In 1610, Galileo Galilei turned his newlyinvented telescope on Mars, seeing only atiny reddish disk. Christiaan Huygenssketched the first crude maps of the planet

in 1659 (Figure 1), noting that the visibledark and bright markings drifted across the

disk in about 24 hours, indicating the length ofthe Martian day was similar to Earth’s. By the 18thcentury, astronomers were routinely mappingclouds, polar caps, and surface markings thatseemed to change with the seasons. Dark markingswere assumed to be oceans and seas; similarassumptions were applied to features on the Moon.In 1877, Italian astronomer Giovanni Schiaparellidrew maps showing a global network ofinterconnected dark lines (Figure 2). His termcanali—Italian for channels—was mistranslated intoEnglish as “canals,” leading to a “life on Mars”hypothesis that held sway for many decades. Anannual “wave of darkening,” in which the darksurface features became larger, darker, and betterdefined in the spring and summer seasons, wasassumed to result from the springtime growth ofvegetation nourished by water collected from theretreating polar ice caps. The supposed canals weretaken as evidence of an advanced Martiancivilization’s global-scale engineering project,designed to carry the springtime melt from the polarcaps to irrigate forests and fields in the temperateregions.

Since the mid-1960s, observations by spacecrafthave allowed Earthbound scientists to beginunraveling the mysteries of the Red Planet and tofinally put many old myths to rest. In 1965, theMariner 4 spacecraft zipped within 6,200 miles

(10,000 km) of Mars and radioed back a scant 22images collected over half an hour. This smallsampling revealed a lunarlike surface scarred withimpact craters (Figure 3) and sheathed in a thincarbon dioxide atmosphere. Scientific interest in thegeological and possible biological history of Marswaned—it was assumed Mars was as “dead” as theMoon. The planned missions to Mars continued,however, and in 1971 Mariner 9 became the firstspacecraft to orbit another planet. Over the next year,7,329 images allowed the entire surface to be map-ped. To the delight of scientists, a host of fantasticlandscapes was revealed. Mars became known as aworld of great extremes. Its features include thelargest known volcano in the solar system at about360 miles across (600 km) and 15 miles high (25km)—nearly three times taller than Mount Everest; acanyon system 2,500 miles long (4,000 km), hundredsof miles across, and up to 4 miles deep (7 km)—onEarth, it would span North America; and surfacesthat range from those being slowly covered by dustto those whipped by intense dust storms (Figure 4,before and during a dust storm).

In 1976, the Viking missions (two orbiters as wellas landers in two locations) yielded several years ofboth surface and global observations. The Vikinglanders carried out the first “biology experiments”on the surface. Although the somewhat ambiguousresults are still debated, the scientific consensus isthat the tests revealed exotic chemical processes

Unraveling the Mysteries of Mars

By Dr. Steven Lee

For as long as humans havelooked up into the nightsky, the planet Mars has

sparked imaginations.For thousands of years,it was merely a blood-

red star wanderingacross the heavens. In

early cultures, it conjuredup images of war and

bloodshed. Egyptians named itHar décher, “the Red One.” The

Babylonians called it Nergal, “the Starof Death.” The Greeks considered it Ares,

“the Fiery One,” and to Romans it wasthe god of war, called Mars.

Figure 1

Figure 2

Figure 3

Figure 4

THE RED ARROW IN THE MARINER 4IMAGE (ABOVE) POINTS TO THELOCATION OF THE MARS GLOBALSURVEYOR IMAGE (RIGHT). THE 1999IMAGE SHOWS FEATURES ONLY 10 FEET(3 METERS) ACROSS—AN IMPROVEMENTIN RESOLUTION OF ABOUT 400 TIMES.

rather than biological activity. Recent years have seenongoing exploration of Mars. In 1997, the Pathfindermission delivered the first roving vehicle to theMartian surface, while the Mars Global Surveyor(arriving 1997) and Mars Odyssey (arriving 2001)missions are continuing the orbital reconnaissancewith unprecedented spy-cameralike images of thesurface (Figure 3).

From the accumulated spacecraft observations, ithas become obvious that Mars is a very dynamicworld but one very different than previouslyimagined. At 4,222 miles (6,794 km) Mars is abouthalf the diameter of Earth, and the surface gravity is38 percent of Earth’s. The Martian day lasts 24 hoursand 37 minutes (Huygens was close in hisestimation!), but the year is 687 days long. Therotation axis is tilted 25.1 degrees (similar to Earth’s23.5 degrees), but each Martian season is about twiceas long as the terrestrial counterpart. The atmosphereis very different—composed of 95 percent carbondioxide and with a surface pressure less than onepercent that found at sea level on Earth (equivalent toflying in an aircraft at an altitude of about 80,000feet). On the surface, the daytime high temperaturesrarely climb above the freezing point of water butplunge to at least -220 degrees F (-140 degrees C)every night. In the winter, the polar regions becomecold enough to freeze carbon dioxide out of theatmosphere causing dry ice “snow” to fall! Undercurrent conditions, liquid water cannot exist for longon the surface and will either freeze or quicklyevaporate. Only about a hair’s thickness of watervapor exists in the Martian atmosphere, but HubbleSpace Telescope observations have revealed that isenough to form widespread bands of water-iceclouds at some times of the year (Figure 5).

The Mars Global Surveyor and MarsOdyssey missions are busy revisingmany of the “common wisdom”aspects of Mars that werelargely based on theMariner and Vikingresults. Near-surfacewinds seem to bemore effective thanonce thought,formingenormous sanddunes in manyregions. Hugetornadolike dust

devils sweep dust from the surface to high in theatmosphere. Most intriguing is the evidence forrecent gullies, perhaps carved by running water; to ageologist, “recent” could be 10,000 years ago oryesterday. Several hundred examples of these gullieshave been discovered (Figure 6), supporting the ideathat subsurface aquifers containing liquid water orbrines may exist in many areas. The life-on-Marspendulum is swinging intothe maybe camp once again.

Many of the missionsplanned for the comingdecade will attempt to“follow the water” in searchof environments that mayhave once supported—ormay still support—microbiallife. In early 2004, two MarsExploration Rovers will actas robotic field geologists,roaming for several monthsand examining two separatelanding sites. Arriving in2006, the Mars Reconnais-sance Orbiter will expand onthe legacy of orbitalexploration. Later in thedecade, a “smart lander”may place one or morerovers within drivingdistance of a fresh gully,perhaps determining if near-surface deposits of waterare present. There are plans for a mission—perhapsas early as 2013—that will return a few hundredgrams of Martian soil and rock samples for detailedanalyses in laboratories on Earth. At that point, the

Golden Age of Mars exploration will benearly complete and serious plans

can begin for human missions. Stay tuned because theRed Planet is slowly but

surely revealing itssecrets, and many

fantastic newdiscoveries aresure to come inthe months andyears ahead.

Dr. Steven Lee isDMNS curator ofplanetary science. Healso serves as acoinvestigator on theMars Color Imager, acamera system slatedto be launched on theMars ReconnaissanceOrbiter in 2005.

Figure 5

Figure 6