolute bounds are placed on the phase space of the libration-orbit coupling through the use of zero-velocity curves that exist in the system. These zero-velocity curves are used to derive a sufficient condition for when the satellite's libration is bound to less than 90 degrees. When this condition is not satisfied so that circulation of the satellite is possible, the initial conditions at zero libration angle are determined which lead to circulation of the satellite. Exact analytical conditions for circulation and the maximum libration angle are derived for the case of a small satellite in orbits of any eccentricity.Comment: Submitted to Celestial Mechanics and Dynamical Astronom

Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbræ, Greenland

The recent loss of Jakobshavn Isbræ’s extensive floating ice tongue has been accompanied by a change in near terminus behavior.The recent loss of Jakobshavn Isbræ’s extensive floating ice tongue has been accompanied by a change in near terminus behavior. Calving currently occurs primarily in summer from a grounded terminus, involves the detachment and overturning of several icebergs within 30 – 60 min, and produces long-lasting and far-reaching ocean waves and seismic signals, including ‘‘glacial earthquakes’’. Calving also increases near-terminus glacier velocities by 3% but does not cause episodic rapid glacier slip, thereby contradicting the originally proposed glacial earthquake mechanism. We propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving.We thank J. Brown and D. Maxwell for field assistance, and S. Anandakrishnan, A. Behar, and R. Fatland for loaning GPS receivers. Comments from editor E. Rignot and reviewers S. O’Neel and T. Pfeffer improved the manuscript. Logistics and instrumental support were provided by VECO Polar Resources, UNAVCO, and PASSCAL. Seismic analysis was done with the Matlab waveform object package written by C. Reyes (http://www.giseis.alaska.edu/Seis/EQ/tools/matlab/). Funding was provided by NASA’s Cryospheric Sciences Program (NNG06GB49G), the U.S. National Science Foundation (ARC0531075), the Swiss National Science Foundation (200021-113503/1), the Comer Science and Education Foundation, and a CIFAR IPY student fellowship under NOAA cooperative agreement NA17RJ1224 with the University of Alaska.Ye

Minimum Energy Configurations in the NN-Body Problem and the Celestial Mechanics of Granular Systems

Minimum energy configurations in celestial mechanics are investigated. It is shown that this is not a well defined problem for point-mass celestial mechanics but well-posed for finite density distributions. This naturally leads to a granular mechanics extension of usual celestial mechanics questions such as relative equilibria and stability. This paper specifically studies and finds all relative equilibria and minimum energy configurations for $N=1,2,3$ and develops hypotheses on the relative equilibria and minimum energy configurations for $N\gg 1$ bodies.Comment: Accepted for publication in Celestial Mechanics and Dynamical Astronom

Observing calving-generated ocean waves with coastal broadband seismometers, Jakobshavn Isbræ, Greenland

We use time-lapse photography, MODIS satellite imagery, ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from Jakobshavn Isbræ, Greenland, can generate ocean waves that are detectable over 150 km from their source.We use time-lapse photography, MODIS satellite imagery, ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from Jakobshavn Isbræ, Greenland, can generate ocean waves that are detectable over 150 km from their source. The waves, which are recorded seismically, have distinct spectral peaks, are not dispersive and persist for several hours. On the basis of these observations, we suggest that calving events at Jakobshavn Isbræ can stimulate seiches, or basin eigenmodes, in both Ilulissat Icefjord and Disko Bay. Our observations furthermore indicate that coastal, land-based seismometers located near calving termini (e.g. as part of the new Greenland Ice Sheet Monitoring Network (GLISN)) can aid investigations into the largely unexplored, oceanographic consequences of iceberg calving.Funding for this project was provided by NASA’s Cryospheric Sciences Program (NNG06GB49G), the US National Science Foundation (ARC0531075, ARC0909552 and ANT0944193), the Swiss National Science Foundation (200021-113503/1) and a Cooperative Institute for Arctic Research (CIFAR) International Polar Year (IPY) student fellowship under US National Oceanic and Atmospheric Administration (NOAA) cooperative agreement NA17RJ1224 with the University of Alaska. The seismic data were col- lected and distributed by the Greenland Ice Sheet Monitoring Network (GLISN) federation and its members: data from GDH were collected by the Geological Survey of Denmark and Greenland (GEUS); data from ASI, ILU and SUMG were collected by GEOFON; data from SFJ/SFJD were collected by GEUS, GEOFON, Incorporated Research Institutions for Seismology (IRIS) and the Comprehensive Test-Ban Treaty Organization (CTBTO); and data from ILULI were collected by ETH. We thank J. Brown and D. Podrasky for assistance with fieldwork and D.R. MacAyeal and E.A. Okal for discussions that led to and improved the manuscript. The manuscript benefited from the comments of O. Sergienko, an anonymous reviewer and editor P. Christoffersen.Ye

Spatio-temporal variations in seasonal ice tongue submarine melt rate at a tidewater glacier in southwest Greenland

Submarine melting of tidewater glaciers is proposed as a trigger for their recent thinning, acceleration and retreat. We estimate spring submarine melt rates (SMRs) of Kangiata Nunaata Sermia in southwest Greenland, from 2012 to 2014, by examining changes in along-fjord freeboard and velocity of the seasonal floating ice tongue. Estimated SMRs vary spatially and temporally near the grounding line, with mean rates of 1.3 ± 0.6, 0.8 ± 0.3 and 1.0 ± 0.4 m d−1 across the tongue in 2012, 2013 and 2014, respectively. Higher melt rates correspond with locations of emerging subglacial plumes and terminus calving activity observed during the melt season using time-lapse camera imagery. Modelling of subglacial flow paths suggests a dynamic system capable of rapid re-routing of subglacial discharge both within and between melt seasons. Our results provide an empirically-derived link between the presence of subglacial discharge plumes and areas of high spring submarine melting and calving along glacier termini

The Puromycin Route to Assess Stereo- and Regiochemical Constraints on Peptide Bond Formation in Eukaryotic Ribosomes

We synthesized a series of puromycin analogues to probe the chemical specificity of the ribosome in an intact eukaryotic translation system. These studies reveal that both d-enantiomers and β-amino acid analogues can be incorporated into protein, and provide a quantitative means to rank natural and unnatural residues. Modeling of a d-amino acid analogue into the 50S ribosomal subunit indicates that steric clash may provide part of the chiral discrimination. The data presented provide one metric of the chiral and regiospecificity of mammalian ribosomes

Conformational Plasticity of proNGF

Nerve Growth Factor is an essential protein that supports neuronal survival during development and influences neuronal function throughout adulthood, both in the central and peripheral nervous system. The unprocessed precursor of NGF, proNGF, seems to be endowed with biological functions distinct from those of the mature protein, such as chaperone-like activities and apoptotic and/or neurotrophic properties. We have previously suggested, based on Small Angle X-ray Scattering data, that recombinant murine proNGF has features typical of an intrinsically unfolded protein. Using complementary biophysical techniques, we show here new evidence that clarifies and widens this hypothesis through a detailed comparison of the structural properties of NGF and proNGF. Our data provide direct information about the dynamic properties of the pro-peptide and indicate that proNGF assumes in solution a compact globular conformation. The N-terminal pro-peptide extension influences the chemical environment of the mature protein and protects the protein from proteolytic digestion. Accordingly, we observe that unfolding of proNGF involves a two-steps mechanism. The distinct structural properties of proNGF as compared to NGF agree with and rationalise a different functional role of the precursor

After DART: Using the First Full-scale Test of a Kinetic Impactor to Inform a Future Planetary Defense Mission

NASA’s Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ∼10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos’s response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor β, showing that a particular direction-specific β will be directly determined by the DART results, and that a related direction-specific β is a figure of merit for a kinetic impact mission. The DART β determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos’s near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction