The Atmosphere of Venus (2) ...

Different wavelengths of light penetrate the atmosphere to varying degrees and therefore may be used to study different layers of the clouds and atmosphere.
Viewing at Different Wavelengths
For example, the following images show the clouds of Venus imaged in visible light (with a violet filter), ultraviolet light, and infrared light respectively.



Technically Speaking: The Deuterium to Hydrogen Ratio

An important indication that Venus once had more water that has been lost to space lies in an observation of the Pioneer Venus space probe (1978). It found that the ratio of the mass-2 isotope of hydrogen (deuterium) to the mass-1 isotope is enhanced by a factor of 100 in the atmosphere of Venus relative to Earth. Since more massive deuterium would have more chance to be retained by the atmosphere after water vapor is broken down than normal hydrogen, the UV dissociation of water vapor should lead to an enhanced concentration of deuterium, as observed.

Absence of Water Vapor
The clouds contain little water vapor, and there is scant evidence for water in any form on Venus. It is believed that the absence of water is because most water that may have initially been present on Venus made its way to the upper atmosphere. There it was broken down by UV sunlight and interactions with cosmic rays and the solar wind into oxygen and hydrogen. Because the hydrogen is very light, it is easily lost to space, while the oxygen left behind combines with other chemicals to form compounds in the atmosphere and on the surface.

Reversible and Irreversible Processes
The breakdown of water into hydrogen and oxygen with subsequent loss of the hydrogen to space is an example of what we call an irreversible process: once the water is broken down and the hydrogen lost, it is not possible to reconstitute the original water. The atmosphere of Venus has been shaped strongly by such irreversible processes. In contrast, the Earth's atmosphere, oceans, and crust are coupled by processes that are characteristically cyclic and reversible. The enormous differences between Earth and Venus are strongly influenced by this contrast.