The helium abundance of Neptune from Voyager measurements

Abstract

Voyager radio occultation and infrared spectrometer measurements are used to obtain an estimate of the helium abundance in the atmosphere of Neptune. It is found that the shape of the measured spectrum cannot be well matched by spectra calculated from atmospheric models that include only gaseous opacity. The most plausible explanation of the observed spectral shape appears to require the existence of an additional opacity source associated with clouds or hazes. The data can be fit with either a tropospheric or a stratospheric cloud model. If a tropospheric cloud is invoked, an optical thickness at 200 cm−1 between 1 and 8 is required, depending on the particle size assumed. For a stratospheric cloud an optical thickness between 0.2 and 0.8 is required. Optical constants for methane ice are used, but ethane has similar optical properties in this spectral region. Assuming no modification to the temperature profile is required other than a molecular weight adjustment, a combination of gas and cloud opacities produces a good fit to the spectrum for a helium mole fraction qHe = 0.190 ± 0.032, corresponding to a mass fraction Y = 0.32 ± 0.05. Within the measurement uncertainties the Neptune helium abundance may be compatible with that of Uranus and, marginally, with the protosolar value. However, models in which the outer part of the primitive solar nebular was enriched in CO and N2, leading to an enhanced atmospheric He/H2 ratio, cannot be ruled out at this time.

The Planets and Titan Observed by ISO

“…The derived helium abundance, 14.9 +1.7 −2.2 %, confirms, with smaller error bars, previous determination obtained from the combined analysis of Voyager-2 IRIS and radio occultation experiment (Conrath et al, 1991). This result is consistent with the protoplanetary estimate of the helium abundance, as expected from the internal models of Neptune which predict no differenciation (Marley, 1999).…”

EPIC Simulations of Bright Companions to Neptune's Great Dark Spots

“…Figure 7 of Conrath et al (1991) shows a weak dependence in synthetic spectra of methane cloud models in the 220 to 325 cm −1 wavenumber range where the atmosphere is tranparent enough to allow emission from the underlying clouds to contribute to the outgoing radiation field. They applied the method to Voyager IRIS spectra covering latitudes near 39-45 • S and found that radii fall between 0.3 and 30.0 µm.…”

“…2 (digitized from Fig. 1 of Conrath et al 1991). To increase grid resolution, we reduce the physical domain size in all three directions while maintaining about the same number of grid points (Table I), and the layers are spaced proportionally to log θ instead of log p, which yields three layers (vertical index k = 6, 7, and 8 in Fig.…”

Detection of the Methyl Radical on Neptune

“…to model opacity are described in et al (1998). CH 3 Be zard The mean stratospheric molecular weight adopted in our modeling is 2.39, corresponding to a He mixing ratio of 0.19 (Conrath et al 1991). Spectral radiances were calculated for disk-averaged conditions and converted to Ñuxes using a value of 3.87 arcsec2 for NeptuneÏs solid angle.…”