Sky & Telescope - December 2019 - 28

Red Planet Forecast

7KHWULJJHUIRUJOREDO
GXVWVWRUPVRQ0DUV
LVVWLOODP\VWHU\
GLOBAL STORM These images
from the Mars Global Surveyor
orbiter show a 2001 regional dust
storm in the basin Hellas (left,
basin is bright oval feature) that
within a few days exploded across
the planet, ultimately veiling the
planet in dust (right).

D EC E M B E R 2 019 * SK Y & TELESCOPE

appears to initiate many of the dust storms on Mars is the
wind stress, a drag exerted on the surface by the passing wind.
If the wind stress is above a certain threshold, the wind can
lift the tiny (smoke-size) grains of dust that cover the high
albedo regions of the planet, keeping that dust airborne for
weeks or even months at a time. In fact, the famous reddish
sky of Mars photographed by landers and rovers is evidence
that there is always some dust suspended in the Martian
atmosphere. The oxidized iron in the dust grains absorbs most
of the sunlight's blueish wavelengths and scatters its reddish
ones, giving the Martian sky its distinctive tan hue. Indeed,
if there were no dust in the atmosphere, the sky would be
dark blue or perhaps even black (comparable to being on the
Moon) because the atmosphere is so thin.
The airborne dust also gives Mars its distinctive blueish
sunrises and sunsets. When the Sun is low in the sky, its
light has to pass through much more of the atmosphere to
reach a rover's cameras than it does at other times of day, and
many more of the redder hues of sunlight are scattered away.
Changes in the amount of dust in the Martian atmosphere
determine how blue the sunrise or sunset is - an ironic and
visually stunning contrast to the varying blushes of sunrise
and sunset on our own planet. Dust is not only a key part of
Mars's weather, it is a key part of Mars's character.

Long-Term Forecast
Just like on Earth, the distinction between weather (day-today environmental and meteorological phenomena) and
climate (decades- or centuries-long weather trends) is crucial
to understanding Mars. Surface, atmospheric, and satellite
observations of the Red Planet are helping us to measure the
sol-to-sol weather as well as to learn about long-term climate
changes. Although Mars's climate is now far less volatile than
Earth's, there is ample evidence in the geologic and atmospheric record that the planet's environment wasn't always

N ASA / JPL / MSSS

polar caps and traveling along storm tracks, lifting dust as
they brush across the surface. Others form more locally, as
winds move across hills or canyons and become turbulent, or
as warmer air rises out of craters or cooler air descends down
mountain ﬂanks.
The combined MOC and MARCI observational record
shows that smaller dust storms frequently merge into larger
regional storms. However, we still don't fully understand
what occasionally causes them to merge into even larger
hemispheric or truly global dust storms that shroud the surface from telescopic and orbital view and blot out the surface's view of the Sun.
Astronomers observed many such enormous storms in
the 19th and 20th centuries; among the more recent ones
are those in 1971 just before NASA's Mariner 9 spacecraft
went into orbit, in 1977 during the Viking Lander missions,
in 2001, and in 2007. The most recent global dust storm
on Mars began in June 2018 and lasted for several months,
turning daytime into night in the skies above NASA's solarpowered rover Opportunity and ultimately killing that robot
after more than 14 years of successful operations on the Red
Planet (S&T: Sept. 2019, p. 24).
As with the regional storms, the trigger for global dust
storms on Mars is still a mystery. What we do know is that,
unlike for smaller storms, there is a preferred season for the
largest ones: Most occur during the "heat" of southern summer. The timing indicates that the increased energy of the
southern summer's more intense sunlight fuels these enormous tempests. However, just like for hurricane season on
Earth, there are also relatively chaotic local to regional effects
from the atmosphere and surface that frustrate global dust
storm forecasting. It's not yet possible to predict exactly how
many large dust storms will occur each Martian year, or even
whether a global-scale storm will occur at all.
On a smaller scale, the important weather factor that

Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019