VOL. 82, NO. 28 JOURNALOF GEOPHYSICALRESEARCH SEPTEMBER30, 1977 Structureof the Atmosphereof Mars in Summerat Mid-Latitudes ALVIN SEIFF AND DONN B. KIRK AmesResearch Center, NASA, Moffett Field, California94035 The structureof Mars' atmospherewas measured in situby instrumentson boardthe two Viking landersfrom an altitudeof 120km to nearthe surface.The two entrieswere separated by 178ø in longitude,25 ø in latitude,45 dayselapsed time, and 6 hoursin Marslocal time. Atmosphere structure was verywell defined by the measurementsand was generally similar at thetwo sites.Viking 1 and2 surface pressuresand temperatures were 7.62 and 7.81 mbar and 238øK and 226øK, respectively, while pressures at theelevation of the referenceellipsoid were 6.74 and 6.30 mbar. Mean temperature decreased with a lapserate of about1.6øK/km, significantly subadiabatic, from above the boundary layer to about40 km, thenwas near isothermal but with a large-amplitudewave superimposed, attributed to thediurnal thermal tide. The meanprofile appears to be governedby radiativeequilibrium. Differences between the two temperatureprofiles are dueto diurnaleffects in the boundarylayer, a smallcooling of the Viking2 profileup to 40 km dueto latitudeand season, and effects bf timeof day,latitude, terrain, and season on thewave structure. The density data merge well with those of theupper-atmosphere mass spectrometer to definea continuousprofile to 200 km. The temperaturewave continues above 100 km, increasingin amplitudeand wavelength. INTRODUCTION ing dataof comparablequality to that normallyobtained from Knowledgeof the structureof the neutralatmosphere of meteorologicalsounding techniques [Seiffet al., 1973].A de- Mars has advancedrapidly in the age of spaceastronomy. scriptionand discussionof the techniquesand instruments Remotesensing experiments from flyby and orbitingspace- whichhave been applied to the Vikingmission has been given craft have indicatedthe mean surfacepressure to be near 5 [Seiff, 1976]. mbar, muchlower than was previouslyaccepted (see, for ex- This paper reportsthe resultsof the atmospherestructure ample,Ktiore et at. [1972]),and in conjunctionwith ground- measurementsfrom Viking 1 and 2 as of May 1977. The basedspectroscopy have indicated that the atmosphereis pre- analysisof the data is not yet completein all respects,but we dominantlyCO•,. Remotesensing has also indicatedthat the expectthat the materialwe presentherein will be substantially temperaturestructure up to 45-km altitude is highly variable, unchangedby furtheranalysis. Two topicsnot yet readyfor dependenton latitude, season,and the dust content of the reportingare omittedmdataon thewinds encountered during atmosphere[Ktiore et at., 1972;Hanel et at., 1972]. entry and descent of the two landers and data on the terrain The temperature data have been limited to the lowest 35 km underthe entry trajectories.These will be the subjectof later (occultation)and 45 km (IR sounding)and have beensome- reports.Preliminary accounts of the atmosphericdata have whatpuzzling and hardto assessbecause of their diversity.It beengiven [Nier et at., 1976;Seiff and Kirk, 1976]. hasnot beenpossible to sayconclusively what accuracy, tem- peratureresolution, and altituderesolution should be assigned INSTRUMENT DESCRIPTION to these results. The overall definitionof the structureof Mars' atmosphere The USSR spacecraftMars 6 made some measurementsof to be describedherein was a result of synthesisof data from the atmospherestructure during its entryinto Mars in 1974 severalinstruments. In the upper regionsof the experiment, [Kerzhanot)ich,1977]. Very usefulinformation was obtained, altitudes from 120 to 26 kin, a set of three-axis accelerometers althoughit was limited by the lack of direct temperature measured the atmospheric density as a function of altitude sensing,an extensiveradio blackout, and accelerationsensing from the vehicledeceleration. To calibrate this measurement,a confinedto four points. It confirmedthe magnitudeof the thorough and extensiveground test programwas conductedto surfacepressure obtained from remote sensing, measuring 5.45 definethe drag coefficientas a function of velocity and Rey- + 0.3 mbar, and put boundson the temperaturestructure, nolds number in an atmosphereof CO2. Thesetests were made indicatinga lapse rate of 2.9øK/km up to 33 km and an with models of the Viking entry configurationin free flight isothermalmiddle atmosphere at 149ø + 8øKto 90 km in early through a ballistic range. springin the southernhemisphere (-24 ø latitude).There was During parachutedescent, which began nominally at 6 km, significantuncertainty in temperaturesbelow 30 km, with a pressure and temperature were directly measured. Also maximum of +18 ø at 29 km. throughout the high-speedentry the flow stagnationpressure The Viking missionprovided an opportunity for in situ was measured, and below about 25 km the flow recovery measurementsof the atmosphere,and experimentsto make temperature(a temperatureclosely related to stagnationtem- useof that opportunitywere describedby Nier et at. [1972]. perature) was directly sensed. The atmospherestructure measurement approach, initially The lander sytemsprovided three kinds of data which were proposedin 1962 [Seiff, 1963], was the subjectof intensive important to the determinationof atmospherestructure: (1) studyand development in theensuing 9 years,culminating in a altitudes, from a radar altimeter; (2) attitude changedata, testflight of the experimentin the earth'satmosphere in 1971. from gyroscopes,which were usedin the determinationof the Thisexperiment showed that thetechniques for measuringthe trajectoriesand for analysisof wind effects;and (3) velocity atmosphereduring high-velocity entry were capable of provid- data during parachute descent,from a three-axis Doppler radar. Copyright¸ 1977by the AmericanGeophysical Union. The nominal measurementaltitudes, telefnetry resolution, Paper number 7S0499. 4364 SEIFF AND KIRK: STRUCTUREOF THE ATMOSPHEREOF MARS 4365 TABLE 1. Viking Atmosphere Structure Instruments Measurement Telemetry Sample Altitude Sensors Altitudes, km Resolution Interval, s Resolution, km Accelerometer 120-0 A V = 0.0127 m/s 0.1 0.006-0.1 Pressure Aeroshell* 90-6 0.16 and 0.74 mbar 0.2 0.01-0.5 Parachuter 4.5-1.5 0.085 mbar 0.5 0.03 Temperature Aeroshell 27-6 1.2øK 1.0 0.01-0.1 Parachute 3.8-1.5 1.2øK 0.5 0.03 Radar altimeter 132-0 5 m 0.2 0.01-0.2 Doppler radar (TDLR) 5-0 0.06 m/s 1.0 0.012 Gyros 250-0 0.0008ø 0.1 0.006-0.1 *The aeroshellphase of the experimentis from entry to nominally 6-km altitude. t The parachutephase is from 6 km to descentengine ignition at 1.5 km. sampling intervals, and altitude resolution of all these mea- obliquely upward. Thesesensors were retainedwith the lander surementsare shownin Table 1. These were generallymore through the landing and provided necessaryguidance data as than adequatefor atmosphericdefinition. The accuracyof well as scientificdata on the atmosphere. definitionof temperatureand pressuregradients with altitude The pressureinlet during high-speedentry was at the nomi- in the parachutedescent was constrainedby telemetryresolu- nal flow stagnation point on the heat shield at the entry atti- tion, as discussedbelow, but it was possibleto definethese tude. The aeroshell temperature sensor was deployed at a gradientsto the order of 1% or 2% on pressureand to within velocity of 1.1 km/s through the surface of the conical heat 0.1øK/km on temperature,or better. shieldto a position safelyoutside the aeroshellboundary layer, The locations of the sensors on the landers are shown in where it could sensethe atmosphericrecovery temperature, Figure 1. The accelerometersand gyros were located within without convectiveinfluence by the heat shield. the inertialreference unit, externalto the landerbody, along The pressureinlet during parachute descentwas mounted the z axis,which was in the nominallyvertical plane, pointing on an edge of the lander body, with a Kiel probe geometry facing into the theoretical flow direction at that location (Fig- ure lb). This insured that the stagnationpressure was sensed. (O) AEROSHELL PHASE The temperature sensor in this phase was mounted on the inboard edgeof the footpad on landing leg 2, whereit sampled LAA• .ACCELEROMETERS the temperature of oncoming stream tubes well away from thermal contact with the lander. This location also ensured a vigorousflow over the sensingelements at essentiallythe de- SENSOR• • .• scentvelocity. The location of the Doppler radar (terminal descentand landing radar (TDLR)) on the lander bottom is also indicated. Two altimeter antennaswere provided, one on the surfaceof the heat shield,used during high-speedentry, and the other on the lander bottom (LAA), usedduring parachute descent. _ The accelerationsensors were derivedfrom guidancequality ANTENNA'>?3"•TEMPERATURE accelerometersmanufactured by Bell Aerospace Company SENSOR (Bell model IX). They senseacceleration by electromagnetic- (b) PARACHUTE PHASE ally constraininga test mass to a precisenull position. The restoring force is provided by a current flowing in a coil • p SENSOR•=• mounted in the test mass, which reacts against the field of a • •_ LANDER •----•111 permanent magnet in the sensor.The nulling current is the measure of the acceleration. The scale factor accuracy achieved is believed to have been better than 0.02%, with bias uncertainties < 100 3tg. _ The temperature sensorswere multiple fine wire (0.0127-cm diameter) thermocouples,directly exposedto the atmospheric flow. They were