high gain parabolic antenna was also mounted on the underside of Mariner and kept pointed toward the Earth. An omnidirectional antenna atop the spacecraft would broadcast at times that the spacecraft was rolling or tumbling out of its proper orientation, to maintain contact with Earth; as an unfocused antenna, its signal would be much weaker than the primary. Mariner also mounted small antennas on each of the wings to receive commands from ground stations. Temperature control was both passive, involving insulated, and highly reflective components; and active, incorporating louvers to protect the case carrying the onboard computer. At the time the first Mariners were built, no test chamber existed to simulate the near-Venus solar environment, so the efficacy of these cooling techniques could not be tested until the live mission. Scientific instruments Background At the time of the Mariner project's inception, few of Venus' characteristics were definitely known. Its opaque atmosphere precluded telescopic study of the ground. It was unknown whether there was water beneath the clouds, though a small amount of water vapor above them had been detected. The planet's rotation rate was uncertain, though JPL scientists had concluded through radar observation that Venus rotated very slowly compared to the Earth, advancing the long-standing (but later disproven) hypothesis that the planet was tidally locked with respect to the Sun (as the Moon is with respect to the Earth). No oxygen had been detected in Venus' atmosphere, suggesting that life as existed on Earth was not present. It had been determined that Venus' atmosphere contained at least 500 times as much carbon dioxide as the Earth's. These comparatively high levels suggested that the planet might be subject to a runaway greenhouse effect with surface temperatures as high as , but this had not yet been conclusively determined. The Mariner spacecraft would be able to verify this hypothesis by measuring the temperature of Venus close-up; at the same time, the spacecraft could determine if there was a significant disparity between night and daytime temperatures. An on-board magnetometer and suite of charged particle detectors could determine if Venus possessed an appreciable magnetic field and an analog to Earth's Van Allen Belts. As the Mariner spacecraft would spend most of its journey to Venus in interplanetary space, the mission also offered an opportunity for long-term measurement of the solar wind of charged particles and to map the variations in the Sun's magnetosphere. The