NASA said Friday that a mysterious spot shown on a photograph sent back to Earth by the Mars rover Curiosity was the crash landing of its rocket stage.

Scientists and enthusiasts were furiously speculating since Monday about the nature of the small spot that looked like a dust cloud in a picture Curiosity snapped seconds after it parked in a crater.

The smudge disappeared in a photo taken 40 minutes later.

"It was a wonderful coincidence" that the car-sized rover was able to capture the impact, said engineer Steve Sell of the US space agency's Jet Propulsion Laboratory, which is managing the $2.5 billion mission.

A heat shield protected the Curiosity rover during its fiery entry into Mars's atmosphere, a supersonic parachute deployed to slow it down and the spacecraft back shell separated.

A rocket-powered backpack was then fired before the one-ton rover was lowered to the surface by nylon tethers. The sky crane was designed to detach and crash somewhere to the north.

"It's just interesting from an engineering standpoint. The thing hit the ground pretty hard (100 miles per hour, or 160 kilometers per hour)," Sell told AFP about the backpack.

"Our predictions were right on, and it allows us to know that we did a good job with our models."

NASA researchers have fine-tuned their analysis of the data Curiosity sent back on its last few minutes of flight. They said the exact time of the landing was 0531 GMT on Monday and not 0532 GMT as initially announced.

Curiosity was also found to have landed about 1.4 miles to the east of where it had been expected to touch down, "which is very close," NASA engineer Gavin Mendeck said.

The Mars Science Laboratory is a nuclear-powered vehicle that is designed for a two-year mission on Mars to hunt for signs of past life, though scientists hope it will last at least twice that long.