David A. Glenar

Metallic species, oxygen and silicon in the lunar exosphere: Upper limits and prospects for LADEE measurements

[1]xa0The only species that have been so far detected in the lunar exosphere are Na, K, Ar, and He. However, models for the production and loss of species derived from the lunar regolith through micrometeoroid impact vaporization, sputtering, and photon-stimulated desorption, predict that a host of other species should exist in the lunar exosphere. Assuming that loss processes are limited to ballistic escape, photoionization, and recycling to the surface, we have computed column abundances and compared them to published upper limits for the Moon. Only for Ca do modeled abundances clearly exceed the available measurements. This result suggests the relevance of some loss processes that were not included in the model, such as the possibility of gas-to-solid phase condensation during micrometeoroid impacts or the formation of stable metallic oxides. Our simulations and the recalculation of efficiencies for resonant light scattering show that models for other species studied are not well constrained by existing measurements. This fact underlines the need for improved remote and in situ measurements of the lunar exosphere such as those planned by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Our simulations of the LADEE neutral mass spectrometer and visible/ultraviolet spectrometer indicate that LADEE measurements promise to provide definitive observations or set stringent upper limits for all regolith-driven exospheric species. We predict that observations by LADEE will constrain assumed model parameters for the exosphere of the Moon.

Search for a high‐altitude lunar dust exosphere using Clementine navigational star tracker measurements

During the 1994 Clementine lunar mapping mission, portions of 25 orbits were dedicated to a search for lunar horizon glow (LHG) using the spacecraft star tracker navigation cameras. Previous putative detections of LHG were believed to result from forward scattering of sunlight by exospheric dust grains with radiiu2009≈u20090.1u2009µm, observable above the limb from within the shadow of the Moon near orbital sunrise or sunset. We have examined star tracker image sequences from five Clementine orbits in which the limb occulted the Sun, and was at least partially shadowed from earthshine, minimizing the chance of stray light contamination. No LHG appears in the image data, or in any of the net brightness images, after subtraction of a reference zodiacal light model. However, some of the images display faint excess limb brightness that appears to be solar streamer structure. Therefore, we derive upper limits for the amount of dust in the lunar exosphere that could be hidden by these brightness fluctuations using a dust-scattering simulation code and simple exponential dust profiles defined by surface concentration n0 and scale height H. Simulations using grains of radius 0.1u2009µm show that fluctuations in the observed excess brightness can be matched by a dust exosphere with a vertical column abundance n0H of 5–30u2009cm−2 and overlying mass <10−12u2009gu2009cm−2. These dust upper limit estimates are highly dependent on assumed grain size due to the rapid increase in per-grain brightness with grain radius.

An AOTF-LDTOF spectrometer suite for in situ organic detection and characterization

We discuss the development of a miniature near-infrared point spectrometer, operating in the 1.7–4 mm region, based on acousto-optic tunable filter (AOTF) technology. This instrument may be used to screen and corroborate analyses of samples containing organic biomarkers or mineralogical signatures suggestive of extant or extinct organic material collected in situ from planetary surfaces. The AOTF point spectrometer will be paired with a laser desorption time-of-flight (LDTOF) mass spectrometer and will prescreen samples for evidence of volatile or refractory organics before the laser desorption step and subsequent mass spectrometer measurement. 1 2 We describe the prototype AOTF point spectrometer instrument and present laboratory analysis of geological samples of known astrobiological importance. An initial mineral and rock sample suite of planetary relevance was used in the laboratory for baseline testing. To this, we will add a complement of astrobiologically relevant biosignatures from a variety of well-characterized geomicrobial study sites. We also describe LDTOF analysis of kaolinite and serpentine specimens, which are both highly relevant to the Martian surface mineralogy and the aqueous history of the planet. The AOTF-LDTOF instrument pairing offers the powerful advantage of cross-checked chemical analyses of individual samples, which can reduce chemical and biological interpretation ambiguities.

Rapid assessment of high value samples: An AOTF-LDTOF spectrometer suite for planetary surfaces

We discuss the development of a miniature near-infrared point spectrometer, operating between 1.7-3.45 μm, based on acousto-optic tunable filter (AOTF) technology. This instrument may be used to screen and corroborate analyses of samples containing organic biomarkers or mineralogical signatures suggestive of extant or extinct organic material collected in situ from planetary surfaces. The AOTF point spectrometer will be paired with a laser desorption time-of-flight (LDTOF) mass spectrometer and will prescreen samples for evidence of volatile or refractory organics before the laser desorption step and subsequent mass spectrometer measurement. We describe the AOTF point spectrometer instrument and present laboratory analysis of geological samples of known astrobiological importance. We also present LDTOF spectra of the same samples analyzed with the AOTF, which highlights the value of a comparative data set with the two instruments. We discuss plans for the integration of the two instruments, which is scheduled to take place in the first half of 2012. The AOTF-LDTOF instrument pairing offers the powerful advantage of cross-checked chemical analyses of individual samples, which can reduce chemical and biological interpretation ambiguities.

Infrared acousto-optic tunable filter point spectrometer for detection of organics on mineral surfaces

Abstract. A prototype infrared (IR) acousto-optic tunable filter (AOTF)-based point spectrometer has been designed for examining and analyzing potential biological samples collected in situ from the planets or other solar system objects. The reflectance spectrometer operates at a wavelength range of 1.6 to 3.6 μm, which is diagnostic of minerals and organics, and inspects a 1-mm sized spot on the sample. The tuning component is the AOTF that has been utilized in a variety of spectral detection applications. The instrument’s specification and design approach including the selected components is described. The data acquisition system, the electronic components, and their interconnections are presented. The instrument’s radiometric performance is examined and described by a noise equivalent reflectance value of 0.13% that is obtained from the laboratory measurements. The device has been demonstrated by measuring the reflectance spectra for a variety of geological samples and comparing the results with the United States Geological Survey data.

Results from an integrated AOTF-LDTOF spectrometer suite for planetary surfaces

On future landed missions to Mars and small solar system bodies, efficient sample prescreening will be necessary to select interesting targets for further analysis by analytical instruments with very limited time and power resources. Near infrared spectroscopy is well suited for rapid and non-invasive identification of mineral classes, and for determining the possible presence of organic molecules. Here we describe a miniature acousto-optic tunable filter (AOTF) point spectrometer that is tunable from ~1.6 - 3.6 μm. It identifies minerals associated with aqueous environments at sample scales of ~1 mm, as well as organic molecules and volatiles. The AOTF point spectrometer was integrated with a laser desorption time-of-flight (LDTOF) mass spectrometer developed at NASAs Goddard Space Flight Center, and can be used to prescreen samples for evidence of organics before the laser desorption step and subsequent mass spectrometer measurement. The LDTOF mass spectrometer provides pulsed-laser desorption and analysis of refractory organic compounds up to 150,000 Da on a spatial scale of 50-100 μm, determined by the laser spot size at the target. The recent integration of the two instruments allowed for coincident spectral measurements of geologic samples; follow-up measurements from the LDTOF were taken from an identical region on the samples of interest, allowing for a direct comparison between the two complementary data sets. We present measurements of a standard sample suite consisting of sulfates, carbonates, clay minerals, and iron oxides. We also compare AOTF and LDTOF spectra of calcite, as well as gypsum doped with phthalic acid and valine, and discuss the relationship between reflectance spectra acquired by the AOTF and the LDTOF mass spectra. Finally, we discuss measurements made of irradiated ices such as those found in areas of high astrobiological interest like Europa.

Absence of a detectable lunar nanodust exosphere during a search with LRO's LAMP UV imaging spectrograph

The Lyman-Alpha Mapping Project (LAMP) UV spectrograph on board the Lunar Reconnaissance Orbiter (LRO) performed a campaign to observe the Moons nanodust exosphere, evidence for which was provided by the Lunar Atmosphere and Dust Environment Explorer (LADEE) Ultraviolet and Visible Spectrometer (UVS) during the 2014 Quadrantid meteoroid stream. These LADEE/UVS observations were consistent with a nanodust exosphere modulated by meteoroid impacts. LRO performed off-nadir maneuvers around the peak of the 2016 Quadrantids, in order to reproduce, as closely as possible, the active meteoroid environment and observing geometry of LADEE/UVS. We analyzed LAMP spectra to search for sunlight backscattering from nanodust. No brightness enhancement attributable to dust, of any size, was observed. We determine an upper limit for dust column concentration of ~105xa0cm−2 for grains of radius ~25xa0nm, and an upper limit for dust column mass of ~10−11xa0gxa0cm−2, nearly independent of grain size for radii <100xa0nm.

A comparative study of in situ biosignature detection spectroscopy techniques on planetary surfaces

We demonstrate the biosignature detection capabilities of several classes of instruments, including a compact laser desorption/ionization time-of-flight mass spectrometer, an acousto-optic tunable filter IR point spectrometer, a laser-induced breakdown spectrometer, and a scanning electron microscope. We collected biotic and abiotic calcite, gypsum, and manganese oxide samples from Fort Stanton Cave to identify the presence of biomarkers with each instrument class. We find evidence of biologic activity in these samples including the presence of organic molecules, macroscopic and microscopic morphological features consistent with fossilized mircobes, and the presence of trace elements consistent with the biotic precipitation of minerals. The identification of extant or extinct microbial life is best supported by a suite of biosignatures, rather than a single observation. We demonstrate the unique biosignature detection results of each instrument class and discuss the importance of developing an instrument suite for future landed astrobiology missions on other planetary surfaces.

Demonstration of a portable AOTF IR spectrometer for in situ exploration of planetary surfaces

We discuss the development of a portable NIR reflectance point spectrometer based on acousto-optic tunable filter (AOTF) technology for future in situ geologic investigations of planetary surfaces. AOTFs are low power devices that operate on the principle of diffraction in a birefringent crystal. We have demonstrated the efficacy of this technique in extreme subterranean environments using the Portable AOTF Spectrometer for Astrobiology (PASA) to detect spectral biosignatures consistent with microbial alteration of geologic samples. PASA measures the IR reflectance spectrum of geologic samples in the 1.6 - 3.6 μm range with a resolution of λ/Δλ ≈ 250 - 400. We present the development and specifications of PASA, results from these expeditions, and discuss potential applications of a portable NIR point spectrometer based on AOTF technology for future landed or roving missions to other Solar System bodies.

A balloon-borne Acousto-Optic Tunable Filter imaging camera for planetary science investigations

A balloon-borne Acousto-Optic Tunable Filter (AOTF) hyperspectral imager is ideally suited to address numerous outstanding questions in planetary science. The spectral agility, narrowband wavelength selection, tolerance to the near-space environment, and spectral coverage afforded by AOTFs would enable investigations not feasible from ground-based facilities. A notional AOTF imager design includes both visible and near-infrared channels to take full advantage of the spectral coverage of an AOTF.We explore an example use case of synoptic observations of clouds on the giant planets using the visible channel of such an instrument. Although technical challenges such as detector cooling would require further performance modeling, an AOTF hyperspectral imager is a logical choice for giant planet imaging investigations from a balloon platform. The ability to rapidly acquire hyperspectral image cubes, thereby obtaining spectra of all locations on the planet that could elucidate atmospheric structure and dynamical processes, offers a unique advantage over traditional imaging techniques.