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The Seasons of Mars
The Seasons of Mars

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Not only does the Red Planet have seasons just like ours, they're also easy to see, writes Paul Abel

Mars by Jimmy E.

Mars by Jimmy E.

The changing of the seasons here on Earth can be quite spectacular. In many regions the whole landscape is transformed by the onset of spring: as the cold winter snows retreat and the days become longer, trees come into blossom and hibernating animals awaken. The people of ancient civilisations were particularly keen observers of these cycles, since their existence depended upon knowing when to plant seeds and harvest crops. It is no wonder that they constructed huge monuments to mark important dates like the spring equinox.

The well-defined seasons of winter, spring summer and autumn are the result of the Earth's 23.5° axial tilt. If the northern hemisphere is tilted towards the Sun it experiences summer, while the southern hemisphere experiences winter. Over the course of a year, both hemispheres experience all of the seasons, although because Earth's orbit is slightly elliptical they are not of equal length — summer in the northern hemisphere is longer than it is in the south.

Seasonal Similarity

Earth is not alone in having an axial tilt — all of the planets do, from Mercury's fractional 0.03° to Uranus' massive 82.2°. But Mars' tilt is the closest to Earth's at 25.2° and as such it experiences similar seasons — although being farther from the Sun means they last for longer.

Consequently, Mars is a very dynamic world, where the changing of the seasons can produce dramatic, planet-wide changes that are visible in small telescopes.

Mars has been closely scrutinised by Earthbound astronomers from the middle of the 19th century onwards, and although our early ideas about Martian seasonal changes being due to vegetation were wrong, the fundamental idea that its seasons have a lasting effect on its features is correct. All you need to do to see just how much Mars' surface has changed is compare a map of Mars from the 1950s to a current one.

In order to keep track of seasonal changes, we need to devise a calendar for Mars. Astronomers use a quantity called Martian solar longitude, or 'Ls', which is measured in degrees. Mars' orbit is divided into 12 months represented by 30° segments. For Mars' northern hemisphere, 0-90° is spring time, 90-180° is summer, 180-270° is autumn and 270-360° is winter. Just like Earth, the Martian hemispheres experience opposite seasons, so if it is spring in the northern hemisphere it will be autumn in the south.

Month by Month

By the end of May, Mars will be a reasonably large 18 arcseconds, and its northern hemisphere will be slightly tilted towards us. By the time of opposition, Martian solar longitude has a value of 157°, so it is will be early summer in the north. You can look up the value of Ls in the freeware program WinJupos (www.grischa-hahn.homepage.t-online.de).

The large northern polar ice cap will now have shrunk dramatically, appearing as a small white patch in the far north. As the cap recedes, a dark band can usually be made out surrounding it. This is the Lowell band, which can sometimes be very prominent and appears a dark brown in color. With a 6-inch or larger telescope you may also be able to see an equatorial cloud band, which can give the northern part of Syrtis Major a distinctly bluish cast.

The melting of the north polar cap returns a lot of volatiles into the Martian atmosphere, and over the following months you should keep a look out for bright white clouds and fog patches. There are a number of well known places where they form: the Hellas, Argyre and Chryse basins for example. Sometimes they form around the vast shield volcano Olympus Mons or the other volcanoes in the Tharsis Plateau. Clouds and fogs sometimes encroach into the darker albedo features; you can enhance their contrast using a blue filter such as a Wratten number 80 (W80). If they form on the sunward limb, you can watch them dissipate as Mars rotates and the late morning Sun disperses them.

By the end of July, solar longitude on Mars will be 195°. Summer will have now given way to autumn and it will be slowly getting colder in the north. However, spring will have arrived in the south and the southern ice cap will now start retreating, and by the start of August the dust storm season will be underway. This is the time to keep a look out for storms which usually start off as small orange clouds. They may remain small or they can continue to develop until they have encompassed the entire globe, hiding all but the most prominent features.

By October, the southern hemisphere will be tilted towards us. It will be spring in the south, and if you have access to an 8-inch telescope you should be able to follow the shrinking of the south polar cap over the next couple of months. By the end of November, Martian solar longitude will be 271°, marking summer solstice for the southern hemisphere. The Martian disc will now be small, but imagers with large telescopes should still be able to pick out any dust storms.

Mars' Shifting Ice Caps

The extent of Mars' frozen poles varies greatly with the seasons.

The polar ice caps of Mars can be very striking and can be glimpsed in a 4-inch telescope when prominent. The caps wax and wane with the planet's seasons. At the time of opposition, it will be summer in the north, and so the northern ice cap will have shrunk to a small white patch, and it will probably be quite difficult to see.

As we move into September, Mars' southern hemisphere will start to tilt towards us. It will be spring in the south, and from the end of October onwards you should be able to watch the large south polar cap start to shrink. A blue Wratten 80 filter will help enhance the polar regions. By December, Mars will be quite small, and so you'll need at least an 8-inch telescope and a magnification of 250x or more to see the south polar cap.

Dust in the Wind

Tiny dust storms can develop into planet-wide events.

Although the Martian atmosphere is not very substantial, it can still produce winds that can develop into dust storms. The storms begin as small orange clouds, and can grow quite rapidly. There are three categories:

  1. Local: Small yellow-orange clouds usually less than 2,000km in size, but can encompass whole regions (such as the Hellas basin).
  2. Regional: These are much larger and may affect a whole hemisphere.
  3. Global: These cover the entire planet, and may hide the surface features for many months at a time.

In 2001, a dust storm was observed in the Hellas basin and developed into a global event shortly after. Another famous global dust storm occurred in 1971, obscuring the entire planet just as the Mariner 9 spacecraft arrived.

Dust storm season usually starts at a Martian solar longitude of 180°, so for this apparition the season starts in early July and will continue for the rest of the year. Small yellow clouds show up well in a red Wratten 25 filter, while a green filter can help to enhance larger storms.

A Martian Calendar

The 12 months of Mars, and some events to expect in them:

Martian Month Martian Solar Longitude (Ls) Event
1 0-30° Spring equinox (northern hemisphere) at Ls=0
2 30-60° Autumn in the south
3 60-90° Northern ice cap shrinking; white cloud activity
4 90-120° Summer solstice (northern hemisphere) at Ls=90
5 120-150° White cloud activity still possible (March, April)
6 150-180° Autumn equinox (northern hemisphere) at Ls=180 (June)
7 180-210° Start of dust storm season (July, early August)
8 210-240° Spring in the south (August, early September)
9 240-270° Winter solstice (northern hemisphere) at Ls=270 (October, November)
10 270-300° Southern hemisphere now tilted towards us (November, December)
11 300-330° Winter for the northern hemisphere; Mars is now very small
12 330-360° End of dust storm season

About The Writer
Dr. Paul Abel is an astronomer at the University of Leicester. Listen to him on BBC Sky at Night magazine's Virtual Planetarium.

Copyright Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Details
Date Taken: 08/17/2016
Author: Paul Abel, BBC Sky at Night Magazine
Category: Astronomy

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