Sky and Telescope - July 2018 - 26

Mars Opposition

on how the Martian winds move bright surface dust around.
The most obvious southern hemisphere features include Syrtis
Major, an ancient shield volcano shaped like the subcontinent India; the "chicken drumstick" combination of Sinus
Sabaeus and Sinus Meridiani (east of Syrtis Major); the great
bands of Mare Tyrrhenum and Mare Cimmerium that stretch
west of Syrtis Major; the vast and amoebic Mare ErythraeumAurorae Sinus complex; and the dark eye of Solis Lacus. In the
northern hemisphere you can't miss the dark thumb of Mare
Acidalium and Niliacus Lacus located at the same longitudes
as Mare Erythraeum.
But identifying albedo markings can take time. Most are
subtle and difﬁcult to pick out against the glaring orange
landscape, but with practice, they become easier to see until
you recognize them like continents on a globe. To make the
task easier, observe the planet with as much magniﬁcation
allowed by the seeing conditions. I've always found a red
#23A ﬁlter a big help in boosting their contrast.
This is especially true when viewing Mars's "boring"
hemisphere, located between about longitudes 110° and
240°, which includes the narrow polar-hugging stripe of
Mare Sirenum and the low-contrast volcanoes of the Tharsis
Plateau. Orographic clouds often cover Olympus Mons, the
planet's largest extinct volcano, making it look like a pale,
white pustule in the ochre desert.

Clouds, Hazes, and Dust Storms
One of the most exciting aspects of observing Mars is discovering its similarities to Earth. Both planets have their share

SHARPEST VIEW These
images captured during
the planet's 2012 apparition show many of Mars's
dark albedo markings.
The planet rotates every
24 hours, 38 minutes.
Since that's very similar
to Earth's rotation, we
view nearly the same
hemisphere from night to
night. If you observe Mars
at 11 p.m. from the same
place night after night, the
planet will appear to rotate
in retrograde (backward)
over a period of about six
weeks. If you see Syrtis
Major front and center
at 11 p.m. the first night,
you will see it in that same
spot about six weeks later.
To see a different side of
the planet, you need to
observe Mars at a different time of night or from a
different longitude. South
is up.

Sinus
Sabaeus

p YELLOW STORM RISING With frequent observing, you can detect
changes in the day-to-day appearance of Mars. For example, a dust
storm erupted on the planet in October 2005. By recording images each
night, S&T Equipment Editor Sean Walker was able to track the progress
of the yellowish clouds as they traveled across the planet's dark surface.
South is up.

of clouds, fog, and mist. On Mars, these often appear as narrow bands of white haze along either the morning or evening
limb where the Sun is just rising or setting. In my experience, these are the most common clouds visible in amateur
telescopes, but watch for isolated puffs and high-altitude
clouds that hug Martian volcanoes. A blue #80A ﬁlter will
enhance their visibility.
In the opposite hemisphere, autumn will be underway
during Earth's summer and fall, with clouds forming over
the north polar cap (NPC) and beyond to create the NPH.
By October, the NPH may extend as far south as latitude
50° north, giving the "top" of Mars a diffuse, off-white cast
as if it had been dipped in milk. Isolated clouds can appear
anytime, especially as the SPC shrinks through the summer
and fall.

Margaritifer
Sinus

Sinus
Meridiani
Limb
haze

Mare
Sirenum

Mare
Erythraeum

Olympus
Mons

Aurorae
Sinus
Chryse
Niliacus
Lacus
Mare
Acidalium

North
Polar Cap

Mare
Cimmerium
Olympus
Mons

Evening
clouds

J U LY 2 0 1 8 * S K Y & T E L E S C O P E

Mare
Tyrrhenum
Syrtis
Major

Hyblaeus

Utopia

Hellas

Sky and Telescope - July 2018

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