20 May 2014 High sensitivity trace gas sensor for planetary atmospheres: miniaturized Mars methane monitor
Author Affiliations +
J. of Applied Remote Sensing, 8(1), 083625 (2014). doi:10.1117/1.JRS.8.083625
Highly sensitive trace gas measurements in planetary atmospheres can yield information about a planet’s atmosphere and surface. One prominent example is methane in the Martian atmosphere, which could originate biogenically and provides answers to one of the most intriguing questions in planetary science: “Does life currently exist on Mars?” Recently, <italic<in situ</italic< measurements by the Mars Science Laboratory (MSL) have resulted in an upper limit of 1300 parts per trillion by volume (pptv), whereas previous measurements using terrestrial telescopes and an instrument orbiting Mars reported significantly higher values of 10,000 pptv or more. These results are not necessarily contradictory, due to the possibility of spatial and temporal variability of the trace gas concentration. Thus, more measurements will be required to gain clarity. The concept of a miniaturized Mars methane monitor, a high spectral resolution, midinfrared spectrometer observing the sun through the Mars atmosphere from either the Mars surface, a Mars balloon or plane, or a Mars orbiting satellite is presented. The instrument would measure atmospheric methane and water vapor volume mixing ratios with equal or higher precision than the tunable laser spectrometer on MSL. The spectrometer concept uses the spatial heterodyne spectroscopy technique, which has previously been used for ground- and space-based observations of the Earth’s atmosphere.
Englert, Stevens, Brown, Harlander, DeMajistre, and Marr: High sensitivity trace gas sensor for planetary atmospheres: miniaturized Mars methane monitor



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 (CH4) 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.23.4 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 CH4, water (H2O), and carbon dioxide (CO2). Using such solar occultation measurements, high-precision VMRs of CH4 and H2O can be retrieved. Even though we concentrate on CH4 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 CO2, 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 CO2 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.


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.


Bandpass Selection

The spectral region between 3200 and 3240 nm is well suited for this measurement because it contains molecular absorption lines of CH4, H2O, and CO2 as shown in Fig. 1. The simultaneous measurement of all the three atmospheric components is highly desirable for this instrument concept. CH4 and H2O are of scientific interest and the CO2 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 CH4 measurement sensitivity.

Fig. 1

Line strengths of H2O, CH4, and CO2.8 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 CH4 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 1×1deg pointed at the sun. Important model characteristics are given in Table 1.

Table 1

Radiative transfer model characteristics.

Solar fluxStandard Zero Air Mass Solar Spectral Irradiance.9 This solar spectrum has only a few samples over the wavelength range considered. High spectral resolution structure in the solar spectrum is not accounted for in the radiative transfer calculations. (For the performance estimate of the instrument, we accounted for the high spectral resolution structure of the solar spectrum using the exoatmospheric solar spectrum from the ACE-FTS instrument on SCISAT-110)
TemperatureRef. 11
PressureRef. 11
Trace gas concentrationsMSO SDT report;12 methane mixing ratio: 10,000 pptv
Absorption line parametersHITRAN 20088
Line profileVoigt
AerosolsTotal OD between the surface and 5 km
Aerosol scattering phase function0.9 asymmetry factor, strongly forward scattering
Radiative transfer algorithmDISORT13—plane parallel discrete ordinate method (solar zenith angle maximum ∼65  deg)

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 [W/m2/nm/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 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 CH4 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 1×1deg 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 (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.


Conceptual Instrument Design


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 30mm×1mm and a beam angle of 1.5deg×5deg at the gratings, with the smaller angle in the dispersion plane. The silicon immersion gratings have a groove density of 1150g/mm and a blaze angle of about 32.8 deg. This results in a maximum acceptable etendue of about 0.068mm2sr. Further specifications of the optical spectrometer components are given in Table 2. The design uses an imaging detector with a total illuminated area of 7.67mm×0.25mm. 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.

FilterCWL=3231  nmFWHM=13  nm
Lens 1Cylinderf1=infinityf2=156.7  mm
Lens 2Cylinderf1=113.8  mmf2=infinity
Lens 3Cylinderf1=infinityf2=3.5  mm
Lens 4Cylinderf1=infinityf2=77.3  mm
Mirror 1First surfaceGold coating
Mirror 2First surfaceGold coating
Lens 5Sphericalf=602.2  mm
Mirror 3First surfaceGold coating
Mirror 4First surfaceGold coating
Beam splitterMetallic coating
Compensator plateAR coating
GratingGold coating32.8 deg blaze angle1150  g/mm
GratingGold coating32.8 deg blaze angle1150  g/mm
Lens 6Sphericalf=602.2  mm
Mirror 5First surfaceGold coating
Mirror 6First surfaceGold coating
Lens 7Sphericalf=175.9  mm


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/2m 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 2cm1. 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 CH4, CO2, and H2O absorption, assuming a bandpass filter of about 10cm1 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 (2×0.004×150,000×2). Even though midinfrared FPAs with >1024pixels 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, CO2, 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.


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 191mm×173mm×66mm (L×W×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 191mm×173mm×66mm, 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 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.

MaterialDensity (g/cm3)Mass (g)Mass, with 25% margin (g)
Lens 1Si2.331.001.25
Lens 2Si2.331.001.25
Lens 3Si2.331.001.25
Lens 4Si2.331.001.25
Lens 5Si2.331.001.25
Beam splitterZnSe5.424.556.99
Compensator plateZnSe5.424.556.99
Lens 6Si2.331.001.69
Lens 7Si2.331.001.25
Total lenses44.3563.47
Lower lidC-C1.6537.6347.03
Upper lidC-C1.65218.17272.72
Lens tubeC-C1.654.615.76
Lens tubeC-C1.655.797.23
Lens bracketC-C1.653.504.38
Lens holder 1C-C1.654.465.58
Lens holder 2C-C1.654.836.03
Beam splitter and prism mountC-C1.6526.0832.59
Plate holder 1C-C1.654.075.08
Plate holder 2C-C1.654.535.66
Total structure651.99816.07
Other components
Multilayer insulation1522.5
Harness and heaters5062.5
Radiator and heat strap200250
Total mass (g)1461.341839.53

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


Thermal Considerations

Mars surface temperatures can vary by >150°C centered about an average of approximately 63°C. This is a threat to the survival and operation of any exposed Mars-based instrument. The best location for the interferometer is therefore in the thermally controlled interior of a vehicle. The interferometer optics, for example, should be held within approximately 1°C of a design temperature during daytime observing hours. The dimensional stability of a carbon composite structure is generally well behaved for larger temperature ranges, but the beam splitter, lenses, and gratings have temperature-dependent refractive indices and nonnegligible coefficients of expansion, requiring thermal control. Once a target operating temperature is established, it can be maintained assisted by thermal standoffs, multilayer insulation, and PID (proportional, integral, derivative) controlled heaters.

To the first order, the CH4, H2O, and CO2 measurements are not affected by small temperature changes in the instrument since the primary effects are a small change in the Littrow frequency and a slight defocus of the FPA.15 Both of these effects can be corrected in the data analysis if the temperature is stable during an individual exposure since (1) the spectral shape of the measured signal is known (except for the magnitude of the absorption) which allows the determination of the wavelength scale for each measured spectrum from the spectrum itself and (2) the absorption measurement is self-calibrating since the baseline is measured with each exposure. More details on a technique that can be used to correct for the thermal defocus effect in an SHS instrument can be found in Ref. 15. It is worth noting that especially for high fringe frequencies, defocusing will result in a fringe contrast reduction and therefore a decrease in the signal-to-noise ratio, which cannot be recovered. However, a thermally stabilized interferometer will minimize that effect.

Another thermal challenge is the cooling and thermal stability of the FPA in the camera. Since power is likely a limited resource on a planetary instrument platform, passive cooling is a preferred method of removing any heat from the FPA. Depending on the location and orientation of the platform on which this instrument is integrated, an appropriately sized radiator is likely the best choice for rejecting thermal energy. If a passive radiator/heater combination is not sufficient to keep the FPA at the desired temperature throughout the day, single or multistage thermoelectric coolers or closed cycle mechanical coolers can be considered.


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 CH4, CO2, and H2O, 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: 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 CH4, H2O, and CO2 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 CH4 and CO2.

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 CH4 lines that show an absorption of larger than 0.5% (marked as blue diamonds) are used to estimate the uncertainty in retrieving the CH4 VMR from these simulated measurements. The performance estimates are conducted using a Monte Carlo approach with independent sets of noise contributions.

Fig. 9

CH4 and CO2 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 (0.3deg) 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.

Instrument parameterValue
Single exposure time0.010 s
Exposure rate80  s−1
Total measurement time60 s
Telescope aperture0.0001  m2
Maximum filter transmittance50%
Field-of-view1 deg
Optical efficiency0.02
Littrow wavenumber3087.0  cm−1
Array size1024 pixels
Resolving power150,000
Dynamic range14 bits
Read noise3 ADU

The results of the performance estimate for the retrieval of CH4 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.

Solar zenith angle
Aerosol OD10 deg40 deg65 deg
73 ppt57 ppt33 ppt
100 ppt89 ppt71 ppt
847 ppt1000 ppt2100 ppt
1890 ppt3000 ppt8000 ppt
2910 ppt5000 ppt15,000 ppt

The uncertainty estimates given in Table 5 show that for the case of a clear or moderately scattering atmosphere [OD (aerosol OD) 1] and a measurement time of 1 min, the goal of 100 ppt can be achieved. For increasing dust concentrations in the atmosphere (OD>1), the uncertainty of the CH4 VMR measurement increases, as expected.



We conclude that a solar occultation measurement from the Mars surface using a very high resolving power (R=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.

MassOptics and structure: 700 g (conceptual design) Camera: 500 g (estimatea) Harnesses and heaters: 50 g (estimate) MLI: 15 g (estimate) Radiator and heat pipe: <200  g (estimate) Heliostat: highly dependent on platformb
Volume191  mm×173  mm×66  mm (excluding heliostat and radiator)
Power<10  Wc (estimate)
Data rate18.4  kbits/5  min observationd (1024 pixels, 16 bits, summed, averaged, uncompressed, 15% overhead)
ThermalFPA cooled, optics controlled to roughly 1°C


The mass of comparable COTS cameras is in the range of 5 kg, where a significant fraction of the mass is the cooler, the housing, and structure. An integrated space flight instrument will be able to benefit from significant savings compared to these bench-top models.


On a rover, the heliostat could be based on the PanCam mast of the Mars Exploration Explorer.


The power budget consists of detector power, electronics power, power to operate the heliostat, and power for thermal management. Except for the thermal management, the power needs are expected to be quite limited. Depending on the accommodation on the platform, the thermal management might require a more significant amount of heater power. More detailed studies using more specific platform data are needed to derive a better estimate.


On-board compression is likely to reduce the data volume by about 50%. If the data rate is severely limited, longer on-board averaging or more on-board processing can be considered.

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.


This work was supported by the NASA Planetary Instrument Definition and Development Program (PIDDP).



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Christoph R. Englert received a graduate degree in physics at the Technical University of Munich, Germany, in 1996 and a doctorate degree in physics at the University of Bremen, Germany, in 1999. Currently, he is the head of the Geospace Science and Technology Branch of the Naval Research Laboratory (NRL) Space Science Division (SSD) and has been working on several NRL space-flight projects since 1999, including the SHIMMER instrument on the STPSat-1 satellite, for which he was the principal investigator. He is predominantly interested in space-based atmospheric sensors.

Michael H. Stevens earned a PhD degree in earth and planetary science from the Johns Hopkins University in 1992. For two years, he was a research associate at the SSD of the NRL. Since then, he has been a research physicist at NRL’s SSD. He is an expert in spectroscopy and is author or coauthor of over 60 peer-reviewed publications on the terrestrial atmosphere or planetary atmospheres.

Charles M. Brown received a PhD degree in chemical physics in 1971. Since then, he has been at NRL’s SSD. He has enjoyed over 40 years of laboratory and space flight work involving spectroscopic instruments. He has contributed to numerous instruments including MAHRSI for the Shuttle and BCS and EIS on the Yohkoh and Hinode satellites. Currently, he is working with SHS instruments such as REDDI and MIGHTI to measure upper atmospheric winds.

John M. Harlander earned a PhD degree in physics from the University of Wisconsin-Madison in 1991. While at Wisconsin he and Fred Roesler began developing spatial heterodyne spectroscopy for applications in atmospheric science and astronomy. His 20+ years collaboration with the US NRL has resulted in the development of numerous ground-based and space-borne instruments for atmospheric remote sensing. Currently, he is a professor of physics at St. Cloud State University.

Robert DeMajistre obtained a PhD degree in computational sciences and informatics from George Mason University in 2005. Currently, he is a member of the principle professional staff at the Johns Hopkins University Applied Physics Laboratory. He specializes in the development of algorithms for space-based remote sensing and has worked on applications ranging from cloud remote sensing to the characterization of plasma in the outer heliosphere. Currently, he is a member of the AGU.

Kenneth D. Marr received a Bachelor of Science degree in physics from Principia College and his doctorate in plasma physics from the Massachusetts Institute of Technology. Currently, he is an NRC postdoctoral in residence with the Geospace Science and Technology branch of the NRL SSD. His residence began in November 2011, working on the thermal properties of DASH interferometers. His main interests include spectroscopy, high-speed imaging systems, robotics, and fiber optics.

Christoph R. Englert, Michael H. Stevens, Charles M. Brown, John M. Harlander, Robert DeMajistre, Kenneth D. Marr, "High sensitivity trace gas sensor for planetary atmospheres: miniaturized Mars methane monitor," Journal of Applied Remote Sensing 8(1), 083625 (20 May 2014). http://dx.doi.org/10.1117/1.JRS.8.083625
Submission: Received ; Accepted




Atmospheric sensing



Gas sensors

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