All eyes on Mars

Scientists mimic Red planet in the lab to explain conditions on its surface

CAMBRIDGE - In a basement laboratory on Oxford Street, Harvard scientists seeking insight into Mars are shooting metal discs down a 20-foot-long gun barrel, pulverizing slivers of ice and rock.

"It is a very destructive way of doing science," said Sarah T. Stewart, the director of the Harvard Shock Compression Laboratory, who uses an oversized rifle to recreate the shockwaves that reverberate through a planet when a meteor crashes down.

Slamming objects together may seem more child's fantasy than laboratory life, but such brief, cataclysmic experiments can give researchers insight into everything from how materials hold up under extreme conditions to the chemistry of other worlds.

Some examples: At the National Ignition Facility being built at Lawrence Livermore National Laboratory in California, 192 laser beams will focus on a target of hydrogen fuel, creating enough pressure to trigger fusion. In 2005, NASA scientists slammed a probe into a comet, to learn about comets' innards. Imperial College London announced this spring a new Institute for Shock Physics, with the aim of elucidating questions about how tsunamis form or what's happening at a planet's core.

"This is the only method we have on this planet, to create certain pressure and temperature conditions that exist in our universe," said Yogendra Gupta, director of the Institute for Shock Physics at Washington State University. "If you want to understand what is inside Jupiter - well, you take a spaceship and go to the center of Jupiter," or, he said, you could stay at home and generate high pressures in your lab, mimicking the conditions found on other planets.

At Harvard, the experiments start with an initial pop and some clanging, and are over in a flash. Other than the lights occasionally swaying overhead, there's little evidence that something approximating a planetary collision has occurred.

This is science at its grittiest: a gun powered by helium gas and gunpowder that sends a projectile flying at speeds of up to 6,000 miles per hour, blowing a carefully prepared sample to smithereens.

Using lasers, the scientists measure how fast their bullet, called a sabot, is going when it collides with the sample. Then, through gauges embedded in the sample, temperature sensors, and lasers, they monitor the event, which typically lasts just a microsecond.

Sometimes, Stewart uses a special, enclosed recovery capsule that keeps the sample from scattering all over the chamber and allows her to measure how the impact affects fundamental properties, such as magnetism.

The entire apparatus, custom-built and designed according to Stewart's specifications, is about 40 feet long, stretching from the barrel where the projectile starts its journey to a recovery tank filled with rags that slows it down. The cannon shoots in vacuum, so there is no sonic boom when researchers fire the projectile, and experiments are over in a flash.

Then, Stewart manages to do a kind of otherworldly forensics.

"You can take that understanding and go to the planetary scale," she said.

Using data collected from the rovers and satellites that troll the surface of Mars, Stewart creates computer models to better understand how impact craters might tell a larger story of the planet, giving clues about everything from what's underneath the surface to how the atmosphere has changed.

For example, scientists have been intrigued by craters on Mars that are surrounded by what looks like a mud splat, where material ejected from the crater have flowed from the bull's-eye of the impact. Stewart uses the data beamed down from space, combined with her work on Earth shooting discs of ice and basalt to predict what types of icy layers may lie beneath the surface. She also works to predict and understand when and how frozen crater lakes may have formed, and models how much water might have been vaporized on impact - hoping to better understand the Martian atmosphere.

Planetary scientists have also been intrigued by Mars's magnetic field.

By pummeling a piece of magnetic material in a recovery capsule, Stewart has begun to model how collisions change magnetic properties, to try to figure out whether impact craters may hold explanations for Mars's peculiar magnetic field.

Shock physics is a niche area of science, where a lifetime of experimentation may add up to less than a second of data. But the explosive experiments provide scientists with a unique opportunity to mimic everything from ballistics to planetary collision to the pressures at a planet's core.

Paul Asimow, an associate professor of geology and geochemistry at the California Institute of Technology, for example, is trying to better understand the boundary between the earth's core and mantle - a place he could never get by drilling.

Even when scientists can get there, it turns out that shattering tiny samples with a 20-foot-long gun can add valuable insight.

So, as the Phoenix Mars Lander continues to examine the surface of the planet, scooping up soil and rasping away at ice layers, Harvard scientists are busy in their basement laboratory.

"We shoot the gun as often as we can," Stewart said.

Carolyn Y. Johnson can be reached at cjohnson@globe.com.