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CHAPTER 15 POTENTIAL YIELDS OF BIOLOGICAL RELEVANCE FROM REMOTE INVESTIGATIONS OF MARS CARL SAGAN INTRODUCTION Short of actually landing instruments on Mars, several other instrumental platforms are available for remote investigations of that planet. The bulk of our information about Mars has been obtained by astronomical observa- tions from the surface of the Earth. Balloon astronomy is just beginning to be exploited. Orbiting Astronomical Observatories are scheduled to be launched beginning 1966 or 1967. Launchings of Mars fly-bys have been attempted, with partial success. The capability for launching a spacecraft that can be placed into orbit about Mars may be at hand within a few years. It is clear that each of these platforms cannot be given equal sup- port, nor should they be. For a given purpose, there is usually one optimal platform. While it is not impossible, there seems little prospect of directly and unambiguously detecting and characterizing life on Mars by any means short of landing on the planet. But remote observations are capable of determining physical parameters of the Martian environment that are con- straints on any indigenous Martian ecology; testing biological hypotheses put forth to explain Martian phenomena; providing significant information for the selection of sites for landing missions; and detecting topographical or chemical artifacts or products of indigenous biological activity. 264

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Potential Yields of Remote Investigations of Mars 265 All conceivable methods of investigation involve the use of electro- magnetic radiation and, excepting the use of microwave and, possibly, optical radar, all observations are passive. It is not yet possible to do anything to Mars; we must wait for Mars to do something by itself, and then observe it in flagrante delicto. At a given electromagnetic frequency, and from a given resolution element on the Martian surface, the acquired information may be radiometric or polarimetric—we ask how much radia- tion there is, and how it is polarized. If we scan in frequency, our observa- tions become spectrometric; if we scan in angle, the measurements become cartographic. For a brief discussion of some of these techniques, the reader is referred to Chapter 1 of The Atmospheres of Mars and Venus (NAS- NRC Pub. 944, 1961), or to any standard astronomy text. Generally speaking, the most useful techniques of the present and the immediate future seem to be these: ultraviolet spectroscopy and polarime- try; optical cartography, spectroscopy and polarimetry; near infrared spec- troscopy and cartography; far infrared cartography and radiometry; and micowave cartography, polarimetry and spectroscopy. For fundamental reasons, some of these techniques are better applied from certain platforms than from others. Thus, ground-based ultraviolet studies shortward of 3300 A are essentially impossible, because of absorp- tion in the terrestrial ozonosphere. Balloons generally rise to no more than 120,000 feet, which is well below the top of the ozonosphere; therefore, balloon platforms have limited usefulness for ultraviolet work. The simplest platform for ultraviolet studies of Mars is, then, an Earth satellite. Because of turbulence in the Earth's atmosphere, astronomical "seeing" limits the angular resolution possible in optical photography, even from small ground-based telescopes. Since the turbulent elements are primarily in the lower terrestrial troposphere, observations from balloon altitudes can avoid essentially all of the seeing difficulties. Infrared observations at almost all wavelengths, from the surface of the Earth, must customarily allow for absorption by substances in the terres- trial atmosphere, primarily carbon dioxide and water; but at some wave- lengths such other substances as methane and ozone may also become troublesome. Where electronic transitions, or the rotational components of a vibration-rotation band, can be resolved, the Doppler effect is gen- erally used to separate telluric from planetary absorption features. Absorp- tion by substances uniformly mixed in the atmosphere cannot usually be avoided at balloon altitudes, but absorption by water, which, because of its condensation properties, is restricted to the terrestrial troposphere, can be avoided by using stratospheric balloons. Thus, except for wavelengths where carbon dioxide absorbs—in the near infrared, at 2.7/i, 4.3/i, the 10^ region, and the 15/i region—the atmosphere is essentially clear, for most

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266 APPROACHES AND REMOTE OBSERVATIONS spectroscopic infrared purposes, at stratospheric altitudes. In particular, infrared planetary spectroscopy beyond 20/t is now becoming possible, although it should be mentioned that there are no critical observations of Mars that suggest themselves in this wavelength region. The terrestrial atmosphere is generally transparent at centimeter and longer wavelengths, but some absorption by water and other substances in the millimeter range provides a case for microwave observations from Earth satellites. Because of the great weights involved, there has been no devel- opment of equipment for astronomical observations at microwave fre- quencies with balloons. Balloons and fly-bys have an inherent disadvantage that ground-based observatories, Earth satellites, and Mars orbiters do not. Much of the evidence relevant to the possibility of life on Mars has to do with seasonal or secular changes on that planet. However, the useful observational life- times of individual balloon flights (about 12 hours) or fly-bys (several hours) are so small that observations of these changes from such platforms become impossible. Several such vehicles could be flown, but intercali- bration and other problems make this a less promising method. But the useful lifetime of ground-based instruments, Earth satellites and, probably, planetary orbiters, is large enough to allow significant seasonal and synop- tic observations to be performed. Ground-based observations have a number of great advantages which, while obvious, are difficult to belabor. Ground-based astronomical observa- tories can be operated and maintained by men who have the capability of changing the mode of operation and goal of an experiment at short notice if the observations seem to warrant it. The limitations in payload are gen- erally not restrictive, so that much larger apertures and much more sophis- ticated ancillary apparatus and cryogenic systems are possible than with other platforms. Also, ground-based observations are by far the cheapest. Some of the most promising potential sources of information about Mars, such as infrared and microwave spectroscopy, are energy-limited. Thus, experimenters use the radiation incident from the planet as a whole, since any attempt at topographical resolution (which is at least possible at infra- red wavelengths) engenders a serious loss in signal-to-noise ratio. But if a given instrument were capable of flying close to the planet Mars, then its topographical resolution could be greatly improved, without any corre- sponding energy loss. In many cases, the disadvantages inherent in fly-bys and orbiters, such as low scientific payload and low data transmission capability, are more than offset by the high topographical resolution that these platforms make possible. In the case of optical cartography, the advantage of fly-bys and orbiters is due to the fact that with a modest aperture, linear surface resolutions can be obtained that are impossible to obtain from greater distances because

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Potential Yields of Remote Investigations of Mars 267 of the diffraction limitation on optical resolution. This point is illustrated in Tables 1 and 2. Table 1 shows the topographical resolution on the surface of Mars that can be obtained photographically with large telescopes from the surface of the Earth during the 1965-1971 oppositions; these resolutions range from 700 to 400 km, improving with time, as the favor- able opposition of 1971 is approached. During the same opposition, a smaller 36-inch telescope above the terrestrial atmosphere can obtain resolutions that are more than an order of magnitude superior, because it is not beset by seeing problems and can work at the limit of its diffraction disk. There are plans to perform video reconnaissance of Mars with a 36-inch telescope carried by balloon above the turbulence in the Earth's atmosphere. This system, directed by Pro- fessor Martin Schwarzschild of Princeton University, is called Stratoscope II. The photographic resolution that Stratoscope II should be able to obtain routinely is comparable or superior to the best recorded visual (human eye) observations made with telescopes of larger aperture from those terrestrial observatories with the best seeing conditions—for example, Pic du Midi, in the Pyrenees. With a given optical system, visual resolution is generally superior to photographic resolution because the human ob- server is able to reject the moments of bad seeing and remember the few moments of superb seeing. However, it is clearly preferable, in order to eliminate the personal equation and to permit recording and comparison of results, to perform photographic observations. Some further light can be shed on reports of rectilinear fine markings on the surface of Mars by such balloon observations, although the best visual observations suggest that the rectilinear features are spurious. Table 2 shows linear resolutions on the surface of Mars that can be obtained by telescopes of various apertures carried on fly-by vehicles or orbiters. It is assumed here, as in the discussion of balloon observations above, that the stability of the platforms is consistent with the diffraction- limited resolution of their telescopes. At 20,000 km from the center of Mars, the planet would subtend an angle of 21° in the sky, and small tele- scopes would be capable of resolutions of several tens of meters. Much closer to Mars, at a distance of 5,000 km from its center (and about 1620 km from the surface), the planet subtends an angle of 85°, and resolutions down to some meters are possible with the apertures indicated. Such reso- lutions are very impressive, and vastly superior to any obtainable from the vicinity of the Earth. To detect life on Earth unambiguously with an optical resolution of a few tens of meters, at a random location, is far from a trivial problem. If by chance an appropriate location—e.g., a city, or cultivated farmland —were selected, the presence of life would be immediately obvious (see Chapter 9). But to insure a reasonable expectation of detecting life, a

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270 APPROACHES AND REMOTE OBSERVATIONS systematic survey of the Earth would be required. The same is true of Mars. However, here we encounter problems of data transmission rate. Table 3, taken from The Atmospheres of Mars and Venus (op. cit.), indicates the characteristic number of bits required for a range of pictures with various degrees of fineness of detail. Currently tractable bit trans- mission rates from the vicinity of Mars seem to be between 20 and 300 bits per second. We see that 20 bits per second implies the capability of trans- mitting one ordinary television picture of Mars per day; 300 bits per second implies perhaps 15 such pictures. The question of improvement of the data transmission capability is discussed in the section on Mars biological orbiters below. Infrared resolutions that can be obtained with telescopes of 12" to 36" aperture, and microwave resolutions that can be obtained with antennae of aperture between 3 and 30 meters are also shown in Table 2 for a variety of presumed periapsis distances from Mars. The resolutions are four to five orders of magnitude better than the corresponding resolutions obtainable from the vicinity of the Earth. A REPORT ON SCIENTIFIC STUDIES WITH MARTIAN ORBITERS With this as background, we now proceed to a discussion of the scien- tific investigations of biological relevance that might be performed with projected Martian orbiters in the period 1969-1971. This study, reported below, was made for the Bioscience Programs Division, National Aero- nautics and Space Administration by a Working Group on Mars Orbiters. The participants were: Carl Sagan (chairman), Frank D. Drake (Cornell University), Richard M. Goody (Harvard University), Donald P. Hearth (NASA), John Martin (Jet Propulsion Laboratory), William M. Sinton (Lowell Observatory), W. G. Stroud (Goddard Space Flight Center, NASA), and Andrew T. Young (Harvard College Observatory). The direct detection of life on Mars from an orbiter vehicle is considered unlikely, but there are promising possibilities for determining physical parameters of biological relevance as boundary conditions on the ecology of Martian organisms, and for detecting surface phenomena that may be due to the activities of Martian organisms. The 1969-1971 time period is especially favorable, both because of the moderate energy requirements of the trajectories, and because the southern hemisphere wave of darkening will reach its maximum extent within a few months of projected spacecraft arrival times. For experiments of biological significance, the principal advantages of a

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272 APPROACHES AND REMOTE OBSERVATIONS Mars orbiter are the prospects of (1) obtaining high topographical resolu- tion; (2) observing at phase angles greater than 43° (and thus, the night hemisphere of Mars); and (3) detecting seasonal changes on Mars. Because of the importance of the seasonal changes, a long scientific life- time of the orbiter is highly desirable, a three-month lifetime being perhaps the minimum acceptable value, and a four- to six-month lifetime being preferable (these are terrestrial months). In order to investigate the wave of darkening in its photometric, polarimetric, and possibly spectrometric aspects, it is important that the spacecraft be in orbit and functioning when Mars is at about 10° heliocentric longitude; and that it be functioning for at least 90 days prior to, or at least 90 days subsequent to this time. A more nearly ideal orbiter is one that functions for a six-month period, cen- tered on heliocentric longitude 10°. It is recognized that there is a trade-off among the lifetime of the orbiter, the weight available for scientific experiments, and the data rate for com- munication back to Earth. If any of these three parameters can be im- proved by the development of Earth-based facilities, this improvement should be strongly encouraged. It appears that for a cost small compared with that required for even a modest exploitation of the 1969-1971 op- portunity, very great improvements in ground-based communication facili- ties can be made. As a rough example, for about $40 million, a phased array of twenty 85-foot radio antennas or three 210-foot antennas can be constructed, which give approximately a twentyfold improvement in the data rate obtainable from any given spacecraft. With such a facility, a data rate of 30 bits per second received from the vicinity of Mars with equipment now planned can be increased to 600 bits per second. The construction of even one such facility would permit a major improvement in our ability to obtain information from Mars. If any serious thought is to be given to high-resolution television pictures with reasonable surface coverage from the vicinity of Mars in the 1969-1971 time period, such im- provement of ground-based communication facilities is mandatory. It is also recognized that the higher energy required for a circular orbit implies a smaller scientific payload. Since the payload losses involved seem to be more serious than the diminished topographical resolution that an elliptical orbit implies, elliptical orbits are considered acceptable. A polar orbit, especially with a significant precession of the line of apsides during the orbiter's scientific lifetime, is scientifically much more desirable than an equatorial orbit because, among other reasons, of the great biological interest in the receding boundaries of the Martian polar caps. It is recommended that instruments intended for Martian orbiters be de- signed, ground-tested, and then test-flown in the vicinity of the Earth well

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Potential Yields of Remote Investigations of Mars 273 before the projected launch date. In many cases, significant scientific in- formation about the Earth can also be obtained from such flight tests. In the following discussion of possible instrumental techniques for 1969 and 1971 Mars orbiters, it will be assumed that the available scientific payload is in the range 50 to 200 pounds, and that the data rate lies in the range 30 to 200 bits per second. The minimum scientific lifetime acceptable is about three months, and the smaller the periapsis consistent with planetary quarantine requirements, the better. A highly elliptical orbit with a periapsis of 1000 km is useful from many points of view. It may be desir- able to shut down some experiments when the orbiter is far from periapsis, if this can extend the useful life of the orbiter. Ultraviolet Spectroscopy and Photometry The primary scientific goals of this technique are: (a) determination of the ultraviolet flux at the Martian surface, (b) search for the biologically significant gases O2 and O3, (c) search for and topographical characteriza- tion of acetaldehyde (CH.tCHO), a gas suspected from infrared studies, and possibly relevant to Martian biological activities. It is our opinion that such experiments of biological interest can be per- formed almost as well, and much sooner, by rockets and by Orbiting Astronomical Observatories; or by other techniques at different wavelengths. In particular, a search for acetaldehyde can be performed with higher reliability in the infrared, if high spectroscopic resolution or an appropriate selective-absorption gas cell filter is used. The search for oxygen and ozone can be made in the infrared and ultraviolet from the vicinity of the Earth. A computation of the ultraviolet flux at the surface of Mars requires spectral measurements of moderately high topographical resolution, but only relatively poor spectroscopic resolution. It is recommended that a vigorous program of ultraviolet spectroscopy of Mars from the vicinity of the Earth be performed, so that these important parameters can be de- termined without burdening the scientific payload of Mars orbiters late in the decade. Spectrometers can be launched from stabilized platforms by modest rockets such as the Aerobee. Orbiting Astronomical Observatories appear to have quite adequate capabilities for the above ultraviolet observa- tions of the planet Mars. This is particularly true for the Goddard scan- ning spectrometer planned for OAO-2. It is recommended that a firm commitment for the few hours' required observing time be obtained at an early date. There should be no difficulty in finding qualified scientists to interpret the data.

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274 APPROACHES AND REMOTE OBSERVATIONS Television Potential experiments of biological relevance include (a) a detailed characterization of the nuclei and other fine structure details in the Martian dark areas, and (b) a detailed mapping of the seasonal progress of the wave of darkening. Experience with photographs obtained in the Tiros series (.—106 bits per picture) and from the Mercury capsules (—109 bits per picture) indicates that the reliable determination of life on Earth from orbital altitudes is a difficult undertaking. In order to have a high probability of detecting life on Mars by television pictures, a minimum resolution of 1 meter seems indicated. Such resolutions may be barely practical for an orbiter in the 1969 and 1971 missions. Whether more tractable resolutions in the 10-meter range are biologically significant is an unde- cided question. The potential gains may conceivably be very high; on the other hand, the cost in terms of data transmission rate is also very high. It is a question of assessment of a priori probabilities, and, as antici- pated, the group was divided on this issue. Television is not required in order to provide a firm topographical localization for other kinds of observations—for example, infrared spectro- scopy. The expected topographical resolution of infrared spectrometric observations is comparable to the visible topographical resolution of ex- isting maps of Mars. Correlation of dark areas with identifiable topo- graphic features—for example, large impact craters—might be significant in attempting to interpret the dark areas and their seasonal changes. If the factor of 20 improvement in data transmission rate by construction of large ground-based communication facilities is obtained, then —2000 bits per second becomes feasible from the vicinity of Mars. This is roughly 1.7 X 108 bits per day, and implies that —100 Tiros-quality photographs of Mars could, under these circumstances, be transmitted to Earth per day. A more modest television experiment, which is capable of determining with fairly high resolution secular and seasonal changes in the fine struc- ture of the Martian dark areas, is a 1- or 2-line scan of the planetary sur- face. A statistical analysis of the intensity correlation of several scans of the same region, but spaced in time, will give some indication of the seasonal and secular variations of the surface features in two dimensions. Such a device, put into orbit for several months, can give information on the variations of fine structure of the Martian dark areas at a minimal cost in weight and data transmission. The group felt that because most of the reported color changes on Mars

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Potential Yields of Remote Investigations of Mars 275 appear to be psychophysiological in origin, and because of the intrinsic ambiguity in interpreting any authentic detection of color changes on Mars, color determinations should be given low priority, especially in view of the high data transmission cost that multicolor photography en- tails. Optical Polarimetry Polarimetric observations have the capability (a) of suggesting the composition of the Martian bright areas, and the distribution of this com- position; and (b) of suggesting the composition, albedo, and granularity of the Martian dark areas. This last may be directly related to the presence of organisms on Mars. Polarimetry at visible and at microwave frequencies appears to be the only practicable method of detecting remotely the effects of individual small organisms on Mars directly, although even here, the hypothetical organisms can only be detected en masse. Because of the necessity of positioning interference fringes over the disk of Mars, ordinarily only very poor topographical resolution can be obtained from Earth-based visual polarimetry, and astronomical "seeing" limits the resolution of photographic or photoelectric polarimetry. The improvement of resolution obtainable from an orbiter is more than an order of magnitude. In addition, orbiter polarimetry permits observations beyond phase angle 43°, angles which are inaccessible from the Earth. Extension of the fractional polarization-phase angle curve beyond 43° may provide a discriminant among competing candidates for Martian surface composition. Dollfus has suggested that if a complete extension of the polarization curve to 180° is for some reason impossible, observations at 90° should be a significant composition discriminant. A modest improvement in existing polarimeters would permit measure- ment not merely of intensity and linear polarization, but instead, of all four Stokes parameters. The usefulness of such a Stokes meter could be substantially increased if it were to observe at three wavelengths—for example, in the far red or near infrared, where the contrast between the bright and dark areas is high and the effective scattering in the atmosphere is negligible; in the blue, where the Martian blue haze will dominate; and in the yellow. A polarimeter or Stokes meter is an especially attractive instrument for a Mars orbiter because: it is light-weight; conservative in data rate transmission; performs measurements which can, uniquely, be performed only in the vicinity of Mars and only from an orbiter; performs measure- ments on the physical environment; and is capable of detecting very small

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276 APPROACHES AND REMOTE OBSERVATIONS hypothetical organisms themselves. A determination of particular areas on Mars that undergo especially striking seasonal polarization changes might be important in site selection for eventual landing missions. Infrared Radiometry An infrared radiometry observing in the 8-13/x region is capable of de- termining surface temperatures. Longer wavelengths (around 20/t) are better for seeking local temperature anomalies on the night side of Mars. At present, nighttime temperatures can be inferred only from tempera- ture-time curves for the illuminated hemisphere of Mars. The nighttime temperatures provide a boundary condition on Martian organisms, since they are related to the condensation and concentration of water at the surface of the planet. An infrared bolometer in the 8-13/t or 20/x region also has the capability of detecting hot spots on Mars, if they occupy a sizable fraction of the field of view. Hot spots are significant as sites of geothermal activity and possible penetration of the hypothesized Martian permafrost layer. Such locales are presumptive loci of indigenous bio- logical activity, and are therefore relevant to site selection for eventual landing missions. These scientific returns are, however, certainly not significant enough to justify an orbiter for radiometry alone. But because of its light weight and parsimonious data transmission requirement, such a device should nevertheless be considered, especially if the orbiter pay- load is small. It is also possible, by observations of the same area of the planet at different local times of day, that detailed information of high topographical resolution can be obtained on the thermal properties of the Martian sur- face. Similar information can also be obtained by microwave observa- tions. Infrared Spectroscopy Infrared spectroscopy is potentially an extremely useful technique for biologically significant observations from a Mars orbiter. The following observations can, in principle, be obtained: (a) An improvement of at least an order of magnitude in our knowledge of the topographical distribution of the Sinton bands, if they are a bona fide Martian feature. (b) Determination of possible seasonal or secular variation in the in- tensity or wavelength distribution of such bands, again with high topo- graphical resolution. Seasonal variations in the intensity of the Sinton bands correlated with the wave of darkening would provide much stronger evidence for life on Mars than we have at the present time.

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Potential Yields of Remote Investigations of Mars 111 (c) Determination of other organic bands in the same (3.4-3.7^) wave- length region. Earth-based observations of the Sinton bands, because of their low topographical resolution, are actually an integration of contribu- tions from a large area of the Martian disk. Therefore, relatively under- abundant organic molecules will have their spectral features diluted. High topographical resolution may permit such underabundant molecular species to be observed. (d) Observations with high topographical resolution—for example, at 2.7/u or at 6.2jj.—of the distribution of atmospheric water vapor over the disk of Mars. One of the most attractive explanations of the wave of darkening is as the response of Martian organisms to a discontinuous in- crease in the availability of moisture in the Martian atmosphere. While there are plausible arguments for this, it is, at the present time, not known whether the wave of darkening in detail follows the front of the wave of water vapor suspected to be traveling from the vaporizing to the re-forming polar ice cap. (e) A search for organic functional groups other than methyls, methyl- enes, or aldehydes, by observing in the 4-6/i region at the dawn limb of the planet. Here, the temperatures are still so low that reflection domi- nates emission, and surface organic functional groups such as NH2 and C = O can be detected, if their abundance is comparable to that of CH. It will be important to determine the detailed topographical distribution and possible seasonal variations of these bands, to determine, for example, whether the organic matter on Mars is uniformly distributed over the disk, or whether some functional groups become more abundant at cer- tain times and places. (f) A search for other atmospheric gases of biological significance. Methane and carbon monoxide are photodecomposition products of acetaldehyde. Methane and nitrous oxide are metabolic products of terrestrial microorganisms, and their identification, localization, and sea- sonal variation may be relevant to Martian biological activities. It is clear from the preceding characterization that infrared spectro- scopy holds great potential for orbiter observations of Mars. Several types of "spectrometers" are conceivable. These include interferometers, multiplexing with multiple filter and multiple detector arrays, wedges, and conventional grating and prism spectrometers. A generally ideal wave- length range is 1.0 to 7.0/i. A more narrowly circumscribed but still useful range is 2.4 to 3.8^. The spectroscopic resolution required is about 0.04 or 0.05/i at 3.5/i, and may deteriorate to about O.I/i at 5/i. If a character- istic orbital velocity is 2 km per second, and spectra of areas 100 km on a side are desired, scan times <50 seconds are acceptable. Since the abundances of rare substances are being sought, the device should be

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278 APPROACHES AND REMOTE OBSERVATIONS capable of detecting one per cent depressions in the continuum. Since seasonal variations of absorption features are being sought, reliable cali- bration over a three- to six-month operating lifetime must be obtained. It is possible that the desert areas, whose spectral characteristics are ex- pected to change little during the Martian wave of darkening, may provide one natural calibration source. For a reliable identification of such gases as acetaldehyde and water vapor, it is recommended that selective-absorp- tion gas cell filter calibrations be considered, first from Earth-based ex- periments, and then possibly from Mars orbiters. It is important that investigations be performed on the long-term re- liability of detectors and other spectrometer components in simulated Mars trajectories and Mars orbits. It also must be determined whether high signal-to-noise ratios can be obtained by radiation cooling in Mars orbit. An investigation of the comparative merits of various dispersing systems is necessary. Because of the considerable importance of such a system, and because of its attendant complexity, the development of significant redundancy and "fail-safe" capability is recommended. Searches for molecular oxygen on Mars can be made by observations of such forbidden transitions as those at 0.75 and 1.28/i. Since the lines are weak, the observed radiation will arise close to the surface of the planet, permitting a determination of total oxygen abundance not con- veniently obtained at ultraviolet frequencies. Such observations of Mars can also usefully be performed from the vicinity of the Earth. Active and Passive Microwave Radiometry and Polarimetry Passive observations at several microwave wavelengths are capable of determining with high topographical resolution (a) the brightness tem- perature at several different levels beneath the Martian surface. By determining the variation in intensity of the two plane-polarized compo- nents as a function of angle of incidence at a fixed wavelength, the emis- sivity can be determined, thereby giving (b) the true thermometric tem- peratures at these depths. The polarization also gives some index of (c) the surface roughness and its seasonal variation, and, through the dielec- tric constant, (d) the surface composition. Seasonal variations in surface roughness may be evidence of biological activity. Because of the inclemency of the Martian surface environment, a de- tailed characterization of the immediate Martian subsurface environment down tp a depth of about one meter is of biological relevance. Measure- ments of the temperature on the dark side would permit a direct determi- nation of that parameter for the first time, and in addition, such a survey may uncover hot spots of possible biological significance. The topo-

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Potential Yields of Remote Investigations of Mars 279 graphical resolution obtainable from the orbiter greatly exceeds that which can be obtained from the Earth. Active radar observations might be thought useful in searching for the suspected subsurface permafrost layer; however, since the dielectric con- stant of ice is comparable to that of most common rocks, radar does not appear to be a promising technique for the detection of Martian perma- frost. Radar altimetry could be useful in characterizing the topography of the Martian surface. The potential returns from active radar seem to be so small, considering the very large power and weight requirements, that we do not recommend that radar be flown on early Mars orbiter missions.* On the other hand, the wide range of possible information obtainable from a passive micro- wave device makes it a promising instrument. A relatively large antenna is necessary, of aperture about 10 feet, but its weight can be small. The * Prof. V. R. Eshleman has suggested, in the following remarks, that bistatic radar may be a much more useful instrument for planetary exploration: ". . . However, even more meaningful radar studies of the surface could be done with less weight than a microwave radiometer, or by use of such a radiometer, by using transmissions from the Earth. That is, even if the weight, power, and space were available for an on-board radar system, I believe that it should still receive very low priority as compared with the bistatic radar mode involving Earth-based transmissions and reception in the orbiter. "The separation of transmitter and receiver in the bistatic mode of operation offers inherent advantages for studies which could be done neither from the ground alone nor from an orbiter alone. These include measurements of the scattering in various directions for various angles of incidence (including polarization effects), self-calibration of reflection coefficients and dispersive and Doppler frequency shifts made possible by the comparison of the direct ray from the Earth and the energy reflected from the planet, and occultation measurements of the atmosphere and ionosphere of the planet. "Bistatic radar at several wavelengths would complement and extend the meas- urements outlined for passive microwave radiometry and polarimetry. Reflectivity as a function of depth, dielectric constant (a Brewster angle measurement would be particularly sensitive), loss-tangent, and large-scale (Fresnel-zone-size) and small-scale (wavelength-size) surface irregularities could be studied from measures of signal strength, frequency and time-delay spectra, and polarization. Phase path, group path, and amplitude measurements during probe occultation would provide a refractivity profile for the atmosphere and ionosphere, and this would give im- portant information related to pressure, temperature, composition, and ionization reactions. "It appears possible that much of the radiometry system itself could be used, on a time sharing basis, for the bistatic radar receivers, and in fact, the ground- based facilities discussed above for the increase in communication capacity might also be used as the ground terminal for the bistatic radar system. Basic differences for active and passive operation of the orbiter receivers include: (a) narrower bandwidth for active work; (b) desirability of using both pulses and CW for active work; (c) possible desirability of including lower frequencies in the active work; and (d) possibility in the active mode of using delay and frequency spectrum instead of antenna directivity for topographic resolution."

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280 APPROACHES AND REMOTE OBSERVATIONS microwave equipment itself can be comparable to that used in the Mariner II mission. It should be reemphasized that since passive polarization measurements can determine some index of surface roughness, seasonal variations in surface roughness should also be detectable. Such observa- tions can conceivably be related directly to Martian biological activity in the centimeter size range. Conclusions Significant experiments relevant to biology can be performed from a Mars orbiter in the 1969 and 1971 opportunities. These experiments, for various reasons, cannot be performed from the vicinity of the Earth, or from Mars fly-bys. An orbiter has an intrinsic capability for high topo- graphical resolution over a significant fraction of the planetary surface; for observations of seasonal changes; and for measurements at large phase angles. Below are four suggested experimental packages agreed upon by the participants, in order of increasing weight, data rate requirements, and complexity. We believe that Payload 1 represents a significant set of orbiter experiments for a scientific package in the 20-pound range. Payload 4, which appears to be feasible in 1969 and 1971 only if ground- based communication facilities are improved, provides a wide variety of significant experiments, and still weighs in the 200-pound range. Payload 1: Optical polarimeter 1- or 2-line optical scanner Infrared radiometer Payload 2: Optical polarimeter 1- or 2-line optical scanner Infrared radiometer Infrared spectrometer Payload 3: Optical polarimeter 1- or 2-line optical scanner Infrared spectrometer Microwave radiometer for passive observations and for bistatic radar Payload 4: Optical polarimeter Infrared spectrometer Microwave radiometer for passive observations and for bistatic radar 106 bit-per-picture television

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Potential Yields of Remote Investigations of Mars 281 In order to take advantage of the scientific opportunities for 1969- 1971 Mars orbiters, it is recommended that immediate instrument de- velopment and early near-Earth test flights with these devices be per- formed. In addition, the following undertakings are strongly recom- mended: (1) development of large, ground-based, phased-array deep space com- munication facilities to improve substantially data transmission rates from the vicinity of Mars; (2) performance of high-angular-resolution ultraviolet spectroscopy of Mars from the vicinity of the Earth, particularly from rockets with stabilized platforms and from the Orbiting Astronomical Observatories; (3) observations of Mars with gas cell filter calibration to check the identification of the 3.69/i feature; (4) a search in the near infrared for molecular oxygen on Mars. ADDITIONAL REMARKS Thus, as we see from the report presented here in the preceding pages, the Earth satellite experimental procedure that the Mars Orbiter Work- ing Group found of greatest importance is ultraviolet spectroscopy of Mars; such an experiment is well within the capabilities of equipment designed for the Orbiting Astronomical Observatories, which have primary scientific missions in stellar and galactic astronomy. In addition to the near infrared investigations of molecular oxygen and the 3.69/i feature that were recommended, several other promising ground- based observational programs can be suggested. By observing intrinsically weak spectral lines of water vapor (so band saturation is avoided), it seems possible that changes in the precipitable water content of the Martian atmosphere of 50 per cent or more can be detected over the disk of Mars with large ground-based telescopes. In such searches for water vapor, topographical resolutions of some 20 per cent of the planet are possible. A significant improvement in signal-to-noise ratio and topographical reso- lution of the Sinton bands in the 3.5/i region can be obtained if a Michelson interferometric spectrometer and a liquid-helium-cooled detector (for ex- ample, mercury- or gold-doped germanium semiconductive detectors, or germanium bolometers) were used with the largest-aperture telescopes available. Of these three improvements in technique—large apertures, superior spectrometers, and sensitive detectors—no two in combination (much less, all three together) have been used in investigations of the Sinton bands. Using 82-inch apertures and smaller, cooled lead sulfide

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282 APPROACHES AND REMOTE OBSERVATIONS detectors, and traditional spectrometers, Kuiper has recently obtained spectra of Mars in the region short of 2/i that are an order of magnitude better than previous work. Substantial improvement in ground-based spec- troscopic techniques seems feasible, if properly encouraged. Infrared observations in the 0.7-2.2^ region are also greatly needed, to provide a firm determination of the carbon dioxide partial pressure and the total surface pressure on Mars. Ground-based observation of possible organic groups in the A > 4^ region does not seem very promising, because at these wavelengths, Mars is ob- served primarily in emission. In order to observe these wavelengths in reflected sunlight, observations must be made close to the morning limb, and this requires high topographical resolution, which is not available from the ground. Since one of the most exciting of possible remote observations of Mars relevant to biology would be a seasonal variation in the intensity and wave- length distribution of the Sinton bands, if indigenous to Mars, a 3.3-3.8^ synoptic patrol of Mars is desirable. Since, however, the angular diameter of Mars is very small at phase angles far from opposition, such a patrol requires an optimum combination of aperture, spectrometer and detector. There seems little prospect of obtaining large amounts of major observa- tory time for this project, because the majority of astronomers are occu- pied with other endeavors. The desirability of related observations, how- ever, may provide some impetus for the construction of a moderately large planetary observatory. Ground-based radar observations of Mars have already provided a crude map of the Martian surface, and have indicated the presence of reflectivity anomalies. Future higher-resolution studies promise significant new knowledge. It is not impossible that indigenous biological activity can change the surface texture of Mars. It is possible to detect not only the conformation of surface texture, but also changes in this conformation, by microwave interferometric observations from the Earth. Conceivably, it may be possible to detect growth of Martian plants by ground-based radio astronomy, as Frank Drake has suggested. Thus a number of experiments naturally suggest themselves for each observational platform: observations in the infrared and microwave do- mains from the ground, infrared spectroscopic and optical photographic observations from balloons, ultraviolet spectrometric observations from Earth satellites, and a wide range of observations from fly-bys, and espe- cially, orbiters, as discussed in the above report.