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Jennings, Donald E.

Publications and source records attributed to Jennings, Donald E..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2020

50 Years of Spaceflight with Fourier Transform Spectrometers (FTS) Built at NASA GSFC

Over the past 50 years, NASA Goddard Space Flight Center (GSFC) has been developing, building, testing and flying a series of Fourier Transform Spectrometers (FTS). This began with the IRIS instruments on the Earth-orbiting Nimbus satellites and progressed to more sophisticated designs optimized for interplanetary spacecraft sent to Mars and later to the outer solar system. Adaptions have been made over time, including progressively higher spectral resolution, sensitivity, numbers of detectors and complexity. Instrument operating temperatures have decreased to enable remote sensing of the cold giant planet systems. In this paper we describe the historical evolution of this instrument line, comparing and contrasting different aspects such as optical design and materials, detector types and data handling. We conclude by looking towards the future. At present the CIRS-Lite prototype is being tested at NASA GSFC for potential use on a future mission to the ice giants, Uranus and Neptune. Surpassing the previous performance of the Voyager IRIS instruments remains challenging, and new technologies that could enable these measurements are discussed.

Nixon, Conor A.↗
NASA NTRS2019

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

From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close flybys of its largest moon, Titan. Cassini’s Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, profiled Titan in the thermal infrared (7–1000 μm) throughout the entire 13 yr mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hr. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titan’s atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future intercomparison of CIRS results with those of other Cassini instruments and ground-based observations.

Nixon, Conor A.↗
NASA NTRS2019

Ethane in Titan's Stratosphere from Cassini CIRS Far- and Mid-Infrared Spectra

The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission in the far- and mid-infrared (from 10 to 1500 cm(exp −1)), enabling spatiotemporal studies of ethane on Titan across the span of the Cassini mission from 2004 through 2017. Many previous measurements of ethane on Titan have relied on modeling the molecule’s mid-infrared ν(sub 12) band, centered on 822 cm(exp −1). Other bands of ethane at shorter and longer wavelengths were seen, but have not been modeled to measure ethane abundance. Spectral line lists of the far-infrared ν(sub 4) torsional band at 289 cm(exp −1) and the mid-infrared ν(sub 8) band centered at 1468 cm(exp −1) have recently been studied in the laboratory. We model CIRS observations of each of these bands (along with the ν(sub 12) band) separately and compare the retrieved mixing ratios from each spectral region. Nadir observations of the ν(sub 4) band probe the low stratosphere below 100 km. Our equatorial measurements at 289 cm(exp −1) show an abundance of (1.0±0.4)x10(exp −5) at 88 km from 2007 to 2017. This mixing ratio is consistent with measurements at higher altitudes, in contrast to the depletion that many photochemical models predict. Measurements from the ν(sub 12) and ν(sub 8) bands are comparable to each other, with the ν(sub 12) band probing an altitude range that extends deeper in the atmosphere. We suggest that future studies of planetary atmospheres may observe the ν(sub 8) band, enabling shorter wavelength studies of ethane. There may also be an advantage to observing both the ethane ν(sub 8) band and nearby methane ν(sub 4) band in the same spectral window.

Lombardo, Nicholas A.↗
NASA NTRS2017

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.

Jennings, Donald E.↗
NASA NTRS2016

Dual-Telescope Multi-Channel Thermal-Infrared Radiometer for Outer Planet Fly-By Missions

The design of a versatile dual-telescope thermal-infrared radiometer spanning the spectral wavelength range 8-200 microns, in five spectral pass bands, for outer planet fly-by missions is described. The dual- telescope design switches between a narrow-field-of-view and a wide-field-of-view to provide optimal spatial resolution images within a range of spacecraft encounters to the target. The switchable dual-field- of-view system uses an optical configuration based on the axial rotation of a source-select mirror along the optical axis. The optical design, spectral performance, radiometric accuracy, and retrieval estimates of the instrument are discussed. This is followed by an assessment of the surface coverage performance at various spatial resolutions by using the planned NASA Europa Mission 13-F7 fly-by trajectories as a case study.

Aslam, Shahid↗
NASA NTRS2015

Evolution of the Far-Infrared Cloud at Titan's South Pole

A condensate cloud on Titan identified by its 220 cm−1 far-infrared signature continues to undergo seasonal changes at both the north and south poles. In the north, the cloud, which extends from 55 N to the pole, has been gradually decreasing in emission intensity since the beginning of the Cassini mission with a half-life of 3.8 years. The cloud in the south did not appear until 2012 but its intensity has increased rapidly, doubling every year. The shape of the cloud at the south pole is very different from that in the north. Mapping in 2013 December showed that the condensate emission was confined to a ring with a maximum at 80 S. The ring was centered 4deg from Titanʼs pole. The pattern of emission from stratospheric trace gases like nitriles and complex hydrocarbons (mapped in 2014 January) was also offset by 4deg, but had a central peak at the pole and a secondary maximum in a ring at about 70 S with a minimum at 80 S. The shape of the gas emission distribution can be explained by abundances that are high at the atmospheric pole and diminish toward the equator, combined with correspondingly increasing temperatures. We discuss possible causes for the condensate ring. The present rapid build up of the condensate cloud at the south pole is likely to transition to a gradual decline from 2015 to 2016. Key words: molecular processes - planets and satellites: atmospheres - planets and satellites: composition - planets and satellites: individual (Titan) - radiation mechanisms: thermal

Cassini↗
NASA NTRS2012

Seasonal Disappearance of Far-Infrared Haze in Titan's Stratosphere

A far-infrared emission band attributed to volatile or refractory haze in Titan's stratosphere has been decreasing in intensity since Cassini's arrival in 2004. The 220 cm(sup -1) feature, first seen by the Voyager Infrared Interferometer Spectrometer, has only been found in Titan's winter polar region. The emission peaks at about 140 km altitude near the winter stratospheric temperature minimum. Observations recorded over the period 2004-2012 by the Composite Infrared Spectrometer on Cassini show a decrease in the intensity of this feature by about a factor of four. Possible seasonal causes of this decline are an increase in photolytic destruction of source chemicals at high altitude, a lessening of condensation as solar heating increased, or a weakening of downwelling of vapors. As of early 2012, the 220 cm(sup -1) haze has not yet been detected in the south. The haze composition is unknown, but its decrease is similar to that of HC3N gas in Titan's polar stratosphere, pointing to a nitrile origin.

Jennings, Donald E.↗
NASA NTRS2012

Celebrating One Year of Atmospheric Evolution on Titan Since Voyager with Cassini/CIRS

Seven years after Cassini's Saturn orbit insertion, we have in hand almost a complete picture of the stratospheric evolution within a Titanian year by combining Voyager 1 Infrared Radiometer Spectrometer (IRIS) measurements from 1980, Cassini Composite Infrared Spectrometer (CIRS) continuous recordings from 2004 to 2010 and the intervening ground-based and space-borne observations with ISO (Coustenis et al 2003). We have re-analyzed the Voyager l/IRIS data acquired during the 1980 encounter, 30 years (one Titan revolution) before 2010, with the most recent spectroscopic data releases and haze descriptions (Vinatier et al 2010, 2012) by using our radiative transfer code (ART). The re-analysis confirms the Vl/IRIS retrievals by Coustenis and Bezard (1995) and updates the abundances for all molecules and latitudes based on new temperature, haze and spectroscopic parameters. ART was also applied to all available CIRS spectral averages corresponding to more than 70 flybys binned over 10 deg in latitude for both medium (2.5 cm(exp -1) and higher (0.5 cm(exp -1) resolutions and from nadir and limb data both. In these spectra, we search for variations in temperature (following the method in Achterberg et al 2011) and composition at northern (around 50 deg N), equatorial and southern (around 50 deg S) latitudes as the season on Titan progresses and compare them to the new Vl/IRIS, ISO and other ground-based reported composition values (Coustenis et al., 2012, in prep). Other latitudes were examined in previous papers (e.g. Coustenis et al 2010).

Coustenis, A.↗
NASA NTRS2012

Extrasolar Planet Transits Observed at Kitt Peak National Observatory

We obtained J-, H-, and JH-band photometry of known extrasolar planet transiting systems at the 2.1 m Kitt Peak National Observatory Telescope using the FLAMINGOS infrared camera between 2008 October and 2011 October. From the derived light curves we have extracted the midtransit times, transit depths and transit durations for these events. The precise midtransit times obtained help improve the orbital periods and also constrain transit-time variations of the systems. For most cases the published system parameters successfully accounted for our observed light curves, but in some instances we derive improved planetary radii and orbital periods. We complemented our 2.1 m infrared observations using CCD z0-band and B-band photometry (plus two H(alpha) filter observations) obtained with the Kitt Peak Visitor Center Telescope, and with four H-band transits observed in 2007 October with the NSO's 1.6 m McMath-Pierce Solar Telescope. The principal highlights of our results are (1) Our ensemble of J-band planetary radii agree with optical radii, with the best-fit relation being RpRJ0:0017 0:979RpRvis. (2) We observe starspot crossings during the transit of WASP-11HAT-P-10. (3) We detect starspot crossings by HAT-P-11b (Kepler-3b), thus confirming that the magnetic evolution of the stellar active regions can be monitored even after the Kepler mission has ended. (4) We confirm a grazing transit for HAT-P-27WASP-40. In total, we present 57 individual transits of 32 known exoplanet systems.

Kitt Peak↗
NASA NTRS2011

Kepler and Ground-Based Transits of the exo-Neptune HAT-P-11b

We analyze 26 archival Kepler transits of the exo-Neptune HAT-P-11b, supplemented by ground-based transits observed in the blue (B band) and near-IR (J band). Both the planet and host star are smaller than previously believed; our analysis yields Rp = 4.31 R xor 0.06 R xor and Rs = 0.683 R solar mass 0.009 R solar mass, both about 3 sigma smaller than the discovery values. Our ground-based transit data at wavelengths bracketing the Kepler bandpass serve to check the wavelength dependence of stellar limb darkening, and the J-band transit provides a precise and independent constraint on the transit duration. Both the limb darkening and transit duration from our ground-based data are consistent with the new Kepler values for the system parameters. Our smaller radius for the planet implies that its gaseous envelope can be less extensive than previously believed, being very similar to the H-He envelope of GJ 436b and Kepler-4b. HAT-P-11 is an active star, and signatures of star spot crossings are ubiquitous in the Kepler transit data. We develop and apply a methodology to correct the planetary radius for the presence of both crossed and uncrossed star spots. Star spot crossings are concentrated at phases 0.002 and +0.006. This is consistent with inferences from Rossiter-McLaughlin measurements that the planet transits nearly perpendicular to the stellar equator. We identify the dominant phases of star spot crossings with active latitudes on the star, and infer that the stellar rotational pole is inclined at about 12 deg 5 deg to the plane of the sky. We point out that precise transit measurements over long durations could in principle allow us to construct a stellar Butterfly diagram to probe the cyclic evolution of magnetic activity on this active K-dwarf star.

Deming, Drake↗
NASA NTRS2011

Miniature, Low-Power, Waveguide Based Infrared Fourier Transform Spectrometer for Spacecraft Remote Sensing

Fourier transform spectrometers have a venerable heritage as flight instruments. However, obtaining an accurate spectrum exacts a penalty in instrument mass and power requirements. Recent advances in a broad class of non-scanning Fourier transform spectrometer (FTS) devices, generally called spatial heterodyne spectrometers, offer distinct advantages as flight optimized systems. We are developing a miniaturized system that employs photonics lightwave circuit principles and functions as an FTS operating in the 7-14 micrometer spectral region. The inteferogram is constructed from an ensemble of Mach-Zehnder interferometers with path length differences calibrated to mimic scan mirror sample positions of a classic Michelson type FTS. One potential long-term application of this technology in low cost planetary missions is the concept of a self-contained sensor system. We are developing a systems architecture concept for wide area in situ and remote monitoring of characteristic properties that are of scientific interest. The system will be based on wavelength- and resolution-independent spectroscopic sensors for studying atmospheric and surface chemistry, physics, and mineralogy. The self-contained sensor network is based on our concept of an Addressable Photonics Cube (APC) which has real-time flexibility and broad science applications. It is envisaged that a spatially distributed autonomous sensor web concept that integrates multiple APCs will be reactive and dynamically driven. The network is designed to respond in an event- or model-driven manner or reconfigured as needed.

Hewagama, TIlak↗
NASA NTRS2011

Seasonal Changes in Titan's Surface Temperatures

Cassini's extended mission has provided the opportunity to search for seasonal variations on Titan. In particular, surface temperatures are expected to have shifted significantly in latitude during the completed portion of the mission. Spectra recorded by the Composite Infrared Spectrometer (CIRS) during the nominal mission (2004-08) and the Equinox mission. (2008-10) have already shown changes in temperature. CIRS has detected a seasonal shift in the latitudinal distribution of surface brightness temperatures by comparing zonal averages from two time segments, one period in late northern winter centered on L(sub s) approximately 335 deg and a second period centered on the equinox (L(sub s) approximately 0 deg.). The earlier period had a meridional distribution similar to that previously reported: 93.5 K at the equator, 91.7 K at 85 S and 899 K at 85 N. The newly measured distribution near equinox shows a cooling in the south and a warming in the north, both by about 0.5 K. We estimate that. the centroid of the distribution moved from approximately 16 S to 7 S between the two periods. This gives a seasonal lag behind insolation of delta L(sub s) approximately 13 deg. The CIRS equinox results are consistent with those of Voyager IRIS, which encountered Titan in November 1980, just following the previous northern equinox (L(sub s) = 10 deg.). When compared with predictions from general circulation models, seasonal variations of surface temperature can help constrain the identification of surface materials. Our measurements most closely match the case of a porous ice regolith treated by Tokano, but with some apparent differences between the northern and southern hemispheres. CIRS will extend its study of seasonal variations in surface temperature on Titan as Cassini continues through northern spring.

Jennings, Donald E.↗
NASA NTRS2010

Titan's Surface Temperatures from Cassini CIRS

The surface brightness temperature of Titan can be measured from Cassini through a spectral window at 19 microns where the atmosphere is low in opacity. The Composite Infrared Spectrometer (CIRS) on Cassini observes this wavelength in its far-infrared channel. Because the Cassini tour has provided global coverage and a range of viewing geometries, CIRS has been able to go beyond the earlier flyby results of Voyager IRIS Near the equator, CIRS measures the zonally-averaged surface brightness temperature to be 917 K, very close to the temperature found at the surface by Huygens. Latitude maps show that Titan's surface temperatures drop off by about 2 K toward the south and by about 3 K toward the north. This temperature distribution is consistent with Titan's late northern winter when the data were taken. As the seasons progress, CIRS is continuing to search for corresponding changes in the temperatures of the surface and lower atmosphere. CIRS is also extending global mapping to both latitude and longitude to look for correlations between surface temperatures and geological features.

Jennings, Donald E.↗
NASA NTRS2010

Surface Temperatures on Titan; Changes During the Cassini Mission

Surface brightness temperatures on Titan measured by the Composite Infrared Spectrometer (CIRS) aboard Cassini span the period from late northern winter to early spring. The CIRS observations cover all latitudes and can be used to study meridional changes with season. CIRS previously reported surface temperatures from 2004-2008 which were 93.7 K at the equator with decreases of 2 K toward the south pole and 3 K toward the north pole'. From a comparison of the equinox period with the earlier data, CIRS can now detect a seasonal shift in the latitudinal distribution of temperatures. Around the time of the equinox the meridional distribution was more symmetric about the equator than had been found earlier in the mission. The equatorial surface temperatures remained close to 94 K, but in the south the temperatures had decreased by about 0.5 K and in the north had increased by about 0.5 K. The CIRS equinox results are similar to what was seen near the previous vernal equinox by Voyager IRIS Z. The observed surface temperatures can help constrain the type of surface material by comparison with predictions from general circulation models. Of the three cases treated by Tokano t , our measurements most closely match a porous-ice regolith. As Cassini continues through Titan's northern spring CIRS will extend its temporal and spatial coverage and will continue to search for seasonal variations in surface temperature.

Jennings, Donald E.↗
NASA NTRS2009

C-12/C-13 Ratio in Ethane on Titan and Implications for Methane's Replenishment

The C-12/C-13 abundance ratio in ethane in the atmosphere of Titan has been measured at 822 cm(sup -1) from high spectral resolution ground-based observations. The value 89(8), coincides with the telluric standard and also agrees with the ratio seen in the outer planets. It is almost identical to the result for ethane on Titan found by the composite infrared spectrometer (CIRS) on Cassini. The C-12/C-13 ratio for ethane is higher than the ratio measured in atmospheric methane by Cassini/Huygens GCMS, 82.3(l), representing an enrichment of C-12 in the ethane that might be explained by a kinetic isotope effect of approximately 1.1 in the formation of methyl radicals. If methane is being continuously resupplied to balance photochemical destruction, then we expect the isotopic composition in the ethane product to equilibrate at close to the same C-12/C-13 ratio as that in the supply. The telluric value of the ratio in ethane then implies that the methane reservoir is primordial.

Jennings, Donald E.↗
NASA NTRS2009

Thermal Infrared Spectroscopy of Saturn and Titan from Cassini

The Cassini spacecraft completed its nominal mission at Saturn in 2008 and began its extended mission. Cassini carries the Composite Infrared Spectrometer (CIRS); a Fourier transform spectrometer that measures the composition, thermal structure and dynamics of the atmospheres of Saturn and Titan, and also the temperatures of other moons and the rings.

Jennings, Donald E.↗
NASA NTRS2009

Titan Surface Temperatures as Measured by Cassini CIRS

Thermal radiation from the surface of Titan reaches space through a spectral window of low opacity at 19-microns wavelength. This radiance gives a measure of the brightness temperature of the surface. Composite Infrared Spectrometer' (CIRS) observations from Cassini during its first four years at Saturn have permitted latitude mapping of zonally averaged surface temperatures. The measurements are corrected for atmospheric opacity using the dependence of radiance on emission angle. With the more complete latitude coverage and much larger dataset of CIRS we have improved upon the original results from Voyager IRIS. CIRS measures the equatorial surface brightness temperature to be 93.7+/-0.6 K, the same as the temperature measured at the Huygens landing site. The surface brightness temperature decreases by 2 K toward the south pole and by 3 K toward the north pole. The drop in surface temperature between equator and north pole implies a 50% decrease in methane saturation vapor pressure and relative humidity; this may help explain the large northern lakes. The H2 mole fraction is derived as a by-product of our analysis and agrees with previous results. Evidence of seasonal variation in surface and atmospheric temperatures is emerging from CIRS measurements over the Cassini mission.

Jennings, Donald E.↗