The Cleanskin impact structure, Northern Territory and Queensland, Australia: A reconnaissance study

ABSTRACT We report on the Cleanskin structure (18°10′00″S, 137°56′30″E), situated at the border between the Northern Territory and Queensland, Australia, and present results of preliminary geological fieldwork, microscopic analyses, and remote sensing. The Cleanskin structure is an eroded complex impact structure of ~15 km apparent diameter with a polygonal outline caused by two preexisting regional fault sets. The structure has a central uplift of ~6 km diameter surrounded by a rather shallow ring syncline. Based on stratigraphy, the uplift in the center may not exceed ~1000 m. The documentation of planar deformation features (PDFs), planar fractures (PFs), and feather features (FFs) in quartz grains from sandstone members of the Mesoproterozoic Constance Sandstone confirms the impact origin of the Cleanskin structure, as proposed earlier. The crater was most likely eroded before the Cambrian and later became buried beneath Cretaceous strata. We infer a late Mesoproterozoic to Neoproterozoic age of the impact event. In this chapter, the Cleanskin structure is compared with other midsized crater structures on Earth. Those with sandstone-dominated targets show structural similarities to the Cleanskin structure.

The Starry Universe of Jacques Cassini: Century-old Echoes of Kepler

This paper discusses measurements of the apparent diameter and parallax of the star Sirius, made in the early 18th century by Jacques Cassini, and how those measurements were discussed by other writers. Of particular interest is how other writers accepted Cassini’s measurements, but then discussed Sirius and other stars as though they were all the same size as the sun. Cassini’s measurements, by contrast, required Sirius and other stars to dwarf the sun—something Cassini explicitly noted, and something that echoed the ideas of Johannes Kepler more than a century earlier.

Remarkable Spiral Galaxies in Dark Energy Survey Data Release 2

A visual study of spiral galaxies from a subset of spiral galaxies in the Dark Energy Survey Data Release 2 finds that a significant number show long tails of baryonic matter, often much longer than the apparent diameter of the galaxy. Examples from less than 10% of the candidates are shown here and their possible origin is discussed. The tails were only seen connected to spiral galaxies; no examples of tails connected to elliptical galaxies or to artifacts were found. In many examples the tail is associated with what appears to be a colliding galaxy, but in many others there is no sign of one. An intriguing possibility is that in the latter cases the tails are produced by an encounter with an unseen object, either a massive black hole or a compact galaxy with mostly dark matter.

Size of particles ejected from an artificial impact crater on asteroid 162173 Ryugu

A projectile accelerated by the Hayabusa2 Small Carry-on Impactor successfully produced an artificial impact crater with a final apparent diameter of 14.5 ± 0.8 m on the surface of the near-Earth asteroid 162173 Ryugu on April 5, 2019. At the time of cratering, Deployable Camera 3 took clear time-lapse images of the ejecta curtain, an assemblage of ejected particles forming a curtain-like structure emerging from the crater. Focusing on the optical depth of the ejecta curtain and comparing it with a theoretical model, we infer the size of the ejecta particles. As a result, the typical size of the ejecta particles is estimated to be several centimeters to decimeters, although it slightly depends on the assumed size distribution. Since the ejecta particles are expected to come from a depth down to ~1 m, our result suggests that the subsurface layer of Ryugu is composed of relatively small particles compared to the uppermost layer on which we observe many meter-sized boulders. Our result also suggests a deficit of particles of less than ~1 mm in the subsurface layer. These findings will play a key role in revealing the formation and surface evolution process of Ryugu and other small Solar System bodies.

Measuring gas vesicle dimensions by electron microscopy

ABSTRACTGas vesicles (GVs) are cylindrical or spindle-shaped protein nanostructures filled with air and used for flotation by various cyanobacteria, heterotrophic bacteria, and Archaea. Recently, GVs have gained interest in biotechnology applications due to their ability to serve as imaging agents and actuators for ultrasound, magnetic resonance and several optical techniques. The diameter of GVs is a crucial parameter contributing to their mechanical stability, buoyancy function and evolution in host cells, as well as their properties in imaging applications. Despite its importance, reported diameters for the same types of GV differ depending on the method used for its assessment. Here, we provide an explanation for these discrepancies and utilize electron microscopy (EM) techniques to accurately estimate the diameter of the most commonly studied types of GVs. We show that during air drying on the EM grid, GVs flatten, leading to a ~1.5-fold increase in their apparent diameter. We demonstrate that GVs’ diameter can be accurately determined by direct measurements from cryo-EM samples or alternatively indirectly derived from widths of flat collapsed and negatively stained GVs. Our findings help explain the inconsistency in previously reported data and provide accurate methods to measure GV dimensions.

Arecibo Science Highlights of Observatory Planetary Radar Observations: 2019-2020

<p>We present a summary of the radar experiments performed with the Arecibo Observatory Planetary Radar system during 2019-2020.  Located in Puerto Rico (18° 20' 36.6" N, 66° 45' 11.1" W) the Arecibo Observatory S-band (2380 MHz) radar system is capable of transmitting up to 1MW of power and uses the William E Gordon Telescope antenna of 305 m. The planetary radar science group focuses on performing follow-up (post discovery) observations of  known small bodies as well as recently discovered ones. Priority is given to objects on the CNEOS Sentry impact risk list, those classified as Potentially Hazardous (PHA’s) and those that are potential spacecraft mission targets (NHATS). Although currently operating at 35% power capacity, Arecibo has observed 92 objects since September 2019 to abstract submission date, distributed as: 61  recently discovered objects, 28 PHA’s, 2 planets and 1 comet. We present here some science highlights of  this year's observations of near-Earth objects (NEOs), including radar delay-Doppler images of 2020BX12, 2011WN15, 481394 (2006 SF6) and 162082 (1998HL1).</p> <p><strong>Introduction</strong><br />The Arecibo Observatory is the largest and most powerful planetary radar system in the world, successfully observing  up to 130 asteroids a year. Funded by the NASA-NEO Observations program, the ground-based observations done using the S-band (2380 MHz, 12.6 cm) radar systems are a highly cost effective and rapid tool to constrain physical and dynamical properties of the targets in comparison to space missions. This Instrument has the capability of transmitting a signal with or without  phase modulation, providing extremely accurate astrometry measurements (range and radial velocity) on newly discovered objects, and track changes in the orbit of previously observed ones, such as those due to non-gravitational perturbations. Besides orbital characterization, radar data provides constraints on the object's size and rotation rate, is responsible for the discovery of satellites [1,2]  and for some cases can identify the shape and near-surface (meter-scale) structures up to a few wavelengths deep. </p> <p><strong>Methods</strong><br />The S-band system transmits a circularly polarized wave, and receives both the same-sense circular (SC) and opposite-sense circular polarization (OC) as transmitted. Radar observations usually start by a continuous-wave measurement to obtain the Doppler frequency spectrum of the echo. The measured Doppler spectrum bandwidth provides initial limits for rotation period and the object’s apparent diameter. From the measured received backscattered power in these two orthogonal states of polarization, it's possible to calculate the target's circular polarization ratio. Defined as the ratio of the SC and OC echo and commonly used as an indicator of the surface reflection properties. For targets with a relatively high signal-to-noise-ratio (SNR) we use phase modulation to produce delay-Doppler images, with range resolution as fine as 7.5 m per pixel in some cases. These images aid in the estimation of objects' diameter and provide an idea of the body's shape.</p> <p><strong>Results</strong><br />Some highlights of our observations include: 162082 (1998 HL1) observed on October 25-28, 2019 with a delay-Doppler resolution of 75 m/px, its apparent diameter is estimated at 270 m, and its rotation period at approximately 11 hrs. Contact binary 481394 (2006SF6) was observed on November 11-15, 2019, with a delay-Doppler resolution of 7.5 m/px showing a maximum visible extent of 240 m. The rotation period is estimated to be 11.3 hrs and  it was observed at various orientations. 2011 WN15 was observed on December 12-13, 2019, with a delay-Doppler resolution of 7.5 m/px providing an estimate on diameter of 900 m, and a rotation period of up to 4 hours. 2020 BX12 observation on February 4-5, 2020, led to the discovery of a secondary body, images with delay-Doppler resolution of 7.5 m/px, showed a diameter of 165 m for the primary and no more than 70 m for the secondary. The apparent rotation period for the primary is about 2.8 hrs and 49 hrs or less for the secondary.</p> <p><strong>Acknowledgements:</strong><br />The Arecibo Planetary Radar Program is fully supported by NASA’s Near-Earth Object Observations Program in NASA’s Planetary Defense Coordination Office through grant no. 80NSSC19K0523 awarded to University of Central Florida (UCF). UCF manages the National Science Foundation facility under a cooperative agreement with Yang Enterprises, Inc. and Universidad Ana G. Méndez.</p> <p><strong>References</strong><br />[1] Benner, L.A., Nolan, M.C., Margot, J., Brozovic, M., Ostro, S.J., Shepard, M.K., Magri, C., Giorgini, J.D. and Busch, M.W., 2008, September. Arecibo and Goldstone radar imaging of contact binary near-Earth asteroids. In DPS (pp. 25-03).<br />[2] Rivera-Valentin, E.G., Taylor, P.A., Virkki, A. and Aponte-Hernandez, B., 2017. (163693) Atira. CBET, 4347, p.1.</p>

A Novel Method to Calculate Objects’ Apparent Size

The angular diameter is the angle subtended by a generic object – an apple or a star – to the eye of an observer, and it describes how large the object appears from a given viewpoint. The angular diameter represents a powerful tool for distance calculations starting from directly measurable information and it finds application in several contexts varying from cosmography to architecture. In this article, the author proposes a novel equation to calculate the apparent diameter of whatever object. This equation defines a relation between the initial distance R0 at which the observed object is located and the object’s apparent diameter. Based on the preliminary tests conducted, the model seems to faithfully portrait this relation with respect to measured values, also at the astronomical scale, thus considering the Earth-Moon distance, where, the absolute error detected is about 0.56%. The tests highlighted also a dependency between the results accuracy and the measurement conditions suggesting a high level of sensibility linked to the initial magnification effect produced by the retina or the artificial lens employed.

Shatter cones: (Mis)understood?

Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and “double” cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationshipDsc= 0.4Da, whereDscis the maximum spatial extent of in situ shatter cones, andDais the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.