Mars surface and atmospheric temperature during the 2001 global dust storm

Icarus 175 (2005) 23–31 www.elsevier.com/locate/icarus

Mars surface and atmospheric temperature during the 2001 global dust storm Mark A. Gurwell a,∗ , Edwin A. Bergin b , Gary J. Melnick a , Volker Tolls a a Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA b University of Michigan, Department of Astronomy, 830 Dennison Building, Ann Arbor, MI 48109, USA

Received 16 January 2004; revised 29 September 2004 Available online 2 March 2005

Abstract The dramatic growth and evolution of the 2001 martian global dust storm were captured using the Submillimeter Wave Astronomy Satellite (SWAS). While the lower and middle atmosphere (pressures greater than 50 µbar, up to ∼ 45 km altitude) showed rapid heating of up to 40 K, the average surface brightness temperature plummeted by ∼ 20 K at the peak of the storm. The storm appears to have had little impact on the global temperature structure at altitudes above ∼ 10 µbar (∼ 60 km).  2004 Elsevier Inc. All rights reserved. Keywords: Mars, atmosphere; Radio observations; Atmospheres, structure

1. Introduction Mars at opposition is a popular target for both amateur and professional astronomers, as it dramatically increases in apparent size. The 21 June 2001 opposition promised to be a good one, with a maximum diameter just under 21 arcsec, the largest it had been for several years. A few days after closest approach, astronomers of both flavors noted the rise and growth of a global dust storm from smaller regional storms in and around the Hellas Basin (Tytell, 2001). The aerocentric longitude at storm initiation was LS = 185◦ , the earliest ever noted for a global dust storm (Smith et al., 2002). The appearance of this storm just after opposition was serendipitous, for it allowed for detailed remote observations from the near-Earth environment in addition to being extensively monitored from Mars orbit by the instruments on board the Mars Global Surveyor (MGS) spacecraft. Global dust storms occur only around martian perihelion; due to the ellipticity of the orbit of Mars solar heating is at a * Corresponding author.

E-mail address: [email protected] (M.A. Gurwell). 0019-1035/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2004.10.009

maximum during this season. The increased solar energy is thought to be a driver for raising of dust, and perhaps is responsible for allowing some small, regional storms to grow into a global storm, though the exact mechanism is not understood (see reviews by Zurek et al. (1992) and Fernández (1998)). In particular, dust storms that become global do not occur every perihelion. There has been considerable theoretical and observational study of the seasonal and water cycles on Mars and on the interannual repeatability and long term variability of global atmospheric phenomena; this effort has been heightened since the arrival of Mars Global Surveyor in 1997 (e.g., Zurek and Martin, 1993; Clancy et al., 1996, 2000; Cantor et al., 2001; Smith et al., 2001a, 2001b; Smith, 2004; Pankine and Ingersoll, 2004). Millimeter and submillimeter observations of the martian atmosphere have played a critical role in the developing understanding of the seasonal cycle, and are particularly useful for temperature retrieval. The source function is nearly linear with temperature. Radiative transfer calculations are simple, as direct scattering and emission by martian dust is negligible due to the small particle sizes relative to the observing wavelength (typical mean diameter of ∼ 2 µm; Tomasko et al., 1999). Observations of carbon monoxide

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have been used to measure the global atmospheric temperature over long time periods, bridging the gap between the Viking and Mars Global Surveyor eras (e.g., Clancy et al., 1990). Submillimeter observations can be used to probe temperatures from the surface to greater than 80 km in altitude, typically higher than other retrieval methods, and thus probing a little studied portion of the atmosphere (e.g., Clancy and Sandor, 1998). Temperature measurements from CO observations recorded the growth and decay of a large global dust storm in 1994 (Clancy et al., 1994, 2000). In addition, the global water abundance and vertical distribution have been measured from observations of H2 O, HDO, and H2 18 O (Clancy et al., 1992; Gurwell et al., 2000; Burgdorff et al., 2000; Encrenaz et al., 2001a, 2001b). Here we report on observations of the martian atmosphere during 2001 using unique data obtained with the Submillimeter Wave Astronomy Satellite (SWAS). Observations of rotational lines of water and carbon monoxide were used to measure the evolution of global atmospheric temperature from the surface to 75 km during the 2001 dust storm.

2. Observations SWAS is an orbiting radio telescope designed to measure submillimeter-wave radiation simultaneously in two receiver bands. Submillimeter transitions of molecular oxygen and atomic carbon near 490 GHz are observable in the lower frequency band. The H2 16 O ground state and 13 CO (5-4) rotational transitions, or alternately the H2 18 O ground state rotational transition, are available within the higher frequency band centered near 550 GHz (Melnick et al., 2000; see Table 1 for details on the SWAS instrument). While optimized to observe the interstellar medium, SWAS has proven useful for observations of water in the Solar System as well (e.g., Bergin et al., 2000; Neufeld et al., 2000). In particular, SWAS measurements were used to determine the globallyaveraged vertical distribution of water vapor on Mars during the 1999 opposition (LS ∼ 130◦ ), concluding that the water Table 1 The SWAS instrumenta Parameter

Value

Telescope Beam size: 490 GHz 553 GHz Spectrometer Spectral lines:b Receiver 1: Receiver 2:c

54 × 68 cm diameter 3.5 × 5.0 3.3 × 4.5 AOS, 1.5 MHz resolution O2 (3,1-3,2) H2 18 O(110 -101 ) 13 CO(5-4) H2 16 O(110 -101 )

487.249 GHz 547.676 GHz 550.926 GHz 556.936 GHz

a Adapted from Melnick et al. (2000). b Lines of interest for Mars observations. c 13 CO (lower sideband) and H 16 O (upper sideband) are observed. 2

profile was near 100% saturation above ∼ 8 km (Gurwell et al., 2000). Mars was observed by SWAS over the period 23 May to 13 September 2001, (aerocentric longitude LS = 166◦ –233◦ ) in a weekly cycle. The large SWAS beam did not spatially resolve Mars at any time. The sub-Earth latitude varied only slightly around the martian equator, and the SWAS observations are dominated by emission from low latitudes (roughly ±30◦ ). The appearance of Mars changed throughout the observation period (see Table 2), with the apparent size and disk center local timing varying significantly. Observations at the start of the period were centered in the early afternoon, while those at the end of the period were centered in the mid-morning. Over the course of the observations the mean surface atmospheric pressure of Mars increased from 5.3 to 6.6 mbar with the change in LS (Hess et al., 1980; Zurek et al., 1992). Data were collected during SWAS orbit segments that were typically 20–30 min, of which 40% was on-source integration time, 40% was reference calibration integration time, and 20% was spent in spacecraft slewing and settling. The full data set consists of several hundred orbit segments spread over the 114 day observing period. In a typical observing day, seven orbit segments were utilized in a spectral mode for observations with the acousto-optical spectrometer (AOS), using the two receivers simultaneously. Receiver 1 was tuned to place the 487 GHz oxygen transition in the lower sideband for all seven segments. For receiver 2, two of the segments were obtained with the 13 CO and H2 16 O lines in the passband, with a change in the local oscillator (LO) setting between the two orbits of ∼ 230 MHz. This LO shift allowed for a wider effective bandwidth coverage of the H2 16 O line core (the full line profile is several GHz in width). The five remaining orbit segments were obtained with the H2 18 O line in the passband. The receiver temperature for the H2 18 O tuning is nominally 4000 K double sideband (DSB), while for the 13 CO/H2 16 O tunings it is substantially less, about 2200 K DSB (Melnick et al., 2000). The 5:2 ratio in integration time between the two major tunings provides nearly identical radiometric noise levels. After LS 220◦ , the time-intensive H2 18 O observations were discontinued in order to focus on 13 CO and H2 16 O observaTable 2 Observational parameters for Mars on selected dates Date, 2001

LS a (◦ )

Diameter (arcsec)

Local timeb (hr)

Sub-Earth latitude (◦ )

23 May 01 Jun 21 Junc 01 Jul 01 Aug 01 Sep 13 Sep

166.1 171.0 182.2 187.9 206.3 225.3 232.9

17.9 19.3 20.8 20.5 17.0 13.3 12.2

13:05 12:41 11:41 11:11 09:59 09:21 09:11

−0.6 +0.7 +4.5 +6.1 +6.8 +1.7 −1.3

a Aerocentric longitude. b Apparent local time at disk center. c 2001 martian opposition.

SWAS observations of the 2001 global dust storm on Mars

tions as Mars receded and the source antenna temperature dropped. In addition to the spectral mode, SWAS has a continuum mode using detectors that sample the same IF signal fed into the AOS. These detectors are capable of much more rapid calibration and readout than the AOS. SWAS continuum mode observations typically used the standard chopping calibration technique (see Melnick et al., 2000), utilizing the 2-Hz chopping secondary. However, it was determined that a specialized ‘chop-nod’ mode, which placed the target alternately in the primary beam and in the chopped reference beam, was more effective for minimizing fluctuations in the measured brightness due to receiver gain variations as well as standing waves (Tolls et al., 2004). This reduction in fluctuations provided significant improvement in relative continuum sensitivity, at the expense of a loss in efficiency as the chopped beam moves between the primary and reference positions, as well as a loss when Mars is in the reference beam, which is slightly defocused. Dedicated calibration measurements on Jupiter have quantified this loss factor relative to the standard spectroscopic mode, and have been verified to be consistent with observations of Venus and Mars. This allowed the relative continuum to be placed on the same antenna temperature scale as the spectroscopic observations. Approximately two orbit segments per week were spent performing these sensitive continuum observations with both receivers, with nominal frequencies of 490 and 553.5 GHz. In the remainder of this article, data obtained in this operational mode are described as ‘chop-nod continuum’ data.

3. Data reduction Initial calibration of the SWAS data provided twiceweekly chop-nod continuum band measurements as well as daily average DSB spectra, correctly aligned in frequency for each observed transition to account for spacecraft and planet orbital motions. The chop-nod continuum measurements were calibrated to remove the efficiency loss factors described above and were on the same DSB antenna temperature scale as the calibrated spectra. 3.1. Continuum The 490 GHz DSB chop-nod continuum observations are sensitive to thermal emission from the top few millimeters of the martian surface, averaged over the apparent disk, and represent our primary absolute calibration signals. The 553 GHz DSB chop-nod continuum observations sampled the surface (near 551 GHz) as well as atmospheric emission from within the broad water line (near 557 GHz), and are not useful for primary calibration. They are useful for relative calibration of the spectroscopic data (see below). The 490 GHz chop-nod continuum observations were calibrated using a model of the whole disk brightness temperature that is derived from a thermal study of the martian

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surface and subsurface at centimeter wavelengths as a function of season and local time of day (Rudy et al., 1987). The model was extended into the millimeter and submillimeter bands for analysis of previous SWAS observations of Mars (Gurwell et al., 2000). The model has been adjusted to provide slightly higher temperatures (∼ 8 K) than previously calculated in the submillimeter based upon multi-line CO observations (R.T. Clancy, personal communication). This adjustment points to a slightly inadequate modeling of the temperature profile in the top 1 mm of the martian regolith. Given that the model was developed from 2 and 6 cm observations, such an error is not unreasonable. For the 2001 opposition, the model calculates a disk average Planck brightness temperature of 231 ± 8 K, with a mean physical temperature of 260 ± 12 K. Using chop-nod continuum data obtained prior to 26 June 2001 (e.g., before the start of the 2001 global dust storm), the SWAS aperture efficiency at 490 GHz was determined to be 0.69 ± 0.04, in agreement with that determined from the 1999 Mars observations (Gurwell et al., 2000). Figure 1 presents the 490 GHz chopnod continuum observations as a function of LS compared with the model prediction. The radiative transfer model was also used to calculate the disk average brightness temperature versus time at 553 GHz for comparison with data from the high-frequency receiver. 3.2. Spectra To verify that the spectroscopic and chop-nod continuum observations were consistent with each other, the daily average spectra were integrated in frequency to create a second type of continuum measurement, which we refer to as the AOS band-average. These AOS band-average measurements show more variation from day to day than would be expected simply from radiometric noise (see Fig. 1 for 490 GHz data comparison), and point to known receiver gain instabilities. Nevertheless, this comparison shows that over the total observing period the spectroscopic observations are on the same absolute scale as the chop-nod continuum observations. With this finding, we used the H2 18 O AOS band-average observations to measure the 553 GHz aperture efficiency to be 0.65 ± 0.04, in agreement with the assumed aperture efficiency from Gurwell et al. (2000). This aperture efficiency is also valid for the H2 16 O and 13 CO data. Given the small signal of Mars (antenna temperature of ∼ 2 K at opposition) relative to the system temperature (∼ 2000 K) the receiver gain fluctuations with time are significant (as shown by the scatter in the AOS band-averages in Fig. 1), since our radiative transfer modeling and analysis depends upon the accurate measure of the single sideband line-to-continuum (LTC) ratio. For the spectra to be fully analyzed these gain fluctuations needed to be calibrated. The individual daily average spectra were given offset corrections to force them to agree with an interpolated fit to the chop-nod continuum observations. This procedure maintains the proper spectral SNR and lineshape while benefiting from

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Fig. 1. Comparison of model double sideband antenna temperature (line) with chop-nod continuum mode data (circles) and AOS band-average measurements (crosses; see text for details). The primary variation in antenna temperature is due to the changing size of Mars (and from that the total flux) during the observations, peaking during opposition on 21 June 2001. Day-to-day variations in the continuum level of the AOS band-average data dominate on the short-term. The chop-nod continuum data have significantly less intrinsic variation, and show a clear deviation from the model starting near LS = 185◦ .

the increased sensitivity of the chop-nod continuum observations. With our knowledge of the high-frequency aperture efficiency we can also remove the effects of the gain fluctuations from the H2 16 O/13 CO DSB spectra using the chop-nod continuum measurements at 553 GHz. This is significant as there is no other direct measure of the 553 GHz surface emission in the spectroscopic data, which is dominated by Mars’ atmospheric emission across the entire upper sideband. The SWAS receivers were designed to achieve an upperto-lower sideband ratio of 1:1 (i.e., a balanced response to each sideband), and in-orbit observations verify this to be true to within a few percent (Melnick et al., 2000). Each DSB spectrum is therefore considered to be a simple addition of the spectrum from each of the two sidebands. For the H2 18 O DSB spectra, the single spectral line is narrow relative to the bandpass and conversion to LTC spectra is straightforward. The H2 16 O/13 CO DSB spectra are more complicated, with the relatively narrow 13 CO line in the lower sideband and the H2 16 O line (far broader than the spectral coverage) in the upper sideband. Conversion of the spectra followed the procedure developed in Gurwell et al. (2000). First, for 13 CO, the broad wings of the water line were subtracted from each DSB spectrum. Second, using the radiative transfer model of the surface emission and the 553 GHz aperture efficiency, the surface continuum antenna temperature was estimated and added. Finally, division by the continuum antenna temperature produced an accurate, calibrated LTC spectrum. For H2 16 O, we simply subtract the continuum sur-

face emission from the opposite sideband, then divided by that continuum vale. Because the velocity smeared 13 CO line does appear in these H2 16 O LTC spectra, we limited our analysis to ±200 MHz from the H2 16 O line center, eliminating confusion from the overlapped 13 CO line. To increase the channel to channel signal-to-noise ratio (SNR) on the spectra, a running average over 18 days (∼ 10◦ of LS ) was performed for each of the 114 days of observation for the three spectral lines (H2 18 O, H2 16 O, and 13 CO). Figure 2 presents representative LTC spectra for three times covered by SWAS measurements. The spectra show clear variation in lineshape and strength; these changes are related to changes in the atmosphere and surface emission during the observing period.

4. Continuum observations: global surface temperature The 490 GHz continuum brightness temperature versus time, calculated from the DSB chop-nod continuum antenna temperature, and the radiative transfer model prediction for the SWAS observing period are presented in Fig. 3a. The model shows a smooth decrease in surface brightness temperature with time starting around LS = 172◦ (early June 2001). This decrease occurs because the sub-Earth local time changes from the early afternoon (with relatively warm surface temperatures) to mid-morning (with significantly colder temperatures).

SWAS observations of the 2001 global dust storm on Mars

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Fig. 2. Representative fully calibrated LTC spectra from three periods for each of the three molecular lines observed. Left (LS = 175◦ ): prior to initiation of the 2001 global dust storm. Center (LS = 188◦ ): Just after the storm begins major growth. Right (LS = 201◦ ): approximate time of peak dust opacity in the southern hemisphere (LS = 200◦ , Smith et al., 2002). Each molecular line shows significant evolution in shape, which provide the primary observational indications of change in the martian atmospheric temperature structure. A best fit model spectrum, the final result of the iterative inversion process, is overlayed on each observed spectrum. The spectra were smoothed over 18 day periods (∼ 10◦ of LS ).

Before and at opposition (21 June 2001), the variation in brightness temperature with time of the calibrated SWAS continuum observations agree well with the thermal model. After opposition the measured continuum brightness temperature drops at a substantially faster rate than predicted by the model. By LS = 195◦ , the SWAS-measured continuum is 15 K cooler than the model, and by LS = 206◦ , the continuum reaches a maximum deviation of 21 K. After LS = 206◦ , the surface brightness rises substantially, and by LS = 220◦ the observations and model are in close agreement. The 2001 martian global dust storm was observed in detail by the Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument, which provided consistent observations of dust, water column, and temperature of the surface and atmosphere at local times of 0200 and 1400. Analysis of TES results show that the atmospheric dust loading increased rapidly from the initial storm outbreak (LS = 185◦ ) until it became ‘planet encircling’ (LS = 193.5◦ ). Dust opacity peaked at different times between the two hemispheres, with the maximum occurring in the southern hemisphere at LS = 200◦ , followed by the northern maximum at LS = 210◦ (Smith et al., 2002; Smith, 2004). As measured by SWAS, the drop in disk averaged brightness temperature (denoting a drop in the average physical temperature of the top few millimeters of the martian surface) coincides well with the TES-derived dust opacity increase. Dust lofted into the

atmosphere reduces the solar insolation at the surface, lowering the daytime surface temperature (Smith et al., 2002; Smith, 2004). Furthermore, TES-derived globally averaged ‘daytime’ (1400 local time) and ‘nighttime’ (0200 local time) surface temperatures during 2001 show strong deviations from temperatures obtained at comparable LS during different Mars years, starting at LS = 185◦ . The daytime surface temperature drops over 20 K, reaching a minimum near 230 K at LS ∼ 210◦ . Conversely, the nighttime surface temperature rises roughly 20 K, reaching a maximum at the same time as the daytime minimum. To first order, the dust storm’s major effect on surface temperature is to reduce the diurnal variation by roughly a factor of two at storm maximum. Surface temperatures gradually return to levels near nominal, but only after LS ∼ 260◦ . SWAS measurements show a maximum surface brightness drop occurring roughly mid-way between the northern and souther hemisphere peak dust periods, followed by a return to near normal disk average surface brightness temperatures by LS = 220◦ , a significantly more rapid decay than seen in looking at the TES daytime and nighttime surface temperatures. This does not indicate that the SWAS and TES results are in disagreement, however. Using a simple model of the surface temperature variation as a function of local time and LS based upon both the martian thermal model and the TES daytime and nighttime average surface

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Fig. 3. (A) Disk-average Planck brightness temperature of the martian surface as a function of aerocentric longitude during the SWAS observing period. SWAS chop-nod continuum mode observations are denoted by the circles with error bars. The solid line represents the model continuum brightness temperature assuming a typical (e.g., dust storm free) atmosphere. Annotated dotted lines refer to observations of atmospheric dust as measured by the Mars Global Surveyor Thermal Emission Spectrometer instrument (Smith et al., 2002): (I) Storm begins rapid growth from smaller regional storms. (II) Storm becomes ‘planet encircling,’ or global. (III) Peak TES-measured average dust optical depth in the atmosphere in the southern hemisphere. (IV) Peak average dust optical depth in the atmosphere in the northern hemisphere. (B) The global-average atmospheric temperature of Mars at three isobaric levels retrieved during analysis of the SWAS spectra as a function of aerocentric longitude. Greyscale represents the formal statistical error of the solution; there is an overall potential bias error on the temperature scale of about 10 K (from estimated error of the thermal model). Averaging in time of the spectra from which the temperature profiles were retrieved causes the apparent increase in atmospheric temperature at 0.5 mbar to precede the start of the storm itself; the rise is found to be consistent with the storm initiating at LS = 185◦ .

temperatures, we find that the ‘rapid temperature recovery’ seen in the SWAS temperatures can be qualitatively reproduced, and is due primarily to the changing view of Mars as seen from Earth orbit. As the storm progressed, SWAS saw progressively earlier local times (Table 2), and the increased nighttime surface temperatures more closely compensated for the decreased daytime temperatures in the disk average. We cannot exactly match the SWAS continuum brightness drop of 21 K with the simple model. This may suggest that the temperature drop in the immediate subsurface is even larger than at the surface, or that the simple model used here is not sufficiently detailed for exploring the subsurface thermal structure as a function of local time during a rapidly evolving dust storm. There is good agreement in timing between the detailed global average TES surface temperature changes and the SWAS continuum measurement changes during the storm. That the SWAS thermal variations can be reasonably explained using the precise TES measurements is important confirmation of the accuracy of the SWAS continuum observations. The SWAS spectroscopic analyses depend strongly on the accuracy of the LTC spectra, which would be com-

promised if the continuum values were not well calibrated and understood.

5. Spectroscopic observations: global atmospheric water and temperature The measured lineshape from a planetary atmosphere is a complex function of the vertical profiles of temperature and absorber abundances. This information is “encoded” in the lineshape through pressure broadening, and can be retrieved within certain bounds through suitable numerical inversion or radiative transfer modeling. As mentioned in the introduction, there is a long history of analyzing millimeter and submillimeter spectra to retrieve the global average temperature structure (e.g., Clancy et al., 1990). The process utilized here is a relatively straightforward application of those techniques. For the inversion of the spectral data into temperature profiles, the distribution of the absorbers H2 O and 13 CO was needed. The variation of carbon monoxide with season on Mars is relatively well understood; the long chemical

SWAS observations of the 2001 global dust storm on Mars

lifetime leads to a nearly uniform total abundance as a function of time and altitude (Clancy and Nair, 1996). The ratio 12 C/13 C = 89 in carbon monoxide is also well known from millimeter studies (e.g., Clancy et al., 1990). The 13 CO (5-4) transition has a zenith optical depth of about 2.7 in the line center, and the spectral resolution and SNR of the SWAS observations allows for a determination of temperature up to ∼ 35 km in the atmosphere of Mars. The abundance and distribution of water vapor, on the other hand, is highly variable. The atmospheric vapor content and profile is largely controlled by the atmospheric temperature: during colder seasons, such as northern spring and summer, the water vapor is constrained to a relatively short well-mixed column, with an altitude of 100% saturation occurring ∼ 10 km above the surface. During the warmer seasons near perihelion, this saturation altitude can rise to 30 km or more (Clancy et al., 1996; Clancy and Nair, 1996). Above the saturation altitude, it has been shown that water vapor follows a distribution governed by the saturation vapor pressure law, essentially governed by the temperature profile in the upper atmosphere (Gurwell et al., 2000). The ground state transition of water has a high intrinsic strength, and even under aphelion (cold) conditions the line center zenith opacity exceeds 1000. The high optical depth of the transition causes the observed absorption lineshape to be several GHz wide, much broader than the SWAS spectrometer. SWAS observations of water vapor are therefore sensitive to altitudes above ∼ 15 km. Depending on the upper atmospheric temperature, the line may probe altitudes up to 80 km or more. The water distribution can be further constrained by the observations of the H2 18 O transition. The intrinsic line strength for this transition is nearly identical to that of the main isotope (H2 16 O), but due to the small relative abundance of the heavier species, the line center opacity is between 1 and 5 (depending on the total column of water vapor). We have assumed that the ratio of H2 16 O/H2 18 O = 685, consistent with a reanalysis of Kuiper Airborne Observatory observations of Mars (Bjoraker et al., 1989; Krasnopolsky et al., 1996). Using an iterative radiative transfer scheme, we have self-consistently retrieved the water column and temperature profile from the SWAS spectra. We have assumed that the water profile is uniformly mixed below a saturation altitude (a free parameter) and follows 100% saturation above that altitude. Therefore, in this inversion process both the atmospheric temperature and the water distribution in the upper atmosphere are closely linked. Line strength parameters were obtained from the JPL Molecular Spectroscopy database (Pickett et al., 1998). Line broadening parameters were obtained from Bauer et al. (1989) for the water lines, and Varanasi (1975) for the carbon monoxide line. Under the water vapor distribution assumptions given above, the temperature retrieval extended from the surface to about 90 km, with ∼ 1 scale height vertical resolution (about 10–12 km).

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Best fit solution profiles are provided for each of the observed spectra presented in Fig. 2. 5.1. Water column Throughout the observing period, we find that water vapor in the martian atmosphere is well described as a constant 12 ± 6 precipitable microns (pr. µm). In comparison, MGS TES measurements during this period show the global water column abundance to drop from 13 to 11 pr. µm. Despite the large error bars on the SWAS results, the excellent agreement of the SWAS measurements with the highprecision TES measurements are an indication that the inversion process works adequately. The current results also show improved consistency compared to those found during the 1999 opposition results, when SWAS and TES global water vapor abundances differed by a factor of about 2 (Smith, 2002). The improvement is primarily derived from the addition of the H2 18 O observations, which are significantly more sensitive to the total column of water vapor than the H2 O observations (where the sensitivity is limited to above 15 km due to the broad line not fitting in the SWAS bandpass). 5.2. Atmospheric temperature Fig. 3b presents the retrieved globally averaged temperature structure of the atmosphere at three isobaric levels as a function of LS . The three isobars of 0.5 mbar, 50 and 5 µbar, correspond to altitudes near 25, 46, and 66 km, respectively, in the pre-storm atmosphere (and near 28, 53, and 75 km during the peak of the storm). Before opposition the atmosphere was relatively cool with temperatures near 180 K at 0.5 mbar (∼ 25 km). Beginning at LS ∼ 180◦ , temperatures started to rise, sharply at 0.5 mbar and less so at 50 µbar. As the spectra were averaged over ∼ 10◦ of LS , the SWAS measurements are consistent with a temperature increase actually occurring near LS = 185◦ , consistent with the TES determination of the start of the rapid growth of the storm (Smith et al., 2002). Temperatures at 0.5 mbar increased approximately 40 K by LS = 200◦ and maintained these warm levels (with some fluctuation) throughout the SWAS observing period. Temperatures at 50 µbar developed increases of 20 K by LS = 200◦ , and up to 30 K by LS ∼ 220◦ . In contrast, temperatures at 5 µbar km) fluctuated (demonstrating the true noise level of our inversion results) but did not appear to change at a significant level during the storm. The SWAS measurements show that the storm had a dramatic impact on global temperatures at altitudes corresponding to pressures
50 µbar, but essentially none at altitudes corresponding to pressures  10 µbar. The TES instrument mapped the temperature structure of the atmosphere up to about 35 km using nadir observations throughout the storm period. Global atmospheric changes have been presented for the 0.5-mbar pressure level (Smith et al., 2002; Smith, 2004), providing an opportunity to verify

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the accuracy of the SWAS data inversion procedure. Unlike for the martian surface, there is only a small diurnal variation in temperature of the atmosphere, both before and during the storm. Therefore, comparison of SWAS temperatures to the latitudinally averaged TES 0.5-mbar temperatures should show good direct agreement, mostly independent of the SWAS viewing geometry. Taking into account the somewhat large errors on the retrievals, we find that the SWAS results are consistent with the TES results. TES retrievals showed the 0.5-mbar temperature to be around 180 K prior to the storm initiation, with an abrupt increase of about 40–45 K starting at LS = 185◦ (Smith et al., 2002; Smith, 2004). The good agreements between the SWAS and TES 0.5-mbar temperatures indicate that the temperature retrieval process we utilized works well, and that temperatures obtained at higher altitudes (such as 50 and 5 µbars) are accurate.

Using SWAS measurements, we find that after the storm began, surface brightness temperatures dropped up to 20 K on the visible disk, suggesting the physical surface temperature dropped by roughly the same amount. These results are consistent with MGS TES-derived surface temperatures. SWAS measurements appear to show a quicker return to nominal surface temperatures, but qualitatively this behavior is driven by the geometry of the changing sub-Earth local time. SWAS spectroscopic results have been analyzed in a self-consistent manner, using submillimeter observations of H2 O, 13 CO, and H2 18 O. We find that the low to middle atmosphere experienced a strong heating of over 40 K at 0.5 mbar and up to 30 K at 50 µbar during the height of the global dust storm. There appeared to be no global thermal effects of the storm above the 10 µbar level.

5.3. Water saturation altitude

Acknowledgments

With the rapid increase in atmospheric temperature in the lower atmosphere during the initial phases of the dust storm, there was a corresponding increase in the height of 100% water vapor saturation in the martian atmosphere. Before the storm initiated, the best fit temperature and water vapor abundance suggested water vapor saturation at about 0.5 mbar (∼ 25 km). These results agree well with previous determinations of the mean saturation altitude for this season (Clancy et al., 1996; Smith, 2002). By LS 190◦ , however, the best fit saturation level had risen to approximately 8 µbar (∼ 70 km), though this is poorly constrained. In any event, while the column of water changed very little as far as we could determine through the SWAS observing period (and as verified by the high-precision TES observations), it is clear that the distribution of water changed markedly. The upward mixing of water vapor brought increases of 2 or more orders of magnitude in the water abundance in the upper atmosphere. Martian atmospheric chemical cycling is known to be highly sensitive to the water abundance variation due to changing water saturation altitude, at least on seasonal timescales (e.g., ozone, Clancy and Nair, 1996). We expect that the analogous and rapid rise of the saturation altitude during the 2001 global dust storm likely had strong effects on chemistry due to increased water vapor in the upper atmosphere.

The authors thank D.A. Neufeld and M. Harwit for fruitful discussions on the SWAS data reduction, and R.T. Clancy for wide-ranging and long-term conversations on Mars temperature retrieval. We also thank the two anonymous reviewers of the original submission. E.A.B, G.J.M., and V.T. were supported by NASA contract NAS5-30702.

6. Summary remarks The 2001 martian dust storm was the first to be observed in great detail since the Viking era. While extensive observations were obtained from Mars orbit using the Mars Global Surveyor instruments, the storm’s initiation just after opposition made measurements from Earth more complete than would otherwise have been possible.

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