Difference in sky color in Spirit's first panoramic images, where frames show different levels of darkness, depending on the weather when each frame was taken (light dust conditions on the left, heavy dust on the right). Browse Image

When you look across the dusty, rock-strewn landscape of Mars, gazing toward the eastern hills on the horizon, it may seem as though Mars is right outside your window. With a stretch of the imagination, you could almost step outside, pick up a rock, and examine it for yourself. Scientists and engineers have worked hard to provide us all with this human-scale view, because Spirit sees things much differently from her robot perspective.

"Spirit doesn't see the whole vista around her all at once," says Jim Bell, Professor of Astronomy at Cornell and Lead Scientist for the Panoramic Camera (Pancam) Team. "It takes days for the rover to complete a full 360-degree panorama of the surrounding landscape."

In an exacting sequence, the rover takes smaller pictures that are later "stitched" together into one larger view, or "mosaic." That means that a single mosaic may be built from images taken when there's more dust in the air or at a different times of day. Such changes in environmental conditions and light levels result in a patchwork quilt of varying lights and darks, and ultimately, oranges, butterscotches, and browns.

	Dr. Jim Bell Lead Scientist for the Panoramic Camera (Pancam) Team

Getting the Colors Right Combines Art with Science

"Getting the colors right is not an exact science," says Bell. "Giving an approximate view of what we'd see if we were there involves an artistic, visionary element as well – after all, no one's ever been there before." However, great pains are taken to be as accurate as possible, short of going there ourselves.

To give people a sense of being on Mars, scientists combine views through telescopes, data from past Mars missions, and new information from the current mission to create a color-balanced, uniform scene. Color-corrected mosaics simulate the view a person would see if all the images in the mosaic were taken on the same day, at the same moment.

Dr. Eric De Jong, Lead for the Solar System Visualization Team

In addition, the rovers can take three pictures in a row of the same surface area on Mars using three different primary color filters – red, green, and blue – to make one color image. "It works a little like an inkjet color printer, which combines primary colors to create various shades on paper," explains Eric De Jong, Lead for the Solar System Visualization Team at the Jet Propulsion Laboratory. "Then, we can tweak the color just like you can adjust the color balance on a TV screen at home."

So far, however, the images produced are only approximate martian colors. That's because many of the pictures are taken with set of camera filters that include near-infrared or ultraviolet wavelengths, which our eyes do not perceive. Overall, there are 14 "geology" filters (two additional camera filters were designed exclusively to observe the sun). Scientists find these geology filters extremely useful because they provide maximum contrast for analyzing some of the most interesting geological features at the landing site.

"We almost never choose to take the images in natural color, because that's not as helpful to us scientifically," said Eric. "However, we're able to approximate what humans might see because Jim's team lived and breathed with this camera for many years, experimenting to get the colors in the camera models just right."

	Using the calibration target on each rover, scientists have a good knowledge of how to adjust colors according to the circumstances found by the rover cameras on Mars. Scientists saved half of the material for the silicon colored chips on the calibration targets, so they can make comparisons and accurately measure how the rings and chips on Earth are reflecting light on Mars. Large Image

How accurate are the final colors

Before sending cameras to Mars, the team took more than 100,000 pictures in a vacuum chamber on Earth that simulates martian conditions. They experimented with different levels of light entering the lens, depending on the angle of sun or amount of dust in the air. They then fine-tuned the cameras to respond correctly to temperature changes or anything else that might cause the instrument to vary.

"It's crazy how many pictures we took with the Pancam on Earth, but those images were essential to finding the answers on Mars," Jim recounted. Using a "calibration target" with green, red, blue, and yellow silicon swatches and rings of varying shades of gray, the team monitored how those colors changed under different lighting and environmental conditions in the laboratory.

Each rover carries its own calibration target, and is regularly instructed to take pictures of it. With their earth-bound experience, scientists have a good knowledge of how to adjust colors according to the circumstances found by the rover cameras on Mars. They know how much sunlight the three rings of gray, black, and white reflect on the calibration target in the different filters. They also saved half of the material for the silicon colored chips on the calibration targets, so they can make comparisons and accurately measure how the rings and chips are reflecting light on Mars.

Through all of these color corrections, the unprocessed raw images are not affected, and no science is compromised. In fact, there are many interesting things that can be learned about the surface and atmosphere by studying how their colors change during or between each day.