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Mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of CO2
Icarus 217 (2012) 315–321
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Mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of CO2 Guido Sonnabend a,c,d,⇑, Manuela Sornig b,c,d, Peter Kroetz a,c,d, Dusan Stupar a,c,d a
I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany Rheinisches Institut für Umweltforschung an der Universität zu Köln, Abt. Planetenforschung, 50931 Köln, Germany c National Solar Observatory, Operated by The Association of Universities for Research in Astronomy, Inc. (AURA), Under Cooperative Agreement with The National Science Foundation, United States 1 d The Infrared Telescope Facility, Which is Operated by The University of Hawaii Under Cooperative Agreement No. NNX-08AE38A with The National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program, United States 1 b
a r t i c l e
i n f o
Article history: Received 16 August 2011 Revised 28 October 2011 Accepted 2 November 2011 Available online 19 November 2011 Keywords: Mars, Atmosphere Infrared observations Spectroscopy Atmospheres, Dynamics
a b s t r a c t We present direct observations of Mars zonal wind velocities around northern spring equinox (LS = 336°, LS = 355°, LS = 42°) during martian year 27 and 29. Data was acquired by means of infrared heterodyne spectroscopy of CO2 features at 959.3917 cm 1 (10.4232 lm) and 957.8005 cm 1 (10.4405 lm) using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) at the McMath–Pierce telescope of the National Solar Observatory on Kitt Peak in Arizona and the NASA Infrared Telescope Facility on Mauna Kea, Hawaii between 2005 and 2008. Winds were measured on the dayside of Mars with an unprecedented spatial resolution allowing sampling of up to nine independent latitudes over the martian disk. Retrieved wind velocities depend strongly on latitude and season with values ranging from 180 m/s prograde to 94 m/s retrograde. A comparison of the observational results to predicted values from the Mars Climate Database yield a reasonable agreement between modeling and observation. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Our knowledge about the processes and dynamics in the atmosphere of Mars especially at low altitudes has greatly improved in recent years due to the many space missions to the planet, in particular Mars Express, Mars Global Surveyor, Mars Reconaissance Orbiter (MRO) and the Mars Exploration Rovers. Based on these data and advances in parameterizations of processes relevant to atmospheric physics, a number of Mars general circulation models (GCMs) have evolved (Wilson, 1997; Haberle et al., 1999; Forget et al., 1999; Takahashi et al., 2003; Hartogh et al., 2005; Moudden and McConnell, 2005; Richardson et al., 2006). For the high altitude regions of the atmosphere, however, constraints on these models are needed that can only partially be provided by spacecraft. Ground-based observations of fully resolved molecular transition lines in planetary atmospheres allow the retrieval of physical parameters such as pressure, temperature, molecular abundance, and dynamical properties of the atmosphere from single line profiles (Kostiuk, 1994). Such observations can probe atmospheric regions not otherwise accessible and can provide continuous ⇑ Corresponding author at: I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany. E-mail address: [email protected] (G. Sonnabend). 1 Visiting Astronmer. 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.11.009
coverage over many years to decades not possible by spacecraft missions. The first ground-based measurements of mesospheric winds on Mars were obtained by observations of CO at mm wavelengths in the early 1990s by Lellouch et al. (1991). These and follow-up observations, however, have low spatial resolution due to the large field-of-view (FOV) of the telescope at such wavelengths (Clancy et al., 2006; Cavalié et al., 2008), a limitation which can be partly overcome by interferometric observations (Moreno et al., 2009). Recently, wind retrievals from the application of cloud tracking methods to images recorded by the OMEGA and High Resolution Stereo Camera (HRSC) instruments on MarsExpress and by the Thermal Emission Imaging System (THEMIS) instrument on MRO have been reported by Määttänen et al. (2010) and McConnochie et al. (2010). These CO2 clouds occur at similar altitudes to our measurements in the mesosphere, however, cloud appearance is sparse and can not be predicted. In addition, the cloud season is mostly limited to LS = 0–60° and equatorial latitudes with only few cloud detections outside that range. Ground-based observations at mid-infrared (MIR) wavelengths can provide much a 10–100 times improved spatial resolution over sub-mm or radio observations, providing a diffraction limited fieldof-view (FOV) e.g., <2 arcsec resolution at 10 lm with a 1.5 m-class telescope as achieved in the presented investigation. High spatial resolution is important because GCMs for Mars like those mentioned above predict strong spatial variations in zonal wind
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Fig. 1. Measured sample spectrum (gray) for Mars observations during campaign A taken at the equator and 75° East of the central meridian longitude (CML) demonstrating the structure of the spectra with a spectral resolution of 1 MHz. The black line gives the best fit to data.
velocities. In the case of Mars, zonal wind velocities can be measured directly by using transition lines of CO2, the major constituent of the atmosphere, in the 10 lm spectral region. These lines are detected in absorption vs. the surface brightness from low altitudes in the martian atmosphere (below 3 mbar level). In addition, non-thermal (non-LTE) emission is contributed from the mesosphere (50–100 km altitude), peaking at about 80 km and creating the characteristic profiles shown in Fig. 1. Non-LTE emission from Mars was first detected by Johnson et al. (1976) using IR heterodyne spectroscopy. A frequency shift between the absorption and the emission component can be directly converted to a
line-of-sight velocity and, given the observational geometry, into a zonal wind shear between the two altitude regions (Sonnabend et al., 2005). If the absolute spectral calibration during the observation is known, independent retrieval of the mesospheric wind velocity is possible. The observed range of LS = 330–40° is an interesting time in the martian year since the northern spring equinox marks the predicted turnover from a northern to southern jet configuration in the upper atmosphere of Mars. In Fig. 2 averaged wind velocities for the altitude range of 50–116 km for LS = 330°, 0°, 40° are shown as given by the ’’Mars Climate Database’’ (MCD), version 4 (Millour et al., 2011). Due to the narrow linewidth of the emission features (<50 MHz FWHM), high spectral resolution of Dmm 107 is required to analyze those lines. Such spectral resolution in the infrared can only be achieved by heterodyne techniques. MIR heterodyne observations have proven to be a valuable tool for planetary astronomy since the 1970s and important results have been achieved on Mars, Venus, Saturn, Jupiter and Titan (Fast et al., 2006; Goldstein et al., 1991; Kostiuk et al., 1996, 2001; Sonnabend et al., 2010; Sornig et al., 2008). 2. Instrumentation Observations were carried out using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS). In the heterodyne instrument the signal from the telescope is mixed with a local oscillator (LO) signal by means of a Fabry-Pérot diplexer and then focused on a fast mercury–cadmium–telluride detector. The detector generates the difference or intermediate frequency (IF) signal at radio frequencies which preserves the spectral information contained in the original IR signal. The IF signal can then be amplified and analyzed in great detail by means of an acousto-optical spectrometer providing a bandwidth of 3 GHz and spectral resolution of 1 MHz (0.00003 cm 1). The receiver is based on a quantum cascade laser (QCL) as LO. Depending on the availability of LOs, operation is presently possible between 7 and 13 lm. The LO is locked to a transmission maximum of the diplexer. A frequency stability of 1 MHz of the complete system is provided by locking the diplexer to a commercially available helium–neon laser with a specified stability of better than 108 in 8 h. The absolute frequency calibration is provided through a reference gas cell. Two calibration loads at known temperatures are integrated into the system for absolute intensity calibration of the observed spectra. An integrated optical guide system provides accurate pointing information and allows active telescope tracking. A detailed description of the receiver is given by Sonnabend et al. (2008). 3. Observations Results from three observing runs addressing dynamics on Mars are reported here. The first two took place at the McMath–Pierce telescope on Kitt Peak in December 2005 and November 2007. For the third observation we used the NASA Infrared Telescope Facility (IRTF) on Mauna Kea in March 2008. Note that for observational purposes longitude coordinates on sky are used while for discussion of results we use planetary coordinates giving west longitudes. 3.1. Mars campaign A (Kitt Peak, December 2005)
Fig. 2. Predicted averaged wind velocities from the MCD for LS = 330°, 0°, 40° (top to bottom).
On December 5–8, 2005 we observed Mars at the CO2 P (2) transition at 959.3917 cm 1 using the McMath–Pierce telescope on Kitt Peak, Az. The instrument THIS was mounted on the spectrograph table at the primary focus of the telescope.
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The angular diameter of Mars varied from 16.7 to 15.4 arcsec over the course of the observing period compared to the diffraction-limited field of view (FOV) of the telescope of 1.6 arcsec. The pointing uncertainty was estimated to be below 1 arcsec and verified by observing a stellar source. Mars was 1 month past opposition and the Earth–Mars Doppler shift as given by JPL’s HORIZONS ephemeris website (http://ssd.jpl.nasa.gov/) varied between 9.8 and 11.4 km/s (receding). The acquisition time for an individual spectrum was 10 min. Spectra were co-added during analysis to achieve a sufficient SNR. Mars illumination was 96% with the subsolar point at 21°W of CML and 10.3°S from the equator. Martian season was late northern hemisphere winter at a martian solar longitude of LS 335°. In total, six different latitudes on Mars were observed during 102 individual observations, each lasting from 70 to 200 min. The sampled latitudes ranged from 45°N to 75°S and observations were carried out close to the limb covering a local time range of 3 h. Observing geometry is pictured in Fig. 3 indicating the observed positions at the FOV scaled to the planetary diameter. Observed positions, dates, times, and integration times are given in Table 1. A sample spectrum is shown in Fig. 1. The signal-to-noise ratio (SNR) depends mainly on integration time and the surface temper-
ature at the observed position on Mars which provides the quasiblackbody continuum level observed in the spectra. Observations from 2005 have been published in (Sonnabend et al., 2006) and are given here for completeness. 3.2. Mars campaign B (Kitt Peak, November/December 2007) Following the course of the martian year the next observations were carried out at a solar longitude of LS 354 between November 23 to December 3, 2007 again at the McMath–Pierce telescope. The orbital position of Mars was approximately 1 month before opposition. The topocentric relative velocity between Earth and Mars varied between 4.9 and 7.8 km/s (approaching). Acquisition time for an individual spectra is again 10 min. Mars was almost fully illuminated with a subsolar point at 23°E from CML and 2.5°S. The apparent diameter was between 14.4 and 15.3 arcsec, similar to conditions for the observing run in 2005. The observing geometry is given in Fig. 4 and the relative size of the apparent diameter of the planet and the diffraction-limited FOV of 1.6 arcsec are pictured. Data were provided by 74 single measurements at nine different latitudes on the planet (from 57°S to 57°N) along the morning limb. Relevant data for observations are summarized in Table 2. 3.3. Mars campaign C (Mauna Kea, March 2008) Mars was observed again from March 03 to 06, 2008. Martian season of northern hemisphere spring at a solar longitude of LS 42° completed the desired seasonal coverage. The apparent diameter of Mars varied between 8.6 and 8.9 arcsec. To obtain a spatial resolution comparable to the observations in 2005 and 2007, when martian apparent diameter was larger, we used the NASA IRTF on Mauna Kea, Hawaii, USA, having a 3 m diameter primary mirror and thus providing a diffractionlimited FOV at 10 lm of 0.8 arcsec. The related observing geometry is given in Fig. 5. The martian disk was illuminated approximately 91% with a subsolar point of 33°E from CML and 16°N. The topocentric relative velocity between Mars and Earth was 16.7 km/s. Limb observations at nine different latitudes starting from 57°N to 57°S were performed. Observed positions and relevant observation parameters are summarized in Table 3. In total 35 single measurements resulted in observations of 10 positions on the planet. This time the P (4) line of CO2 at 957.8005 cm 1 was observed.
Fig. 3. Observing geometry of Mars during the Mars campaign A from December 5 to 8, 2005. The relative sizes of apparent diameter of the martian disk (approximately 16 arcsec) and the FOV (1.6 arcsec) are indicated. In addition, the circles give the pointing positions on Mars. Mars was almost totally illuminated (96%) with a subsolar point at 21°W from CML and 10.3°S from the equator labeled by the black star. From limb observations line-of-sight velocities were retrieved for six different latitudes. Note that for observational purposes coordinates on sky are used.
4. Data analysis During both observing runs on Kitt Peak the P (2) line at 959.3917 cm 1 of the 10.4 lm-band of CO2 was observed. Due to availability of a new QCL LO the 75% stronger P (4) line at 957.8005 cm 1 of the same band was observed during the observation run on Mauna Kea providing a higher SNR.
Table 1 Overview of the observed positions on Mars during the Mars campaign A at the McMath–Pierce telescope on Kitt Peak. Given are the position on Mars, observing date and time, martian season and local time, and the total integration time in minutes (from Sonnabend et al., 2006). Mars campaign A (Kitt Peak, December 2005)
1 2 3 4 5 6 7 8
Lat./rel. lon. on sky )
Date
Time (UT)
West longitude on planet (°)
Season (LS)
Mars (LT)
Int. time (min)
45°N/90°W 0°/90°W 0°/90°W 15°S/90°W 35°S/90°W 60°S/90°W 75°S/90°W 75°S/90°W
12/05 12/05 12/05 12/07 12/08 12/07 12/06 12/06
3:07–5:40 1:42–2:51 5:23–8:42 6:28–8:22 1:00–3:00 1:29–3:48 1:00–3:33 6:07–8:40
237–275 196–213 251–298 239–268 142–171 149–182 148–185 223–260
335 335 335 336 338 336 336 336
15:15 16:40 16:40 17:11 17:49 18:24 18:36 18:36
153 69 199 114 120 139 153 153
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Fig. 4. Observing geometry of Mars during campaign B in November 23 and December 03, 2007. The field of view was 1.6 arcsec and the angular diameter of the martian disk was 15 arcsec. In addition, the circles give the pointing positions on Mars. Mars was almost totally illuminated and the black star marks the subsolar point at 23°E from CML and 2.5°S. Limb observations were accomplished at nine different latitudes from 57°S to 57°N to retrieve line-of-sight velocities. Note that for observational purposes coordinates on sky are used.
Measured single integrations were added to provide spectra with similar SNR. Integration time of the resulting spectra varied between 30 and 300 min depending on observing conditions (e.g. chosen position on Mars, weather, airmass). The spectra were modeled and fitted using the BEAMINT/CODAT radiative transfer code developed at NASA Goddard Space Flight Center (Hewagama et al., 1998). BEAMINT/CODAT combines a layer-by-layer radiative transfer modeling engine with an algorithm to combine the contribution from sub-resolution segments of the instrument beam to form the overall spectrum. This can be a noticeable improvement over a single point mean viewing angle model, especially when the viewing geometry is such that the beam sees contribution from a wide range of planetary longitudes. We used a 16-element beam model for the analysis of our observations. BEAMINT/CODAT accepts planetary parameters, observation circumstances, molecular and thermal height profiles, and a molecular line atlas. The non-LTE component is not currently included in the radiative transfer code but added as a Gaussian profile describing the shape of the non-LTE emission. Parameters can be iterated until a best fit of a model spectrum to an observed spectrum is achieved. Uncertainties based on correlation between free parameters and the variance
between the observed and model spectra are returned. The telluric contribution to the spectra was modeled using the terrestrial radiative transfer package GENLN2 (Edwards, 1992). For the presented results, we first generated an average value for the Mars surface temperature, weighted by the beam distribution and the brightness temperature of the surface area within the FOV. Surface pressure and atmospheric thermal profile were extracted from the MCD. The frequency position, width and intensity of the Gaussian profile representing the non-LTE feature were iterated along with a scaling factor representing the total loss of signal due to misalignment, clouds, etc. until the best fit to the observed data was reached. Analysis methodology was different for the first observing run in 2005 than for following runs in 2007 and 2008 due to implementation of a reference gas cell into the instrument. The gas cell provides an absolute frequency calibration for the non-LTE emission relative to the gas cell spectrum and therefore information about the absolute wind velocity (independent of the absorption line). Without the reference cell the frequency shift between the martian CO2 absorption and emission feature gives the wind velocity shear between high and low altitudes with the observing geometry taken into account. Wind velocities at the low altitudes were taken from the MCD and averaged over the FOV. Analysis of the contribution functions (see example plotted in Fig. 6) for the absorption line profile shows that most contribution to the absorption line originates below the 3 mbar pressure level. During campaign B two ethylene absorption lines from the reference gas cell were present (at 959.4075 cm 1 and 959.3072 cm 1) within the observed bandwidth and used for determination of the absolute frequency position of the CO2 line. In campaign C The 957.7641 cm 1 ethylene line was used for absolute frequency calibration. In both cases the fitted frequency offsets were then converted to eastward zonal wind velocities, meaning positive sign for wind going from West to East on the planet, taking into account Mars solid-body rotation and assuming a horizontal zonal flow. Determination of the altitude of the emission-forming region is not possible from the data itself. Recent modeling of the non-LTE processes in the atmospheres of Mars predict a peak of the emission at an altitude of 80 km (Lopez-Valverde, 2011) (see also Fig. 7). For campaign C, due to an instrumental problem all observations past transit of Mars had to be discarded. The problem was caused by an instability in the optical guide camera path resulting in a significant shift between the visible and the IR light path and thus an uncontrolled observing position on Mars. Overviews of the retrieved frequency shifts and the zonal wind velocities are presented in the Tables 4–6 and in Fig. 8 below. The uncertainties include the 1-r fitting errors, the precision of the calibration line (0.3 MHz), the AOS calibration (0.2 MHz), and the maximum deviation due to the time step in the Doppler-shift calculation (0.3 MHz).
Table 2 Overview of the observed positions on Mars during the Mars campaign B at the McMath–Pierce telescope on Kitt Peak. Given are the position on Mars, observing date and time, martian season and local time, and the total integration time in minutes. Mars campaign B (Kitt Peak, November/December 2007)
1 2 3 4 5 6 7 8 9 10
Lat./rel. lon. on sky
Date
Time (UT)
West longitude on planet (°)
Season (LS)
Mars (LT)
Int. time (min)
57°N/90°E 45°N/90°E 33°N/90°E 11°N/90°E 0°/90°E 11°S/90°E 33°S/90°E 45°S/90°E 45°S/90°E 57°S/90°E
11/27 11/28 12/03 11/27 11/28 11/24 11/25 11/23 12/03 11/25
11:20–11:47 9:25–13:51 10:30–12:07 9:31–11:03 6:27–9:08 9:00–14:12 5:48–7:45 9:05–11:14 12:16–14:01 9:36–11:14
10–16 332–37 301–325 338–1 282–322 353–70 293–322 358–30 316–342 346–10
354 354 357 354 354 352 353 352 357 353
6:58 7:04 7:13 7:24 7:32 7:39 7:54 8:01 7:55 8:07
27 266 97 92 161 312 117 129 105 98
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Fig. 6. Examples of contribution functions for to the martian CO2 absorption line profile. Analysis showed that most of the contribution to this line originates below the 3 mbar pressure level. The different lines indicate the contributions from the absorption line depending on distance form the line center. Fig. 5. Observing geometry for Mars campaign C during March 03 to March 06, 2008. Mars was almost totally illuminated but this time the subsolar point at 33°E from CML and 16°N was close to the western limb. The FOV was 0.8 arcsec as indicated by the circles on an angular diameter of the martian disk of 9 arcsec. Nine different latitudes were addressed for limb observations to gain line-of-sight wind velocities, indicated by the circles as well. Note that for observational purposes coordinates on sky are used.
5. Data interpretation and comparison In the following we will compare our results to model predictions as well as other observations. 5.1. Comparison to model predictions In order to compare the model predictions to the observations we extracted wind values from the website of the Mars Climate Database (MCD) (http://www-mars.lmd.jussieu.fr/). The corresponding season, latitude/longitude and Mars universal time (MUT) were used. The altitudinal distribution was weighted according to the contribution to the non-LTE feature as shown in Fig. 7. For the uncertainties of the predicted values we use the full month standard deviation as given by the MCD. Since the MCD website only provides average values for a full martian month we constructed two cases each for campaigns A and B since they both fall between 2 months in LS. The comparison for all three runs is shown in Fig. 8. In general, the predicted values agree with the observed data within the error bars for most observed positions. In all three campaigns a non-agreement between data and model
Fig. 7. Derivative of upward radiances of the P (2) line at its line core. The four lines correspond to solar solar zenith angles = 0°, 60°, 80°, and 88°, as indicated (from Lopez-Valverde, 2011). Contribution to the line peaks at 80 km.
occurs for high northern latitudes. In campaign A the mid latitudes show a better agreement to month 12 (LS = 330–360°) while the higher latitudes fall closer to month 11 (LS = 300–330°). In campaign B most values fall in between month 12 and month 1 (LS = 0–30°). Here we also see a significant deviation between observed and predicted value for the latitude 11° North were we measure a prograde wind while the model predicts a retrograde wind and the values do not agree within the error bars. In
Table 3 Overview of the observed positions on Mars during the Mars campaign C at the NASA IRTF on Mauna Kea in Hawaii, US. Given are the position on Mars, observing date and time, martian season and local time, and the total integration time in minutes. Mars campaign C (Mauna Kea, March 2008)
1 2 3 4 5 6 7 8 9 10
Lat./rel. lon. on sky
Date
Time (UT)
West longitude on planet (°)
Season (LS)
Mars (LT)
Int. time (min)
57°N/90°W 57°N/90°W 45°N/90°W 33°N/90°W 15°N/90°W 0°/90°W 15°S/90°W 33°S/90°W 45°S/90°W 57°S/90°W
03/06 03/06 03/05 03/06 03/05 03/05 03/05 03/06 03/05 03/06
2:35–3:17 7:40–8:17 2:48–4:48 4:28–5:23 1:53–2:37 7:10–8:00 8:08–9:01 6:36–7:20 4:57–6:50 8:30–9:26
232–241 306–314 244–273 259–272 232–242 310–321 323–336 292–303 277–305 320–333
42 42 41 42 41 41 41 42 41 42
15:53 15:53 15:52 15:51 15:49 15:48 15:46 15:44 15:44 15:42
42 37 120 55 44 50 53 44 113 56
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Table 4 Overview of the retrieved wind velocities in the mesosphere of Mars during the observing campaign A. Given are the position on the planet, date and the total integration time of observation. The given frequency shift is the relative shift mlos-rel between absorption and emission line with a positive sign for resulting upper atmosphere wind towards the observer. The given eastward zonal wind velocity vzonal takes the low altitude wind component into account (see text for details). Martian season was LS = 335–338 (from Sonnabend et al., 2006). Mars campaign A (Kitt Peak, December 2005)
1 2 3 4 5 6 7 8
Lat./long
Date
tint (min)
45°N/limb 0°/limb 0°/limb 15°S/limb 35°S/limb 60°S/limb 75°S/limb 75°S/limb
12/05 12/05 12/05 12/07 12/08 12/07 12/06 12/06
320 160 400 320 200 280 320 400
mlos-rel (MHz)
vzonal (m/s)
2.2 ± 1.4 6.8 ± 1.1 4.5 ± 0.9 3.0 ± 1.6 0.1 ± 1.0 2.2 ± 1.1 2.1 ± 1.6 4.1 ± 2.4
27 ± 17 80 ± 13 53 ± 10 36 ± 19 1 ± 12 25 ± 12 25 ± 19 51 ± 29
Table 5 Overview of the retrieved wind velocities in the mesosphere of Mars during the Mars campaign B in 2007. Given are the position on the planet, date and the total integration time of observation. The absolute frequency shifts mlos, with a positive sign for wind towards the observer, and the eastward zonal wind velocity vzonal are given as well. Martian season was LS = 352–357. Mars campaign B (Kitt Peak, November and December 2007)
1 2 3 4 5 6 7 8 9 10
Lat./long
Date
tint (min)
mlos (MHz)
vzonal (m/s)
57°N/limb 45°N/limb 33°N/limb 11°N/limb 0°/limb 11°S/limb 33°S/limb 45°S/limb 45°S/limb 57°S/limb
11/27 11/28 12/03 11/28 11/28 11/25 11/25 11/23 12/03 11/25
20 160 100 180 140 240 240 160 120 120
17.3 ± 6.0 7.0 ± 3.4 2.7 ± 2.1 2.1 ± 1.1 3.5 ± 1.6 6.4 ± 1.7 9.0 ± 1.45 1.1 ± 2.0 2.9 ± 1.5 1.6 ± 1.9
180 ± 63 73 ± 35 28 ± 22 22 ± 12 37 ± 17 67 ± 19 94 ± 14 12 ± 21 30 ± 16 17 ± 20
Table 6 Overview of the retrieved wind velocities in the mesosphere of Mars during the Mars campaign C in 2008. Given are the position on the planet, date and the total integration time of observation. The absolute frequency shifts mlos, with a positive sign for wind towards the observer, and absolute line-of-sight wind velocities vzonal are given. Martian season was LS = 41–42. Mars campaign C (Mauna Kea, March 2008)
1 3 4 5
Lat./long
Date
tint (min)
57°N/limb 45°N/limb 33°N/limb 15°N/limb
03/06 03/05 03/06 03/05
40 80 80 60
mlos (MHz) 8.1 ± 2.0 2.6 ± 1.3 1.6 ± 1.4 6.1 ± 1.6
vzonal (m/s) 85 ± 21 27 ± 13 17 ± 15 64 ± 17
campaign C most values agree within the uncertainties except the high latitude position at 60°N, however, the data set is very limited due to the technical failure mentioned above. The used averaging of the MCD values is certainly only a first step on the way to a detailed comparison. Such a study which will include a full calculation of the observational FOV and its geometrical representation in the martian atmosphere including a study of the predicted variabilities is currently underway. It will also include a more detailed analysis of the altitudinal dependance of the non-LTE region to various atmospheric parameters. 5.2. Comparison to other observations A comparison to other direct observations of winds is difficult due to the strong dependency of the zonal winds on latitude, local
Fig. 8. Comparison of retrieved wind values to predicted values from the MCD. Plotted is the eastward zonal wind component vs. latitude for all three campaigns. For campaign A and B comparison is given for two martian months. The wind values from the MCD were averaged between 50 and 116 km according the contribution function shown in Fig. 7. The error plotted for the database is the full month standard deviation as given by the MCD website.
time, and season on the planet. Very few overlaps between our observations and either sub-mm observations (compare Sonnabend et al., 2006) or cloud tracking results were identified. McConnochie et al. (2010) reports winds at LS = 48° and latitude 14°S of 65 to 50 m/s with a confidence interval of 35 to 80 m/s while we find at LS = 42° and latitude 15°N a value of 58 ± 14 m/s. However, our observations occured in Martian Year (MY) 29 while the cloud tracking value was extracted from an observation in MY 27. Määttänen et al. (2010) also reports wind measurements from cloud tracking during MY 27–29. However, no overlaps with our LS ranges were identified. For LS = 10–30° values at low latitudes are found in the range of 60 to 95 m/s which is in agreement to the value of 58 m/s we observe at latitude 15°S shortly after. 6. Conclusions The presented data set provides a unique view of the dynamics of the upper atmosphere of Mars. Only IR heterodyne spectroscopy provides the necessary high spectral and spatial resolution needed for direct Doppler wind observations. In general, there is a lack of wind observations (at all levels and locations) that can be used for GCM validation and which are essential also for validation of higher order diagnostics (i.e. vorticity or divergence). Due to the high sophistication of the available GCMs a very detailed comparison is required which calls for high temporal and spatial resolution of the observational data. The comparison between our observations and the MCD shows a reasonable agreement especially in low and mid latitudes. The winds seem to be underestimated by the model at higher latitudes.
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However, to improve the comparison it will be necessary to extract the data from the model in a similar way as the FOV ‘‘extracts’’ the measured winds from the martian atmosphere. This task is currently under way and will help to better interpret and analyze the observed discrepancies. A direct comparison to other observations is difficult. For future campaigns we will try to arrange for coordinated observations especially with orbiting spacecraft. Coordinated observations between space and ground based observers can provide complementary datasets greatly enhancing the scientific merit of the individual results. In addition, observations we will focus on short term variabilities especially in the equatorial regions where the modeling is most difficult due to the breakdown of the thermal wind equation and on the completion of the data set for a full martian year. Acknowledgments Observations were carried out at the National Solar Observatory’s McMath–Pierce telescope on Kitt Peak/Arizona and the NASA Infrared Telescope Facility on Mauna Kea/Hawaii. We would like to thank the staff of both observatories for their extraordinary support during the observing run. This work was supported by the German Deutsche Forschungsgemeinschaft (DFG), special Grants 494 and SO879/1-1 and SO879/1-2. References Cavalié, T. et al., 2008. Vertical temperature profile and mesospheric winds retrieval on Mars from CO millimeter observations. Comparison with general circulation model predictions. Astron. Astrophys. 489, 795–809. Clancy, R.T., Sandor, B.J., Moriarty-Schieven, G.H., Smith, M.D., 2006. Mesospheric winds and temperatures from JCMT sub-millimeter CO line observations during the 2003 and 2005 Mars oppositions. In: Forget, F., Lopez-Valverde, M.A., Desjean, M.C., Huot, J.P., Lefevre, F., Lebonnois, S., Lewis, S.R., Millour, E., Read, P.L., Wilson, R.J. (Eds.), Mars Atmosphere Modelling and Observations, pp. 135–139. Edwards, D.P., 1992. GENLN2: A general line-by-line atmospheric transmittance and radiance model. Version 3.0: Description and users guide. Technical report. Fast, K., Kostiuk, T., Hewagama, T., A’Hearn, M.F., Livengood, T.A., Lebonnois, S., Lefèvre, F., 2006. Ozone abundance on Mars from infrared heterodyne spectra. Icarus 183, 396–402. Forget, F. et al., 1999. Improved general circulation models of the martian atmosphere from the surface to above 80 km. J. Geophys. Res. 104, 24155–24176. Goldstein, J.J., Mumma, M.J., Kostiuk, T., Deming, D., Espenak, F., Zipoy, D., 1991. Absolute wind velocities in the lower thermosphere of venus using infrared heterodyne spectroscopy. Icarus 94, 45–63. Haberle, R.M. et al., 1999. General circulation model simulations of the Mars Pathfinder atmospheric structure investigation/meteorology data. J. Geophys. Res. 104, 8957–8974.
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Hartogh, P. et al., 2005. Description and climatology of a new general circulation model of the martian atmosphere. J. Geophys. Res. (Planets) 110, E11008. Hewagama, T. et al., 1998. Spectral line analysis for planetary atmospheric dynamics retrieval. In: Bulletin of the American Astronomical Society, p. 1093. Johnson, M.A., Betz, A.L., McLaren, R.A., Townes, C.H., Sutton, E.C., 1976. Nonthermal 10 micron CO2 emission lines in the atmospheres of Mars and Venus. Astrophys. J. 208, L145–L148. Kostiuk, T., 1994. Physics and chemistry of upper atmospheres of planets from infrared observations. Infrared Phys. Technol. 35, 243–266. Kostiuk, T. et al., 1996. Stratospheric ammonia on Jupiter after the SL9 Collision. Icarus 121, 431–441. Kostiuk, T. et al., 2001. Direct measurement of winds of Titan. Geophys. Res. Lett. 28, 2361–2364. Lellouch, E., Rosenqvist, J., Goldstein, J.J., Bougher, S.W., Paubert, G., 1991. First absolute wind measurements in the middle atmosphere of Mars. Astrophys. J. 383, 401–406. Lopez-Valverde, M.A., Sonnabend, G., Sornig, M., Kroetz, P., 2011. Modelling the atmospheric CO2 10-lm non-thermal emission in Mars and Venus at high spectral resolution. Planet. Space Sci. 59, 999–1009. Määttänen, A. et al., 2010. Mapping the mesospheric CO2 clouds on Mars: MEx/ OMEGA and MEx/HRSC observations and challenges for atmospheric models. Icarus 209, 452–469. McConnochie, T.H. et al., 2010. THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology. Icarus 210, 545–565. Millour, E. et al., 2011. An improved Mars Climate Database. Mars Atmos.: Model. Observ., 268. Moreno, R., Lellouch, E., Forget, F., Encrenaz, T., Guilloteau, S., Millour, E., 2009. Wind measurements in Mars’ middle atmosphere: IRAM Plateau de Bure interferometric CO observations. Icarus 201, 549–563. Moudden, Y., McConnell, J.C., 2005. A new model for multiscale modeling of the martian atmosphere, GM3. J. Geophys. Res. (Planets) 110, E04001. Richardson, M.I., Newman, C.E., Toigo, A.D., 2006. PlanetWRF – A flexible, multiscale model for planetary atmospheres. In: Bulletin of the American Astronomical Society, p. 625. Sonnabend, G., Wirtz, D., Vetterle, V., Schieder, R., 2005. High-resolution observations of martian non-thermal CO2 emission near 10 lm with a new tuneable heterodyne receiver. Astron. Astrophys. 435, 1181–1184. Sonnabend, G., Sornig, M., Krötz, P.J., Schieder, R.T., Fast, K.E., 2006. High spatial resolution mapping of Mars mesospheric zonal winds by infrared heterodyne spectroscopy of CO2. Geophys. Res. Lett. 33, L18201. Sonnabend, G., Sornig, M., Kroetz, P., Stupar, D., Schieder, R., 2008. Ultra high spectral resolution observations of planetary atmospheres using the Cologne tuneable heterodyne infrared spectrometer. J. Quantit. Spectrosc. Radiat. Trans. 109, 1016–1029. Sonnabend, G., Kroetz, P., Sornig, M., Stupar, D., 2010. Direct observations of Venus upper mesospheric temperatures from ground based spectroscopy of CO2. Geophys. Res. Lett. 37, L11102. Sornig, M. et al., 2008. Venus upper atmosphere winds from ground-based heterodyne spectroscopy of CO2 at 10 lm wavelength. Planet. Space Sci. 56, 1399–1406. Takahashi, Y.O., Fujiwara, H., Fukunishi, H., Odaka, M., Hayashi, Y., Watanabe, S., 2003. Topographically induced north-south asymmetry of the meridional circulation in the martian atmosphere. J. Geophys. Res. (Planets) 108, 5018. Wilson, R.J., 1997. A general circulation model simulation of the martian polar warming. Geophys. Res. Lett. 24, 123–126.
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Mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of CO2 Guido Sonnabend a,c,d,⇑, Manuela Sornig b,c,d, Peter Kroetz a,c,d, Dusan Stupar a,c,d a
I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany Rheinisches Institut für Umweltforschung an der Universität zu Köln, Abt. Planetenforschung, 50931 Köln, Germany c National Solar Observatory, Operated by The Association of Universities for Research in Astronomy, Inc. (AURA), Under Cooperative Agreement with The National Science Foundation, United States 1 d The Infrared Telescope Facility, Which is Operated by The University of Hawaii Under Cooperative Agreement No. NNX-08AE38A with The National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program, United States 1 b
a r t i c l e
i n f o
Article history: Received 16 August 2011 Revised 28 October 2011 Accepted 2 November 2011 Available online 19 November 2011 Keywords: Mars, Atmosphere Infrared observations Spectroscopy Atmospheres, Dynamics
a b s t r a c t We present direct observations of Mars zonal wind velocities around northern spring equinox (LS = 336°, LS = 355°, LS = 42°) during martian year 27 and 29. Data was acquired by means of infrared heterodyne spectroscopy of CO2 features at 959.3917 cm 1 (10.4232 lm) and 957.8005 cm 1 (10.4405 lm) using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) at the McMath–Pierce telescope of the National Solar Observatory on Kitt Peak in Arizona and the NASA Infrared Telescope Facility on Mauna Kea, Hawaii between 2005 and 2008. Winds were measured on the dayside of Mars with an unprecedented spatial resolution allowing sampling of up to nine independent latitudes over the martian disk. Retrieved wind velocities depend strongly on latitude and season with values ranging from 180 m/s prograde to 94 m/s retrograde. A comparison of the observational results to predicted values from the Mars Climate Database yield a reasonable agreement between modeling and observation. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Our knowledge about the processes and dynamics in the atmosphere of Mars especially at low altitudes has greatly improved in recent years due to the many space missions to the planet, in particular Mars Express, Mars Global Surveyor, Mars Reconaissance Orbiter (MRO) and the Mars Exploration Rovers. Based on these data and advances in parameterizations of processes relevant to atmospheric physics, a number of Mars general circulation models (GCMs) have evolved (Wilson, 1997; Haberle et al., 1999; Forget et al., 1999; Takahashi et al., 2003; Hartogh et al., 2005; Moudden and McConnell, 2005; Richardson et al., 2006). For the high altitude regions of the atmosphere, however, constraints on these models are needed that can only partially be provided by spacecraft. Ground-based observations of fully resolved molecular transition lines in planetary atmospheres allow the retrieval of physical parameters such as pressure, temperature, molecular abundance, and dynamical properties of the atmosphere from single line profiles (Kostiuk, 1994). Such observations can probe atmospheric regions not otherwise accessible and can provide continuous ⇑ Corresponding author at: I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany. E-mail address: [email protected] (G. Sonnabend). 1 Visiting Astronmer. 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.11.009
coverage over many years to decades not possible by spacecraft missions. The first ground-based measurements of mesospheric winds on Mars were obtained by observations of CO at mm wavelengths in the early 1990s by Lellouch et al. (1991). These and follow-up observations, however, have low spatial resolution due to the large field-of-view (FOV) of the telescope at such wavelengths (Clancy et al., 2006; Cavalié et al., 2008), a limitation which can be partly overcome by interferometric observations (Moreno et al., 2009). Recently, wind retrievals from the application of cloud tracking methods to images recorded by the OMEGA and High Resolution Stereo Camera (HRSC) instruments on MarsExpress and by the Thermal Emission Imaging System (THEMIS) instrument on MRO have been reported by Määttänen et al. (2010) and McConnochie et al. (2010). These CO2 clouds occur at similar altitudes to our measurements in the mesosphere, however, cloud appearance is sparse and can not be predicted. In addition, the cloud season is mostly limited to LS = 0–60° and equatorial latitudes with only few cloud detections outside that range. Ground-based observations at mid-infrared (MIR) wavelengths can provide much a 10–100 times improved spatial resolution over sub-mm or radio observations, providing a diffraction limited fieldof-view (FOV) e.g., <2 arcsec resolution at 10 lm with a 1.5 m-class telescope as achieved in the presented investigation. High spatial resolution is important because GCMs for Mars like those mentioned above predict strong spatial variations in zonal wind
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Fig. 1. Measured sample spectrum (gray) for Mars observations during campaign A taken at the equator and 75° East of the central meridian longitude (CML) demonstrating the structure of the spectra with a spectral resolution of 1 MHz. The black line gives the best fit to data.
velocities. In the case of Mars, zonal wind velocities can be measured directly by using transition lines of CO2, the major constituent of the atmosphere, in the 10 lm spectral region. These lines are detected in absorption vs. the surface brightness from low altitudes in the martian atmosphere (below 3 mbar level). In addition, non-thermal (non-LTE) emission is contributed from the mesosphere (50–100 km altitude), peaking at about 80 km and creating the characteristic profiles shown in Fig. 1. Non-LTE emission from Mars was first detected by Johnson et al. (1976) using IR heterodyne spectroscopy. A frequency shift between the absorption and the emission component can be directly converted to a
line-of-sight velocity and, given the observational geometry, into a zonal wind shear between the two altitude regions (Sonnabend et al., 2005). If the absolute spectral calibration during the observation is known, independent retrieval of the mesospheric wind velocity is possible. The observed range of LS = 330–40° is an interesting time in the martian year since the northern spring equinox marks the predicted turnover from a northern to southern jet configuration in the upper atmosphere of Mars. In Fig. 2 averaged wind velocities for the altitude range of 50–116 km for LS = 330°, 0°, 40° are shown as given by the ’’Mars Climate Database’’ (MCD), version 4 (Millour et al., 2011). Due to the narrow linewidth of the emission features (<50 MHz FWHM), high spectral resolution of Dmm 107 is required to analyze those lines. Such spectral resolution in the infrared can only be achieved by heterodyne techniques. MIR heterodyne observations have proven to be a valuable tool for planetary astronomy since the 1970s and important results have been achieved on Mars, Venus, Saturn, Jupiter and Titan (Fast et al., 2006; Goldstein et al., 1991; Kostiuk et al., 1996, 2001; Sonnabend et al., 2010; Sornig et al., 2008). 2. Instrumentation Observations were carried out using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS). In the heterodyne instrument the signal from the telescope is mixed with a local oscillator (LO) signal by means of a Fabry-Pérot diplexer and then focused on a fast mercury–cadmium–telluride detector. The detector generates the difference or intermediate frequency (IF) signal at radio frequencies which preserves the spectral information contained in the original IR signal. The IF signal can then be amplified and analyzed in great detail by means of an acousto-optical spectrometer providing a bandwidth of 3 GHz and spectral resolution of 1 MHz (0.00003 cm 1). The receiver is based on a quantum cascade laser (QCL) as LO. Depending on the availability of LOs, operation is presently possible between 7 and 13 lm. The LO is locked to a transmission maximum of the diplexer. A frequency stability of 1 MHz of the complete system is provided by locking the diplexer to a commercially available helium–neon laser with a specified stability of better than 108 in 8 h. The absolute frequency calibration is provided through a reference gas cell. Two calibration loads at known temperatures are integrated into the system for absolute intensity calibration of the observed spectra. An integrated optical guide system provides accurate pointing information and allows active telescope tracking. A detailed description of the receiver is given by Sonnabend et al. (2008). 3. Observations Results from three observing runs addressing dynamics on Mars are reported here. The first two took place at the McMath–Pierce telescope on Kitt Peak in December 2005 and November 2007. For the third observation we used the NASA Infrared Telescope Facility (IRTF) on Mauna Kea in March 2008. Note that for observational purposes longitude coordinates on sky are used while for discussion of results we use planetary coordinates giving west longitudes. 3.1. Mars campaign A (Kitt Peak, December 2005)
Fig. 2. Predicted averaged wind velocities from the MCD for LS = 330°, 0°, 40° (top to bottom).
On December 5–8, 2005 we observed Mars at the CO2 P (2) transition at 959.3917 cm 1 using the McMath–Pierce telescope on Kitt Peak, Az. The instrument THIS was mounted on the spectrograph table at the primary focus of the telescope.
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The angular diameter of Mars varied from 16.7 to 15.4 arcsec over the course of the observing period compared to the diffraction-limited field of view (FOV) of the telescope of 1.6 arcsec. The pointing uncertainty was estimated to be below 1 arcsec and verified by observing a stellar source. Mars was 1 month past opposition and the Earth–Mars Doppler shift as given by JPL’s HORIZONS ephemeris website (http://ssd.jpl.nasa.gov/) varied between 9.8 and 11.4 km/s (receding). The acquisition time for an individual spectrum was 10 min. Spectra were co-added during analysis to achieve a sufficient SNR. Mars illumination was 96% with the subsolar point at 21°W of CML and 10.3°S from the equator. Martian season was late northern hemisphere winter at a martian solar longitude of LS 335°. In total, six different latitudes on Mars were observed during 102 individual observations, each lasting from 70 to 200 min. The sampled latitudes ranged from 45°N to 75°S and observations were carried out close to the limb covering a local time range of 3 h. Observing geometry is pictured in Fig. 3 indicating the observed positions at the FOV scaled to the planetary diameter. Observed positions, dates, times, and integration times are given in Table 1. A sample spectrum is shown in Fig. 1. The signal-to-noise ratio (SNR) depends mainly on integration time and the surface temper-
ature at the observed position on Mars which provides the quasiblackbody continuum level observed in the spectra. Observations from 2005 have been published in (Sonnabend et al., 2006) and are given here for completeness. 3.2. Mars campaign B (Kitt Peak, November/December 2007) Following the course of the martian year the next observations were carried out at a solar longitude of LS 354 between November 23 to December 3, 2007 again at the McMath–Pierce telescope. The orbital position of Mars was approximately 1 month before opposition. The topocentric relative velocity between Earth and Mars varied between 4.9 and 7.8 km/s (approaching). Acquisition time for an individual spectra is again 10 min. Mars was almost fully illuminated with a subsolar point at 23°E from CML and 2.5°S. The apparent diameter was between 14.4 and 15.3 arcsec, similar to conditions for the observing run in 2005. The observing geometry is given in Fig. 4 and the relative size of the apparent diameter of the planet and the diffraction-limited FOV of 1.6 arcsec are pictured. Data were provided by 74 single measurements at nine different latitudes on the planet (from 57°S to 57°N) along the morning limb. Relevant data for observations are summarized in Table 2. 3.3. Mars campaign C (Mauna Kea, March 2008) Mars was observed again from March 03 to 06, 2008. Martian season of northern hemisphere spring at a solar longitude of LS 42° completed the desired seasonal coverage. The apparent diameter of Mars varied between 8.6 and 8.9 arcsec. To obtain a spatial resolution comparable to the observations in 2005 and 2007, when martian apparent diameter was larger, we used the NASA IRTF on Mauna Kea, Hawaii, USA, having a 3 m diameter primary mirror and thus providing a diffractionlimited FOV at 10 lm of 0.8 arcsec. The related observing geometry is given in Fig. 5. The martian disk was illuminated approximately 91% with a subsolar point of 33°E from CML and 16°N. The topocentric relative velocity between Mars and Earth was 16.7 km/s. Limb observations at nine different latitudes starting from 57°N to 57°S were performed. Observed positions and relevant observation parameters are summarized in Table 3. In total 35 single measurements resulted in observations of 10 positions on the planet. This time the P (4) line of CO2 at 957.8005 cm 1 was observed.
Fig. 3. Observing geometry of Mars during the Mars campaign A from December 5 to 8, 2005. The relative sizes of apparent diameter of the martian disk (approximately 16 arcsec) and the FOV (1.6 arcsec) are indicated. In addition, the circles give the pointing positions on Mars. Mars was almost totally illuminated (96%) with a subsolar point at 21°W from CML and 10.3°S from the equator labeled by the black star. From limb observations line-of-sight velocities were retrieved for six different latitudes. Note that for observational purposes coordinates on sky are used.
4. Data analysis During both observing runs on Kitt Peak the P (2) line at 959.3917 cm 1 of the 10.4 lm-band of CO2 was observed. Due to availability of a new QCL LO the 75% stronger P (4) line at 957.8005 cm 1 of the same band was observed during the observation run on Mauna Kea providing a higher SNR.
Table 1 Overview of the observed positions on Mars during the Mars campaign A at the McMath–Pierce telescope on Kitt Peak. Given are the position on Mars, observing date and time, martian season and local time, and the total integration time in minutes (from Sonnabend et al., 2006). Mars campaign A (Kitt Peak, December 2005)
1 2 3 4 5 6 7 8
Lat./rel. lon. on sky )
Date
Time (UT)
West longitude on planet (°)
Season (LS)
Mars (LT)
Int. time (min)
45°N/90°W 0°/90°W 0°/90°W 15°S/90°W 35°S/90°W 60°S/90°W 75°S/90°W 75°S/90°W
12/05 12/05 12/05 12/07 12/08 12/07 12/06 12/06
3:07–5:40 1:42–2:51 5:23–8:42 6:28–8:22 1:00–3:00 1:29–3:48 1:00–3:33 6:07–8:40
237–275 196–213 251–298 239–268 142–171 149–182 148–185 223–260
335 335 335 336 338 336 336 336
15:15 16:40 16:40 17:11 17:49 18:24 18:36 18:36
153 69 199 114 120 139 153 153
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Fig. 4. Observing geometry of Mars during campaign B in November 23 and December 03, 2007. The field of view was 1.6 arcsec and the angular diameter of the martian disk was 15 arcsec. In addition, the circles give the pointing positions on Mars. Mars was almost totally illuminated and the black star marks the subsolar point at 23°E from CML and 2.5°S. Limb observations were accomplished at nine different latitudes from 57°S to 57°N to retrieve line-of-sight velocities. Note that for observational purposes coordinates on sky are used.
Measured single integrations were added to provide spectra with similar SNR. Integration time of the resulting spectra varied between 30 and 300 min depending on observing conditions (e.g. chosen position on Mars, weather, airmass). The spectra were modeled and fitted using the BEAMINT/CODAT radiative transfer code developed at NASA Goddard Space Flight Center (Hewagama et al., 1998). BEAMINT/CODAT combines a layer-by-layer radiative transfer modeling engine with an algorithm to combine the contribution from sub-resolution segments of the instrument beam to form the overall spectrum. This can be a noticeable improvement over a single point mean viewing angle model, especially when the viewing geometry is such that the beam sees contribution from a wide range of planetary longitudes. We used a 16-element beam model for the analysis of our observations. BEAMINT/CODAT accepts planetary parameters, observation circumstances, molecular and thermal height profiles, and a molecular line atlas. The non-LTE component is not currently included in the radiative transfer code but added as a Gaussian profile describing the shape of the non-LTE emission. Parameters can be iterated until a best fit of a model spectrum to an observed spectrum is achieved. Uncertainties based on correlation between free parameters and the variance
between the observed and model spectra are returned. The telluric contribution to the spectra was modeled using the terrestrial radiative transfer package GENLN2 (Edwards, 1992). For the presented results, we first generated an average value for the Mars surface temperature, weighted by the beam distribution and the brightness temperature of the surface area within the FOV. Surface pressure and atmospheric thermal profile were extracted from the MCD. The frequency position, width and intensity of the Gaussian profile representing the non-LTE feature were iterated along with a scaling factor representing the total loss of signal due to misalignment, clouds, etc. until the best fit to the observed data was reached. Analysis methodology was different for the first observing run in 2005 than for following runs in 2007 and 2008 due to implementation of a reference gas cell into the instrument. The gas cell provides an absolute frequency calibration for the non-LTE emission relative to the gas cell spectrum and therefore information about the absolute wind velocity (independent of the absorption line). Without the reference cell the frequency shift between the martian CO2 absorption and emission feature gives the wind velocity shear between high and low altitudes with the observing geometry taken into account. Wind velocities at the low altitudes were taken from the MCD and averaged over the FOV. Analysis of the contribution functions (see example plotted in Fig. 6) for the absorption line profile shows that most contribution to the absorption line originates below the 3 mbar pressure level. During campaign B two ethylene absorption lines from the reference gas cell were present (at 959.4075 cm 1 and 959.3072 cm 1) within the observed bandwidth and used for determination of the absolute frequency position of the CO2 line. In campaign C The 957.7641 cm 1 ethylene line was used for absolute frequency calibration. In both cases the fitted frequency offsets were then converted to eastward zonal wind velocities, meaning positive sign for wind going from West to East on the planet, taking into account Mars solid-body rotation and assuming a horizontal zonal flow. Determination of the altitude of the emission-forming region is not possible from the data itself. Recent modeling of the non-LTE processes in the atmospheres of Mars predict a peak of the emission at an altitude of 80 km (Lopez-Valverde, 2011) (see also Fig. 7). For campaign C, due to an instrumental problem all observations past transit of Mars had to be discarded. The problem was caused by an instability in the optical guide camera path resulting in a significant shift between the visible and the IR light path and thus an uncontrolled observing position on Mars. Overviews of the retrieved frequency shifts and the zonal wind velocities are presented in the Tables 4–6 and in Fig. 8 below. The uncertainties include the 1-r fitting errors, the precision of the calibration line (0.3 MHz), the AOS calibration (0.2 MHz), and the maximum deviation due to the time step in the Doppler-shift calculation (0.3 MHz).
Table 2 Overview of the observed positions on Mars during the Mars campaign B at the McMath–Pierce telescope on Kitt Peak. Given are the position on Mars, observing date and time, martian season and local time, and the total integration time in minutes. Mars campaign B (Kitt Peak, November/December 2007)
1 2 3 4 5 6 7 8 9 10
Lat./rel. lon. on sky
Date
Time (UT)
West longitude on planet (°)
Season (LS)
Mars (LT)
Int. time (min)
57°N/90°E 45°N/90°E 33°N/90°E 11°N/90°E 0°/90°E 11°S/90°E 33°S/90°E 45°S/90°E 45°S/90°E 57°S/90°E
11/27 11/28 12/03 11/27 11/28 11/24 11/25 11/23 12/03 11/25
11:20–11:47 9:25–13:51 10:30–12:07 9:31–11:03 6:27–9:08 9:00–14:12 5:48–7:45 9:05–11:14 12:16–14:01 9:36–11:14
10–16 332–37 301–325 338–1 282–322 353–70 293–322 358–30 316–342 346–10
354 354 357 354 354 352 353 352 357 353
6:58 7:04 7:13 7:24 7:32 7:39 7:54 8:01 7:55 8:07
27 266 97 92 161 312 117 129 105 98
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Fig. 6. Examples of contribution functions for to the martian CO2 absorption line profile. Analysis showed that most of the contribution to this line originates below the 3 mbar pressure level. The different lines indicate the contributions from the absorption line depending on distance form the line center. Fig. 5. Observing geometry for Mars campaign C during March 03 to March 06, 2008. Mars was almost totally illuminated but this time the subsolar point at 33°E from CML and 16°N was close to the western limb. The FOV was 0.8 arcsec as indicated by the circles on an angular diameter of the martian disk of 9 arcsec. Nine different latitudes were addressed for limb observations to gain line-of-sight wind velocities, indicated by the circles as well. Note that for observational purposes coordinates on sky are used.
5. Data interpretation and comparison In the following we will compare our results to model predictions as well as other observations. 5.1. Comparison to model predictions In order to compare the model predictions to the observations we extracted wind values from the website of the Mars Climate Database (MCD) (http://www-mars.lmd.jussieu.fr/). The corresponding season, latitude/longitude and Mars universal time (MUT) were used. The altitudinal distribution was weighted according to the contribution to the non-LTE feature as shown in Fig. 7. For the uncertainties of the predicted values we use the full month standard deviation as given by the MCD. Since the MCD website only provides average values for a full martian month we constructed two cases each for campaigns A and B since they both fall between 2 months in LS. The comparison for all three runs is shown in Fig. 8. In general, the predicted values agree with the observed data within the error bars for most observed positions. In all three campaigns a non-agreement between data and model
Fig. 7. Derivative of upward radiances of the P (2) line at its line core. The four lines correspond to solar solar zenith angles = 0°, 60°, 80°, and 88°, as indicated (from Lopez-Valverde, 2011). Contribution to the line peaks at 80 km.
occurs for high northern latitudes. In campaign A the mid latitudes show a better agreement to month 12 (LS = 330–360°) while the higher latitudes fall closer to month 11 (LS = 300–330°). In campaign B most values fall in between month 12 and month 1 (LS = 0–30°). Here we also see a significant deviation between observed and predicted value for the latitude 11° North were we measure a prograde wind while the model predicts a retrograde wind and the values do not agree within the error bars. In
Table 3 Overview of the observed positions on Mars during the Mars campaign C at the NASA IRTF on Mauna Kea in Hawaii, US. Given are the position on Mars, observing date and time, martian season and local time, and the total integration time in minutes. Mars campaign C (Mauna Kea, March 2008)
1 2 3 4 5 6 7 8 9 10
Lat./rel. lon. on sky
Date
Time (UT)
West longitude on planet (°)
Season (LS)
Mars (LT)
Int. time (min)
57°N/90°W 57°N/90°W 45°N/90°W 33°N/90°W 15°N/90°W 0°/90°W 15°S/90°W 33°S/90°W 45°S/90°W 57°S/90°W
03/06 03/06 03/05 03/06 03/05 03/05 03/05 03/06 03/05 03/06
2:35–3:17 7:40–8:17 2:48–4:48 4:28–5:23 1:53–2:37 7:10–8:00 8:08–9:01 6:36–7:20 4:57–6:50 8:30–9:26
232–241 306–314 244–273 259–272 232–242 310–321 323–336 292–303 277–305 320–333
42 42 41 42 41 41 41 42 41 42
15:53 15:53 15:52 15:51 15:49 15:48 15:46 15:44 15:44 15:42
42 37 120 55 44 50 53 44 113 56
320
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Table 4 Overview of the retrieved wind velocities in the mesosphere of Mars during the observing campaign A. Given are the position on the planet, date and the total integration time of observation. The given frequency shift is the relative shift mlos-rel between absorption and emission line with a positive sign for resulting upper atmosphere wind towards the observer. The given eastward zonal wind velocity vzonal takes the low altitude wind component into account (see text for details). Martian season was LS = 335–338 (from Sonnabend et al., 2006). Mars campaign A (Kitt Peak, December 2005)
1 2 3 4 5 6 7 8
Lat./long
Date
tint (min)
45°N/limb 0°/limb 0°/limb 15°S/limb 35°S/limb 60°S/limb 75°S/limb 75°S/limb
12/05 12/05 12/05 12/07 12/08 12/07 12/06 12/06
320 160 400 320 200 280 320 400
mlos-rel (MHz)
vzonal (m/s)
2.2 ± 1.4 6.8 ± 1.1 4.5 ± 0.9 3.0 ± 1.6 0.1 ± 1.0 2.2 ± 1.1 2.1 ± 1.6 4.1 ± 2.4
27 ± 17 80 ± 13 53 ± 10 36 ± 19 1 ± 12 25 ± 12 25 ± 19 51 ± 29
Table 5 Overview of the retrieved wind velocities in the mesosphere of Mars during the Mars campaign B in 2007. Given are the position on the planet, date and the total integration time of observation. The absolute frequency shifts mlos, with a positive sign for wind towards the observer, and the eastward zonal wind velocity vzonal are given as well. Martian season was LS = 352–357. Mars campaign B (Kitt Peak, November and December 2007)
1 2 3 4 5 6 7 8 9 10
Lat./long
Date
tint (min)
mlos (MHz)
vzonal (m/s)
57°N/limb 45°N/limb 33°N/limb 11°N/limb 0°/limb 11°S/limb 33°S/limb 45°S/limb 45°S/limb 57°S/limb
11/27 11/28 12/03 11/28 11/28 11/25 11/25 11/23 12/03 11/25
20 160 100 180 140 240 240 160 120 120
17.3 ± 6.0 7.0 ± 3.4 2.7 ± 2.1 2.1 ± 1.1 3.5 ± 1.6 6.4 ± 1.7 9.0 ± 1.45 1.1 ± 2.0 2.9 ± 1.5 1.6 ± 1.9
180 ± 63 73 ± 35 28 ± 22 22 ± 12 37 ± 17 67 ± 19 94 ± 14 12 ± 21 30 ± 16 17 ± 20
Table 6 Overview of the retrieved wind velocities in the mesosphere of Mars during the Mars campaign C in 2008. Given are the position on the planet, date and the total integration time of observation. The absolute frequency shifts mlos, with a positive sign for wind towards the observer, and absolute line-of-sight wind velocities vzonal are given. Martian season was LS = 41–42. Mars campaign C (Mauna Kea, March 2008)
1 3 4 5
Lat./long
Date
tint (min)
57°N/limb 45°N/limb 33°N/limb 15°N/limb
03/06 03/05 03/06 03/05
40 80 80 60
mlos (MHz) 8.1 ± 2.0 2.6 ± 1.3 1.6 ± 1.4 6.1 ± 1.6
vzonal (m/s) 85 ± 21 27 ± 13 17 ± 15 64 ± 17
campaign C most values agree within the uncertainties except the high latitude position at 60°N, however, the data set is very limited due to the technical failure mentioned above. The used averaging of the MCD values is certainly only a first step on the way to a detailed comparison. Such a study which will include a full calculation of the observational FOV and its geometrical representation in the martian atmosphere including a study of the predicted variabilities is currently underway. It will also include a more detailed analysis of the altitudinal dependance of the non-LTE region to various atmospheric parameters. 5.2. Comparison to other observations A comparison to other direct observations of winds is difficult due to the strong dependency of the zonal winds on latitude, local
Fig. 8. Comparison of retrieved wind values to predicted values from the MCD. Plotted is the eastward zonal wind component vs. latitude for all three campaigns. For campaign A and B comparison is given for two martian months. The wind values from the MCD were averaged between 50 and 116 km according the contribution function shown in Fig. 7. The error plotted for the database is the full month standard deviation as given by the MCD website.
time, and season on the planet. Very few overlaps between our observations and either sub-mm observations (compare Sonnabend et al., 2006) or cloud tracking results were identified. McConnochie et al. (2010) reports winds at LS = 48° and latitude 14°S of 65 to 50 m/s with a confidence interval of 35 to 80 m/s while we find at LS = 42° and latitude 15°N a value of 58 ± 14 m/s. However, our observations occured in Martian Year (MY) 29 while the cloud tracking value was extracted from an observation in MY 27. Määttänen et al. (2010) also reports wind measurements from cloud tracking during MY 27–29. However, no overlaps with our LS ranges were identified. For LS = 10–30° values at low latitudes are found in the range of 60 to 95 m/s which is in agreement to the value of 58 m/s we observe at latitude 15°S shortly after. 6. Conclusions The presented data set provides a unique view of the dynamics of the upper atmosphere of Mars. Only IR heterodyne spectroscopy provides the necessary high spectral and spatial resolution needed for direct Doppler wind observations. In general, there is a lack of wind observations (at all levels and locations) that can be used for GCM validation and which are essential also for validation of higher order diagnostics (i.e. vorticity or divergence). Due to the high sophistication of the available GCMs a very detailed comparison is required which calls for high temporal and spatial resolution of the observational data. The comparison between our observations and the MCD shows a reasonable agreement especially in low and mid latitudes. The winds seem to be underestimated by the model at higher latitudes.
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