A father-daughter science team has found what they say are the oldest known impact craters on Mars, ghostly structures that could only be discerned with special software and the latest elevation data.

Images obtained by SPACE.com reveal hints of circular outlines and subtle depressions that appear to be craters created during tremendous asteroid or comet impacts that pummeled the Red Planets original crust 4 billion years ago or more. The features have since been mostly buried or eroded away.

If the entombed craters exist as suspected, then the current visible surface of Mars does not represent the original crust, as some scientists have thought.

A graphics program turned elevations into different colors. By shifting and stretching the colors to study various ranges of elevation change, the researchers spotted faintly detectable features they call quasi-circular depressions, or QCDs. The Freys figure these depressions are craters from early times before the Noachian period, which may date back about 4 billion years and is the oldest identified geological time period on Mars.

"We are talking about crust that's actually older than whats seen at the surface," Erin Frey said. "We can't assign absolute ages because we don't know how far back these subsurfaces go."

Mars, Earth and the other planets are thought to have formed about 4.5 or 4.6 billion years ago. A period of heavy bombardment likely ensued, as countless rocks were cleared from the fledgling solar system. A record of the bombardment remains on the Moon, where little erosion or geologic activity takes place.

But on Mars and Earth, figuring out what happened more than about 3 billion years ago is very, very difficult. Until now, there was little firm evidence about the original early crust of Mars.

Erin Freys work focused on a region near the Hellas Basin of Mars, considered by previous studies to be old. It suggests an intense period of asteroid and comet impact preceded the Noachian period. Most of the evidence was covered over by later impacts and billions of years of geologic processes.

Herb Frey looked at the entire planet. He found large depressions, all more than 125 miles (200 kilometers) in diameter, buried under dust, volcanic material and other sediments in the relatively crater-free northern lowlands of Mars, as well as in the visibly cratered southern highlands. He said its "a very squirrelly business" trying to assign ages to the features theyve seen.

He suggests, however, that the northern lowlands were formed very early by massive impact events. "It fits in with the time scale," he said.

"Erin's results show that the assumption many people have made that the oldest visible surface units go back to 4.6 billion years ago is wrong," the father told SPACE.com. "That means the absolute time scales people have tried to use are probably wrong. It also means there is a recoverable history on Mars that we cannot easily see, except in terms of the crater record. The surface we see is not the original crust of Mars, but something younger."

How does a high school student get involved in such important work?

"Each year of high school I was required to complete a science project, and my contacts at the Goddard Space Flight Center allowed me to study unusual topics," Erin Frey said. "Beginning in freshman year, I was introduced to the topography of Mars and completed research on a similar subject. I continued my work through 10th grade, which eventually turned into this paper."

The paper also won 1^st Place at a county science and engineering fair. The Freys presented their findings last week at a meeting of the Geological Society of America in Denver.