Martian Reality - A Story of Catastrophe?

A Contrast in Scales

Mars has a diameter about half that of Earth and its surface gravity is about 38% of Earth's. For this reason, Mars has geological features that are simply staggering in Earth terms. There are no mountain ranges as such, but there are volcanoes of monumental proportions, and one can be seen in this image as a dark blob in the lower left limb of the planet. It also has fissures or rift valleys in the surface that make the Grand Canyon look like a hair-line fracture! There is evidence of old impact craters of colossal size, the largest is in the southern hemisphere and is known as the Hellas Basin or Planitia. Hellas is around 1200 km (750 miles) in diameter. Recent ideas about Martian geology suggest that the features we see are no coincidence, so lets look at some of them and see what they tell us about Mars.

Think about this: Why can geological features on Mars be bigger than the same type of features on Earth? Explanation

Vast Volcanoes

This image shows the largest volcano we know of, Olympus Mons. Placed on Earth it would cover France. Its main caldera or crater could hold London or Sao Paulo and it stands some 22 km high. It literally sticks out into space. In the Viking picture you can see early morning clouds around its base.

Mars has several other extremely large volcanoes, and they are all concentrated in a particular area called the Tharsis Bulge. Present evidence suggests that they are all long-since extinct, the last eruptions probably being more than 60 million years ago.

Here is a comparison of the highest features of Earth and Mars. The blue line is the sea level equivalent.

© Image: The Guardians of the Millennium

Think about this: How do you know, from this picture of the Olympus Mons caldera (left) that the volcano had a series of eruptions, but that each was smaller than the one before? Explanation




Enormous Valleys

On the edge of the bulge, just below the equator, one of Mars' most spectactular features can be found - the Valles Marineris, named after the Mariner 9 spacecraft that discovered it. Valles Marineris, if placed on Earth, would reach from New York to Los Angeles. It is 250 km wide and 15 km deep in places. The Grand Canyon, the largest such feature on Earth, is insignificant by comparison. Although the Marineris is the largest such valley, there are many similar fissure systems all around the edge of the Tharsis Bulge.

© Image: The Guardians of the Millennium

Enormous areas, particularly in the northern part of the planet, show signs of catastrophic flooding. The northern hemisphere of the planet is much lower on average than the southern hemisphere. Pictures of the surface from orbiting spacecraft have shown vast river systems, flood plains and even coastlines in this lowland area. But there is no obvious indication as to what happened to the water that caused the flood. It is not visible now.

Think about this: If you were in the middle of the Valles Marineris, could you see the wall of the canyon? Explanation

Deep-Frozen Ice Caps

Mars has large icecaps. These are so big they can be seen in a moderate-sized astronomical telescope, such as a school may have access to. This image shows the edge of the ice fields at the north polar cap. Glaciers can be clearly seen, suggesting flow and movement, but not necessarily melting.

It has been suggested that most of the water is locked up as ice at the poles, but this ice is ALL water and solid carbon dioxide. Mars' orbit is more elliptical than Earth's, so the southern winter is colder than the northern one. However, recent information from Mars Global Surveyor have shown that there is less ice than previously anticipated, so the big mystery is: where has it all gone?

Think about this: If you could melt some ice on Mars what would happen to the water? Explanation

Extensive Cratering

The other characteristic of Mars is cratering. Certain areas, particularly in the southern hemisphere as seen here, are extremely heavily cratered. They tend to look darker, in pictures. The areas that are not cratered, show signs of geological activity - volcanic eruptions, water flow and dust deposit through wind action. Because the cratering is so extensive and many craters are large - ie older - it means that the surface of Mars has remained unchanged for billions of years. Craters are remnants of the period of formation of the Solar System.

Think about this: A meteorite strikes the surface of Mars at 30° to the the horizontal. What shape will the crater be? Explanation

Character Reference

Mars' mass is about 11% that of the Earth, and its average density is much lower, at about 3.9 tonnes per cubic metre as opposed to 5.5 for Earth. This implies a much smaller metallic core for Mars. This seems correct, for Mars has a magnetosphere which is weaker than the Earth's, though its rate of rotation is almost identical to Earth. Recent NASA data from Global Surveyor indicate the magnetosphere is much stronger than that detected by the Viking Landers. So maybe Mars has weak Van Allen type structures around it and will also experience aurorae, but so far no such phenomena have been observed.

In orbital terms, Mars is a wanderer. It takes 687 days to orbit the Sun, and its orbital eccentricity is eight times that of the Earth. Its mean orbit is 228,000,000 km (142,500,000 miles) but it perihelion is 207 million km (129 million miles) and its aphelion is 249 million km (156 million miles). This results in the southern summer being colder than the northern summer. Over time, the eccentricity of the orbit also varies a good deal and this could cause all sorts of climatic variations over periods of millions of years.

Mars is tidally locked with the Earth's orbit, so that over a few years the two planets arrive back in the same relative positions. This means that you could set a spacecraft in motion to rendezvous with Mars and return to Earth, and so on, ad infinitum. A trip each way would take about two years and would require no energy expenditure once the motion was established. This is called a Hohman transfer orbit.

Some of Mars' characteristics are uncannily Earthlike. Its inclination to the ecliptic is only 1.85 deg, so it is almost on the ecliptic, Its rotational period is 24 Hr and 37 minutes and 22 seconds and its tilt or obliquity is just under 24° (Earth 23.5°). This means Mars has distinct seasons much as Earth, but each one lasts around 5 months!

Mars' shape is also far from spherical. It's polar radius is 15 km (9 miles) less than its equatorial radius. This gives it an ellipticity twice that of Earth's. In addition, the Tharsis Bulge covers around a quarter of the planet and this distorts its shape by several kilometres.

Think about this: Why is Mars' southern winter longer than its northern winter? Explanation

Iron Master

Mars' soil is largely orange or red-brown, as it has a high iron oxide content - it is rusty. This gives the overall planet its reddish colouring, which is clearly visible in the night sky on occasions. With an average relative magnitude of -1.52 it is one of the brightest objects in the sky (variations -1,-2.8, with favourable oppositions every 15 to 17 years). However, its albedo is low, being less than half that of Earth and only a fraction that of Venus. The reason is that the surface - the rock - reflects the sunlight, but this is always less shiny than cloud cover or a thick atmosphere.

This Viking Orbiter image shows part of the Valles Marineris, and the typical colour of much of lowland Mars.

Think about this: Early astronomers noticed that areas of Mars change colour with the seasons. Why is this? Explanation

A Bad Atmosphere?

© Image: NASA/JPL

The Martian atmosphere, like that of Venus, is largely carbon dioxide, but the pressure is very low - about 1/100th that of Earth. This means it is very thin. Despite this, Mars still experiences strong winds, dust storms of emormous size and whirlwinds. The dust storms, in particular, can last for weeks on end and may whip up rocky or sandy particles that are deposited on top of ice - thus making the surface appear rocky when it is in fact, icy. These dust storms are not confined to the equatorial regions, which are warmer and therefore might be expected to be more active.

However, the thin atmosphere is not capable of maintaining the solar heating experienced by ALL Earth and Venus. On average, Mars gets only 43% of the sunlight Earth experiences, so we could reasonably have expected it to be cooler. The actual average equatorial temperature is about 20 C below zero - livable but a little chilly. The maximum Mars might reasonably expect under the right circumstances, is something between zero C and plus twenty. Low temperatures at night are typically around 60 C to 80 C below in the equatorial region, and perhaps down to 125 C below towards the poles.

The image was returned by the Pathfinder mission on sol 39. Other images reveal blues, mauves and pinks in the evening sky. As on Earth, dust plays an important part in tinting the sunset.

Think about this: Carbon dioxide is a greenhouse gas - it "collects" heat. Why doesn't it warm Mars up? Explanation

First Contact

In 1976 two Viking spacecraft made land-fall on Mars.

The Viking 1 landing site was in the Chryse Planitia, about 22N. This is not far from the 1997 Pathfinder mission, so comparative weather measurements have been possible.

The Viking landers were fitted with various experiments and monitoring equipment, including cameras, a life detection experiment, a soil analysis kit and a weather station. They returned the first data from the surface of another planet. Although Viking 1's life experiment gave positive results, this was later discounted. The experiment depended upon detecting the production of gasses - the by-product of living processes - after nutrients had been added to a soil sample which had been scooped off the surface. Gasses were found, but these are now beleived to be the result of ordinary chemistry rather than organic chemistry or life processes. The deciding factor was that no organic compounds were detected by other analysis, using different experimental techniques.

This conclusion, however, is still disputed by a few scientists, who maintain that life was actually found.

This picture is from the Viking 2 lander. The lander came down in the northern hemisphere (approx 48° N) in Utopia Planitia, not far from the Mie crater. Viking 2 got no results from the life experiment; but over the period of several months that it operated, the landscape changed with the seasons as can be seen in this image. It shows water and carbon dioxide frost on the surface.

The first weather data from the landers enabled windspeed measurements to be made, dust to be monitored and a temperature profile to be compiled. One thing that caused a great deal of discussion was the colour of the sky. NASA-JPL scientists eventually decided it would look a sort of butterscotch colour, and tinted the images accordingly. Recent images from the Pathfinder mission, however, suggest the sky is bluish in general; but can display all the variation of the Earth's skies, particularly at sunset.

Think about this: What gasses are most commonly produced by life processes? Explanation

First Drive

© Image: NASA/JPL

The Pathfinder mission, which landed just above the equator in the northern hemisphere, recorded temperatures from around -3 C to -80 C, with a good day-time temperature being about -12 C. This picture shows the Sojourner Rover investigating Barnacle Bill. Bill is thought to be made of a rock similar to Andesite, which is usually found at tectonic subduction zones on Earth.

Sojourner left us with a series of major puzzles about Mars which are still being worked on. Is there any common thread here, anything that may explain the observations?


© Image: NASA/JPL

This image is from the Pathfinder mission which landed in one of the flood plain outflows, the Ares Valley. Here the Sojourner robot rover examines a rock called Yogi, by the JPL team.

Pathfinder landed not far from Viking 1 and confirmed much of Viking's data associated with the weather. It landed in a flood plain - the outflow of the Ares Valley, which is east of the Valles Marineris. This area is strewn with rocks and boulders that were washed down in a catastrophic flood. It is thought that the whole area was under several metres of water for a long period, perhaps 1000 years.

Think about this: Why was the rock Yogi so called? Explanation

A Global Survey

Over the next decade we will see a number of missions visit Mars. We may get answers about the planet. But certainly we will get more questions. Already the Mars Global Surveyor is on site and sending back amazing pictures like these, which have resolutions of just metres - you can see very small details as little as a metre across in some cases.

© Images: NASA and MSSS

Olympus Mons (left) shows the shield and caldera.

(Right) How was this bathtub-shaped depression made? It does not appear to be a crater - it has no walls or ejecta. One idea is that it is a collapsed volcanic lava tube - an underground tunnel along which molten rock can flow.

Think about this: How might you tell how new, in geological terms, an area of Mars is? Explanation

Fear and Dread

Mars has two moons - Phobos and Deimos - fear and dread. They are very small and are of irregular shape - like potatoes - rather than spherical. They are believed to be captured asteroids.

The Viking 1 Orbiter collected this image of linear features on the surface of Mars' satellite Phobos. They appear to be related to the formation of the crater Stickney, which is not in the picture, but somewhere to the bottom left. Phobos is 20 x 28 x 23 km and is the closer of the two to Mars. It orbits at a height of just 6000 km, three times per sol, and it may crash down within the next 20,000 years according to some calculations.

The Viking 2 Orbiter also took images of Deimos. This one was taken from 1400 km away. Like Phobos, Deimos is cratered, but many seem to be partly buried by dust. The mini-moon is about 15 x 12 x 10 km. It orbits 20,000 km above Mars and orbits about once every sol and a quarter.

Is it possible that Mars had more mini-moons in the past, perhaps, and did they come crashing down?

Think about this: Would you be able to see Phobos and Deimos in the Martian sky? Explanation

Features from Above

This sequence of images is derived from the Viking Orbiter photographs and show some of the characteristic features of Mars.

Splat or rampart craters are more common on Mars than ray craters. Here, the muddy remains of a "splat" impact is clearly seen in several overlapping layers. Note that the rock below the top-soil is lighter than the surface layer, and has been thrown out in the butterfly-wing pattern of the splat. All this indicates that the soil was wet or icy when the crater was made.

This crater, which is 28 km across, has a high central peak and is one of the largest in this part of Mars, the northern lowlands. Larger craters, bigger than 50 km across, do not have a peak in the middle. Also, it should be noted that craters are nearly always circular, no matter how shallow the angle of impact. The only exception is when meteorites strike at low speed, say hundreds of kilometers per hour rather than thousands, and the angle is very shallow. Typical impact speeds of meteorites and comets is around 35 kilometres per second!

River systems, now dry, are clearly seen in this image of the lower Ares valley where they flow out onto the flood plains of Chryse Planitia. Islands and washed out craters are visible. It was a little to the north of here that the Pathfinder mission touched down.

Apollinaris Patera is a volcano on the south western edge of the Tharsis Bulge. It has an enormous fan-shaped lava flow emanating from the axehead-shaped caldera. To the west the scarp of the Avernus Rupes casts a dark shadow. This enormous cliff structure is an unusual formation in this type of volcano, which usually creates a gently sloping shield, hundreds of kilometers in diameter, before ending in a low scarp or rupes. The volcano is about 5 kilometres (3 miles) high and the caldera is around 80 kilometres (50 miles) across.

Orcus Patera. This looks like an elongated crater, but most planetary scientist believe it is a large lava outflow. Slow sticky material has seeped from the ground and spread out leaving a large flat crater-like structure, which has now filled with dust to give a smooth appearance.

Another type of volcano is the Tholus. This cone peak in the centre of the picture is Ceraunius Tholus. It is located 2000 km east of Olympus Mons. It is unusual in that it has a bathtub crater associated with it, to the north. Elongated depressions like this are rarely formed by meteorites, but it does exhibit the butterfly wing pattern of an impact. A lava channel flows from the caldera to the crater, and a flow fan is just visible in the crater, where lava has entered. Were the impact and the eruption linked events?

Note the cracks in the terrain to the west. This is the Tractus Fossae. These are associated with the edge of the Tharsis Bulge. The large patera to the north east is Uranius Patera and the smaller cone north of Ceraunius is Uranius Tholus. Note the pyramid shaped hill east of Ceraunius.

Think about this: In this image, which came first, craters or river? Explanation





A Common Thread

A mercator projection contour map of Mars. Cool colours are lowland, hot colours are high. The Hellas Planitia is the deep blue circle to the south. The Tharsis Bulge is the big red patch. The Valles Marineris cuts into the Tharsis Bulge on the eastern side, like a split in the planet's fabric. Note how the north is all lowland and the south is nearly all highland. This image was created from data collect by Mars Global Surveyor. © Image: NASA and MSSS

Is there any common thread in what we have seen of Mars - something that tells us why it is like it is?

A clue may come from the Hellas Basin. This is directly opposite the Tharsis Bulge. Whatever made this huge crater was extremely large and hit with colossal force. It has been proposed by some planetary scientists, that the impactor went so deep that it pushed up a piece of the mantle on the opposite side of the planet, thus forming the Tharsis Bulge and leaving giant fissures in and around the edge. Not only that, a good deal of the northern crust of the planet may have been blasted off into space, thus leaving the northern terrain lower than the southern. In addition, such a departure of the surface could well have ripped or sucked away much of the atmosphere.

The energy of the impact would have dissipated as heat and this would have melted ice and caused a catastophic release of water, thus forming the wadis and flood plains we see. With a thinner atmosphere, the water could then boil off into space.

This theory is not accepted by many planetary scientists yet, but it may be on the right track. Watch out for developments. Information from the Web sites of NASA, ESA and NASDA will keep you up to date.


A Contrast in Scales: Because the gravity is lower. Rocks are the same strength on Mars as on Earth, but they are lighter. Therefore, larger rock structures can be created before they collapse under their own weight.

Vast Volacanoes: There are several caldera rings which overlay one another. Later ones destroy parts of earlier ones. But the later ones are smaller, so it is reasonable to suppose that later eruptions were less fierce.

Enormous Valleys: Probably not. Although the cliffs are up to 15 kilometres high, in the centre of the valley they are 125 kilometres away. Mars is half the diameter of Earth, so the horizon would be about 16 kilometres for people of average height. Even these great high cliffs would not be tall enough to appear over the horizon.

Deep-Frozen Ice Caps: Some of the water would immediately boil away in the thin atmosphere, the remainder would re-freeze.

Extensive Cratering: Round. Craters are always round unless the impact is very slow and the angle of approach is very shallow.

Character Reference: Mars' orbit is quite elliptical, and during the southern winter it is further from the Sun. Kepler's Laws state that planets move slower when further from the Sun. Newton's Law of Gravity says why.

Iron Master: Three reason: Cloud formation and dissipation. Frost condensation and evaporation. Dust movement from one region to another.

A Bad Atmosphere: It does! Mercury, the planet closest to the Sun, has no atmosphere. Mercury's dark side, the side facing away from the Sun, has a temperature only a few degrees above absolute zero (-273 C). Contrast that with Mars, which is four times as far away, and we find that Mars' coldest temperature is about -125 C. So something must keep the planet warm - ie the atmosphere. This temperature is about as good as at the thin atmosphere can manage at 228 million kilometres from the Sun.

First Contact: Animals: Carbon dioxide and methane. Plants: Oxygen.

First Drive: It looks like the famous Yogi bear's head in profile.

A Global Survey: By the number and size of the craters. The more cratered the older the terrain, and generally the larger the craters the older the terrain. Uncratered or sparsely cratered landscapes are usually the result of flood washout, volcanic lava flow or dust deposition.

Fear and Dread: From Mars you should certainly be able to see Phobos with the naked eye, and probably in daylight. It might be about as bright as Venus. Deimos is much smaller and more than twice as far away, but it should look as bright as the star Sirius.

Features from Above: The craters. They cause islands in the river. Note how the river cuts channels in the terrain. This area of Mars is close to the Pathfinder landing site.

Mars: vital statistics

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