Astronomy Picture of the Day
August 27, 2012

High TAU at Gale - Sol 3
High TAU at Gale - Sol 3

Credits: NASA/JPL - MER Curiosity - Credits for the additional process.: Dr Gianluigi Barca/Lunar Explorer Italia/IPF

In general terms, when we talk about Opacity, (including the so-called Atmospheric Opacity - or TAU) we mean the Measure of Impenetrability of a given Element (or of a combination of Elements) to a given Radiation (in particular, that Radiation which we call "Visible Light"). In Radiative Transfer, the term Opacity describes the Absorption and Scattering of a Radiation as it occurs in (through) a Medium, such as a Plasma, Dielectric, Shielding Material, Glass, or, as the case in point, that specific combination of different Gases and Particles which forms an Atmosphere. On the other hand, an Opaque Object is neither Transparent (meaning that it allows all the received Light to pass through it), nor Translucent (meaning, in this case, that it allows only a certain quantity of the received Light to pass through it).

In general, when the Light strikes an interface between two (or more) Elements, some of it can be reflected, some absorbed, some scattered, and the rest transmitted. The Reflection, in particular, can be Diffuse (this is the case, for instance, of Light reflecting off a White - Clear - Surface), or Specular (for example, light reflecting off a mirror). However, an Opaque Substance CANNOT transmit Light and, therefore, it can only Reflect, Scatter, or Absorb all of it. The Opacity also depends on the Frequency of the Light being considered (for instance, some kind of Glass, while transparent in the Visual Range, is largely opaque to UltraViolet Light) and its most extreme Frequency-Dependence can be seen in the Absorption Lines (which are all black) of the Gases forming distant Stars and Nebulae.

This frame has been additionally processed and colorized in Natural Colors (such as the colors that a perfect human eye - or an Electronic Eye - would actually perceive if someone were on the Surface of Mars, near the NASA - Mars Exploration Rover - Mars Laboratory "Curiosity", and then looked towards the Horizon and Sky), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team.

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