Solar Ultraviolet Bursts

The term “ultraviolet (UV) burst” is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016–2017.     

A Pre-Landing Assessment of Regolith Properties at the InSight Landing Site

This article discusses relevant physical properties of the regolith at the Mars InSight landing site as understood prior to landing of the spacecraft. InSight will land in the northern lowland plains of Mars, close to the equator, where the regolith is estimated to be \(\geq3\mbox{--}5~\mbox{m}\) thick. These investigations of physical properties have relied on data collected from Mars orbital measurements, previously collected lander and rover data, results of studies of data and samples from Apollo lunar missions, laboratory measurements on regolith simulants, and theoretical studies. The investigations include changes in properties with depth and temperature. Mechanical properties investigated include density, grain-size distribution, cohesion, and angle of internal friction. Thermophysical properties include thermal inertia, surface emissivity and albedo, thermal conductivity and diffusivity, and specific heat. Regolith elastic properties not only include parameters that control seismic wave velocities in the immediate vicinity of the Insight lander but also coupling of the lander and other potential noise sources to the InSight broadband seismometer. The related properties include Poisson’s ratio, P- and S-wave velocities, Young’s modulus, and seismic attenuation. Finally, mass diffusivity was investigated to estimate gas movements in the regolith driven by atmospheric pressure changes. Physical properties presented here are all to some degree speculative. However, they form a basis for interpretation of the early data to be returned from the InSight mission.     

Tetrahedral Robotics for Space Exploration

A reconfigurable space filling robotic architecture has a wide range of possible applications. One of the more intriguing possibilities is mobility in very irregular and otherwise impassable terrain. NASA Goddard Space Flight Center is developing the third generation of its addressable reconfigurable technology (ART) tetrahedral robotics architecture. An ART-based variable geometry truss consisting of 12 tetrahedral elements made from 26 smart struts on a wireless network has been developed. The primary goal of this development is the demonstration of a new kind of robotic mobility that can provide access and articulation that complement existing capabilities. An initial set of gaits and other behaviors are being tested, and accommodations for payloads such as sensor and telemetry packages are being studied. Herein, we describe our experience with the ART tetrahedral robotics architecture and the improvements implemented in the third generation of this technology. Applications of these robots to space exploration and the tradeoffs involved with this architecture will be discussed.     

Analysis of Regolith Properties Using Seismic Signals Generated by InSight’s HP<Superscript>3</Superscript> Penetrator

InSight’s Seismic Experiment for Interior Structure (SEIS) provides a unique and unprecedented opportunity to conduct the first geotechnical survey of the Martian soil by taking advantage of the repeated seismic signals that will be generated by the mole of the Heat Flow and Physical Properties Package (HP³). Knowledge of the elastic properties of the Martian regolith have implications to material strength and can constrain models of water content, and provide context to geological processes and history that have acted on the landing site in western Elysium Planitia. Moreover, it will help to reduce travel-time errors introduced into the analysis of seismic data due to poor knowledge of the shallow subsurface. The challenge faced by the InSight team is to overcome the limited temporal resolution of the sharp hammer signals, which have significantly higher frequency content than the SEIS 100 Hz sampling rate. Fortunately, since the mole propagates at a rate of \(\sim1~\mbox{mm}\) per stroke down to 5 m depth, we anticipate thousands of seismic signals, which will vary very gradually as the mole travels.Using a combination of field measurements and modeling we simulate a seismic data set that mimics the InSight HP3-SEIS scenario, and the resolution of the InSight seismometer data. We demonstrate that the direct signal, and more importantly an anticipated reflected signal from the interface between the bottom of the regolith layer and an underlying lava flow, are likely to be observed both by Insight’s Very Broad Band (VBB) seismometer and Short Period (SP) seismometer. We have outlined several strategies to increase the signal temporal resolution using the multitude of hammer stroke and internal timing information to stack and interpolate multiple signals, and demonstrated that in spite of the low resolution, the key parameters—seismic velocities and regolith depth—can be retrieved with a high degree of confidence.     

Development of the Primary Sorption Pump for the SEIS Seismometer of the InSight Mission to Mars

We report on the development of a passive sorption pump, capable of maintaining high-vacuum conditions in the InSight seismometer throughout the duration of any extended mission. The adsorber material is a novel zeolite-loaded aerogel (ZLA) composite, which consists of fine zeolite particles homogeneously dispersed throughout a porous silica network. The outgassing species within the SEIS evacuated container were analyzed and the outgassing rate was estimated by different methods. The results were used to optimize the ZLA composition to adsorb the outgassing constituents, dominated by water, while minimizing the SEIS bakeout constraints. The InSight ZLA composite additionally facilitated substantial CO₂ adsorption capabilities for risk mitigation against external leaks in Mars atmosphere. To comply with the stringent particle requirements, the ZLA getters were packaged in sealed containers, open to the SEIS interior through \(1~\upmu\mbox{m}\)-size pore filters. Results from experimental validation and verification tests of the packaged getters are presented. The pressure forecast based on these data, corroborated by rudimentary in situ pressure measurements, infer SEIS operational pressures not exceeding \(10^{-5}~\mbox{mbar}\) throughout the mission.     

Experimental study of the aerodynamics of a wing with various configurations of wingtip triangular extension

In this paper, we discuss the main ways of improving the aircraft aerodynamics. The results of a physical experiment are presented that is aimed to verify the theoretical results obtained earlier on the possible improvement in a seaplane wing model lift-to-drag ratio, by using a deflectable triangular extension along the wing leading edge near the wingtip. We confirm the slight effectiveness of using the wingtip leading edge triangular extensions on the nature of flow around the wing.     

JIRAM,the Jovian Infrared Auroral Mapper

JIRAM is an imager/spectrometer on board the Juno spacecraft bound for a polar orbit around Jupiter. JIRAM is composed of IR imager and spectrometer channels. Its scientific goals are to explore the Jovian aurorae and the planet’s atmospheric structure, dynamics and composition. This paper explains the characteristics and functionalities of the instrument and reports on the results of ground calibrations. It discusses the main subsystems to the extent needed to understand how the instrument is sequenced and used, the purpose of the calibrations necessary to determine instrument performance, the process for generating the commanding sequences, the main elements of the observational strategy, and the format of the scientific data that JIRAM will produce.     

Rayleigh Wave Ellipticity Modeling and Inversion for Shallow Structure at the Proposed InSight Landing Site in Elysium Planitia,Mars

The SEIS (Seismic Experiment for Interior Structure) instrument onboard the InSight mission will be the first seismometer directly deployed on the surface of Mars. From studies on the Earth and the Moon, it is well known that site amplification in low-velocity sediments on top of more competent rocks has a strong influence on seismic signals, but can also be used to constrain the subsurface structure. Here we simulate ambient vibration wavefields in a model of the shallow sub-surface at the InSight landing site in Elysium Planitia and demonstrate how the high-frequency Rayleigh wave ellipticity can be extracted from these data and inverted for shallow structure. We find that, depending on model parameters, higher mode ellipticity information can be extracted from single-station data, which significantly reduces uncertainties in inversion. Though the data are most sensitive to properties of the upper-most layer and show a strong trade-off between layer depth and velocity, it is possible to estimate the velocity and thickness of the sub-regolith layer by using reasonable constraints on regolith properties. Model parameters are best constrained if either higher mode data can be used or additional constraints on regolith properties from seismic analysis of the hammer strokes of InSight’s heat flow probe HP³ are available. In addition, the Rayleigh wave ellipticity can distinguish between models with a constant regolith velocity and models with a velocity increase in the regolith, information which is difficult to obtain otherwise.     

The Modern Near-Surface Martian Climate: A Review of In-situ Meteorological Data from Viking to Curiosity

We analyze the complete set of in-situ meteorological data obtained from the Viking landers in the 1970s to today’s Curiosity rover to review our understanding of the modern near-surface climate of Mars, with focus on the dust, CO₂ and H₂O cycles and their impact on the radiative and thermodynamic conditions near the surface. In particular, we provide values of the highest confidence possible for atmospheric opacity, atmospheric pressure, near-surface air temperature, ground temperature, near-surface wind speed and direction, and near-surface air relative humidity and water vapor content. Then, we study the diurnal, seasonal and interannual variability of these quantities over a span of more than twenty Martian years. Finally, we propose measurements to improve our understanding of the Martian dust and H₂O cycles, and discuss the potential for liquid water formation under Mars’ present day conditions and its implications for future Mars missions. Understanding the modern Martian climate is important to determine if Mars could have the conditions to support life and to prepare for future human exploration.