Simultaneous Spectral Temporal Adaptive Raman Spectrometer - SSTARS

Raman spectroscopy is a prime candidate for the next generation of planetary instruments, as it addresses the primary goal of mineralogical analysis, which is structure and composition. However, large fluorescence return from many mineral samples under visible light excitation can render Raman spectra unattainable. Using the described approach, Raman and fluorescence, which occur on different time scales, can be simultaneously obtained from mineral samples using a compact instrument in a planetary environment. This new approach is taken based on the use of time-resolved spectroscopy for removing the fluorescence background from Raman spectra in the laboratory. In the SSTARS instrument, a visible excitation source (a green, pulsed laser) is used to generate Raman and fluorescence signals in a mineral sample. A spectral notch filter eliminates the directly reflected beam. A grating then disperses the signal spectrally, and a streak camera provides temporal resolution. The output of the streak camera is imaged on the CCD (charge-coupled device), and the data are read out electronically. By adjusting the sweep speed of the streak camera, anywhere from picoseconds to milliseconds, it is possible to resolve Raman spectra from numerous fluorescence spectra in the same sample. The key features of SSTARS include a compact streak tube capable of picosecond time resolution for collection of simultaneous spectral and temporal information, adaptive streak tube electronics that can rapidly change from one sweep rate to another over ranges of picoseconds to milliseconds, enabling collection of both Raman and fluorescence signatures versus time and wavelength, and Synchroscan integration that allows for a compact, low-power laser without compromising ultimate sensitivity

Time-resolved Raman spectroscopy for in situ planetary mineralogy

Planetary mineralogy can be revealed through a variety of remote sensing and in situ investigations that precede any plans for eventual sample return. We briefly review those techniques and focus on the capabilities for on-surface in situ examination of Mars, Venus, the Moon, asteroids, and other bodies. Over the past decade, Raman spectroscopy has continued to develop as a prime candidate for the next generation of in situ planetary instruments, as it provides definitive structural and compositional information of minerals in their natural geological context. Traditional continuous-wave Raman spectroscopy using a green laser suffers from fluorescence interference, which can be large (sometimes saturating the detector), particularly in altered minerals, which are of the greatest geophysical interest. Taking advantage of the fact that fluorescence occurs at a later time than the instantaneous Raman signal, we have developed a time-resolved Raman spectrometer that uses a streak camera and pulsed miniature microchip laser to provide picosecond time resolution. Our ability to observe the complete time evolution of Raman and fluorescence spectra in minerals makes this technique ideal for exploration of diverse planetary environments, some of which are expected to contain strong, if not overwhelming, fluorescence signatures. We discuss performance capability and present time-resolved pulsed Raman spectra collected from several highly fluorescent and Mars-relevant minerals. In particular, we have found that conventional Raman spectra from fine grained clays, sulfates, and phosphates exhibited large fluorescent signatures, but high quality spectra could be obtained using our time-resolved approach

Constraining the Origin of the Jupiter Trojans by In Situ Measurement of Volatiles, Minerals, and Ices

As the KISS Trojans program comes to a close, we report here on our achievements in this venture that began with a KISS workshop in 2012, “In Situ Science and Instrumentation for Primitive Bodies”. The original workshop brought together a diverse group (see Appendix B) that set out to tackle an ambitious goal – to find a way to test predictions of dynamical models (such as the Nice model, named after the founding research group in Nice, France), that have recently led to a radically new understanding of solar system formation. We aimed to do so through interdisciplinary collaboration between the planetary dynamics communities that have formulated (and largely dominated discussion of) these new ideas, and the meteoritics and cosmochemistry communities who would no doubt be involved in any in situ mission to an outer solar system body

In Situ Science and Instrumentation for Primitive Bodies

Our study began with the goal of developing new methods to test the radically new understanding of solar system formation that has recently emerged, and to identify innovative instrumentation targeted to this purpose. In particular, we were seeking to test predictions of dynamical models such as the Nice model (after the founding research group in Nice, France), and to do so through interdisciplinary collaboration between the planetary dynamics communities that have formulated (and largely dominated discussion of) these new ideas, and the meteoritics and cosmochemistry communities who will be most involved in any in situ mission to an outer solar system body. Our study was principally focused on coming up with explicit tests of the predictions of these new dynamical models of solar system evolution. The key outcome of our first workshop was the realization that fundamental work is needed before these two communities—dynamics and meteoritics/cosmochemistry—are really ready to come to a collective understanding of early solar system evolution. Planetary dynamics examines solar system history through the orbital properties of large populations of bodies, but says little specific about any one of them. In fact, at present it appears that there is nothing you could learn about any one body that this community would consider to be a concrete test of the Nice model (or another similarly broad model of solar system evolution). On the other hand, people who study planetary materials through meteoritics and in situ missions are strongly focused on the idiosyncratic properties of individual bodies but don’t actually know how to identify the properties of a primitive body that depend upon its orbital evolution. Without such tools, it isn’t clear how this community can turn insights regarding one body into statements about broad classes of related bodies. This is a frustrating moment in the study of solar system evolution—both the dynamics and meteoritics/cosmochemistry communities have well developed and consequential hypotheses about solar system evolution, but it isn’t obvious that either knows how to make a concrete statement that is testable by the other. Our reaction to this impasse was to step back from the narrow problem of testing the Nice model as a whole (or similar specific dynamical models) and ask whether there might be specific instances—particular bodies or groups of bodies—where we could forge a link between the dynamical and meteoritic/cosmochemical approaches. If so, this could serve as a foundation that will eventually lead to a synthesis of the dynamical and cosmochemical understanding of solar system evolution. The key, we imagine, is to find a case where dynamical approaches lead to clear predictions about mineralogical or chemical properties of individual bodies, so that mineralogical or cosmochemical approaches could test those predictions through in situ or remote observations. There was consensus amongst our team that we should be able to use dynamics to predict the chemistry of a primitive body based on knowledge of where the body originated in the solar nebula and the thermal history it has undergone. We are in a unique position to make this new type of connection between dynamical models and chemistry because of the diverse backgrounds represented in our group, which includes dynamicists, astronomers, geochemists, cosmochemists, spectroscopists, mineralogists, and instrument developers. For our second workshop, we further expanded our team to address new directions, specifically drawing on expertise in geochemistry of returned samples and meteorites. Throughout our study, we had extensive discussions about the composition of primitive bodies, where in many cases little is known from telescopic observations. Moreover, there is no known meteorite collection of materials from the most relevant group of parent bodies (e.g., D-types – Trojan asteroids, irregular satellites, Phobos and Deimos, and some outer main belt asteroids). Trojan asteroids were identified as the most interesting target because they represent a large reservoir of D-types that can potentially be linked to origins in the outer solar system (primitive Kuiper belt). Dynamical histories have not yet made specific predictions about the chemistry of these bodies because the field is still in its infancy and there has been little interaction between dynamicists and chemists. We concluded that we need to develop our own theoretical framework starting from the beginning—what are the starting materials? How were they processed during and after migration? Then, we need to actually do the lab work to simulate these materials and look for markers. A search for these markers would be the basis of the science motivation for future missions to these bodies. Because of the current lack of knowledge about the compositions of these bodies, we found that choosing a specific suite of in situ instruments to develop for such a mission would be premature at this point. (For a primer on in situ instruments for planetary surface exploration, see Appendix B). It is understood that any mission to the Trojans would operate under extreme constraints of mass and power so that it would not be possible to send all possible instrumentation to characterize the surface. Hence, we must develop the theoretical and laboratory framework first so that we can tailor the instruments to the most important measurements. The expected significance of the identification of these markers (the topic of our follow-on proposal) is that it would have implications for all future missions to small bodies (not just the Trojans). It is understood that in order to gain the most detailed knowledge of both chemical and isotopic compositions of small bodies, sample return would be preferred. However, if we can identify one or several very specific markers, it will become feasible to search for these with a small suite of in situ instruments at a number of target bodies. Or, even better, it may be possible for us to identify spectral properties that can be observed remotely. Our goal is to work our way to an understanding of these sorts of dynamically important signatures

N-Type delta Doping of High-Purity Silicon Imaging Arrays

A process for n-type (electron-donor) delta doping has shown promise as a means of modifying back-illuminated image detectors made from n-doped high-purity silicon to enable them to detect high-energy photons (ultraviolet and x-rays) and low-energy charged particles (electrons and ions). This process is applicable to imaging detectors of several types, including charge-coupled devices, hybrid devices, and complementary metal oxide/semiconductor detector arrays. Delta doping is so named because its density-vs.-depth characteristic is reminiscent of the Dirac delta function (impulse function): the dopant is highly concentrated in a very thin layer. Preferably, the dopant is concentrated in one or at most two atomic layers in a crystal plane and, therefore, delta doping is also known as atomic-plane doping. The use of doping to enable detection of high-energy photons and low-energy particles was reported in several prior NASA Tech Briefs articles. As described in more detail in those articles, the main benefit afforded by delta doping of a back-illuminated silicon detector is to eliminate a "dead" layer at the back surface of the silicon wherein high-energy photons and low-energy particles are absorbed without detection. An additional benefit is that the delta-doped layer can serve as a back-side electrical contact. Delta doping of p-type silicon detectors is well established. The development of the present process addresses concerns specific to the delta doping of high-purity silicon detectors, which are typically n-type. The present process involves relatively low temperatures, is fully compatible with other processes used to fabricate the detectors, and does not entail interruption of those processes. Indeed, this process can be the last stage in the fabrication of an imaging detector that has, in all other respects, already been fully processed, including metallized. This process includes molecular-beam epitaxy (MBE) for deposition of three layers, including metallization. The success of the process depends on accurate temperature control, surface treatment, growth of high-quality crystalline silicon, and precise control of thicknesses of layers. MBE affords the necessary nanometer- scale control of the placement of atoms for delta doping. More specifically, the process consists of MBE deposition of a thin silicon buffer layer, the n-type delta doping layer, and a thin silicon cap layer. The n dopant selected for initial experiments was antimony, but other n dopants as (phosphorus or arsenic) could be used. All n-type dopants in silicon tend to surface-segregate during growth, leading to a broadened dopant-concentration- versus-depth profile. In order to keep the profile as narrow as possible, the substrate temperature is held below 300 C during deposition of the silicon cap layer onto the antimony delta layer. The deposition of silicon includes a silicon- surface-preparation step, involving H-termination, that enables the growth of high-quality crystalline silicon at the relatively low temperature with close to full electrical activation of donors in the surface layer

Method for growing a back surface contact on an imaging detector used in conjunction with back illumination

A method is provided for growing a back surface contact on an imaging detector used in conjunction with back illumination. In operation, an imaging detector is provided. Additionally, a back surface contact (e.g. a delta-doped layer, etc.) is grown on the imaging detector utilizing a process that is performed at a temperature less than 450 degrees Celsius

Hubble Ultraviolet Spectroscopy of Jupiter Trojans

We present the first ultraviolet spectra of Jupiter Trojans. These observations were carried out using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and cover the wavelength range 200-550 nm at low resolution. The targets include objects from both of the Trojan color subpopulations (less-red and red). We do not observe any discernible absorption features in these spectra. Comparisons of the averaged UV spectra of less-red and red targets show that the subpopulations are spectrally distinct in the UV. Less-red objects display a steep UV slope and a rollover at around 450 nm to a shallower visible slope, whereas red objects show the opposite trend. Laboratory spectra of irradiated ices with and without H$_{2}$S exhibit distinct UV absorption features; consequently, the featureless spectra observed here suggest H$_{2}$S alone is not responsible for the observed color bimodality of Trojans, as has been previously hypothesized. We propose some possible explanations for the observed UV-visible spectra, including complex organics, space weathering of iron-bearing silicates, and masked features due to previous cometary activity.Comment: 7 pages, 4 figures, accepted by A

Hubble Ultraviolet Spectroscopy of Jupiter Trojans

We present the first ultraviolet spectra of Jupiter Trojans. These observations were carried out using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and cover the wavelength range 200–550 nm at low resolution. The targets include objects from both of the Trojan color subpopulations (less-red and red). We do not observe any discernible absorption features in these spectra. Comparisons of the averaged UV spectra of less-red and red targets show that the subpopulations are spectrally distinct in the UV. Less-red objects display a steep UV slope and a rollover at around 450 nm to a shallower visible slope, whereas red objects show the opposite trend. Laboratory spectra of irradiated ices with and without H_2S exhibit distinct UV absorption features; consequently, the featureless spectra observed here suggest H_2S alone is not responsible for the observed color bimodality of Trojans, as has been previously hypothesized. We propose some possible explanations for the observed UV-visible spectra, including complex organics, space weathering of iron-bearing silicates, and masked features due to previous cometary activity

Miniaturized time-resolved Raman spectrometer for planetary science based on a fast single photon avalanche diode detector array

We present recent developments in time-resolved Raman spectroscopy instrumentation and measurement techniques for in situ planetary surface exploration, leading to improved performance and identification of minerals and organics. The time-resolved Raman spectrometer uses a 532 nm pulsed microchip laser source synchronized with a single photon avalanche diode array to achieve sub-nanosecond time resolution. This instrument can detect Raman spectral signatures from a wide variety of minerals and organics relevant to planetary science while eliminating pervasive background interference caused by fluorescence. We present an overview of the instrument design and operation and demonstrate high signal-to-noise ratio Raman spectra for several relevant samples of sulfates, clays, and polycyclic aromatic hydrocarbons. Finally, we present an instrument design suitable for operation on a rover or lander and discuss future directions that promise great advancement in capability