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Martian Diaries

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Anatomy of a Decision
By Jeffrey Marlow


Marks of Laser Exam on Martian Soil
The Chemistry and Camera (ChemCam) instrument on NASA's Mars rover Curiosity used its laser to examine side-by-side points in a target patch of soil, leaving the marks apparent in this before-and-after comparison.

At 1:45 PM, news seeped into the science theme group rooms that Curiosity's schedule had a very appealing opening: a two-hour science block. It was the kind of opportunity that the team dreams of, a chance to unleash the analytical force of Curiosity's payload on the martian surface.

But gaining consensus among the room's two-dozen scientists while working within the spacecraft's constraints is not easy. That task fell to the day's Mineralogy Team Lead, Darby Dyar. "We're going to start talking about target selection in 10 minutes," she says, moving among the rooms.

Participating Scientists Kevin Lewis and Becky Williams man the two-monitored computer at the front of the Geology Theme Group room. They're the first scientists to see the data beamed back from Mars, and they move images between monitors, scrolling and dragging to the beat of the debate.

As the team stares at the martian landscape projected on the wall, Dyar works the room, trying to figure out which instruments are available and how many targets could be analyzed. It's an exercise in restrained advocacy: Dyar wants to fight for as aggressive a science program as possible, but she is well aware that overall mission health is the top priority, engineering check-ups included.

She returns with a proclamation: ChemCam - the rock-vaporizing laser that determines elemental compositions - is the tool of choice, and the open science block allows for four measurements. "Let's go ahead and start brainstorming some potential targets," says Dyar, uncapping a black Sharpie.

What was previously a passive browsing session instantly turns into a Black Friday stampede, and the team quickly converges on three possibilities: an assessment of nearby rocks, a detailed study of a small depression filled with smaller fine-grained particles, or a side-by-side comparison of compressed dirt (in Curiosity's tracks) with unaffected dirt nearby.

Other team members aren't particularly impressed by current offerings: "Drive on!" comes a shout from the back of the room. The request is mostly tongue-in-cheek since Curiosity is staying put for the day, and there's plenty of good science to be done at the site.

Lewis doesn't pull any punches. "We're always going to have rover tracks, every day, by definition," he says. "Let's focus on the interesting rocks while we've got them." But it's a moot point: as Williams runs the numbers and pivots the model rover on the computer screen, she notices that the tracks are too far away, out of the running. The depression and rock targets remain on the table.

David Des Marais, a veteran astrobiologist who speaks when he has something to say, campaigns for the gravel pit. "It could be a little crater, a dust filled crater," he speculates. "Mars could have done the work of isolating this size fraction for us, so that could be nice."

The rocks are presented systematically, and observations are encouraged from the crowd. Rock N198: "darker," "fractured, "kind of looks layered to me." N197: "broken cluster of rocks," "dusty," "that one is not interesting." N201: "small," "angular," "weird."

There isn't a lot of information to work with when forming these lists of characteristics, so the scientists offer their bids liberally, like hedge fund managers at a Banksy auction. But there are limits: an "external crust," is proposed for N196, drawing an incredulous response from the rest of the team. "I don't know about that," says Lewis with diplomatic restraint.

Dyar glances at her watch and initiates a straw poll. It's a tough line to walk - encouraging debate while pushing the team to reach a consensus - but Dyar manages it expertly. "So we seem to have settled on the gravel pit," she says after the votes have been cast, seeming to create the consensus as much as report it.

Over the next several minutes, the team develops the experimental plan, deciding precisely where to aim and how to arrange the observations. Des Marais points out the danger of generalizing from a single measurement. "If you're looking at a heterogeneous material, you need a lot of observations to average out those differences," he notes, encouraging an array of closely spaced measurements.

Dyar offers the rock advocates, who are looking a little morose, a consolation prize: "How about we place three measurements in the fine-particle depression, and one on a rock?"

"Sold!" comes the enthusiastic response from the back of the room. N198, the dark, possibly fractured rock is the team's top choice; N201 is the backup.

At this point, ChemCam instrument collaborator Pierre Yves Meslin enters the fray. Meslin is today's ChemCam representative to the science team, responsible for making sure the scientific program is technically feasible. He knows the instrument inside and out; he knows the optimal angles, the best target distances, the way the laser ablates different types of targets.

"So is this a 5-minute measurement?" asks Lewis, pointing to a line of code in the nascent plan.

"Yeah, I'll have to check, but that should be right," Meslin responds. He retreats to the ChemCam team room and the scientists exhale.

It's 2:53 PM, and the plan is ready.

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