Laboratory studies of thermal space weathering on airless bodies

Deriving the surface composition of Mercury from remote sensing hyper spectral data is a challenging task. Mercury’s surface has a low iron abundance, which complicates the application of “traditional” space weathering approach. In addition the high temperatures on Mercury lead to previously unseen changes in the spectral characteristics, which we call “thermal space weathering”. The Planetary Emissivity Laboratory (PEL) at DLR in Berlin was setup specifically to study the effects of high temperatures on the spectral characteristics of planetary analog materials. It allows characterizing “thermal space weathering” and adds temperature as another important factor for the creation of spectral libraries. Thermal space weathering can produce reversible as well as irreversible changes in the spectral characteristics of materials. In comparison to “traditional space weathering” it acts on much shorter timescales. We are going to present a number of examples for thermal space weathering effects in the visible as well as infrared spectral range

Compositional Diversity Among Primitive Asteroids

Spectroscopic observations from the ultraviolet to the mid-infrared have revealed new and diagnostic differences among primitive asteroids. We review the spectral characteristics of these asteroids and their inferred compositional and physical properties. Primitive asteroids throughout the belt show carbon-rich compounds, varying degrees of aqueous alteration and even surface ice; recent observations provide significant new constraints on composition, thermal inertia, and other surface properties. New mid-infrared connections between primitive asteroids and interplanetary dust particles indicate that the latter sample a larger fraction of main belt asteroids than meteorites. Links with the composition of comets are consistent with a proposed continuum between primitive asteroids and comets. Two sample-return missions, OSIRIS-REx and Hayabusa 2, will visit primitive near-Earth asteroids (NEAs). Most spacecraft-accessible NEAs originate in the inner asteroid belt, which contains several primitive asteroid families and a background of primitive asteroids outside these families. Initial results from these families offer a tantalizing preview of the properties expected in the NEAs they produce. So far, primitive asteroids in the inner belt fall into two spectral groups. The first group includes the Polana-Eulalia families, which show considerable spectral homogeneity in spite of their dynamical and collisional complexity. In contrast, the Erigone and Sulamitis families are spectrally diverse and most of their members show clear 0.7 microns hydration features. The two sample-return targets (101955) Bennu and (162173) Ryugu, most likely originated in the Polana family.Comment: 31 pages, 11 figures, chapter 5 in Primitive Meteorites and Asteroids, Physical, Chemical, and Spectroscopic Observations Paving the Way to Exploratio

Mineralogy and Surface Composition of Asteroids

Methods to constrain the surface mineralogy of asteroids have seen considerable development during the last decade with advancement in laboratory spectral calibrations and validation of our interpretive methodologies by spacecraft rendezvous missions. This has enabled the accurate identification of several meteorite parent bodies in the main asteroid belt and helped constrain the mineral chemistries and abundances in ordinary chondrites and basaltic achondrites. With better quantification of spectral effects due to temperature, phase angle, and grain size, systematic discrepancies due to non-compositional factors can now be virtually eliminated for mafic silicate-bearing asteroids. Interpretation of spectrally featureless asteroids remains a challenge. This paper presents a review of all mineralogical interpretive tools currently in use and outlines procedures for their application.Comment: Chapter to appear in the Space Science Series Book: Asteroids IV, 51 pages, 7 figures, 2 table

The Mid-IR Spectral Effects of Darkening Agents and Porosity on the Silicate Surface Features of Airless Bodies

We systematically measured the mid-IR spectra of different mixtures of three silicates (antigorite, lizardite, and pure silica) with varying effective porosities and amounts of darkening agent (iron oxide and carbon). These spectra have broad implications for interpretation of current and future mission data for airless bodies, as well as for testing the capabilities of new instruments. Serpentines, such as antigorite and lizardite, are common to airless surfaces, and their mid-IR spectra in the presence of darkening agents and different surface porosities would be typical for those measured by spacecraft. Silica has only been measured in the plumes of Enceladus and presents exciting possibilities for other Saturn-system surfaces due to long range transport of E-ring material. Results show that the addition of the IR-transparent salt, KBr, to simulate surface porosity affected silicate spectra in ways that were not predictable from linear mixing models. The strengthening of silicate bands with increasing pore space, even when only trace amounts of KBr were added, indicates that spectral features of porous surfaces are more detectable in the mid-IR. Combining iron oxide with the pure silicates seemed to flatten most of the silicate features, but strengthened the reststrahlen band of the silica. Incorporating carbon with the silicates weakened all silicate features, but the silica bands were more resistant to being diminished, indicating silica may be more detectable in the mid-IR than the serpentines. We show how incorporating darkening agents and porosity provides a more complete explanation of the mid-IR spectral features previously reported on worlds such as Iapetus

HD 145263: Spectral Observations of Silica Debris Disk Formation via Extreme Space Weathering?

We report here time domain infrared spectroscopy and optical photometry of the HD145263 silica-rich circumstellar disk system taken from 2003 through 2014. We find an F4V host star surrounded by a stable, massive 1e22 - 1e23 kg (M_Moon to M_Mars) 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 superflare 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.Comment: 41 Pages, 5 Figures, 5 Tables, Accepted for publication in the Astrophysical Journa

Assessment of mapping exposed ferrous and ferric iron compounds using Skylab-EREP data

The author has identified the following significant results. The S190B color photography is as useful as LANDSAT data for the mapping of color differences in the rocks and soils of the terrain. An S192 ratio of 0.79 - 0.89 and 0.93 - 1.05 micron bands produced an apparently successful delineation of ferrous, ferric, and other materials, in agreement with theory and ratio code studies. From an analysis of S191 data, basalt and dacite were separated on the basis of differences in spectral emissivity in the 8.3 - 12 micron region