Undersea Chesapeake Crater Offers Hints to Mars Life

Kimberly Johnson
for National Geographic News

June 30, 2008

A 35-million-year-old crater under Chesapeake Bay is offering new insights into possible locations for life on Mars. But it also has the potential to threaten the area's groundwater supplies with contamination, according to a new study.

The pit, discovered about 15 years ago, is considered one of the best-preserved examples of the roughly 200 known impact structures on Earth. (See a map of the area.)

The full crater has a diameter of about 56 miles (90 kilometers). That's an oddly large size for a crater believed formed by a comet or asteroid only about 1.3 miles (2 kilometers) wide, said study co-author Gregory Gohn of the U.S. Geological Survey.

At the time, the eastern shore of the U.S. lay near where Richmond, Virginia, sits today, so the object struck a 650-foot-deep (200-meter-deep) sea, he added.

The projectile vaporized upon impact, releasing intense heat for more than 100 miles (161 kilometers), Gohn said.

The collision blew open a crater that was initially about 12.4 miles (20 kilometers) wide and 4.3 miles (7 kilometers) deep.

But the crater walls were not strong enough to support the massive pit and collapsed further, creating a larger pit and allowing salty seawater and broken rock to became trapped in a subsurface cavity.

The crumbling rock stirred up raw materials that allowed life to bloom inside the crater after the waters finally cooled, hinting that life on Mars may also have flourished because of impacts.

(Related: "Any Possible Mars Water or Life Is Deep Below Surface" [May 15, 2008].)

Now, however, the salt water lies dangerously close to freshwater aquifers that supply the local population, the new study found.

Salty Contamination

In 2007 an international team of researchers began drilling about 6 miles (9 kilometers) from the crater's center. Using diamond-containing steel drill bits, the scientists were able to capture rock samples up to a mile (1.8 kilometers) into the impact structure.

It was a laborious process spanning three months of round-the-clock drilling, Gohn said.

But the hard work paid off. Brine found in the impact structure contained roughly twice as much salt as regular seawater, the researchers say.

Communities around the bay largely rely on freshwater aquifers and could potentially draw brine into the wells if they're overpumped.

Water managers have now started to take the crater into account, Gohn added.

Death and Life

The research also represents the first microbial sampling within a crater structure, scientists say.

The results show that the crater had both destructive and beneficial effects on local marine life.

The heat of the impact probably exceeded 250 degrees Fahrenheit (120 degrees Celsius)—the temperature traditionally used to sterilize bacteria—explained team member Mary Voytek, a USGS microbiologist.

"Any organisms living there would not be able to survive," she said.

Some areas of the crater remain too salty be habitable, she added, but life likely returned in other crater locations about 10,000 years after the impact when temperatures started to cool.

Now in some portions of the impact structure, such as the upper edges and deep in the belly, ten to a hundred times as many organisms are present as there are at similar depths outside of the pit, she said.

The release of broken and porous rock material appeared to act much like churning a compost pile or tilling a field, freeing fresh organic nutrients from plants killed during the impact, she explained.

As the material filled in the crater, a habitable space was created for organisms in what had been nutrient-poor solid rock, she said.

"If you get habitable temperatures, space, and nutrients, you can get things to grow back and grow back well," Voytek said.

Mars Connection

The findings, which were released this week in the journal Science, could hold clues about the role craters played in the evolution of life on early Mars, Voytek said.

The red planet's surface is currently inhospitable to life because of high levels of ultraviolet radiation and frigid temperatures, but material and a suitable environment could have been introduced into the subsurface during impacts.

The new data suggest that "there's a higher probability we'll find life in the subsurface" on Mars, she said.

David Kring of NASA's Lunar and Planetary Institute was not involved in the research.

"The recent drilling project is an excellent probe of the crater's geology, and this study reveals important details of the crater's formation," he said. "The crater is possibly an analog for impacts that may have occurred long ago on Mars, when the planet may have had intermittent seas."

Elisabetta Pierazzo, a planetary physicist at the Planetary Science Institute in Tucson, Arizona, was also not part of the new study.

She said that the findings help unravel how underwater life reacts to external bombardments.

"This is very important for studying Mars, where a subsurface biosphere may have existed in the past."