By Jonathan Amos
Science correspondent, BBC News, San Francisco
The Egg (l) and its companion obtained by multibeam echosounder bathymetry
Portuguese scientists have found a depression on the Atlantic Ocean floor they think may be an impact crater.
The roughly circular, 6km-wide hollow has a broad central dome and has been dubbed the "Fried Egg" because of its distinctive shape.
It was detected to the south of the Azores Islands during a survey to map the continental shelf.
If the Fried Egg was made by a space impactor, the collision probably took place within the past 17 million years.
This is the likely maximum age of the basaltic sea-floor rock which harbours the feature.
"To be sure, we need to take samples and make a profile of the sediment layers to determine if there really is a central uplift from an impact," explained Dr Frederico Dias from EMEPC (Task Group for the Extension of the Portuguese Continental Shelf).
"We need also to see all the signatures that are consistent with a high velocity impact, like glasses from melting and, of course, debris; and what are called shatter cones (shocked rocks)," he told BBC News.
Dr Dias described the putative impact feature here at the American Geophysical Union's (AGU) Fall Meeting, the world's largest annual gathering of Earth scientists.
The Fried Egg was first identified in data gathered by a 2008 multibeam echosounder hydrographic survey. A further cruise from September to November this year confirmed its presence.
It lies under 2km of water about 150km from the Azores archipelago.
The depressed ring sits roughly 110m below the surrounding ocean bottom, with the circular dome-shaped central uplift 3km in diameter and with a base-to-top height of some 300m.
Central peaks are often associated with meteorite impacts and form when the compressed crater floor rebounds. A peak is not definitive proof of an impact, however.
A volcanic origin for the Fried Egg seems unlikely because the Portuguese team has not been able to find any lava flows within the structure or on its surroundings.
Interestingly, there is another - but much smaller - feature just 3-4km to the west of the egg.
"It's just by the side. If the Fried Egg is a crater, this could be a crater also," speculated Dr Dias.
Dr Dias and colleagues are examining gravity and magnetic data gathered during September's cruise. A third expedition to the area early next year will use a remotely operated vehicle (ROV) to try to retrieve samples from the ocean floor for analysis.
The Portuguese team detailed the currently available Fried Egg data on a poster at the AGU meeting. Other researchers who came to view the information were split on the impact theory, Dr Dias said.
"Even if it's not an impact crater it's still a very interesting feature," he told the BBC.
The EMEPC is working under the United Nations Convention on the Law of the Sea to establish the true extent of Portuguese territorial waters.
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