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Mars Nomenclature


Mars Compared To Earth

Mars is the most Earth like planet in our solar system: it has mountains, valleys, ancient dry rivers, lakebeds, and seas that once held salty water, and it may have liquid water under its surface. In addition Mars has had glaciers form and move across its surface during ice ages in the past, and it has experienced volcanism as well. Mars has also an atmosphere, although much less dense than ours, as the average atmospheric pressure on Mars is only about 7% of the Earth's.

NASA's Curiosity rover has determined that that Mars could have supported a habitable environment.

In addition NASA's Mars Global Surveyor (MGS) has shown that the surface of Mars was shaped by plate tectonics, just like the Earth. Plate tectonics work in the following way. Continents are embedded on plates that sit on top a layer of molten rock called the mantle. On the Earth, molten rock rises from the mantle at ocean ridges, such as the Mid-Atlantic Ridge, and is pushing the North American plate away from the European plate. In other cases ocean ridges push continental plates towards each other where they collide together.

Unlike the Earth, where this process continues today, plate tectonics no longer occurs on Mars. However, when this process occurred on Mars it formed some of its' most prominent features. For example, on the Earth the Hawaiian Islands were formed as a plate moved slowly over hotspot in the mantle. It is believed that the same process formed the Tharsis volcanoes on Mars, including Olympus Mons, the largest volcano in the solar system. Another well known feature on Mars, The Valles Marineris, is an example of a tectonic plate being pulled apart.

NASA's Mars Exploration Rovers have found evidence that Mars once had liquid water on its surface, and new research suggests that salty liquid water could remain on the Martian surface for an extended period of time. This is because salty water can exist in a liquid form at colder temperatures than pure water, so it won't evaporate as quickly or freeze.

For example pure water freezes at 32 Fahrenheit, while salty water, depending upon its salt content, may not freeze until the temperature reaches -58 degrees Fahrenheit. The surface temperature on Mars varies from a low of -220 degrees Fahrenheit at the polar region during the winter to a high of 80 degrees Fahrenheit during the summer, so it possible there are some locations that salty water could exist on the surface.

In addition NASA's rovers have shown that Mars atmosphere has cirrus-like clouds like those seen in Earth's atmosphere. Like Earth it has four seasons, roughly twice the length as those on Earth, so the Martian year is 687 days long. The Martian day is 24 hours, 37 minutes, and 23 seconds long, slightly longer than the Earth's. Although it is less than half the size of Earth in diameter, its land surface area is very similar to Earth's.

The NASA Phoenix Mars Lander touched down near the north poler region and detected water-ice near the Martian surface.

Here is a link to all of my Mars drawings and observing reports on my web site.

Drawing made May 16, 1999 using Astro-Physics 7.1" f/9 EDT refractor on homemade Dobsonian-style mount.

Observing Mars

Mars can be a challenging planet to observe. One reason is that the disk of Mars even at very favorable oppositions is only 25 seconds in diameter, while for some oppositions it can be less than 14 seconds in diameter. By contrast Jupiter's disk can be almost 50 seconds in diameter. Also, due to Mars orbit, the closest oppositions occur when Mars is in the southern most constellations of Sagittarius and Scorpius, which is low in the sky for Northern Hemisphere observers where seeing can be more of a problem.

This is important because the higher a celestial object is in the sky the less atmosphere you will be looking through and the better the seeing tends to be. When I observed Mars during the 2003 opposition I found that because it was low in sky I was limited to at best around 30 minutes to observe and make a sketch of the planet due to seeing. Normally when I observe a planet if the seeing is good I may spend 15 - 20 minutes studying the detail that visible before I begin to make a sketch of it. It the seeing is good to excellent and there is other interesting detail visible I may spend an hour and a half or more observing and making additional sketches of the planet.

Further, unlike Jupiter and Saturn, which have oppositions every year, Mars only comes to opposition once every 26 months. So observers do not have as much of an opportunity to become familiar with Martian detail and train their eyes to see it. In addition, Mars can present its equator, north polar region, or south polar region to Earth during its oppositions, so not every feature is visible each time, and those features that are visible can appear different. The graphic below shows the oppositions of Mars from 2010-2025.

This graphic was created by a Curt Renz and used with his permission.

Another issue is dust storms that can grow and blot out all of the surface detail for months at a time, as occurred during the 2001 opposition of Mars. Despite all of this, Mars is a very interesting planet to observe and study as its surface and atmospheric features change.

Martian Surface Features

The Martian surface markings are caused by differences in reflectivity of large areas of rock and dust. Early Mars observers adopted the convention of referring to the lighter areas as "Lands" and the darker areas as "Mare" or "Seas". Other terms that were used include "Lacus" or "Lake", "Sinus" or "Curve", and "Mons" or "Mountain".

There are notable differences between the Martian hemispheres. For example, the Northern Hemisphere is smoother and lower in elevation than the Southern Hemisphere. By contrast, the Southern Hemisphere is heavily cratered, and on average about three miles higher in elevation than its northern counterpart.

Windstorms can sometimes move the dust resulting in the lighter and darker areas appearing different from one season to another and one opposition to another. One example is how Solis Lacus has changed in appearance since the 1986 opposition as shown in the following sketches.

Note that the differences in color between the 1986, 1988, and 2003 Mars nomenclature sketches are due to different color pencils being used to prepare the sketches rather than a change in the color of the Martian surface features. Also note that south is at the top in each of the sketches:

Compare the sketches below from 1986, 1988, and 2003. Even taking into the account that the tilt of Mars was not the same in 1986 and 2003, and the fact that different telescopes and magnifications were used to make the sketches, Solis Lacus appeared larger in 2003 and its shape was more elongated than in 1986:

Drawing made July 25, 1986 using Astro-Physics 6" f/9 refractor on a Byers 58 German equatorial mount:


Drawing made September 11th, 1988 Astro-Physics 7" f/9 refractor on an Astro-Physics 706 German equatorial mount. Magnification 202x - 234x:


Drawing made August 24th, 2003 Astro-Physics 5.1" f/8.35 EDF refractor on home made Dobsonian style mount. Magnification 185x - 231x.

Other areas that often show variations include Syrtis Major, Sinus Sabaeus and Sinus Meridiani.

The north and south poles of Mars are covered by ice caps composed of frozen water and carbon dioxide. These caps (South Polar Cap, or SPC, and North Polar Cap, or NPC) thaw during the spring in their respective Martian hemispheres, and reform during the autumn. As the ice caps melt and retreat their edges can become jagged and "Rima" or "Fissures" or "Rifts" become visible in the cap. During the 2014 opposition the North Polar Region will be tilted towards the Earth and the NPC will be thawing.

In addition the melting polar cap often has a thin, dark collar around it known as Lowell's Band. This feature is most likely the dark underlying rock, which is exposed by winds blowing the dust off of the surrounding rock. Here is a sketch made on August 13th, 1988 using an Astro-Physics 7" f/9 refractor on an Astro-Physics 706 German equatorial mount which shows both a rift in the SPC as well as Lowell's Band at the top of this sketch:

Polar hood clouds (South Polar Hood, or SPH, and North Polar Hood, or NPH) usually precede the growth of the caps, and disappear around the vernal equinoxes. Polar hoods can appear as bright as the caps themselves. In order to determine whether it is the polar cap or polar hood cloud try comparing the view through a red filter (W23A or W25) or a green filter (W58) vs. blue filter (W80A). If the feature disappears in either the red or green filter it is a polar hood cloud. In the sketch above the NPH is at the bottom while a limb cloud was off to its left. During the 2005 opposition of Mars the North Polar Hood had a prominent blue color to it.

Martian Atmospheric Features

Mars atmosphere is very thin compared to Earth and is composed mostly of carbon dioxide or CO2 (~95%). However while the Martian atmosphere is thin it supports a variety of atmospheric features, including clouds, hazes, fogs, and winds which can sometimes create vast dust storms that blot out surface features for many months.

There are different kinds of clouds in the Martian atmosphere, and the amount of cloudiness appears to be related to the sublimation of the polar caps and ice in the Argyre and Hellas basins into the atmosphere (sublimation is the process of a solid, such as ice, going directly into a gaseous state rather than first going into a liquid state such as water).

The best way to observe clouds are to use a blue filter such as a Wratten (W) W80A, W38, or W38A (more information on filters to use for Mars is presented below). Some of the clouds visible in the Martian atmosphere include discrete clouds, which appear regularly over certain areas, such as Hellas, Chryse, Libya, and Syrtis Major. One well-known discrete cloud is the "Syrtis Blue Cloud". This cloud circulates around the Libya basin and across Syrtis Major. The best time to look for it is when Syrtis Major is near the limb of Mars. If you observe this cloud with a yellow filter (W8, W12, W15) it sometimes appears vivid green in color.

There are also orographic clouds, which are often visible over Olympus Mons and other volcanoes on the Tharsis plateau in the so-called W-shaped cloud formation. Orographic clouds are formed when moist air is forced to rise over a mountain, which causes condensation to occur and forms clouds. When clouds obscure Olympus Mons it is called Nix Olympia, as shown in near the center in the following sketch made using an Astro-Physics 5.1" f/8.35 EDF refractor on homemade Dobsonian-style mount:

Additional features visible in the above sketch include at the top of the sketch the South Polar Hood Clouds, Mare Cimmerium and Mare Sirenum, and Solis Lacus on the following limb. Also Chryse was visible on following limb, with Mare Boreum and North Polar Cap visible at the bottom of the sketch.

Limb clouds or limb haze (sometimes referred to as limb brightenings or limb arcs) often appears on both the preceding and following limb of the Mars. They are caused by scattered light from dry ice and dust particles in the upper atmosphere of Mars. Here is how limb clouds and limb haze appeared during an observation made on October 2, 1988 using Astro-Physics 7" f/9 refractor on an Astro-Physics 706 German equatorial mount:

Other atmospheric features include frost patches and surfaces fogs, which form near the surface in the cold temperatures of the Martian night. As the Sun warms the surface during the day the fogs tend to dissipate by midmorning, while frosts may remain for most of the Martian day.

Fogs usually form in valleys, basins, upper slopes, and depressions, while frost form on plateaus, deserts, floors of large craters, and mountains. If you are unsure if you are observing surface frosts, surface fogs, or clouds higher up in the atmosphere, try using a green filter (W58) and compare it to the view through a blue filter (W38, W38A, W80A). If it is brighter through the green filter it is most likely a surface frost or fog. If it is brighter in the blue filter it is most likely an upper atmospheric features such as clouds or haze.

Note that frost patches and surfaces fogs are sometimes referred to as morning and evening clouds. Morning clouds are seen on the following limb of Mars, while evening clouds are seen on the preceding limb.

Dust storms usually form during the warmest part of the Martian Southern Hemisphere, but can occur during any season. Dust storms are common on Mars and often form and dissipate within a few days or weeks. However in some cases they can grow to encompass the entire planet and obscure all surface detail.

For example, during the 2001 Mars's opposition dust storms began in the Hellas basin shortly after the planet's closet approach to Earth. This dust storm eventually spread across the entire globe and bloated out all surface detail, much to the chagrin of Mars's observers everywhere.

The same thing happened to me in the 1970's when I was observing Mars with a
Criterion RV-6 6" f/8 reflector. Initially I could not understand why the telescope performed so well when observing Jupiter and Saturn yet on Mars I could only see a featureless disk, when a couple of years earlier I had been able to see more detail with my small 60mm refractor. I found out later that a dust storm had broken out on Mars.

To help determine if a dust storm is occuring try a yellow filter (W8, W12, W15), orange filter (W21), and red filters (W23A and W25). If you are unsure if you have detected a dust storm try a yellow, orange, or red filter: if they appear brighter than it is a dust storm. Another hint that a dust storm may be underway is that surface features that are usually dark and well defined appear less prominent and have lower contrast, particularly when viewed with an orange (W21) or red filter (W23A, W25).

On a smaller scale than dust storms are dust devils, which were shown to move across the Martian surface in a movie made by NASA's Mars Exploration Rover Spirit. Scientists now believe that dust devils may play a role in the formation of dust storms.

Other kinds of storms that occur in the Martian atmosphere include low pressure systems (or cyclonic storm) which are common during the late summer and fall. Sometimes these storms are located near the polar caps and can be large enough to observe from Earth in amateur-sized telescopes. For example during the 1999 opposition of Mars I observed large a storm near the North Polar Cap (NPC) as shown in this sketch made with an Astro-Physics 7.1" f/9 EDT refractor on homemade Dobsonian-style mount using a magnification 203x-289x with Baader binoviewer. It appeared just to the left of the NPC at the bottom of this sketch:

The storm was somewhat oval in shape, white in color with a dark border, in particular a dark line dividing it from the NPC. Also, it appeared large, but not quite as large as the polar cap. I had the impression this feature was slightly higher in elevation then the NPC, but dismissed this as an optical illusion. After looking at my Mars map back inside later I assumed it was Baltia, which had perhaps become frost covered. However, after checking with someone at the Mars ALPO section it appears what I saw was the large cyclonic storm reported on Mars in late April.

More rare are equatorial cloud bands (ECBs), which appear as broad, diffuse hazy strips along the Martian equator (as denoted by the dashed lines in the sketch below), and are difficult to observe. Equatorial cloud bands may be limb clouds seen edge on. Here is a sketch I made of the feature that I made during the 1999 opposition of Mars using an Astro-Physics 7.1" f/9 EDT refractor on homemade Dobsonian-style mount with a magnification of 203x-289x:

The ECBs appeared to be a light colored band that extended across the equator from Chryse on the preceding limb to the following limb. It was faint yet noticeable, and was more pronounced with a light blue filter, which suggested it was an atmospheric phenomena. It reminded me of an equatorial belt on Jupiter or Saturn, but much fainter.

I had not seen this feature before, and after completing the drawing went back inside and checked the various Mars maps that I had, as well as books on Mars. I could only find one drawing that showed a similar feature, but it had been made with a 30" telescope at 560x magnification, and it was noted that this feature is exceedingly rare. So I thought that perhaps my observation was incorrect, and did not report it.

However, around a year and a half later I attended a star party where a well-known planetary astrophotographer was showing images of Mars he had taken with a 10" aperture telescope during the 1999 opposition. One of them showed what appeared to be the ECBs that I saw visually. After his talk ended I asked him if he could see the ECBs through his telescope and he indicated that he had, and that he felt that I should be able to see it in my 7.1". So I then reported my observation and submitted my sketch to the Mars ALPO section.

In the September 2005 issue of Sky & Telescope magazine there is an article on the Mars Apparition of 2005. On the top of page 70 there are some images of Mars taken by Donald Parker using a 16" reflector that show the ECBs. They are most noticeable in the images made in blue light. I found that a blue filter made them easier for me to see them as well, but they were not as bright as shown in his image.

On page 72 of the article the author discusses ECBs and notes that: "These delicate, wispy streaks are difficult to observe with ground-based telescopes". I feel one reason I was able to see them in my 7.1" aperture telescope was because I make sketches at the eyepiece, which helps to train my eye to see more detail. If you would like to learn more about sketching please refer to my article on An Introduction to Planetary Drawing. As an added bonus, sketching provides you also with a permanent record so you can refer to them to see how planetary features have changed year to year.

Moons of Mars

Mars, the Roman God of War and Agriculture, has two moons named for his sons, Phobos (Fear) and Deimos (Panic). These moons are most likely are asteroids that were captured by Mars' gravity, and were discovered by Asaph Hall in 1877 using a 26" Clark refractor while working at the U.S. Naval Observatory. Hall began his astronomy career at the Harvard College Observatory with The Bonds from 1857 until 1862.

On October 29th - 30th, 2005 Mars is at its closet to Earth, and Deimos will be at magnitude 12.1, and Phobos at magnitude 11.0, so this will be a good time to search for them. The problem in seeing them is the glare from Mars. So the best way to try and observe these moons is to look for them when they are at their greatest elongation's (or distance) from Mars, and to use an occulting bar, or place Mars just outside of the field of view. The Sky and Telescope Magazine website has an article and a JavaScript utility to help you find the moons of Mars.

An occulting bar can be made using strip of aluminum foil and taping it across the field stop at the bottom of the eyepiece. Then when observing position Mars behind the bar to reduce the glare and search for the moons. Placing Mars just outside the field of view is another way to search for the moons. Observers have reported being able to see these moons using telescopes with apertures as small as 6".

Color Filters For Mars

Color filters help to bring out the different aspects of Mars, both atmospheric and surface features. Filters are listed by their Kodak Wratten (or W) number. A good selection may include the following, and to get started you may only need one of each within these colors: yellow (W8, W12, and W15), orange (W 21), light red (W23A), red (W25), violet (W47), green (W58), and blue (W38, W38A, W80A).

Note that as you move from the red end of spectrum to the blue end you are seeing less of the surface detail and more atmospheric detail. If you are unsure if a feature is near the surface or higher in the atmosphere try using a green and blue filter. If it is brighter in the green filter it is most likely near the surface such as ground frost, while if it is brighter in the blue it is likely to be higher in the atmosphere.

Here are some specific uses of each of these filters relative to the Martian feature:

Yellow (W8, W12, W15) - helps to brighten desert regions, as well as detect dust clouds, and darkens blue and brown colored features.

Orange (W21), light Red (W23A), and Red (W25) - enhances contrast and definition of surface features. Depending upon the aperture of your telescope a W25 may be too strong, so smaller aperture telescopes may find W23A a better match.

Magenta (W30, W32) - enhances both surface and atmospheric features such as clouds. In addition enhances polar region features, red and blue features and darkens green ones. The Baader Planetarium Moon & Skyglow filter seems to enhance both surface and atmospheric detail. The Baader Planetarium Semi-APO filter also works well for observing Mars.

Green (W58) - enhances areas of frost and fogs on the surface, as well as polar features.

Blue (W38, W38A, W80A) - enhances atmospheric features such as clouds, limb hazes, equatorial cloud bands, and polar cloud hoods. In addition it darkens reddish features.

Violet (W47) - aids in the detection of Martian atmospheric blue clearing.

The best quality colored filters I have owned or used are made by Baader Planetarium.

Eyepieces for Observing Mars

Everyone has their own favorite eyepieces for observing Mars but here is a review of different eyepieces I have used when observing the planets.


Article 2000 - 2014, Eric Jamison, All rights reserved.


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