Observations of Mars previously reported in 10 narrow bands between 3150 Å and 1.06 μ and in UBV are analyzed for brightness variations which correlate with longitude of the central meridian. Such an effect is found for λ ≥ 5000 Å, with some evidence for such a correlation at λ = 4570 Å. The data are then corrected to the mean (over longitude) brightness and a linear phase curve fitted to those observations with phase angle i ≥ 15°. An opposition effect (anomalous brightening at small phase angles) is found for wavelengths λ ≤ 5500 Å, in contrast to a result previously reported. The magnitude at zero phase, phase coefficient, and monochromatic albedo are computed for Mars as a function of wavelength.

It has been suggested that the discrepancy between radio occultation determinations of the Martian atmospheric surface pressure (3.8 to 7 mb) and those deduced from optical polarization measurements and a simple Rayleigh atmosphere model (about 10 mb) are the results of sub-micron sized aerosols in the Martian atmosphere. Based on observed viewing angle dependence of the polarization of the Martian disk in the visual range, a Mie scattering analysis has been made utilizing the measured complex index of refraction of limonite and bulk solid CO2. The results of this study indicate that limonite aerosols alone are unsatisfactory to explain the viewing angle observations, whereas solid CO2 (and H2O ice) aerosol spheres, having a dominant particle radius range between 0.28 and 0.35 μ, could bring planetary and laboratory observations into compatibility. It is suggested, further, that solid CO2 aerosols could explain limb brightening in the blue spectral range. Various distributions of solid CO2 and H2O Mie particles with radii up to 0.35 μ show an opposition effect. However, the role of these aerosols in explaining the Mars opposition observations is very dependent on the optical properties of the underlying Mars surface material.

Mie scattering calculations have been made for atmospheric aerosols having various indices of refraction to determine their possible contribution to a Martian opposition effect, such as that reported by O'Leary in 1967. Neither substances with a real index between 1.20 and 1.50, such as ice, water, or solid CO2, nor highly absorbing materials, such as limonite, can produce the observed effect. Submicron-sized spherical particles with refractive indices of 1.55 to 2.00 do, on the other hand, exhibit a marked increase in reflectivity at small phase angles and might be responsible for the enhanced brightness at the shorter wavelengths.

Ground-based polarimetric data taken near maximum elongation are presented. These data are analyzed assuming an optically thin, Rayleigh scattering atmosphere, and a surface whose polarization varies inversely as the surface albedo. The best fit to the data yields an optical depth for Rayleigh scattering corresponding to a surface pressure of 6 ± 1 mb, if carbon dioxide is the principal constituent. There is no need to postulate the existence of fine dust in the Martian atmosphere. This method is potentially capable of resolving elevation differences on the Martian surface.

The similarity rules to compare atmospheres with anisotropic and isotropic scattering are reviewed. Omission of a narrow diffraction peak in the scattering pattern is permitted and corresponds to a special application of the similarity rules. It is shown that the extrapolation length for conservative scattering can be found with great precision from a formula involving only ω2 and ω3. An asymptotic expression for high-order scattering in a semi-infinite atmosphere is given and it is shown by some examples that this expression can be used to find by interpolation the terms of any order. Finally, the way in which the contrast between a dark and bright area near the center of the disk of Mars is affected by an overlying haze is computed for isotropic and for anisotropic scattering.

Lines of the 2-0 and 3-0 bands of carbon monoxide and (many) bands of carbon dioxide appear prominently in the Connes' Mars spectrum [1]. Five carbon dioxide bands were measured to construct a curve of growth for CO2 lines formed in the Martian atmosphere [2]. A similar curve of growth was constructed for the 2-0 band of carbon monoxide. From these curves, we have computed the rotational temperature of the atmosphere, the surface pressure, and the abundance of CO and CO2. The surface pressure is found to be approximately equal to the CO2 partial pressure, i.e. ps ~ 5 mb. The CO concentration by volume was found to be slightly less than one part per thousand.

A method to determine independently the Martian surface pressure from measurements of individual lines in the 1.2030 and 1.2177 μ CO2 bands is presented. Observations obtained during the 1967 apparition and some preliminary results of the 1969 apparition observations yield CO2 abundances near 100 m-atm and surface pressures of 4–8 mb.

All previous published equivalent widths of the Martian CO2 bands including the 1967 apparition coverage at McDonald Observatory are reduced to CO2 abundances using the same curve of growth parameters. The corresponding CO2 partial pressures (assuming a pure CO2 atmosphere), along with the regression curves of the north polar cap, are presented as functions of Martian heliocentric longitudes, Ls. A correlation is noted between the maximum CO2 abundance and minimum north cap diameter and the decrease in CO2 abundance when the northern polar haze begins to reform. Preliminary results of the 1969 apparition coverage at McDonald Observatory are also presented.

High dispersion spectroscopic observations of Mars, carried out with the new 107-inch and 82-inch Struve telescopes at McDonald Observatory, were used for detection of relative elevation differences on the surface of Mars. The dispersion of the spectra was 4.4 Å/mm for the 0.87 CO2 band and 3.5 Å/mm for the CO2 bands in the 1 μ region. Spatial resolution during the 1969 opposition was about 20° by 10° for 0.87 μ and 15° by 15° on the planet for the 1.03 μ and 1.05 μ bands. The results are shown in Tables I, II, III and Figures 2 and 3. The observed differences in elevation are on the order of 10 km, which is in good agreement with the latest radar measurements of the topographic features on the Martian surface.

Observations of a carbon dioxide band at 1.2206 μ (8192.6 cm−1) made at the opposition of Mars in June, 1969, are used with laboratory data to derive a value for the pressure at the surface of Mars. This band is especially suitable because it is sensitive to pressure, lying in the transition region of the curve of growth, but is relatively free of telluric contamination. The pressure derived is , corresponding to the Martian desert region Amazonis.

Using a five-channel spectrometer, the strengths of two carbon dioxide bands at 1.5753 and 1.6057 μ (6347.8 and 6227.9 cm−1) were compared over a Martian dark area (Mare Acidalium) and a nearby bright area in Amazonis. The bands were 1.32 times stronger in the dark area than in the bright area. Interpreting this as evidence for elevation differences on the Martian surface, it is found that the dark Mare Acidalium is 2.5 km lower in elevation than the bright area with which it was compared.

The discovery of water on Mars is a lot like the discovery of America. Both were done many times by many different people. In both cases many of the ‘discoveries’ were either fictitious or difficult to verify. Regarding the discovery of America, a grammar school teacher of mine once pointed out that Columbus was the effective discoverer of America. After he did it, people believed it was so. More importantly, people began to act on the basis of his discovery.

Spectra of Mars, centered on the 8200 Å band of H2O, have been obtained using the coudé spectrograph of the 107-inch telescope at McDonald Observatory with reciprocal dispersion of 1.9 Å/mm. The plate scale (4.4 arc sec/mm) and angular resolution (3″–6″) were sufficient to measure the strength of the Doppler-shifted H2O lines at 5 points across the disk.

The spectra were obtained in March and April 1969, when the apparent diameter of Mars was 10″ to 16″ during mid-summer of the northern hemisphere; the Doppler shift at 8200 Å varied from –0.42 Å to –0.28 Å. On the first plate, obtained on March 27, 1969 the abundance reached a maximum of about 48 μ precipitable H2O at 30° to 40° north latitude and decreased to about 20 μ at 30° south latitude. The second plate, taken on April 28, 1969 showed the same north-south decrease in abundance but the total amounts were about two-thirds of the March abundances.

Little variation in Martian atmospheric H2O abundance was observed during three months prior to the 1969 opposition.

Spectra of Mars were presented which were taken as part of the LPL high altitude program [1] with the NASA CV990 jet. The spectra were taken at about 40 000′ (12 km) altitude. Two sets of five flights each were made around May 1, 1969 and June 1, 1969. In each case the first flight was a Moon comparison flight and the other four were Mars flights, with some Moon if the Moon could be observed.

A 12″ telescope received light from a gyroscopically controlled heliostat and directed it into a Block Associates rapid scan interferometer. (For details see Reference [2].) The interferometer had a resolution of 5 cm−1. The present spectra are, however, only reduced for a resolution of 8 cm−1 due to limitations in our computer operations.

The spectra taken around June 1 are of better quality since Mars was brighter at that time. The CO2 bands at 1.4 μ, 1.6 μ, 2.0 μ, and 2.7 μ all show up very nicely. The isotopic C13 companion band of 003 as well as the ordinary forbidden isotopic band of O16C12O18 (002) can be detected. No water vapor at 1.4 μ or 1.8 μ remains after taking the ratio with the Moon. Some possible trace remains at 2.7 μ. Quantitative analysis as well as reductions of the interferograms to higher resolution are in progress.

In the absence of juvenile liquid water, condensation and subsequent melting of ice are the only means of producing liquid water on the Martian surface. However, the evaporation rate is so high that the available heat sources cannot melt ice on Mars. Melting might occur only in concentrated solutions of strongly deliquescent salts.

Emission features from ionized carbon dioxide and carbon monoxide were measured in the 1900- to 4300-Å spectral region. The Lyman-α 1216-Å line of atomic hydrogen and the 1304-, 1356-, and 2972-Å lines of atomic oxygen were observed.

Spacecraft tracking data can be used to determine some parameters of importance to recent developments in celestial mechanics. Integrated Doppler data are available from both Mariner VI and VII. These data are phase coherent and are accurate to two one thousandths of a cycle at S-band over a 10 min integration time. In terms of a range rate error this is better than 0.2 mm/sec. Round trip range data are also available. They are accurate to 100 nsec in absolute accuracy and have a random error component of about 30 nsec, or about 4.5 m in one-way range.

Only data near the closest approach of the spacecraft to Mars are capable of yielding information on that planet, but other data are useful for investigations into the ratio of the masses of the Earth and Moon, the ephemeris of the Earth, and both orbital and propagation effects of general relativity. For purposes of this conference, however, the only results reported are those which pertain directly to Mars.

The problem in using the Mariner VI and VII encounter data is that significant non-gravitational forces were acting on the spacecraft.

On Mariner VI, the trajectory was essentially a gravitational one until about 35 minutes before closest approach when the system to cool the infrared spectrometer was turned on. This system expelled hydrogen and nitrogen gas through a Joule Thompson cryostat and as a result imparted a thrust on the spacecraft of something on the order of 100 dyne, or a total velocity change in the trajectory of about 0.2 m/sec. Unfortunately the system did not operate properly on Mariner VI; it is known that the thrusting from the infrared system occurred over a period of about six days after encounter. In normal operation, the thrust level would decrease to an insignificant level after a few hours.

On Mariner VII, the infrared cooling system worked properly but an unknown event which occurred some 5 days before encounter imparted a thrust to the spacecraft for at least several days prior to closest approach.

There is a possibility, in fact, that a thrust was applied to the spacecraft for several weeks after encounter. Combining this effect with the normal thrusting from the infrared system, makes the scientific analysis of the Mariner VII tracking data a near impossibility.

Despite these difficulties with Mariner VII, some interesting results have been obtained with Mariner VI. For example, the universal gravitational constant times the mass of Mars GM♂ has been determined from the acceleration on the spacecraft as it approached the planet. The best value of GM♂ is 42 828.3 ± 1.0. Also, by combining the range data from Mariner VI with data from the radar range measurements of Mars obtained by R. Goldstein this year at Goldstone, California, it has been possible to prepare a topographical map of Mars between 3° and 12° north latitudes. This map predicts that the immersion of Mariner VI into earth occultation should occur at a point on Mars which is 3393.6 km from the center of the planet. An independent calculation of this distance by A. Kliore, who used the Mariner VI occultation data, is in excellent agreement with the calibrated radar topography.

The energy radiated by Mars in the wavelength bands 8–12 and 18–25 μ has been measured during the flyby of Mariner 1969 with linear resolution of 50 km at closest approach. From the laboratory energy calibrations and assuming unit emissivity, the temperature of the surface materials has been derived. The main results are summarized as follows:

1. (a) The temperatures near the equator (Mariner 6), as function of local time, on the average agree with the values expected on the basis of the gross thermophysical properties of the soil determined from ground-based measurements.

2. (b) The temperature fluctuations around their mean seem to correspond to variations in albedo over the classical features of the planet, with the possible exception of one area.

3. (c) The lowest temperatures measured by channel 2 of Mariner 7, during the polar swath, were 148 K, while at the edge of the polar cap the temperature was 230 K.

A small amount of instrumental responsivity to off-axis radiation will change these nominal temperatures slightly, in the general sense that the low temperatures, when properly corrected, will decrease by a few degrees, while the high temperatures will increase by a similar amount.

Before the space era, Mars was thought to be like the earth; after Mariner 4, Mars seemed to be like the moon; Mariners 6 and 7 have shown Mars to have its own distinctive features, unknown elsewhere within the solar system.

The successful flyby of Mariner 4 past Mars in July 1965 opened a new era in the close-range study of planetary surfaces with imaging techniques. In spite of the limited return of data, Mariner 4 established the basic workability of one such technique, which involved use of a vidicon image tube, on-board digitization of the video signal, storage of the data on magnetic tape, transmission to the earth at reduced bit rate by way of a directional antenna, and reconstruction into a picture under computer control. Even though the Mariner 4 pictures covered only about 1% of Mars's area, they contributed significantly to our knowledge of that planet's surface and history [1, 2, 15, 17].

The similarity theory of atmospheric boundary layers is applied to an estimate of the form of vertical profiles of average wind velocity and potential temperature in the atmospheres of the terrestrial planets in day- and night-time conditions.

It is then considered, as in the case of the earth, that the magnitude of the turbulent heat flux qT during the day is about 0.1 of q(1 – A), where q is the solar constant for the planet and A is its albedo; at night, qT is several times smaller still. The friction velocity u* is taken equal to 2–5% (depending upon the stratification) of the mean wind velocity in the free atmosphere, which was adopted from previous calculations (Golitsyn, 1968).

The boundary layers in the atmospheres of Mars and Venus and in the hypothetical atmosphere of Mercury are examined in detail. Sharp temperature drops are characteristic of Mars within a few tens of meters from the surface, attaining a magnitude of several tens of degrees, especially during the day. Large changes of the wind velocity also take place in this thin lower layer. This effect results from the low density of the Martian atmosphere.

For Venus, owing to the very high density of the atmosphere, the stratification is close to neutral, i.e., the temperature profile is close to the adiabatic one and the wind profile is of a logarithmic shape.

Owing to high winds, the stratification on Mercury must also be close to neutral with respect to the wind (the profile being close to the logarithmic), but because of the expected low density, the temperature changes near the ground may still be very great.