Cassini Composite Infrared Spectrometer (CIRS) Observations of Titan 2004–2017

Saturn has an obliquity of 267 and an orbital period of 29.5 Earth yr, so it has seasons that are approximately 7.4 terrestrial yr in length. Titan orbits in Saturn's equatorial plane with a negligible axial tilt relative to its orbit, so has seasons of the same length as Saturn. When Cassini arrived at Saturn in 2004 July, the season was northern winter. Cassini was originally planned to have a prime mission (PM) from 2004–2008; eventually, this was extended to 2010, which encompassed Saturn's equinox in 2009. A second and final extension then continued the mission through 2017, thereby reaching Saturn's northern summer solstice that year (Figure 1). Finally, on 2017 September 15, the spacecraft exhausted all its fuel and was destroyed by a planned entry into Saturn to prevent the possibility of a later, uncontrolled impact with a moon. The long duration of this 13 yr mission thus enabled Cassini to experience almost two full seasons on Saturn and Titan, which has proved crucial for understanding the seasonal and even interannual change (by comparison to other data sets such as Voyager) in their atmospheres (Lockwood & Thompson 2009; Coustenis et al. 2013).

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During the mission, the spacecraft changed its orbital inclination relative to the Saturn ring plane (equatorial plane) continuously (Figure 2), so as to have equal opportunities to rendezvous with the moons (requiring low inclination) and to view the rings (requiring high inclination). Flybys of Titan were used as gravity-assist maneuvers, changing the spacecraft inclination while minimizing fuel expenditure. The effect on Titan observations was twofold: (i) frequent flyby opportunities and (ii) almost every flyby geometry was different, in terms of encounter range at closest approach and trajectory (subspacecraft track on Titan). This implied that each flyby had to be individually designed for unique science observations/instrument operations, and that the possible atmospheric and surface coverage was dictated by the particular orbital geometry.

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Flybys of Titan are divided into two categories: "targeted" encounters (r < 100,000 km) and "untargeted" or distant encounters (r > 100,000 km). All of these encounters may be identified by a Cassini orbit number; in addition, the targeted encounters are also given a flyby number in the format Tn—see data table in Appendix B. For example, the T6 flyby occurred on orbit 13 at a range of 3660 km, while the last encounter of the mission on orbit 292 was at a range of 119,733 km and therefore does not have a "T" number. Several exceptions to the naming convention must be noted. The very first, untargeted Titan encounter at a range of 339,123 km immediately following Saturn orbit insertion (SOI) is given the special designation "T0," on orbit 0. Immediately following T0, the first several orbits, originally containing T1 and T2, were redesigned to accommodate a more distant flyby for the Huygens probe data relay. This entailed adding an additional orbit; therefore, encounters T1 and T2 became TA, TB and TC, with the rest of the planned tour continuing using the already designated numbers from T3 onward.

Figure 3 shows a histogram of flyby ranges; approximately one-third of targeted flybys (41/127) were at ranges <1000 km, and a further one-third (39/127) occurred at ranges 1000–1500 km, still inside the atmosphere defined by the exobase at 1500 km (Yelle et al. 2008; Vuitton et al. 2019). Therefore, on 63% of Titan targeted flybys (those where r < 100,000 km) in situ measurements of the atmosphere were possible, as well as remote sensing on approach and departure. The remaining approximately one-third (47/127) of targeted flybys were at ranges 1500–100,000 km, along with 14 more distant encounters.

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A detailed description of the instrument is given in Jennings et al. (2017), while some key facts are given here that are most relevant to the Titan observation planning. The CIRS instrument was a dual spectrometer, which used a field-splitting beam splitter to direct the incoming light from a 50 cm diameter telescope into mid- and far-infrared spectrometers. These functioned in tandem, sharing a common mirror carriage mechanism that defined the spectral resolution through its distance of travel, from a lowest apodized resolution of 15.5 cm⁻¹ to a highest apodized resolution of 0.5 cm⁻¹. The lowest resolutions required the shortest movements (4.5 s), while the highest resolutions required the longest movements (52 s). Intermediate resolutions were possible, with the most common medium resolution being 2.75 cm⁻¹ (12 s). This created a trade-off: acquiring many low-resolution spectra was desirable in some circumstances—for example during observations of strong gas emissions such as methane—and enabled rapid repositioning for mapping purposes. High-resolution spectra required longer acquisition times, and therefore a substantial dwell time on source to build up significant a signal-to-noise ratio (S/N). This was desirable when measuring weaker gas emissions of less abundant species that required a higher resolution to isolate.

A second important consideration was the number and configuration of the pixels, as shown in Figure 4. The far-infrared focal plane, known as FP1, was a single large pixel similar to Voyager IRIS that was optimized for sensitivity to light from 10 to 600 cm⁻¹ (1000-17 μm). ¹³ The mid-infrared reception was very different from that of Voyager IRIS and used twin 1 × 10 mercury–cadmium–telluride arrays sensitive to 600–1100 cm⁻¹ (FP3, photoconductive-type detectors, 17–9 μm) and 1100–1400 cm⁻¹ (FP4, photovoltaic-type detectors, 9–7 μm). The optical boresights were closely aligned with the spacecraft −Y direction, while the mid-infrared arrays were aligned along the Z axis, and FP1, FP3, and FP4 were offset in the X direction (Nixon et al. 2009b). The implication was that the −Y direction was pointed at Titan for optical measurements, while rotating the spacecraft about the Z axis swept the mid-infrared arrays across the sky to perform "pushbroom" mapping. Subsequent offsetting in X permitted multiple, parallel sweeps.

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A further factor in observation design was detector readout. CIRS had 11 simultaneous readout channels: one for FP1, and five each for FP3 and FP4. This meant that typically only half of the mid-infrared detectors could be used at a time. Readout modes for the mid-infrared included odd detectors only (1, 3, 5, 7, 9 on each of FP3 and FP4), even detectors only (2, 4, 6, 8, 10 on each array), or center mode (4–8 on FP3 and 3–7 on FP4). A typical observation alternated back and forth between the even and odd readout modes on successive scans to allow for the fullest spatial sampling, known as "blink" mode. However, a "pair" mode was also available that utilized all 10 detectors on each array by reading them out in five pairs (1 + 2, 3 + 4, 5 + 6, 7 + 8, 9 + 10). Pair mode effectively created double-size detector pixels that may be harder to model in certain circumstances, but had the advantage of using the maximum possible amount of incoming flux—a $\sqrt{2}$ advantage over the other modes that was used to improve the S/N. A graphical summary is shown in Figure 5.

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A final note regarding the instrument is that frequent calibration data were required in addition to science observations. Only radiometric (flux) calibration was taken in flight, to enable the conversion from detector counts to physical radiance units, and comprised two types. The first was "dark sky" or "deep space" observations of the 2.73 K background (sky background avoiding planets, moons, the Sun, IR-bright stars, etc.), equivalent to zero radiance for the purpose of CIRS, and the second was a warm internal target (shutter) that was periodically emplaced into the beam path for the mid-infrared only (FP3 and FP4). ¹⁴ These flux calibration observations were made sometimes before, sometimes after, or occasionally interspersed within longer science observations; and sometimes while slewing the spacecraft to reach a target point. Later in the mission, the normal practice became to concentrate the calibration observations in dedicated blocks of time (normally 6–8 hr) during downlink of spacecraft data to Earth when the instrument was usually pointing at empty space, and previous practice of taking calibration data during science observations diminished. This new paradigm created longer, more homogeneous blocks of calibration data, at the expense of the calibration data being slightly more remote in time from the science observations that they would later be used to calibrate. When using CIRS data, care must be taken to sift out calibration observations from science data. For further details, see Jennings et al. (2017).

Science overview: The far-infrared limb temperature scan (FIRLMBT) observation was the closest observation to Titan, occurring at (5–15) × 10³ km (15–45 minutes from closest approach). At 30 minutes from closest approach, FP1 resolved ∼40 km on Titan's limb, or about 80% of an atmospheric scale height (∼50 km). The observation was designed to allow for several vertical profiles of temperature to be obtained via measurement of N₂–N₂ collision-induced absorption (CIA) or opacity at 50–150 cm⁻¹, focusing on pressure levels of 8–100 mbar in the lower stratosphere and upper troposphere (Flasar et al. 2004; Sylvestre et al. 2018)—see Figure 10.

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Implementation: The lowest spectral resolution of CIRS was used (15.5 cm⁻¹), which enabled a rapid spectrum acquisition time (5 s). A turn rate of ∼40 μrad s⁻¹ meant that the FOV moved by only 0.2 mrad, or 1/20 of a pixel, during a single spectrum. Therefore, at least 10 spectra can be coadded (2 mrad) without loss of spatial resolution, typically considered to be one-half of the detector size (i.e., Nyquist sampling). Each limb scan covered ∼28 mrad, taking about 11 minutes. Allowing for repositioning at the start and end of the scan, two scans were typically achieved in the nominal 30 minute window. The two scans were notionally positioned 10° apart in latitude, although this was not achievable if the flyby was at high inclination. Due to the customization of each Titan flyby through negotiation with other Cassini teams, the FIRLMBT observation was sometimes shorter or longer than 30 minutes, in which case the scan rate was adjusted accordingly (up or down). If the required scan rate exceeded 50 μrad s⁻¹, only one scan was implemented, and/or the observation was merged with the adjacent FIRLMBAER observation.

Science overview: The far-infrared aerosol scan (FIRLMBAER) was the second-closest observation to Titan, occurring at (15–25) × 10³ km (45–75 minutes from closest approach). Like FIRLMBT, this was also a limb scan observation designed to measure vertical profiles of aerosol opacity in the range 250–600 cm⁻¹. Due to the differing spectral dependence of CIA, aerosols, and clouds (condensates), the vertical profile can be isolated and measured (Teanby et al. 2009a; de Kok et al. 2010; Anderson & Samuelson 2011; Anderson et al. 2018). By considering multiple flybys, latitudinal and temporal variations of aerosol and condensates may be inferred (Jennings et al. 2012a, 2012b, 2015). FIRLMBAER data also provide important constraints for modeling nadir-viewing observations, where vertical information is more ambiguous.

Implementation: From 2004 to 2010, two scans separated by 5° on the horizon were implemented in the 30 minute window, covering a radial distance of 51 mrad, or about 1000 km from −100 to +900 km relative to the surface. The scan required was consequently rapid: ∼55 μrad s⁻¹. From 2010, the observation was redesigned to focus on altitudes −100 to +600 km, because the signal became too weak for detection at higher altitudes. A slower scan rate was also employed (∼28 μrad s⁻¹) to increase the S/N. Also, the number of scans was reduced from two to one (Figure 11), as it was found that similar aerosol information could be obtained from the FIRLMBT observations, and therefore, it became desirable to focus on high fidelity rather than greater spatial coverage.

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Science overview: The far-infrared limb integration constituted the third type of the original FIRLMB observation group, designed prior to orbit insertion. This observation type was implemented from 75 to 135 minutes from closest approach, or at a range of (25–45) × 10³ km. In contrast to FIRLMBT and FIRLMBAER, the FIRLMBINT was not a scan (slew), but rather a sit-and-stare observation (or integration) at a series of fixed pointings relative to Titan. The objective was to obtain measurements of trace gas concentrations at two altitudes to obtain a basic vertical gradient. In particular, measurements of the gases CO (30–70 cm⁻¹), C₂N₂ (233 cm⁻¹), and H₂O (∼150–250 cm⁻¹; de Kok et al. 2007b; Cottini et al. 2012b; Lellouch et al. 2014) were of interest, because they do not have spectral bands detectable by CIRS in the mid-infrared. However, C₃H₄ (328 cm⁻¹) and C₄H₂ (228 cm⁻¹) were also measured (Sylvestre et al. 2018), as well as a weak band of HC₃N at 499 cm⁻¹. FIRLMBINTs have also been used to characterize aerosols and condensates (ices; de Kok et al. 2007a, 2010; Samuelson et al. 2007; Anderson et al. 2010, 2014, 2016, 2018; Anderson & Samuelson 2011; Jolly et al. 2015).

Implementation: The FIRLMBINT was implemented as two fixed integrations at 125 and 225 km above the limb (later, a third, intermediate point at 175 km was added as a separate observation: see FIRLMBWTR). The highest spectral resolution of CIRS was used, 0.5 cm⁻¹, requiring 52 s acquisition times for a single spectrum. The observation proceeded by pointing for nominally 13 minutes (15 spectra) at 125 km, then 13 minutes at 225 km, followed by a repeat of the two positions. Due to the changing range from Titan, two shorter visits at each altitude (Figure 12) were preferred instead of one longer visit, ensuring that the spatial footprint at each altitude was not too dissimilar.

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Science overview: The far-infrared limb condensate integration (FIRLMBCON) was designed to address the gap in resolution between the high spectral resolution integrations (FIRLMBINT, 0.5 cm⁻¹), and the low-resolution scans (FIRLMBT and FIRLMBAER, 15 cm⁻¹). The lower resolution of the aerosol scans was insufficient to resolve condensate (ice) features in the spectrum, such as HC₃N at 506 cm⁻¹, while the high-resolution integrations had sufficient spectral resolution but an insufficient S/N and altitude information. Data from the FIRLMBCON observation has been used to infer the presence of C₄N₂ ice at 478 cm¹ (Anderson et al. 2016, 2018).

Implementation: The observation was implemented twice, on T67 and T118 (see Table 5), as a modified FIRLMBINT from 135 to 75 minutes from closest approach on the inbound approach of the flyby. The spectral resolution was set to 3.0 cm⁻¹, with three dwells at 125, 175, and 225 km. See Figure 13.

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Science overview: Water had previously been detected on Titan by the Infrared Space Observatory (Coustenis et al. 1998), which determined a disk-average abundance. The use of CIRS data, which averaged over multiple FIRLMBINTs to provide a simple vertical profile from abundances retrieved at 125 and 225 km, permitted the first measurement of water on Titan's limb (Cottini et al. 2012b). It was later suggested (S. Hörst 2012, private communication) that a third, intermediate data point at 175 km would help to better distinguish between photochemical model profiles.

Implementation: As with the FIRLMBCON, the FIRLMBWTR was performed as a modified FIRLMBINT in the same time/distance window of 75–135 minutes from closest approach. In this case, however, the entire 1 hr period was spent integrating at a single altitude of 175 km, intermediate to the usual two FIRLMBINT altitudes, at 0.5 cm⁻¹ resolution. Due to the very weak water emission, three 1 hr observations at low latitudes were scheduled on T100, T123, and T125 with the intention that these would later be combined to provide a single measurement at 175 km. See Figure 14.

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The coverage of CIRS far-infrared limb observations is shown in Figure 15. Observations (symbols) largely track the limb stationary nodes (points). These provide a huge improvement over the previous limb observations by Voyager 1 (Coustenis et al. 1991) both in latitude coverage and in time. While the latitude sampling over the entire mission is excellent, different latitudes are mostly sampled at different times, preventing a true global snapshot from being obtained at any one epoch. It is clear from the pattern where the inclined orbits occur (2008–2010 and 2013–2015), where limb viewing is restricted to low latitudes as the flybys took the spacecraft over the polar regions. The consequence is that there are some gaps in spatial and temporal coverage that hinder our attempts to understand the formation and breakup of the polar vortices.

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Science overview: The mid-infrared limb map (MIRLMBMAP) observation was designed to measure vertical profiles of temperature in Titan's stratosphere from ∼120–500 km, or 5.0 to 0.005 mbar, primarily by modeling/inversion of the ν₄ band emissions of CH₄ centered at 1304 cm⁻¹ (Achterberg et al. 2008a, 2008b, 2011; Teanby et al. 2012, 2017). MIRLMBMAPs have also been used to measure the vertical profile of the most abundant trace gases, such as HCN, C₂H₂, and HC₃N (Teanby et al. 2007), and to observe dynamical redistribution over Titan's changing seasons (Teanby et al. 2012; Vinatier et al. 2015).

Implementation: At 140,000 km range, the mid-infrared arrays 3 mrad in length had a projected size of ∼420 km. The arrays were positioned perpendicular to Titan's limb (+/−Z direction perpendicular to the edge of the disk). Two successive and overlapping pointing altitudes were used with the array centers at 100 km and then 350 km, which also allowed for pointing error by the spacecraft of up to 1 mrad (although in practice pointing accuracy was always better than 0.5 mrad.) If pointing was exact, the arrays covered altitudes −120 to +570 km over both positions. Dwells were performed at each altitude for ∼4 minutes using the fast acquisition, low spectral resolution mode (15 cm⁻¹), with the FP3/4 arrays "blinking" between odd and even detector readout on alternate spectra to allow for maximum possible vertical information. The arrays were then repositioned to a different limb location. This was notionally an increment of 5° in latitude, although as flyby inclination increased, the horizon circle unavoidably transitioned from latitude (most useful) to longitude (less useful). This may be understood by considering that when viewing Titan from the equatorial plane, the horizon circle includes all latitudes, while from a vantage point above either pole, the horizon circle is the equator, permitting only limb viewing of a single latitude (but multiple longitudes). Altogether, some 15–18 vertical profiles were typically obtained in a 4 hr observation window (see Figure 16).

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Science overview: The mid-infrared limb integrations were designed to measure a single vertical profile of trace gases from ∼100–500 km, including hydrocarbons, nitriles, CO₂, and other species using high spectral resolution (0.5 cm⁻¹). Many of these were detected on CIRS FP3 (600–1100 cm⁻¹). FP4 provided vertical temperature information at approximately the same location (although the two arrays were actually side by side, so the locations were not identical). MIRLMBINT data have resulted in numerous publications describing vertical and temporal mapping of trace gases (Teanby et al. 2007, 2008a, 2012, 2017; Vinatier et al. 2007a, 2010b, 2015, 2018; Nixon et al. 2009a; Lombardo et al. 2019b), aerosols (Vinatier et al. 2010a, 2012) and benzene ice (Vinatier et al. 2018). In addition, these data proved invaluable for new detections such as propene (Nixon et al. 2013a; Lombardo et al. 2019a) and many isotopologues of previously known gas species including H¹³CN and HC¹⁵N (Vinatier et al. 2007b), ¹³CH₄ and ¹³CH₃D (Bézard et al. 2007; Nixon et al. 2008a, 2012b), H¹³CCH and C₂HD (Coustenis et al. 2008; Nixon et al. 2008a), ¹³CH₃ ¹²CH₃ (Nixon et al. 2008a), H¹³CCCN (Jennings et al. 2008), ¹³CO₂ and CO¹⁸O (Nixon et al. 2008b), and H¹³CCCCH and HC¹³CCCH (Jolly et al. 2010).

Implementation: MIRLMBINT was similar to the MIRLMBMAP; however, only a single limb location (latitude) was observed, again at two altitudes together covering approximately −100 to +600 km. As with the complementary far-infrared limb integration (FIRLMBINT), the two positions were observed twice for ∼1 hr each to reduce the difference in projected array size at the two altitudes that would otherwise be incurred due to the spacecraft approaching/receding from Titan. See Figure 17.

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Science overview: While the mid-infrared limb integrations were successful in measuring vertical profiles of many known trace gases, CIRS scientists later wished to search more intensively for new, undetected gases and isotopes that may have even weaker signals undetectable in the MIRLMBINTs. As it was impossible to increase spectral resolution beyond the maximum (0.5 cm⁻¹), the other option was to increase the S/N by acquiring more spectra, including the use of the pair mode (see Section 2.2). Results from modeling of MIRLMBPAIR data to search for trace gases and measure isotopes are described in Nixon et al. (2010b, 2012b, 2013b)

Implementation: The solution adopted to increase the S/N was to position the arrays parallel to the disk edge, so that all pixels were close to the same altitude (actually, there was a small difference between the pixels at the array ends, which are farther from the horizon, and those at the center). Then all pixels from either FP3 or FP4 could be coadded into a single spectrum. The spectra were acquired at 0.5 cm⁻¹ resolution (52 s scans), and the pixels were read out in pair mode, doubling the effective number of spectra compared to the usual odd/even modes that only read out half the pixels at a time. The arrays were maintained at a single position throughout the observation, with the lower array (either FP3 or FP4) at a fixed altitude; see Figure 18. The observation was repeated on four occasions: twice at low latitude and twice at high latitude. At each latitude, there were two observations: one with FP3 at low altitude ("bottom") and FP4 above, and a second observation with the reverse configuration (summarized in Table 2).

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Figure 19 shows the spatial and temporal coverage of the mid-infrared limb observations during the mission. Of principal note is that the limb maps (MIRLMBMAP, blue bars) have a relatively complete coverage in latitude and season. However, as with the far-infrared limb observations, there are some gaps (e.g., late 2008 to early 2009, late 2010, mid 2014) where the highest northern and southern latitudes are not sampled due to the inclined spacecraft orbits. Limb integrations (MIRLMBINT) also exhibit this pattern, although overall there is repeat coverage of low, medium, and high latitudes in each hemisphere during the mission, providing an excellent reference data set for understanding atmospheric circulation and composition.

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Science overview: The far-infrared nadir map observation was designed primarily to measure the temperature of Titan's surface using a spectral window at ∼530 cm⁻¹ (19 μm) where the opacity of both aerosols and collision-induced gas absorption is low (Jennings et al. 2009, 2011, 2016; Cottini et al. 2012a). However, these observations have also been used to measure the spatial variation of condensates (Jennings et al. 2012a, 2015; see also FIRLMBAER). Tropospheric temperatures may also be obtained from the N₂–N₂ CIA region at 50–150 cm⁻¹ (Lellouch et al. 2014), and the N₂–H₂ CIA regions around 350 and 600 cm⁻¹ have been used by Bézard & Vinatier (2019) to infer the H₂ mole fraction and ortho-to-para ratio in the troposphere.

Implementation: The observation nominally takes place in the period 02:15 to 05:00 (HH:MM) from closest approach, when the spatial footprint of FP1 is about 200–400 km (see Figure 6). The FP1 detector was typically scanned slowly in a north–south or east–west direction across a diameter of the disk, starting from a position off the limb on dark sky and ending on a dark sky position situated off the disk on the opposite side. The spectral resolution was 15.0 cm⁻¹, and the scan speed was ∼7 μrad s⁻¹. See Figure 20.

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Variations: the 2–5 hr time window from Titan closest approach was often requested by other instruments, including RADAR, VIMS, and ISS, resulting in changes to the default template, whereby CIRS might have a shorter time than the nominal 2 hr 45 minutes. In these cases, the scans may have been shortened to cover half a diameter only or to cover a specific part of the visible hemisphere such as Xanadu. Therefore, extracting the exact pointing for the observations from the CIRS archive in the PDS is important.

The FIRNADMAP observation was very similar to a UVIS-designed slow scan observation (EUVFUV scan) that took place typically 2–7 hr from closest approach to map airglow across an entire hemisphere by sweeping a linear detector array. CIRS acted as a "rider" taking data on these observations, and they are considered equivalent to the FIRNADMAP for CIRS data analysis purposes. The CIRS ride-along observations with EUVFUV were initially labeled in the form CIRS_nnnFIRNADMAPnnn_UVIS (where "nnn" are numbers) but later switched to CIRS_nnnEUVFUVnnn_UVIS to further distinguish these from the CIRS-designed FIRNADMAPs (see Appendix E).

Coverage of CIRS FIRNADMAP observations in cylindrical projection is shown in Figure 21, divided into early (2004–2010) and late (2010–2017) mission phases for clarity of viewing. Due in part to the map projection, and also the typically equatorial viewing geometry from the spacecraft, there is substantial "stretching" of the FOV footprint near the poles. Figure 22 shows the same information but plotted in polar projection, producing less distortion, although the stretching of the FOV footprint at high latitudes is still evident where the spacecraft was viewing from low latitudes. Finally, Figure 23 shows the coverage of the complementary UVIS EUVFUV maps in both rectangular and polar projection for the entire mission.

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Science overview: The far-infrared nadir composition integrations (FIRNADCMP) were designed to complement the far-infrared limb integrations (FIRLMBINT) by providing latitude–longitude spatial coverage with high spectral resolution and S/N, although without vertical resolution. The principal science goals were to measure the abundances of HCN, CO, H₂O, and CH₄ through their far-infrared rotational lines (de Kok et al. 2007b; Lellouch et al. 2014); hydrocarbons (C₃H₄, C₄H₂) and nitriles (C₂N₂, HC₃N) can also be measured (de Kok et al. 2008; Teanby et al. 2009b; Sylvestre et al. 2018). Due to the time and distance from closest approach (nominally 9–13 hr, or (180–260) × 10³ km), these became the most frequent and numerous of all CIRS Titan observations. In addition to the desired FP1 science, large amounts of FP3 and FP4 data were acquired in nadir mode at 0.5 cm⁻¹ resolution. These FP3 and FP4 data were used for many purposes: to map latitude variations of trace gases (e.g., Coustenis et al. 2007, 2010; Teanby et al. 2010a; Bampasidis et al. 2012; Coustenis et al. 2013, 2016, 2018; see also MIDIRTMAP), to measure isotopic ratios of hydrocarbons (Nixon et al. 2008a), and to search for new species (Jolly et al. 2015).

Implementation: The FP1 detector was positioned at approximately 45°–60° emission angle, or about two-thirds of the way between the disk center and the disk edge. Where possible, the detector was rotated so that FP3 and FP4 were also on the disk. The instrument then dwelled for typically ∼90 minutes, bracketed on either side by shorter integrations on deep space, about 1000 km above the limb. Observations of more than 3–4 hr were broken up with an additional one, or in some circumstances by two deep space calibration observations of about 30 minutes between the science time blocks on Titan's disk. See Figure 24.

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Figures 25 and 26 show mission coverage of the far-infrared nadir composition integrations in cylindrical and polar projections, respectively. It is evident that these numerous observations achieved excellent spatial and temporal coverage. See also Appendix F for a complete listing of FIRNADCMP observations.

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Science overview: The mid-infrared temperature map observation was designed as a map of the visible hemisphere at medium spectral resolution (3 cm⁻¹) primarily to allow temperature retrievals from the ν₄ band of methane at 1305 cm⁻¹. Subsequently, the temperatures retrieved could be converted into wind fields via the thermal wind equation, allowing for Titan's changing global circulation to be tracked. MIDIRTMAP observations have proved essential for mapping of Titan's global stratospheric temperature and wind fields; see, for example, Flasar et al. (2005) and Achterberg et al. (2008b, 2011). Due to the excellent spatial coverage and medium spectral resolution, MIDIRTMAP observations have been widely used for not only temperature retrievals, but also for mapping the more abundant trace gases such as C₂H₂, HCN, and C₂H₆ (Teanby et al. 2006, 2008b, 2009c, 2010b, 2017, 2019; Coustenis et al. 2007, 2013; Bampasidis et al. 2012), and for measuring Titan's total emitted power (Li et al. 2011; Li 2015). The combined latitudinal and longitudinal coverage has been used to determine a tilt in the atmospheric rotation axis relative to Titan's solid body from the temperature field (Achterberg et al. 2008a) and trace gases (Teanby et al. 2010c). In addition, medium spectral resolution FP1 data from the MIDIRTMAPs have been used for retrievals of Titan's H₂ abundance from the H₂–N₂ dimer at ∼360 cm⁻¹ (Courtin et al. 2012).

Implementation: MIDRTMAP was a "workhorse" observation for CIRS that was performed on almost every flyby on either the inbound leg of the flyby, the outbound leg, or both. This observation was commonly used because the range at 13–19 hr from C/A ((260–480) × 10³ km) was not in high demand for measurements by other instruments, with the exception of cloud monitoring by ISS. The observation was performed using the "pushbroom mapping" method, where the FP3 and 4 arrays were slowly scanned across the visible disk in several (typically four to seven) parallel tracks to map the entire disk. The scan rate was ∼4 μrad s⁻¹ and tracks overlapped slightly (∼20%) to prevent any gaps in coverage. In the early part of the mission, the observations were usually preceded and followed by a "stare" (integration) on deep space significantly away from the atmosphere. Later, this function was performed instead by dedicated deep space calibration observations ("DSCAL") by CIRS made during spacecraft downlinks (data relay to Earth), so the "embedded" deep space calibration blocks within observations gradually disappeared from usage. See Figure 27.

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Variations: The label "TEMPMAP" was used early in the mission for more distant MIDIRTMAP observations that fell outside of a canonical TOST period—a segment of the Cassini timeline designated as a Titan encounter time block. These typically have lower spatial resolution (i.e., larger detector footprints on Titan) than normal MIDIRTMAPs, and correspondingly fewer and shorter angular scans of the arrays to cover the disk, but otherwise accomplish the same mid-infrared nadir-mapping goal. After the end of the PM, from 2008 onward, the TEMPMAP designation was deprecated, and all observations of this type became MIDIRTMAPs, or the time was used for integrations instead.

In the late mission, many MIDIRTMAPs were cut short by downlinks that increasingly were moved inward in time, shortening the Titan observation block (a.k.a. the "TOST segment," after the TOST working group) especially on the unlit (night) side, whether inbound or outbound. In these cases, MIDIRTMAPs that were notionally 6 hr in length were sometimes cut down to 3–4 hr, resulting in only partial disk maps. In the final months of the mission, during the "F-ring" and "proximal" orbits at high inclination with repeated distant Titan encounters, MIDIRTMAPs were often performed as multiple short blocks, interspersed with ISS "mosaic" observations designed to search for clouds.

Coverage of mid-infrared temperature maps in latitude and time is shown in Figure 28. Aside from a loss of high latitude coverage from 2010 to 2012 due to spacecraft-viewing geometry, overall coverage during the mission is excellent, permitting a wide-ranging survey of Titan's atmospheric dynamics (winds and circulation). For a full list of these observations, see Appendix G.

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Overview: These were the most distant Titan observations performed by CIRS, occurring at distances (0.5–2.0) × 10⁶ km. They were very distant integrations at high spectral resolution (0.5 cm⁻¹), usually designed to measure a single section (either N–S or E–W) of trace gas abundances across the disk (Teanby et al. 2006, 2008b, 2010b). The COMPMAP (composition map) name was used when the observation occurred in a regular TOST segment, while in the later mission phases the name TEA was used instead (Titan Exploration at Apoapse) when the observation took place in a non-TOST observation block, and usually at greater range than COMPMAP. See also the observation listing in Appendix H.

Implementation: The FP3/4 arrays were positioned to span Titan's disk in one to five positions, with long dwells at each position to build up the S/N. COMPMAP varieties tended to be at somewhat closer distances than TEAs and typically had two or more pointings (Figure 29), whereas the TEAs had only one (Figure 30).

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Variations: Several very distant TEA observations were specially designed to place Titan entirely within the FP1 pixel for comparison with far-infrared unresolved observations made with ISO (Coustenis et al. 1998) and Herschel (Moreno et al. 2012), as published in Bauduin et al. (2018; see Figure 31).

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The PDS Atmospheres node is the primary delivery point for CIRS data, which can be found here:

https://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Cassini/inst-cirs.html.

Data search tools include the Event Calendar and Master Schedule. Image cubes showing coverage of individual observations are contained in the EXTRAS/CUBE_OVERVIEW subdirectory of individual data volumes, which are labeled by year and month: e.g., "cocirs_0401" is the volume for "Cassini Orbiter, CIRS, 2004 January." Data are stored in the DATA/TSDR area of the volumes, while documentation, including a detailed User Guide to the CIRS data set, is included in the DOCUMENT area.

Note: CIRS data at the Rings node is stored in a space-minimizing binary format, with fixed-length records for most ancillary and pointing information, and variable length records for interferogram and spectra. A different format is used at the Rings Node.

CIRS data is also stored at the Rings Node:

https://pds-rings.seti.org/cassini/cirs/

It is important to note that the data are reformatted by the Rings Node compared to the Atmospheres Node, offering some advantages in readability at the cost of more storage space in bytes. Ancillary data records are stored in ASCII rather than binary format, while the interferograms and spectra are provided as fixed-length (as opposed to variable length) binary records. The remainder of the archive volumes—directories other than DATA—is the same as that at the Atmospheres Node, as delivered by the CIRS team. The data may be browsed, and is also searchable using the OPUS tool: https://tools.pds-rings.seti.org/opus/#/.