Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..649y&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Physics
Scientific paper
The Cassini observations reveal the presence of a detached haze layer in Titan's mesosphere at an altitude of 520 km, well above the stratosphere. Observations of scattered light made by the Imaging Science Subsystem (ISS) [1] reveal a clearly defined layer encircling low and mid-latitude regions. The aerosol layer is also detected in stellar occultation measurements of UV extinction by the UltraViolet Imaging Spectrometer (UVIS) [2,3]. The haze is a global and permanent feature of Titan's atmosphere. Furthermore the location of the detached haze layer is coincident with and the likely cause of a local maximum in the temperature profile measured by the Huygens Atmospheric Structure Instrument (HASI) [4]. This temperature inversion is also permanent and global, having been detected in several ground-based stellar occultations [5]. The correlation between the extinction profile and temperature profile imply that the detached haze cannot be due to condensation, as previously suggested [3]. Analysis of the observed optical properties (Fig. 2) implies that the average size of particles in the detached layer is <45 nm, with an imaginary index < 0.3 at 187.5 nm. Using the solutions shown in Fig. 2 we calculate the imaginary index, number density, sedimentation velocity, and mass flux for the detached haze. The results are shown in Fig. 3. Non-LTE calculations of the temperature perturbation due to the detached haze show that the average size of the haze particles must be greater than 35 nm or implied heating rates are far too large (Fig. 4). Calculation of a suite of thermal structure models as a function of assumed particle size show that the observed temperature rise implies a mean particle radius greater than 35 nm. Thus, we conclude that the particle radius in the detached layer must be in the 35- 45 nm range. Consideration of sources for the haze also implies larger particle sizes. As shown in Fig. 3, a mean particle radius less than 35 nm implies a mass flux greater than 3.2×10-14 g cm-2 s-1. Only the solar flux below 145 nm is absorbed above 500 km and could therefore contribute to the detached haze layer. Absorption of longer wavelength solar radiation, which contributes to the photocatalytic destruction of CH4 and the formation of C2H6, occurs far too deep in the atmosphere to be a factor. The column-integrated photoabsorption rate at wavelengths less than 145 nm corresponds to a globally-averaged mass production rate of 9×10-14 g cm-2 s-1. This value is a factor of ~3-5 times larger than our estimate of the mass flux in the detached layer. This implies that thermospheric chemistry could account for most of the haze in Titan's atmosphere as long as the efficiency is tens of percent. It also follows that particles sizes much smaller than 35 nm are not realistic, because the required efficiency for aerosol production from photochemistry is far too large. It is striking that the particle size of 35-45 nm in the detached layer is approximately equal to the radii of ~50 nm for the monomers in the aggregate aerosols that populate the main layer [6]. In addition, the mass flux of 1.9-3.2×10-14 g cm-2 s-1 derived here is approximately equal to the mass of 0.5-2.0×10-14 g cm-2 s-1 [7] derived for the main layer. This suggests that the main layer is formed by coagulation and sedimentation of the particles in the detached haze layer. It follows that aerosols on Titan are formed primarily in the thermosphere, rather than the stratosphere as assumed in many pre-Cassini studies. This is consistent with the detection of negatively charged aerosols in the thermosphere by Cassini/CAPS [8]. It suggests that further investigations into the formation of haze should concentrate on the high energy radical and ion chemistry in the thermosphere. These conclusions are supported by microphysical aerosol models that couple the detached haze layer and the main haze layer and extend into the thermosphere, to be presented in the accompanying talk [9]. These models also explain the minimum in extinction near 500 km that defines the detached layer. The results presented here are described in more detail in [10].
Lavvas Panayiotis
Vuitton Veronique
Yelle Roger V.
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