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D Gautier

Publications and source records attributed to D Gautier.

At least 19 recordsLinked to original sources
PubMed2005

The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe.

Saturn's largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellite's surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.

Argon↗
PubMed2005

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.

Atmosphere↗
PubMed2004

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.

Atmosphere↗
PubMed2004

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.

Acetylene↗
PubMed2004

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.

Journal Article↗
PubMed1998

Deuterium in comet C/1995 O1 (Hale-Bopp): detection of DCN.

Deuterated hydrogen cyanide (DCN) was detected in a comet, C/1995 O1 (Hale-Bopp), with the use of the James Clerk Maxwell Telescope on Mauna Kea, Hawaii. The inferred deuterium/hydrogen (D/H) ratio in hydrogen cyanide (HCN) is (D/H)HCN = (2.3 +/- 0.4) x 10(-3). This ratio is higher than the D/H ratio found in cometary water and supports the interstellar origin of cometary ices. The observed values of D/H in water and HCN imply a kinetic temperature >/=30 +/- 10 K in the fragment of interstellar cloud that formed the solar system.

Deuterium↗
PubMed1998

A determination of the HDO/H2O ratio in comet C/1995 O1 (Hale-Bopp).

Deuterated water (HDO) was detected in comet C/1995 O1 (Hale-Bopp) with the use of the James Clerk Maxwell Telescope on Mauna Kea, Hawaii. The inferred D/H ratio in Hale-Bopp's water is (3.3 +/- 0.8) x 10(-4). This result is consistent with in situ measurements of comet P/Halley and the value found in C/1996 B2 (Hyakutake). This D/H ratio, higher than that in terrestrial water and more than 10 times the value for protosolar H2, implies that comets cannot be the only source for the oceans on Earth.

Deuterium Oxide↗
PubMed1997

Insulin responses to intravenous glucose, intravenous arginine and a hyperglycaemic clamp in ICA-positive subjects with different degrees of glucose tolerance.

The relationship between altered insulin secretion and impaired glucose tolerance was studied in 32 non-obese subjects aged 14-49 years with islet-cell antibodies (ICA) and fasting blood glucose below 7.9 mmol/l, using oral (OGTT) and intravenous (IVGTT) glucose tolerance tests. Glucose tolerance was normal in 19 subjects, impaired (IGT) in 4 and satisfied diabetic criteria in 9. Fifteen of these subjects and 8 ICA-negative controls also underwent a hyperglycaemic clamp (10 mmol/l) and a glucose-potentiated IV arginine bolus. Acute insulin response to IVGTT and insulin and C-peptide responses to the hyperglycaemic clamp and the arginine bolus were dramatically lower (p < 0.001) in diabetic and IGT subjects than in ICA-positive patients with normal glucose tolerance and control subjects. Insulin responses to the three tests were inversely correlated with plasma glucose levels and the area under the curve of OGTT. The correlations between the degree of glucose tolerance and insulin responses to IVGTT, the hyperglycaemic clamp and the arginine bolus were virtually identical. It is concluded that insulin responses to the three stimuli were severely altered in ICA-positive patients with impaired glucose tolerance or asymptomatic diabetes, normal in normotolerant ICA-positive subjects, and correlated with glucose tolerance.

Administration, Oral↗
PubMed1996

Spectroscopic evidence for interstellar ices in comet Hyakutake.

Volatile compounds in comets are the most pristine materials surviving from the time of formation of the Solar System, and thus potentially provide information about conditions that prevailed in the primitive solar nebula. Moreover, comets may have supplied a substantial fraction of the volatiles on the terrestrial planets, perhaps including organic compounds that played a role in the origin of life on Earth. Here we report the detection of hydrogen isocyanide (HNC) in comet Hyakutake. The abundance of HNC relative to hydrogen cyanide (HCN) is very similar to that observed in quiescent interstellar molecular clouds, and quite different from the equilibrium ratio expected in the outermost solar nebula, where comets are thought to form. Such a departure from equilibrium has long been considered a hallmark of gas-phase chemical processing in the interstellar medium, suggesting that interstellar gases have been incorporated into the comet's nucleus, perhaps as ices frozen onto interstellar grains. If this interpretation is correct, our results should provide constraints on the temperature of the solar nebula, and the subsequent chemical processes that occurred in the region where comets formed.

Extraterrestrial Environment↗
PubMed1995

Chemical and thermal response of Jupiter's atmosphere following the impact of comet Shoemaker-Levy 9.

In July 1994, the collisions of the fragments of comet Shoemaker-Levy 9 with Jupiter resulted in dramatic changes in the planet's atmosphere. Observations of the events suggest that the composition and thermal properties of the atmosphere were considerably modified at the impact sites, with the changes persisting for times lasting from minutes to weeks (see, for example, refs 1-4). Here we report observations of the impact sites at millimetre wave-lengths, which reveal strong emission lines associated with carbon monoxide, carbonyl sulphide and carbon monosulphide. The abundance of carbon monoxide in the jovian atmosphere is normally very low; carbonyl sulphide and carbon monosulphide, on the other hand, have not hitherto been detected. We find that the largest fragments (G and K) each produced approximately 10(14) g of carbon monoxide, 3 x 10(12) g of carbonyl sulphide and 3 x 10(11) g of carbon monosulphide, most probably by shock-induced chemical reactions. Our observations also place firm constraints on the thermal response of Jupiter's stratosphere to the impacts.

Atmosphere↗
PubMed1995

Photochemical modeling of Titan's atmosphere

We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984, Astrophys. J. Suppl. 55, 465-506), the most significant recent change in the reactions used is the updated methane photodissociation scheme (Mordaunt et al. 1993, J. Chem. Phys. 98(3), 2054-2065). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990, Icarus, 85, 43-57) using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992, Icarus 100, 512-526). We have run the model in both steady-state and diurnal modes, with 62 speices involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.

Atmosphere↗
PubMed1995

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 Météorologie 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.

Atmosphere↗
PubMed1995

Titan's atmosphere composition: certainties and speculations.

Firm results concerning the thermal structure, the composition, the seasonal effects of the atmosphere of Titan, as well as the superotation of its stratosphere are reviewed. The nature of the surface of the satellite, the possible presence of argon in the atmosphere and the structure and composition of clouds and aerosols are, among other topics, still speculative. The implications of the observed deuterium enrichment on the origin of ices in the outer part of the nebula are controversial.

Argon↗
PubMed1994

Use of runs test to assess cardiovascular autonomic function in diabetic subjects.

OBJECTIVE: We suggest a simple, noninvasive method to assess the autonomic function in diabetic subjects. The method requires only a monitoring of heart rate (HR) with subjects in the sitting position. RESEARCH DESIGN AND METHODS: Sixty diabetic subjects, 44 men and 16 women, between 20-80 years of age, were recruited, chronologically, for this study. Subjects treated for high blood pressure were not included. Their autonomic function was assessed by the total score of five classical cardiovascular function tests. In the same subjects and in 44 healthy subjects, blood pressure and HR were determined from beat to beat by the Finapres system with subjects in the sitting position. We examined the randomness of the HR changes by calculating the zeta statistic of the runs test on 1,000 successive HR readings (the zeta value is low if the HR changes are random). When the HR changes are random, we consider that the autonomic control of HR is impaired. RESULTS: The zeta values of HR changes were significantly lower in diabetic subjects compared with normal subjects (2.98 +/- 0.97 vs. 3.54 +/- 0.97, P < 0.004). In diabetic subjects, the zeta value was closely correlated to the total score of disautonomy (r = -0.66, P < 0.0001, after correction for age effect) and to the office systolic blood pressure (r = -0.43, P < 0.001). CONCLUSIONS: The zeta value of HR changes might be a marker of the autonomic function in diabetic subjects.

Adult↗
PubMed1994

Ambulatory blood pressure in diabetic subjects.

Ambulatory blood pressure (ABP) was measured every 15 min for 24 h in 82 diabetic subjects aged 35 to 79 years and in 66 healthy controls having the same age and office blood pressure. The autonomic control in diabetic subjects was evaluated by the total score of five cardiovascular function tests (a high score means an autonomic neuropathy). The diurnal cycle of BP was assessed by the difference of BP between daytime and nighttime (delta BP = BP in the day - BP in the night). The variability of BP was evaluated by the standard deviations of the readings. Compared with control subjects, diabetic subjects had the same 24-h mean level of BP, a smaller delta BP, and an increased variability during the daytime; however, the differences were in the limit of statistical significance. Clearcut results were obtained in diabetic subjects with autonomic neuropathy. In the latter, the score of autonomic neuropathy was (1) negatively correlated to delta SBP (systolic) and delta DBP (diastolic) (r = 0.44, P = .0004 and r = 0.46, P = .0004, respectively) and (2) positively correlated to the variability of SBP and DBP during the daytime (r = 0.46, P .0004 and r = 0.29, P = .03, respectively). In diabetic subjects, mean level and variability of ABP were positively correlated to urinary microalbumin. The relationships were the most significant when one relates microalbuminuria to the level of SBP in the night (r = 0.42, P < .0003) and to the variability of SBP in the day (r = 0.32, P = .008).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

[Action of trandolapril on the blood glucose balance and microalbuminuria in hypertensive diabetics].

The aim of this study was to evaluate the action of trandolapril on blood glucose control and microalbuminuria in mild to moderate hypertensive in patients with non-insulin-dependent diabetes. Sixty-seven patients, aged between 33 and 79, were enrolled. After a two week placebo run-in period, treatment with trandolapril as monotherapy was given for 3 months. The dose of trandolapril was adjusted between 1 and 4 mg/day according to antihypertensive response. Patients were assessed clinically and by laboratory investigations each month. Two patients were excluded from efficacy analysis because of major protocol deviations. Mean DBP fell, under the influence of treatment, from 101 +/- 5 mmHg to 82 +/- 7 mmHg (p < 0.0001) and mean SBP from 171 +/- 9 mmHg tp 147 +/- 11 mmHG (p < 0.0001). At three months, 54 patients (84%) had a DBP < or = 90 mmHg. Microalbuminuria decreased significantly (p = 0.03) during treatment. Microalbuminuria returned to normal in 11 of the 13 patients in whom the baseline value was above 21 micrograms/min and increased to above normal in 2 of the 26 patients who had a normal baseline value. Blood glycosylated hemoglobin, fructosamine, glucose and creatinine, and creatinine clearance remained stable. Plasma potassium rose slightly in 7 patients. Six adverse events were reported (4 coughs, 1 peripheral edema, 1 plantar mal perforans). One patient died from pulmonary embolism. In conclusion, trandolapril is an effective antihypertensive agent in hypertensive diabetics. Trandolapril causes a significant decrease in microalbuminuria and does not interfere with blood glucose control in these patients.

Aged↗