with each other; photographs of the upper and lower cloud layers corroborated this hypothesis. Mariner 10s ultraviolet photographs were an invaluable information source for studying the churning clouds of Venus' atmosphere. The mission researchers believed the cloud features they photographed were located in the stratosphere and upper troposphere, created by condensation; they also concluded that the contrast between darker and lighter features was due to differences in the cloud's absorptivity of UV light. The subsolar region was of particular interest: as the sun is straight overhead, it imparts more solar energy to this area than other part of the planet. Compared to the rest of the planet's atmosphere, the subsolar region was highly active and irregular. "Cells" of air lifted by convection, each up to wide, were observed forming and dissipating within the span of a few hours; some had polygonal outlines. The gravity assist was also a success, coming well within the acceptable margin for error. In the four hours between 16:00 and 20:00 UTC on 5 February 1974, Mariner 10s heliocentric velocity dropped from to . This changed the shape of the spacecraft's elliptical orbit around the Sun, so that the perihelion now coincided with the orbit of Mercury. First Mercury flyby The spacecraft flew past Mercury three times. The first Mercury encounter took place at 20:47 UTC on 29 March 1974,at a range of , passing on the shadow side. Second Mercury flyby After looping once around the Sun while Mercury completed two orbits, Mariner 10 flew by Mercury again on 21 September 1974, at a more distant range of below the southern hemisphere. Third Mercury flyby After losing roll control in October 1974, a third and final encounter, the closest to Mercury, took place on 16 March 1975, at a range of , passing almost over the north pole. End of mission With its maneuvering gas just about exhausted, Mariner 10 started another orbit of the Sun. Engineering tests were continued until 24 March 1975, when final depletion of the nitrogen supply was signaled by the onset of an un-programmed pitch turn. Commands were sent immediately to the spacecraft to turn off its transmitter, and radio signals to Earth ceased. Mariner 10 is presumably still orbiting the Sun, although its electronics have probably been damaged by the Sun's radiation. Mariner 10 has not been spotted or tracked from Earth since it stopped transmitting. The only ways