UPDATE 1-'Missing' US governor was on private Argentina trip

* Sanford's aides had said he was hiking on U.S. trail

* Secretive absence seen hurting presidential aspirations (Adds quote, background, paragraphs 3-4, 14)

By Matthew Bigg

ATLANTA, June 24 (Reuters) - South Carolina Governor Mark Sanford returned to the United States on Wednesday from a secret private trip to Argentina, ending days of speculation over his whereabouts and raising questions about his judgment.

Sanford is the chairman of the Republican Governors' Association and a prominent fiscal conservative who has been talked about as a potential Republican candidate for the 2012 U.S. presidential election.

Analysts said Sanford's six-day disappearance, during which he apparently failed to inform aides of his whereabouts, could damage his chances if he decided to run in 2012.

Republican strategist Rich Galen said the issue would "absolutely" torpedo any presidential aspirations. "To have someone who is willing to take a hike, literally or figuratively, I just think is a disqualification," he said.

When media reported Sanford's whereabouts were unknown since last Thursday and that even his wife did not know where he was, his aides said he was hiking on the Appalachian Trail in the eastern United States to get away for a break after a tough state legislative session.

During his absence, some South Carolina politicians accused him of abdicating responsibility for state affairs.