Planetary geologist Herb Frey of NASA's Goddard Space Flight Center and his daughter Erin Frey, a junior at South River High School, in Edgewater, Md., will be explaining their discoveries in two consecutive presentations at the annual meeting of the Geological Society of America on Sunday, October 27, in Denver, CO.

In both studies Mars Orbiting Laser Altimeter data was loaded into a graphics program that allowed colors to be assigned to different elevations. By manually shifting and stretching the colors to study various ranges of elevation change, they were able to detect very subtle quasi-circular depressions, or "QCDs" for short. The Freys contend that these are craters from early times before the Noachian (pronounced "no-ACK-ee-en") -- the name for the oldest identified geological time period on Mars.

What it all means, says Erin Frey, is that the oldest visible Martian crust is not really the oldest. "We are talking about crust that's actually older," she said.

Something from a period they are calling the "Pre-Noachian."

Erin Frey focused her search for Pre-Noachian craters on a relatively small area near the Hellas Basin of Mars that has been previously considered Early in age. What she found were subtle signs that there was a yet earlier episode of intense impacts, largely covered over by subsequent cratering and billions of years of geological processes.

Herb Frey took a similar approach in studying the entire planet for signs of very early large impacts. In the flat northern lowlands of Mars, for instance, he found QCDs buried by eons of accumulated dust, volcanic materials, and other sediments. Placing the cratered surfaces in order, Herb Frey says that it appears the buried lowlands are older than the visible cratered surface of the Noachian southern highlands, and that the QCDs his daughter found in the highlands are older still.

How old? "We can't assign absolute ages because we don't know how far back these subsurfaces go," says Erin Frey.

What's more, despite previous geologists assigning ages of about four-billion-years old to the Noachian and the Southern Highlands, it's just a guess based on the order in which things appeared to have occurred on Mars and what's known about similarly cratered crust on the Moon.

"It's actually a very squirrelly business," said Herb Frey of attempts to assign actual ages to Martian features without actual rock samples isotopically. Still, there are some things the older crust can tell us about the history of Mars.

For one thing, says Herb Frey, the evidence of earlier impact periods means that some process that fit in to a relatively short interval formed the oddly low northern hemisphere of Mars. That means hypotheses of plate tectonics and large-scale motion in Mars' mantle may be less likely -- since they tend to be very slow processes.

Alternatively, what becomes more likely is that the northern lowlands were formed by a massive impact event.

"It fits in with the timescale," he said.

Until samples from the cratered terrains can be studied in labs, there are experiments that can be sent to Mars that will help clear things up, says Herb Frey. One would be a series of seismic sensors that could listen to Mars quakes and use the information to discern the interior structure of the planet -- as is done on Earth. European and US scientists are already planning such a project, says Frey.

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