The Dynamic Quasiperpendicular Shock: Cluster Discoveries

The physics of collisionless shocks is a very broad topic which has been studied for more than five decades. However, there are a number of important issues which remain unresolved. The energy repartition amongst particle populations in quasiperpendicular shocks is a multi-scale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. The most important processes take place in the close vicinity of the major magnetic transition or ramp region. The distribution of electromagnetic fields in this region determines the characteristics of ion reflection and thus defines the conditions for ion heating and energy dissipation for supercritical shocks and also the region where an important part of electron heating takes place. In other words, the ramp region determines the main characteristics of energy repartition. All these processes are crucially dependent upon the characteristic spatial scales of the ramp and foot region provided that the shock is stationary. The process of shock formation consists of the steepening of a large amplitude nonlinear wave. At some point in its evolution the steepening is arrested by processes occurring within the shock transition. From the earliest studies of collisionless shocks these processes were identified as nonlinearity, dissipation, and dispersion. Their relative role determines the scales of electric and magnetic fields, and so control the characteristics of processes such as ion reflection, electron heating and particle acceleration. The determination of the scales of the electric and magnetic field is one of the key issues in the physics of collisionless shocks. Moreover, it is well known that under certain conditions shocks manifest a nonstationary dynamic behaviour called reformation. It was suggested that the transition from stationary to nonstationary quasiperiodic dynamics is related to gradients, e.g. scales of the ramp region and its associated whistler waves that form a precursor wave train. This implies that the ramp region should be considered as the source of these waves. All these questions have been studied making use observations from the Cluster satellites. The Cluster project continues to provide a unique viewpoint from which to study the scales of shocks. During its lifetime the inter-satellite distance between the Cluster satellites has varied from 100 km to 10000 km allowing scientists to use the data best adapted for the given scientific objective. The purpose of this review is to address a subset of unresolved problems in collisionless shock physics from experimental point of view making use multi-point observations onboard Cluster satellites. The problems we address are determination of scales of fields and of a scale of electron heating, identification of energy source of precursor wave train, an estimate of the role of anomalous resistivity in energy dissipation process by means of measuring short scale wave fields, and direct observation of reformation process during one single shock front crossing.     

Near Surface Stratigraphy and Regolith Production in Southwestern Elysium Planitia,Mars: Implications for Hesperian-Amazonian Terrains and the InSight Lander Mission

The presence of rocks in the ejecta of craters at the InSight landing site in southwestern Elysium Planitia indicates a strong, rock-producing unit at depth. A finer regolith above is inferred by the lack of rocks in the ejecta of 10-m-scale craters. This regolith should be penetrable by the mole of the Heat Flow and Physical Properties Package (HP³). An analysis of the size-frequency distribution (SFD) of 7988 rocky ejecta craters (RECs) across four candidate landing ellipses reveals that all craters >200 m in diameter and \({<}750 \pm 30\ \mbox{Ma}\) in age have boulder-sized rocks in their ejecta. The frequency of RECs however decreases significantly below this diameter (\(D\)), represented by a roll-off in the SFD slope. At \(30\ \text{m} < D < 200\ \text{m}\), the slope of the cumulative SFD declines to near zero at \(D < 30\ \text{m}\). Surface modification, resolution limits, or human counting error cannot account for the magnitude of this roll-off. Rather, a significant population of <200 m diameter fresh non-rocky ejecta craters (NRECs) here indicates the presence of a relatively fine-grained regolith that prevents smaller craters from excavating the strong rock-producing unit. Depth to excavation relationships and the REC size thresholds indicate the region is capped by a regolith that is almost everywhere 3 m thick but may be as thick as 12 to 18 m. The lower bound of the thickness range is independently confirmed by the depth to the inner crater in concentric or nested craters. The data indicate that 85% of the InSight landing region is covered by a regolith that is at least 3 m thick. The probability of encountering rockier material at depths >3 m by the HP³ however increases significantly due to the increase in boulder-size rocks in the lower regolith column, near the interface of the bedrock.     

H0323+022: Classification as a BL Lac and EXOSAT coordinated observations

The object H0323+022 (Doxsey et al. 1983) has been shown to be a BL Lac object by virtue of a diversity of observational characteristics at radio, optical, and x-ray wavelengths, in agreement with the conclusion of Margon and Jacoby (1984). Multi-frequency coordinated observations of this highly variable object with EXOSAT in September 1984 found it to be in a faint quiescent state (1/3; Jy at 5 keV and V=16.55). Preliminary results from the latter observations are presented.     

On applying the extended Kalman filter to nonlinear regressionmodels

In using an extended Kalman filter to estimate the parameters of a nonlinear regression model, the order in which the measurements are processed can be important, as the filter cannot always be expected to produce a satisfactory global fit when processing the measurements in the causal order in which they occur. To obtain a better fit, the possibility is explored of using a sequential state estimator in an offline mode to process the measurements in a random order rather than in the causal order in which they occur     

Minimal requirements for molecular information transfer.

The information transfer properties of nucleic acids are largely due to the properties of the bases themselves, for which it would appear evolution has selected a highly favorable set of molecules. In aqueous solution, it is possible that this task can only be carried out efficiently by heterocyclic, condensed-ring structures fairly similar to the purines and pyrimidines. Alterations in backbone structure, however, whether in the mode of linkage or in the nature of the sugar residue, are much less critical. Such alternatives could have played a role in the emergence of life on Earth and should be considered likely in extraterrestrial environments. The possible existence of low-efficiency, information transfer systems based on other structures entirely (proteins, clay minerals, etc), remains an interesting speculation for which no evidence is presently available.     

THE CLUSTER SCIENCE DATA SYSTEM (CSDS) – A NEW APPROACH TO THE DISTRIBUTION OF SCIENTIFIC DATA

Cluster is an ESA/NASA four-spacecraft mission designed to study plasma processes in three dimensions using the combined data from eleven instruments on each spacecraft. This mission requires the combination of many measured parameters, and the Cluster community have taken the unprecedented step of establishing a set of high quality data products from all instruments at spin (~ 4 s) resolution which will be produced and distributed throughout the mission lifetime. The Cluster Science Data System (CSDS) is based on a set of eight data centres which are implemented and funded through national programmes. As part of CSDS, a Joint Science Operations Centre (JSOC) has been established to facilitate the commanding of the 44 instruments. It is co-located with the UK data centre at the Rutherford Appleton Laboratory (RAL), Didcot, United Kingdom. ESA's contribution to CSDS includes the provision of the CSDS User Interface, a dedicated network (CSDSnet) to interconnect the data centres, and the co-ordination of all activities at CSDS level. A wide scientific community wishing to use Cluster data will have differing data rights, experience and means of access. Users will also include those working with data sets from other missions, e.g., Soho, Geotail, Wind, Polar, Interball, and Equator-S. The Cluster Science Data System is primarily designed to support multi-instrument and multi-spacecraft data analysis and it is distributed across six national data centres in Europe, one in the USA, and one in China. CSDSnet will be used to interconnect the European data centres, the Joint Science Operations Centre at Didcot and the spacecraft Operations Control Centre at ESOC in Darmstadt.     

The use of mineral crystals as bio-markers in the search for life on Mars.

Photographs that depict presumed fluvial features on the martian surface have led geologists to hypothesize that water flowed across the early martian terrain. From this, it has been further hypothesized that the surface and atmospheric conditions on early Mars were similar to those on early Earth. Because the oldest fossil evidence of life on Earth dates back to this early period, at least 3.5 billion years ago, the possibility exists that the early Martian environment could have also been conducive to the origin of life. To investigate this possibility, universal signatures or bio-markers indicative of past (or present) biological activity must be identified for use in the search for life on Mars. Several potentially applicable biomarkers have been identified and include: organics (e.g., specific classes of lipids and hopanes), suites of specific inorganic and organic compounds, as well as the isotopic ratios of C, N, and S. Unfortunately, all of these bio-markers may be of biologic or abiotic origin; these origins are often difficult to distinguish. Thus, the discovery of any one of these compounds alone is not a bio-marker. Because minerals produced under biologic control have distinctive crystallographies, morphologies, and isotopic ratios that distinguishable from abiotically produced minerals with the same chemical composition, and are stable through geologic time, we propose the use of minerals resulting from biologically controlled mineralization processes as bio-markers.     

Nucleic acid analogues and the origins of replication.

Recent interest in the properties of "nucleic acid-like structures" has been stimulated by difficulties encountered in the synthesis and nonenzymatic oligomerization of nucleotides. However, none of the newly proposed monomers has yet been synthesized in a plausibly prebiotic manner. Arguments are presented that analogues based on 8-hydroxymethyladenine and 5-hydroxymethyluracil are promising candidates for primitive nucleotide precursors.