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On Mars: Exploration of the Red Planet. 1958-1978
 
 
MEASURING THE ATMOSPHERE
 
 
 
[374] The water-vapor-mapping investigation was designed to map the distribution of water vapor over the planet and to determine the pressure of the atmosphere at the level where vapor is present. Understanding the distribution of water vapor is crucial to understanding the geological features of Mars and the possibility of the existence of life. Viking's measurements of water vapor varied, depending on the location, season, and time of day.
 
Specialists discovered a direct correlation between elevation and the amount of water vapor present, with the lowest points on the planet having the greatest concentrations and the highest features the minimum. More water vapor was found during the summer season than during winter, when it was barely perceptible. In regions of rough terrain, there were marked daily variations in water vapor, and C. Barney Farmer and his team believed the variations were attributable to local phenomena-shifting wind patterns, dust, or a thin cloud or haze that is present at dawn but dissipates by noon. For example, early in the first mission one site was monitored over a six-hour period. The water vapor content in the atmosphere rose steadily from dawn until noon. This water could have been brought into the area from another region by the wind, or the haze or dust in the air could have affected the instrument's measurements. Whatever the cause for the change, the increase would be considered minute when compared to Earth's atmosphere with 1000 times as much moisture. 22
 
During the Viking primary mission, the Martian water vapor underwent a gradual redistribution, the latitude of the maximum amounts moving from the north polar region toward the equator. Interestingly, while the amounts of vapor at some latitudes changed dramatically, the total global water remained almost constant at the equivalent of about one cubic kilometer of ice. The largest amounts observed were found over the dark polar region, which is inaccessible to Earth-bound observers. Maximum vapor column abundances of about 100 precipitable micrometers were measured adjacent to the residual cap itself-a very large amount considering the temperature of the surface and atmosphere in this region." The Mars atmospheric water detector also confirmed the conclusion that the residual cap is made of frozen water and that the atmosphere above it is saturated with vapor during the polar summer. 23
 
Orbital science investigations had given a better grasp of the global nature of Mars, and the entry science experiments provided the first direct measurements of the physical and chemical composition of the planet's atmosphere. The scientists were for the first time "getting their hands on" some more tangible data. Entry science investigations consisted of measurements by the retarding potential analyzer, the upper-atmosphere mass spectrometer, lander accelerometers, the aeroshell stagnation-pressure instrument, and the recovery temperature instrument. The analyzer had been designed to study the nature of the ionosphere. The mass spectrometer was to provide mass spectra for the constituents of the upper atmosphere. [375] Three of the instruments-the lander accelerometers, the aeroshell stagnation-pressure instrument, and the recovery temperature instrument-made up the lower-atmosphere structure experiment, which measured the density, temperature, and pressure profile of the atmosphere as the lander approached the surface. As with other experiments and Viking hardware, the entry investigations had been based on the common "Mars engineering model" adopted early in the project. That model described the nature of the planet as it was believed to be, from the best knowledge then available. As Jerry Soffen recounted, the model was developed to set the boundaries for design, prescribing the atmospheric envelope, the variety of possible surfaces, range of textures, radiation environment, etc. This "working manual" was constantly reviewed by scientists both within and outside the project and used by all the engineers. The Mars engineering model "was an excellent crossroads for scientists and engineers," With the mission definition, it "truly spelled out what we were trying to do and the planetary constraints we believed existed." 24
 
The lander's mode of descent altered several times before touchdown, and the entry instruments operated during different phases of the entry process. At separation, the lander capsule-consisting of the aeroshell and basecover surrounding the lander-was deorbited by ignition of the deorbit engines. The capsule began the first part of its descent trajectory through the undisturbed interplanetary medium of ions and electrons. The interplanetary medium streams away from the sun at hypersonic velocities in what is called solar wind. Closer to the planet, the lander capsule passed through a disturbed region where the solar wind is diverted to flow around and past Mars, Beneath this zone of interaction lay the Martian ionosphere, a region of charged atomic particles. It was in the ionosphere, 3 minutes after the completion of the deorbit burn, that the retarding potential analyzer began 18 sampling sequences, during which 71 seconds of data were collected.
 
Entry has been arbitrarily defined as starting at 250 kilometers, although the atmosphere is only readily apparent from about 91 kilometers. From separation to entry required about 3 hours. At entry, the lander capsule was oriented with the aeroshell and its heatshield facing the direction of travel; before the atmosphere exerted an appreciable drag, the capsule would accelerate to about 16000 km per hour. Almost l hour before the lander reached the 250-km mark, the upper-atmosphere mass spectrometer was turned on for a 30-minute warmup period. The spectrometer and the retarding potential analyze would continue to take measurements until the capsule system sensed 0.05 gravity, at which time they would shutdown. The capsule-mounted temperature sensor was then deployed. With pressure sensors (deployed 10 minutes before entry), it would continue to function until the aeroshell was jettisoned (12 seconds after the radar altimeter sensed an altitude of 5.9 km).
 
At about 27 km above the surface, the capsule reached its peak deceleration and for a time its path leveled off into a long glide, because of the [376] aerodynamic lift provided by the aeroshell. As the effects of atmospheric friction and gravity overcame the lift, the capsule resumed descent. By the time its radar altimeter indicated an altitude of 6.4km per hour, the capsule was traveling slowly enough (an estimated 1600 km per hour) to deploy the parachute. Seven seconds later, the aeroshell separated from the lander, and the remaining lift in the lightened aeroshell permitted it to drift well away from the landing site. Twelve seconds after aeroshell separation, the lander legs were deployed, at which time the footpad temperature sensor began collecting data, doing so until touchdown. 25
 
From the retarding potential analyzer, new information about the Martian ionosphere was collected through measurements of the solar wind electrons and ionospheric electrons, the temperatures of the electrons, and composition, concentrations, and temperatures of positive ions. At the higher altitudes, the analyzer examined the interaction of the solar wind and the upper atmosphere. The planet's weak (or non-existent) magnetic field permits the solar wind to penetrate closer to the surface of Mars than its does to Earth's surface. Data obtained during descent indicates that singly ionized molecular oxygen (O2+) is the major element of the upper atmosphere, with peak concentration at an altitude of 130km. Singly ionized molecular oxygen is about nine times as abundant as singly ionized carbon dioxide (CO2+), the primary ion produced by the interaction of sunlight with the Martian atmosphere. This new finding lends support to theoretical analyses by M. B. McElroy and J.C. McConnell, which call attention to the reaction of atomic oxygen with CO2+ that would produce carbon monoxide and the more stable ion O2+. The temperature of the observed ions at 130km was about -113°C. 26 Viking measurements of O+ ions moving away from the planet coupled with Mariner 9 observations of hydrogen escaping from the planet's upper atmosphere suggest that the planet has been losing the basic ingredients for water for billions of years. Perhaps some of the water that once carved the massive channels on the surface of Mars slowly escaped in the form of ionized hydrogen and oxygen.
 
The upper-atmosphere mass spectrometer obtained data about the identities and concentrations of the various gases from 230 to 100 km. As expected, the main constituent of the upper atmosphere is carbon dioxide, with small amounts of nitrogen, argon, carbon monoxide, oxygen, and nitric oxide. Taken together, what do these upper atmospheric measurements suggest? The discovery of nitrogen was a particularly pleasant surprise. As Tobias Owen of the nuclear analysis team commented, the search for nitrogen in the Martian atmosphere goes back several decades, and he was "delighted" that they finally had found it. When he first became interested in Mars during the 1950s, "it was an established doctrine that the pressure on Mars was eighty-five millibars, plus or minus three millibars, and that the atmosphere was well over ninety-five percent nitrogen.'' As time passed, predictions changed; both the surface pressure and the amount of nitrogen decreased. As the estimated amount of carbon dioxide grew to more than 95 percent of the gas in the atmosphere, detection of any nitrogen [377] seemed unlikely. This outlook was disheartening to the exobiologists who believed that nitrogen was an essential ingredient in any environment in which life might have evolved. But the upper-atmosphere mass spectrometer did detect nitrogen. Happily, Toby Owen said, "And now we finally got it; it's really there." 27
 
Michael McElroy of the entry science team went even further. According to him, Mars was a very "cooperative" planet, and it had given the Viking scientists some bonus information. Beyond defining the chemical composition of the atmosphere, they discovered some "clues as to the evolution of the planet from its isotopic abundance." Mars has more of the heavy form of nitrogen than does Earth, which allows specialists to theorize that Mars is "remarkably Earth-like although it has gone through a different evolutionary history." McElroy explained that there are two abundant isotopes of nitrogen: Mass 14, which is the common form, and Mass 15, which is less common. They are both present in Earth's atmosphere and in the Martian atmosphere, but Mars has rather more of the heavy component than does Earth. The implication is that Mars must have lost the light material over time. The initial amount of nitrogen on Mars was apparently similar to the initial amount on Earth, but slightly lower gravity on Mars allowed the lighter nitrogen to escape. Perhaps Mars has "evolved to a larger extent than the Earth because of this escape process." 28
 
While the presence of 2.5 percent nitrogen in the atmosphere opened the door for speculation about possibilities of organic material, the levels of argon led to other theories, some of which were contradictory to the one used to explain the presence of nitrogen. Argon was measured at 1.5 percent, considerably less than indicated by the indirect measurements made by the Soviet Union with its Mars 6 mission in 1974. The discovery that Soviet scientists were mistaken was welcome to Klaus Biemann and his colleagues on the molecular analysis team, because it relieved their worry that argon might choke the gas chromatograph-mass spectrometer. The low amount of argon in the atmosphere would not prevent that instrument from performing a series of atmospheric analyses units way to the surface before it could be contaminated by organic compounds from the Martian soil. 29
 
A low concentration of argon also had significant implications when it came to reconstructing the early Martian atmosphere. The two common isotopes of argon are argon-36 and argon-40. The former is an inert element produced in the interior of stars such as our sun, and the latter is created during the radioactive decay of potassium-40. Both isotopes have been released over time from the rocks of planets, and it is generally held that the relative amount of the two says something about how the atmosphere evolved. For Mars, this theory poses some interesting problems and questions. Toby Owen proposed the following scenario during a 28 July 1976 Viking science symposium at JPL. Using the Earth's atmospheric history as a guide, Owen argued that one could by analogy plot the evolution of the Martian atmosphere hack over time. One way to make this analysis for the two planets was to use argon-36 as the common piece of information. It was [378] assumed that Earth and Mars were formed at the same time and from the same inventory of gases in the solar nebula. If that is true, then Earth and Mars should have about the same ratio of argon-36 and argon-40 in their atmospheres. They do not. Earth is relatively poor in argon-36; it is held that this gas was lost early in the evolution of the terrestrial atmosphere. Scientists thought that they could deduce from the amount of argon-36 in the Martian atmosphere the gases that have been lost. Viking measurements indicate that the planet should have lost 10 times the amount of carbon dioxide and nitrogen now measured in the atmosphere. But the loss was not out into space; it was hidden in some form on the planet itself. Ten times the present amount of carbon dioxide constitutes a considerable amount of material to hide. Owen reported: "I'm suggesting that somewhere between land 10 times the present amount of CO2 is missing on Mars....and some fraction could still be present in the form of CO2 trapped in the [polar] caps. The other part of this reconstruction, which is interesting, is that it implies a couple of tens of meters of water on the surface which must also be sequestered somewhere." 30 The water could have become permafrost, but this explanation disagrees with the theory that the water left the planet in the form of ionized hydrogen and oxygen.

Although no general agreements have been reached on how the upper atmosphere of Mars was formed, one point seems certain: that atmosphere was significantly different in the past. Just as the evolution of Earth's atmosphere helped determine the nature of its environment, the evolution of Mars is linked with the development of its atmosphere. As Jerry Soffen concluded: "It appears that there was a considerably denser atmosphere in the past, somewhere between 10 and 50 times the present value of 7.5 millibars at the surface. This denser atmosphere would account for the possibility of the ancient river [beds] seen from the orbiter." 31 Whatever explanation the scientific community comes to accept, Viking has made two points very clear-the Red Planet's environment has not been static, and in the past was very dynamic.

 
The lower atmosphere structure experiment provided vertical profiles of the density, pressure, and temperature of the atmosphere from an altitude of 90 km to the surface. Accelerometers, part of the lander's inertial reference unit, acted as sensors for the initial measurements from which the density profile was derived. The profile was determined by observing the retardation of the capsule's descent by atmospheric drag. Pressure and temperature measurements came at first from the two instruments in the aeroshell. Because of the high initial velocities of the lander capsule, the pressure sensor determined the pressure of the atmospheric molecules against the aeroshell surface; the actual pressures were determined analytically later. In a similar fashion, the temperature probe, near the outer rim of the aeroshell, measured the temperature of molecules flowing around the aeroshell. During the parachute phase of the descent, after the aeroshell had been jettisoned, the lander's pressure and temperature sensors provided this information.
 
[379] Altitude data for construction of profiles came from the radar altimeter. A by-product of the radar altimeter measurements was information about the terrain beneath the lander. The terminal descent and landing radar system, which controlled the very last stage of the landing, also measured the extent to which the lander drifted because of winds above the point of touchdown. Pressure and temperature variations were measured by the two landers at selected intervals during the descent (table 53). The temperature in the region between 200 and 140 km above the surface averaged about -93°C; for the region between 120 and 28 km it was - l30°C. At touchdown, the Viking 1 atmospheric temperature was about -36°C, and Viking 2 's reading was -48°C. 32
 
 
 
 

Table 53
Structure of Martian Atmosphere

Viking 1

Viking 2

Altitude

Pressure

Temperature

Pressure

Temperature

(km)

(mb)

(°C)

(mb)

(°C)

120.0

0.000 004 14

- 136.85

0.000 001 99

- 157.15

108.0

0.000 018 40

- 126.75

0.000 013 00

- 152.05

96.0

0.000 080 20

- 127.25

0.000 066 00

- 122.95

84.0

0.000 387 00

- 128.95

0.000 288 00

- 131.75

72.0

0.002 050 00

- 134.05

0.001680 00

- 142.25

60.0

0.009 110 00

- 127.65

0.008 540 00

- 135.85

48.0

0.044 500 00

- 124.55

0.039 200 00

- 102.45

36.0

0.198 000 00

- 107.05

0.158 000 00

- 108.75

28.0

0.483 000 00

- 89.35

0.404 000 00

- 99.95

4.5

5.160 000 00

- 51.05*

5.222 000 00

- 51.95

4.0

5.390 000 00

- 50.53

5.483 000 00

- 51.55

3.5

5.635 000 00

- 48.45

5.747 000 00

- 51.05

3.0

5.885 000 00

- 46.65

6.015 000 00

- 50.55

2.5

6.150 000 00

- 44.85

6.282 000 00

- 50.05

2.0

6.427 000 00

- 43.05

6.564 000 00

- 49.55

1.5

6.707 000 00

- 41.35

6.853 000 00

- 49.15

1.0

6.994 000 00*

- 39.45*

7.160 000 00*

- 48.55*

0.5

7.301 000 00*

- 37.65*

7.480 000 00*

- 48.05*

0.0

7.620 000 00*

- 35.85*

7.820 000 00*

- 47.55*

 
*Extrapolated.
 
SOURCE: Alvin Seiff and Donn B. Kirk, "Structure of the Atmosphere of Mars in Summer at Mid-Latitudes," Journal of Geophysical Research 80 (30 Sept. 1977): 4367, 4371.
 
 
[380] Compared to the scientific instruments aboard the orbiter or the lander, the entry experiments were very short-lived. They operated only during the descent to the surface. Still, these instruments provided investigators with several new insights into the Martian environment and clues that, when coupled with orbital and lauded data, would help frame new hypotheses about the evolution of the planet.
 
As interesting as the orbital pictures and measurements were and as informative as the entry data instruments were, the best was to come. Science aside for a moment, the reception of the first pictures from the lander cameras had to be the most exciting event for many project participants, scientists and engineers alike. For the public, the surface pictures were certainly the main event.