Donald Walter Strecker

Near-infrared spectra of the Galilean satellites - Observations and compositional implications

Abstract We have obtained reflectivity spectra of the trailing and leading sides of all four Galilean satellites with circular variable filter wheel spectrometers operating in the 0.7- to 5.5-μm spectral interval. These observations were obtained at an altitude of 41,000 ft from the Kuiper Airborne Observatory. Features seen in these data include a 2.9-μm band present in the spectra of both sides of Callisto; the well-known 1.5-μm and 2.0-μm combination bands and the previously more poorly defined 3.1-μm fundamental of water ice observed in the spectra of both sides of Europa and Ganymede; and features centered at 1.35 ± 0.1, 2.55 ± 0.1, and 4.05 ± 0.05 μ m noted in the spectra of both sides of Io. In an effort to interpret these data, we have compared them with laboratory spectra as well as synthetic spectra constructed with a simple multiple-scattering theory. We attribute the 2.9-μm feature of Callistos spectra primarily to bound water, with the product of fractional abundance of bound water and mean grain radius in micrometers equaling approximately 3.5 × 10 −1 for both sides of the satellite. The fractional amounts of water ice cover on the trailing side of Ganymede, its leading side, and the leading side of Europa were found to be 50 ± 15, 65 ± 15, and 85% or greater, respectively. The bare ground areas on Ganymede have reflectivity properties in the 0.7- to 2.5-μm spectral region comparable to those of Callistos surface and also have significant quantities of bound water, as does Callisto. Interpretation of the spectrum for the trailing side of Europa is complicated by magnetospheric particle bombardment which causes a perceptible broadening of strong bands, but the ice cover on this side is probably comparable to that on the leading side. These irradiation effects may be responsible for much of the difference in the visual geometric albedos of the two sides of Europa. Minor, but significant, amounts of ferrous-bearing material (either ferrous salts or alkali feldspars but not olivines or pyroxenes) account for the 1.35-μm feature of Io. The two longer wavelength bands are most likely attributable to nitrate salts. Ferrous salts and nitrates can jointly also account for much of the spectral variation in Ios visible reflectivity, thereby eliminating the need to postulate large quantities of sulfur. The absence of noticeable features near 3-μm wavelength in Ios spectra leads to upper bounds of 10% on the fractional cover of water and ammonia ice and 10 −3 on the relative abundance of bound water and hydroxylated material on Io. The two sides of Io have similar compositions. We suggest that the systematic increase in fractional water ice cover from Callisto to Ganymede to Europa is bought about by variations in efficiencies of recoating the satellites surface by interior water brought to the surface, and by the deposition of extrinsic dust. The most important component of the latter is debris, derived from the outer irregular satellites of Jupiter, which impacts the Galilean satellites at relatively low velocities. Europa has the largest water ice cover because its crust is thinnest and thus the frequency of water recoating is the greatest, and because it is farthest from the sources of low-velocity dust. While models which depict Ios surface as consisting primarily of very fine-grained ice are no longer viable, we are unable to definitively distinguish between the salt assemblage and alkali feldspar models. The salt model can better account for Ios reflectivity spectrum from 0.3 to 5 μm, but the absence of appreciable quantities of bound water and hydroxylated material may not be readily understood within the context of that model.

For infrared spectrophotometry of Jupiter and Saturn

Abstract Infrared spectral observations of Mars, Jupiter, and Saturn were made from 100 to 470 cm−1 using NASAs G. P. Kuiper Airborne Observatory. Taking Mars as a calibration source, we determined brightness temperatures of Jupiter and Saturn with approximately 5 cm−1 resolution. The data are used to determine the internal luminosities of the giant planets, for which more than 75% of the thermally emitted power is estimated to be in the measured bandpass: for Jupiter LJ = (8.0 ± 2.0) × 10−10 L⊙ and for Saturn LS = (3.6 ± 0.9) × 10−10. The ratio R of thermally emitted power to solar power absorbed was estimated to be RJ = 1.6 ± 0.2, and RS = 2.7 ± 0.8 from the observations when both planets were near perihelion. The Jupiter spectrum clearly shows the presence of the rotational ammonia transitions which strongly influence the opacity at frequencies ≲250 cm−1. Comparison of the data with spectra predicted from current models of Jupiter and Saturn permits inferences regarding the structure of the planetary atmospheres below the temperature inversion. In particular, an opacity source in addition to gaseous hydrogen and ammonia, such as ammonia ice crystals as suggested by Orton, may be necessary to explain the observed Jupiter spectrum in the vicinity of 250 cm−1.

A Determination of the Composition of the Venus Clouds from Aircraft Observations in the Near Infrared

Abstract We summarize the evidence showing that the first optical depth of the Venus cloud layer is composed of a water solution of sulfuric acid, including our earlier aircraft observations of Venus’ reflectivity in the 1–4 μm region obtained at a phase angle of 120° (Pollack et al.). Analyses of these aircraft results indicated that of all the proposed cloud candidates only a sulfuric acid solution with a concentration of 75% or more H2SO4, by weight was consistent with the observed 3 µm cloud feature. We present new aircraft observations of Venus obtained in the 1–4 µm region at a phase angle of 40° and in the 3–6 µm region at a phase angle of 136°. Comparing the two sets of observations in the 1–4 µm region, we find a striking phase effect: the reflectivity is much lower in the 3 µm region and there is a much more marked decline between 1.3 and 2.5 µm for the data obtained at the smaller phase angle. The observations made at the 40° phase angle are consistent with the theoretical behavior of a sulfuri...

Mars: Far-infrared spectra and thermal-emission models

Abstract Spectra of Mars from 100 to 360 cm −1 were obtained during three different observation periods from NASAs Kuiper Airborne Observatory. Also, a new thermal model was constructed for the surface of Mars, and synthetic spectra were computed from the models to compare with the observations. The models include the effects of a dusty atmosphere which absorbs, scatters, and reradiates energy. The synthetic spectra show significant effects on disk-averaged brigthness temperatures, as well as absorption features, due to silicate dust. The spectra of Mars, which are ratios of Mars to the Moon, do not fit the synthetic spectra unless the surface emissivities of Mars and the Moon have different dependencies on wavelenght. A possible explanation for this behavior is a difference in soil particle-size distributions between Mars and the Moon, with Mars being depleted in large particles compared to the Moon. Small particles are consistent with clay minerals which have been suggested elsewhere as constituents of the Martian surface.

The brightness temperatures of Saturn and its rings at 39 microns

We have resolved the relative rings-to-disk brightness (specific intensity) of Saturn at 39 μm (δλ ≃ 8 μm) using the 224-cm telecscope at Mauna Kea Oservatory, and have also measured the total flux of Saturn relative to Jupiter in the same bandpass from the NASA Learjet Observatory. These two measurements, which were made in early 1975 with Saturns rings near maximum inclination (b′ ≃ 25°), determine the disk and average ring (A and B) brightness in terms of an absolute flux calibration of Jupiter in the same bandpass. While present uncertainties in Jupiters absolute calibration make it possible to compare existing measurementsunambiguously, it is nevertheless possible to conclude the following: (1) observations between 20 and 40 μm are all compatible (within 2σ) of a disk brightness temperature of 94°K, and do not agree with the radiative equilibrium models of Trafton; (2) the rings at large tilt contribute a flux component comparable to that of the planet itself for λ ≲ 40 μm and (3) there is a decrease of ∼22% in the relative ring: disk brightness between effective wavelengths of 33.5 and 39 μm.

Identifying organic molecules in space: the AstroBiology Explorer (ABE) MIDEX misson concept

In this paper we review our current state of knowledge regarding the identity of organic and related compounds in the interstellar medium (ISM). The remote detection and identification of organics is ideally suited to the technique of infrared spectroscopy since such data can be obtained telescopically and this spectral range encompasses the fundamental vibrational modes of common molecular bonds. Despite recent advances in our knowledge of the organic component of the ISM, we are still far from understanding the distribution, abundance and evolutionary inter- relationship of these materials within our galaxy and the universe as a whole. Many of these issues can be addressed by the acquisition of new infrared spectra. We briefly describe a potential new Explorer-class space mission capable of obtaining such data, the AstroBiology Explorer (ABE) which consists of a space observatory capable of obtaining spectra in the 2.5-16.0 micrometers range at a spectral resolution of (Delta)

Far Infrared Spectral Observations of the Galactic Center Region from the Gerard P. Kuiper Airborne Observatory

lamda/(lambda) equals 2000-3000. ABE would be capable of addressing outstanding problems in Astrochemistry and Astrophysics that are particularly relevant to Astrobiology and addressable via astronomical observation. ABE would have approximately one year lifetime during which it would obtain a coordinated set of infrared spectroscopic observations of large numbers of galaxies, stars, planetary nebulae, interstellar clouds, young star planetary systems and objects within our own Solar System.

Properties of the clouds of Venus, as inferred from airborne observations of its near-infrared reflectivity spectrum

Low resolution spectra in the region 45–250 microns of SgrA and SgrB2 were obtained in July/August, 1975, with the 91 cm telescope on the Gerard P. Kuiper Airborne Observatory. The spectra were taken with a single-beam Michelson interferometer utilizing a Mylar beam-splitter and a Ge-Ga bolometer with a beam diameter of 1.4 min. FWHM. Preliminary results give mean brightness temperatures for SgrA of roughly 90°K and for SgrB2 of roughly 40°K. SgrA was observed with a resolution of approximately 6 cm-1, while the resolution for B2 is approximately 3 cm-1. In addition, the HII region NGS 7538 was also observed at approximately 9 cm-1 resolution. The data will be presented and discussed.