Data Set Information
DATA_SET_NAME CH3D ICE ABSORPTION COEFFICIENTS V1.0
DATA_SET_ID EAR-X-I2041-5-CH3DICESPEC-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : Quoting from the Icarus paper: 'Samples were mixed at room temperature in [a 2 liter] volume using CH4 and N2 gases supplied in high pressure cylinders by Airgas Specialty Gases. These were connected through separate pressure regulators and valves to the mixing volume. The reported purities were >:99.99% for the CH4 and >:99.9% for the N2. To these gases, we could add CH3D purchased in a lecture bottle from Sigma Aldrich (product number 490237, with a reported purity of >:98%). Unlike in water, proton-deuteron exchange is negligible in methane at room temperature and below, so we did not need to consider the formation of other isotopomers such as CH2D2 and CHD3 (Sigma Aldrich Stable Isotope Department, personal communication 2010). After mixing gases to the desired composition at room temperature in the mixing volume, we opened a valve to allow the gas to flow into the empty, cold cell, condensing it as a liquid. We froze this liquid by reducing its temperature, maintaining a vertical thermal gradient of about 2 K across the 15 mm diameter sample by means of heaters (see Fig. 1), so that it would freeze from the bottom upward, with the location of the freezing front being controlled by the cell temperature. Each new mixture requires some experimentation to find appropriate cooling rates, but rates for freezing samples were typically in the range of -0.01 to -0.1 K/min. It was sometimes necessary to freeze an initial polycrystalline mass and then melt almost all of it to obtain a small seed crystal before re-freezing slowly, in order to obtain an optical-quality ice sample. After a suitable sample had been frozen, we removed the thermal gradient by smoothly shifting temperature-control heating from the upper to the lower heating element over a period of 10-20 min. Using only the bottom heater for temperature control resulted in a thermal gradient across the sample of just a few tenths of a Kelvin. 'Spectra were recorded with a Nicolet Nexus 670 Fourier transform infrared (FTIR) spectrometer at a sampling interval of 0.24 cm-1, resulting in a spectral resolution of 0.6 cm-1 (measured full width at half maximum of unresolved lines). The spectrometer beam was focused to a few mm spot inside the cell. We typically averaged over 100 spectral scans to improve the signal/noise ratio. After a sample spectrum had been recorded, we would ramp to a new temperature, at rates typically in the range of 0.1-0.5 K/min. We recorded spectra through our ice samples at every multiple of 10 K between 40 K and the host ice melting points (90.7 K for CH4 ice, 63.1 K for N2 ice). 'Before each ice sample was prepared we also recorded spectra through the empty, cold cell, and did the same after each sample was eliminated. Filled-cell spectra were divided by empty-cell spectra to remove gross effects of lamp emission, detector sensitivity, and absorptions by the windows and air, resulting in approximate transmission spectra T(k). These spectra are affected by subtle, spurious slopes from a variety of sources. Wavelength-dependent refractive index contrasts exist between ice and cell windows leading to slightly different transmission through the ice-window interface than through the vacuum-window interface. The room temperature refractive index n(k) of sapphire decreases gradually with wavelength from about 1.76 to 1.59 from 1.0 to 5.5 microns (Malitson et al., 1958; Gervais, 1991) but methane ice shows almost no wavelength dependence in its refractive index (except at wavelengths near 3.3 microns where absorption is so strong that we measure no transmission whatsoever; Pearl et al., 1991). If we knew the temperature dependent n(k) of both sapphire and ice, and the ice-window interfaces were solely responsible for these slopes, we could easily correct for the effect. But as the ice and the surrounding cell contract on cooling, each with their own distinct temperature-dependent coefficients of thermal expansion, the sample is stressed and can fracture or pull away from the windows, opening additional ice-vacuum interfaces that can produce wavelength-dependent scattering that varies with temperature, with thermal history, and with location within the sample. In addition to imparting slopes, these effects lead to a decline in overall transmission, by as much as a factor of three in an ice sample cooled relatively rapidly from 90 to 40 K. Slow drifts in lamp filament temperature or detector sensitivity over the course of experiments lasting multiple days can also contribute spurious slopes. Slopes arising from any combination of the above factors were removed by fitting a line or low-order polynomial to continuum regions adjacent to absorption bands to be quantified and dividing by this function to 'straighten out' the continuum. The continuum-corrected transmission spectra were then converted to Lambert absorption coefficient spectra alpha(lambda) via the Beer-Lambert absorption law, rearranged as alpha(lambda) : -ln(T(lambda))/d, where d is the path length through the cell (d : 5.4 +- 0.1 mm for all experiments reported in this paper). 'We added a small amount of CH3D to CH4 to produce a deuterium-enriched sample that remained dominated by normal CH4 absorptions. Addition of 0.5% CH3D (D/H ratio of 1.25 x 10-4) produced ice having sufficiently strong CH3D absorption to be easily measurable, but not so strong as to be saturated in transmission though our 5.4 mm optical path length. We were not able to directly measure the D/H ratio in our ice sample, so we assumed zero fractionation between the gas phase and the liquid condensed from it, and again zero fractionation between the liquid and the solid crystallized from it. 'We subtracted the ordinary methane absorption coefficients to isolate the contribution of the CH3D. The resulting CH3D ice absorption coefficients were divided by the 0.5% concentration of the CH3D (assuming the composition was unchanged by condensation and freezing) to obtain effective absorption coefficients for CH3D in methane as if it were pure CH3D. These values can be combined with ordinary CH4 ice absorption coefficients, scaled by their relative abundances, to approximate absorption coefficients for arbitrary CH3D/CH4 mixing ratios. 'We performed an experiment with CH3D diluted in the hexagonal beta phase of N2 ice. N2 ice melts at 63.1 K, so this experiment spanned a smaller range of temperatures. Uncertainty over the CH3D concentration in the ice presented even more of a challenge with this experiment. The CH3D/N2 ratio mixed in the gas phase in the mixing volume could not be expected to remain unchanged in the ice, for two reasons. First, methane is much less volatile than nitrogen. On condensing the N2-dominated gas into the cell as a liquid at about 65 K, some of the deuterated methane could have condensed as frost somewhere in the inlet tube rather than making it into the cell. Second, compositional gradients appear on freezing, as a result of the separation between liquidus and solidus curves of the binary phase diagram of nitrogen and methane (e.g., Prokhvatilov and Yantsevich, 1983; unfortunately, the two curves are difficult to distinguish in their figure). Quirico and Schmitt (1997) assumed that the integrated absorptions of CH4 bands remain unchanged on dilution in N2 ice in order to estimate the composition of their samples. Making the same assumption for CH3D, we used the integrated absorption of the 4.56 micron CH3D band to estimate the CH3D fraction in our mixed CH3D + N2 ice sample as 0.0024 +- 0.0002 (about a factor of two below its gas phase abundance of 0.005). As before, we subtracted the absorption coefficients of the host N2 ice, studied in an otherwise identical separate experiment in our laboratory, to isolate the CH3D absorptions.
DATA_SET_RELEASE_DATE 2014-01-13T00:00:00.000Z
START_TIME 2010-07-21T12:00:00.000Z
STOP_TIME 2010-08-25T12:00:00.000Z
MISSION_NAME SUPPORT ARCHIVES
MISSION_START_DATE 2004-03-22T12:00:00.000Z
MISSION_STOP_DATE N/A (ongoing)
TARGET_NAME CH3D DILUTED IN CH4 (METHANE) ICE
CH3D DILUTED IN N2 (NITROGEN) ICE
TARGET_TYPE TERRESTRIAL SAMPLE
INSTRUMENT_HOST_ID LAB9884
INSTRUMENT_NAME NICOLET NEXUS 670 FTIR SPECTROMETER
INSTRUMENT_ID I2041
INSTRUMENT_TYPE SPECTROMETER
NODE_NAME Small Bodies
ARCHIVE_STATUS LOCALLY_ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview : Temperatures uncertainty is about +- 0.5K. Cell thickness uncertainty is about +- 0.1 mm. Composition uncertainty is not well determined.
CITATION_DESCRIPTION Grundy, W.M., Morrison, S.J., Bovyn, M.J., Tegler, S.C., and Cornelison, D.M., CH3D ice absorption coefficients V1.0. EAR-X-I2041-5-CH3DICESPEC-V1.0. NASA Planetary Data System, 2014.
ABSTRACT_TEXT This data set is the result of laboratory transmission spectroscopy experiments performed within a sealed cell in the ice laboratory at Northern Arizona University. We record spectra at wavelengths from 1 to 6 microns to study CH3D bands at 2.47, 2.87, and 4.56 microns (as indicated by '25', '29' and '45' in the data file names). We report temperature-dependent absorption coefficients of these bands when the CH3D is diluted in CH4 ice and also when it is dissolved in N2 ice. These absorption coefficient spectra can be combined with data from the literature to simulate arbitrary D/H ratio absorption coefficients for CH4 ice and for CH4 in N2 ice. The data were originally published in 2011 in Icarus, vol. 212, pp. 941-949, and that paper should be consulted for details about laboratory procedures and limitations of the data.
PRODUCER_FULL_NAME WILL GRUNDY
SEARCH/ACCESS DATA
  • SBN Comet Website
    		•