Catherine de Bergh

Surface ices and the atmospheric composition of Pluto

Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.

Deuterium on Mars: The Abundance of HDO and the Value of D/H

Deuterium on Mars has been detected by the resolution of several Doppler-shifted lines ofHDO near 3.7 micrometers in the planets spectrum. The ratio of deuterium to hydrogen is (9 � 4) x 10-4; the abundance of H20 was derived from lines near 1.1 micrometers. This ratio is enriched on Mars over the teiluric value by a factor of6 � 3. The enrichment implies that hydrogen escaped more rapidly from Mars in the past than it does now, consistent with a dense and warm ancient atmosphere on the planet.

Ices on the Surface of Triton

The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Tritons surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.

Deuterium on Venus: Observations From Earth

Absorption lines of HDO and H2O have been detected in a 0.23-wave number resolution spectrum of the dark side of Venus in the interval 2.34 to 2.43 micrometers, where the atmosphere is sounded in the altitude range from 32 to 42 kilometers (8 to 3 bars). The resulting value of the deuterium-to-hydrogen ratio (D/H) is 120 � 40 times the telluric ratio, providing unequivocal confirmation of in situ Pioneer Venus mass spectrometer measurements that were in apparent conflict with an upper limit set from International Ultraviolet Explorer spectra. The 100-fold enrichment of the D/H ratio on Venus compared to Earth is thus a fundamental constraint on models for its atmospheric evolution.

Titan: Discovery of Carbon Monoxide in Its Atmosphere

The 3-0 rotation-vibration band of carbon monoxide in the near-infrared spectrum of Titan has been identified, and a reflecting layer model mixing ratio of carbon monoxide to molecular nitrogen of 6 x 10–5 has been determined. This result supports the probable detection of carbon dioxide by Samuelson and his co-workers and strengthens possible analogies between the atmosphere of Titan and conditions on primitive Earth.

Constraints on the Composition of Trojan Asteroid 624 Hektor

Abstract We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3–3.6 μm, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no discernible 3-μm absorption band. Such a band would indicate the presence of OH − or H 2 O-bearing silicate minerals, or macromolecular carbon-rich organic material of the kind seen on the low-albedo hemisphere of Saturns satellite Iapetus. The absence of spectral structure is itself indicative of the absence of the nitrogen-rich tholins (which show a distinctive absorption band attributed to N–H). The successful models in this study all incorporate magnesium-rich pyroxene (Mg, Fe SiO 3 ), which satisfactorily matches the red color of Hektor. Pyroxene is a mafic mineral common in terrestrial and lunar lavas, and is also identified in Main Belt asteroid spectra. An upper limit to the amount of crystalline H 2 O ice (30-μm grains) in the surface layer of Hektor accessible to near-infrared remote sensing observations is 3 wt%. The upper limit for serpentine, as a representative of hydrous silicates, is much less stringent, at 40%, based on the shape of the spectral region around 3 μm. Thus, the spectrum at 3 μm does not preclude the presence of a few weight percent of volatile material in the uppermost surface layer of Hektor. Below this “optical” surface that our observations probe, any amount of H 2 O ice and other volatile-rich materials might exist. All of the models we calculated require a very low-albedo, neutral color material to achieve the low geometric albedo that matches Hektor; we use elemental carbon. If elemental carbon is present on Hektor, it could be of organic or inorganic origin. By analogy, other D-type asteroids could achieve their red color, low albedo, and apparent absence of phyllosilicates from compositions similar to the models presented here. Our models appear to demonstrate that organic solids are not required to match the red color and low albedos of D-type asteroids.

The abundance of sulfur dioxide below the clouds of Venus

We present a new method for determining the abundance of sulfur dioxide below the clouds of Venus. Absorption by the 3ν3 band of SO2 near 2.45 µm has been detected in high-resolution spectra of the night side of Venus recorded at the Canada-France-Hawaii telescope in 1989 and 1991. The inferred SO2 abundance is 130±40 ppm at all observed locations and pertains to the 35–45 km region. These values are comparable to those measured by the Pioneer Venus and Venera 11/12 entry probes in 1978. This stability stands in contrast to the apparent massive decrease in SO2 observed at the cloud tops since these space missions. These results are consistent with laboratory and modelling studies of the SO2 destruction rates in the lower atmosphere of Venus. The new spectroscopic technique presented here allows a remote monitoring of the SO2 abundance below the clouds, a likely tracer of Venusian volcanism.

The region of the 3ν3 band of methane

Abstract The spectrum of methane near 9000 cm −1 , the region of the 3 ν 3 band, has been recorded at Meudon Observatory with a Fourier transform spectrometer under high resolution. Intensity measurements at two different temperatures, 149 and 295 K, have allowed us to identify two new vibration bands by determining the lower-state quantum numbers J of the transitions. About 100 lines are now assigned in this range, including P and Q branches. Furthermore, the first detailed rotational analysis of the 3 ν 3 band has been made; nine parameters of the band have been determined. The standard deviation of the differences between observed and computed wavenumbers for 45 lines of the 3 ν 3 band is only 0.045 cm −1 . It is found that the observed 45 lines of the 3 ν 3 band correspond to the sublevel l 3 = 3 and C v = F 2 .

The Surface Compositions of Triton, Pluto, and Charon

Neptune’s satellite Triton, and the planet-satellite binary Pluto and Charon, are the most distant planetary bodies on which ices have been directly detected. Triton and Pluto have very similar dimensions and mean densities, suggesting a similar or common origin. Through Earth-based spectroscopic observations in the near-infrared, solid N 2, CH 4, H 2 O, and CO have been found on both bodies, with the additional molecule CO 2 on Triton. N 2 dominates both surfaces, although the coverage is not spatially uniform. On Triton, the CH 2 and CO are mostly or entirely frozen in the N 2 matrix, while CO 2 may be spatially segregated. On Pluto, some CH 4 and the CO are frozen in the N 2 matrix, but there is evidence for additional CH 2 in a pure state, perhaps lying as a lag deposit on a subsurface layer of N 2. Despite their compositional and dimensional similarities, Pluto and Triton are quite different from one another in detail. Additional hydrocarbons and other volatile ices have been sought spectroscopically but have not yet been detected. The only molecule identified on Pluto’s satellite Charon is solid H 2 O, but the spectroscopic data are of low precision and admit the presence of other ices such as CH 4.

Deuterium enrichment in the primitive ices of the protosolar nebula.

We have estimated the D/H ratio that may have been present in the primitive ices in the protosolar nebula. Using observations of the CH3D/CH4 ratio in the outer planets, we developed two simple but limiting models which constrain the amount of dilution that deuterated volatiles which were contributed to the planetary atmospheres by evaporated primordial ices may have undergone by mixing with the original hydrogen envelopes. The models suggest that the D/H ratio in these ices was probably somewhere between a few times 10(-4) and 10(-3). These planetary-atmosphere-derived results are compared with other solar system bodies thought to contain primitive material and with D/H ratios observed in interstellar polyatomic molecules.