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Getting To Know Mars
By Greg Bryant

Published in the April 1999 issue of Universe
 
 



Since the beginning of this decade, the Red Planet Mars has attracted an increasing amount of attention. Notwithstanding the failure of the Mars Observer mission in 1993, Mars captured the imagination of the public in July 1997 with the landing of Mars Pathfinder and its rover, Sojourner. For a few days, it made the news each day and night, and we could follow its explorations and findings. Two months later, the Mars Global Surveyor probe arrived in orbit, albeit it an eccentric one. It has just settled into a circular path around Mars, and its official mapping program should have begun by the time you read this. Three other spacecraft (two American and one Japanese) are currently on their way to Mars.

It is in this setting of renewed scientific exploration that we see Mars coming to opposition again in our skies and with Mars becoming brighter and larger, we should take the opportunity to set our sights on the Red Planet as frequently as possible. A lot can happen when you are observing Mars and it warrants observing on more than one weekend per month, especially since dark skies are not required.

My profile on the ASNSW in the February issue of Sky & Telescope magazine stated that we "do something new virtually every week". Whether you are a newcomer, an experienced hand, and/or a regular at the Society's two observing sites, you can maximise your enjoyment of astronomy by observing the sky more frequently and under different settings.

No Telescope Required

Mars is bright - it was known to the ancients as a wanderer across the sky. In 1609, Johannes Kepler analysed the accurate observations of Tycho Brahe (a Danish astronomer to whom Kepler was an assistant) and, after trying to reconcile the observations with a presumed circular orbit, he concluded that the orbit of Mars was actually elliptical. From this he established the first of his three laws of planetary motion: the orbit of a planet is an ellipse with the Sun at one focus of the ellipse.

You don't have to repeat the precise observations and calculations of Brahe and Kepler, but you can take a relaxed approach to following Mars. The first step is to identify the planet in the sky.

When a planet is at opposition - that is, the Earth lies between the Sun and the planet - the rule of thumb is that the planet rises when the Sun sets, and sets when the Sun rises - a bit like the Full Moon. In the days, weeks, and months leading up to the time of opposition, the planet doesn't rise until some time after sunset. Similarly, after the time of opposition, the planet is already above the eastern horizon when the Sun has set, but the planet sets before the Sun rises in the morning. These rules apply for Mars. At the beginning of April, Mars is rising just after 7 pm, but by the beginning of May (ie. after opposition), it is above the horizon by 5 pm, before sunset.

Mars is brighter than magnitude -1 between the beginning of April and early June. It will be the brightest point of light in the eastern evening sky. Furthermore, its reddish colour will give it away. During April, Mars moves from the constellation of Libra into Virgo, where it remains until late July. A project that you might like to pursue is to plot the position of Mars on your charts, without referring to an ephemeris beforehand, seeing how the planet moves among the stars.

The 1999 Opposition

Although Mars is visible as a naked-eye planet at any time of the year except when it is in conjunction with the Sun, the best time for telescopic observations of Mars is during the months around the time of opposition, when Mars is close to Earth and its apparent diameter is at its largest. Mars reaches opposition on 25th April, and a few days later, on 2nd May, it is at its closest to Earth (0.58 AU) when its disk will be 16.18 arcseconds wide.

As far as oppositions of Mars come and go, this is barely middle-of-the-road. Table 1 lists the oppositions and closest approaches of Mars between 1986 and 2005. From that, you can see that we are just coming out of a trough of unfavourable oppositions. The 1999 opposition is better than those of 1993, 1995, and 1997, but it still doesn't beat 1990, let alone the oppositions of the late 1980s and early next decade.

(Table 1)
Opposition                              Nearest To Earth                        
                                                        
Date                    Dec     Date                        Distance            Diameter
                                                        AU      Million km      
                                                        
1986    July 10         -27.7   1986    July 16         0.4036  60.37           23.19
1988    September 28    - 2.1   1988    September 22    0.3932  58.81           23.81
1990    November 27      22.6   1990    November 20     0.5169  77.33           18.11
1993    January 07       26.3   1993    January 03      0.6261  93.66           14.95
1995    February 12      18.2   1995    February 11     0.6757  101.08          13.85
1997    March 17          4.7   1997    March 20        0.6594  98.64           14.2
1999    April 24        -11.6   1999    May 01          0.5785  86.54           16.18
2001    June 13         -26.5   2001    June 21         0.4502  67.34           20.79
2003    August 28       -15.8   2003    August 27       0.3727  55.76           25.11
2005    November 07      15.9   2005    October 30      0.4641  69.42           20.17
Why do oppositions of Mars differ? You'll recall that Kepler determined that Mars had an elliptical orbit rather than a circular orbit. Indeed, all the planets have elliptical orbits, with differing amounts of eccentricity. Earth has an eccentricity of 0.017, meaning that its distance from the Sun varies between 147 million km and 152 million km. Perihelion occurs in January and aphelion occurs in July. Mars' eccentricity, on the other hand, is 0.093 - a difference in solar distance of more than 42 million km. When we observe Mars at opposition around the time of its perihelion, known as a perihelic opposition, Mars displays a larger disk because it is closer to Earth than when we observe it at an aphelic opposition.

Experienced observers of Mars know that the planet is worth observing whenever its disk is greater than 6 arc-seconds. As April opens, Mars diameter is already greater than 14 arc-seconds. It reaches its maximum of 16.2 arc-seconds on 2nd May and remains greater than 14 arc-seconds until the end of May. Even by the end of June, it is still greater than 11 arc-seconds in diameter.

Referring back to Table 1, you will notice that forthcoming oppositions will be even better. In 2003, Mars will display a disk of 25.11 arcseconds. Oppositions of Mars that display a disk diameter greater than 25 arc-seconds are rare. The last was in August 1924 when a disk of 25.10 arc-seconds was on offer. Prior to that, August 1845 saw a disk of 25.09 arc-seconds and August 1766 had 25.08 arc-seconds. Indeed, if you go back over the last two thousand years, there is no occasion when Mars was as close to Earth as it will be in August 2003. The best opposition of Mars in the history of the telescope is just four years away - and it won't be until the 24th Century that Mars is even closer! We live in fortunate times.

Observing Mars

So far, we've discussed the circumstances in which Mars will present itself to us this year. If you have a telescope, what do you do?

Textbooks tell you that planetary observing is best done with a large refractor or a large Newtonian reflector. However, good views are obtainable from Schmidt-Cassegrains, so use what you have. At the end of the day, what you see on Mars will be limited by the atmospheric seeing - Earth's more so than Mars'. To study the planet will mean spending a lot of time at the eyepiece waiting for those brief moments when the seeing improves.

A telescope as small as 6 - 8 cm in size will show the polar caps and the major dark areas. Serious work can be done with telescopes ranging from 15 cm upwards.

Colour filters are important in getting the most out of your Martian observing. If you don't have any, don't worry. You will still see features on Mars without filters, just as you can see galaxies and planetary nebulae without deep sky filters. However, an investment in colour filters would be worth considering. They improve the contrast and they can help in identifying the nature of the feature you are seeing.

Red and orange filters are best for observing the surface of Mars. As you move from the red to the blue end of the spectrum, less of the Martian surface and more of the Martian atmosphere is seen. Daniel Troiani, the Mars Section Coordinator of the Association of Lunar and Planetary Observers (ALPO), recommends that a basic set of filters should include red or orange (Wratten 25 or 23A), green (W58), blue-green (W64), blue (W38A or W80A), and violet (W47). If you don't have a large aperture telescope, you may find that the W25 (red) and W47 (violet) filters are too dark. In this case, a yellow (W15) filter may suffice.

According to Troiani, colour filters assist your observing in the following ways, depending on what colour you use:

When you're observing Mars, why not take the opportunity to sketch what you see. It helps you focus on the detail and provides a record for comparison from night to night and between Martian oppositions. ALPO recommends that a circle of 4.2 cm diameter on white paper be used. With a soft pencil, lightly draw the largest markings. An eraser can be used to lighten an area. A particularly bright spot is best indicated with a dashed line. Don't forget to note the time and date on your sheet of paper as soon as the larger areas are sketched in. After the major features and the polar cap have been drawn, add the fine details.

Since the perihelic oppositions of the late 1980s, a new tool for recording Martian oppositions has entered the reach of the amateur - a CCD camera. A few of our members already own such a camera and there has been talk for years of the Society actually purchasing one. Crago Observatory would be the obvious location for such a setup. If you have CCD equipment, you can use a variety of filters, one after each other, reeling off quick exposures.

Identifying Surface Features

Whenever you observe the sky, there is always a degree of satisfaction in identifying what you are seeing. The same applies for Mars, when you can turn a dark smudge on the disk into a named feature.

In identifying features on Mars, you need to know what side of the planet you are looking at. Table 2 gives details of the longitude of the central meridian - an imaginary line that runs down the centre of the disk from the north pole to the south pole. The table gives the longitude on Mars for 0:00 UT. This corresponds to 10:00 am in New South Wales.

(Table 2)
        April 1999      May 1999
Date    Central         Central
0hr UT  Meridian        Meridian
1       344             79
2       335             71
3       326             62
4       317             53.2
5       308             44.4
6       299             35.7
7       290             26.9
8       281             18.1
9       272             9.3
10      264             0.5
11      255             351.7
12      246             342.9
13      237             334.1
14      228             325.2
15      220             316.4
16      211             307.5
17      202             298.7
18      193             289.8
19      184             280.9
20      176             272
21      167             263.1
22      158             254.1
23      149             245
24      141             236
25      132             227
26      123             218
27      114             209
28      106             200
29      97              191
30      88              182
31                      173
To find the longitude at the time you are observing, convert the current time into Universal Time (ie. subtract 10 hours as AEST is 10 hours ahead of UT). Consulting the table, add 14.62 degrees for each whole hour since 0:00 UT, and 0.244 degrees for each additional minute.

Example: what is the longitude of the central meridian of Mars at 11:20 pm on 25th April?

Converting the time to UT = 13:20 UT on 25th April. The longitude at 0:00 UT on 25th April is 131 degrees. Adding 13 x 14.62 (hours) and 20 x 0.244 (minutes) gives a longitude value of 326 degrees.

Having calculated the longitude that you are observing, compare it to one of the diagrams whose central meridian is closest to what you are observing. If you are looking for further features, consult a more detailed map of Mars (the April issues of Astronomy and Sky & Telescope magazine each have maps).

Mars' period of rotation is just 40 minutes longer than Earth. This has implications for observing the entire planet. Given that you would only be able to observe about half of the Martian surface during the course of one night (taking into account that observations of Mars low in the sky would be compromised by atmospheric turbulence), then a program of observing Mars should be conducted over many nights.

What To Look For

In recording your Martian observations, or planning what to look for, there are four main things to keep an eye on:

1) Albedo Features: The surface markings on Mars are essentially variations in the albedo (reflectivity) of the soil. There is a relationship with the actual topography of Mars, but it is small. The brighter areas are covered with fine dust whereas the darker regions are probably rock or coarser sand. The Martian winds blow dust continuously, so many areas change their size and shape. These changes can be seasonal or even long-term.

Yearly variations are known to occur in such areas as the Elysium region, Sabaeus-Meridiani, Solis Lacus, and Syrtis Major. The latter is the most prominent dark area on Mars and is a large wedge-shape found just north of the Martian equator. During this century, its width has varied on a seasonal basis: maximum during the northern summer and minimum during the northern winter. However, observations by amateurs and the Hubble Space Telescope have shown little variation in the 1990s. What's happening this year? The northern summer on Mars began on 29th January, and autumn begins on 31st July. Can you detect any changes during the next few months?

The Solis Lacus region has seen some interesting changes over the years. Also known as the "Eye of Mars" because of its resemblance to the pupil of an eye, the region expanded and darkened in 1975.

In 1977, amateurs discovered a new dark feature in the Aetheria desert at 240 degrees longitude, 25 degrees north. It was subsequently found on images taken by the Viking Orbiter in 1975 - apparently undetected by Viking scientists.

2) North Polar Cap: The northern hemisphere of Mars tilts up to 23 degrees towards Earth this year, providing a good view of the north polar cap shrinking. However, you may see the cap appear to regrow as the water vapour lost from the shrinking cap causes clouds to form.

3) Clouds: The Martian atmosphere is in a constant state of change, and has been studied with interest in the last few decades. A study of cloud observations since the 1960s suggests that the appearance of water ice crystal clouds and surface fogs is more frequent in the northern spring and summer than in the southern spring and summer. Consequently, there may be plenty of clouds during this apparition (hopefully, they'll be Martian clouds rather than the variety closer to home).

Before opposition, observers should be watching for the famous W clouds, so-called because the clouds grow so large that they form a "W" shape. These were noticed by E. C. Slipher in 1954 to be forming every late-spring afternoon over the Tharsis-Amazonis region. A decade later, it was proposed that the clouds were orographic in origin, and this was confirmed in 1971 when Mariner 9 images showed the clouds forming as wind passed over the volcanoes Olympus Mons, Ascraeus Mons, and Pavonis Mons.

Another cloud formation of note is the Syrtis Blue Cloud. Originally called the "Blue Scorpion" by Father Angelo Secchi in 1858, this cloud moves around the Libya basin and across Syrtis Major, changing the colour of the dark feature to an intense blue. The cloud usually appears during the late spring and early summer of Mars' northern hemisphere and was noticed at the last two apparitions in 1995 and 1997. It is best seen when Syrtis Major is near the limb of the planet and a yellow filter combines with the blue cloud to present a vivid green colour!

Mars exhibits morning and evening clouds. Morning clouds are bright patches of surface fog or frosty ground. The fog dissipates by mid-morning, but the frost can persist for most of the day. Evening clouds are usually larger and more numerous.

4) Dust Storms: These are double-edge swords to watch. On the one hand, they can be exciting to observe as they form and grow, but if they get too big, they can hide surface features for many months. Some dust storms have been known to cover the whole planet.

Dust storms can occur during virtually any Martian season, but historically they peak during Mars' southern summer. There is a second, smaller peak during Mars' early northern summer, as was the case during the 1984 apparition. Global dust storms are quite rare. Only five have been observed since 1873, and all of these have been since 1956. However, localised versions often start in Elysium, Serpentis-Noachis, Solis Lacus, or Chryse. Already in this apparition, a local dust storm was observed in mid-February. If a dust storm does occur, take note of where it originates, how long it lasts, and how large it becomes.

Making The Most Of Mars

Oppositions of Mars only occur every 26 months, so it's important not to miss the opportunity. Between now and the end of May, Mars' diameter is greater than 14 arcseconds - a size that was never reached during the opposition of 1995. Like any aspect of observing, the more time that you put into observing Mars during these months, the more you will get out of it and the better prepared you will be for future oppositions.

Amateur observations of Mars remain important. As space agencies send probes to Mars, the orbiting cameras will only focus on small regions of the planet at any one particular time. A global perspective is important, and professional telescopes can not afford to monitor Mars continuously.

Our club magazine is a wonderful forum for members' observations. If you undertake any drawings of Mars, or take photographs or CCD images, please submit them to Universe.

Describe the successes you achieve with your observations.

If you haven't yet commenced your own personal exploration of Mars, let tonight be the beginning of a lifelong association with the planet that will be the focus of attention in the coming decades.