CMO/OAA Cahier #02

Blue Haze?



S might be strange to anyone, we seldom refer to the problem or phenomenon of the blue haze and/or blue clearing in this journal, not to say about the blue light photographs. The reason is very simple: we simply don't believe in such an idea of the Blue Haze.

  We have no references about the blue haze at hand, and we cannot therefore dispute with any certain sources, but going back to a popular introduction of the blue haze, the surface of Mars was said to be covered by a blue layer which scatters or absorbs the light composed of shorter wave lengths so that the surface taken through the blue light was almost always opaque concerning the dark markings: it was also said that the blue light image is bigger than the red light image; the fact believed to show the existence of the blue layer at the higher atmosphere. It is however not so often stressed that the comparison is not so easy because the focal lengths are different in blue and red lights and furthermore the exposure time which is needed when we use W47 is much longer than the case we use W25 and hence the image by W47 absorbs more the unnecessary turbulence.

  We may contrarily infer that the mythology of the blue haze was the one when they believed in the existence of the fictitious canals and at those times they believed that the Martian surfaces were full of the white integrated light under the blue layer.
  If we instead suppose that the Martian surface itself absorbs the light of shorter wave length, the situation becomes much simpler. The reason that the blue-light photos don't usually convey the dark markings becomes simply because the soil scarcely emits the blue light, and so our sensibility to the surface goes down as the wave lengths are shortened. In this sense, the contour of the disc needs not be taken through the blue light if the polar cap is properly exposed. If over-exposed in blue light to the extent that the disc contour is forced to show up, then the colour balance will be broken, and the synthesised colour image will not produce the real colour of the surface.

  One may think that since the surface is in high contrast through the red light in the sense that the dark markings become darker, the colour of the dark markings are contrary to red and emit the shorter wave lengths. The fact however does not necessarily prove the colour difference. It is highly possible that the longer the wave length is, the more apparent the light and shade becomes. As once proved by DOLLFUS and FOCAS, the reflectivity difference of the bright area and the dark area increases as the wavelength of the light increases. This contrarily implies that for a shorter wave length the light and shade become quite dimmer.

  We however don't intend to say that the Martian surface does never emit the violet-blue light. This is impossible. Any marking must have a weak "blue ingredient." If we can observe in a full light scale, the blue censor must also work. For example if we approach near the surface it will give a more vivid image to us also in blue light. The reason why at opposition the markings are sometimes appear must thus be just because the difference between the incident and the reflection angle is so small that any lights are reflected back in a full scale. This phenomenon is therefore a kind of opposition effect, and should be common. The dark markings, if appeared in blue, look sometimes deformed, but this must mainly be caused by a casual float of a white veil. Hence the blue clearing, if it implies the clearing of the blue haze, or an existence of a blue-hole, must be a misnomer, and just be said to show a white veiling. If this is so, any statistic of the blue clearing is meaningless, because the opposition effect is very common, and the statistics should be replaced by the survey of the motion of the white veils, which we tentatively call mists. If a float of a mist exists, then the area where the mist prevails is shot through the blue light, and the "reddish" bright area will be enhanced to show its "blue ingredient".

  We thus conclude that the blue photos are still important, but it is not because it shows the blue holes, but it gives us information of the distribution of the whitish or water haze.


CMO No.128 (25 January 1993 issue) p1171

NB: Bill SHEEHAN wrote as follows in his “The Planet Mars” (Arizona University Press, 1996, p120) “We now know, however, that the violet Layer does not exist. The blue clearings have been rather mundanely explained as due to phase angle effects of light scattering by airborne dust, which causes occasional enhancement of the low-contrast differences between the light and dark areas in blue light.*”

 *T E THORPE, Viking Orbiter Observations of the Mars Opposition Effect, Icarus 36 (1978) 204.

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