54 AUSTRALIAN SKY & TELESCOPE February | March 2019
EXPLORING THE SOLAR SYSTEM by Sean WalkerM
ars continues to grace the
evening skies in February, long
past its perihelic opposition
last July, but drawing nearer the
horizon night by night. Although the
Red Planet has shrunk to a diminutive
7.4 arcseconds diameter, at the time
of writing it was still showing off lots
of detail through the eyepiece. That
wasn’t the case in the months before
its closest approach in 15 years. As the
planet was poised to put on a great
show for observers at opposition with a
respectable disk size of 24.3 arcseconds,
its atmosphere had other plans, at least
for a while.
When the planet was transitioning
from a morning to an evening target
in the last days of May, observers
spotted a yellowish dust storm over the
dark albedo feature Mare Acidalium.
While Martian dust storms are not an
uncommon occurrence, they most oftentend to kick up during the summer
months in the planet’s Southern
Hemisphere. At the time, the planet had
just experienced equinox days before,
with the north entering autumn while
its south began its long spring. Seasons
on Mars last roughly twice as long as we
experience them here on Earth.
Within days, the storm moved
south and spread out, obscuring most
of Sinus Meridiani, Oxia Palus and
Margaritifer Sinus. Dust storms are
a double-edged sword, at least from
an observer’s perspective. While it’s
exciting to spot these unpredictable
changes through a telescope, they
can often grow strong enough to
encircle the entire planet, rendering
it a featureless, salmon-hued ball.But on the positive side, these storms
paradoxically can make hard-to-
discern features easily visible for a
short time. For example, as the storm
poured into the great canyon system
of Valles Marineris, it rendered this
normally invisible feature cutting
across Aurorae Sinus visible with
even small telescopes. Amateurs with
telescope apertures as small as 15 cm
were able to clearly resolve the dust-
choked valleys as conditions permitted
in early June.
For several weeks, the storm grew
to global proportions, completely
encircling the planet in the north, while
some albedo features remained visible
in the south, particularly in the region
of Mare Cimmerium. Glimpses ofSMars normally sports high-contrast albedo features and white clouds near opposition (left),
but the global dust storm that erupted in late May 2018 (right) obscured many of the familiar
features around Sinus Meridiani, seen at centre in both of these Hubble Space Telescope images.A dusty apparition
Mars was bustling with activity during the close opposition of 2018.
Mars at Opposition
2016 2018
NASA / ESA / STSCI