We switch now to the region beyond Earth, as we explore the Mysterious Planet Mars, recently upgraded in interest because of the discovery of possible evidence of single-celled, organic matter (life?) in a meteorite (found in the Antarctic), believed to have been ejected into deep space from an impact on ancient Martian crust.
Mars has always been especially intriguing to the peoples of Earth. It has captured our imagination for many reasons, two of which are the once-believed speculation that it held intelligent life because of so-called "canals" and then the famous book by H.G. Wells, "The War of the Worlds", that had martians invading Earth (this was the basis for the "infamous" radio program by Orson Welles that was so realistic that people panicked during the broadcast). Mars is about half the size of Earth, evident in this picture in which the two planets are shown side by side at the same size scale:
The writer (NMS), a geologist/planetologist, has found (through preparing this subsection) Mars to be, acre for acre, the most intriguing and diverse planetary surface of any of the Solar System planets - including Earth. This is due in part to the 100% exposure of a wide variety of geologic features to observation owing to lack of vegetation. But the variety of landforms and process indicators of tectonic, volcanic, impact, water, and wind action are the chief reason.
After the Earth and its Moon, Mars has become the most intensely studied Solar System body because of its similarity to Earth itself (but as a planet that has lost most of its water) and therefore is a candidate for once or still active organic (biologic?) material. More missions have been sent to Mars than to any other planet; these are summarized on this JPL website. Spacecraft today are orbiting it, surface rovers are exploring it onsite, and NASA - as well as the whole world community - is hoping for enough evidence of its special features to warrant a long-term program culminating in Man's setting foot on its surface. Another reason for the attention given to Mars is that it has many features explainable by extrapolation of geologic studies of Earth (and some features without terrestrial counterparts) -- and these are easily "seeable" because of total absence of oceanic and vegetation cover.
Here are four Web sites to consult if you want a quick overview of Mars or images to download: (1),(2), (3), (4). A splendid synopsis of Mars Exploration is found on this Wikipedia website; it lists 42 missions to the planet of which 20 were successful and 3 achieved partial success.
Also there is a NASA JPL video that describes how geologists use comparative planetology methods (based on familiarity with the Earth) to study, in this case, Mars. The speaker, Dr. James Garvin, is now Project Scientist for Mars Exploration at NASA Headquarters. Access it through the JPL Video Site, then the pathway Format-->Video -->Search to bring up the list that includes "Finding Mars on Earth", December 2, 2003. To start it, once found, click on the blue RealVideo link.
Mars's mean diameter (6780 km [4875 miles]) is 0.53 that of Earth, but about double that of the Moon, and has about 1/10th its mass; its volume is about 1/8th that of Earth. The planet, fourth from the Sun, has has a mean density of 3.93 g/cm3 compared with Earth at 5.52 g/cm3; it may have a much small inner core that would be iron-poor, as borne out by the lack of a distinct magnetic field.
A martian day is almost the same as a terrestrial day - it rotates once every 24 hours and 37 minutes (Earth time). Mars takes 1.86 Earth years (685 days) to complete its orbit around the Sun. Its orbit is distinctly elliptical (1.38 A.U. perihelion to 1.67 A.U. aphelion; average distance of 1.52 A.U.). Its rotational axis is tilted 25°, similar to Earth, which causes Mars to have summer and winter seasons. Mars surface temperatures range from -140° to +20° C.
Look below at a beautiful full-face image of a Martian hemisphere, which should make it obvious why Mars is also known as the Red Planet. This mosaic was constructed from reprocessed Viking images. Two conspicuous features, described later, are the huge gash across the face, known as Valles Marineris, and the three great volcanoes in the Tharsis group, on the left.
Compare this with a single (not mosaicked) image of a face of Mars, taken through the Hubble Space Telescope.
Then, look at these three hemispherical views of Mars taken by the electronic camera system on the Hubble Space Telescope (HST) through filters that allow close approximations to true color. The blues along the limbs are somewhat artificial. Note the presence (white) of material at the poles: these are the polar ice caps (water and carbon dioxide ice).
In late August of 2003, Mars became a "hot item" in TV and Newspaper accounts. Mars' orbit is such that most of the time it lies beyond 160 million km (100 million miles) from Earth. But there are rare times when the two planets positions in their orbits place them much closer together. On August 27, 2003 Mars and Earth were at the closest distance (55,690,000 km or 34,500,000 miles) apart. This had not happened until about 60000 years ago but will happen in about 600 years into the third Millenium. Thousands of amateur telescopes, 100s of Observatory telescopes, and the Hubble Space Telescope all trained on the Red Planet (which was just less than Venus as the brightest object in the Sky) and many photographs and electronic images were taken. The one below was made by the Hubble Space Telescope in orbit :
As we shall see on subsequent pages, while volcanic rocks predominate on Mars, there are widespread sedimentary rocks - mostly evaporites and some carbonates. The overall surface state of Mars, judging from these full planet face views, is that it consists of three major color states, each tied to some dominant material: 1) the predominant reddish colored surface, which is now known to be regions in which the rocks, soils and dust are strongly oxidized into phases consisting of hematite, and possibly maghemite (the γ polymorph of Fe2O3) and similar minerals (including the group of hydrated iron oxides going under the name of "limonite", if water at and beneath the surface has "weathered" the hematite); 2) the dark bluish to blackish surface, presumably basaltic bedrock with less iron discoloration, and 3) the whitish areas around the poles, identified as a mix of water and carbon dioxide (the outer coating). Color phase 2) implies that the surface is volcanic bedrock with insufficient iron oxide dust cover to significantly alter the color depicted; this suggests much less onsite alteration of the basalt and the transient nature of dust cover as martian winds remove much of previously deposited red dust (but some may be cyclically deposited during strong dust storms [see next page]). As an example of this dark color phase, look at the Syrtis Major physiographic region. It probably is volcanic crust, likely basalt. It size, however, varies from time to time because winds carry red dust back and forth over its boundaries.
The iron rust responsible for the reds and oranges on much of the martian surface is believed to have largely formed early in the history of the planet when its atmosphere may have been different and water in the air was more abundant. In the last billion years or so, the martian atmosphere has been cold and dry. Under these conditions UV radiation bombardment causes a thin top layer in the soil and rock enriched in the oxygen anion ("superoxide", an oxidant) which is capable of further release of iron from mineral by oxidation. However, this is a much slower process and probably doesn't account for most of the iron coloration. One consequence of superoxides is to create an environment that is anathema (destructive) to bacteria and other microbes. This would mean that life right at the surface cannot tolerate these conditions and, if present today, must dwell 10 centimeters or more below the topmost soil layers (life possibilities on Mars is discussed on page 19-13).
Mars was the first extraterrestrial planet mapped in some detail, solely from telescope observations. Thus, in the 1870s, Giovanni Schiaparelli published the map below, in which many of the features he named are still known as such in today's nomenclature. Although some names are illegible in the figure below, we reproduce two versions of this map for its historical importance:
Schiaparelli called attention to long linear features that he "thought" he could see. Schiaparelli referred to them as "canali" to mean "channels" (of non-human origins), which is the normal meaning in Italian but the English world mistranslated the word into "canals" a la Venice. This led to the widespread belief persistent into the second half of the 20th Century that there might be canals on Mars used by its "inhabitants" to transport water. Thus was born the legend of Martians (e.g. H.G. Wells "War of the Worlds" brought these creatures by spaceship to Earth, only to succumb to the adverse biological conditions on our planet). These canals, of course, disappeared when the good views shown above were obtained.
Sir Percival Lowell, using a telescope he built on a hill in Flagstaff, Arizona, extended this mapping and popularized the notion of canals on Mars. His 1909 map depicts a large number of nearly straight lines. Two versions are shown:
As late as 1940, improved maps of Mars still showed the (fictitious) straight lines on Mars
The probes sent to Mars finally dispelled the martian canal fantasy. It is difficult to reproduce on a Net site a modern map that shows all of Mars with many of its features identified. We will try with this simplified version:
A more detailed version:
We recognize the difficulty of familiarizing yourself with so many feature names and their locations. However, from this list find the following: Olympus Mons; Arsia Mons; Pavonis Mons; Valles Marineris; Alba Terra; Arabia Terra; Noachis Terra; Argyre Planitia; Chryse Planitia; Hellas Planitia; Elysium Planitia; Syrtis Major; Terra Cimmeria.
We now know a great deal about Mars from flybys, orbiting surveyors, and landers. The tricky problem of operating in near realtime makes a spacecraft's orbital insertion around or landing on Mars difficult, owing to some extent just to its distance; this has led to a number of mission failures. But, this list documents the successes achieved in the NASA programs for exploration of Mars: Mariner 4 (1964) --> Mariners 6 & 7 (1969) --> Vikings 1 & 2 (1976) --> Mars Global Surveyor (MGS) (1996) --> Mars Pathfinder (1997) --> Mars Odyssey (2001) --> Mars Exploration Rovers 1 (Spirit) & 2 (Opportunity) (2003) --> Mars Reconnaissance Orbiter (MRO) (2006)--> Mars Polar Lander (renamed Phoenix (2008). ESA, the Soviets, and Japan has sent missions to Mars. If you want to see a complete chronology of all trips to Mars, check out this List of Mars missions.
At the top of the list of what space probes and landers have discovered about the Red Planet is the evidence that Mars has water now in limited abundance and seems to have had a lot more in its past. Today, most of that water is in the polar ice caps and in a thin permafrost zone in martian soils. In earlier times, Mars had sufficient water to carve out canyons and gullies and perhaps even support an ocean. Sedimentary units (distinct bedding) resulting from sediment deposition have been observed over much of the planet. A more extensive atmosphere than now exists probably held water under warmer conditions than are current. However, over time much of the water has been lost to outer space by the action of the solar wind (Mars has a metal core which seems to have solidified so that the shielding magnetic field it once may have had is no longer operative). Surviving water is largely locked up in ice. But the fact that there still is some water, and even more existed earlier, inspires cautious optimism that some signs of life - most types need water - may yet be found.On this page we review the result of the first missions to Mars. The first flyby, by Mariner 4, on July 14, 1965, produced 22 TV-like images covering roughly 1% of the planet. The very first of these is:
Here is another of those TV views and below it is almost the same area covered by the TV camera aboard Mariner 9 (see below) six years later - a good measure of technological progress:
Mariners 6 and 7 passed Mars on July 31 and August 5, 1969, together obtaining 199 images that extended coverage to about 10% of the total surface. A typical Mariner 6 image shows frost-covered parts of the south polar region, with dark craters, furrows, and pits.
A quantum leap in coverage followed the first successful orbiting of another planet, when Mariner 9 arrived on November 13, 1971. Returning more than 7,300 panchromatic images, a wide-angle TV camera, capable of 1-3 km (0.6-1.9 mi) resolution, mapped almost the entire surface, while a narrow-angle TV camera imaged some areas at 100 m (328 ft) resolution. Mariner 9 also carried an IR radiometer, an IR interferometer spectrometer, and a UV spectrometer. Here is an artist's depiction of this pioneering visitor to the Red Planet:
The imaged surface qualifies Mars as one of the most diverse and spectacular planetary bodies in the Solar System. Lacking vegetation and water cover, the easily seen geologic features are often grandiose in scale and sometimes unique. As with most extraterrestrial objects, their interpretation by astrogeologists started by comparing or contrasting features with those known on Earth. Martian features often required innovative explanations.
Two typical Mariner 9 views, as mosaics, show fractures, erosion (stream?) channels, and craters.
One of the grand surprises revealed during Mariner 9's passage was the largest volcano in the Solar System - named Olympus Mons - which previously had only been a suspicious bump in telescope photos. Shown here is one frame, in raw unprocessed format, which displays a part of this huge volcanic structure; a mosaic made from three additional frames is shown on page 19-12.
The Russians launched Mars 2 and 3 probes to that planet in 1971, but acquired only limited data, mainly on the magnetic field. In 1973, the former Soviets sent three more spacecraft to the Red Planet. In March of 1974, Mars 4 failed to orbit the planet, but Mars 5 did, long enough to return data, including images during 10 of its 20 orbits. Mars 6 successfully descended through the thin Mrtian atmosphere but failed during the last second before touchdown.
A quantum leap in the U.S. exploration of Mars began on August 20, 1975, and September 9, 1975, with the launches of Vikings 1 and 2 respectively. Their Orbiter components began circling the planet in late June and early August of 1976, coinciding roughly with America's Bicentennial Celebration. Each carried two identical vidicon (TV) cameras, capable of color imaging (from black and white images taken through color filters) and acquiring stereo scenes at 100 m (328 ft) resolution. They also bore a thermal-mapping infrared spectrometer and a second IR spectrometer designed to detect atmospheric water.
The aforementioned Mars JPL video includes a segment dealing with Viking. Access through the JPL Video Site, then the pathway Format-->Video -->Search to bring up the list that includes "Viking Legacy and Mars Exploration", July 12, 2001. To start it, once found, click on the blue RealVideo link.
As an example of a Viking image, consider this Viking 1 view of Tyrrhena Patera, a volcano built up from both flows and abundant ash, located near the Hellas Basin in the southern hemisphere.
The two Viking images below show color versions of the Volcano Tyrrhena. The first image does not give a sense of relief but when we confound topographic information with this image, a perspective view, marked by strong vertical exaggeration, gives a sense of its height:
As had been observed earlier in the Mariner 9 mission, the martian surface and atmosphere are actually dynamic. This Viking-2 image shows a large dust storm:
Both Vikings also transported separable landing vehicles that descended by parachute throughout the atmosphere. Here is an image of Viking Lander 1:
This map shows the two Lander sites and also that of Pathfinder (see below):
So far, most landings have avoided these pitfalls (the loss of Beagle2 from the Mars Express mission is unexplained). The first ever image (black and white) of the martian surface from the surface is this:
Both TVs also successfully transmitted surface views in color, of the immediate ground, up to the visible horizon.
The two Viking Landers were exceptional ground-truth devices. These panoramas from them reveal a red iron-coated surface, consisting of rocks (the largest > 2 m [6.6 ft]) mixed with sand and dust, some piled into incipient dunes. In other views, some rocks look like vesicular basalt. Note the yellow-brown tones in the lower, dusty layers of the sky's rarified gas envelope (mainly carbon dioxide). This stationary observatory was also equipped with magnets, a seismometer, an X-ray fluorescence spectrometer (that receives samples from a movable scoop), a miniature meteorological station, and three experiments seeking signs of metabolic action (mainly, as gases released or absorbed) by biogenic matter (none found).
After the Vikings, the resumption of martian exploration was vested in the Mars Observer but this spacecraft failed enroute to Mars in 1993. The next visit was by the Mars Global Surveyor, launched in 1996, which orbits the planet taking pictures up to 3 meters in resolution.Three of its prime instruments are the MOC (Mars Orbiting Camera), the MOLA (Mars Orbiter Laser Altimeter), and TES (Thermal Emission Spectrometer). This spacecraft is described on page 19-13 but as a preview sample here is an image that shows at high resolution the interior wall of a small impact crater within the very large crater Newton; the series of channel strips running down the side has been interpreted as evidence for subsurface water emerging at some depth along the wall and eroding these rills (see page 19-13 for details).
JPL's Pathfinder mission landed a Rover on the surface in 1997 to conduct a series of physical and chemical experiments (page 19-13). Then, in October 2001, the Mars Odyssey mission reached the planet and is continuing to gather data with THEMIS, its thermal emission spectrometer, and GRS, a Gamma-Ray Spectrometer, and other instruments. In the first years of the 21st Century, several more U.S. and International spacecraft, both orbiters and landers, have been sent to Mars. The successful ones - Mars Express, Mars Reconnaissance Orbiter, the Mars Exploration Rovers Spirit and Opportunity, and Mars Phoenix - are described on pages 19-13ff. Prior to that, selected images from these missions displaying specific features appear in the next two pages.