In The Field

Phoenix landing: Part of the day IV

ssi.jpg Right now the activities of the Surface Stereo Imager, or SSI, seem a bit mundane — documentary pictures of the robotic arm, bland portraits of dust-free solar arrays — but make no mistake: The SSI is like an orchestra’s conductor, integral to the sweet scientific music that the Phoenix team hopes to make with the other instruments.

The key is that the SSI has two eyes, which give it depth perception. As it takes its panoramic shots, it is also building an elevation model. The model has already allowed mission scientists to spot several polygons, and has shown that one of the polygonal troughs sits within digging reach.

Knowing the precise 3-D terrain also will allow the robotic arm to move with confidence and efficiency. If the arm were to rely on its own single-eyed vision (there’s a camera just above the scoop), it would use shadows for depth perception and move slowly and tentatively, wary of ramming into something.

“The arm is kind of dumb,” says Phoenix PI Peter Smith. “It has to be told where to go.” Adds Mark Lemmon, lead scientist for the SSI: “It lets us know our coordinates easily rather than painfully.”

But the SSI won’t be totally left out of the science. It’s already taking daily pictures of the telltale on the weather mast to gauge windspeeds. And Lemmon told me about two other exciting long-term experiments.

One will involve watching ice melt sublime. After using the scoop to dig a double-wide trench, a swath of ice will be exposed to the summer sun. The SSI will sit back and watch the surface recede until it reaches equilibrium. Knowing the precise shape of the ice surface over time (as well as temperatures and amount of sunshine) will allow the team to back out key parameters for the ice that have long been assumed in modeling the way ice grows and recedes on Mars.

A second experiment would recycle the “trash” of other experiments. As the scoop excavates, it needs to dump dirt somewhere. Some scientists would make these middens useful. They want to build a big dirt pile (using the cone-like shape of the pile to learn something about the size distribution of the soil particles) . Then, every day, the SSI would take a picture of the pile, watching it change in size and shape as the wind does it work — it would thus constrain models for the wind-driven processes on Mars.

Image: University of Arizona

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