|Microscopic Imager with lens at left and frame for MI electronics at right. |
When the Mars Exploration Rovers, Spirit and Opportunity, land on Mars in January 2004, geologists will want a close look at the rocks and soil the rovers encounter. Faced with an interesting rock on Earth, a field geologist might reach into a pocket and pull out a magnifying glass. On Mars, they'll use a custom-built close-up camera called the Microscopic Imager.
"It mimics the view a geologist would have through a hand lens -- some people call it a loupe -- that geologists often use to look at rocks and soils up close," says Ken Herkenhoff, a USGS astrogeologist in Flagstaff. "A lot of us have experience with geologic field work, and the MER mission is our first chance to really do field geology on Mars, so it was thought it would be useful to have a hand lens on the rover."
The Microscopic Imager includes a camera body identical to the others on the rover, similar to a digital camera with a one-megapixel CCD, the digital equivalent of film. The close-up lens and its placement on the end of the rover's robotic arm differentiate the Microscopic Imager from other rover cameras.
"This is the highest-resolution camera that NASA has flown to the surface of Mars," Herkenhoff says. The Microscopic Imager will make useful pictures of objects as small as 90 to 100 micrometers. Sand grains will show up sharply, but grains of dust, at one or two micrometers, will appear as a blur. (The Beagle 2 lander, slated to land on Mars in late December 2003, will carry a close-up camera with much higher resolution, but still not sharp enough to show individual dust grains.)
|The IDD enables scientists to place a suite of four instruments at precisely the right angle against a rock to work as a human geologist would: exposing a fresh rock surface, taking microscopic images, and analyzing the composition of rocks and soil |
Close-up images will help Earth-bound geologists determine the nature of various rocks the rovers encounter, including sedimentary rocks that may have formed in standing water. Rocks formed by volcanic activity often include more than one kind of mineral, and the Microscopic Imager can show those features.
Point 'n' Shoot
The Imager itself cannot move or focus, and takes only grayscale pictures, detecting the same range of wavelengths as the human eye sees. By using the yellowish transparent lens cap as a filter, geologists will be able to derive some color information from Imager photographs.
"It's not really a color camera," Herkenhoff says. "But we can get crude color by closing the dust cover."
However, the robotic arm, or Instrument Deployment Device (IDD), can move the camera precisely, and that capability opens up a wide range of possibilities.
Mars Close Up
|The RAT is a powerful grinding tool that can create a hole 45 millimeters in diameter and up to 5 millimeters deep into a rock on the Martian surface. |
One difficulty of all close-up photography is that the plane that's sharply in focus, called the depth of field, is very slim. Move the camera a few millimeters and the insect or rock you're shooting will be just an out-of-focus blur.
The Microscopic Imager includes a probe to let the arm's computer controls know when the lens is nearly in contact with a surface. Then the arm can pull back from the surface until the object-to-camera distance is ideal for sharp focus.
"The depth of field of the camera is roughly plus or minus three millimeters," Herkenhoff says. "And the accuracy of positioning of the arm is good enough to get us within that depth of field."
But that slim depth of field might only include part of a three-dimensional or slanted rock surface. Here's where the IDD comes to the rescue. The rover's robotic arm can move the camera in tiny, precise steps, the Imager taking a photo at each step.
"We need to take a number of images of a rough target to make sure all parts of that rough surface are in good focus," Herkenhoff says.
"We are now working on software, in collaboration with NASA Ames Research Center, to merge those images into a single well-focused frame," Herkenhoff says. The software detects sharply focused parts of an image-like the way a consumer digital camera performs autofocusing-then combines these "optical slices" into a single image with an apparently large depth of field.
Geologists can also instruct the IDD to move the camera from side to side, taking stereo pictures that the science team can combine to produce an even more precise three-dimensional view.
One important use of the Microscopic Imager will be to take "before and after" pictures of rock and soil surfaces. For example, scientists will want several images of any surface on which they plan to use the Rock Abrasion Tool (RAT). The RAT scrapes the surface to reveal fresh material beneath any weathered or dusty coat.
"That's the plan for a surface that we plan to abrade. We would take pictures before and after. In that way we can see if there's some kind of weathering rind or patina on the rock that we've abraded away," Herkenhoff says.
Herkenhoff and his colleagues hope to begin working with images made by the Microscopic Imager in January 2004.
Written by Stephen Hart
NASA's Astrobiology Magazine