Harold P. Larson

The 1.7- to 4.2-μm spectrum of asteroid 1 Ceres: Evidence for structural water in clay minerals

Abstract A high-resolution Fourier spectrum (1.7–3.5 μm) and medium-resolution spectrophotometry (2.7–4.2 μm) were obtained for Asteroid 1 Ceres. The presence of the 3-μm absorption feature due to water of hydration was confirmed. The 3-μm feature is compared with the 3-μm bands due to water of hydration in clays and salts. It is concluded that the spectrum of Ceres shows a strong absorption at 2.7–2.8 μm due to structural OH groups in clay minerals. The dominant minerals on the surface of Ceres are therefore hydrated clay minerals structurally similar to terrestrial montmorillonites. There is also a narrow absorption feature at 3.1 μm which is attributable to a very small amount of water ice on Ceres. This is the first evidence for ice on the surface of an asteroid.

Detection of water vapor in Halley's comet

Gaseous, neutral H2O was detected in the coma of comet Halley on 22.1 and 24.1 December 1985 Universal Time. Nine spectral lines of thev3 band (2.65 micrometers) were found by means of a Fourier transform spectrometer (λ/▵λ ∼ 105) on the NASA-Kuiper Airborne Observatory. The water production rate was ∼6 x 1028 molecules per second on 22.1 December and 1.7 x 1029 molecules per second on 24.1 December UT. The numbers of spectral lines and their intensities are in accord with nonthermal-equilibrium cometary models. Rotational populations are derived from the observed spectral line intensities and excitation conditions are discussed. The ortho-para ratio was found to be 2.66�0.13, corresponding to a nuclear-spin temperature of 32 K (+5 K, -2 K), possibly indicating that the observed water vapor originated from a low-temperature ice.

Infrared spectra of the Galilean satellites of Jupiter.

Spectra of the four Galilean satellites from 1 to 4 microns were obtained with a Michelson interferometer. The spectra show that the albedo of Io is very nearly constant with wavelength; no absorptions are found in the region observed. Europa and Ganymede have large amounts of water ice on their surface. Callisto shows some faint ice absorptions. Upper limits of 0.5 cm-atm (STP) corresponding to 6 x 10 to the minus 8th power atm partial surface pressure were set for CH4 and NH3 on all four satellites.

The spectrum of Saturn from 1990 to 2230 cm−1: Abundances of AsH3, CH3D, CO, GeH4, NH3, and PH3

We have investigated the mole fractions of six gases (AsH3, CH3D, CO, GeH4, NH3, and PH3) from extensive computer modeling of a high-resolution spectrum of Saturn from 1994 to 2229 cm−1 (4.5 to 5.0 μm). Five new AsH3 spectral features and three previously observed absorptions were detected, leaving no doubt as to the presence of this gas in Saturn. The mole fraction of AsH3 is 3 × 10−9, in agreement with earlier work and 10 times higher than that found in Jupiter. Thirteen lines of CO were detected, including eight new lines. Our CO abundance is lower than previous estimates, which we attribute to an improved atmospheric model. We find that the CO mole fraction is 1 × 10−9 if its distribution is uniform and 2.5 × 10−8 if concentrated in the stratosphere at pressures P 400 mbar, however, we find that the PH3 mole fraction is 7−2+3 × 10−6, which indicates a surprisingly high elemental abundance of P in Saturn. The mole fraction of CH3D is (3.3 ± 1.5) × 10−7, in general agreement with values determined at other wavelengths, which leads to a D/H ratio of (1.7 ± 1.1) × 10−5. Our constraints to the abundances of GeH4 and NH3 are weak. We find that the mole fraction of GeH4 is (4 ± 4) × 10−10 and that of NH3 is ≤ 3 × 10−4. The observed elemental abundances of P and As are important as remnants of the solid component of the solar nebula. Their pattern of enrichment in Jupiter and Saturn appears to be different than that for C and N.

Infrared spectral observations of asteroid 4 Vesta

An ir spectrum of asteroid Vesta, the first of any asteroid, has been recorded at a spectral resolution of 44 cm−1 with a Fourier spectrometer. An electronic absorption band is observed that is assigned to an iron-rich pyroxene (pigeonite) spectroscopically similar to that found in certain eucrites. Other important rock-forming minerals such as olivine and plagioclase feldspar are not observed. There is no evidence for compositional variation with rotational phase angle. This spectroscopic picture of Vesta suggests considerable evolution including the melting and differentiation of silicates.

Temperature dependence of the water-ice spectrum between 1 and 4 microns: Application to Europa, Ganymede and Saturn's rings

Abstract Reflection spectra of water ice from 1 to 4 μm are presented as a function of temperature. It is found that a feature at 6056 cm −1 changes its intensity sufficiently that it can be used as a spectroscopic measure of the ice temperature. A temperature calibration curve of this feature down to 55 K is developed and is used to determine ice temperatures for the Galilean satellites Europa (95±10 K), Ganymede (103±10 K), and the rings of Saturn (80±5 K). The ice temperatures for the Galilean satellites are lower than their measured brightness temperatures, which can be explained by a higher albedo of the ice covered regions relative to the rest of the satellite and possibly a concentration of the ice near the polar caps.

The middle-infrared spectrum of Saturn - Evidence for phosphine and upper limits to other trace atmospheric constituents

Observations of Saturn at high spectral resolution in the middle-IR spectral region (2.5-5.6 microns) were obtained using a Fourier spectrometer at ground-based and airborne observatories. These spectra establish that PH3 exists on Saturn with an abundance at least equal to the solar P/H value, and probably enhanced by about a factor of 2. No evidence is found for gaseous or solid NH3 on Saturn throughout this spectral region, and upper limits to several other molecules (H2O, HCN, SiH4, and GeH4) are determined. Frequent comparisons of the spectral data for Saturn with observations and interpretations of IR studies of Jupiter show that these two planetary atmospheres are chemically similar, with major observational differences accounted for by reduced H2O and NH3 abundances in Saturns colder atmosphere.

The gas composition of Jupiter derived from 5-μm airborne spectroscopic observations

Abstract The atmospheric transmission window between 1850 and 2250 cm −1 in Jupiters atmosphere was observed at a spectral resolution of 0.5 cm −1 from the Kuiper Airborne Observatory. The mole fractions of NH 3 , PH 3 , CH 4 , CH 3 D, CO, and GeH 4 were derived for the 1- to 6-bar portion of Jupiters troposphere using a spectrum synthesis program. Knowledge of the abundances of these gases below the visible clouds is necessary to calculate the global inventory of nitrogen, phosphorus, carbon, and deuterium, which, in turn, may constrain models of Jupiters formation. The N/H ratio is 1.5 ± 0.2 times the value for the Suns photosphere. The P/H ratio for the 5-bar level is between 1.0 and 1.6 times the solar abundance. The weak ν 3 − ν 4 hot band of CH 4 was detected for the first time on Jupiter, thus providing a deep atmospheric value for C/H of 3.6 ± 1.2 times solar. The Jovian deuterium abundance is comparable to that measured in the interstellar medium ( D / H = 1.2 ± 0.5) × 10 −5 . CO appears to be well mixed with a mole fraction of (1.0 ± 0.3) × 10 −9 . Multiple absorption features confirm that GeH 4 is present on Jupiter with a mole fraction of (7.0 −2.0 +4.0 ) × 10 −10 . The observed abundances of CO, GeH 4 , and PH 3 are consistent with models of convective transport from Jupiters deep atmosphere.

Spectroscopic evidence for undifferentiated S-type asteroids

The small solar system bodies broadly defined as asteroids have been considered the parent bodies for most or all meteorites. This association requires that the compositional differences between the various classes of meteorites be convincingly related to the observed compositions of asteroids. An investigation has been conducted regarding the compositional relationship between S-type asteroids and the common types of differentiated and undifferentiated meteorites. It is found that spectroscopic data for S-type asteroids are consistent with a simple, undifferentiated model and with certain variations of a differentiated model. However, the differentiated models for S-type asteroids are not supported by meteoritic evidence. It appears, therefore, that most S-type asteroids are undifferentiated bodies, similar to ordinary chondrites in composition.

Detection of water vapor on Jupiter

High-altitude (12.4 km) spectroscopic observations of Jupiter at 5 microns from the NASA 91.5 cm airborne infrared telescope have revealed 14 absorptions assigned to the rotation-vibration spectrum of water vapor. Preliminary analysis indicates a mixing ratio about 1 millionth for the vapor phase of water. Estimates of temperature (greater than about 300 K) and pressure (less than 20 atm) suggest observation of water deep in Jupiters hot spots responsible for its 5 micron flux. Model-atmosphere calculations based on radiative-transfer theory may change these initial estimates and provide a better physical picture of Jupiters atmosphere below the visible cloud tops.