3. MARTIAN FEATURES

Mars is a world of wonders. Here is a sampler:

Volcanic Mountains

The Tharsis province in the northern hemisphere of Mars contains several stupendous volcanic mountains. The largest of these, Mount Olympus [credit JPL], is in fact the largest mountain known in the solar system. Mount Olympus is a "shield" volcano, with an average slope of only about 5 degrees, but it is enormous -- about 500 km in diameter, or larger than the state of Arizona. The mountain rises 25 km above the surrounding terrain. By comparison, the summit of Mt. Everest is less than 9 km above sea level. The volcanic caldera (basin) at the summit of Mount Olympus is about 60 by 90 km across. The lower slopes end abruptly at a steep scarp, which is between 3 and 6 km high. The origin of this imposing cliff is not at all understood. For more information, see "Volcanoes on Mars" , compiled by James Zimbleman for the Lunar and Planetary Institute (LPI).

Rift Valleys and Landslides

The Mariner Valleys (Valles Marineris), named for the Mariner spacecraft which discovered them, are a series of valleys about 5000 km long, stretching a fifth of the way around Mars. In places they are as deep as 7 km. The Grand Canyon of Arizona, about 450 km long and 1.5 km deep, would almost be lost in one of the tributary canyons of this enormous system. The Mariner Valleys probably began as a tectonic rift with stretching of the Martian crust, and were then widened by wind erosion and landslides. Candor Chasm [credit USGS] is a complex part of the Mariner Valleys system. Landslides [credit Calvin Hamilton, LIP] eat away the the rim of the canyon, particularly where it has been weakened by craters on the plateau. In this image you can see where the landslide deposit flowed out across the floor of the canyon. You can also see horizontal layering of strata in the upper part of the exposed wall of the canyon.

Channels and Valleys.

Two kinds of surface features on Mars look like they were formed by flowing liquid water: outflow channels and valley networks. Outflow channels were apparently eroded by catastrophic floods much larger than anything in the recent geological record on Earth. These channels are concentrated in the lowlands north of the Mariner Valleys in the area of the Plain of Gold (Chryse Planitia). The site for the Mars Pathfinder landing was chosen to be near the mouth of one of these channels, called Ares Vallis. Another example of an outflow channel is Tiu Vallis. The other surface features that appear to have been formed by running water are the valley networks [credit Calvin Hamilton, LPI], which resemble systems of drainage channels on Earth. They are generally confined to the older terrain on Mars. For more information on channels and valleys, see the article by Mike Caplinger, written for the Planetary Society.

"Splosh" Craters.

Many Martian craters larger than about 5 km in diameter are surrounded by peculiar lobes that look very different from the debris ejected by craters on the Moon and Mercury. The Martian crater Yuty (located at 22 degrees N, 34 degrees W) is a good example. Its multiple lobes look like they were made by a "mud flow" across the surface of Mars. They are probably due to the presence of liquid water below the surface. Craters smaller than about 5 km don't have these "fluidized ejecta blankets" presumably because they did not reach deep enough to tap the liquid water. Since 5 km craters eject matter from depths of 600 to 1000 m, this suggests that liquid water exists within a few hundred meters of the surface over large portions of Mars. (Frozen subsurface water may not do the job, because most of the ejected material might not be shocked sufficiently by an impact to melt the ice.) For more information on splosh craters and other impact craters on Mars, see "A Guide to Martian Impact Craters" , compiled by Nadine Barlov and Virgil Sharpton for LPI.

Polar Caps and Laminated Terrain.

South polar cap

Mars has north and south polar ice caps, which wax and wane with the seasons. The south polar cap [credit JPL] is composed mainly of frozen carbon dioxide (dry ice). The north polar cap is believed to be made mainly of water ice. This scene, from Viking orbiter images of the north polar cap, is about 70 km across. The white surface at the top is a layer of ice. The reddish cliff below the icy surface is about 500 m high, and contains about ten alternating layers of ice and dust. These layers may preserve a record of past changes in the Martian climate.

Wind Streaks.

Mars has seasonal dust storms that blow over the entire planet for weeks at a time. These dust storms leave behind hundreds of "wind shadows" on the surface in the wakes of craters and hills. The wind streaks are temporary features, but they can be seen from orbiting spacecraft. They point in the downwind direction from the last big wind storm, and thus serve as convenient "weather vanes" for the scientists who study the Martian climate. Martian global dust storms also fill the air with fine red dust, giving the sky the same color as the ground for years at a time. For more information, see "Winds on Mars", compiled by Steven H. Williams for LPI.

The "Face" on Mars.

Face on Mars

In 1976 the Viking 1 orbiter photographed a mountain about 1.5 km across in the Cydonia region. The mountain had a superficial resemblance to a human face.

"Happy face"

It appears to be a wind-sculpted mesa that owes its appearance to the lighting conditions at the time. Some writers have promoted the idea that the "face" was carved by intelligent beings. Extensive image processing has shown that it is just a mountain. Of the millions of features photographed on the surface of Mars, we expect some to have a chance resemblance to a face. The human tendency for pattern recognition is so well honed that it easily picks out faces and animals in clouds on Earth. See the excellent article "The Face on Mars", written by Sally Stephens for the Astronomical Society of the Pacific , which compares this feature to the "happy face" on Mars (see photo at right), the "Old Man of the Mountain" rock formation in New Hampshire, and the "Man in the Moon".

MGS Face on Mars Latest News! Face on Mars Unmasked! On April 6, 1998, the Mars Global Surveyor took this detailed image from orbit of the so called "face on Mars". It turns out to be an undistinguished geological feature. Light and shadows can do funny things.

The "Inca City"

There's a lot we don't understand about the surface of Mars. This Viking 2 image of an area near the south pole of Mars (82 degrees S, 60 degrees longitude) shows a grid of semi-rectangular ridges (forming "cells"). This area has been given the whimsical name "Inca City" because of its superficial resemblance to the ruined city of Macchu Pichu in Peru. The individual cells are about 4 to 5 km across. The grid may have been formed when wind erosion exposed hardened sediments that originally filled cracks in glacial ice. However, this is just speculation. No one really knows.

Satellites of Mars

Mars has two tiny satellites, Phobos and Deimos . Phobos is about 26 km long, while Deimos is half the size. Both satellites are shaped like potatoes. They are too small to have enough gravity to make them round. They are in effect big rocks. The gravity on Phobos is only about one thousandth that on the Earth. Actually, there are more small irregularly shaped satellites in the solar system (mainly orbiting around Jupiter and Saturn) than large round ones.