Sky & Telescope - January 2021 - 53

observations very challenging.
Blue clearings display a very puzzling
characteristic - they are seldom localized, but usually occur over the entire
visible hemisphere of the planet or were
sometimes reported as a planetwide phenomenon. This is hard to reconcile with
mundane meteorological explanations
like the evaporation of aerosols or the
settling out of dust particles. Even more
confounding is the fact that the absorption or scattering of short wavelengths of
light in the Martian atmosphere should
decrease the brightness of the planet's
polar caps, yet they stand out very clearly
in blue and violet images.
Slipher reported that photographs
taken on the same date at different
locations around the world showed
stark differences in the violet layer,

ranging from virtual opacity to nearly
complete transparency. In his classic
1962 book Mars: The Photographic Story,
he recounted that "The 1956 observations showed plainly that the state of
the blue clearing differed widely at
Lowell Observatory [Arizona], LamontHussey [South Africa], and Mount
Stromlo [Australia], where observations
were made on the same night."
In 1967 Carl Sagan and James Pollack presented very convincing arguments that blue clearings originated in
the atmosphere of Earth rather than
Mars. They noted that the intrinsic
differences in the reﬂectivity of the
albedo features on the Martian surface
are reduced at short wavelengths. Consequently, their outlines, which stand
out so boldly in red and orange light,

4:47 UT

3:20 UT

Syrtis Major

SE A N WA LK ER / S&T (2)

S This pair of Mars images, recorded with a 10-inch telescope through an Astronomik blue filter on
the evening of February 8, 2008, shows the dramatic effect of a slight deterioration of atmospheric
steadiness. At left, very steady seeing clearly revealed Syrtis Major. When the air became more
turbulent nearly an hour-and-a-half later, the feature was almost invisible.

S Though months past opposition, Mars still presents a sizable disk this month. The planet typically displays a bland disk in blue light (right) but watch for particularly steady seeing conditions in
order to identify albedo markings through a blue filter (left).

are very subdued in blue and violet
light. Turbulence in Earth's atmosphere
further reduces this contrast, which is
invariably more pronounced at short
wavelengths. Sagan and Pollack contended that blue clearings were simply
occasions when the seeing improves
sufﬁciently to permit the albedo features to be discerned more clearly visually or photographically, despite their
muted contrast in blue light.
Seeing conditions vary from place to
place and often change rapidly at any
given location. This handily accounts
for the seemingly sudden disappearance
and reappearance of the violet layer over
an entire hemisphere of Mars, as well
as the profound differences in intensity
reported from different observing sites
on the same date.
The purported tendency of blue
clearings to occur near opposition may
be attributable simply to the fact that
Mars is at its closest to Earth at such
times and has the maximum apparent
size, making it easier to record lowcontrast surface features than when
the planet is more distant. Sagan and
Pollack noted that blue clearings tend to
be reported when the largest and most
easily visible dark markings are located
near the center of the disk. In addition,
Mars is often at its highest in the sky
around the time of opposition, increasing the odds of good seeing.
Belief in the violet layer was shattered when the Mariner 6 and 7 spacecraft took close-up photographs of the
planet's topographic features through
color ﬁlters in 1969. The clarity of the
images was not perceptibly diminished
in blue light. The veritable armada of
spacecraft that followed has failed to
detect the presence of any violet layer.
To my surprise, more than a handful of observing handbooks published in
recent years perpetuate the myth of the
violet layer and blue clearings. Perhaps to
some, partaking of the mysterious can be
more gratifying than solving a mystery.
Contributing Editor TOM DOBBINS
often observed Mars through a Wratten
47 filter in hopes of observing the fabled
blue clearing.
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Sky & Telescope - January 2021

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