Payoff from Venus

For Bruce Murray, professor of planetary science, and for his co- workers in the Space Photography Laboratory and at Caltech's Jet Propul- The ultraviolet markings in the atmosphere appearance as seen from an Earth-based sion Laboratory, the more than 3,000 of Venus present a dramatically different telescope (left) and from Mariner 10. close-up photographs of Venus taken by Mariner 10 in February represent the payoff on a long-shot gamble. The emphasis early in the U.S. space program was on radio probing of Venus rather than photographic reconnaissance, so it was a gamble he almost didn't have a chance to take.

The reluctance of the National Aero- nautics and Space Administration to commit a spacecraft to a photographic mission to Venus is understandable, for Venus is obscured by an atmosphere almost 100 times as dense as Earth's. In the early 1960's even the best Earth- based pictures of the planet revealed practically no detail in visible light; how- ever, faint markings were discernible when Venus was photogfiaphed in ultra- violet light. There was considerable controversy over the nature of these markings-whether they were signs of large-scale atmospheric activity, atmo- spheric waves, or just chance groupings of stratospheric clouds with no signifi- cance at all.

Murray became convinced about that same time that the markings were signifi- cant, and he tried twice to get photo- graphic experiments aboard Venus probes before finally succeeding with the 1973 Mercury-Venus flyby. The first time was in 1964 on Mariner 5, which As Mariner 10 flew by Venus at a distance the entire planet, computer-enhanced here was originally designed as a photo- of 453,000 miles, it snapped this picture of to bring out the details.

MARCH-APRIL 1974 graphic spacecraft. But NASA decided atmosphere, where the pressure is 10 to Suomi. "He just had the wrong planet." to remove the television cameras and 20 percent that of Earth. The patterns related equipment to make room for are unlike those of Earth. It is exactly The ddail seen in the ultraviolet photo- another experiment. these differences (plus a few similarities) graphs probably occurs only in the upper that excite Murray and his fellow Venus- 3 to 4 miles d Venus's 40-mile-thick Murray's second attempt came in late watchers. What will interest them even atmosphere, and there is no direct way 1967, when NASA was considering more will be finding out why the fea- of telling from photographs what is redesigning a Mariner-Mars spacecraft tures show up only in ultraviolet light; going on underneath. However, it seems for a flight to Mercury via Venus in nothing but a haze appears under unlikely that there are any Earth-like 1970. But this mission had to compete normal light. storms in the Venusian atmosphere. for funding with the 1971 Mars orbiter and was eventually passed over. One member of the television team, Instead of towering clouds that hover Verner E. Suomi, a meteorologist from over the surface, Venus seems to be Finally, the current Mercury-Venus the University of Wisconsin and a visit- blanketed with smooth cloud layers flyby was approved, and Murray, ing professor of atmospheric sciences covered with haze. Features seen from as head of the television experiment at Caltech, has speculated that the clouds Earth as bands, or streaks, are revealed team, talked NASA into altering the are formed from streams of hot air that by Mariner 10's ultraviolet television cameras to photograph in the invisible spiral out from the equator, move in cameras to have the wispy, elongated ultraviolet region of the spectrum on flat layers toward the planet's north and quality of terrestrial cirrus clouds. the chance that the faint Venusian cloud south poles, and then sink to the surface. They probably circulate above the planet markings were a significant and relatively Suomi feels that, oddly enough, these at about 200 miles an hour and in the stable phenomenon. Furthermore, he cloud patterns resemble those of a theory same direction as the planet but much persuaded them to make changes in the of atmospheric circulation first sug- faster. spacecraft radio system so that gested in 1735 by the British meteor- thousands, not hundreds, of pictures ologist George Hadky to explain The results of an infrared radiometer could be returned, resulting in recording phenomena observed on Earth. How- experiment designed and built by Guido the time variation of the atmosphere. ever, Earth rotates too fast to produce Miinch, professor of astronomy, confirm the simple cloud patterns he predicted, that the massive Venusian atmosphere Murray's expectations as to the but Venus, which rotates just about stores so much heat that in spite of its scientific importance of the markings once every 243 days, demonstrates long night ( 124 days) its dark side is seem to be verified. The markings are Hadley's ideas to perfection. essentially as hot as the side facing the stable and recurring and may be caused Sun. The experiment also reveals that by clouds moving high in Venus's "Hadley's theory was correct," says the difference in cloud-top temperature

Thls series of mosalcs shows the rotailon of the ultrav~oletmark~ngs c~rclethe planet every four days, even though the planet ~tselfrotates that appear In the Venus~anatmosphere (arrows). These marklngs only once every 243 days.

ENGINEERING AND SCIENCE Payoff from Venus Research . . . continued Notes

between the equ'ator and the poles is Earthquake Side Effects small compared to that on Earth. The days are slightly longer during or one that causes a large tsunami or Other experiments aboard Mariner 10 periods of intense earthquake activity seismic sea wave.) In addition to the indicate that planetary Venus is more around the world. Also, during these giant earthquakes, the general level of nearly round than the Earth, the Moon, periods, the earth's poles wander faster. seismic activity was also greater during or Mars; has an electrically charged those periods. upper atmosphere but practically no Don Anderson, professor of geophysics magnetic field; and is possessed of a and director of Caltech's Seismological During thew seismically active periods comet-like tail of protons and electrons Laboratory, made this surprising cor- the earth's rotation rate was abnormally that extends away from the Sun. relation after studying three periods in slow, meaning that the days were history when giant earthquakes were slightly longer. During the 1896-19 11 Mariner 10 has also radioed back prevalent-1835 to 1847, 1896 to 191 1, period the length of the day increased measurements that confirm the presence and 1933 to 1942. (A giant earthquake by about eight milliseconds. The rate is of hydrogen in the Venusian atmosphere. is one of magnitude 8.5 or greater- known back for many centuries from Some of that element is locked into astronomical studies of the motions of sulfuric acid and water molecules in the the stars, sun, moon, and planets. clouds, but its presence as a gas in greater-than-predicted amounts may It is not yet clear whether the earth- challenge one theory that the hydrogen quakes slow the earth's rotation rate, or was released by comets colliding with whether the slowdown activates great the surface of the planet. Another theory earthquakes. But earthquakes are only suggests that the hydrogen was swept part of the story. The much larger move- into the atmosphere by the solar wind- ments involving the shifting of mass particles "blowing" out from the sun. near the earth's surface are probably It also seems clear that this hydrogen responsible for both the earthquakes prevents solar radiation from breaking and changes in the rotation rate. This down the carbon dioxide atmosphere of motion is provided by the activities of Venus into a more Earth-like one. the huge tectonic plates that make up the lithosphere, the outer 45 miles of Important as it is, the information the earth. transmitted from Mariner 10 in its brief encounter with Venus is only a minor Most geologists believe that the earth's by-product of the mission's prime target entire surface down to a depth of 45 -Mercury. The innermost planet of miks is made up of a mosaic of the solar system, Mercury is not much massive plates that move in relation larger than the Moon, yet it is denser to each other, riding and sliding on a than Earth. Because of its position in layer of plastic rock about 80 miles relation to the Sun, it cannot be thick. The largest of these is the observed in detail by Earth-based tele- Pacific Plate, which extends from Japan scopes; in faot, the 750-km-resolution to the west coast of the United States (about 500 miles) observed from Earth Don Anderson and from Alaska southward many is worse than that of the Moon seen by thousands of miles. the naked eye. Mariner 10 is scheduled to execute a week-long picture-taking The Pacific Plate is moving faster than mission in which the spacecraft and its the others and seems to dominate the cameras will zoom in closer and closer earth-rotation effect among the plates. to Mercury, until the final pictures are It is diving under the Aleutian Islands at a resolution of 0.1 km (about 350 and Japan md is scraping past Cali- feet). That is just as good as the best fornia along the San Andreas Fault. Mariner 9 photos of Mars. A piece of the Pacific crust is'diving