The fine dust detected by infrared (IR) emission around the nearby Delta *b Pic analog star HD172555 is very peculiar. The dust mineralogy is composed primarily of highly refractory, nonequilibrium ...materials, with approximately three quarters of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal emission spectra (nonequilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a -3.95+/-0.10. While IR photometry of the system has stayed stable since the 1983 IRAS mission, this steep a size distribution, with abundant micron-sized particles, argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 +/- 0.6 AU (equivalent to 1.9 +/- 0.2 AU from the Sun), within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 X 1019-2 X 1020 kg, equivalent to a 150-200 km radius asteroid. Significant emission features centered at 4 and 8 Delta *mm due to fluorescing SiO gas are also found. Roughly 1022 kg of SiO gas, formed by vaporizing silicate rock, is also present in the system, and a separate population of very large, cool grains, massing 1021-1022 kg and equivalent to the largest sized asteroid currently found in the solar system's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong stellar X-ray activity, and an extended disk of Delta *b meteoroids argues that the source of the observed circumstellar materials is a giant hypervelocity (>10 km s-1) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.
We report here time-domain infrared spectroscopy and optical photometry of the HD 145263 silica-rich circumstellar-disk system taken from 2003 through 2014. We find an F4V host star surrounded by a ...stable, massive 1022-1023 kg (MMoon to MMars) dust disk. No disk gas was detected, and the primary star was seen rotating with a rapid ∼1.75 day period. After resolving a problem with previously reported observations, we find the silica, Mg-olivine, and Fe-pyroxene mineralogy of the dust disk to be stable throughout and very unusual compared to the ferromagnesian silicates typically found in primordial and debris disks. By comparison with mid-infrared spectral features of primitive solar system dust, we explore the possibility that HD 145263's circumstellar dust mineralogy occurred with preferential destruction of Fe-bearing olivines, metal sulfides, and water ice in an initially comet-like mineral mix and their replacement by Fe-bearing pyroxenes, amorphous pyroxene, and silica. We reject models based on vaporizing optical stellar megaflares, aqueous alteration, or giant hypervelocity impacts as unable to produce the observed mineralogy. Scenarios involving unusually high Si abundances are at odds with the normal stellar absorption near-infrared feature strengths for Mg, Fe, and Si. Models involving intense space weathering of a thin surface patina via moderate (T < 1300 K) heating and energetic ion sputtering due to a stellar super-flare from the F4V primary are consistent with the observations. The space-weathered patina should be reddened, contain copious amounts of nanophase Fe, and should be transient on timescales of decades unless replenished.
Spectral modeling of the large infrared excess in the Spitzer IRS spectra of HD 172555 suggests that there is more than 10 super(19) kg of submicron dust in the system. Using physical arguments and ...constraints from observations, we rule out the possibility of the infrared excess being created by a magma ocean planet or a circumplanetary disk or torus. We show that the infrared excess is consistent with a circumstellar debris disk or torus, located at ~6 AU, that was created by a planetary scale hypervelocity impact. We find that radiation pressure should remove submicron dust from the debris disk in less than one year. However, the system's mid-infrared photometric flux, dominated by submicron grains, has been stable within 4% over the last 27 years, from the Infrared Astronomical Satellite (1983) to WISE (2010). Our new spectral modeling work and calculations of the radiation pressure on fine dust in HD 172555 provide a self-consistent explanation for this apparent contradiction. We also explore the unconfirmed claim that ~10 super(47) molecules of SiO vapor are needed to explain an emission feature at ~8 mum in the Spitzer IRS spectrum of HD 172555. We find that unless there are ~10 super(48) atoms or 0.05 M sub(+ in circle) of atomic Si and O vapor in the system, SiO vapor should be destroyed by photo-dissociation in less than 0.2 years. We argue that a second plausible explanation for the ~8 mum feature can be emission from solid SiO, which naturally occurs in submicron silicate "smokes" created by quickly condensing vaporized silicate.
Launched onboard the BepiColombo Mercury Planetary Orbiter (MPO) in October 2018, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is on its way to planet Mercury. MERTIS consists of ...a push-broom IR-spectrometer (TIS) and a radiometer (TIR), which operate in the wavelength regions of 7-14 μm and 7-40 μm, respectively. This wavelength region is characterized by several diagnostic spectral signatures: the Christiansen feature (CF), Reststrahlen bands (RB), and the Transparency feature (TF), which will allow us to identify and map rock-forming silicates, sulfides as well as other minerals. Thus, the instrument is particularly well-suited to study the mineralogy and composition of the hermean surface at a spatial resolution of about 500 m globally and better than 500 m for approximately 5-10% of the surface. The instrument is fully functional onboard the BepiColombo spacecraft and exceeds all requirements (e.g., mass, power, performance). To prepare for the science phase at Mercury, the team developed an innovative operations plan to maximize the scientific output while at the same time saving spacecraft resources (e.g., data downlink). The upcoming fly-bys will be excellent opportunities to further test and adapt our software and operational procedures. In summary, the team is undertaking action at multiple levels, including performing a comprehensive suite of spectroscopic measurements in our laboratories on relevant analog materials, performing extensive spectral modeling, examining space weathering effects, and modeling the thermal behavior of the hermean surface.
We present an analysis of the gas-poor circumstellar material in the HD 113766 binary system (F3/F5, 10-16 Myr), recently observed by the Spitzer Space Telescope. For our study we have used the IR ...mineralogical model derived from observations of the Deep Impact experiment. We find the dust dominated by warm, fine ( similar to 1 mu m) particles, abundant in Mg-rich olivine, crystalline pyroxenes, amorphous silicates, Fe-rich sulfides, amorphous carbon, and colder water ice. The warm dust material mix is akin to an inner main-belt asteroid of S-type composition. The similar to 440 K effective temperature of the warm dust implies that the bulk of the observed material is in a narrow belt similar to 1.8 AU from the 4.4 L unk central source, in the terrestrial planet-forming region and habitable zone of the system (equivalent to 0.9 AU in the solar system). The icy dust lies in two belts, located at 4-9 and 30-80 AU. The lower bound of warm dust mass in 0.1-20 mu m, unk similar to a super(-35) particles is very large, at least 3 x 10 super(20) kg, equivalent to a 320 km radius asteroid of 2.5 g cm super(-3) density. Assuming 10 m particles are the largest present, the lower bound of warm dust mass is at least 0.5 M sub(Mars). Neither primordial nor mature, the dust around HD 113766A originates from catastrophic disruption of terrestrial planet embryo(s) and subsequent grinding of the fragments or from collisions in a young, extremely dense asteroid belt undergoing planetary aggregation. The persistence of the strong IR excess over the last two decades argues for a mechanism to provide replenishment of the circumstellar material on yearly timescales.
This work is part of a project to build an infrared database in order to link IR data of planetary materials (and therefore possible Mercury material) with remote sensing observations of Mercury, ...which will probably be obtained by the MERTIS instrument on the forthcoming BepiColombo mission.
The unique achondrite Northwest Africa (NWA) 7325, which has previously been suggested to represent the first sample from Mercury, was investigated by optical and electron microscopy, and infrared and Raman spectroscopy. In addition, the oxygen, strontium, xenon, and argon isotopes were measured and the abundance of selected trace elements determined. The meteorite is a cumulate rock with subchondritic abundances of HFSE and REE and elevated Sr contents, which underwent a second heating and partial remelting process. Oxygen isotope measurements show that NWA 7325 plots in the ureilite field, close to the ALM‐A trachyandesitic fragment found in the unique Almahata Sitta meteorite breccia. On the other hand, mineralogical investigations of the pyroxenes in NWA 7325 provide evidence for similarities to the lodranites and acapulcoites. Furthermore, the rock is weakly shocked and argon isotope data record ancient (~4.5 Ga) plateau ages that have not been reset. The sample records a cosmogenic exposure age of ~19 Ma. Systematics of Rb‐Sr indicate an extreme early volatile depletion of the precursor material, similar to many other achondrite groups. However, despite its compositional similarities to other meteorite groups, our results suggest that this meteorite is unique and unrelated to any other known achondrite group. An origin for NWA 7325 as a sample from the planet Mercury is not supported by the results of our investigation. In particular, the evidence from infrared spectroscopy indicates that a direct relationship between NWA 7325 and the planet Mercury can be ruled out: no acceptable spectral match between laboratory analyses and remote sensing observations from Mercury has been obtained. However, we demonstrate that infrared spectroscopy is a rapid and nondestructive method to characterize mineral phases and thus an excellent tool for planetary surface characterization in space missions.
The last thirty years of cosmochemistry and planetary science have shown that one major Solar System reservoir is vastly undersampled in the available suite of extra-terrestrial materials, namely ...small bodies that formed in the outer Solar System (>10 AU). Because various dynamical evolutionary processes have modified their initial orbits (e.g., giant planet migration, resonances), these objects can be found today across the entire Solar System as P/D near-Earth and main-belt asteroids, Jupiter and Neptune Trojans, comets, Centaurs, and small (diameter < 200 km) trans-Neptunian objects. This reservoir is of tremendous interest, as it is recognized as the least processed since the dawn of the Solar System and thus the closest to the starting materials from which the Solar System formed. Some of the next major breakthroughs in planetary science will come from studying outer Solar System samples (volatiles and refractory constituents) in the laboratory. Yet, this can only be achieved by an L-class mission that directly collects and returns to Earth materials from this reservoir. It is thus not surprising that two White Papers advocating a sample return mission of a primitive Solar System small body (ideally a comet) were submitted to ESA in response to its Voyage 2050 call for ideas for future L-class missions in the 2035-2050 time frame. One of these two White Papers is presented in this article.
We investigated a suite of impact glass‐bearing rocks using a multi‐analytical approach including visible‐near‐infrared diffuse reflectance spectroscopy, Mössbauer spectroscopy, and powder X‐ray ...diffraction. In order to better understand and interpret the obtained results, we built a database containing physical, chemical, and spectroscopic information on glasses and glass‐bearing materials using new results from this study and published works. We used the database to explore systematic relationships between parameters of interest and finally we applied several machine learning algorithms (support vector machine, random forests, and gradient boosting) to test the possibility to regress the oxidation state of iron from chemical and spectroscopic information. Our results show that even small amounts of mafic crystalline phases have a big influence on the spectral features of glass‐bearing rocks. Samples without mafic crystalline inclusions show the typical spectrum of glasses (two broad and shallow bands roughly centered around 1,100 and 1,900 nm) with minor variations due to bulk chemistry. We described a non‐linear relationship between average reflectance (average reflectance value between 500 and 1,000 nm), FeO + TiO2 content, grain size, and Fe3+/FeTOT. We tested the relation for the finer grain size (0–25 μm), and we qualitatively assessed how it is affected by grain size, Fe3+/FeTOT, and crystal content. Finally, we developed a machine learning pipeline to regress the Fe3+/FeTOT of glass‐bearing materials using the proposed database. Our machine learning calculations give satisfactory results (MAE: 0.0321) and additional data will enable the application of our computational strategy to remotely acquired data to extract chemical and mineralogical information of planetary surfaces.
Plain Language Summary
Glasses and glass‐bearing rocks are abundant on many planetary surfaces. Glass‐bearing rocks form either as a result of rapid cooling of volcanic materials or after shock impact of (large enough) planetary bodies. Their detection from remotely acquired data remains a challenge due to the many variables affecting their spectroscopic features. More data are necessary in order to improve our knowledge of these materials. In this work, we investigated a suite of terrestrial rocks formed after impact/near surface explosion of asteroids. These rocks are characterized by a varied chemistry and different degrees of glass‐crystal ratios, making them very interesting analog planetary materials. We characterized the spectroscopic, chemical, and mineralogical features of these rocks, and we built a database containing similar information retrieved from literature data. The newly compiled database allowed us to uncover relationships between spectroscopic features and chemical/physical features of glass‐bearing materials. Given the extremely complicated physics of the shock phenomena, it is not always easy to extract useful information from the spectra of these materials. For this reason, we applied some machine learning algorithms to the newly compiled database in order to retrieve information of interest. The approach that we use is successful and suggests that when there will be more data in the database, it would be possible to use more sophisticated algorithms to retrieve information of interest from remotely acquired data.
Key Points
The average reflectance in the visible‐near‐infrared spectral range is negatively correlated with FeO + TiO2 content in glassy materials
A database was built merging spectral, chemical, and physical information from several literature data
The redox state of iron in glassy materials can be successfully regressed by a machine learning approach using spectroscopic information
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