Towering volcanoes with nearly vertical slopes jut into the pitch black sky. Jagged mountains and dizzying cliffs dominate a dazzling yellow landscape. From the startling images of Venus released by NASA’s Jet Propulsion Laboratory, you’d think it would be a dramatic place indeed.
Forget it. A few areas of the planet have spectacular relief, but in reality most of the planet’s surface is a fairly flat rolling plain, says David Anderson of Southern Methodist University. Anderson is one of the computer scientists who take the raw data from NASA’s Venus- orbiting spacecraft Magellan--data that mostly exist in the form of zeros and ones--and transform them into what we think of as photographs.
But they’re not photos. The planet’s surface is so obscured by thick clouds that Magellan could not take pictures; it bounced radar pulses off the surface instead. By analyzing the timing and scattering of the returning signals, scientists have created remarkably detailed false-color images of the large-scale topography. The images have been intentionally exaggerated along the vertical scale, typically by a factor of 20. Volcanoes look 20 times taller than they really are, slopes 20 times steeper. And since the brain interprets tall objects as being nearby, mountains 300 miles away--which would be beyond the horizon--seem practically next door. The distortion is not simply for dramatic effect, nor are the computer scientists trying to deceive anyone. Vertical exaggeration helps geologists detect subtle features.
I love these images because they tell me things about Venus that I wouldn’t otherwise have seen, says geologist Vicki Hansen, a colleague of Anderson’s. For example, distorted images revealed shallow moats encircling previously detected volcanic features called coronae. In an unexaggerated image, you wouldn’t see that moat showing up very well, says Hansen. Your eye would be saying, ‘Is it really a dip or is it just the data making a funny little dip there?’ Now, when you look at it with vertical exaggeration, you see very clearly that there is a moat. And that turns out to be very important in terms of understanding how that feature formed.
The exaggerated images have been a boon to scientists, but Anderson wanted to create an image much closer to what Venus actually looks like. Because most of the planet has topography as bland as Nebraska’s, Anderson concentrated on the part of the planet with the most spectacular relief--Maxwell Montes, in the far northern hemisphere, where mountains rise to Himalayan altitudes. His data came from Magellan’s two main scientific instruments, the altimeter and the radar. The altimeter, which took direct measurements of Venus’s elevation every 10 to 15 miles, provided accurate readings of surface elevations over broad regions. Radar provided high-resolution images of the surface, enabling scientists to discern objects as small as a football field while providing information about the relative roughness of the terrain.
Anderson painstakingly combined the two sets of data to produce a realistic topographic map. But that left the question of color. Magellan’s radar is color-blind, and the few Soviet and American surface probes returned only minimal data before succumbing to Venus’s 850-degree temperature and the crushing pressure of its thick atmosphere. The color scheme in JPL images is based on this scant data. The surface is colored yellow because the Soviet probe Venera 13 observed that yellow sunlight filters through the clouds. The sky is jet black by default; there are no data at all on what Venus’s sky actually looks like from the surface. A black sky would be realistic if we were imaging an airless body like the moon, but it’s misleading if we’re imaging Venus, says Anderson. And no planetary scientist believes the surface of Venus is JPL yellow.
The planet’s surface is composed primarily of basalt, a dirt brown volcanic rock that’s common on Earth. We know what basalt looks like here, says Anderson. But what colors look like to the human eye is a complicated question. It has a lot to do not only with the properties of what you’re looking at but also the medium it’s being transmitted through. So we don’t really have any idea what the colors are. Anderson worked with Vicki Hansen and other geologists and geochemists to ensure that his color choices were plausible. Because no one knows what the Venusian sky looks like, he used a simple computer model that conveys the sense that the planet has thick cloud cover while not drawing the eye’s attention away from the topography.
Anderson added the final touches to his image by choosing an advantageous camera angle and creating fog in the background, although Venus’s sky is probably much hazier than Anderson’s image suggests. We decided to put in enough haze to suggest psychologically that this is something big you’re looking at, he says, but not so much that what you’re looking at is obscured. Creating this image was an ambitious undertaking that required nearly two months’ work.
Without the majestic structures of the distorted images, the serene beauty of the landscape and the true-to-life vastness of the planet’s surface become clear. But just how real is this new image?
This is not a photograph, emphasizes Anderson. In creating this image, a lot of assumptions have been made about things we don’t know. In the absence of better data, we’d say it’s substantially scientifically correct, but it’s always possible that there’s something totally unexpected. This image will have to suffice, though, until we can build Venusian probes capable of withstanding the planet’s hellish surface for more than a few hours.