Armed with a combination of detailed maps of Mars' gravity patterns and high-resolution topography maps of the planet's surface, a team of scientists has calculated the rough shape and thickness of the martian crust.

This map combines information from gravity and topography maps to estimate the thickness of Mars' outer crust. The map represents the entire mid-latitudes from 70 degrees south to 70 degrees north. The equator is a horizontal line crossing through the center of the map at zero degrees. The red region toward the left is the Tharsis region, which contains the largest volcanic mountains on the planet. The blue spot on the right side represents the impact crater known as the Hellas basin.

That scientists are able to see below the surface of Mars to make conclusions about the planet's early history is testimony to the strength of gravity measurements. Used in concert with precise terrain maps that show the elevations of land features, the gravity maps become a powerful decoder of secrets that are locked below the surface and hidden in the past.

The way scientists use gravity and topography together can be illustrated in an examination of the Hellas basin.

Nearly 6 miles (9 kilometers) deep and 1,300 miles (2,100 kilometers) across, Hellas is the largest open crater on the planet. Since there is a missing mass of material from the giant hole, the region above it might be expected to have a low gravitational pull. The region is actually very gravitationally flat, as the map below shows. Hellas, centered at 45 degrees south latitude, 70 degrees east longitude, appears in the lower-right quadrant.

The force of Mars' gravitation is measured in units called milli-Gals (mGal), which indicate the acceleration of the Mars Global Surveyor orbiting Mars. When the craft passes over a region that is relatively massive, gravity pulls a little bit stronger on the spacecraft, causing it to dip slightly, and thus, speed up. Over less-massive portions of the planet, the pull of gravity is weaker, and the satellite slows down. One milli-Gal on the map scale signifies a change in the satellite's velocity of one-thousandth of a centimeter per second.

The fact that the gravity is smooth, means that the crater is very old, and the planet has "compensated" for the impact. We tend to think of planetary rocky crusts as hard and unmoving, but during millions of years they are actually somewhat elastic, depending on the thickness and temperature of the crust.

The impact that created Hellas might be compared to draining the water from a swimming pool, Smith said.

"What often happens when people do dumb things like that is the whole thing pops up out of the ground because it's being held in place by the massive amount of water that's in the pool.

"On some level, that's what would have happened when the meteorite impact carved out the Hellas basin," he said. "You create this big basin by removing a lot of material and then there's this enormous buoyancy pressure for material, the base of it to be pushed upwards by material from the sides."

Over hundreds of thousands or millions of years, forces try to lift up the basin, Smith said. It is impossible to say how deep the crater was originally, but it could have been two to three times as deep just after the impact occurred, and the floor has slowly been pushed upward until the whole region reached a gravitational equilibrium.

Fast-forward a couple billion years to a time when the crust had become much thicker and cooler, and thus less malleable. If the basin were to fill with dust and dirt -- from erosion, for example -- the additional material would actually create an area of high gravitational force above the filled basin.

Scientists would call this feature a positive gravity anomaly. It is an anomaly because the high gravity doesn't match the flat topography. It is these anomalies, both positive and negative, that allow scientists to figure out what is going on beneath the surface.

Once the basin had been filled, forces would push downward, attempting to bring the surface to a gravity equilibrium. The colder, stiffer crust might then take longer to equalize than it would have in the planet's early days. If the crust is thick enough, the gravity high might actually become permanent.

"In Hellas's case, nobody ever filled it with anything," Smith said. "If you filled it now with water, or filled it with dust and dirt, it would have a big positive anomaly."

This is what likely happened to the northern hemisphere, Smith said. The north, which is gravitationally very rough, is topographically very smooth, indicating that a rough surface may have been covered with sediment relatively late in its history.