Will the "wave of darkening" soon go the way of Martian canals during the Mariner 9 orbital study of the red planet?
In 1969, Mars photographs from Mariners 6 and 7 showed absolutely no traces of the canals which earlier astronomers believed to be there. This year, another long-favored concept --that a darkening wave seasonally sweeps across Mars--may be proved or disproved in the 90-day study tour of Mariner 9.
The latest Mars spacecraft of the National Aeronautics and Space Administration is due to arrive Nov. 13. Some 5,000 photographs and scientific experiments from as close as 750 miles should shed light on the darkening controversy.
Mariner project scientists themselves are divided on the issue. No Earth-bound observer denies that there is a darkening in spots, if not in a wave, on the face of Mars, but, as one prominent astronomer recently advised Mariner investigators, even these dark spots could be an optical illusion.
"The darkening may well be caused by changes in optical geometry (how we look at the planet) rather than by anything that's occurring on Mars," Dr. William Baum of Lowell Observatory, Flagstaff, Ariz., told the scientists at a recent meeting at Caltech's Jet Propulsion Laboratory. JPL is managing the Mariner Mars project for NASA.
Baum and Bradford Smith of New Mexico State University, Las Cruces, expressed doubt that there was a dark wave which sweeps from each pole toward the Martian equator in each hemisphere's spring. The view of Smith, a member of the Mariner television team, is opposed by another TV investigator, Dr. Gerard deVaucouleurs of the University of Texas.
Mariner 9 will arrive at Mars during the apparent peak of the darkening period (late spring) in the southern hemisphere. It should settle the question whether the darkening is on the surface, in the atmosphere, or is indeed, an optical illusion.
Historically, any astronomers observed the so-called wave as a gradual spread across the face of the planet. Some believed it to be vegetation.
However, since the 1969 Mariner flybys and subsequent Earth-based observations, the darkening has been observed in patches, not necessarily adjacent to each other. This has given rise to another theory that dust storms whipped by high winds cause the changes.
Whether or not Mars could have vegetation--seemingly impossible in light of 1969 Mariner photographs--dark spots might indicate the presence of ozone, Dr. Charles A. Barth, University of Colorado physicist, suggests.
Dr. Barth heads the Mariner ultraviolet spectrometer (UVS) team. The UVS instrument, one of two spectrometers aboard Mariner 9, will identify gases, even molecules, ions and atoms, in the Martian atmosphere. Barth also led the 1969 UVS group which, through Mariner 6 and 7 experiments, found the Martian atmosphere to be dominated (97 per cent) by carbon dioxide, with apparently no nitrogen, and slight traces of ozone at the south polar cap. Ozone is a gaseous form of oxygen.
The amount of Martian ozone measured in 1969 was only 1/100th of the ozone found in Earth's atmosphere, but those readings were made in quick flybys. This time the orbiting spacecraft will afford opportunity for study of two-thirds of the planet and its surrounding atmosphere.
"If we can find ozone in the equatorial regions," Barth says, "it could be an affirmative sign that some oxygen-producing plant or life form might be present."
The 1971 UVS instrument, about three times as sensitive as its 1969 counterpart, detects molecular oxygen as well as ozone. Biological activities generally produce molecular oxygen and often ozone. Final determintion of any sort of life on Mars, however, will depend on a future landed system.
Mariner's other spectrometer operates in the infrared wavelengths and is called IRIS--an acronym for infrared interferometer spectrometer. The IRIS team is led by Dr. R. A. Hanel of NASA's Goddard Space Flight Center, Greenbelt, Md.
Basically, the UVS measures gases from the surface to 600 miles above the planet, the IRIS from the surface to 60 miles, but the instruments have overlapping capabilities.
The two instruments plus the two TV cameras--wide and narrow angle--should provide much new knowledge of the Martian clouds. From Earth and 1969 Mariner photographs, investigators still are uncertain of the makeup of a huge W-shaped cloud and yellow clouds--although the latter are generally believed to be caused by dust storms.
Television experimenters, headed by Harold Masursky of the U.S. Geological Survey, Flagstaff, will devote considerable time to photographing the so-called variable features on the surface and in the atmosphere of Mars. Their primary mission, however, is to photograph and map some 70 per cent of the planet.
Mariner 9 as inserted into Mars orbit on Nov. 13, 1971. An orbit trim was performed two days later as planned to adjust the point of closest approach to Mars (periapsis) toward the middle of the view period of the 210-foot antenna at Goldstone, California. The 210 is the only station capable of receiving a high bit rate of 16,000 bits per second (bps) and centering periapsis in the view period optimizes data return.
An unexpected gravitational anomaly caused by a bulge at the equator of Mars affected the path of the spacecraft in orbit causing the time of periapsis to begin shifting earlier into the Goldstone view period.
To correct this, a second trim was performed on December 30. The trim also provided an adjustment necessitated by the dust storm that covered the entire globe making it impossible to begin mapping operations. Therefore the trim was also designed to raise the periapsis point from 862 miles (1388 kilometers) to 1025 miles (1650 kilometers) to increase the area coverage of the camera system and require fewer pictures for mapping.
Towards the end of December the dust storm began to lift and, beginning Jan. 2, a new mission plan was designed to provide the mapping objective; three 20-day mapping cycles covering 65?o South Latitude to 50?o North Latitude. At the end of this period the 210-foot dish was scheduled to support Apollo 16, and a high bit rate would not be available.
However, a slip of the launch of Apollo allows a fourth 20-day cycle. A limited operation is planned for this cycle in order to conserve attitude control gas, the limiting factor in spacecraft lifetime. A high bit rate of 8,000 bps could be maintained during this period if attitude control gas were used to turn the spacecraft to point the high gain antenna toward Earth. However, current planning is to accept a declining bit rate of 4,000 bps down to 2,000 bps without pointing. The drop in bit rates is caused by the antenna beam shifting away from Earth after end of the basic 90-day mission as the relative position of Earth and Mars changes.
An option exists, however, to maneuver the spacecraft and attain an 8,000 bps rate.
Beginning on April 2, and continuing until June 3, the spacecraft will pass behind Mars on each orbit and be occulted from the sun for up to 95 minutes. This will require operating the spacecraft on battery power during the occultation periods and recharging the battery after each occultation. The spacecraft will also be subjected to changes in temperatures.
During this period, therefore, the spacecraft will be placed in a survival mode and no science will be transmitted. Tracking and engineering data will be obtained on a limited basis.
At the end of the sun occultation period, science data will again be transmitted to Earth but on a reduced basis to conserve attitude control gas. Data playback will require maneuvering the spacecraft and will be limited to once per week. The amount of playback time, and therefore the amount of science data, will be determined by available battery power as the spacecraft will be maneuvered off the sun line to point the high-gain antenna at Earth.
It is planned to conduct a relativity experiment in August and October when the spacecraft is on the opposite side of the sun from Earth.
Mission operations will continue as long as the attitude control gas lasts. Nominal end of the mission is Nov. 13, 1972.
As of January 31, the spacecraft has made 158 orbits of Mars, transmitted 5,174 pictures to Earth, received 21,000 commands and has been operating for 246 days since launch.
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