From Initial Models of Seismicity,Structure and Noise to Synthetic Seismograms for Mars

The InSight mission will land a single seismic station on Mars in November 2018, and the resultant seismicity catalog will be a key component for studies aiming to understand the interior structure of the planet. Here, we present a preliminary version of the web services that will be used to distribute the event and station metadata in practice, employing synthetic seismograms generated for Mars using a catalog of expected seismicity. Our seismicity catalog consists of 120 events with double-couple source mechanisms only. We also provide Green’s functions databases for a total of 16 structural models, which are constructed to reflect one-dimensional thin (30 km) and thick (80 km) Martian crust with varying seismic wave speeds and densities, combined with two different profiles for temperature and composition for the mantle. Both the Green’s functions databases and the precomputed seismograms are accessible online. These new utilities allow the researchers to either download the precomputed synthetic waveforms directly, or produce customized data sets using any desired source mechanism and event distribution via our servers.

     

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.     

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.     

The IBEX-Lo Sensor

The IBEX-Lo sensor covers the low-energy heliospheric neutral atom spectrum from 0.01 to 2 keV. It shares significant energy overlap and an overall design philosophy with the IBEX-Hi sensor. Both sensors are large geometric factor, single pixel cameras that maximize the relatively weak heliospheric neutral signal while effectively eliminating ion, electron, and UV background sources. The IBEX-Lo sensor is divided into four major subsystems. The entrance subsystem includes an annular collimator that collimates neutrals to approximately 7°×7° in three 90° sectors and approximately 3.5°×3.5° in the fourth 90° sector (called the high angular resolution sector). A fraction of the interstellar neutrals and heliospheric neutrals that pass through the collimator are converted to negative ions in the ENA to ion conversion subsystem. The neutrals are converted on a high yield, inert, diamond-like carbon conversion surface. Negative ions from the conversion surface are accelerated into an electrostatic analyzer (ESA), which sets the energy passband for the sensor. Finally, negative ions exit the ESA, are post-accelerated to 16 kV, and then are analyzed in a time-of-flight (TOF) mass spectrometer. This triple-coincidence, TOF subsystem effectively rejects random background while maintaining high detection efficiency for negative ions. Mass analysis distinguishes heliospheric hydrogen from interstellar helium and oxygen. In normal sensor operations, eight energy steps are sampled on a 2-spin per energy step cadence so that the full energy range is covered in 16 spacecraft spins. Each year in the spring and fall, the sensor is operated in a special interstellar oxygen and helium mode during part of the spacecraft spin. In the spring, this mode includes electrostatic shutoff of the low resolution (7°×7°) quadrants of the collimator so that the interstellar neutrals are detected with 3.5°×3.5° angular resolution. These high angular resolution data are combined with star positions determined from a dedicated star sensor to measure the relative flow difference between filtered and unfiltered interstellar oxygen. At the end of 6 months of operation, full sky maps of heliospheric neutral hydrogen from 0.01 to 2 keV in 8 energy steps are accumulated. These data, similar sky maps from IBEX-Hi, and the first observations of interstellar neutral oxygen will answer the four key science questions of the IBEX mission.     

High energy cosmic-ray nuclei results on Ulysses: 2. Effects of a recurrent high-speed stream from the southern polar coronal hole

Since June 1992 the Kiel Electron Telescope on board ULYSSES measures 26-day variations of the order of 6% in the fluxes of high energy H and He. In May 1993 ULYSSES entered into the unipolar region of the southern polar coronal hole, but continued to observe similar effects: increases in the MeV proton channels due to acceleration near the shocks of the corotating interaction region and decreases in the intensity of galactic nuclei associated with the same region. Amplitude variations are presented for different magnetic rigidities and the effects are discussed in view of corotating shock development in a 3-dimensional heliospheric structure.     

Outgassing History and Escape of the Martian Atmosphere and Water Inventory

The evolution and escape of the martian atmosphere and the planet’s water inventory can be separated into an early and late evolutionary epoch. The first epoch started from the planet’s origin and lasted ～500 Myr. Because of the high EUV flux of the young Sun and Mars’ low gravity it was accompanied by hydrodynamic blow-off of hydrogen and strong thermal escape rates of dragged heavier species such as O and C atoms. After the main part of the protoatmosphere was lost, impact-related volatiles and mantle outgassing may have resulted in accumulation of a secondary CO₂ atmosphere of a few tens to a few hundred mbar around ～4–4.3 Gyr ago. The evolution of the atmospheric surface pressure and water inventory of such a secondary atmosphere during the second epoch which lasted from the end of the Noachian until today was most likely determined by a complex interplay of various nonthermal atmospheric escape processes, impacts, carbonate precipitation, and serpentinization during the Hesperian and Amazonian epochs which led to the present day surface pressure.     

Global Gravity Field Recovery Using Solely GPS Tracking and Accelerometer Data from Champ

A new long-wavelength global gravity field model, called EIGEN-1, has been derived in a joint German-French effort from orbit perturbations of the CHAMP satellite, exploiting CHAMP-GPS satellite-to-satellite tracking and on-board accelerometer data over a three months time span. For the first time it becomes possible to recover the gravity field from one satellite only. Thanks to CHAMP'S tailored orbit characteristics and dedicated instrumentation, providing continuous tracking and on-orbit measurements of non-gravitational satellite accelerations, the three months CHAMP-only solution provides the geoid and gravity with an accuracy of 20 cm and 1 mgal, respectively, at a half wavelength resolution of 550 km, which is already an improvement by a factor of two compared to any pre-CHAMP satellite-only gravity field model. This revised version was published online in August 2006 with corrections to the Cover Date.