Análisis del curso 1962-1963.

HASI studied the atmospheric structure of Titan and gave a detailed temperature vertical profile and recorded the surface pressure and temperature (1,467 1 mbar and 93.65

0.25 K, respectively). HASI temperature and pressure sensors probed the atmosphere directly for the first time from an altitude of 1,400 km down to the surface. Indeed, the P, T sensors were deployed after the shield release and sampled the local atmosphere. These parameters help to calibrate measurements from other instruments both on the probe and from the orbiter. The derived HASI temperature vertical profile (Fig. 2.14) is used for instance as a reference when adopting inverse methods to retrieve the temperature and abundance from CIRS data. Following Voyager, HASI found Titan's atmosphere to be a terrestrial analogue in terms of thermal structure consisting of layers of: an exosphere, a mesosphere, a stratosphere and a troposphere, with two major temperature inversions at 40 and 250 km, corresponding to the tropopause (temperatures of 70 K –minimum) and stratopause (temperatures of 186 K - maximum) (Fulchignoni et al. 2005).

Fig. 2.14 - Titan's temperature profile as derived from Huygens/HASI measurements. The several inversions in the upper atmosphere may be due to gravity waves (Fulchignoni et al. 2005).

In addition, gravity waves signatures of 10–20 K in amplitude were recorded above 500 km around an average temperature of 170 K (Fig. 2.14). These temperatures are higher than predicted by the models. In the lower atmosphere, below about 200 km, all current

 

measurements on Titan agree with the Voyager 1 profile (Coustenis et al. 2007).

Another Huygens instrument that investigated the lower layer of the atmosphere but also the surface was the DISR. DISR acquired spectra and high-resolution images of Titan's atmosphere while also measuring the solar radiation in the atmosphere. Except for the imagers, a visible spectrometer, an IR spectrometer, a solar aureole camera, violet photometers, and a sun sensor are parts of the instrument. DISR also had a 20 W lamp (surface science lamp), which switched on during the last stages of the probe's descent (at 700 m altitude) in order to enlighten the surface beneath it. It also revealed traces of hydrocarbon liquids on its surface through complex drainage systems and sent back to the Earth the first images of Titan's surface (Tomasko et al. 1997; 2005).

During the descent of Huygens into Titan’s surface, DISR measured the haze properties of the lower atmosphere (Tomasko et al. 2005) finding the monomer radius to be 0.05 mm; similar to previous estimations. Although some measurements were not in agreement with previous assumptions, such as the haze optical depth that showed larger range from 2 at 935 nm to 4.5 at 531 nm (Tomasko et al. 2009).

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Fig. 2.15 - Mole fractions of CH4, C2H6, C2H2, C2N2, and CO2 with respect to the descent time. The time of

surface impact is indicated by a vertical dashed red line (Image and information credit: Niemann et al. 2010).

In addition, the DISR spectral measurements of the methane mole fraction in the stratosphere were in agreement with CIRS and GCMS measurements of 1.4-1.6% while at altitudes close to 20 m DISR measured it at 5±1% (Tomasko et al. 2005; Lorenz et al. 2006b). Niemann et al. (2010) re-examined the GCMS data and suggested the methane mole fraction to be (1.48 ± 0.09) × 10−2 in the lower stratosphere (139.8–75.5 km) and (5.65 ± 0.18) × 10−2 near the surface (6.7 km to the surface). Additionally, the molecular hydrogen mole fraction was found to be (1.01 ± 0.16) × 10−3 in the atmosphere and (9.90 ± 0.17) × 10−4 on the surface. Isotope ratios were 167.7 ± 0.6 for 14

12

C/13C in methane, and (1.35 ± 0.30) × 10−4 for D/H in molecular hydrogen. The mole fractions of 36Ar and radiogenic 40Ar are found to be (2.1 ± 0.8) × 10−7 and (3.39 ± 0.12) × 10−5, respectively. 22Ne has been tentatively identified at a mole fraction of (2.8 ± 2.1) × 10−7 (Fig. 2.15).

These results have significant implications for formation and evolution models of Titan, as discussed in later Chapters, but also demonstrate also from chromatography and spectral and mass spectroscopy the complexity of Titan’s surface nature and composition.

As presented in Chapter 1, the Cassini-Huygens mission enhanced significantly our knowledge with respect to Titan’s methane cycle and its role in the satellite’s atmosphere. The DISR measurements suggested for the surface a relative humidity of methane at about 45% and in addition to GCMS evidence for evaporation (methane, ethane, acetylene, cyanogen and carbon dioxide) indicated the presence of fluid flows on the surface through rainfall of methane from the atmosphere. Moreover, the DISR observations unveil some surface characteristics, as I will discuss in detail in section 4.1.1, on Chapter 4 where we use it in our radiative transfer simulations.

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Chapter 3

DATA description

This chapter introduces the data in use during my PhD researches. In particular, I have processed Cassini VIMS and RADAR data from several flybys while the Radiative transfer tool I apply for the VIMS data analysis (introduced in Chapter 5) makes use of parameters adapted from Huygens’ DISR, HASI and GCMS, which I do not introduce here and the reader is referred to the paper by Hirtzig et al. (2013) for further details. Of course, I also take into account results from the Cassini cameras and other instruments (with access to the interior for instance) in trying to build a coherent picture of Titan’s atmosphere-surface- interior exchanges.

3.1 Cassini Flybys and observations in relation to the surface

In the previous Chapter 2, I discussed the execution of flybys of Titan and Enceladus by Cassini while it tours the Saturnian system. Table 3.1 lists the so far completed (by July 2013) Cassini flybys of Titan and Enceladus and the operations by its various instruments (in color are the ones related to the surface) starting with the T0 flyby, which was executed right after the Saturn Orbit Insertion.

Table 3.1 - Cassini Titan flybys16 over Titan and Enceladus. The list includes the coded name of the flyby (‘T’ for Titan and ‘E’ for Enceladus), the date of execution and the instrument(s) in operation. The flybys in color correspond to the ones dedicated mainly to the surface investigation.

Year Flyby Date Target Instrument in operation

2004 T0 03 July 2004 Titan VIMS

TA 26 October 2004 Titan VIMS TB 13 December 2004 Titan UVIS TC 14 January 2005 Titan

2005 _{T3 15 February 2005 Titan RSS}

E01 15 February 2005 Enceladus E02 09 March 2005 Enceladus

T4 31 March 2005 Titan ISS/CIRS - 31 March 2005 Enceladus

T5 16 April 2005 Titan INMS/MAPS T6 22 August 2005 Titan CIRS T7 07 September 2005 Titan RADAR T8 28 October 2005 Titan RADAR T9 26 December 2005 Titan UVIS

2006 _{T10 15 January 2006} Titan _UVIS

T11 27 February 2006 Titan RSS T12 18 March 2006 Titan RADAR T13 30 April 2006 Titan UVIS/RADAR T14 20 May 2006 Titan RSS T15 02 July 2006 Titan MAPS T16 22 July 2006 Titan RADAR/UVIS T17 07 September 2006 Titan INMS/VIMS

- 09 September 2006 Enceladus

T18 23 September 2006 Titan INMS/RADAR T19 09 October 2006 Titan RADAR T20 25 October 2006 Titan VIMS

- 09 November 2006 Enceladus

T21 12 December 2006 Titan INMS/RADAR T22 28 December 2006 Titan RSS

2007 _{T23 13 January 2007} Titan _RADAR

T24 29 January 2007 Titan VIMS

16 _{http://saturn.jpl.nasa.gov/mission/flybys/}

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Year Flyby Date Target Instrument in operation

T25 22 February 2007 Titan RADAR T26 10 March 2007 Titan INMS T27 26 March 2007 Titan RSS T28 10 April 2007 Titan RADAR T29 26 April 2007 Titan RADAR T30 12 May 2007 Titan RADAR T31 28 May 2007 Titan RSS T32 13 June 2007 Titan ISS T33 29 June 2007 Titan RSS T34 19 July 2007 Titan ISS T35 31 August 2007 Titan VIMS T36 02 October 2007 Titan INMS/RADAR T37 19 November 2007 Titan INMS/VIMS T38 05 December 2007 Titan RSS T39 20 December 2007 Titan RADAR

2008 _{T40 05 January 2008} Titan _UVIS/VIMS

T41 22 February 2008 Titan RADAR E03 12 March 2008 Enceladus

T42 25 March 2008 Titan INMS T43 12 May 2008 Titan RADAR T44 28 May 2008 Titan RADAR

Cassini Equinox Mission

T45 31 July 2008 Titan RSS E04 11 August 2008 Enceladus

E05 09 October 2008 Enceladus E06 31 October 2008 Enceladus

T46 03 November 2008 Titan RSS T47 19 November 2008 Titan VIMS T48 05 December 2008 Titan RADAR/INMS T49 21 December 2008 Titan RADAR

2009 _{T50 07 February 2009} Titan _RADAR/INMS

T51 27 March 2009 Titan VIMS/ISS T52 04 April 2009 Titan RSS/ISS T53 20 April 2009 Titan CIRS T54 05 May 2009 Titan ISS/VIMS T55 21 May 2009 Titan RADAR T56 06 June 2009 Titan INMS/RADAR T57 22 June 2009 Titan RADAR/INMS T58 08 July 2009 Titan UVIS/RADAR T59 24 July 2009 Titan CAPS T60 09 August 2009 Titan RADAR/ISS T61 25 August 2009 Titan RADAR/MAPS T62 12 October 2009 Titan VIMS/UVIS/CIRS E07 02 November 2000 Enceladus

E08 21 November 2009 Enceladus

Year Flyby Date Target Instrument in operation

T64 28 December 2009 Titan RADAR

2010 _{T65 12 January 2010} Titan _RADAR/INMS

T66 28 January 2010 Titan VIMS/ISS T67 05 April 2010 Titan CIRS E09 28 April 2010 Enceladus RSS E10 18 May 2010 Enceladus UVIS T68 20 May 2010 Titan CIRS T69 05 June 2010 Titan VIMS T70 21 June 2010 Titan MAG T71 07 July 2010 Titan INMS/RADAR/CAPS E11 13 August 2010 Enceladus

T72 24 September 2010 Titan VIMS T73 11 November 2010 Titan VIMS/CAPS E12 30 November 2010 Enceladus RSS E13 21 December 2010 Enceladus

2011 _{T74 18 February 2011} Titan _{RSS/RADAR/CAPS}

T75 19 April 2011 Titan RPWS/CAPS T76 8 May 2011 Titan VIMS/UVIS/CIRS/MAG T77 20 June 2011 Titan RADAR T78 12 September 2011 Titan RSS/UVIS/CAPS E14 01 October 2011 Enceladus

E15 19 October 2011 Enceladus

E16 06 November 2011 Enceladus RADAR/CIRS T79 13 December 2011 Titan CAPS

2012 _{T80 2 January 2012} Titan _{ISS/CIRS/VIMS/RPWS}

T81 30 January 2012 Titan ISS/CIRS T82 19 February 2012 Titan CIRS/VIMS E17 27 March 2012 Enceladus INMS E18 14 April 2012 Enceladus INMS E19 02 May 2012 Enceladus RSS T83 22 May 2012 Titan RADAR/MAG T84 7 June 2012 Titan RADAR/ISS T85 24 July 2012 Titan VIMS/MAG T86 26 September 2012 Titan INMS/UVIS T87 13 November 2012 Titan UVIS/INMS/VIMS/ISS T88 29 November 2012 Titan CIRS/VIMS/ISS

2013 T89 17 February 2013 Titan RSS

T90 05 April 2013 Titan CIRS/VIMS T91 23 May 2013 Titan RADAR T92 10 June 2013 Titan RADAR/ISS/CIRS T93 26 July 2013 Titan VIMS/ISS

From Table 3.1 we can note the extensive use of the instruments dedicated to the surface research such as VIMS, RADAR and ISS, which operate during most of the flybys.

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