30 July 2015

Clyde Tombaugh's Vision of Mars (1959)

Elysium and Tombaugh crater are located near the center of this image from India's Mars Orbiter spacecraft. Image credit: Indian Space Research Organization
Clyde William Tombaugh (1906-1997) was born in Streator, Illinois, and grew up in Burdett, Kansas, where he built his first telescopes. In 1929, Tombaugh joined the staff of Lowell Observatory in Flagstaff, Arizona, to hunt for Planet X, a world which Boston businessman Percival Lowell had predicted should exist beyond Neptune. On 18 February 1930, 24-year-old Tombaugh discovered Pluto.

A little over two weeks ago, on 14 July 2015, the New Horizons spacecraft raced through the Pluto system. The piano-sized probe bore some of Tombaugh's ashes. The New Horizons scientists applied the unofficial name Tombaugh Regio to a prominent surface feature on Pluto.

It is possible that Tombaugh Regio will not win through the formal planetary feature naming process, in part because a feature named for Tombaugh already exists: a crater on Mars, the world to which he devoted far more professional attention than he did Pluto. Tombaugh crater is located at 162° East, just north of the martian equator in Elysium Planitia, a region that Tombaugh believed was important for understanding the surface structure of the Red Planet.

Clyde Tombaugh with a telescope of his own making. Image credit: Wikipedia
Although Pluto became Lowell Observatory's greatest claim to fame, Percival Lowell had founded his observatory in 1894 to find proof of intelligent life on Mars. He had theorized that the aging planet was slowly losing its water, and that the dark lines some astronomers glimpsed on its ochre disk were canals its inhabitants had excavated to distribute melt water from the polar ice caps and stave off encroaching deserts.

Lowell believed that spots strung like beads along the lines were oases, and that irregular dark-colored areas (maria) scattered over the surface were desiccated sea beds. Though rejected by most astronomers (including Tombaugh), Lowell's romantic vision helped to inspire H. G. Wells's novel The War of the Worlds (1898) and the "Barsoom" books of Edgar Rice Burroughs. These tales in turn inspired generations of rocketeers and skywatchers.

In the January 1959 issue of Astronautics, the journal of the American Rocket Society, Tombaugh summarized the prevailing view of Mars surface conditions on the eve of its exploration by spacecraft. He first described three areas where improved data had undermined Lowell's romantic vision.

The first was temperature. Depending on its position in its elliptical orbit around the Sun, Mars receives between 53% and 36% as much solar energy as Earth. Astronomers using telescopes equipped with thermocouples had determined that the temperature on the surface at noon normally barely surpassed the freezing point of water, though it could reach 70° Fahrenheit at noon in the southern hemisphere in summer. Tombaugh added that the temperature regularly swings 200° Fahrenheit from frigid midnight to chilly noon over much of the planet.

Low atmospheric pressure also created problems for Lowell's Mars. Evidence was mounting, Tombaugh wrote, that at its surface Mars had an atmospheric pressure only 10% as great as Earth's sea-level pressure. Enough carbon dioxide was known to exist in the martian atmosphere to give the planet an atmospheric pressure about 1% of Earth's. Many planetary astronomers, Tombaugh added, believed that nitrogen made up the remaining nine-tenths of the martian atmosphere, though none had been detected.

Finally, Mars's surface was likely to be subjected to unhealthy levels of radiation. Planetary astronomers had found no evidence of oxygen in the martian atmosphere, Tombaugh reported. Whatever oxygen Mars had was probably locked up chemically in its crust, giving the planet its characteristic rusty color. Lack of free oxygen meant that Mars would also lack atmospheric ozone, which on Earth creates a shield against solar ultraviolet (UV) radiation. This meant that sterilizing UV radiation from the Sun would reach Mars's surface largely unfiltered.

Tombaugh argued that the dark maria could not be sea bottoms; they would be salt-encrusted if they were, so would appear bright white, not dark. Mars, he added, showed no signs of "a visible dendritic [branching] drainage system" akin to Earth's rivers, so was probably extremely arid. He noted seasonal changes in the maria's color which he attributed to plant life. As the polar cap evaporated in springtime, he wrote, atmospheric moisture would move toward the equator. The martian vegetation in the spring/summer hemisphere would absorb the moisture and change hue.

Tombaugh contended that martian plants had evolved novel ways of withstanding the planet’s cruel conditions. He recounted telescopic observations he made during Mars' close approach to Earth in 1954.
Normally the southern maria range from green to blue in color. The long dark sash, Sabaeus Sinus, running east to west only a few degrees south of the equator, is habitually bluish-green. Amazingly. . .this marking. . .altogether some 2000 miles long. . .suddenly turned to bright lavender or perhaps magenta! The other maria did not. Why? Can vegetation inhabiting this area shield itself by changing pigment to reflect away a sudden influx of lethal radiation?
Sometimes, Tombaugh reported, Mars's cruel conditions could spell catastrophe for even the toughest martian vegetation. He wrote that Syrtis Major,
the principal dark marking on Mars, undergoes some very strange metamorphoses in color. The north half is habitually of a deep blue color, while the southern half is grey-green to blue-green or sometimes a vivid green. I remember. . .when the whole marking became intensely black - totally devoid of color! In the absence of oxygen, dead vegetal matter would not yield to oxidation and decay. Were we seeing dead vegetal matter when the Syrtis turned black?
Tombaugh assured Astronautics readers that he did not believe in Lowell's intelligent Martians, though he hastened to add that he had "seen over 100 of the controversial canals too well, with telescopes of great effective power" to be able to "dismiss them as unreal." He offered an explanation for the planet's linear features that was first advanced by former Lowell collaborator (later Lowell rival) William Pickering in 1904.
Over the ages, Mars must have been hit by many asteroids. Such dreadful collisions must have produced some visible marks. . .Collisions with asteroids a few miles in diameter going at velocities of the order of 15 [miles per second] might well fracture a planet to the bottom of the crust and to radial distances of hundreds or even a few thousand miles. . .Where a fracture line met the surface, a long narrow strip of shattered rock would be produced, and would offer some haven to a hardy form of vegetation . . . [The plants growing in the fracture strip would] make a dark contrast against a light. . .terrain.
Tombaugh theorized that the dark spots Lowell thought were oases are actually asteroid impact craters. The canals, he asserted, divided the planet's entire crust into a "tetrahedron" pattern. As Mars cooled internally and shrank, some faces of the martian tetrahedron slumped. Tombaugh differed from the majority opinion of his time when he argued that other faces had risen to form high plateaus. Many of his contemporaries confidently asserted that Mars lacked any raised landforms. The northern-hemisphere region of Elysium, Tombaugh added, was probably the highest land on the planet. He explained that it
is sharply pentagonal in shape, [and] bounded by five long canals. . .The corners of the pentagon extend 600 geographical miles from the center. During most of the Martian year, Elysium appears much the same as the surrounding desert. By midsummer of the northern hemisphere, this area becomes white with frost except around noon. . .the whitening develops over the entire area, but always stops abruptly at the edges of the pentagon. One is forced to conclude that the five sides represent enormous vertical escarpments - and just where we should expect to see them - along the canals.
After five decades of Mars exploration by robot spacecraft - the first was Mariner IV, which flew past the planet on 14 July 1965, 50 years to the day before the New Horizons Pluto flyby - we know that Elysium is indeed an uplifted region, though not the highest on Mars. That honor goes to the massive Tharsis Plateau, upon which stand the planet's great shield volcanoes. The tallest of these, Olympus Mons, stands some 27 kilometers above the base datum, the martian equivalent of Earth's sea level.

Topographic map of eastern Elysium Planitia showing Tombaugh crater. The image is based on data from  the Mars Orbiter Laser Altimeter (MOLA) carried on the Mars Global Surveyor spacecraft. Low areas are blue and high areas are yellow and red. Image credit: MOLA Science Team/NASA 
We know today that, when Tombaugh's contemporaries detected martian atmospheric carbon dioxide, they had found not a minor atmospheric constituent, but instead virtually Mars's entire atmosphere. We have learned that branching channels are common on Mars, though at a scale invisible to Earth-based telescopes, and that Lowell's canals were products of eyestrain, the mind's tendency to impose patterns on random arrangements of objects, and wishful thinking.

We know also that the dark areas on Mars are mostly sand made from weathering and erosion of volcanic rocks, and that seasonal changes in their color and extent result from obscuring dust storms. We have found cracks in the martian crust, though those associated with asteroid impact craters are only local in extent. The best-known crustal fracture, the 3000-mile-long Valles Marineris canyon system, probably formed through internal stresses associated with the uplift of Tharsis. We know that the planet's overall shape has a pattern, though not one as intricate as Tombaugh's tetrahedron. Rather, Mars has southern highlands and northern lowlands (the latter at least partly underlain by ice, lending credence to the theory that it was once an ocean bottom).

In spite of our improved knowledge, key questions about Mars remain unanswered. We do not know, for example, whether it hosts living organisms. The pitch for piloted Mars exploration which concluded Tombaugh's paper thus remains relevant today.
[W]hy should we be interested in making a trip to Mars?. . .A manned landing on Mars would be a momentous achievement for the human race. It would be a field day for the geologist, biologist, astrophysicist, and meteorologist. They would glean knowledge on the consequences of a set of physical conditions foreign to us. . .Most important, to see at first hand what Nature has done with a world so marginal for life would be of the greatest philosophic and religious value, in helping us to understand our place and our purpose in the Universe.
Astronauts seek signs of past life on Mars. Image credit: NASA/Pat Rawlings
Sources

“Mars - A World for Exploration,” Clyde W. Tombaugh, Astronautics, January 1959, pp. 30-31, 86-93.

Mars and Its Canals, Percival Lowell, The MacMillan Company, 1906.

More Information

Pluto, Doorway to the Stars (1962)

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