The report quoted an unnamed expert as saying that the mark could have been caused by oil expelled through a vent from a piece of equipment on the space station and said Russian experts were investigating the origin of the spot.

The spot of unknown origin posed no danger to the crew or the space station, the expert told Interfax.

The ISS crew, Russian cosmonaut Salizhan Sharipov and American astronaut Leroy Chiao, installed various types of equipment on the exterior of the station during a five-and-a-half-hour space walk, Russian mission control officials said earlier.

They also checked station vents and photographed them during their time outside the space station, they said.

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