Crustal S-Wave Velocity from Apparent Incidence Angles: A Case Study in Preparation for InSight

Retrieval of crustal structure and thickness of Mars is among the main goals of InSight. Here we investigate which constraints on the crust at the landing site can be provided by apparent P-wave incidence angles derived from P-receiver functions. We consider receiver functions for six different Mars models, calculated from synthetic seismograms generated via Instaseis from the Green’s function databases of the Marsquake Service, in detail. To allow for a larger range of crustal thicknesses and structures, we additionally analyze data from five broad-band stations across Central Europe. We find that the likely usable epicentral distance range for P-wave receiver functions on Mars lies between \(35^{\circ}\) and the core shadow, and can be extended to more than \(150^{\circ}\) by also using the PP-phase. Comparison to models for the spatial distribution of Martian seismicity indicates that sufficient seismicity should occur within the P-wave distance range around InSight within the nominal mission duration to allow for the application of our method. Apparent P-wave incidence angles are derived from the amplitudes of vertical and radial receiver functions at the P-wave onset within a range of period bands, up to 120 s. The apparent incidence angles are directly related to apparent S-wave velocities, which are inverted for the subsurface S-wave velocity structure via a grid search. The veracity of the forward calculated receiver functions and apparent S-wave velocities is ensured by benchmarking various algorithms against the Instaseis synthetics. Results indicate that apparent S-wave velocity curves provide valuable constraints on crustal thickness and structure, even without any additional constraints, and considering the location uncertainty and limited data quantity of InSight. S-wave velocities in the upper half of the crust are constrained best, but if reliable measurements at long periods are available, the curves also provide constraints down to the uppermost mantle. Besides, it is demonstrated that the apparent velocity curves can differentiate between crustal velocity models that are indistinguishable by other methods.     

The Origin,Early Evolution and Predictability of Solar Eruptions

Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt.     

Venus: The Atmosphere,Climate, Surface,Interior and Near-Space Environment of an Earth-Like Planet

This is a review of current knowledge about Earth’s nearest planetary neighbour and near twin, Venus. Such knowledge has recently been extended by the European Venus Express and the Japanese Akatsuki spacecraft in orbit around the planet; these missions and their achievements are concisely described in the first part of the review, along with a summary of previous Venus observations. The scientific discussions which follow are divided into three main sections: on the surface and interior; the atmosphere and climate; and the thermosphere, exosphere and magnetosphere. These reports are intended to provide an overview for the general reader, and also an introduction to the more detailed topical surveys in the following articles in this issue, where full references to original material may be found.     

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.     

Assessment of gas temperature fluctuations in the GTE combustion chamber exit section at generalizing experimental data on fuel combustion efficiency

It is suggested that gas composition at every point of the combustion chamber exit section be characterized by the temperature values T _i (“ideal” temperature) corresponding to the local values of the air-to-fuel coefficient α _i under complete fuel combustion (ν _comb ≈ 1). It is assumed that the values of T _i are distributed over the exit section area (gas mass) linearly and the values of T _imax and T _imin can be determined by the experimental data on the gas temperature fields in the combustion chambers. The distribution of temperatures T _i is used when it is necessary to generalize the experimental data on fuel combustion efficiency in GTE combustion chambers.     

Characteristics of the equatorial F-region vertical plasma drift observed during post-sunset and pre-sunrise hours

Post-sunset and pre-sunrise vertical plasma drifts at the equatorial F-region have been investigated using the HF Doppler radar and ionosonde observations. Observed vertical plasma drift features during the sunrise are found to complement that observed during the evening. The post-sunset vertical plasma drift is characterized by an upward enhancement, a pre-reversal enhancement and a reversal in the drift direction. Similarly, the pre-sunrise plasma drift is characterized by a sudden downward excursion followed by an upward turning. The wavelet analysis of the plasma drift shows the presence of fluctuations in the period range 4–32 min and the short period fluctuations are attributed to the atmospheric gravity waves.     

Planning the unplannable: Scenarios on the future of space

This article explores the use of scenario analysis as a methodology to rigorously analyze potential space futures, particularly with respect to space security challenges, in the context of rapid and uncertain change across several dimensions of human space activities. The successful use of scenario analysis in other (e.g. corporate and military) sectors is described and results of an initial scenario analysis workshop are presented. Scenario analysis is recommended as a promising approach to evaluating the long-term consequences of various policy choices in the context of uncertainty, and as a process well-suited to fostering communication and building consensual knowledge among diverse stakeholders.     

Observations on the evolution of satellite launch volume and cyclicality in the space industry

Satellites have been rightly described as the lifeblood of the entire space industry and the number of satellites ordered or launched per year is an important defining metric of the industry's level of activity, such that trends and variability in this volume have significant strategic impact on the space industry. Over the past 40+ years, hundreds of satellites have been launched every year. Thus an important dataset is available for time series analysis and identification of trends and cycles in the various markets of the space industry. This article reports findings of a study for which we collected data on over 6000 satellites launched since 1960 on a yearly basis. We grouped the satellites into three broad categories – defense and intelligence, science, and commercial satellites – and identified and discussed the main trends and cyclical patterns for each of these. Institutional customers (defense and intelligence, and science) accounted for over two-thirds of all satellites launched within our time period (1960–2008), and, in the 1960s and 1970s, they accounted for 90% and 73.5%, respectively. A fair conclusion from this data is that the space industry was enabled by, and grew because of the institutional customers, not commercial market forces. However, when the launch data is examined more closely, a growing influence of the commercial sector is noticeable. Over the past two decades communication satellites accounted for roughly half of all launches, thus reflecting an important shift in the space industry in which the commercial sector is playing an equal role (on a launch volume basis) to that of the institutional market. Cyclical patterns in the satellite launch volume over the past decade are separately discussed before we sum up with a conclusion.     

Effects of heavy particle irradiation and diet on object recognition memory in rats

On long-duration missions to other planets astronauts will be exposed to types and doses of radiation that are not experienced in low earth orbit. Previous research using a ground-based model for exposure to cosmic rays has shown that exposure to heavy particles, such as ⁵⁶Fe, disrupts spatial learning and memory measured using the Morris water maze. Maintaining rats on diets containing antioxidant phytochemicals for 2 weeks prior to irradiation ameliorated this deficit. The present experiments were designed to determine: (1) the generality of the particle-induced disruption of memory by examining the effects of exposure to ⁵⁶Fe particles on object recognition memory; and (2) whether maintaining rats on these antioxidant diets for 2 weeks prior to irradiation would also ameliorate any potential deficit. The results showed that exposure to low doses of ⁵⁶Fe particles does disrupt recognition memory and that maintaining rats on antioxidant diets containing blueberry and strawberry extract for only 2 weeks was effective in ameliorating the disruptive effects of irradiation. The results are discussed in terms of the mechanisms by which exposure to these particles may produce effects on neurocognitive performance.