Christopher D. Barnet
General Sciences Corporation, Laurel MD 20707-2929
John Caldwell, Cindy C. Cunningham
York University, Toronto ON M3J 1P3 Canada
NRC and GSFC/NASA, Greenbelt MD 320771
Keywords: Mars, carbon monoxide, Cameron band, ultraviolet, HST/GHRS
Previous measurements of carbon monoxide (CO) on Mars (Clancy et al., 1983) have shown that its mixing ratio in the Martian atmosphere is variable and that there may be seasonal fluctuations. Further, the altitude distribution of CO is not well constrained.
We have observed Mars with the HST/GHRS, in the spectral vicinity of the Cameron 0-0 band near 2060Å. The CO mixing ratio derived from the present study, 600 300 ppm, is comparable to measurements by some others (Clancy et al. 1990, and Encrenaz et al. 1991) which show CO mixing ratios in the range of 550--800 ppm. However, it is considerably smaller, by a factor of 3--5, than those of Kakar et al. (1977) and of Good & Schloerb (1981).
Measurements of CO from a variety of studies performed during different Martian seasons consistently give CO mixing ratios in the above range. This is puzzling since the main atmospheric constituent, CO, is subject to substantial seasonal variations due to condensation (fall and winter) and sublimation (spring) on the polar caps. Measured variations in CO should result in measurable variations in CO by a factor of 2--4 according to predictions from Mars general circulation models.
The photochemistry, transport properties and thermal structure of the atmosphere of Mars, and the temporal variations of those properties on daily and seasonal time scales, are important to understanding the basic questions about the Martian environment---its origin, evolution and stability. This paper, in which a previously observed species is observed by a new technique, addresses one of these areas---photochemistry---directly.
The photochemistry of Mars is currently uncertain in several respects. One is the question of the suitability of homogeneous (gas phase) chemistry to explain the abundance of trace species, including CO as well as HO, O and O, in the atmosphere. Atreya & Gu (1994,1995) have suggested that some kind of heterogeneous catalytic process, perhaps involving aerosols of ice or dust, may be required to explain the recycling rate of CO on Mars: specifically, aerosol surface chemistry may be required to provide a sink for some of the trace species which participate efficiently in the recycling of CO.
The present CO result, which is derived from ultraviolet spectroscopy, is independent from, but consistent with, other results cited above, all of which were made at much longer wavelengths, where the atmospheric radiative transfer is much different. In principle, our result has the potential to determine the temperature of the Martian atmosphere from rotation-vibration line structure, using radiative transfer calculations where the source function is entirely due to scattering in an optically thin medium, which is specifically independent of thermal radiation. In practice, however, uncertainties in transforming the measured Mars flux to albedos preclude such determination in the present data.
Like the other methods, ours is extremely insensitive to the vertical distribution of CO on Mars.
An ultraviolet spectrum of Mars was obtained by the G200M grating of the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) on April 26, 1993, at 5:04 UT. The grating has a spectral resolution of 25000 and, for this observation, was centered on the Cameron 0-0 CO band between 2060 and 2070Å. The observing parameters for Mars at the time of the observation are summarized in Table 1.
Table 1: Parameters for the April 26, 1993, 5:04 UT Mars Observation
In order to compare the data to radiative transfer atmospheric models, one must convert flux to normalized intensity, . The first step in this process is to divide the HST Mars spectrum by a solar spectrum. We chose the 1983 solar measurements of Hall & Anderson (1991) because theirs are the only ones in this wavelength region with sufficient spectral resolution (0.1Å) not to compromise information in the HST spectrum. Their data, obtained from a balloon near 40km altitude, were extrapolated to zero optical depth by correcting for ozone, O Herzberg continuum absorption, and Rayleigh scattering.
Dividing the Mars spectrum by the solar spectrum requires that both spectra be sampled at the same wavelengths. The Mars spectrum is sampled at 0.08Å, while the solar spectrum is sampled at 0.1Å. We, therefore, used a cubic spline interpolation to calculate the solar flux at the exact wavelengths of the HST data points. The two spectra are shown together in Figure 1. In that figure, the data have been smoothed with a 3-point running mean filter to improve the signal to noise ratio.
Figure: The Mars spectrum (darker line) and the Sun, with arbitrary normalization. Both spectra have been smoothed, and Mars has been shifted by Å with respect to the Sun, as discussed in the text. Near 2062Å, where the CO Cameron band occurs, Mars is noticeably relatively less bright than the Sun. Aliasing effects in solar Fraunhofer lines is apparent.
Before we plotted Figure 1, we had discovered an unexpected relative wavelength shift between the planetary and solar data. To investigate the apparent anomaly, we introduced a systematic variable wavelength shift, , between the two spectra and minimized the square of the sum of the variances in the continuum region, outside the region of the CO band, where we expect real differences to occur in spectral detail. The minimum in the sum of the square of the variances in the continuum region corresponds to a wavelength shift of Å, which is included in Figure 1.
The wavelength shift is not a Doppler effect. At the time of observation, the geocentric velocity of Mars was 16.25kms which corresponds to a redshift in the Mars spectrum of +0.11Å (i.e., Mars is receding from the Earth). This ``best fit'' shift is therefore larger than the Mars radial velocity by a factor of 4, and in the opposite sense.
During the data reduction process, we learned that there has been a systematic problem with the HST ground software system for doing spectroscopy of moving targets. Specifically, the standard target specification process for a planet to be observed with the HST includes not specifying the right ascension and declination of the target in advance but allowing the scheduling system to insert the correct position at the time of observation. That aspect of planet tracking has worked very well, but it had not been realized that there was no corresponding insertion of the correct position in the on-board GHRS Doppler correction software. The latter effectively assumed that all planetary targets were at 0 RA, 0 dec, instead of their correct positions, and applied erroneous Doppler compensation to the data before telemetry to the ground.
In view of this known defect in the data, which are otherwise clearly good, we have not further investigated the wavelength discrepancy between the planetary and solar spectra.
Two features are apparent in Figure 1. First, in the region between 2060 and 2065Å, where the CO Cameron band occurs, Mars is noticeably less bright than the Sun compared to the rest of the spectrum. This is the essential result of this paper, and is discussed in detail below.
It is also obvious in Figure 1 that there is significant aliasing between the two spectra in the solar Fraunhofer lines. This problem has been found in many HST and other planetary spectroscopy programs, and is due to the intrinsic difficulty in matching the characteristics of two different spectrographs. Since it is not possible to observe the Sun with the HST, it is necessary to use a solar spectrum obtained elsewhere. When the planetary spectrum is divided by the solar spectrum to obtain an albedo, the aliasing results in ``noise'' features that are frequently the limiting factor in determining the value of the data. See Figure 3 below, for example.
One potential solution to this problem is to use HST observations of solar substitutes, including planetary satellites. At the time of writing, there are plans to perform just such observations for calibration purposes. One problem is that, especially in the ultraviolet, the signal from satellites is sufficiently weak that there will be significant photon noise in the substitute spectrum. A potential solution to that problem is to use the brightest known planetary satellite which, unfortunately, is also a very difficult object for the HST to track. Efforts will be made to observe the Moon for this purpose, and it is hoped that future papers will benefit from them.
The final albedo spectrum, which is included in Figures 2--5 for comparison with atmospheric models, is
where is the heliocentric distance of Mars in AU; is the solar flux at 1 AU; is the measured intensity of Mars; , is the wavelength shift required to bring the two spectra into coincidence; and is the area of the GHRS aperture, 4 arcsec.
Models of the geometric albedos of Mars were calculated to match the HST observations. The atmospheric code is derived from a doubling and adding algorithm developed at Princeton University by R. E. Danielson, P. G. Wannier, and W. D. Cochran. For the new Mars observations, it was necessary to modify the code to calculate the albedo at the observational phase angle of 36.9.
The albedo of Mars in the wavelength region of the CO Cameron band is dominated by optically thin Rayleigh scattering from CO molecules, with reflection of sunlight from the surface also being significant (Caldwell 1973). In this study, we simulate the Martian environment by two vertically homogeneous layers. The first layer consists of uniformly mixed CO and CO. The second layer, which is modeled as a thick layer of grey, isotropic scatterers, simulates the dark reflecting surface. The free parameters are the abundance of CO, the mixing ratio of CO, and the surface reflectivity.
Since laboratory measurements of absorption cross section are extremely difficult at Martian pressures (typically 7mbar), a theoretical model was employed to simulate the Cameron 0-0 CO band. The theoretical basis of this model and comparisons to laboratory measurements at 185mbar and 215mbar are discussed by White et al. (1993) and by Caldwell et al. (1993). In summary, we are confident that the model successfully reproduces CO line positions and strengths, including the latter as a function of temperature.
The model was calculated at a spectral interval of 0.001Å, which ensures that each rotational line is represented by a minimum of 5 points. It was run for temperatures of 150, 200 and 250K, respectively, and at Martian pressures. It turned out that the data were too noisy to allow estimation of the temperature from the variation of line strengths within the band, so in our best-fitting model below, we used 200K. Individual line profiles were assumed to have Doppler broadening only.
It is computationally prohibitive to calculate the exact atmospheric model at every wavelength point (roughly 10000 per model) within the CO absorption range. Instead, we calculated the continuum model every 5Å\ and for each such subrange, we calculated the atmospheric model for 10 logarithmically-spaced values of absorption, corresponding to the local range of CO absorption. Effectively, the result of this calculation is a two-dimensional grid of albedo as a function of absorption coefficient and wavelength. The true albedo at each wavelength can be found accurately by interpolation within this grid. We verified a segment of our interpolation method with the ``tru'' model calculated at full resolution.
The high resolution atmospheric model was then binned to 0.08Å and both the data and model were smoothed with a five-point running mean for presentation. The treatment of the model is directly comparable to that for the data.
Because the continuum model contains two free parameters, there is some uncertainty in the ``best'' model fit, which we discuss below. Our preferred model has a CO column abundance of 84 meter-amagats (corresponding to a CO surface pressure of 6.25mbar), a CO mixing ratio of 600 ppm, and a grey surface albedo of 0.035. In Figure 2, we show the model for these parameters, both at full resolution and with binning and smoothing. Also shown in this figure are the GHRS Mars data.
Figure: The full resolution (i.e., 0.001Å resolution) model is shown as a light line. These data, binned to 0.08Å and smoothed with a 5 point running mean filter (solid dark curve), are compared with the GHRS data (dashed curve), also smoothed with a 5 point running mean filter. This is the ``best fit'' atmospheric model consisting of a surface pressure of 6.25mbar, 600 ppm of CO, and a surface albedo of 0.2.
The surface albedo and abundance of CO can both be adjusted to fit the continuum level. However, increasing the ratio of CO to surface contribution will cause the slope of the continuum to increase toward shorter wavelengths because the continuum scattering is optically thin and exhibits the dependence of Rayleigh scattering. Conversely, to create a ``flatter'' albedo than shown would require a significantly greater contribution of light from the surface.
In Figure 3, we compare the best fit model with an extreme model having a single layer of CO and no reflecting surface. We require 150 meter-amagats (surface pressure = 11.1mbar) to match the albedo of the Mars spectrum in the case of no surface reflectivity, but it is clear that the slope of the continuum is too large. In general, we find that models with more than 7mbar of CO have too large a slope in the continuum. Also shown in Figure 3 is a model with 84 meter-amagats without a surface, to illustrate the albedo of the surface.
Figure: Three models compared to the GHRS Mars spectrum, which is shown as a broken line. Three features which are probably due to aliasing effects with solar features are indicated as ``solar''. The ``best fit'' model, including non-zero surface reflectivity and 6.25mbar of CO is shown as a dark line; a model with no surface reflectivity and 11.1mbar of CO is shown as a lighter line; a model with 6.25mbar of CO and no surface albedo is included at the bottom of the figure.
We estimate the uncertainty in the CO surface pressure to be about 1mbar. The CO mixing ratio is relatively insensitive to the ratio of albedo concentration of CO and surface since the atmospheric scattering is optically thin in both the extreme models.
In Figure 4, we show two models where the abundance of CO is varied. Both models have 6.25mbar of CO and a surface albedo of 0.035. The light line represents 50% more CO (900 ppm) than the ``best model'' (600 ppm) and the dark line represents 50% less CO (300 ppm). From these more poorly fitting models, we estimate the error on the CO abundance to be 300 ppm.
Figure: Two models are shown to illustrate the sensitivity of the model to the abundance of CO. The dark line has 50% more CO and the light line has 50% less CO than the ``best fit'' model.
Although the molecular band model shows some variation of absorption coefficient with temperature, the low signal-to-noise of the GHRS data and the binning to 0.08Å preclude determination of the temperature of the absorbing CO. We chose the 200K model for our analysis based on temperature soundings and average conditions on Mars.
In Figure 5, we show the temperature effect. Clearly, the greatest spectral difference between the models with different temperature occurs in the long wavelength part of the band, between 2065 and 2070Å, where several solar aliasing features occur. (See Figure 1, above.) The aliasing problem precludes distinguishing between the temperature models in the current reduction of the data.
Figure: Two models are shown to illustrate the sensitivity of the model to the temperature of CO. The dark curve (deeper absorption) is the absorption of CO at 250K and the lighter curve is at 150K.
We have demonstrated that measurement of the difficult CO 0-0 Cameron band is feasible with the HST. Future observations covering the different Mars seasons will be an important HST science objective in the coming years. With the advent of long-slit spectroscopy with the Space Telescope Imaging Spectrograph (STIS), the observations can be very efficient in covering different latitude regions on the planet. If a suitable solar substitute object, preferably the Moon, can be observed with the HST, the resulting albedo curves will be much less noisy than the one presented herein.
Accurate and comprehensive studies of the chemical properties of Mars with the HST will be an important part of its future planetary program.
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