Sky & Telescope - January 2021 - 52

To this day observers report the unpredictable phenomenon known as the blue clearing. Spacecraft views like the above image from the European Space Agency's Rosetta
probe conclusively prove that it is an illusion arising in the atmosphere of Earth rather
than that of Mars.

The "Blue
Clearing"
on Mars
This legendary phenomenon
is a product of Earth's
atmosphere rather than the
Red Planet's.

ate in September 2020, the renowned
observer Stephen O'Meara reported
that he was enjoying unusually clear
views of Martian surface features
through a blue ﬁlter. His remarks called
to mind reports of a strange atmospheric veil on Mars that endured well
beyond the dawn of the Space Age.
Early in the 20th century a number
of astronomers began to photograph
the planets through color ﬁlters. In the
United States this work was conducted
by Carl Otto Lampland and Earl C.

JANUARY 2 021 * SK Y & TELESCOPE

Slipher at Lowell Observatory, Robert
Trumpler and William Wright at Lick
Observatory, and Frank Ross at Mount
Wilson Observatory. Russian astronomers Aristarkh Belopolsky and Gavriil
Tikhov undertook similar investigations
at Pulkovo Observatory.
Mars displayed the
most dramatic differences in appearance
through color ﬁlters.
In red, orange, and yellow light the planet's
albedo features - the
bright "continents"
and dusky "seas" in
19th-century parlance
- stand out boldly.
In the blue region of
the spectrum around
wavelengths shorter
than 450 nanometers,
Mars often appears
uniformly bright and
devoid of features
except for diffuse
clouds and hazes near
the morning and

evening limbs, as well as in the regions
above the polar caps. This bland appearance was attributed to the presence of
a mysterious "violet layer" composed of
aerosols or exceedingly ﬁne particles of
dust or ice high in the tenuous Martian
atmosphere that selectively absorbs or
scatters light of short wavelengths.
The violet layer was regarded as
a permanent feature of the Martian
atmosphere until 1937. For a period of
several days around the date of opposition that year, Slipher's blue-light photographs, which required much longer
exposures than in red light, captured
the planet's conspicuous albedo features
Syrtis Major and Sabaeus Sinus so
clearly that he reported that they might
have been mistaken for images taken
through a yellow ﬁlter. He attributed
this strange transient clarity to the sudden disappearance of the violet layer.
In the decades that followed,
Slipher and many other Mars observers reported additional "blue clearings"
that varied in intensity and lasted from
a few hours to several days. According
to Slipher, ". . . there is deﬁnite tendency for pronounced blue clearings to
occur more often near opposition."
Looking for these events by carefully examining the planet through a
Wratten 47 (blue-violet) ﬁlter became
standard practice for several generations
of dedicated visual
observers. The reddish
surface materials of
Mars reﬂect only 5%
to 7% of incident blue
light. Combined with
the fact that the sensitivity of the human
eye falls off in this
region of the spectrum, the excessive
dimming of the planet's image through
this dense ﬁlter made
W Earl C. Slipher of Lowell
Observatory was first to
record distinct albedo
markings on Mars in blue
light. This "blue clearing"
became a phenomenon to
watch for each apparition.

M A RS: ESA; SLIPHER: TERENCE SPENCER

JANUARY 2021 OBSERVING
Exploring the Solar System by Thomas A. Dobbins

Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021