Vertical structures induced by embedded moonlets in Saturn's rings: the gap region

We study the vertical extent of propeller structures in Saturn's rings. Our focus lies on the gap region of the propeller and on non-inclined propeller moonlets. In order to describe the vertical structure of propellers we extend the model of Spahn and Sremcevic (2000) to include the vertical direction. We find that the gravitational interaction of ring particles with the non-inclined moonlet does not induce considerable vertical excursions of ring particles, but causes a considerable thermal motion in the ring plane. We expect ring particle collisions to partly convert the lateral induced thermal motion into vertical excursions of ring particles. For the gap region of the propeller, we calculate gap averaged propeller heights on the order of 0.7 Hill radii, which is of the order of the moonlet radius. In our model the propeller height decreases exponentially until viscous heating and collisional cooling balance. We estimate Hill radii of 370m and 615m for the propellers Earhart and Bleriot. Our model predicts about 120km for the azimuthal extent of the Earhart propeller at Saturn's 2009 equinox, being consistent with values determined from Cassini images

Size distribution of particles in Saturn's rings from aggregation and fragmentation

Saturn's rings consist of a huge number of water ice particles, with a tiny addition of rocky material. They form a flat disk, as the result of an interplay of angular momentum conservation and the steady loss of energy in dissipative inter-particle collisions. For particles in the size range from a few centimeters to a few meters, a power-law distribution of radii, $\sim r^{-q}$ with $q \approx 3$, has been inferred; for larger sizes, the distribution has a steep cutoff. It has been suggested that this size distribution may arise from a balance between aggregation and fragmentation of ring particles, yet neither the power-law dependence nor the upper size cutoff have been established on theoretical grounds. Here we propose a model for the particle size distribution that quantitatively explains the observations. In accordance with data, our model predicts the exponent $q$ to be constrained to the interval $2.75 \le q \le 3.5$. Also an exponential cutoff for larger particle sizes establishes naturally with the cutoff-radius being set by the relative frequency of aggregating and disruptive collisions. This cutoff is much smaller than the typical scale of micro-structures seen in Saturn's rings

Structure of the 80S Ribosome from Saccharomyces cerevisiae—tRNA-Ribosome and Subunit-Subunit Interactions

AbstractA cryo-EM reconstruction of the translating yeast 80S ribosome was analyzed. Computationally separated rRNA and protein densities were used for docking of appropriately modified rRNA models and homology models of yeast ribosomal proteins. The core of the ribosome shows a remarkable degree of conservation. However, some significant differences in functionally important regions and dramatic changes in the periphery due to expansion segments and additional ribosomal proteins are evident. As in the bacterial ribosome, bridges between the subunits are mainly formed by RNA contacts. Four new bridges are present at the periphery. The position of the P site tRNA coincides precisely with its prokaryotic counterpart, with mainly rRNA contributing to its molecular environment. This analysis presents an exhaustive inventory of an eukaryotic ribosome at the molecular level

Attention as Practice:Buddhist Ethics Responses to Persuasive Technologies

The “attention economy” refers to the tech industry’s business model that treats human attention as a commodifiable resource. The libertarian critique of this model, dominant within tech and philosophical communities, claims that the persuasive technologies of the attention economy infringe on the individual user’s autonomy and therefore the proposed solutions focus on safeguarding personal freedom through expanding individual control. While this push back is important, current societal debates on the ethics of persuasive technologies are informed by a particular understanding of attention, rarely posited explicitly yet assumed as the default. They share the same concept of attention, namely an individualistic and descriptive concept of attention that is a cognitive process, an expendable resource, something that one should control individually. We step away from a negative analysis in terms of external distractions and aim for positive answers, turning to Buddhist ethics to formulate a critique of persuasive technology from a genuinely ethical perspective. Buddhist ethics points at our attention’s inescapable ethical and ontological embeddedness. Attention as practice requires “the right effort” to distinguish desirable and undesirable states, the “right concentration” to stop the flow we are caught in, and the “right mindfulness” to fortify the ability to attend to the present situation and keep in mind a general sense of life’s direction. We offer input for further philosophical inquiry on attention as practice and attention ecology. We put forward comfort/effort and individualism/collectivism as two remaining central tensions in need of further research.</p

Structural basis for the binding of IRES RNAs to the head of the ribosomal 40S subunit

Some viruses exploit internal initiation for their propagation in the host cell. This type of initiation is facilitated by structured elements (internal ribosome entry site, IRES) upstream of the initiator AUG and requires only a reduced number of canonical initiation factors. An important example are IRES of the virus family Dicistroviridae that bind to the inter-subunit side of the small ribosomal 40S subunit and lead to the formation of elongation-competent 80S ribosomes without the help of any initiation factor. Here, we present a comprehensive functional and structural analysis of eukaryotic-specific ribosomal protein rpS25 in the context of this type of initiation and propose a structural model explaining the essential involvement of rpS25 for hijacking the ribosome

Literary-theoretical Transformations of Social Models

This study investigates transformations of classical antiquity oikonomia and chrematistics from the Middle Ages to the present-day.From an ancient- historical, philosophical, literary and cultural-science perspective, it reconstructs exemplary acquisitions and reinterpretations of economic knowledge. The study argues that the modern economy has benefited from transformation relationships with the oikonomia of classical antiquity, which exhibit no unambiguously economic, efficient and profit-maximising character. For this reason, in addition to actual historical aspects, our interest also includes issues relating to the poetology of economic knowledge, the metaphorology and scenaristics of the house, the theoretical, narrative and literary representation economies and the promotion of ‘economy’ to an ordering category per se