Engineering topics

Courtin, R.

Publications and source records attributed to Courtin, R..

13 records

NASA NTRS2018

Titan's Cold Case Files - Outstanding Questions After Cassini-Huygens

The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the prime mission (2004-2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox mission (2008-2010) and Solstice mission (2010-2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the new discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Saturn and Titan, and to stimulate new experimental, observation and theoretical investigations in the interim, before such missions arrive again at Titan.

Nixon, C. 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 NTRS2014

Scientific Rationale for Saturn's in Situ Exploration

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases׳ abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn's atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn's bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn's upper troposphere may help constraining its bulk O/H ratio. We compare predictions of Jupiter and Saturn's bulk compositions from different formation scenarios, and highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn's stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations, all based on an entry probe that would descend through Saturn's stratosphere and troposphere under parachute down to a minimum of 10 bar of atmospheric pressure. We finally discuss the science payload required on a Saturn probe to match the measurement requirements.

Galileo↗

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.↗

NASA NTRS2008

Titan Surface Temperatures from Cassini CIRS

Thermal radiation from the surface of Titan reaches space through a spectral window at 19-microns wavelength. After removing the effects of the atmosphere, measurement of this radiance gives the brightness temperature of the surface. The Composite Infrared Spectrometer (CIRS) has made such measurements during the Cassini prime mission. These observations cover a wide range of emission angles, thereby constraining the contributions from atmospheric radiance and opacity. With the more complete latitude coverage and much larger dataset, we have been able to improve upon the original results from Voyager IRIS. CIRS measures an equatorial surface brightness temperature, averaged over longitude, of 93.7 +/- 0.6 K. This agrees with the HASI temperature at the Huygens landing site. The latitude dependence of surface brightness temperature exhibits an approximately 2 K decrease toward the South Pole and 3 K decrease toward the North Pole. The lower surface temperatures seen at high latitudes are consistent with conditions expected for lake formation.

Jennings, Donald E.↗

NASA NTRS2008

Titan's Surface Brightness Temperatures and H2 Mole Fraction from Cassini CIRS

The atmosphere of Titan has a spectral window of low opacity around 530/cm in the thermal infrared where radiation from the surface can be detected from space. The Composite Infrared spectrometer1 (CIRS) uses this window to measure the surface brightness temperature of Titan. By combining all observations from the Cassini tour it is possible to go beyond previous Voyager IRIS studies in latitude mapping of surface temperature. CIRS finds an average equatorial surface brightness temperature of 93.7+/-0.6 K, which is close to the 93.65+/-0.25 K value measured at the surface by Huygens HASi. The temperature decreases toward the poles, reaching 91.6+/-0.7 K at 90 S and 90.0+/-1.0 K at 87 N. The temperature distribution is centered in latitude at approximately 12 S, consistent with Titan's season of late northern winter. Near the equator the temperature varies with longitude and is higher in the trailing hemisphere, where the lower albedo may lead to relatively greater surface heating5. Modeling of radiances at 590/cm constrains the atmospheric H2 mole fraction to 0.12+/-0.06 %, in agreement with results from Voyager iris.

Jennings, Donald E.↗

NASA NTRS2008

Titan's Surface Temperatures Measured by Cassini CIRS

A large fraction of 19-micron thermal radiation from the surface of Titan reaches space through a spectral window of low atmospheric opacity. The emergent radiance, after removing the effect of the atmosphere, gives the brightness temperature of the surface. This atmospheric window is covered by the far-infrared channel of the Composite Infrared spectrometer1 (CIRS) on Cassini. In mapping Titan surface temperatures, CIRS is able to improve upon results of Voyager IRIS, by taking advantage of improved latitude coverage and a much larger dataset. Observations are from a wide range of emission angles and thereby provide constraints on the atmospheric opacity and radiance that are used to derive the surface temperature. CIRS finds an average equatorial surface brightness temperature of 93.7+/-0.6 K, virtually identical to the HASI temperature at the Huygens landing site. Mapping in latitude shows that the surface temperature decreases toward the poles by about 2 K in the south and 3 K in the north. This surface temperature distribution is consistent with the formation of lakes seen at high latitudes on Titan.

Jennings, Donald E.↗

NASA NTRS1996

Numerical modelling of the circulation of superrotating atmospheres: Venus and Titan

Atmospheric superrotation is known to exist on Venus where the cloud cover, at 70 km above the surface, rotates about 60 times faster than the solid planet. Superrotation is strongly suspected on Titan on the basis of numerical studies and observations. The numerical simulation of superrotation is described. It is shown how the general circulation models, originally developed for terrestrial meteorology and climatology, can be used to infer the circulation in such atmospheres. The emphasis is on the case of Titan for which a specific general circulation model was developed.

Hourding, F.↗

NASA NTRS1995

Numerical simulation of the general circulation of the atmosphere of Titan

The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Meteorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.

NASA Discipline Exobiology↗

NASA NTRS1985

The D/H ratio in Saturn's atmosphere from Voyager IRIS spectra

A selection of 1656 spectra recorded between 180 and 2300 mm in the 20 to 40 N latitude range by the Voyager interferometer IRIS were analyzed. The CH3D/H2 and CH4/H2 ratios are determined from a best fit of the radiances measured in the nu6 and nu4 fundamental bands, respectively centered at 1161 and 1304 mm. Results are CH3D/H2 = 3.9 + or - 2.5 times 10 to the minus 7th power and CH4/H2 = 4.5 + 2.4, -1.9 x 0.001. The uncertainty includes random errors due to instrumental noise and possible systematic errors in the assumed temperature profile and spectroscopic parameters. The D/H isotopic ratio is determined from abundance ratios from the expression D/H = 1/4f (CH3D/H2)/(CH4/H2) where f accounts for equilibrium deuterium fractionation between the molecular hydrogen and methane phases. Beer and Taylor (1973, 1978) estimated f = 1.37 + or - 0.07 for Jupiter assuming a wide range of convective velocities and neglecting catalytic effects from dust particles. The same value was adopted for Saturn and yields D/H = 1.6 +1.3, -1.2 times 10 to the minus 5th power.

Courtin, R.↗

NASA NTRS1984

The composition of Saturn's atmosphere at northern temperate latitudes from Voyager IRIS spectra - NH3, PH3, C2H2, C2H6, CH3D, CH4, and the Saturnian D/H isotopic ratio

The vertical distributions and mixing ratios of minor constituents in the northern hemisphere of Saturn are investigated. Results are obtained for NH3, PH3, C2H2, C2H6, CH3D, and CH4; the D/H ratio is obtained from the CH4 and CH3D abundances. The NH3 mixing ratio in the upper atmosphere is found to be compatible with the saturated partial pressure. The inferred PH3/H2 ratio of 1.4 + or - 0.8 x 10 to the -6th is higher than the value derived from the solar P/H ratio. The stratospheric C2H2/H2 and C2H6/H2 ratios are, respectively, 2.1 + or - 1.4 x 10 to the -7th and 3.0 + or - 1.1 x 10 to the -6th; the latter decreases sharply below the 20-50 mbar level. The results for CH3D/H2 and CH4/H2 imply an enrichment of Saturn's upper atmosphere in carbon by a factor of at least three over the solar abundance. The interpretation of two NH3 lines in the five-micron window suggests a NH3/H2 ratio at the two bar level below the solar value.

Courtin, R.↗

NASA NTRS1983

The C-12/C-13 ratio in Jupiter from the Voyager infrared investigation

An analysis of the v(4) band of CH4 in the spectra recorded by the Voyager 1 IRIS experiment has yielded a C-12/C-13 ratio in Jupiter that is 160 plus 40 or minus 55, or 1.8 plus 0.4 or minus 0.6 times the terrestrial value. It is noted that while no plausible theory predicts such a difference between the C-12/C-13 ratio values of Jupiter and the inner solar system, values of this ratio in the solar neighborhood 4.5 million years ago, inferred from recent interstellar medium measurements, are compatible with the present determination in Jupiter. The Jovian, rather than the terrestrial value, would then be representative of the carbon isotope ratio in the primitive solar nebula.

Courtin, R.↗

NASA NTRS1982

The spectrum of Titan in the far-infrared and microwave regions

The pressure-induced absorptions of gaseous nitrogen (N2) and methane (CH4) are computed on the basis of the collisional lineshape theory of Birnbaum and Cohen (1976). Laboratory data at 300 and 124 K for N2 and at 296 and 195 K for CH4 are used to determine the collisional time constants and their temperature dependence. The spectrum of Titan from the microwave to the far-infrared region (0.1-600/cm) is then modeled using these opacities and a temperature profile of Titan's atmosphere derived from the Voyager 1 radio occultation experiment. The model atmosphere is composed of N2 and CH4, their relative proportions being determined by the vapor pressure law of CH4. A model with gaseous opacity alone is ruled out by the far-infrared observations. An additional opacity, thought to be associated with a methane cloud, is confirmed. The effective temperature of Titan is estimated at 83.2 plus or minus 1.4 K.

Courtin, R.↗