Search

We report our search for methane in the atmosphere of Mars using high-spectral resolution heterodyne spectroscopy in the 7.8 μm wavelength region. Resolving power and frequency precision of >10 6 of the technique enable identification and full resolution of a targeted spectral line in the terrestrial-Mars spectrum observed from the ground. Observations were carried out on two occasions, in April 2010 and May 2012 at the McMath-Pierce Solar Telescope and the NASA Infrared Telescope Facility, respectively. A single line in the ν 4 band of methane at 1282.62448 cm −1 was targeted in both cases. No absorption due to methane was detected and only upper limits of ∼100 ppb for the martian atmospheric methane concentration were retrieved. Lack of observing time (due to weather) and telluric opacity greater than anticipated led to reduced signal-to-noise ratios (SNR). Based on current measurements and calculations, under proper viewing conditions, we estimate an achievable detection limit of ∼10 ppb using the infrared heterodyne technique – adequate for confirming reported detections of methane based on other techniques.

Spectroscopic data obtained by the Infrared Interferometer Spectrometer (IRIS) aboard Voyager 1 and 2 have been re-visited. Using the spectroscopic data and footprints of the IRIS aperture on the planet, we constructed images of the stratosphere of Jupiter at the emission bands of hydrocarbons including , and . Thermal emission from the hydrocarbons on Jupiter originates from a broad region of the stratosphere extending from 1 to 10 millibars. We averaged the data using a bin of 20 degrees of longitude and latitudes in order to increase signal-to-noise ratios. The resultant images show interesting wave structure in Jupiter`s stratosphere. Fourier transform analyses of these images yield wavenumbers 5 - 7 at mid-Northern and mid-Southern latitudes, and these results are different from those resulted from previous ground-based observations and recent Cassini CIRS, suggesting temporal variations on the stratospheric infrared pattern. The comparisons of the Voyager 1 and 2 spectra also show evidence of temporal intensity variations not only on the infrared hydrocarbon polar brightenings of hydrocarbon emissions but also on the stratospheric infrared structure in the temperate regions of Jupiter over the 4 month period between the two Voyager encounters. Short running title: Stratospheric Images of Jupiter derived from Voyager IRIS Spectra.

Infrared heterodyne spectroscopy (λ/Δλ⩾106) enabled the precision measurement of lineshapes of 168 ethylene absorption transitions for 12C2H4 and 88 absorption transitions for 13 C 12 CH 4 . Of the 168 12C2H4 transitions, 96 are assigned to ν7, 17 to ν10, 10 to ν4, and 45 to hotband transitions. The 88 13 C 12 CH 4 transitions have not been assigned. Measurements were made at gas temperatures in the range of 293– 297 K and at pressures in the range of 0.05– 0.5 Torr . Absolute line frequencies and line intensities were retrieved for each of the transitions. Frequencies and absolute intensities of unblended strong lines were determined to better than 5×10 −5 cm −1 and 10%, respectively.

Self- and nitrogen (N2) broadening of ethylene (C2H4) have been measured for the ν 10 20 11,10 ←19 10,9 ethylene transition at 927.01879 cm −1 . The measurements were made using infrared heterodyne spectroscopy with a spectral resolution of ∼1.7×10−4cm−1. At 296 K , the retrieved self-broadening coefficient for the observed transition is 0.1244(92) cm −1 / atm ; the nitrogen-broadening coefficient is 0.0757(99) cm −1 / atm . These results represent the most conservative treatment of experimental and statistical errors. A different purely statistical approach provides consistent values with uncertainties of only a few percent.

[1] High-resolution infrared spectroscopy of ethane (C2H6) emission features formed in the stratosphere of Titan was collected on disc center at 11.74 μm wavelength (851 cm−1) on 15 January 2005 UT. The observations were obtained at the Subaru 8.2 m telescope of the National Astronomical Observatory of Japan on Mauna Kea, Hawaii, using the NASA Goddard Space Flight Center Heterodyne Instrument for Planetary Winds and Composition (HIPWAC). Fully resolved rotational-vibrational transitions of C2H6 were measured with resolving power λ/Δλ ≥ 106 by infrared heterodyne spectroscopy (IRHS). The spectrum is reproduced most effectively by vertical profiles of ethane abundance that are uniform through the stratosphere and enhanced within the mesosphere. Profiles in which there is a significant gradient within the stratosphere are not favored. The retrieved stratospheric ethane mole fraction depends weakly on the form invoked for the mesospheric enhancement. Two forms of the ethane mole fraction profile are found to reproduce the observed spectrum effectively: the best fitting results are obtained with a profile in which the mesospheric ethane concentration increases logarithmically versus decreasing pressure, retrieving a stratospheric ethane concentration of 8.2 ± 2.1 × 10−6 (1σ), increasing proportional to p−1.2 from the stratopause through the mesosphere (p is pressure). A second form of profile, in which the mesospheric ethane concentration is enhanced uniformly by a factor of 9.5, retrieves a stratospheric concentration of 9.7 ± 4.9 × 10−6 (1σ), with the enhancement discontinuity at about one scale height above the stratopause. The retrieved stratospheric mole fraction is consistent with earlier retrievals from IRHS and is somewhat less than contemporaneous retrievals from infrared spectroscopy at lower resolution by the Cassini spacecraft. The retrieved mesospheric concentration is consistent with in situ measurements in Titan's thermosphere made by the Cassini Ion Neutral Mass Spectrometer instrument during the Titan flyby (Waite et al., 2005).

Heterodyne spectroscopy at infrared (IR) wavelengths is a technique well suited for measuring small velocities in the solar atmosphere. An IR heterodyne spectrometer for solar oscillation measurements has been located at the McMath Solar Telescope of the Kitt Peak National Solar Observatory. It is now being used for single point Doppler shift measurements of 11 micron OH absorption features formed in the upper photosphere, with sampling rates as high as 33 mHz. The instrument employs a stabilized CO2 laser permitting absolute velocity measurements with an uncertainty of < 10 ms-1.

Published data on a select group of SiO maser sources have been analyzed for velocity variations as a function of phase. No apparent correlation was found to a level of about 2 km/s. This places constraints on the location of the maser molecules. Such a correlation should be present at some level. The implications for future high resolution infrared measurements are discussed.