Composite infrared spectrometer (CIRS) on Cassini.

The Cassini spacecraft orbiting Saturn carries the composite infrared spectrometer (CIRS) designed to study thermal emission from Saturn and its rings and moons. CIRS, a Fourier transform spectrometer, is an indispensable part of the payload providing unique measurements and important synergies with the other instruments. It takes full advantage of Cassini's 13-year-long mission and surpasses the capabilities of previous spectrometers on Voyager 1 and 2. The instrument, consisting of two interferometers sharing a telescope and a scan mechanism, covers over a factor of 100 in wavelength in the mid and far infrared. It is used to study temperature, composition, structure, and dynamics of the atmospheres of Jupiter, Saturn, and Titan, the rings of Saturn, and surfaces of the icy moons. CIRS has returned a large volume of scientific results, the culmination of over 30 years of instrument development, operation, data calibration, and analysis. As Cassini and CIRS reach the end of their mission in 2017, we expect that archived spectra will be used by scientists for many years to come.

Composite infrared spectrometer (CIRS) on Cassini: publisher's note.

This publisher's note renumbers the reference list in Appl. Opt.56, 5274 (2017)APOPAI0003-693510.1364/AO.56.005274.

Simple parametric model for intensity calibration of Cassini composite infrared spectrometer data.

Accurate intensity calibration of a linear Fourier-transform spectrometer typically requires the unknown science target and the two calibration targets to be acquired under identical conditions. We present a simple model suitable for vector calibration that enables accurate calibration via adjustments of measured spectral amplitudes and phases when these three targets are recorded at different detector or optics temperatures. Our model makes calibration more accurate both by minimizing biases due to changing instrument temperatures that are always present at some level and by decreasing estimate variance through incorporating larger averages of science and calibration interferogram scans.

Identifying sampling comb changes in Fourier transform spectrometers with significant self-emission and beam splitter absorption.

For accurate calibration of Fourier transform spectrometers we must constrain or resample the interferogram data to an invariant sampling comb. This can become challenging when instrument self-emission is significant and beam splitter absorption is present. The originally-sampled interferogram center-burst position can move due not only to sampling comb changes, but also to an interaction between the strength of an external target and the so-called anomalous phase (the two ports of the interferometer contribute center-bursts at different locations, and the relative weighting of the two ports varies with the strength of the external target). We present a model of the anomalous phase to enable partitioning of changes in observed center-burst location between sampling comb changes and anomalous phase effects.

Titan's atmospheric temperatures, winds, and composition.

Temperatures obtained from early Cassini infrared observations of Titan show a stratopause at an altitude of 310 kilometers (and 186 kelvin at 15 degrees S). Stratospheric temperatures are coldest in the winter northern hemisphere, with zonal winds reaching 160 meters per second. The concentrations of several stratospheric organic compounds are enhanced at mid- and high northern latitudes, and the strong zonal winds may inhibit mixing between these latitudes and the rest of Titan. Above the south pole, temperatures in the stratosphere are 4 to 5 kelvin cooler than at the equator. The stratospheric mole fractions of methane and carbon monoxide are (1.6 +/- 0.5) x 10(-2) and (4.5 +/- 1.5) x 10(-5), respectively.

Temperatures, winds, and composition in the saturnian system.

Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites.

Jupiter's atmospheric composition from the Cassini thermal infrared spectroscopy experiment.

The Composite Infrared Spectrometer observed Jupiter in the thermal infrared during the swing-by of the Cassini spacecraft. Results include the detection of two new stratospheric species, the methyl radical and diacetylene, gaseous species present in the north and south auroral infrared hot spots; determination of the variations with latitude of acetylene and ethane, the latter a tracer of atmospheric motion; observations of unexpected spatial distributions of carbon dioxide and hydrogen cyanide, both considered to be products of comet Shoemaker-Levy 9 impacts; characterization of the morphology of the auroral infrared hot spot acetylene emission; and a new evaluation of the energetics of the northern auroral infrared hot spot.

An intense stratospheric jet on Jupiter.

The Earth's equatorial stratosphere shows oscillations in which the east-west winds reverse direction and the temperatures change cyclically with a period of about two years. This phenomenon, called the quasi-biennial oscillation, also affects the dynamics of the mid- and high-latitude stratosphere and weather in the lower atmosphere. Ground-based observations have suggested that similar temperature oscillations (with a 4-5-yr cycle) occur on Jupiter, but these data suffer from poor vertical resolution and Jupiter's stratospheric wind velocities have not yet been determined. Here we report maps of temperatures and winds with high spatial resolution, obtained from spacecraft measurements of infrared spectra of Jupiter's stratosphere. We find an intense, high-altitude equatorial jet with a speed of approximately 140 m s(-1), whose spatial structure resembles that of a quasi-quadrennial oscillation. Wave activity in the stratosphere also appears analogous to that occurring on Earth. A strong interaction between Jupiter and its plasma environment produces hot spots in its upper atmosphere and stratosphere near its poles, and the temperature maps define the penetration of the hot spots into the stratosphere.

Artificial diamond as a broadband infrared beam splitter for fourier transform spectroscopy: improved results.

We show that an artificially grown diamond plate with an improved surface flatness serves as an infrared beam splitter to wavelengths as short as 2.2 microm (4500 cm(-1)).

Artificial diamond as a broadband infrared beam splitter for fourier transform spectroscopy.

We show that an artificially grown diamond plate serves as a very broadband beam splitter from approximately 7 mum, where the intrinsic diamond absorption becomes important, to the submillimeter region (a 250-mum wavelength or longer).

Crystalline quartz as a far-infrared beam splitter for Fourier transform spectroscopy.

We show that room-temperature, crystalline quartz is a useful material for a beam splitter for spectroscopy in the far infrared, ~60-mum wavelength and longer. We compare such a beam splitter with the traditional far-infrared candidates: Mylar, polarizing, and lamellar beam splitters.

Uniform time-sampling Fourier transform spectroscopy.

We show that uniform time sampling of both the reference and the target channels in a continuous scanning Fourier transform spectrometer is a simple and versatile way of extending the Nyquist limit shorter than the wavelength of the reference channel. We also discuss the benefits of recording the reference channel when intensity calibrating the target data.

Infrared detector responsivity measured simultaneously at multiple frequencies.

Rapid automated frequency response of IR detectors can be measured to get responsivity, noise, and NEP using multiple harmonics of the IR detector response.

Cryogenic Fourier spectrometer for measuring trace species in the lower stratosphere.

A cryogenic Fourier transform spectrometer has been built to measure thermal emission of the earth's limb from a balloon-borne platform. Liquid nitrogen cooling of the spectrometer and liquid helium cooling of the detectors has provided sufficient sensitivity to detect, at 5-15 microm, fifteen molecular species relevant to stratospheric ozone chemistry. The spectral resolution achieved, 0.022 cm(-1), is the best yet attained for emission mode data at these wavelengths. The philosophy behind the design of the optical and electronic systems is presented, followed by an analysis of the performance achieved during balloon flight.

Infrared spectroscopy of the lower stratosphere with a balloon-borne cryogenic Fourier spectrometer.

The IR limb emission of the lower stratosphere has been measured using a balloon-borne liquid nitrogencooled Michelson interferometer with liquid helium-cooled Si:Ga detectors. Portions of the thermal emission spectrum have been recorded between 650 and 2000 cm(-1) with an unapodized spectral resolution of 0.03 cm(-1). This is the highest spectral resolution limb emission thus far obtained. A preliminary description is given of these data along with a discussion of the significant features. Species identified to date include CO(2), O(3), CFCl(3), CF(2)Cl(2), H(2)O, CH(4), HNO(3), N(2)O, NO(2), and ClONO(2). A tentative identification is made for NO, representing the first direct spectroscopic detection of NO in emission.