Sky & Telescope - October 2022 - 60

TAKESHI SATO
Planetary Event
with temperatures very far above those
known in terrestrial volcanism. "
McLaughlin offered a very plausible
alternative, suggesting that " Perhaps it
would be worthwhile to explore the possibility
of a solar reflection from oriented
ice crystals suspended in the Martian
atmosphere. . . "
Earthly Analog
Ice crystals high in Earth's atmosphere
form hexagonal shapes resembling thin
slices cut from a pencil. They slowly
descend like falling leaves, their faces
aligned parallel to the ground by aerodynamic
drag. A layer of millions of these
horizontal plates can act as a giant mirror,
producing a dazzling specular reflection
known as a subsun. Resembling the glint
off the surface of a calm body of water,
subsuns are often seen from aircraft flying
above a deck of cirrostratus clouds. These thin, translucent
veils of ice crystals form at altitudes above 6,000 meters and
can often cover the sky before the passage of a warm front.
In 1969, Victor Davydov of the Sternberg Astronomical
Institute in Moscow published a pair of papers supporting
McLaughlin's hypothesis that flares might be reflections
of sunlight off layers of aligned ice crystals floating in the
Martian atmosphere. He attributed their fleeting duration to
the planet's rotation, which displaces a reflector by about 3
km every minute. Davydov's list of flare observations included
sightings by Soviet astronomers in 1924 and 1956, helping to
dispel the widespread impression that Japanese observers had
a virtual monopoly on seeing them.
Prediction and Validation
Knowing that specular reflections appear halfway between
the sub-Sun and sub-Earth points on Mars, we conferred in
late 2000 with a group of dedicated planetary observers to
Martian-Flare Sightings
Date
June 4, 1937
December 8, 1951
July 1, 1954
July 24, 1954
November 7, 1958
November 11, 1958
November 21, 1958
June 7, 2001
June 8, 2001
LATER SIGHTING Noted planetary observer and
telescope maker Ichiro Tasaka witnessed a pair of
flares during the 1958 apparition of Mars.
analyze the Mars-Earth-Sun geometry of
historical flare observations.
Our calculations indicated that the
flares at Edom Promontorium sighted by
Saheki and McClellan in 1954 occurred
near zero phase angle illumination, with
almost vertical angles of incidence and
reflection. At Edom Promontorium the
Sun was near the local zenith, separated
from the position of Earth in the Martian
sky by less than two degrees - ideal
geometry for producing a reflection from
a horizontal surface like a layer of atmospheric
ice crystals.
The Mars-Earth-Sun geometry of other
flares suggested reflectors with a modest but
appreciable tilt. All but one of the flares at
Tithonius Lacus required a highly inclined
reflector, but this scenario wasn't difficult to envision because
the area is a maze of branching canyons with steep walls.
We also realized that for a few days early in June 2001,
when Mars would be near opposition, the Mars-Earth-Sun
geometry would be almost identical to when flares appeared
at Edom Promontorium in 1954. The sub-solar and sub-Earth
points would virtually coincide at Edom Promontorium and
cross the center of the Martian disc when the planet would be
at a reasonable altitude above the horizon for observers in the
eastern and central United States.
Our findings were published in the May 2001 issue of Sky
& Telescope. We urged readers to monitor Edom Promontorium
early the following month and organized an expedition
to observe the event from a site in the Florida Keys that promised
clear skies and steady seeing. Our team included luminaries
Don Parker, Matt (Tippy) D'Auria, Don Troiani, and
Richard Schmude of the Association of Lunar and Planetary
Observers, editors Rick Fienberg, Gary Seronik, and Carolyn
Time (UT)
19:44
Observer
Mayeda
21:00
13:15
4:32
15:03
15:05
13:35 and 13:50
6:40
7:00 and 7:53
60 OCTOBER 2022 * SKY & TELESCOPE
Saheki
Saheki
McClelland
Tanabe
Fukui
Tasaka
Florida Expedition
Florida Expedition
Location
Sithonius Lacus
Tithonius Lacus
Edom Promontorium
Edom Promontorium
Southwest edge of Tithonius Lacus
Northeast of Solis Lacus
Edom Promontorium and Northern Hellas
Edom Promontorium
Edom Promontorium
Sky & Telescope - October 2022

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