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 concentration of cosmic dust beyond the vicinity of Earth could be explored as well. Experiments Experiments for the measurement of Venus and interplanetary space included: A crystal microphone for measurement of the density of cosmic dust, mounted on the central frame. A proton detector for counting low-energy protons in the solar wind, also mounted on the central frame. Two Geiger-Müller (GM) tubes and an ion chamber, for measuring high-energy charged particles in interplanetary space and in the Venusian equivalent of Earth's Van Allen Belts (which were later shown not to exist). These were mounted on Mariner's long axis to avoid the magnetic fields of the control equipment as well as secondary radiation caused by cosmic rays hitting the metal structure of the spacecraft. An Anton special-purpose GM tube, for measuring lower energy radiation, particularly near Venus, also mounted away from the central frame. A three-axis fluxgate magnetometer for measuring the Sun's and Venus' magnetic fields, also mounted away from the central frame. A microwave radiometer, a diameter, deep, parabolic antenna designed to scan Venus up and down at two microwave wavelengths (19 mm and 13.5mm), slowing down and reversing when it found a hot spot. The 19 mm wavelength was for measuring the temperature of the planet's surface while the 13.5mm wavelength measured the temperature of Venus' cloudtops. The instrument was mounted just above