1.

Introduction

One of the most intriguing and challenging questions in Mars science is undoubtedly whether life currently exists on Mars. It has long been recognized that high-precision measurements of methane (${\mathrm{CH}}_4$) volume mixing ratio (VMR) and its spatial and temporal distributions in the Martian atmosphere will provide invaluable insight to this question. Recently, in situ measurements by the Mars Science Laboratory (MSL) have resulted in an upper limit of 1300 parts per trillion by volume (pptv),¹ whereas previous measurements using terrestrial telescopes and an instrument in Mars orbit reported significantly higher values of 10,000 pptv or more.^2–4 These results are not necessarily contradictory, due to the possibility of spatial and temporal variability of the trace gas concentration.^5,6 Thus, more measurements will be required to gain clarity.

The case of methane on Mars illustrates that, in general, high-precision and high-accuracy measurements of trace gases in planetary atmospheres are a powerful scientific tool. Thus, miniaturized, highly sensitive instruments providing these data are required to optimize the limited resources available on planetary missions. Here, we present a conceptual design for a miniaturized, high-resolution spectrometer to view the sun through the Martian atmosphere to measure the midinfrared absorption of atmospheric $\mathrm{C}{\mathrm{H}}_4$, water (${\mathrm{H}}_2\mathrm{O}$), and carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$). Using such solar occultation measurements, high-precision VMRs of $\mathrm{C}{\mathrm{H}}_4$ and ${\mathrm{H}}_2\mathrm{O}$ can be retrieved. Even though we concentrate on $\mathrm{C}{\mathrm{H}}_4$ in this study, the instrument concept can be applied to other trace gases, provided suitable spectral absorption features are available.

The desired instrument has to have several key properties. First, it requires high spectral resolution to unambiguously separate the absorption lines of the different molecular species. Second, it requires a high signal-to-noise ratio for relatively weak absorption features. Third, it needs to be able to account for multiple scattering by aerosols in the Martian atmosphere, which results in an effective increase in path length through the atmosphere and therefore could result in systematic uncertainties in the VMR retrievals. Finally, it has to be compact and robust to be a suitable candidate for a planetary mission.

Considering these requirements, a spatial heterodyne spectroscopy (SHS) instrument,⁷ measuring high spectral resolution atmospheric transmittance by directly viewing the sun through the atmosphere, is well suited for this measurement challenge. It allows very high spectral resolution within a compact package and it requires no moving spectrometer parts. Accounting for multiple scattering is achieved by simultaneously measuring $\mathrm{C}{\mathrm{O}}_2$, the dominant Martian atmospheric molecule, at virtually the same wavelength, which provides an air mass along the same effective absorption path length through the atmosphere. Thus, the simultaneous $\mathrm{C}{\mathrm{O}}_2$ measurement allows for an accurate methane VMR determination.

In the following section, we first describe the science and measurement requirements that drive this concept design. In Sec. 3, we discuss the bandpass selection, and in Sec. 4, we show the key design aspects of the miniaturized Mars methane monitor (MMMM) concept. The performance estimates for the instrument are given in Sec. 5 followed by a brief summary.

In this concept study, we targeted the deployment of the instrument on the surface of Mars; however, a similar instrument can be operated from a Mars balloon, aircraft, or even from orbit.

2.

Science Requirement

Any future mission will have science requirements defining the precision and accuracy of the VMR measurements and the time that is available to make the measurement. These science requirements will drive the size, weight, power, and data rate of the selected instrument. To anticipate such requirements for upcoming missions, we consider the previous measurements that report a range of VMRs between an upper limit of 1300 pptv all the way to several ten thousands of pptv as discussed above. In addition, we consider the capabilities of the tunable laser spectrometer on MSL which was designed for a precision of 300 pptv assuming a 15 min integration time, and which, with the help of sample analysis at Mars, is expected to perform down to a few parts per trillion.⁶

The observations to date suggest that measurement durations of one sol (24.6 h) or less should be sufficient. However, when considering instrumental properties like detector dark noise, thermal drifts, etc., shorter integration times are desirable for individual measurements. Individual measurements can subsequently be averaged to increase the measurement precision at the expense of the temporal resolution. Using the SHS instrument concept, which uses an imaging electro-optic detector such as a charge-coupled device, we estimate that an integration time on the order of minutes is desirable.

Considering the above, we concluded that a precision requirement of 100 pptv, combined with an integration time of 1 min is a realistic requirement for a future mission.

3.

Bandpass Selection

The spectral region between 3200 and 3240 nm is well suited for this measurement because it contains molecular absorption lines of $\mathrm{C}{\mathrm{H}}_4$, ${\mathrm{H}}_2\mathrm{O}$, and $\mathrm{C}{\mathrm{O}}_2$ as shown in Fig. 1. The simultaneous measurement of all the three atmospheric components is highly desirable for this instrument concept. $\mathrm{C}{\mathrm{H}}_4$ and ${\mathrm{H}}_2\mathrm{O}$ are of scientific interest and the $\mathrm{C}{\mathrm{O}}_2$ measurement can be used to infer accurate VMRs in the presence of single and multiple scattering by atmospheric aerosols. The wavelength region between about 3230 and 3232 nm is most suitable for this measurement, primarily because it contains the strongest methane lines resulting in the highest $\mathrm{C}{\mathrm{H}}_4$ measurement sensitivity.

Fig. 1

Line strengths of ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{H}}_4$, and $\mathrm{C}{\mathrm{O}}_2$.⁸ The highlighted region contains absorption features of all three species, which enables the simultaneous measurement. A suitable instrument passband includes a roughly 2-nm wide interval containing the strongest $\mathrm{C}{\mathrm{H}}_4$ lines toward the right (long wavelength) edge of the highlighted region above.

Using the selected spectral interval, we calculate the expected signal at the instrument, viewing the sun from the planet’s surface. The calculations use a field-of-view of $1\times 1\mathrm{deg}$ pointed at the sun. Important model characteristics are given in Table 1.

Table 1

Radiative transfer model characteristics.

In particular, we model three quantities: (1) direct flux: solar flux transmitted through the atmosphere; (2) diffuse downward flux: scattered sunlight moving downward onto the planetary surface; and (3) near sun radiance: diffuse radiance from the direction near the sun [$\mathrm{W}/{\mathrm{m}}^2/\mathrm{nm}/\mathrm{sr}$], which is dominated by the forward scattering aerosols. The sum of the three components represents the expected signal. The radiative transfer calculations represent different aerosol optical depths (ODs) and solar zenith angles. Figure 2 shows a typical set of results. Figure 2(a) shows that for clear sky (aerosol $\mathrm{OD}=0.1$) and a solar zenith angle of 10 deg, the spectral power density is dominated by direct sunlight. Figure 2(b) illustrates the result for a solar zenith angle of 65 deg, which shows a factor of 2.25 reduction of the overall signal, but stronger $\mathrm{C}{\mathrm{H}}_4$ absorption, due to the longer absorption path, as is illustrated in the scaled (dashed) trace. Figures 2(c) and 2(d) show, for a solar zenith angle of 10 deg, the decrease in total signal for increasing OD caused by an increased atmospheric aerosol concentration.

Fig. 2

Radiative transfer calculation results for a telescope on the Martian surface with a square $1\times 1\mathrm{deg}$ field-of-view pointed at the sun. The panels (a), (c), and (d) show the total spectral power density and the direct and diffuse components for a solar zenith angle of 10 deg and different aerosol optical depths (ODs). Panel (b) shows the result for a clear sky ($\mathrm{OD}=0.1$) and for a solar zenith angle of 65 deg with the signal scaled (dashed) for comparison of the relative absorption shown in panel (a).

As expected, the radiative transfer calculations show that larger solar zenith angles result in increased trace gas absorption and therefore larger signal levels (relative change in signal within the absorption feature). The results of these radiative transfer calculations are used in the performance estimates discussed in Sec. 5.

4.

Conceptual Instrument Design

4.1.

Optical Design

In this section, we focus on the design of the interferometer, the entrance optic that is necessary to interface with a heliostat and the exit optics, which relay the signal from the interferometer to the focal plane array (FPA). Since heliostats, which point the field-of-view toward the sun, have been implemented before and fundamentally consist only of flat mirrors and a miniaturized sun sensor, we did not further investigate them here. For example, a heliostat based on the PanCam Mast Assembly¹⁴ would be more than adequate.

Here, we focus on demonstrating that a suitable spectrometer with a resolving power of 150,000 can be built in a very small and light package using the SHS technique. The conceptual design is shown in Fig. 3. Figure 3(a) shows the interferometer together with a 5-in. diameter compact disc for scale. Figure 3(b) shows the overall optical design concept, which uses thin lenses for the entrance and exit optics. Figures 3(c) and 3(d) illustrate the conceptual optical design with appropriate fold mirrors to minimize the size of the overall package. Figure 4 shows another view of the interferometer as modeled by the optical design software.

Fig. 3

Optical design. (a) Interferometer comprising a beam splitter and two immersion gratings. In the background, a 5-in. diameter compact disk is shown for size comparison. (b) Overall entrance and exit optics concept using thin lenses and employing an interference bandpass filter. (c) Top view of the optical design using appropriate fold mirrors to minimize the size of the overall package. (d) Side view of the design shown in (c).

Fig. 4

Illustration of the spatial heterodyne spectroscopy interferometer with selected rays of the beam. The incoming and outgoing beams are folded using flat mirrors as shown by the arrows [see Fig. 3(d)].

The interferometer has an illuminated grating area of $30\mathrm{mm}\times 1\mathrm{mm}$ and a beam angle of $1.5\mathrm{deg}\times 5\mathrm{deg}$ at the gratings, with the smaller angle in the dispersion plane. The silicon immersion gratings have a groove density of $1150\mathrm{g}/\mathrm{mm}$ and a blaze angle of about 32.8 deg. This results in a maximum acceptable etendue of about $0.068\mathrm{m}{\mathrm{m}}^2\mathrm{sr}$. Further specifications of the optical spectrometer components are given in Table 2. The design uses an imaging detector with a total illuminated area of $7.67\mathrm{mm}\times 0.25\mathrm{mm}$. With 1024 pixels across the detector, this results in a pixel pitch of about 7.5 μm.

Table 2

Specifications of optical components ordered by their position in the beam path. All of the infrared transmitting optics are silicon except the beam splitter, which is ZnSe.

Filter	$\mathrm{CWL}=3231\mathrm{nm}$	$\mathrm{FWHM}=13\mathrm{nm}$
Lens 1	Cylinder	$f_1=\text{infinity}$	$f_2=156.7\mathrm{mm}$
Lens 2	Cylinder	$f_1=113.8\mathrm{mm}$	$f_2=\text{infinity}$
Lens 3	Cylinder	$f_1=\text{infinity}$	$f_2=3.5\mathrm{mm}$
Lens 4	Cylinder	$f_1=\text{infinity}$	$f_2=77.3\mathrm{mm}$
Mirror 1	First surface	Gold coating
Mirror 2	First surface	Gold coating
Lens 5	Spherical	$f=602.2\mathrm{mm}$
Mirror 3	First surface	Gold coating
Mirror 4	First surface	Gold coating
Beam splitter	Metallic coating
Compensator plate	AR coating
Grating	Gold coating	32.8 deg blaze angle	$1150\mathrm{g}/\mathrm{mm}$
Grating	Gold coating	32.8 deg blaze angle	$1150\mathrm{g}/\mathrm{mm}$
Lens 6	Spherical	$f=602.2\mathrm{mm}$
Mirror 5	First surface	Gold coating
Mirror 6	First surface	Gold coating
Lens 7	Spherical	$f=175.9\mathrm{mm}$

4.2.

Narrow Passband Filter

Like many other spectrometer types, SHS spectrometers require a bandpass filter limiting the spectral region that is accepted by the spectrometer. In the case of SHS, the bandpass filter is generally used to perform two tasks:

• (1) Since the spatial frequency of the fringes created by the interferometer is a function of the absolute difference between the Littrow wavenumber of the interferometer and the incident wavenumber, the fringes have the same spatial frequency for wavenumbers that are equally spaced from the Littrow wavenumber,⁷ similar to the lower and upper sidebands of a traditional heterodyne spectrometer, where the Littrow wavenumber is replaced by the wavenumber of the local oscillator. Thus, a bandpass filter is generally used to limit the incident signal to either the upper or lower sideband, by removing the signal on one side of the Littrow wavenumber. If a signal from both sides of the Littrow wavenumber is admitted, the recovered spectrum will typically be the superposition of the upper and lower sidebands. The recovered spectrum will then appear folded around the Littrow wavenumber.

• (2) The number of elements of the FPA in the dispersion direction limits the number of spatial fringe frequencies that can be recovered unambiguously by the Fourier transform of the recorded interferogram (Nyquist sampling theorem). With $N$ elements, $N/2+1$ independent fringe frequencies can be recovered (0 to $N/2$ fringes across the FPA). Higher fringe frequencies result in aliasing of the recovered spectrum, where a signal with a fringe frequency of $N/2+m$ fringes across the FPA will appear as an aliased signal at $N/2-m$ fringes across the FPA. The aliasing effect results in a folding of the spectrum at the sampling limit ($N/2$) and a reduced visibility, depending on $m$.

As shown in Fig. 5, the desired bandpass of the MMMM instrument is no larger than about 2 nm in wavelength, which corresponds to a wavenumber interval of about $2{\mathrm{cm}}^{-1}$. This is $<0.1\%$ of the center wavelength of a hypothetical passband filter.

Fig. 5

Illustration of the strategy employed for the MMMM to avoid signal contamination of the $\mathrm{C}{\mathrm{H}}_4$, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{H}}_2\mathrm{O}$ absorption, assuming a bandpass filter of about $10{\mathrm{cm}}^{-1}$ is not available. The black line is the solar spectrum multiplied with the atmospheric absorption of the spectral region of interest and the filter transmittance. The side band folding around the Littrow wavenumber and the aliasing around the sampling limit is indicated by the dot-dashed lines. The uncontaminated region containing all the spectral information of interest is highlighted as the hashed area.

However, our discussions with a leading manufacturer of infrared interference filters revealed that in this wavelength region, due to the available production, monitoring, and metrology techniques, the filter width is currently limited to about 0.4% full width at half maximum (FWHM) of the center wavenumber. Covering a wavenumber region of approximately two times the FWHM of the filter is necessary to avoid signal aliasing while using only one sideband. Therefore, an SHS spectrometer with a resolving power of 150,000, would require an FPA of about 2400 elements ($2\times 0.004\times \mathrm{150,000}\times 2$). Even though midinfrared FPAs with $>1024\text{pixels}$ are beginning to enter the commercial market, we did not pursue this option in an effort to minimize technical risk. Instead, we explored whether a certain amount of sideband folding and aliasing is acceptable without contaminating the narrow wavelength region of interest, which will enable the selection of an FPA with 1024 pixels in one-dimension.

Figure 5 shows that this is indeed possible. Assuming a Hann-shaped filter transmittance and allowing a roughly equal amount of sideband folding around the Littrow wavenumber and aliasing around the Nyquist limit, a central uncontaminated region remains, which can be used to recover the methane, $\mathrm{C}{\mathrm{O}}_2$, and water vapor signatures. Figure 5 illustrates a solar spectrum multiplied with the atmospheric transmittance in the wavelength region of interest and the filter transmittance in black. The folding (red) and aliasing (blue) on the short- and long-wavelength sides of the filter transmittance are shown as dot-dashed traces. The region in the center, which is not affected by either spectral superposition (contamination), is highlighted. This region contains all the previously modeled molecular absorption features of interest.

The sideband folding and aliasing on both sides of the bandpass are balanced on both sides of the filter passband so that the wavenumbers of interest are approximately in the middle of the available fringe frequencies. This region of spatial frequencies, as opposed to the highest or lowest spatial frequencies, is least likely to be affected by systematic errors.¹⁵

The disadvantage of a larger than desired bandpass filter is that the multiplex noise level in the recovered spectrum is larger when compared to the case with an optimal, narrower filter width. This effect is included in the performance estimate of the MMMM instrument described in Sec. 5.

4.3.

Mechanical Design

The mechanical design is based on the folded optical layout shown in Fig. 3 and includes a compact and light-weight structure to house the optics, detector, and electronics. This design meets the fundamental instrument requirements and was used to determine the instrument size and mass properties. A final flight design will have to be customized to the payload platform.

Different configurations and views of the three-dimensional assembly model for the MMMM are shown in Figs. 6(a)–6(d). The main structural component is the optical bench, which is a stiffened plate. It carries the optics on both sides. Lightweight covers on the top and bottom sides complete the instrument enclosure. Figure 6(a) shows the assembly with the covers. Figure 6(b) shows the assembly with the top cover removed, revealing the entrance optic, which is mounted off of the optical bench, and the fold mirrors that direct the beam into the interferometer. The interferometer components, gratings and beam splitter, are indicated in green and blue, respectively. A top view is shown in Fig. 6(c). After exiting the interferometer, the beam is again folded underneath the optical bench, which is illustrated in Fig. 6(d). The detector is located at this level, as indicated in Fig. 6.

Fig. 6

Different views of the mechanical design as described in the text. The dimensions are $191\mathrm{mm}\times 173\mathrm{mm}\times 66\mathrm{mm}$ ($L\times W\times H$).

A carbon-carbon composite material is used for the structure because of its light weight, high strength, low coefficient of thermal expansion, and reasonable thermal conductivity.

The input telescope, not shown in Fig. 6, as discussed before, can be as simple as a system of flat folding mirrors, the first of which may be actuated to track the sun and direct its light to the input filter and first lens. Otherwise, simple lenses could be added to shape the field-of-view. As this item critically depends on the accommodations, it has not been detailed further in this study.

For the mass estimates, fasteners are neglected and are assumed to be replaced by epoxy glue wherever possible. Harness and connectors are also minimal since they are limited to heaters and temperature sensors and there are no mechanisms inside the structure.

The structure has a rough envelope of $191\mathrm{mm}\times 173\mathrm{mm}\times 66\mathrm{mm}$, and a total mass of approximately 700 g. Although it is believed that the mass of this structure can be further reduced by a more detailed analysis, adding a $\sim 25\%$ growth margin gives a total mass of under 1.0 kg, excluding the heliostat, focal plane assembly, and any potentially required thermal hardware like radiators and heat straps. The mass breakdown is detailed in Table 3.

Table 3

Detailed mass table of the MMMM instrument, excluding a heliostat, the focal plane assembly, and thermal hardware. The mass is given in grams, while individual masses below 1 g are rounded up to 1 g. All optics component dimensions are increased by 1 mm (or 25% if lighter than 1 g) to accommodate their mounting.

	Material	Density ($\mathrm{g}/{\mathrm{cm}}^3$)	Mass (g)	Mass, with 25% margin (g)
Optics
Filter	ZnSe	5.42	1.00	1.04
Lens 1	Si	2.33	1.00	1.25
Lens 2	Si	2.33	1.00	1.25
Lens 3	Si	2.33	1.00	1.25
Lens 4	Si	2.33	1.00	1.25
Mirror	Quartz	2.65	1.00	1.87
Mirror	Quartz	2.65	1.00	1.87
Lens 5	Si	2.33	1.00	1.25
Mirror	Quartz	2.65	1.00	1.37
Mirror	Quartz	2.65	1.00	1.37
Beam splitter	ZnSe	5.42	4.55	6.99
Compensator plate	ZnSe	5.42	4.55	6.99
Grating	Si	2.33	11.07	14.18
Grating	Si	2.33	11.07	14.18
Lens 6	Si	2.33	1.00	1.69
Mirror	Quartz	2.65	1.11	2.51
Mirror	Quartz	2.65	1.00	1.92
Lens 7	Si	2.33	1.00	1.25
Total lenses			44.35	63.47
Structures
Lower lid	C-C	1.65	37.63	47.03
Upper lid	C-C	1.65	218.17	272.72
Lens tube	C-C	1.65	4.61	5.76
Bracket	C-C	1.65	14.46	18.08
Lens tube	C-C	1.65	5.79	7.23
Lens bracket	C-C	1.65	3.50	4.38
Lens holder 1	C-C	1.65	4.46	5.58
Lens holder 2	C-C	1.65	4.83	6.03
Beam splitter and prism mount	C-C	1.65	26.08	32.59
Clamp	C-C	1.65	0.48	0.60
Clamp	C-C	1.65	0.48	0.60
Clamp	C-C	1.65	3.41	4.27
Bench	C-C	1.65	320.35	400.44
Plate holder 1	C-C	1.65	4.07	5.08
Plate holder 2	C-C	1.65	4.53	5.66
Total structure			651.99	816.07
Other components
Multilayer insulation			15	22.5
Harness and heaters			50	62.5
Detector			500	625
Radiator and heat strap			200	250
Total mass (g)			1461.34	1839.53

The mass of a heliostat, which could be based on the Mars exploration rover PanCam Mast,¹⁴ is not included in the above data.

5.

Performance Estimate

The performance of the MMMM instrument is estimated by simulating the response of the experiment to the incident photon flux reaching the surface of Mars, given several dust opacities and several solar zenith angles. For each of these cases, a noise free interferogram is generated, based on the given incident flux for a 1-deg square field-of-view pointed at the sun. To this interferogram, noise is added based on photon statistics (shot noise) and realistic readout noise (0.02% of the dynamic range). Both readout noise and dark noise are small compared to the photon shot noise due to the large solar signal. Finally, these interferograms are used to derive spectra, thus simulating observations. This process introduces both the instrumental resolution (instrumental line shape function) and the expected noise into the simulation. Based on these simulated observations, we can estimate the uncertainty in the retrieval of the atmospheric methane VMR.

As discussed in Sec. 4.2 and illustrated in Fig. 5, the measured spectrum contains a signal from the opposite sideband and aliased signal on either end of the retrieved wavenumber interval, while the lines of interest, originating from $\mathrm{C}{\mathrm{H}}_4$, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{H}}_2\mathrm{O}$, are in the central part of the retrieved spectrum that is not influenced by these “folding” effects. This is illustrated in Fig. 7, along with a simulated measurement and the simulated noise in the retrieved spectrum.

Fig. 7

The top part of this figure (thin black line) shows an example of a high resolution incident flux, referenced to the left axis (aerosol optical depth: $\mathrm{OD}=1$, solar zenith angle: 40 deg). The flux was created using the solar atlas of ACE¹⁰ and the radiative transfer calculations performed within the central wavelength region containing $\mathrm{C}{\mathrm{H}}_4$, ${\mathrm{H}}_2\mathrm{O}$, and $\mathrm{C}{\mathrm{O}}_2$ absorption lines. The thick black line is the retrieved spectrum that clearly shows the folding effects on both edges that introduce extra signal in these regions. The depth of the measured absorption is also noticeably smaller than for the high resolution case, due to the instrumental line shape function of the instrument. The red trace indicates a simulated observation with noise, and the blue trace, referenced to the right axis, represents the noise in the retrieved spectrum. This noise simulation assumes photon shot noise, detector dark, and read noise, and the instrument and measurement parameters listed in Table 2.

Figure 8 shows a smaller wavelength section from a simulated spectrum. The effect of the convolution with the instrumental line shape function of the spectrometer is apparent. The high resolving power of 150,000 is sufficient to clearly separate the absorption features for both $\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{C}{\mathrm{O}}_2$.

Fig. 8

Close-up of a retrieved spectrum is shown in Fig. 7.

Figure 9 illustrates the simulated absorption measurement, after removing the spectral shape of the sun. The $\mathrm{C}{\mathrm{H}}_4$ lines that show an absorption of larger than 0.5% (marked as blue diamonds) are used to estimate the uncertainty in retrieving the $\mathrm{C}{\mathrm{H}}_4$ VMR from these simulated measurements. The performance estimates are conducted using a Monte Carlo approach with independent sets of noise contributions.

Fig. 9

$\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{C}{\mathrm{O}}_2$ absorption, calculated from the simulated spectrum by removing the features included in the spectral shape of the solar signal. The methane absorption feature of $>0.5\%$ (marked with blue diamonds) and the corresponding noise were used for the sensitivity estimate of the instrument.

For this Monte Carlo approach, 100 measurement simulations (interferograms) were created using different random noises of equal magnitude, containing photon shot noise, read noise, and dark noise. Using analytical error propagation, we determined that the statistical uncertainty in the noisy interferogram is equivalent to the uncertainty in the associated absorption feature.¹⁶ Next the standard deviations of these simulated methane absorption features (marked in Fig. 9) were converted into an equivalent VMR uncertainty. Note that for optically thin conditions, as encountered here, the absorption and VMR have a nearly linear relationship.

Table 4 summarizes the instrument parameters that were used for the measurement simulations. For these measurement simulations, the following conservative assumptions are made: the field-of-view on the sky (1 deg) is larger than the angle subtended by the sun ($\sim 0.3\mathrm{deg}$) and the etendue of the telescope is smaller than the maximum etendue that can be accepted by the interferometer.

Table 4

Instrument parameters used for measurement simulations.

The results of the performance estimate for the retrieval of $\mathrm{C}{\mathrm{H}}_4$ VMR are given in Table 5 for five atmospheric aerosol conditions and three solar zenith angles.

Table 5

Performance predictions for five different aerosol scenarios and three solar zenith angles assuming a total measurement time of 60 s. The signal-to-noise is calculated using the total incident signal and the corresponding noise. The mixing ratio numbers are the uncertainties in the CH4 retrieval that result from the simulated signal-to-noise of the spectrum and the simulated CH4 absorption.

The uncertainty estimates given in Table 5 show that for the case of a clear or moderately scattering atmosphere [OD (aerosol OD) $\le 1$] and a measurement time of 1 min, the goal of 100 ppt can be achieved. For increasing dust concentrations in the atmosphere ($\mathrm{OD}>1$), the uncertainty of the $\mathrm{C}{\mathrm{H}}_4$ VMR measurement increases, as expected.

6.

Conclusions

We conclude that a solar occultation measurement from the Mars surface using a very high resolving power ($R=\mathrm{150,000}$) compact spatial heterodyne spectrometer will satisfy the science requirements we anticipate to be leveraged on future Mars missions (100 pptv, 60 s, for a clear atmosphere). We presented a conceptual instrument design using state-of-the-art, available components, so that no technology development is required for the implementation of the instrument.

The main resource requirements are summarized in Table 6.

Table 6

Main instrument resource requirements.

Further refinements of the instrument are expected to result in weight reductions by a more aggressive, lightweight aluminum structure design or by choosing a different structure material, such as carbon fiber. Further information about the actual platform is needed to finalize the thermal design and the sun pointing telescope.