Editing Mariner 10 (section)

=== Venus flyby ===
The spacecraft passed Venus on 5 February 1974, the closest approach being {{convert|5768|km|sp=us}} at 17:01 UTC. It was the twelfth spacecraft to reach Venus and the eighth to return data from the planet,<ref name=nssdc1/> as well as the first mission to succeed in broadcasting images of Venus back to Earth.{{sfn|Ulivi|Harland|2007|p=181}} ''Mariner 10'' built upon observations made by ''Mariner 5'' six years earlier; importantly, ''Mariner 10'' had a camera whereas the prior mission lacked one.{{sfn|Reeves|1994|p=244}} As ''Mariner 10'' veered around Venus, from the planet's night side to daylight, the cameras snapped the probe's first image of Venus, showing an illuminated arc of clouds over the north pole emerging from darkness. Engineers initially feared that the star-tracker could mistake the much brighter Venus for Canopus, repeating the mishaps with flaking paint. However, the star-tracker did not malfunction. Earth occultation occurred between 17:07 and 17:11 UTC, during which the spacecraft transmitted X-band radio waves through Venus' atmosphere, gathering data on cloud structure and temperature.{{sfn|Murray|Burgess|1977|pp=61-64}}{{sfn|Dunne|Burgess|1978|pp=61-63}} Although Venus's cloud cover is nearly featureless in visible light, it was discovered that extensive cloud detail could be seen through Mariner's ultraviolet camera filters. Earth-based ultraviolet observation had shown some indistinct blotching even before ''Mariner 10'', but the detail seen by Mariner was a surprise to most researchers. The probe continued photographing Venus until 13 February 1974{{sfn|Murray|Burgess|1977|p=79}} Among the encounter's 4165 acquired photographs, one resulting series of images captured a thick and distinctly patterned atmosphere making a full revolution every four days{{sfn|Reeves|1994|p=244}} just as terrestrial observations had suggested.{{sfn|Dunne|Burgess|1978|p=68}}

The mission revealed the composition and meteorological nature of the atmosphere of Venus. Data from the radio science experiment measured the extent to which radio waves passing through the atmosphere were refracted, which was used to calculate the density, pressure, and temperature of the atmosphere at any given altitude.<ref name=Howard/> Overall, atmospheric temperature is higher closer to the planet's surface, but ''Mariner 10'' found four altitudes where the pattern was reversed, which could signify the presence of a layer of clouds.{{sfn|Dunne|Burgess|1978|p=65}} The inversions occurred at the {{cvt|56|,|61|,|63|, and|81|km|sp=us}} levels,{{sfn|Giberson|Cunningham|1975|p=726}} confirming previous observations made by the ''Mariner 5'' encounter.<ref name=Howard/> The ultraviolet spectrometers identified the chemical substances that comprise Venus' atmosphere.{{sfn|Giberson|Cunningham|1975|p=725}} The elevated concentration of atomic oxygen in the upper atmosphere showed that the atmosphere is stratified into upper and lower layers that do not mix with each other; photographs of the upper and lower cloud layers corroborated this hypothesis.{{sfn|Dunne|Burgess|1978|p=65}} ''Mariner 10''{{’}}s 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.<ref name=Murray-Belton/> 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 {{convert|500|km|sp=us}} wide, were observed forming and dissipating within the span of a few hours; some had polygonal outlines.<ref name=Murray-Belton/>

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 10''{{'}}s heliocentric velocity dropped from {{convert|82,785|mph|km/s|abbr=on|order=flip|sp=us}} to {{convert|72,215|mph|km/s|abbr=on|order=flip|sp=us}}.<ref name=No18/> This changed the shape of the spacecraft's elliptical orbit around the Sun,{{sfn|Ulivi|Harland|2007|p=181}} so that the perihelion now coincided with the orbit of Mercury.<ref name=No18/>

<gallery class="center">
File:Mariner 10's encounter with Venus (diagram).jpg|Venus encounter
File:Venus Mariner10 UV.png|Venus in UV light. This image is a mosaic of two frames.
File:Venus-m10.jpg|Venus cloud patterns
File:Venus-real color.jpg|Venus in real colors, processed from clear and blue filtered images
File:PIA23791-Venus-RealAndEnhancedContrastViews-20200608 (cropped).jpg|Color composite  using orange and ultraviolet filters with modern image processing software 
</gallery>