Australian Sky & Telescope - April 2016__

60 AUSTRALIAN SKY & TELESCOPE APRIL 2016

more water vapour and clouds

become more frequent planet-wide.

Discrete clouds often recur at

the same places, especially Hellas,

Chryse and Libya. The ‘Syrtis Blue

Cloud’ circulates around the Libya

basin and across Syrtis Major; it

is best seen when these features

are near the limb. Viewing this

bluish cloud through a yellow filter

might cause Syrtis Major to appear

distinctly green.

Orographic clouds sometimes

form over windblown Martian

mountains, like orographic clouds

on Earth. Enormous Olympus

Mons and the three other shield

volcanoes of the Tharsis plateau

are especially prone to them. So is

Elysium Mons in Elysium on the

planet’s other side.

Limb brightenings (‘limb arcs’)

are caused by dust and dry-ice

crystals scattering light high in the

Martian atmosphere.

Morning clouds are bright,

isolated patches of surface

fog or frosty ground near the

morning limb (the celestial east

or ‘following’ side). Fogs usually

dissipate by midmorning as Mars

rotates; frosts may persist for most

of the day.

Evening clouds have the same

appearance as morning clouds but

occur on the planet’s preceding limb.

WHICH SIDE ARE YOU SEEING?
One side of Mars is fairly blank;the other is
more feature-rich. To identify surface features
you see through the eyepiece, you’ll need to
know which side of Mars is facing you.
To find out, enter the time and date when
you plan to observe into our MarsProfiler,
atSkyandTelescope.com/marsprofiler.

Choose the correct orientation to match your

telescope, and compare the map section you

get you to the map and disks printed at left.

Out observing, you’ll find that Mars

presents nearly the same face from night

to night. This is because the Martian day is

only 38 minutes longer than Earth’s. To see

other parts of Mars, you have to observe at

adifferent time of night, travel to a different

longitude on Earth or wait for a week or

more to pass.If viewed at the same time

of night from the same place, Mars takes

somewhat more than a month to complete

one retrograde (backward) ‘rotation’.

This year the North Polar Cap is tipped

into view a little less than seen here. The

cap should again have shrunk to expose its

dark collar. The vast Hellas Basin, south

of Syrtis Major, often fills with clouds and

frosts; when Syrtis Major faces us, Hellas

is often mistaken for the South Polar Cap.

When Syrtis Major rotates to the limb,

observers sometimes spot the ‘Syrtis Blue

Cloud’. It’s presumably due to the blueing

of hazes and low clouds near any limb,

combined with the effect of an unusually

dark background, rather than some

chemical peculiarity of the cloud itself.

When the four great Tharsis volcanos aren’t

capping themselves with orographic clouds,

they sometimes appear as the opposite: tiny

ground-colored dots sticking up through a

blanket of lower clouds. Amateur stacked-

video imaging can bring out fine details like

these that often escape visual observers.

DAMIAN PEACH

()

Apr. 28, 201 Apr. 22, 201 Apr. 1, 201

Hellas

Arsia Mons

Pavonis

Mons

Ascraeus

Mons

Olympus

Mons

Hellas

Syrtis

Major

Syrtis

Major

North

Polar Cap

Limb

haze Morning clouds

Evening

clouds

Clouds

over

Elysium

Dust storms can occur during

almost any Martian season, but

they’re uncommon during the

southern hemisphere winter,

meaning now. The critical

diagnostic of a dust storm is a

relatively bright patch moving

and obscuring dark features that

were previously well defined. Dust

storms may change appearance

from one night to the next.

For More Information

Jeff Beish offers his detailed

rundown of this Mars apparition,

with a calendar of what to watch

for as the Martian seasons change

through 2 0 16, at alpo-astronomy

.org/jbeish/ 201 6_MARS.htm

The next opposition of Mars

will be even better. Mars will be at

the peak of its 16-year opposition

cycle, and when it’s closest to

Earth on July 31, 2018, it will be

24.3′′ in diameter. That’s nearly the

maximum possible: 25.1′′, which it

attained in August 2003. ✦

When moisture-
laden winds
blow up and
over broad, high
Olympus Mons, a
stationary hood of
clouds (arrowed)
sometimes
forms. The 19th-
century Mars
mapper Giovanni
Schiaparelli
named this
bright spot
Nix Olympica,
“the snow of
Olympus,” though
he knew nothing
of its origin. It was
pure coincidence
that it turned out
to be a mountain.
Dark Mare
Sirenum is above
centre. South is
up in all images. M. F. MOBBERLEY

Red Green Blue

Red Planet