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.
L
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.
52
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
Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
Sky & Telescope - January 2021 - Contents
Sky & Telescope - January 2021 - 3
Sky & Telescope - January 2021 - 4
Sky & Telescope - January 2021 - 5
Sky & Telescope - January 2021 - 6
Sky & Telescope - January 2021 - 7
Sky & Telescope - January 2021 - 8
Sky & Telescope - January 2021 - 9
Sky & Telescope - January 2021 - 10
Sky & Telescope - January 2021 - 11
Sky & Telescope - January 2021 - 12
Sky & Telescope - January 2021 - 13
Sky & Telescope - January 2021 - 14
Sky & Telescope - January 2021 - 15
Sky & Telescope - January 2021 - 16
Sky & Telescope - January 2021 - 17
Sky & Telescope - January 2021 - 18
Sky & Telescope - January 2021 - 19
Sky & Telescope - January 2021 - 20
Sky & Telescope - January 2021 - 21
Sky & Telescope - January 2021 - 22
Sky & Telescope - January 2021 - 23
Sky & Telescope - January 2021 - 24
Sky & Telescope - January 2021 - 25
Sky & Telescope - January 2021 - 26
Sky & Telescope - January 2021 - 27
Sky & Telescope - January 2021 - 28
Sky & Telescope - January 2021 - 29
Sky & Telescope - January 2021 - 30
Sky & Telescope - January 2021 - 31
Sky & Telescope - January 2021 - 32
Sky & Telescope - January 2021 - 33
Sky & Telescope - January 2021 - 34
Sky & Telescope - January 2021 - 35
Sky & Telescope - January 2021 - 36
Sky & Telescope - January 2021 - 37
Sky & Telescope - January 2021 - 38
Sky & Telescope - January 2021 - 39
Sky & Telescope - January 2021 - 40
Sky & Telescope - January 2021 - 41
Sky & Telescope - January 2021 - 42
Sky & Telescope - January 2021 - 43
Sky & Telescope - January 2021 - 44
Sky & Telescope - January 2021 - 45
Sky & Telescope - January 2021 - 46
Sky & Telescope - January 2021 - 47
Sky & Telescope - January 2021 - 48
Sky & Telescope - January 2021 - 49
Sky & Telescope - January 2021 - 50
Sky & Telescope - January 2021 - 51
Sky & Telescope - January 2021 - 52
Sky & Telescope - January 2021 - 53
Sky & Telescope - January 2021 - 54
Sky & Telescope - January 2021 - 55
Sky & Telescope - January 2021 - 56
Sky & Telescope - January 2021 - 57
Sky & Telescope - January 2021 - 58
Sky & Telescope - January 2021 - 59
Sky & Telescope - January 2021 - 60
Sky & Telescope - January 2021 - 61
Sky & Telescope - January 2021 - 62
Sky & Telescope - January 2021 - 63
Sky & Telescope - January 2021 - 64
Sky & Telescope - January 2021 - 65
Sky & Telescope - January 2021 - 66
Sky & Telescope - January 2021 - 67
Sky & Telescope - January 2021 - 68
Sky & Telescope - January 2021 - 69
Sky & Telescope - January 2021 - 70
Sky & Telescope - January 2021 - 71
Sky & Telescope - January 2021 - 72
Sky & Telescope - January 2021 - 73
Sky & Telescope - January 2021 - 74
Sky & Telescope - January 2021 - 75
Sky & Telescope - January 2021 - 76
Sky & Telescope - January 2021 - 77
Sky & Telescope - January 2021 - 78
Sky & Telescope - January 2021 - 79
Sky & Telescope - January 2021 - 80
Sky & Telescope - January 2021 - 81
Sky & Telescope - January 2021 - 82
Sky & Telescope - January 2021 - 83
Sky & Telescope - January 2021 - 84
Sky & Telescope - January 2021 - Cover3
Sky & Telescope - January 2021 - Cover4
Sky & Telescope - January 2021 - SA1
Sky & Telescope - January 2021 - SA2
Sky & Telescope - January 2021 - SA3
Sky & Telescope - January 2021 - SA4
Sky & Telescope - January 2021 - SA5
Sky & Telescope - January 2021 - SA6
Sky & Telescope - January 2021 - SA7
Sky & Telescope - January 2021 - SA8
Sky & Telescope - January 2021 - SA9
https://www.nxtbook.com/nxtbooks/aas/st_202410
https://www.nxtbook.com/nxtbooks/aas/st_202409
https://www.nxtbook.com/nxtbooks/aas/st_202408
https://www.nxtbook.com/nxtbooks/aas/st_202407
https://www.nxtbook.com/nxtbooks/aas/st_202406
https://www.nxtbook.com/nxtbooks/aas/st_202405
https://www.nxtbook.com/nxtbooks/aas/st_202404_qr
https://www.nxtbook.com/nxtbooks/aas/st_202404
https://www.nxtbook.com/nxtbooks/aas/st_202403
https://www.nxtbook.com/nxtbooks/aas/st_202402
https://www.nxtbook.com/nxtbooks/aas/st_202401_aus
https://www.nxtbook.com/nxtbooks/aas/st_202401
https://www.nxtbook.com/nxtbooks/aas/st_202312_aus
https://www.nxtbook.com/nxtbooks/aas/st_202312
https://www.nxtbook.com/nxtbooks/aas/st_202311
https://www.nxtbook.com/nxtbooks/aas/st_202310
https://www.nxtbook.com/nxtbooks/aas/st_202309
https://www.nxtbook.com/nxtbooks/aas/st_202308
https://www.nxtbook.com/nxtbooks/aas/st_202307
https://www.nxtbook.com/nxtbooks/aas/st_202306
https://www.nxtbook.com/nxtbooks/aas/st_202305
https://www.nxtbook.com/nxtbooks/aas/st_202304
https://www.nxtbook.com/nxtbooks/aas/st_202304_qr
https://www.nxtbook.com/nxtbooks/aas/st_202303
https://www.nxtbook.com/nxtbooks/aas/st_202302
https://www.nxtbook.com/nxtbooks/aas/st_202301
https://www.nxtbook.com/nxtbooks/aas/st_202212
https://www.nxtbook.com/nxtbooks/aas/st_202211
https://www.nxtbook.com/nxtbooks/aas/st_202210
https://www.nxtbook.com/nxtbooks/aas/st_202209
https://www.nxtbook.com/nxtbooks/aas/st_202208
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com