Evolution of anticodons

Anticodons are trinucleotides in transfer RNA (tRNA) molecules. The latter carry amino acids for insertion into the polypeptide sequences of proteins during the translation of messenger RNA (mRNA) molecules. Messenger RNA molecules are transcribed from genes. Evolution of tRNA molecules has resulted in a set of anticodons for the 20 amino acids that are used in protein synthesis. This set of anticodons is slightly different in mitochondrial codes from the set that is used in the nuclear “universal” code. Theories for the evolution of the code include frozen accident, doublet expansion, repeating triplets and coevolutionary distribution. The number of codons has always been fixed at 64 by mathematical rules, but because an anticodon may pair with more than one codon, the number of anticodons is only 54 in the universal code, is smaller in mitochondrial codes, and was probably even smaller in archetypal primitive codes. Evidence of anticodon evolution can be seen by comparing mitochondrial codes with the universal code. Codes used by very primitive organisms that are now extinct might have specified fewer amino acids than are now used.     

Genetic modification and selection of microorganisms for growth on Mars

Genetic engineering has often been suggested as a mechanism for improving the survival prospects of terrestrial microoganisms when seeded on Mars. The survival characteristics that these pioneer microorganisms could be endowed with and a variety of mechanisms by which this can be achieved are discussed, together with an overview of some of the potential hurdles that must be overcome. Also, a number of biologically useful properties for these microorganisms are presented that could facilitate the initial human colonisation and ultimately the planetary engineering of Mars.     

Towards a Global Unified Model of Europa’s Tenuous Atmosphere

Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.     

Daytime Ionosphere Retrieval Algorithm for the Ionospheric Connection Explorer (ICON)

The NASA Ionospheric Connection Explorer Extreme Ultraviolet spectrograph, ICON EUV, will measure altitude profiles of the daytime extreme-ultraviolet (EUV) OII emission near 83.4 and 61.7 nm that are used to determine density profiles and state parameters of the ionosphere. This paper describes the algorithm concept and approach to inverting these measured OII emission profiles to derive the associated \(\mathrm{O}^{+}\) density profile from 150–450 km as a proxy for the electron content in the F-region of the ionosphere. The algorithm incorporates a bias evaluation and feedback step, developed at the U.S. Naval Research Laboratory using data from the Special Sensor Ultraviolet Limb Imager (SSULI) and the Remote Atmospheric and Ionospheric Detection System (RAIDS) missions, that is able to effectively mitigate the effects of systematic instrument calibration errors and inaccuracies in the original photon source within the forward model. Results are presented from end-to-end simulations that convolved simulated airglow profiles with the expected instrument measurement response to produce profiles that were inverted with the algorithm to return data products for comparison to truth. Simulations of measurements over a representative ICON orbit show the algorithm is able to reproduce hmF2 values to better than 5 km accuracy, and NmF2 to better than 12% accuracy over a 12-second integration, and demonstrate that the ICON EUV instrument and daytime ionosphere algorithm can meet the ICON science objectives which require 20 km vertical resolution in hmF2 and 18% precision in NmF2.     

Planned Products of the Mars Structure Service for the InSight Mission to Mars

Space Science Reviews - The InSight lander will deliver geophysical instruments to Mars in 2018, including seismometers installed directly on the surface (Seismic Experiment for Interior Structure,...     

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

Massive stars, at least \(\sim10\) times more massive than the Sun, have two key properties that make them the main drivers of evolution of star clusters, galaxies, and the Universe as a whole. On the one hand, the outer layers of massive stars are so hot that they produce most of the ionizing ultraviolet radiation of galaxies; in fact, the first massive stars helped to re-ionize the Universe after its Dark Ages. Another important property of massive stars are the strong stellar winds and outflows they produce. This mass loss, and finally the explosion of a massive star as a supernova or a gamma-ray burst, provide a significant input of mechanical and radiative energy into the interstellar space. These two properties together make massive stars one of the most important cosmic engines: they trigger the star formation and enrich the interstellar medium with heavy elements, that ultimately leads to formation of Earth-like rocky planets and the development of complex life. The study of massive star winds is thus a truly multidisciplinary field and has a wide impact on different areas of astronomy.In recent years observational and theoretical evidences have been growing that these winds are not smooth and homogeneous as previously assumed, but rather populated by dense “clumps”. The presence of these structures dramatically affects the mass loss rates derived from the study of stellar winds. Clump properties in isolated stars are nowadays inferred mostly through indirect methods (i.e., spectroscopic observations of line profiles in various wavelength regimes, and their analysis based on tailored, inhomogeneous wind models). The limited characterization of the clump physical properties (mass, size) obtained so far have led to large uncertainties in the mass loss rates from massive stars. Such uncertainties limit our understanding of the role of massive star winds in galactic and cosmic evolution.Supergiant high mass X-ray binaries (SgXBs) are among the brightest X-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations.This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.     

用于低速风洞飞行器气动导数试验的绳牵引并联支撑系统

 为了用同一套绳牵引并联支撑系统来实现低速风洞静导数和动导数实验,采用理论与实验相结合的方法,总结了在WDPSS-8项目中所做的研究工作。结果表明,传统的杆支撑系统存在如支架对空气流场的影响等不可避免的缺陷;张线支撑系统很适合用于飞行器的静导数实验中,但它不能用在飞行器的动导数实验中;绳牵引并联支撑系统WDPSS-8能成功用于静导数实验中,且它在动导数实验中有潜在用途。WDPSS-8项目中的理论问题都得到解决,但实验方面还有不少问题有待解决。     

A solar extreme ultraviolet telescope and spectrograph for Shuttle/Spacelab

An instrument for advanced studies of the solar corona is described. Its optical system provides nearly stigmatic imaging of selected portions of the Sun over the spectral range from 22.5 to 44.0 nm. Both spectroheliograms and emission line profiles of coronal features will be obtained over a wide range of coronal temperatures.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.This paper was presented at the conference by U. Feldman.     

Comparative study of the dust emission of 19P/Borrelly (Deep space 1) and 1P/Halley

Images obtained by the Miniature Integrated Camera and Imaging Spectrometer (MICAS) experiment onboard the Deep Space 1 spacecraft which encountered comet 19P/Borrelly on September 22nd 2001 show a dust coma dominated by jets. In particular a major collimated dust jet on the sunward side of the nucleus was observed. Our approach to analyse these features is to integrate the observed intensity in concentric envelopes around the nucleus. The same procedures has been used on the Halley Multicolour Camera images of comet 1P/Halley acquired on March 14th 1986. We are able to show that at Borrelly the dust brightness dependence as a function of radial distance is different to that of Halley. At large distances both comets show constant values as the size of the concentric envelopes increases (as one would expect for force free radial outflow). For Halley the integral decreases as one gets closer to the nucleus. Borrelly shows opposite behaviour. The main cause for Halley's intensity distribution is either high optical thickness or particle fragmentation. For Borrelly, we have constructed a simple model of the brightness distribution near the nucleus. This indicates that the influence of deviations from point source geometry is insufficient to explain the observed steepening of the intensity profile close to the nucleus. Dust acceleration or fragmentation into submicron particles appear to be required. We also estimate the dust production rate of Borrelly with respect to Halley and compare their dust to gas ratios.