AerosolsopticalpropertiesinTitan’s detachedhazelayerbeforetheequinox B. Seignovert1, P. Rannou1, P. Lavvas1, T. Cours1 and R.A. West2,1 1GSMA, UMR CNRS 6089, Université de Reims Champagne-Ardenne, France, 2JPL, California Institute of Technology, Pasadena, California, USA Contact: univ-reims@seignovert.fr Introduction First observed in the 80’s by Voyager 1 and 2 ﬂybys [1, 2], Titan’s atmosphere presents a complex stratiﬁed succession of haze layers above the main haze. One of these layers, the Detached Haze Layer (DHL), presented a large extent at 350 km and could be seen all along the limb surrounding Titan between 90◦S up to 45◦N. Voyager 2 radial intensity scans at high phase angles [3] reveal an important depletion of the aerosol particle density below this layer. The arrivial of the Cassini spacecraft in the Saturnian System in 2004, was a unique opportunity to investigate the persistence of the DHL over time [4]. The Imaging Science Subsystem (ISS) instrument on board performs a continuous survey of the Titan’s atmosphere. In 2011, West et al. [5] conﬁrm the persistence in time of the DHL at the equator at 500 km before the equinox followed by a collapse at 380 km in 2008. Recent observations conﬁrmed its disappearance in 2012 and a new emergence is reported in early 2016 [6]. This seasonal cycle, predicted by previous Global Circulation Models (GCM) [7, 8, 9], tended to be conﬁrmed by theses recent observations. In this study our purpose is to characterize the physical properties of the DHL and its latitudinal variability during the 2005-2007 period. Observations To determine the properties of the aerosols in the DHL, we made a survey over the ISS images taken with the Nar- row Angle Camera (NAC) before its collapse in 2008 [5]. We limit our analysis on the CL1-UV3 ﬁlter (λ=338 nm) where only the aerosols scatter and to minimize the mul- tiple scattering. Table 1: List of ISS NAC images used in CL1-UV3 ﬁlter ISS ID Date Phase Pix. Scale N1506288442 2005/09/24 145◦ 5.5 km N1506300441 2005/09/25 151◦ 5.5 km N1509304398 2005/10/29 155◦ 6.1 km N1521213736 2006/03/16 68◦ 7.3 km N1525327324 2006/05/03 147◦ 7.2 km N1540314950 2006/10/23 120◦ 5.7 km N1546223487 2006/12/31 66◦ 7.4 km N1551888681 2007/03/06 143◦ 7.9 km N1557908615 2007/05/15 25◦ 7.8 km N1557919415 2007/05/15 25◦ 8.2 km N1559282756 2007/05/31 20◦ 7.7 km N1562037403 2007/07/02 14◦ 7.8 km N1567440117 2007/09/02 20◦ 7.9 km N1570185840 2007/10/04 27◦ 7.3 km N1571476343 2007/10/19 80◦ 8.2 km Figure 1: (left) Overexposed and contrasted ISS NAC N1551888681. (right) Same image inverted with geo-reference grid, sub-spacecraft point (SC), Spice Kernels predicted center (SK) and photometric equator (PE). Vertical proﬁle, altitude and light curve For each image the limb proﬁles are extracted by 5◦bins in latitude on the illuminated side of Titan and smooth with a moving average. The local maximum I/F corresponding to the DHL is located in altitude and intensity. The DHL presents a very high stability in altitude (500±8 km) for all latitude lower than 45◦N. The I/F light curve content information about the shape of the aerosols. 400 420 440 460 480 500 520 540 0.005 0.02 0.03 0.05 0.01 Altitude (km) I/F () ISS data Moving average proﬁle Average box DHL peak Oct Jan 2006 Apr Jul Oct Jan 2007 Apr Jul Oct 90 ◦S 60 ◦S 30 ◦S 0 ◦ 30 ◦N 60 ◦N 450 465 465 480 495 495 495 495 495 510 510 510 510 450 460 470 480 490 500 510 DHL altitude (km) 0.001 0.01 0◦ 20◦ 40◦ 60◦ 80◦ 100◦ 120◦ 140◦ 160◦ 180◦ I/F () Phase angle () DHL peak Figure 2: (left) N1551888681 vertical proﬁle between 0 to 5◦N. (middle) Altitude of the DHL as function of time and latitude. (right) Light curve on the local maximum I/F between 0 and 5◦N. Atmospheric and scattering model We consider that above the DHL, the atmosphere of Titan is optically thin and the incoming ﬂux from the Sun is not signiﬁcantly attenuated down to the DHL, then the output ﬂux can be simply described as: I/F = ω · P(θ) 4 · 1 −e−τext with τext