357 research outputs found
COLLIDE: Collisions into Dust Experiment
The Collisions Into Dust Experiment (COLLIDE) was completed and flew on STS-90 in April and May of 1998. After the experiment was returned to Earth, the data and experiment were analyzed. Some anomalies occurred during the flight which prevented a complete set of data from being obtained. However, the experiment did meet its criteria for scientific success and returned surprising results on the outcomes of very low energy collisions into powder. The attached publication, "Low Velocity Microgravity Impact Experiments into Simulated Regolith," describes in detail the scientific background, engineering, and scientific results of COLLIDE. Our scientific conclusions, along with a summary of the anomalies which occurred during flight, are contained in that publication. We offer it as our final report on this grant
NanoRocks: Design and Performance of an Experiment Studying Planet Formation on the International Space Station
In an effort to better understand the early stages of planet formation, we
have developed a 1.5U payload that flew on the International Space Station
(ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016.
This payload, named NanoRocks, ran a particle collision experiment under
long-term microgravity conditions. The objectives of the experiment were (a) to
observe collisions between mm-sized particles at relative velocities of
1~cm/s, and (b) to study the formation and disruption of particle clusters
for different particle types and collision velocities. Four types of particles
were used: mm-sized acrylic, glass, and copper beads, and 0.75 mm-sized JSC-1
lunar regolith simulant grains. The particles were placed in sample cells
carved out of an aluminum tray. This tray was attached to one side of the
payload casing with three springs. Every 60~s, the tray was agitated and the
resulting collisions between the particles in the sample cells were recorded by
the experiment camera.
During the 18 months the payload stayed on ISS, we obtained 158 videos, thus
recording a great number of collisions. The average particle velocities in the
sample cells after each shaking event were around 1 cm/s. After shaking
stopped, the inter-particle collisions damped the particle kinetic energy in
less than 20~s, reducing the average particle velocity to below 1 mm/s, and
eventually slowing them to below our detection threshold. As the particle
velocity decreased, we observed the transition from bouncing to sticking
collisions. We recorded the formation of particle clusters at the end of each
experiment run. This paper describes the design and performance of the
NanoRocks ISS payload.Comment: 8 pages, 8 figure
Regolith behavior under asteroid-level gravity conditions: low-velocity impact experiments
The dusty regolith covering the surfaces of asteroids and planetary
satellites differs in size, shape, and composition from terrestrial soil
particles and is subject to very different environmental conditions.
Experimental studies of the response of planetary regolith in the relevant
environmental conditions are thus necessary to facilitate future Solar System
exploration activities. We combined the results and provided new data analysis
elements for a series of impact experiments into simulated planetary regolith
in low-gravity conditions using two experimental setups: the Physics of
Regolith Impacts in Microgravity Experiment (PRIME) and the COLLisions Into
Dust Experiment (COLLIDE). Results of these experimental campaigns found that
there is a significant change in the regolith behavior with the gravity
environment. In a 10-2g environment (Lunar g levels), only embedding of the
impactor was observed and ejecta production was produced for most impacts at >
20 cm/s. Once at microgravity levels (<10-4g), the lowest impact energies also
produced impactor rebound. In these microgravity conditions, ejecta started to
be produced for impacts at > 10 cm/s. The measured ejecta speeds were lower
than the ones measured at reduced-gravity levels, but the ejected masses were
higher. The mean ejecta velocity shows a power-law dependence on the impact
energy with an index of ~0.7. When projectile rebound occurred, we observed
that its coefficients of restitution on the bed of regolith simulant decrease
by a factor of 10 with increasing impact speeds from ~5 cm/s up to 100 cm/s. We
could also observe an increased cohesion between the JSC-1 grains compared to
the quartz sand targets
Meter-Sized Moonlet Population in Saturn\u27s C Ring and Cassini Division
Stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph reveal the presence of transparent holes a few meters to a few tens of meters in radial extent in otherwise optically thick regions of the C ring and the Cassini Division. We attribute the holes to gravitational disturbances generated by a population of similar to 10 m boulders in the rings that is intermediate in size between the background ring particle size distribution and the previously observed similar to 100 m propeller moonlets in the A ring. The size distribution of these boulders is described by a shallower power-law than the one that describes the ring particle size distribution. The number and size distribution of these boulders could be explained by limited accretion processes deep within Saturn\u27s Roche zone
Robust estimation of microbial diversity in theory and in practice
Quantifying diversity is of central importance for the study of structure,
function and evolution of microbial communities. The estimation of microbial
diversity has received renewed attention with the advent of large-scale
metagenomic studies. Here, we consider what the diversity observed in a sample
tells us about the diversity of the community being sampled. First, we argue
that one cannot reliably estimate the absolute and relative number of microbial
species present in a community without making unsupported assumptions about
species abundance distributions. The reason for this is that sample data do not
contain information about the number of rare species in the tail of species
abundance distributions. We illustrate the difficulty in comparing species
richness estimates by applying Chao's estimator of species richness to a set of
in silico communities: they are ranked incorrectly in the presence of large
numbers of rare species. Next, we extend our analysis to a general family of
diversity metrics ("Hill diversities"), and construct lower and upper estimates
of diversity values consistent with the sample data. The theory generalizes
Chao's estimator, which we retrieve as the lower estimate of species richness.
We show that Shannon and Simpson diversity can be robustly estimated for the in
silico communities. We analyze nine metagenomic data sets from a wide range of
environments, and show that our findings are relevant for empirically-sampled
communities. Hence, we recommend the use of Shannon and Simpson diversity
rather than species richness in efforts to quantify and compare microbial
diversity.Comment: To be published in The ISME Journal. Main text: 16 pages, 5 figures.
Supplement: 16 pages, 4 figure
Characterization and identification of productivity-associated rhizobacteria in wheat
The rhizosphere is populated by a numerous and diverse array of rhizobacteria, and many impact productivity in largely unknown ways. Here we characterize the rhizobacterial community in a wheat variety categorized according to shoot biomass using 16S rRNA pyrosequencing abundance data. Plants were grown in homogenized field soil under greenhouse conditions, and DNA was extracted and pyrosequenced, resulting in 29,007 quality sequences. Operational taxonomic units (OTUs) that were significantly associated with biomass productivity were identified using an exact test adjusted for the false-discovery rate. The productivity deviation expressed as a percentage of the total mean square for regression (PMSR) was determined for each OTU. Out of 719 OTUs, 42 showed significant positive associations and 39 showed significant negative associations (q value, </=0.05). OTUs with the greatest net positive associations, by genus, were as follows: Duganella, OTU 43 and OTU 3; Janthinobacterium, OTU 278; Pseudomonas, OTU 588; and Cellvibrio, OTU 1847. Those with negative associations were as follows: Bacteria, OTU 273; Chryseobacterium, OTU 508; Proteobacteria, OTU 249; and Enterobacter, OTU 357. Shoot biomass productivity was strongly correlated with the balance between the overall abundances of positive- and negative-productivity-associated OTUs. High-productivity rhizospheres contained 9.2 significant positives for every negatively associated rhizobacterium, while low-productivity rhizospheres showed 2.3 significant negatives for every positively associated rhizobacterium. Overall rhizobacterial community diversity as measured by the Chao1, Shannon, and Simpson indexes was nonlinearly related to productivity, closely fitting a wavelike cubic equation. We conclude that shoot biomass productivity is strongly related to the ratio of positive- to negative-productivity-associated rhizobacteria in the rhizosphere. This study identifies significant OTUs composing the productive and unproductive rhizobacterial communities.Peer reviewedPlant and Soil SciencesStatistic
Diversity and abundance of solitary and primitively eusocial bees in an urban centre: a case study from Northampton (England)
The apparent reduction of solitary and primitively eusocial bees populations has remained a huge concern over the past few decades and urbanisation is considered as one of the factors affecting bees at different scales depending on bee guild. As urbanisation is increasing globally it necessitates more research to understand the complex community dynamics of solitary and primitively eusocial bees in urban settings. We investigated the urban core of a British town for diversity and abundance of solitary bees using standardized methods, and compared the results with nearby meadows and nature reserves. The study recorded 48 species within the town, about 22 % of the total species and 58 % of the genera of solitary bees in the United Kingdom. Furthermore we found the urban core to be more diverse and abundant in solitary and primitively eusocial bees compared to the meadows and nature re-serves. Of particular note was an urban record of the nationally rare Red Data Book species Coelioxys quadridentata and its host Anthophora quadrimaculata. This research demonstrates that urban settings can contribute significantly to the conservation of solitary and primitively eusocial bees in Britain
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