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NASA's James Webb Space Telescope:
C. A . Nixon (N ASA) R.K. Achterberg (UMD) M. аdАmkovics (UCB) B. BИz ard (Obs Paris) G. L. Bjoraker (NASA) T. Cornet (ESA) A . Hayes (Cornell) E. Lellouch (Obs Paris) M.T. Lemmon (UofA) S. Rodriguez (Paris-Diderot) C. Sotin (JPL) N. A . Teanby (Bristol) E. Tur tle (APL) R. A . West (JPL)

Obser vations of Titan
Af ter the end of the Cassini mission in 2017, the James Webb Space Telescope (JWST ) beginning in 2018 will provide an impor tant tool for keeping track of the changing seasons on Saturn's moon, Titan. As Saturn slowly circles the Sun, Titan undergoes a 29.5 (Ear th) year annual cycle, and will be in nor thern summer when JWST begins obser vations. Spectral imaging using the integral field units (IFUs) of NIRSpec and MIRI, and targeted filter imaging with NIRCam will allow long-term monitoring of the changing spatial distributions of gases, clouds and ha zes, and thereby reveal the interplay of chemistr y and dynamics in response to the seasonal cycle.

Example Titan Science Investigations:
· Atmospheric Composition: With a near-IR spectral resolution ~10 в higher than Cassini VIMS, NIRSpec can investigate the spatial distribution of tropospheric methane, while MIRI can make a detailed sur vey of stratospheric gases in the mid-IR (Fig. 1) · Clouds: A cloud monitoring campaign at selected near-IR wavelengths will build on the long-term database of clouds tracked by Cassini and groundbased obser vatories (see overleaf). · Hazes: NIRSpec in IFU mode can spectrally image (R~2700, 0.1'') Titan over seasonal time scales, allowing monitoring of seasonal changes in ha ze distribution. · Sur face Temperatures: At 19 m Tit an's atmosphere is mostly transparent, allowing MIRI to me asure the disk-average sur face temperature over time. · Sur face changes: JWST NIRSpec and NIRCam can see Titan's sur face in methane-free spectral `windows', allowing monitoring of sur face albedo changes due to rainfall, geologic activit y or sea shrinkage.

Figure 1: JWST will probe Titan's complex chemistr y. Photochemistr y of N2 and CH4 leads to the production of hydrocarbons and other organic molecules and haze par ticles. JWST will measure these both in the atmosphere and sur face deposits. Credit: ESA/ATG medialab.


Figure 2: Separation of terrain t ypes with NIRSpec. The spectral diversit y of terrains on Titan as recorded by the Cassini VIMS instrument (thin lines), compared to brightness saturation limits for various resolutions, single-frame read-out, of NIRSpec in integral field mode (thick lines).

Figure 4: Resolving Titan's disk with NIRSpec IFU imaging. The t ypical angular size of Titan (0.8" ) is compared to the NIRSpec resolution at 2 m (0.1" ), showing that approximately 8в8 pixels are obtained on the solid body disk. The visible atmosphere adds another ~500 km (1 pixel) in size. Image shows par t of the IFU 3"x3" field (30x30 pixels). Figure 3: Sur face and cloud imaging with NIRCam. Predicted appearance of Titan in various filters. Some shor ter wavelength filters (e.g. 0.9 m) will saturate at the single-frame readout time. Longer wavelength filters will allow detection of sur face features and seasonal monitoring of the atmosphere (gas, clouds and haze). Credits: Fig.s 2 & 3. produced by S. Rodriguez, UniversitИ de Paris-Diderot, for the JWST Titan Focus Group. Fig. 4 from Norwood et al., "Solar System Observations with JWST", 2014.

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