Observing Mars Surface Atmosphere and Moons

Even at its best, Mars is a comparatively tiny target. And for northern hemisphere observers, when Mars is at its very best, it stays low in the south at its most favorable oppositions. Patience is critical in observing Mars because those few moments when the planet is in steady air only come along a few times in a night and only last for a fraction of a second. But for those who wait, the detail Mars can show during those few precious seconds can be absolutely phenomenal. Then if you are patient, persistent and paying attention to the fine detail, some of the solar system's most massive formations come into view. One feature of Mars that does jump out is its color, especially in a telescope. Mars' bright orange-red hue will easily trip the cones of your eyes even when not at it's brightest.

With Mars at its best, watch the planet's changing markings as the night wears on. Mars rotates on its axis once each 24 hours, 37 minutes, or just over a half-hour longer than Earth takes. So Mars' surface features parade across the face of the disk during the night. Compare the features as they slide by to a map of the Martian surface and see if you can identify approximately where the historic explorations of Viking, Pathfinder and the MER's took place. The surface of Mars you will eventually realize presents an interesting dichotomy. You will not be able to see anywhere near the kind of detail that you did in the Moon, but you can learn to see fine detail in the Martian geography. The south is primarily bright highlands similar to those of the Moon, except for the high iron content of the rocks. The north is primarily low-lying plain. There is a sharp dividing line between the two, which will become evident as you watch the planet rotate. The average elevation sharply falls off by several thousand meters. There is no good theory to explain how this happened. In addition the highland hemisphere is very heavily cratered while the lowland hemisphere is very smooth. For every crater in the north larger than 28 kilometers there are 15 in the south. This too is one of Mars's great mysteries.

Like the Moon and Mercury, Mars has no plate tectonic activity as evidenced by the lack of folded mountain formations that so abound on Earth. This means that local hot spots in the mantle can sit under the same area of crust for millennia and build massive volcanoes. One of the great challenges that amateurs tried to undertake during the close approach of Mars in 2003 was to try and sight the massive Tharsis Montes and Olympus Mons. These are massive shield volcanoes that dwarf anything on Earth. Olympus Mons is the largest mountain in the solar system, as big as Texas and towering some 80,000 feet above the plain that surrounds it. The forces that formed the Tharsis Montes also created an enormous crack in the surface known as Valles Marineris, the solar system's largest known canyon. It stretches for 4,000 kilometers around the planet and is a much as seven kilometers deep in places.

Maps of Mars and freeware computer programs abound on the Internet that will show you the planet's geography and what side of the planet will be visible. You can watch the planet's parade of high and lowlands go by as regular as clockwork each day. As you look at the Martian surface, try if you can to imagine the scale of some of the features at which you are looking. Mars is home to the largest volcanoes in the solar system, the deepest canyons and somewhere on or below that surface there might be large quantities of frozen water. The European probe Mars Express has found what scientists believe is an enormous subsurface ocean of frozen water; a critical discovery that future probes will follow up on.

Speaking of water, watch the behavior of the exposed polar cap. The European Mars Express probe has been watching the polar caps closely since arriving at Mars in December 2003. The caps each consist of a permanent component that consists of alternating layers of ice and dirt. The mechanism that creates the layering is still unknown but a likely culprit would seem to be the unstable nature of Mars' axial tilt. The planet's axis is currently inclined just over 25 degrees with respect to the plane of its orbit, slightly more than Earth's is. Earth's axial inclination is stable however. Though Earth's axis precesses in a circle over time, the angle of inclination does not change with respect to its orbit. Mars wobbles like a top about to fall over, its axial inclination approaching angles as great as 60 degrees over many millennia. This causes dramatic changes in climate over many millions of years and leads to the kind of layering that we see in the cap. Each layer represents a different epoch of climatological and geological change. During the winter, a overlying cap of carbon dioxide ice overlies the permanent cap. During the summer months the carbon dioxide cap completely sublimes back into the atmosphere. At the Viking landing sites, this process of building the polar caps and vaporizing them again in the summer was found to vary atmospheric pressure by as much as 25% over the course of a Martian year. As the cap sublimates away with the onset of spring and summer, it may begin to break up into segments around the outer pole. Using high magnification you can view this effect in medium telescopes with high magnification. As the caps melt and atmospheric pressure rises, winds will begin to pick up in the higher latitudes. This can be a prime time to watch out for dust storms on the planet.

Mars' atmosphere also offers the patient and skilled observer some interesting details. The atmosphere is excruciatingly thin with a mean surface pressure of only about 7 millibars (sea level pressure on Earth is 1013.2 millibars, on Venus it is 90,000 millibars). Locally though pressures can vary dramatically. At the peak of Olympus Mons, the pressure is barely 1 millibar while in the deepest basins pressure can build as high as 9 millibars. The makeup of the atmosphere is almost entirely carbon dioxide gas that was not absorbed into the rocks during the planet's formation. Small amounts of nitrogen, argon, water vapor and oxygen are also present. The atmosphere can on occasion present water vapor clouds in thin white wisps in the upper atmosphere. Much more dramatic are when the winds kick up huge dust storms that begin in a plain and within a few days completely envelop the entire planet. Such storms are most likely when Mars is near the Sun and the sublimating ice cap pumps large amounts of gas into the atmosphere. Massive global storms were visible during the planet's close oppositions in 1971 and 2001.

There is more to Mars than just Mars. Two tiny moons also accompany the planet, discovered by the American astronomer Asaph Hall. Phobos is the larger of the two moons. Hall discovered it on August 18, 1877. It is irregular in shape measuring about 27 by 22 by 18 kilometers in diameter. The moon orbits Mars at a mean distance of only 9,378 kilometers from the center of Mars. This puts the moon lower than the synchronous orbit altitude so Phobos revolves around Mars faster than Mars rotates. In fact the moon is so low that not every location on the side of the planet facing Phobos can see it. Where it can be seen, Phobos moves across the sky in the opposite direction of everything else, rising in the west and setting in the east three times each day as the satellite scoots around the planet three times each day. Phobos is also condemned to destruction. Because Phobos orbits below the synchronous orbit altitude24, tidal forces are dragging it down towards Mars at a rate of about 1.8 meters per century. Within fifty million years it will be gone (barely a blink of an eye astronomically speaking), either dragged down to the surface or more likely broken up into a ring.

Though it is the smaller moon, Deimos was actually discovered first, six days before Phobos. Deimos orbits above the synchronous orbit altitude, so tidal forces are actually driving it slowly away from Mars, just as our own Moon is drifting slowly away. Deimos is also irregular in shape and measures about 15 by 12 by 11 kilometers. The little rock orbits Mars every thirty hours at a distance of 23,459 kilometers from Mars' center and is only visible to an observer on the surface as a very bright star moving across the background of the stars. Both are small rocky bodies similar to the rocky asteroids that orbit in the belt just beyond the orbit of Mars. There is no completely satisfactory theory that explains the presence of the moons because the gravity of Mars should not be strong enough to capture and hold them. But there they are, faithfully circling Mars. You can see them both in telescopes of 8 inches and larger under dark skies and favorable conditions. Phobos is the brighter of the two and can become as bright as magnitude 10 during the closest oppositions. But Phobos orbits so close to Mars that it can be difficult to see in the glare of the planet. Deimos circles out farther from the planet and thus is not as likely to be washed by Mars' brilliance, but is two magnitudes fainter putting it near the limit of an 8-inch scope under dark skies.

Mars offers many treats for the observer. Its surface is readily visible, it has an atmosphere that can show clouds and storms to the patient observer and it presents a constantly changing face to the observer with some of the most astonishing terrain features in the solar system. For the skilled observer willing to work, Mars's two moons can be teased out of the planet's glare. Lets go to Mars and have a look.

24 If a satellite orbits a planet so low that it revolves faster than the planet rotates, then the satellite loses energy to the planet's rotation and falls. If the satellite is high enough that it revolves slower than the planet rotates, then the satellite gains energy from the planet and drifts higher while the planet's rotation slows.

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