The Mariner 2 spacecraft did not enter Venus’s atmosphere, but it was nonetheless able to determine that t?

Question

The Mariner 2 spacecraft did not enter Venus’s atmosphere, but it was nonetheless able to determine that the atmosphere is very dry. How was this done?

Answer 1

The scientific instruments onboard the Mariner spacecraft were two radiometers (microwave and infrared), a micrometeorite sensor, a solar plasma sensor, a charged particle sensor, and a magnetometer.
These instruments were designed to measure the temperature distribution on the surface of Venus, as well as making basic measurements of Venus' atmosphere.

One of the things being measured by these instruments was water vapour (using the spectrometer and the radiometers). Water vapour has a specific "signature" in the electromagnetic spectrum that can be detected. And there was very little water vapour detected in the atmosphere.
Add to that the surface temperature measurements of about 450 degrees Celsius, water doesn't exist on Venus in any appreciable amounts. The water that was detected is theorized to be from geological outgassing.

Answer 2

Remote sensing with an infrared radiometer and listening to the radio emissions from the planet was how Mariner 2 determined that Venus' atmosphere was both extremely hot and very dense. When the spacecraft passed behind the planet, it beamed radio signals back at Earth. As the signals faded then returned as Mariner 2 flew behind the planet as seen from Earth, they gave us information on Venus' atmosphere. When it was shown that the surface temperature was 850 degrees Farenheit and up, it became clear no liquid water could possibly exist there. The radiometer also revealed information about the composition and nature of the atmosphere, as well as how much heat was getting to the planet as well as leaving it. The instrument saw almost no signs of water, but plenty of CO2.

Answer 3

Many gasses such as carbon dioxide, methane, and water vapor can be detected in an atmosphere by observing which wavelengths of infrared light are absorbed by it. I don't know the details of the Mariner mission but it would have been possible to aim an infrared spectrometer from the night side towards the limb of the planet as the sun comes into view. This data would provide information about the presence and concentration of many gasses including water vapor. If I remember correctly astronomers had been doing this from earth long before space craft were sent there and we already had an idea that Venus was hot and dry.

Answer 4

From the spectrum of light and radio waves coming from the Planet I would think.