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.
M MINAMI & T NAKAJIMA
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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