DETROIT (AP) _ The research submarine that helped retrieve the remains of the space shuttle Challenger is turning its sights to the Great Lakes, where scientists will seek to unveil the secrets of its depths.

The month-long journey of the Seward-Johnson and its Johnson-Sea Link II sub is the second of five planned annual trips through the Great Lakes sponsored by the National Oceanic and Atmospheric Administration.

''The gee-whiz phase is over,'' Richard Cooper, director of the undersea research program at the University of Connecticut, said in a breifing Wednesday on the eve of the ship's planned departure for Lake Huron early today.

The crew of 26 includes eight scientists, including Cooper and his brother, William Cooper, chairman of the Michigan State University zoology department.

Last summer, the Johnson-Sea Link II enabled humans for the first time to see the bottom of Lake Superior firsthand.

The highlight of that four-week expedition was the discovery that Superior's black, frigid depths teem with life, including a small scaleless fish, the sculpin, that carves moonscapes in the lake floor with its tail.

After the Challenger disaster Jan. 28, the ship and its submarine were the only non-military vessels that took part in the search for debris.

Scientists were to begin today by examining the air, water and sediment of Huron for pollutants including PCBs and dioxins, William Cooper said.

Dow Chemical Co. of Midland, which has polluted the lake from factories on the American and Canadian sides, will have a scientist aboard, officials said.

The spawning grounds of the burbot - a type of freshwater cod - and the sculpin are known, as are the places where the adults live, but ''one of the mysteries of the Great Lakes is the whereabouts of the juvenile fish,'' said Doug Lee of the Michigan State zoology department.

To find these juveniles, scientists will use an electronic rod to send ''a small, irritating current into rocks,'' where they may be hiding, Lee said.

Today's first stop was scheduled for Six Fathoms Bank in the middle of Lake Huron off Black River, a troubled spawning area that is not producing as much lake trout as it should, he said.

Geochemical and biological research will be conducted at the second stop, Lake Huron's Mackinac Basin off Rogers City. More lake trout spawning research will be conducted at Spectacle Reef near Bois Blanc Island off Hammond Bay.

Eventually, the scientists will cruise Lake Superior again, exploring the deepest part of the lake off Munising where, at 1,319 feet below the surface and more than 700 feet below sea level, the temperature hovers just above freezing even in mid-summer.

Deepwater canyons carved by glaciers near Caribou Island also will be explored.

William Cooper said the information from Lake Superior, the least polluted of the Great Lakes, will be particularly useful as a base from which to compare the evolution of pollution in the other four lakes.

''It's not a pollution-free control, but it's as close as we can get,'' he said.

The Michigan Sesquicentennial Commission, Michigan State and the Michigan Polar Equator Club on Wednesday gave the crew a fluorescent orange plastic tube bearing a time capsule to be placed at Lake Superior's deepest spot for recovery in 50 years.