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.     

Lunar astrobiology: a review and suggested laboratory equipment

In October of 2005, the European Space Agency (ESA) and Alcatel Alenia Spazio released a "call to academia for innovative concepts and technologies for lunar exploration." In recent years, interest in lunar exploration has increased in numerous space programs around the globe, and the purpose of our study, in response to the ESA call, was to draw on the expertise of researchers and university students to examine science questions and technologies that could support human astrobiology activity on the Moon. In this mini review, we discuss astrobiology science questions of importance for a human presence on the surface of the Moon and we provide a summary of key instrumentation requirements to support a lunar astrobiology laboratory.     

Oxidant enhancement in martian dust devils and storms: implications for life and habitability

We investigate a new mechanism for producing oxidants, especially hydrogen peroxide (H2O2), on Mars. Large-scale electrostatic fields generated by charged sand and dust in the martian dust devils and storms, as well as during normal saltation, can induce chemical changes near and above the surface of Mars. The most dramatic effect is found in the production of H2O2 whose atmospheric abundance in the "vapor" phase can exceed 200 times that produced by photochemistry alone. With large electric fields, H2O2 abundance gets large enough for condensation to occur, followed by precipitation out of the atmosphere. Large quantities of H2O2 would then be adsorbed into the regolith, either as solid H2O2 "dust" or as re-evaporated vapor if the solid does not survive as it diffuses from its production region close to the surface. We suggest that this H2O2, or another superoxide processed from it in the surface, may be responsible for scavenging organic material from Mars. The presence of H2O2 in the surface could also accelerate the loss of methane from the atmosphere, thus requiring a larger source for maintaining a steady-state abundance of methane on Mars. The surface oxidants, together with storm electric fields and the harmful ultraviolet radiation that readily passes through the thin martian atmosphere, are likely to render the surface of Mars inhospitable to life as we know it.     

Experimental Investigation of InSight HP<Superscript>3</Superscript> Mole Interaction with Martian Regolith Simulant

The InSight mission launches in 2018 to characterize several geophysical quantities on Mars, including the heat flow from the planetary interior. This quantity will be calculated by utilizing measurements of the thermal conductivity and the thermal gradient down to 5 meters below the Martian surface. One of the components of InSight is the Mole, which hammers into the Martian regolith to facilitate these thermal property measurements. In this paper, we experimentally investigated the effect of the Mole’s penetrating action on regolith compaction and mechanical properties. Quasi-static and dynamic experiments were run with a 2D model of the 3D cylindrical mole. Force resistance data was captured with load cells. Deformation information was captured in images and analyzed using Digitial Image Correlation (DIC). Additionally, we used existing approximations of Martian regolith thermal conductivity to estimate the change in the surrounding granular material’s thermal conductivity due to the Mole’s penetration. We found that the Mole has the potential to cause a high degree of densification, especially if the initial granular material is relatively loose. The effect on the thermal conductivity from this densification was found to be relatively small in first-order calculations though more complete thermal models incorporating this densification should be a subject of further investigation. The results obtained provide an initial estimate of the Mole’s impact on Martian regolith thermal properties.     

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.     

A Hitch-hiker’s Guide to Stochastic Differential Equations

In this review, an overview of the recent history of stochastic differential equations (SDEs) in application to particle transport problems in space physics and astrophysics is given. The aim is to present a helpful working guide to the literature and at the same time introduce key principles of the SDE approach via “toy models”. Using these examples, we hope to provide an easy way for newcomers to the field to use such methods in their own research. Aspects covered are the solar modulation of cosmic rays, diffusive shock acceleration, galactic cosmic ray propagation and solar energetic particle transport. We believe that the SDE method, due to its simplicity and computational efficiency on modern computer architectures, will be of significant relevance in energetic particle studies in the years to come.     

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.     

Extended Pile Driving Model to Predict the Penetration of the Insight/HP<Superscript>3</Superscript> Mole into the Martian Soil

The NASA InSight mission will provide an opportunity for soil investigations using the penetration data of the heat flow probe built by the German Aerospace Center DLR. The Heat flow and Physical Properties Probe (HP³) will penetrate 3 to 5 meter into the Martian subsurface to investigate the planetary heat flow. The measurement of the penetration rate during the insertion of the HP³ will be used to determine the physical properties of the soil at the landing site. For this purpose, numerical simulations of the penetration process were performed to get a better understanding of the soil properties influencing the penetration performance of HP³. A pile driving model has been developed considering all masses of the hammering mechanism of HP³. By cumulative application of individual stroke cycles it is now able to describe the penetration of the Mole into the Martian soil as a function of time, assuming that the soil parameters of the material through which it penetrates are known. We are using calibrated materials similar to those expected to be encountered by the InSight/HP³ Mole when it will be operated on the surface of Mars after the landing of the InSight spacecraft. We consider various possible scenarios, among them a more or less homogeneous material down to a depth of 3–5 m as well as a layered ground, consisting of layers with different soil parameters. Finally we describe some experimental tests performed with the latest prototype of the InSight Mole at DLR Bremen and compare the measured penetration performance in sand with our modeling results. Furthermore, results from a 3D DEM simulation are presented to get a better understanding of the soil response.     

Auroral Substorms: Search for Processes Causing the Expansion Phase in Terms of the Electric Current Approach

Auroral substorms are mostly manifestations of dissipative processes of electromagnetic energy. Thus, we consider a sequence of processes consisting of the power supply (dynamo), transmission (currents/circuits) and dissipations (auroral substorms-the end product), namely the electric current line approach. This work confirms quantitatively that after accumulating magnetic energy during the growth phase, the magnetosphere unloads the stored magnetic energy impulsively in order to stabilize itself. This work is based on our result that substorms are caused by two current systems, the directly driven (DD) current system and the unloading system (UL). The most crucial finding in this work is the identification of the UL (unloading) current system which is responsible for the expansion phase. A very tentative sequence of the processes leading to the expansion phase (the generation of the UL current system) is suggested for future discussions.

1. (1)
   The solar wind-magnetosphere dynamo enhances significantly the plasma sheet current when its power is increased above \(10^{18}~\mbox{erg}/\mbox{s}\) (\(10^{11}\) w).
    
2. (2)
   The magnetosphere accumulates magnetic energy during the growth phase, because the ionosphere cannot dissipate the increasing power because of a low conductivity. As a result, the magnetosphere is inflated, accumulating magnetic energy.
    
3. (3)
   When the power reaches \(3\mbox{--}5\times 10^{18}~\mbox{erg}/\mbox{s}\) (\(3\mbox{--}5\times 10^{11}\) w) for about one hour and the stored magnetic energy reaches \(3\mbox{--}5\times10^{22}\) ergs (\(10^{15}\) J), the magnetosphere begins to develop perturbations caused by current instabilities (the current density \({\approx}3\times 10^{-12}~\mbox{A}/\mbox{cm}^{2}\) and the total current \({\approx}10^{6}~\mbox{A}\) at 6 Re). As a result, the plasma sheet current is reduced.
    
4. (4)
   The magnetosphere is thus deflated. The current reduction causes \(\partial B/\partial t > 0\) in the main body of the magnetosphere, producing an earthward electric field. As it is transmitted to the ionosphere, it becomes equatorward-directed electric field which drives both Pedersen and Hall currents and thus generates the UL current system.
    
5. (5)
   A significant part of the magnetic energy is accumulated in the main body of the magnetosphere (the inner plasma sheet) between 4 Re and 10 Re, because the power (Poynting flux \([ \boldsymbol{E} \times \boldsymbol{B} ])\) is mainly directed toward this region which can hold the substorm energy.
    
6. (6)
   The substorm intensity depends on the location of the energy accumulation (between 4 Re and 10 Re), the closer the location to the earth, the more intense substorms becomes, because the capacity of holding the energy is higher at closer distances. The convective flow toward the earth brings both the ring current and the plasma sheet current closer when the dynamo power becomes higher.
    

This proposed sequence is not necessarily new. Individual processes involved have been considered by many, but the electric current approach can bring them together systematically and provide some new quantitative insights.

     

Interior Evolution of Mercury

The interior evolution of Mercury—the innermost planet in the solar system, with its exceptional high density—is poorly known. Our current knowledge of Mercury is based on observations from Mariner 10’s three flybys. That knowledge includes the important discoveries of a weak, active magnetic field and a system of lobate scarps that suggests limited radial contraction of the planet during the last 4 billion years. We review existing models of Mercury’s interior evolution and further present new 2D and 3D convection models that consider both a strongly temperature-dependent viscosity and core cooling. These studies provide a framework for understanding the basic characteristics of the planet’s internal evolution as well as the role of the amount and distribution of radiogenic heat production, mantle viscosity, and sulfur content of the core have had on the history of Mercury’s interior. The existence of a dynamo-generated magnetic field suggests a growing inner core, as model calculations show that a thermally driven dynamo for Mercury is unlikely. Thermal evolution models suggest a range of possible upper limits for the sulfur content in the core. For large sulfur contents the model cores would be entirely fluid. The observation of limited planetary contraction (∼1–2 km)—if confirmed by future missions—may provide a lower limit for the core sulfur content. For smaller sulfur contents, the planetary contraction obtained after the end of the heavy bombardment due to inner core growth is larger than the observed value. Due to the present poor knowledge of various parameters, for example, the mantle rheology, the thermal conductivity of mantle and crust, and the amount and distribution of radiogenic heat production, it is not possible to constrain the core sulfur content nor the present state of the mantle. Therefore, it is difficult to robustly predict whether or not the mantle is conductive or in the convective regime. For instance, in the case of very inefficient planetary cooling—for example, as a consequence of a strong thermal insulation by a low conductivity crust and a stiff Newtonian mantle rheology—the predicted sulfur content can be as low as 1 wt% to match current estimates of planetary contraction, making deep mantle convection likely. Efficient cooling—for example, caused by the growth of a crust strongly in enriched in radiogenic elements—requires more than 6.5 wt% S. These latter models also predict a transition from a convective to a conductive mantle during the planet’s history. Data from future missions to Mercury will aid considerably our understanding of the evolution of its interior.