The Relative Humidity of Mars' Atmosphere

VOL. 84, NO. B14 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 30, 1979 The Relative Humidity of Mars' Atmosphere DONALD W. DAVIES Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91103 As a general rule, Mars' atmosphere contains as much water vapor as it can hold on a daily basis; it reache saturation at night, and the vapor appears to be distributed throughout the lowest several kilometers. Interesting exceptions to this occur when there are temperature inversions in the arctic springtime and during dust-storm activity. As contrasted to the northern hemisphere there appears to be no local source of water in the southern temperate and arctic areas. Although the amount of water vapor in Mars' atmosphere is small by terrestrial standards, the relative humidity can be quite high. The most obvious manifestations of high-relative humidity are the H20 ice clouds near topographic features seen by Mariner 9 [Leovy et al., 1973] and the Viking orbiters [Briggs et al., 1977], the occasional morning fogs seen by Viking [Briggs et al., 1977], and the north polar hood long seen by earth-based observers and shown to be composed of water ice clouds by Mariner 9 [Briggs and Leovy, 1974]. Measurements made by the Mars atmospheric water detector (Mawd) on Viking Orbiter 2 showed large amounts of water vapor over the north polar remnant cap during the northern summer [Farmer et al., 1976]. Although the atmosphere is free of clouds at that t'u'ne, comparison of the water vapor amounts to an estimate of the atmospheric temperature profile based on surface and 20-km temperatures from the infrared thermal mapper (IRTM) on the same spacecraft [Kieffer et al., 1976] implied that the atmosphere was near saturation. A model of the seasonal behavior of the water va- por over the north pole was developed [Davies et al., 1977] which successfully reproduced the rapid decrease in water abundance in the northern fall. The model depended on the atmosphere being near saturation during this time. For the geometric pole of the planet the atmospheric temperature profile (and therefore the saturation vapor pressure and total vertical column of water vapor) is much easier to estimate than for other areas of the planet because the insolation is not varying rapidly. At other latitudes the daily surface temperature extremes (+40øC is not uncommon) coupled with our ignorance of the detailed dynamical behavior of the Martian boundary layer, make accurate calculations of the atmospheric temperature profiles impossible. Nevertheless there is a strong suggestion in the seasonal behavior of the water vapor at other points on the planet that the atmospheric vapor may be as much as the atmosphere can hold on a daily time scale [Davies, 1979a]. During the course of the two Viking orbiter missions there have been many occultations of the spacecraft (the spacecraft passing behind Mars as seen from the earth). By analyzing the radio signals from the spacecraft as the signals passed through Mars' atmosphere just before occultation, it is possible to obtain temperature profiles of Mars' atmosphere. In practice, extracting a temperature profile places very stringent requirements on the data quality, with the result that only a small fraction of the occultations can be used for temperature determination. Lindal et al. [this issue] have gathered together the most re- Copyright 1979 by the American Geophysical Union. liable temperature profiles obtained by the two Viking orbiters; here, we will compare these temperature profiles and the two Viking lander entry profiles to water vapor measurements made by the Viking orbiter Mawd instruments. As will be shown this comparison yields information on the relative humidity, daily atmospheric temperature variations, control of the observed amounts of vapor with latitude and season, and sources of water. DESCRIPTION OF THE DATA Since the earth is in the same general direction as the sun when viewed from Mars, radio occultations occur near the morning or evening terminators. For temperature profile de- terminations, only entry occultations are useable; for nonpolar areas, entry occultations occurred only near the evening terminator. For a more detailed description of the occultation results see Lindal et al. [this issue]. The estimated error in the temperature derived from the occultation profiles used here is 3 K (lo). The two Viking lander entry profiles were obtained from Seiffand Kirk [ 1977]. To obtain a corresponding measurement of the water vapor one would ideally like to have measurements made at the same latitude, longitude, season, and local time. Unless otherwise noted, 'corresponding' has been defined to mean within 5 ø of latitude and longitude, 2«ø of planetocentric longitude (Ls), and any local time. Global maps and detailed seasonal histories of selected areas indicate that geographical and seasonal intervals of this size should not introduce significant er- rors. Since the Mawd instruments operate by measuring the spectral properties of sunlight reflected off of the surface of Mars [Farmer et al., 1977], and the occultations occur near the terminator (indeed some of them are in the dark), it is not pos- sible to get measurements of vapor that correspond in local time to the occultation measurements. Fortunately, it appears that there is little diurnal variation in water vapor content [Davies, 1979a]; therefore measurements made during the day should be applicable to the local time corresponding to the oc- cultation. Table 1 contains the locations and times of the occultation measurements, as well as of the two entry measurements. The identification of the occultation measurements is consistent with the paper by Lindal et al. [this issue]. The first three digits are the spacecraft orbit number, followed by N (for entry), and then followed by the spacecraft number (1 or 2). The column labeled Ls is the planetocentric longitude (season): 0 ø is the northern spring equinox; 90 ø, summer solstice; 180 ø, fall equinox; and 270 ø, winter solstice. The local time is in units of one twenty-fourth of a Mars day (0 = midnight). The mea- Paper number 9C1036. 0148-0027/79/009C- 1036501.00 8335

8336 DAVIES: SECOND MARS COLLOQUIUM Occulta- TABLE 1. Summary or Relative Humidity Data tion Profile Measure- Local Number ments Ls Latitude Longitude Time Vapor, prpm Saturation Measured Group 1 VL1 101 22 48 16.2 450 11 2 2 VL2 8 48 226 9.8 250 26 2 3 181N2 209-65 147 4.3 3.5 3-6 lb 4 182N2 209-65 175 4.0 1.6 3-6 lb 5 292N1 236-63 278 3.3 32,--15 lb 6 224N2 237-42 70 2!.3 215 9 3 7 225N2 237-40 68 21.2 400 9 3 8 228N2 239-37 63 21.1 1000 8 3 9 233N2 242-31 52 20.9 1500 8 3 10 331N1 261-26 73 20.9 1900 8 3 11 364N1 281 13 247 20.0 50* 9 4 367N1 283 17 197 19.9 160' 9 4 13 391N1 297 38 237 19.8 86*,--7 4 14 392N1 298 38 227 19.8 17' 4 15 471N2 14 66 103 5.2 1.3 /,-)+0.2 la 16 486N2 21 73 358 2.6 0.1... o.1 la f,9+0.2 17 487N2 21 73 349 2.6 0.06... 0.1 la 18 488N2 22 73 339 2.6 0.06 v. --0.1 n,9+0.2 la 19 490N2 23 73 319 2.6 0.3 u. --0.1 n,9+0.2 la 20 491N2 23 73 308 2.7 0.6 n,9+0.2 la 21 493N2 24 73 284 2.9 0.4 v. --O. 1 n,9+o.2 la 22 494N2 24 72 272 3.1 0.3... o. la f,3+0.3 23 495N2 25 72 259 3.2 0.5... 0. la f '1+0.3 24 496N2 25 72 248 3.4 0.4... o.1 la 25 504N2 29 62 147 5.0 1.4 f A+0.3... o.2 la 26 668N1 74 27 59 16.0 115 10 2 27 669N1 75 27 49 16.0 220 11 2 28 670N1 75 27 39 16.0 190 15 2 VL 1 is the Viking Lander 1, and VL2 is the Viking Lander 2. *These temperature profiles could hold substantial amounts of water above 20 km. Value given is for 0-20-km altitude. surements have been combined into four groups whose members have similar seasons and latitudes. Figure 1 displays the latitude and Ls of the measurements included in Table 1. Since many of the observations occur in the arctic winter, the edge of the annual CO2 polar cap has also been indicated from data in James et al. [1979] and from P. B. James (private communication, 1979). In addition the two global dust storms have been indicated. Figures 2, 4, 5, and 6, corresponding to groups 1-4, display the temperature profiles identified in Table 1. They are essen- tially smoothed versions of the occultation and near-surface entry profiles displayed with a common altitude scale, relative to the surface instead of relative to the center of Mars. For each temperature profile the vapor pressure of water was integrated from the surface to 20 km, and that number is listed in Table 1 in the column labeled 'saturation.' The measured water vapor is obtained from measurements made by Mawd on the Viking orbiters, as mentioned before. The Mawd measures the total vertical column abundance of water vapor and provides no direct information on the ex- I I I I I I I I I I I I I I I I I - CO 2 cap edge DUST STORM DUST STORM NO. 1 NO. 2 tat a3 +10 -- -10-30 ß ß - -70-90 I I I I I 0 20 40 60 80 100 CO 2 cap e dge" '"'",,,,,, 0 140 160 180 200 220 240 260 280 300 320 340 360 SEASON (Lst degrees) Fig. 1. I atitude and seasonal coverage of occultation and lander entry temperature profiles. The approximate edges of the annual CO2 caps and the times of the two dust storms are shown.

DAVIES: SECOND MARS COLLOQUIUM 8337 tent or altitude of the vapor. If one assumes a certain vapor vertical extent, the minimum allowable atmospheric temperature can be calculated. For example, if the total vertical column abundance was 10 pr/zm (precipitable micrometers of water) and the assumed extent was 5 km, the vapor density would be 2 pr/zm/km which corresponds, at saturation, to an atmospheric temperature of 201 K. The figures include dashed vertical lines corresponding to the minimum allowable atmospheric temperature (before condensation occurs) corresponding to different assumptions regarding the vertical extent of the vapor. DISCUSSION The four groups of data will be discussed separately and are as follows. Group 1. Arctic Spring There are actually two series of observations included here: observations in the north arctic from Ls = 14 ø to Ls = 29 ø, and observations in the south at Ls = 209 ø and 236 ø. These profiles (Figure 2) are very cold and, with a few exceptions, are over CO2-frost-covered ground, have substantial temperature inversions, and appear to be saturated with water vapor. In the north, profiles 16-24 (Figure 2, b-j) are several degrees north of the edge of the annual CO2 cap as recorded by Viking imaging observations (P. B. James, private communication, 1979), and all show a temperature inversion near the surface, consistent with a CO2-frost cover, and a broad maximum temperature of 170-180 K at altitudes from --,3 to 7 km. Profiles 15 and 25 (Figure 2, a and k) are very close to the edge of the CO2-frost cover measured by the Viking orbiter olo o o ] 20 k I I ' 16 (a) 471N2 WATER, pr/zrn/km,oo - 486N2 - - (b) - 01101 J 10 [ 17 1100 487N2 (,:) - I I I -'l / 488N2 -- (d) 490N2 (,,) - i I i 491N2 - (f) - 2O I I I I J 493N2 - - (g) { i { i I - 494N2 - - (h) - 8 4 I I } 1-496N2 - ß 504N2 (k) I I I 1811 N2 (i) I { I { 292N 1-0',) -, i I I It I I I I TEMPERATURE, K Fig. 2. Arctic spring temperature profiles (solid curves) with minimum temperatures deduced from water vapor amounts (dashed vertical lines). The orbit and spacecraft are indicated in upper right. (a-k) North arctic. (l-n) South arc- tic.

8338 DAVIES: SECOND MARS COLLOQUIUM E 10 7 5 4 i I I I DUST STORMS J /! ',: /i 1 I i 50 200 250 300 350 SEASONt LSt degrees Fig. 3. Seasonal history of water vapor at -65 ø latitude for comparison to occultation measurements 181N2 and 182N2. The dashed curve is the measured water amount; the solid curve has been corrected for dust obscuration. cameras. From the shape of the profiles it appears that 25 may be over CO2 frost but that 15 is not. High northern latitudes could only be observed with good viewing conditions from Viking Orbiter 2 periapsis; consequently, the observations we obtained tended to be high-resolution small-area observations, with very little spatial overlap with the occultation measurements. To get water measurements corresponding to profiles 16-24, all measurements (independent of longitude) at 72ø-73øN were used to obtain the measured values. The errors quoted take into account the scatter in the measurements and should include the error in- troduced by not using the appropriate longitude. Comparison of the north arctic profiles in Figure 2 (a-k) with the corresponding vapor amounts shows that the atmosphere is basically saturated, and therefore the observed water vapor is determined by the temperature of Mars' atmosphere at the latitude and season of the observations. Temperature profiles 3-5 (181N2, 182N2, and 292N 1, Fig- ure 2, l-n) were obtained in the south arctic spring, and although they are about 10 K warmer, also appear to be saturated. Profiles 3 and 4 were taken right after the onset of the first large dust storm, at a time when the measured water amounts in the south arctic were rising rapidly (probably also due to the effects of the dust storm). Dust in the atmosphere tends to cause the Mawd measurement of water vapor to err on the low side. This error can be estimated by modeling the scattering process and calculating, for the geometry of a particular observation, the reduction in observed water vapor. The measured amount can then be corrected. An example of this process is given in Davies [1979a]. When the opacities and air mass are not excessive this correction process appears to work well, and the water vapor measurements in Table 1 include corrections for the effects of opacity. (The opacity was estimated from measurements made by the two Viking landers, [Pollack et al., 1979].) Corrections to profiles 181N2 and 182N2 (Figures 2, l and m) are rather uncertain, however, since for these observations the air mass was quite large (the measurements were taken near the terminator) and the atmospheric opacity was large. To illustrate this, Figure 3 shows the complete seasonal history of water vapor amount near -65 o with and without corrections for atmospheric opacity. At the time of profiles 3 and 4 (181N2 and 182N2) the correction for opacity was very large (a factor of 2). Notwithstanding the uncertainty of the water measurements for observations 181N2 and 182N2 the south arctic area also appears to be saturated with water vapor in the springtime, at least to Ls ' 240 ø. In both the north and south arctic the water is not concentrated near the surface but is distrib- uted over several kilometers starting above the inversion layer at --,2 kin. Group 2. Late Spring/Early Summer at North Temperate Latitudes There are three sets of observations included in this group: the Viking 1 lander profile, the Viking 2 lander profile, and three occultation profiles (26-28) (Figure 4). Viking 1 landing. This is a late afternoon profile and probably represents the maximum daily temperature. If the middle WAT ER pr.m/ m 0ll01010100 01011010 lick) 01011010100 VL1 VL2 - -- \ 668N1 16 o) (b) - (½) - i I 4 - - II - -:,, : I i, i\, : - : : :, :Ji,,, : --, :, 150 170 190 2 I0 230 250 52o,,l :, 6 i)n I - I 670N 16 - (e) I - 8 I I I I I I I I I I I Ill I' 170 190 210 250 170 190 210 230 50 230 150 250 TEMPERATUREt K Fig. 4. North temperate latitude late spring and early summer temperature profiles. Same conventions as Figure 2. (a), temperature profile from the landing of Viking 1 lander; (b), Viking 2 landing; (c-e), occultation profiles taken a little less than 1 Mars year later near Viking lander 1.

DAVIES: SECOND MARS COLLOQUIUM 8339 20 110 i 1 0 10 I 100 224N2 6 (o) WATER, pr/ m/ m 110 I 10 10 100-225N2 - (b) Ol o ß 1 1 o lo loo - : 8N2 - i I I I 1 -- i i i - I J 1,50 170 190 210 230 250 16 I 331N1 I ( ) - i i 50 170 190 210 230 250 15o 17o 19o 21o 230 250 TEMPERATURE K Fig. 5. South temperate latitude, spring, dusty atmosphere temperature profiles. Same conventions as Figure 2. atmosphere (region above the diurnal boundary layer) experiences a 10 K peak-to-peak daily temperature variation, then the water vapor seen could be mixed to about 10 km and will be near saturation at night. This is consistent with the vertical distribution of vapor over the Viking 1 lander site deduced from measurements made about a month later [Davies, 1979a]. Viking 2 landing. This profile was taken earlier in the day and, consequently, is closer to the minimum daily temperature than the Viking 1 lander profile. In addition, being farther north, the daily temperature variation is expected to be smaller. Consequently, it is not surprising to see the measured vapor amount closer to the saturation amount. Again, vapor could be mixed to 6-10 km and would be near saturation at night. Occulation profiles 26-28 (668N1, 669N1, and 670N1). These profiles are taken close to the Viking 1 lander site, about the same local time as the Viking lander landing measurements (maximum daily temperature) but earlier in the season (Ls = 75 ø as opposed to Ls = 101 o for the landing). These profiles are unique in that they show a very large lapse rate, approaching the adiabatic lapse rate of 4-5 K/km. With a measured near-surface atmospheric temperature of 230-235 K and a large lapse rate, the measured amount of water must be confined to the lowest 2 or 3 km, in contrast to the measurements mentioned for the Viking 1 landing, taken later in the season. Again, the atmosphere must be close to saturation at night. Confining the water vapor to the lowest 2 or 3 km has implications for the topographic control of the measured vapor. During seasons when the lapse rate is large, one could expect to see abrupt changes in elevation reflected in changes in the vertical vapor amount. There are suggestions of this effect in the water vapor distributions, but they are not very clear-cut. Group 3. South Temperate Latitudes, Spring, Dusty Atmosphere These measurements were all taken in the early evening (2100 local time). The average relative humidity at that time is very low, ranging from 5% to less than 0.5%. Profile 6 (224N2, Figure 5a), the earliest and most southern of the group, has the highest relative humidity, and if the diurnal amplitude were relatively large (20 K), then it is possible the atmosphere could reach saturation at night. The other profiles (7-10, Figure 5, b-e) get progressively warmer at midaltitude with no more vapor. It is likely that these locations do not get to saturation at night (a drop of up to 40 K being required to saturate the atmosphere). Therefore apparently there is no source of water at these locations that can supply vapor to the atmosphere. This is in striking contrast to other areas of the planet (e.g., the north arctic area), where water vapor concentrations can rise very rapidly in response to a warming atmosphere [Davies, 1979b]. In addition it also implies that the dust in the atmosphere there contains no water ice. Group 4. North Equatorial/Temperate Dust Storm Profiles This group consists of two pairs of profiles, 11 and (364N1 and 367N1 Figure 6, a-b) and 13 and 14 (391N1 and 392N 1 Figure 6, c-d). Both pairs were obtained about 2000 lo- cal time; consequently, they are close to maximum temperature. The first pair, taken near the beginning of the second 110 110 01100 I 364N'1 16 I (a) ',, I I I I II 391N1 J / ( ) ; / II ml 8 ii 1.50 170 190 210 230 2.50 WATER, pr/zm/km TEMPERATURE, K 110ll 1 I 0 10 100 - I 367N 1 - I ', ',,,,, -- 392N 1 ] ( (b) Fig. 6. North equatorial and temperate dust storm temperature profile. Same conventions as Figure 2.

8340 DAVIES: SECOND MARS COLLOQUIUM dust storm, shows basically an isothermal atmosphere. With the measured water amounts, which may be somewhat underestimated because of the obscuring effects of the dust, the atmosphere must be near saturation at night. The second pair of profiles shows a striking monotonic increase of temperature with altitude and a very cold surface. This inverted temperature profile must be due to an optically thick layer of dust high in the atmosphere, causing absorption of solar radiation at 10-20-km altitude and a drop in heating of the surface. In order that the atmosphere contains the order of 10 pr/zm of vapor, the vapor must be concentrated in the region above 10-km altitude. Again, these temperature profiles are expected to be maximum daily temperatures, and the question of diurnal variation in temperature arises. Fortunately, the important altitude in this case is quite high, and measurements obtained by the Viking orbiters' IRTM 15- m channel can be used to estimate diurnal temperature fluctuations. Measurements made at the same Ls, but at slightly different longitudes, indicate a 20-km temperature of 205-210 K at the local time corresponding to the occultation profiles and a similar temperature early in the morning (F. Palluconi, private communication, 1979). It appears from these measurements that the diurnal amplitude in this case is probably less than 5 K peak to peak. Therefore it is possible for the water to remain as vapor all day, although with the water mixed up with the large amounts of dust it is also entirely possible that what we are seeing is a saturated mixture of vapor and icy dust grains. At this season there is evidence for high-altitude poleward atmospheric transport [Martin and Kieffer, 1979]. The flow apparently subsides in the arctic area where it experiences a cooling episode and then must return with a southward flow at lower altitude. Since the cooler air can hold only a fraction of the original water vapor, the water will condense out in the arctic, probably taking the dust grains with it. Therefore we may have the interesting situation where the air at low altitude (returning from the pole) is relatively free of dust, while the region above about 10 km is quite dusty. CONCLUSIONS AND GENERALIZATIONS 1. At most seasons and latitudes the atmosphere is holding all it can on a daily basis (i.e., 100% relative humidity at night). The exception is the south temperate and arctic summer (see point 4 ). 2. At most seasons and latitudes the vapor is distributed throughout about the lowest scale height. The exceptions are (a) regions where the lapse rate is large (clear atmosphere equatorial sites?) (in this case the water must be confined to the lowest 1 or 2 km); (b) very dusty conditions where a large (40 K) temperature inversion exists in the atmosphere from 10-20 km (in this case the vapor (plus perhaps ice) exists in the region above 10-km altitude); and (c) the water in the spring polar regions over CO2 frost is above the temperature inversion at -- 2 km. 3. In times of dust storm activity the water vapor and the dust exist at the same altitude. This is a consequence of the fact that the atmospheric temperature at the altitude of the dust is generally greater than in other regions of the atmosphere. In addition a reduction in temperature causes a reduction in atmospheric vapor which may cause a reduction in dust by condensing on and precipitating the dust grains. 4. The source of vapor for the north and south arctic regions appears to be different. As the north arctic atmospheric temperature rises, the atmospheric water vapor is able to keep up, even after the amounts exceed quantities anywhere else on the planet. In the south the water vapor is able to keep up with atmospheric holding capacity only until the vapor amounts match the amounts of vapor already present in the equatorial region. Then, even though the temperature profiles imply the atmosphere could hold much more, the amount in the atmosphere stops increasing. It appears that there is an essentially unlimited local supply of water in the north arctic, but that atmospheric vapor in the south arctic area comes from the equatorial region of the planet by atmospheric transport. Acknowledgments. The author would like to thank G. Lindal and D. Sweetnam for providing the occultation data prior to publication and for helpful discussions regarding its accuracy, to C. B. Farmer and D. D. LaPorte who conceived and designed the Mawd instrument, and to R. Zurek for discussions atmospheric dynamics. This paper represents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract number NAS 7-100, sponsored by NASA. REFERENCES Briggs, G. A., and C. B. Leovy, Mariner 9 observations of the Mars north polar hood, Bull. Amer. Meteorol. Soc., 55, 278-296, 1974. Briggs, G., K. Klaasen, T. Thorpe, and J. Wellman, Martian dynamical phenomena during June-November 1976: Viking orbiter imaging results, J. Geophys. Res., 82, 41-4149, 1977. Davies, D. W., The vertical distribution of Mars water vapor, J. Geophys. Res., 84, 2875-2879, 1979a. Davies, D. W., Water in Mars' polar areas: Its sources, sinks, and transport, in Proceedings of the Second Colloquium Planetary Water and Polar Processes, in press, 1979b. Davies, D. W., C. B. Farmer, and D. D. LaPorte, Behavior of volatiles in Mars' polar areas: A model incorporating new experimental data, J. Geophys. Res., 82, 3815-3822, 1977. Farmer, C. B., D. W. Davies, and D. D. LaPorte, Mars: North summer ice cap--water vapor observations from Viking 2, Science, 194, 1339-1341, 1976. Farmer, C. B., D. W. Davies, A. L. Holland, D. D. LaPorte, and P. E. Doms, Mars: Water vapor observations for the Viking orbiters, J. Geophys. Res., 82, 4225-4248, 1977. James, P. B., G. A. Briggs, A. Spruck, and J. Barnes, Seasonal recession of Mars south polar cap as seen by Viking, J. Geophys. Res., 84, 2889-2922, 1979. Kieffer, H. H., S.C. Chase, Jr., T. Z. Martin, E. D. Miner, and F. D. Palluconi, Martian north pole summer temperatures: Dirty water ice, Science, 194, 1341-1344, 1976. Leovy, C. B., G. A. Briggs, and B. A. Smith, Mars atmosphere during the Mariner 9 extended mission: Television results, J. Geophys. Res., 78, 4252-4266, 1973. Lindal, G. F., H. B. Hotz, D. N. Sweetman, Z. Shippony, J.P. Brenkle, G. V. Hartsell, R. T. Spear, and W. H. Michael Jr., Viking radio occultation measurements of the atmosphere and topography of Mars; Data acquired during one Martian year of tracking, J. Geophys. Res., this issue. Martin, T. Z., and H. H. Kieffer, Thermal infrared properties of the Martian atmosphere, 2, The 15-/tm band measurements, J. Geophys. Res., 84, 2843-2852, 1979. Pollack, J. B., D. S. Colburn, F. M. Flasar, C. E. Carlston, D. Pidek, and R. Kahn, Properties and effects of dust particle suspended in the Martian atmosphere, J. Geophys. Res., 84, 2929-2945, 1979. Seiff, A., and D. B. Kirk, Structure of the atmosphere of Mars in summer at mid-latitudes, J. Geophys. Res., 82, 4364-4378, 1977. (Received April 4, 1979; revised June 18, 1979; accepted June 25, 1979.)