The flyby of Pluto and Charon by the New Horizons spacecraft provided high-resolution images of cratered surfaces embedded in the Kuiper belt, an extensive region of bodies orbiting beyond Neptune. ...Impact craters on Pluto and Charon were formed by collisions with other Kuiper belt objects (KBOs) with diameters from ~40 kilometers to ~300 meters, smaller than most KBOs observed directly by telescopes. We find a relative paucity of small craters ≲13 kilometers in diameter, which cannot be explained solely by geological resurfacing. This implies a deficit of small KBOs (≲1 to 2 kilometers in diameter). Some surfaces on Pluto and Charon are likely ≳4 billion years old, thus their crater records provide information on the size-frequency distribution of KBOs in the early Solar System.
Surface compositions across Pluto and Charon Grundy, W. M.; Binzel, R. P.; Buratti, B. J. ...
Science (American Association for the Advancement of Science),
03/2016, Letnik:
351, Številka:
6279
Journal Article
Recenzirano
Odprti dostop
The Kuiper Belt hosts a swarm of distant, icy objects ranging in size from small, primordial planetesimals to much larger, highly evolved objects, representing a whole new class of previously ...unexplored cryogenic worlds. Pluto, the largest among them, along with its system of five satellites, has been revealed by NASAs New Horizons spacecraft flight through the system in July 2015, nearly a decade after its launch.
Craters are probes of planetary surface and interior properties. Here we measure depths, widths, and spacing of circumferential ring‐graben surrounding the two largest multiring impact structures on ...Europa, Tyre and Callanish. We estimate formation conditions including the ice shell structure. The radial extension necessary to form these graben is thought to be caused by asthenospheric drag of warmer, more ductile ice and/or water flowing toward the excavated center of the crater, under a brittle‐elastic lithospheric lid. Measurements of graben depths from stereo‐photoclinometric digital elevation models result in estimates of displacement, strain, and stress experienced by the ice shell. Graben widths are used to estimate the intersection depth of the bounding normal faults, a quantity related to the brittle‐ductile transition depth that approximates elastic shell thickness during crater collapse. Heat flows at the time of crater formation as well as ice lithosphere and total shell thickness are thus also constrained. Average widths and depths tend to decrease with increasing distance from the structure center, while inter‐graben spacing generally increases. Varied assumptions yield plausible total conductive ice shell thickness estimates between 4–8 and 2.5–5 km for Tyre and Callanish, respectively, and heat flows of ∼70–115 (±30) mW m−2 for realistic thermal conductivities, consistent with other geophysical estimates for Europa. Higher heat flows are consistent with thin (≲10 km), conductive ice shells and impact breaching, or penetration of the stagnant lid for a convecting ice shell. Callanish, geologically younger, formed in a time or region of greater heat flow than Tyre.
Plain Language Summary
Jupiter's moon Europa has an outer icy shell overlying a global subsurface ocean. The thickness of this icy shell controls the appearance of impact craters. The shell thickness is not well known, and may change over time, but geological indicators suggest it is on the order of 10 km thick or more. This is a relatively thin layer for craters to form in, compared the hundred‐km or thicker ice shells on most other icy bodies in the solar system. The two largest europan impacts created sets of circumferential ring‐faults that take the form of graben/troughs in the outer portions of their structures. We use measurements of the graben widths and depths to derive minimum estimates of the ice lithosphere (upper brittle portion) and total shell thicknesses and heat flows on Europa at the time the structures formed. The measurements are consistent with a minimum ice shell thickness between 2.5 and 8 km and high to exceptionally high heat flows. Greater total ice shell thicknesses are permitted if the shell is convectively overturning at depth. Notably, our heat flow estimates for Callanish (the younger structure) are uniformly higher than for Tyre (the older), contrary to expectations for Europa.
Key Points
Circumferential ring‐graben depths, widths, and spacing measured for the two largest known impact structures on Europa: Tyre and Callanish
Graben widths give plausible depths to the brittle‐ductile transition of ∼2–3 km at the time of and under the conditions of impact
Heat flows are high, dependent on fault structure and thermal conductivity, but consistent with impact breaching of a thin ice shell or lithosphere
Abstract
We augment the heliospheric network of galactic cosmic ray (GCR) monitors using 2012–2017 penetrating radiation measurements from the New Horizons (NH) Pluto Energetic Particle Spectrometer ...Science Investigation (PEPSSI), obtaining intensities of ≳75 MeV particles. The new, predominantly GCR observations provide critical links between the Sun and Voyager 2 and Voyager 1 (V2 and V1), in the heliosheath and local interstellar medium (LISM), respectively. We provide NH, Advanced Composition Explorer (ACE), V2, and V1 GCR observations, using them to track solar cycle variations and short-term Forbush decreases from the Sun to the LISM, and to examine the interaction that results in the surprising, previously reported V1 LISM anisotropy episodes. To investigate these episodes and the hitherto unexplained lagging of associated in situ shock features at V1, propagating disturbances seen at ACE, NH, and V2 were compared to V1. We conclude that the region where LISM magnetic field lines drape around the heliopause is likely critical for communicating solar disturbance signals upstream of the heliosheath to V1. We propose that the anisotropy-causing physical process that suppresses intensities at ∼90° pitch angles relies on GCRs escaping from a single compression in the draping region, not on GCRs trapped between two compressions. We also show that NH suprathermal and energetic particle data from PEPSSI are consistent with the interpretation that traveling shocks and corotating interaction region (CIR) remnants can be distinguished by the existence or lack of Forbush decreases, respectively, because turbulent magnetic fields at local shocks inhibit GCR transport while older CIR structures reaching the outer heliosphere do not.
Evidence from Arrokoth and comets strongly suggests a very low density for this and similar small Kuiper belt objects. Plausible compositions imply high porosities, in excess of 70%, and low ...compaction crush strengths. If so, impact craters on Arrokoth (especially Sky, its largest) formed largely by compaction of pore space and material displacement. This is consistent with geological evidence from New Horizons imaging. High porosity reduces cratering efficiency in the gravity regime whereas compaction moves it toward crush strength scaling and increased efficiency. Compaction also guarantees that most impactor kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. Monte Carlo simulations of Sky‐forming conditions indicate that the momentum imparted likely separated Arrokoth's two lobes, but displacement was limited by dissipation at the neck between them. Unusual strength properties are not required to preserve Arrokoth's bilobate configuration.
Plain Language Summary
It has become apparent over the last few years that small asteroids and comets are very underdense compared with the materials they are made of. This means that their total porosities are likely quite high, in excess of 70%, both as tiny voids within particles (so‐called microscopic porosity) and spaces between particles (macroscopic porosity). But none are likely as porous as the distant denizens of the Kuiper belt such as Arrokoth (visited by the New Horizons spacecraft in 2019). This paper concerns impact craters on Arrokoth and similar small bodies, and the rather unusual effects expected. Imagine a fluffy (fine powder) snowball striking a much larger fluffy snowball, only that the snow is not pure ice but a mixture of porous icy, rocky, and carbon‐rich particles. Even at high velocities (>100 s of m/s) craters should mostly form by compacting pore space and pushing material away from the impact point, not the traditional blasting of ejecta back into space. Similar to crush‐up of an automobile bumper, compaction helps to protect from the potentially catastrophic effects of large impacts, such as complete disruption of the target or breakup of bilobate bodies like Arrokoth, and should be incorporated in future collisional evolution studies.
Key Points
Arrokoth is likely a low density, highly porous, contact binary planetesimal
Impact craters on Arrokoth, and particularly its largest, likely formed as compaction craters, with modest or little ejecta
High porosity acted to protect Arrokoth from catastrophic disruption, ejecta recoil, and lobe dislocation and rearrangement
COVID-19 disproportionately affects racial and ethnic minority groups as well as people in jails and immigration detention centers in the United States. Between April and August of 2020, the mean ...monthly COVID-19 case ratio for ICE detainees was 13.4 times that of the general U.S. population. This study aims to understand the experiences of detained asylum seekers during the pandemic and to provide insight into COVID-19's impact on this population's health. This qualitative study employed first-person, in-depth narratives obtained from 12 asylum seekers, all of whom were detained in immigration detention centers or prisons during the initial surge of the COVID-19 pandemic and were subsequently released. Detained asylum seekers reported inadequate medical care, obstacles to receiving care, an inability to social distance, poor hygiene, restricted movement, and a lack of infection control-- all which increased their risk of contracting and spreading COVID-19 and exacerbated health inequalities brought to the forefront by the pandemic. Advocating for improved disease prevention and screening, prompt access to health care and treatment, cohorting of infectious cases, and community alternatives to detention to decrease the detained immigrant population sizes are crucial to halt communicability of the virus and its subsequent morbidity and mortality in this vulnerable population.
We use Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images to characterize boulder populations around six small (<1 km), young (<200 Ma) impact craters near spacecraft landing sites. The ...Narrow Angle Camera boulder counts are used to analyze how boulder distributions vary around craters of different sizes and ages. These comparisons inform how various properties affect the distance to which boulders are ejected and the size and density of boulders produced by an impact event. The counts show that boulder population densities decrease with crater age, with few boulders remaining at craters older than a few hundred million years, consistent with results of other studies of boulder degradation rates on the Moon. Variations in boulder distributions around younger craters may provide information regarding impact conditions; South Ray crater has a larger population of small boulders than the larger North Ray crater, which could be explained by variations in impact velocity. Large craters generally excavate more boulders than smaller craters, and the size of the largest boulder ejected is related to crater size by a power‐law function. Larger boulders occur closer to the crater rim (within 2–4 crater radii), whereas smaller boulders occur at all distances. The density of boulders is greater near the crater rim and decreases with increasing radial distance; this data can aid in establishing safe landing zones for future missions. Analyzing boulder distributions across craters of varying ages allows us to test models of boulder breakdown rates, with implications for understanding the Moon's regolith production rate.
Plain Language Summary
Many of the boulders found on the surface of the Moon are located around impact craters. Over time, these boulders break down, primarily as a result of being bombarded with small meteorites. Correlating the abundance of boulders with the age of the crater they are located around can inform how long it takes for boulders to break down and become part of the regolith (the Moon's soil). We used high‐resolution images from the Lunar Reconnaissance Orbiter Camera to measure and count boulders around six young lunar impact craters. We found that it takes a few hundred million years for boulders to begin disappearing due to degradation. We also analyzed how boulder distributions change with increasing distance from the crater rim, and we find that more small boulders occur at greater distances than large boulders. Understanding how boulder distributions vary around craters of different sizes and ages is important in planning for the safety of future landed missions to the lunar surface.
Key Points
Few boulders remain at craters older than a few hundred million years
Determining boulder size distributions as a function of distance from a crater is key in assessing boulder hazards for future missions
Lunar Reconnaissance Orbiter Camera Narrow Angle Camera boulder counts are consistent with Diviner rock abundance estimates
The major challenges to fabricate quantum processors and future nano-solid-state devices are material modification techniques with nanometer resolution and suppression of statistical fluctuations of ...dopants or qubit carriers. Based on a segmented ion trap with mK laser-cooled ions we have realized a deterministic single-ion source which could operate with a huge range of sympathetically cooled ion species, isotopes or ionic molecules. We have deterministically extracted a predetermined number of ions on demand and have measured a longitudinal velocity uncertainty of 6.3 m/s and a spatial beam divergence of 600 microrad. We show in numerical simulations that if the ions are cooled to the motional ground state (Heisenberg limit) nanometer spatial resolution can be achieved.
There is consensus among many medical school deans that exposure to human rights is a necessary component of physician training 78, however little is known about the impact of engagement in human ...rights programs on students' personal and professional development 1516171819202122232425262728.
This study aimed to examine medical students' experiences in the Mount Sinai Human Rights Program (MSHRP), their motivations for involvement, and the possible influence of engagement on their professional identities, personal growth, and career choices.
Through semi-structured interviews, this qualitative study gathered the experiences of 15 fourth year and recently graduated medical students who participated in the comprehensive training, research, and direct service opportunities provided by the program. Responses were coded using a content analysis approach.
The results of this research highlight the motivations behind students' involvement in a medical human rights program, as well as the challenges they experienced engaging with this work. The study captured students' perceptions of the role of the program on their personal growth, clinical skills, and career vision. Nearly all the students interviewed indicated they developed important, clinically applicable skills that enhanced their traditional medical education. Students indicated that their participation directly influenced their professional identities and future career directions by reinforcing previous interests in human rights and social justice work, impacting medical specialty and residency program selections and fostering commitment to working with immigrant populations.
The results of this study indicate that longitudinal involvement with the MSHRP contributed to the acquisition of important clinical skills that were not otherwise attained in students' early medical education. Findings suggest that there is significant opportunity for clinical and leadership development outside the traditional preclinical and clinical setting, and that exposure to human rights education shapes students' professional identities and career paths. Finally, the findings highlight the essential role of human rights and social justice in medical education.
We observe the phase space trajectory of an entangled wave packet of a trapped ion with high precision. The application of a spin-dependent light force on a superposition of spin states allows for ...coherent splitting of the matter wave packet such that two distinct components in phase space emerge. We observe such motion with a precision of better than 9% of the wave packet extension in both momentum and position, corresponding to a 0.8 nm position resolution. We accurately study the effect of the initial ion temperature on the quantum entanglement dynamics. Furthermore, we map out the phonon distributions throughout the action of the displacement force. Our investigation shows corrections to simplified models of the system evolution. The precise knowledge of these dynamics may improve quantum gates for ion crystals and lead to entangled matter wave states with large displacements.
PDF
