the atmosphere The atmosphere of Titan is optically thick in both visible and near infrared wavelengths; the atmosphere only begins to clear out in the neighborhood of 1500 nm where the optical depth falls to 0.5. The main source of this opacity is the atmospheric tholin haze which consists of fractal-like agglomerations of several hundred photolyzed organic monomers with a radius of 0.05 microns. Between 30 and 150 km altitude, the number density of these particles was approximately constant at several tens of particles per cm 3. The ground, which became visible through the haze at~42 km altitude, dominates the radiative transfer properties of the atmosphere below 20 km. There are hints of finer structure in this crossover region. A thin cloud-like layer is visible in images near 21 km altitude and can be seen in atmospheric “strip data” as a 2% intensity feature relative to the cross-section of the SLI profile. Even so, there is evidence of tholin haze all the way to the ground in contrast to the clear space that had been expected in the troposphere. There is a reversal in wind direction near the surface. the surface At first glance, the surface looks incredibly earth-like showing landforms reminiscent of shorelines. The surface temperature of Titan is 94K (-179°C). Therefore, in this environment the water ice detected in the Downward Looking Visible and Infrared Spectrometers (DLVS and DLIS) behaves rheologically like rock on earth. Close to the surface, the DISR instrument used a lamp to illuminate the ground to obtain a higher signal to noise ratio for the collection of surface spectra and the methane mixing ratio. The best fit for the ground spectra was found to be mixed tholins and water ice. Gaseous methane was detected just above the ground with a mixing ratio of about 5% which corresponds to a “relative humidity” of about 48%. Huygens landed in a low, dark region bordering white highlands reminiscent of a paleoshoreline. At the landing site, the ice “rocks” are rounded, suggestive of their transport and erosion by fluid flow. The surface around the landing site appears to be covered by gravel-sized ice with some component of methane due to the high mixing ratio in the atmosphere above. J.E. Moores, M.G. Tomasko, P.H. Smith, B. Bezard, M. Combes, A. Coustenis, C. DeBergh, L. Doose, F. Gleim, U. Keller, E. Lellouch, B. Rizk, L. Soderblom, N. Thomas, R. West [ correspondance: abstract On January 14, 2005 the Huygens probe descended into the atmosphere of Titan, the largest moon of Saturn. The joint ESA-NASA mission had been dispatched to follow up on images obtained by the Voyager spacecraft in November of Now, almost 25 years later, the first high-resolution views of the surface have been revealed using the Descent Imager/Spectral Radiometer (DISR). The Titanian terrain is characterized by brighter upland regions next to darker low-lying plains. This upland terrain is crossed by numerous dendritic channels, suggestive of fluid flow perhaps due to precipitation of methane from the atmosphere or from liquid methane springs. The DISR experiment also carried a powerful instrument package for analyzing the atmosphere including upward and downward looking visible and infrared spectrometers, a solar aureole imager and a sun sensor which was designed to track the azimuth of the spinning probe through the descent. From the data collected we can conclude that the thick (1.5 bars at the surface) atmosphere has an optical depth of near 6 at 530 nm decreasing to 2 at 900 nm mainly due to the scattering and absorption of incident radiation by fractal- like tholin particles probably produced by photochemistry in the high atmosphere. Nearer to the surface we find evidence for a possible cloud-like layer between 20 and 30km of altitude. titan facts Largest satellite of Saturn Numerous detatched haze layers in the high atmosphere Highly polarizing 1.5 bar N 2 atmosphere 5 th largest rocky body in the solar system (between ganymede and mercury) mission facts CASSINI-HUYGENS launch: October 15, 1997 aboard a Titan IV-B/Centaur At 5.9 Tones, the largest interplanetary spacecraft ever built Huygens probe is the first craft to land on an outer solar system body Huygens carries 6 instruments: DISR – Descent Imager/Spectral Radiometer: Measures the spectra of the atmosphere and surface GCMS – Gas Chromatograph Mass Spectrometer: Measures abundances of chemical species and isotopes in the bulk atmosphere ACP – Aerosol Collector and Pyrolizer: Determines chemical composition of aerosols DWE – Doppler Wind Experiment: Radio sounding of the atmosphere (No data due to loss of channel A) HASI – Huygens Atmosphere Structure Instrument: Physical and electrical properties of the atmosphere, includes a microphone SSP – Surface Science package: Determines physical properties of the surface using a diverse suite of instruments Separation from Cassini orbiter on December 25, 2004 Atmospheric entry and descent on January 14, 2005 Probe carrier signal was picked up on earth for over 3 hours after entry instrument facts DISR is comprised of 11 separate instruments including 5 spectrometers and 4 imagers feeding into one CCD. It is one of the most complex instruments ever flown on a spacecraft SPECTROMETERS: ULVS+DLVS – Upward and dow nward looking visible spectrometers DLIS+ULIS – Upward and downward looking infrared spectrometers UVP – Ultraviolet photometer SUN SENSOR – Photometer to determine sun orientation and optical depth IMAGERS: SLI – Side looking imager MRI – Medium Resolution Imager HRI – High Resolution Imager Solar Aureole Imager SURFACE SCIENCE LAMP Built by the University of Arizona / Lockheed Martin the descent Prior to atmospheric entry, the probe was awakened and performed a successful aerobreaking maneuver before deploying a series of parachutes to slow its 8875 second descent to the surface. Unfortunately, this descent was not without problems. A software error prevented one of the two communications channels from functioning onboard the Cassini orbiter. This failure affected the DISR experiment more severely then other systems since imaging data was not sent redundantly on both channels. As a result, of the 1215 images captured and transmitted by the DISR imaging system, only 606 were received on Earth. Additionally, the motions of the probe in the upper atmosphere were more severe than expected. As a result, the sun sensor had a difficult time locking onto the sun which made determining azimuths of images difficult. This was exacerbated by the rotation of the spacecraft which reversed direction early on in the mission. As a result, a shadow bar meant to protect the solar aureole imager from the direct solar beam was less effective then planned. Despite these limitations, valuable data were obtained all the way down through the atmosphere. We also have the added bonus of a successful landing which allowed the first pictures to be taken from the surface of Titan. The Huygens probe descends through the thick hazy atmosphere of Titan [Artwork courtesy of ESA] The Huygens probe on the surface of Titan [ Artwork courtesy ESA ] Detached haze layers in the upper atmosphere of Titan [Cassini Imaging team] Figure Box 1: Changing conditions during descent [ A ] A Profile of Altitude vs Mission Time [ B ] Rotation Rate vs. Mission Time – a critical parameter that defines the coverage of DISR [ C ] Changing upward irradiance – affects the number of photons the detector has to work with Figure Box 2: Bulk atmospheric properties detected by DISR. [ A ] Optical depth vs. altitude [ B ] Wind speed and direction during descent (derived from ground tracks) [ C ] Cloud deck discovered at 21km altitude – a 1/1000 optical depth feature demonstrates the sensitivity of DISR