Ammonia Ice Cloud Structure at the Galileo Probe Entry Site

L.A. Sromovsky (U. of Wisconsin), M.T. Lemmon (U. of Arizona), A.D. Collard (U. of Wisconsin)

The Galileo Probe Net Flux Radiometer (NFR) measured in channel A (3 - 200 tex2html_wrap_inline17 m) a radiative heating above 600 mb consistent with a large-particle NH tex2html_wrap_inline19 cloud of optical depth 2 at 0.5 tex2html_wrap_inline17 m. Evidence for this cloud also appears in the large correlated variation of the NFR solar channels between deployment and 500-600 mb, an effect expected from probe spin-modulation of direct solar beam input at the edge of the NFR field of view. The way the modulations decayed with depth seemed to require an optical depth of 1.5-2 of cloud material extending down to the 500-600 mb level (Sromovsky et al., Science 272, 1996), in accord with the NH tex2html_wrap_inline19 ice cloud base inferred from remote observations (West et al., Icarus 65, 1986)). However, the low level of particulate scattering measured by the Nephelometer in this region (Ragent et al., Science 272, 1996) has been difficult to reconcile with this picture. Subsequently, a quantitative analysis of the much smaller solar modulations in channel A ( tex2html_wrap_inline25 m) has provided new insights: (1) the ratio of the Channel A modulation amplitude to that of Channel B (0.3-3.5 tex2html_wrap_inline17 m) is a factor of 4-5 larger than would be expected for an unmodified solar spectrum, strongly suggesting that the cloud particles are small enough (< 1 tex2html_wrap_inline17 m) to attenuate B much more than A; and (2) the channel B modulation amplitude is too large to allow that attenuation unless the direct beam view is enhanced by a tilt of the probe spin axis, perhaps by as much as 15-20 tex2html_wrap_inline31 . This scenario still requires a cloud optical depth of 1-2 to explain the A/B ratio, but that cloud need not reach the 600 mb level. The attenuation we initially attributed to accumulating cloud opacity during descent might instead be caused by a decay of the probe pendulum motion, thereby reducing the solar modulation amplitude without requiring local particulate opacity that was not seen by the Nephelometer.