Two Weeks on Mars
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The Problem of perception

As we watched and drew the planet, we posed the question: how did our ancient visual system -- the product of tens and even hundreds of millions of years of evolutionary tinkering, going back to the reptiles - manage with the visualization of a distant planet for whose imaging it was not specifically designed? How would the human eye, brain and hand -- designed for seeing in our terrestrial world -- cope with the challenge of Martian seeing?

In tackling this problem we were giving a Martian face to the age-old problem of perception: how do we know what we know? An eminent neurophysiologist has remarked (Vernon B. Mountcastle, "Brain Science at the Century's Ebb," Daedalus, Spring 1998):

"An ancient problem in natural philosophy and a major research program in present-day neuroscience is to determine the relations between the material order of the world around us and the sensory-perceptual order of our experience. . . . The primary features of the stimuli that impinge upon us - the "distal" stimuli of heat, force, light, sound, and chemical substances - are selectively transduced at the peripheral ends of sensory nerve fibers. Some groups of fibers respond more selectively at lower thresholds than do others to different forms of impinging energy. This tuning [is] sometimes called feature detection [e.g., line-detectors]. Our perceptual experiences are . . . mediate, once-removed, slightly delayed in time, abstracted images determined by the transducing properties of the receptors and the processing properties of the central neural networks they engage."

The "distal" stimulus for us - sunlight reflected from Mars and reaching us after traversing 35 million miles of void - had not changed much from Barnard's and Lowell's time. Also, we had no reason to believe our receptors were any more acute than theirs. But the processing properties of the central neural networks - our brains - were no doubt, and unavoidably, much changed. We could not, in 2003, return to the state of innocence of observers of Mars in 1894, even if we had wanted to. We knew about the "real" features on the surface: the majestic basins, the volcanic plains like Syrtis Major which exposed surfaces of basaltic rock, the splotches of small craters serving as traps for coarser dust scoured off of lava fields, the grand canyon of Mars, the towering shield volcanoes, all of which topography is swathed in a mantling of fine dust, whose distribution is in turn dictated mainly by the direction the prevailing winds blow. The patterns of the surface can appear markedly different depending on the resolution with which they are being studied. Also, as the shifting winds move the dust around, they increase or decrease the contrast.

In the Wratten 25 filter, the contrast of the markings in the Great Refractor was such that the planet looked very much like a huge Harvest or Hunter's Moon. The maria on Mars resembled those of the Moon as seen with the naked-eye. However, instead of the stark circular basins and craters of the lunar landscape, the main Martian leitmotifs consisted of something softer -- triangular patches and gracefully curving wisps and winding streaks - all characteristic of the directions in which the wind has blown surficial materials on the planet. The distinctive planetary physiognomies of the Moon and Mars attest to a difference in physical conditions on these worlds. The Moon is a study in charcoal; Mars requires the use of a soft pencil and a free hand in order to follow its graceful curves and softer, more nuanced attitudes. To describe it in a word is by no means easy, but if one were to choose a word it would be: windblown, swept, tossed, tousled.

The classic observers who studied this nuanced and subtle and gorgeously beautiful but heartrendingly difficult planet pronounced differently on what they saw. One problem of visualizing the planets is "seeing," a blurring of the atmosphere except for short intervals during which the planet's disk suddenly sharpens and one attempts to recall what was revealed and record it in a sketch. Under such conditions, one does not represent the planet as it looks at any one time; instead one builds the impression, rather like the modern videographer's Fourier transform software which selects the best images frame by frame before combining them. R.L. Gregory, the Cambridge psychologist, has written: "drawings do not represent how the object appears through a telescope at any one time. They are a synthesis of very many observations.... They represent a belief in what it is really like."

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