Ptolemy – an Instrument to Measure Stable Isotopic Ratios of Key Volatiles on a Cometary Nucleus

A fundamental goal of cometary studies is to determine the exact relationship between these bodies and the Solar System – the question(s) can be summarised as follows: did comets originate during the same events that spawned the Sun and planets, are they more primitive bodies that record a pre-solar history, or are they interstellar materials collected in relatively more recent times? Now, whatever the origin of comets, it is entirely possible that they could, in part, contain interstellar or pre-solar components – indeed, it seems rather likely in light of the fact that primitive meteorites contain such entities. These particular components are likely to be refractory (dust, macromolecular organic complexes, etc.). Of more relevance to the issues above are the volatile constituents, which make up the bulk of a comet's mass. Since these materials, by their very nature, volatilise during perihelion passage of a comet they can, in some instances, be detected and measured spectroscopically. Perhaps the most useful species for isotopic investigations are C₂, HCN and CN. Unfortunately, spectroscopic measurements can only currently be made with accuracies of ±10 to ±20%. As such it is very often not practical to conclude anything further than the fact that isotopic measurements are compatible with ‘`solar’' values, which tends to imply an origin from the margins of the solar accretion disk. But there is another problem with the spectroscopic measurements – since these are made on gaseous species in the coma (and relatively minor species at that) it is impossible to be certain that these represent the true nuclear values. In other words, if the processes of sublimation, active jetting, and photochemistry in the coma impart isotopic fractionation, the spectroscopic measurements could give a false impression of the true isotope ratios. What is required is an experiment capable of measuring isotopic ratios at the very surface of a comet. Herein we describe the Ptolemy instrument, which is included on the Philae lander as part of the Rosetta mission to 67P/Churyumov-Gerasimenko. The major objective of Ptolemy is a detailed appraisal of the nature and isotopic compositions of all materials present at the surface of a comet.     

On-the-job training mastery test development

The effectiveness of the national aviation security system is highly dependent upon people, especially those employed as checkpoint screeners. The training of these individuals is critical to their performance on the job. The Federal Aviation Administration (FAA) is very interested in enhancing screener training and further improving their readiness for the job. The FAA's aviation security human factors program has developed and assisted in the development of a number of tests and training tools to aid in the selection and training of screeners. Now the program is concentrating on the development of an on-the-job training mastery test to ensure that screener candidates have acquired all of the checkpoint operations skills and abilities needed to be a successful checkpoint screener     

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.     

Radar-Derived Properties of the InSight Landing Site in Western Elysium Planitia on Mars

We carried out an assessment of surface and subsurface properties based on radar observations of the region in western Elysium Planitia selected as the landing site for the InSight mission. Using observations from Arecibo Observatory and from the Mars Reconnaissance Orbiter’s Shallow Radar (SHARAD), we examined the near-surface properties of the landing site, including characterization of reflectivity, near-surface roughness, and layering. In the Arecibo data (12.6-cm wavelength), we found a radar-reflective surface with no unusual properties that would cause problems for the InSight radar altimeter (7-cm wavelength). In addition, the moderately low backscatter strength is indicative of a relatively smooth surface at \({\sim} 10\mbox{-cm}\) scales that is composed of load-bearing materials and should not present a hazard for landing safety. For roughness at 10–100 m scales derived from SHARAD data, we find relatively low values in a narrow distribution, similar to those found at the Phoenix and Opportunity landing sites. The power of returns at InSight is similar to that at Phoenix and thus suggestive of near-surface layering, consistent with a layer of regolith over bedrock (e.g., lava flows) that is largely too shallow (\({<}10\mbox{--}20~\mbox{m}\)) for SHARAD to discern distinct reflectors. However, an isolated area outside of the ellipse chosen in 2015 for InSight’s landing shows faint returns that may represent such a contact at depths of \({\sim} 20\mbox{--}43~\mbox{m}\).     

Geology and Physical Properties Investigations by the InSight Lander

Although not the prime focus of the InSight mission, the near-surface geology and physical properties investigations provide critical information for both placing the instruments (seismometer and heat flow probe with mole) on the surface and for understanding the nature of the shallow subsurface and its effect on recorded seismic waves. Two color cameras on the lander will obtain multiple stereo images of the surface and its interaction with the spacecraft. Images will be used to identify the geologic materials and features present, quantify their areal coverage, help determine the basic geologic evolution of the area, and provide ground truth for orbital remote sensing data. A radiometer will measure the hourly temperature of the surface in two spots, which will determine the thermal inertia of the surface materials present and their particle size and/or cohesion. Continuous measurements of wind speed and direction offer a unique opportunity to correlate dust devils and high winds with eolian changes imaged at the surface and to determine the threshold friction wind stress for grain motion on Mars. During the first two weeks after landing, these investigations will support the selection of instrument placement locations that are relatively smooth, flat, free of small rocks and load bearing. Soil mechanics parameters and elastic properties of near surface materials will be determined from mole penetration and thermal conductivity measurements from the surface to 3–5 m depth, the measurement of seismic waves during mole hammering, passive monitoring of seismic waves, and experiments with the arm and scoop of the lander (indentations, scraping and trenching). These investigations will determine and test the presence and mechanical properties of the expected 3–17 m thick fragmented regolith (and underlying fractured material) built up by impact and eolian processes on top of Hesperian lava flows and determine its seismic properties for the seismic investigation of Mars’ interior.