Astronomy and Space articles
by Martin George of the Launceston Planetarium
4 June 2016
Mars Has Two Seasons, Too
As we shiver somewhat while we currently stare at Mars in the evenings, a question that
many people ponder is whether Mars has seasons. It's a quite frequent question in the
planetarium. Mars certainly does have seasons, but their temperatures and durations are
quite different to those that we experience on Earth.
The seasons on both Earth and Mars are caused by what astronomers call their axial tilt.
This is the angle between the rotation axis of the planet and the vertical through the plane
of its orbit.
For Earth, this angle is currently 23.44 degrees. This figure, which is often quoted as
about 23.5 degrees, is illustrated very well by the way in which globes of the Earth are
constructed, with the line between the north and south poles inclined to the vertical by
that angle.
The reason that this causes seasonal effects is that the axis remains aimed in the same
direction in space, or at least, very nearly so. This means that as the Earth revolves
around the Sun, the northern hemisphere and the southern take turns as being aimed
more toward the Sun.
When it is summer in either hemisphere, the Sun is higher in the sky during the day and
is above the horizon for longer, resulting in more solar energy falling onto a given area of
the Earth's surface.
The same effect occurs on Mars, which has — at least, currently — a surprising similar tilt
to that of Earth. Mars' axis is tipped over by about 25.19 degrees.
On Mars, however, the seasons are longer than those on Earth. The planet takes 687
days to orbit the Sun, which is nearly twice that of Earth.
But there is more: the seasons are of quite unequal length. With the exception of
Mercury, Mars has the most elliptical orbit of all the planets, resulting in its distance from
the Sun varying considerably. In Mars' northern hemisphere, for example, the longest
season is spring, which lasts for about seven Earth months. The shortest is winter, with
about four Earth months.
An important effect of this dramatically varying distance from the Sun is that summer in
the southern hemisphere is notably warmer than in the northern, by some 20 to 30
degrees Celsius. The northern summer days reach only about -20 degrees Celsius,
whereas the summer days in the southern hemisphere can reach as high as about zero.
Astronomy and Space articles
by Martin George of the Launceston Planetarium
4 June 2016
This imbalance between the two hemispheres is thought to be responsible for global dust
storms, for which Mars is famous. They are a problem for astronomers, because they
obscure the view of the surface. I remember vividly my disappointment when Mars was
especially close to Earth in 1971: after excitedly preparing for watching the planet, for
much of the time it appeared to have an completely blank disc, rather than exhibiting its
dark shadings.
NASA's Curiosity Rover, which landed on the Red Planet in August 2012, has been
making a special study of the Martian seasons from its location in Gale Crater. It's near
the Martian equator, at a latitude of about 5 degrees south.
An artist's impression of the
Curiosity Rover on Mars, which has
made a series of measurements of
the effects of the Martian seasons.
Credit: NASA
The rover has been making various measurements of the conditions there, and has
recorded temperatures climbing close to zero Celsius on summer days, with the winter
days reaching only about -30 degrees. It also found that the temperature drops to as low
as around -100 degrees on winter nights. Curiosity also measured variations in the
humidity, which does correlate with the seasons, and the variation in the local methane
concentration, which does not seem to have any connection.
For those keen to follow the Martian seasons, it is currently nearing the end of summer in
the northern hemisphere. The autumnal equinox will take place in a few weeks' time on 4
July, and the northern-hemisphere winter solstice will occur on 28 November.
Despite feeling rather cold on Launceston's evenings at the moment, it makes one feel a
little better when we consider the Martian nights — especially in winter!
Astronomy and Space articles
by Martin George of the Launceston Planetarium
4 June 2016
In the northern Martian spring, avalanches of frost are common.
This image of the north polar region was taken by the Mars
Reconnaissance Orbiter in 2015.
Credit: NASA/JPL-Caltech/University of Arizona
Martin George is the manager of the Planetarium at the Queen Victoria Museum in Launceston.
Article reproduced with the permission of The Mercury newspaper.