Exploring Mars: Educational Brief

Mars is the only planet besides Earth that was ever cut by flowing water or graced by lakes and ponds. Now, that water is frozen at the poles and buried beneath Mars' frigid deserts. In those ancient martian pools, might life have sprung up and prospered? The pools are dry and sterile today, but could life persist in deep and hidden places? Someday, will humans walk those distant deserts, seeking signs of ancient life?

A HISTORY OF EXPLORATION
Humans have known of Mars since before recorded history. Even 3600 years ago, the Babylonians wrote about Mars' looping motion across the sky and changing brightness. Mars was one of five "stars that wandered" among the fixed stars of the night, and was special because of its red color. In ancient India, Mars appeared like a fire in the sky - for many other cultures, its redness recalled the fire and blood of war. In ancient Greece, the red wanderer personified the god of war, "Ares." When the Romans conquered Greece, they adopted this symbol and named the planet for their god of war, "Mars."

Through the Middle Ages, astrologers studied Mars' motions to help them predict the future - if Mars moved unfavorably, wars would be lost! But no one could predict Mars' motion accurately, even using Copernicus' theory (of 1543) that the planets orbit in circles around the Sun. Johannes Kepler solved this puzzle in 1609 when he discovered that Mars orbits the Sun in an ellipse, not a circle. Seventy-five years later, Kepler's solution was crucial to Isaac Newton's discovery of the law of gravity.

While Kepler explained its orbit, Galileo Galilei transformed Mars into a world. In 1609, Galileo first viewed Mars through his newly invented telescope. Although his telescope was no better than a modern toy, it revealed enough to prove that Mars was a large sphere, a world like the Earth. Could this new world be inhabited? As telescopes improved, more of Mars could be seen: polar icecaps, color patterns on its face, clouds, and hazes. These observations all fit a habitable planet, and speculations that Mars was inhabited became more and more believable.

The idea of living martians came to full flower in 1877 when the Italian astronomer Giovanni Schiaparelli observed thin dark lines crossing Mars' bright "continents." He called the lines "canali," (Italian for "channels"), and the word was widely misread as "canals." In the U.S., Percival Lowell seized on the canals as proof of a martian civilization, advanced enough to move water across a whole world. Many scientists agreed, but most thought that the canals were optical illusions. They thought that Mars was too cold and its air too thin to support life as we know it.

Understanding of Mars advanced little from Lowell's time in the late nineteenth century until 1965, when the Mariner 4 spacecraft flew within 10,000 kilometers of the martian surface. Its pictures, the first close-up views of Mars, showed a Moonlike landscape of plains pocked by impact craters. There were no canals or other signs of life. Mariner 4 finally proved that Mars' atmosphere, only 0.7% as thick as the Earth's, was much too thin for life as we know it.

Four years later the twin spacecraft Mariner 6 and Mariner 7 flew by Mars again, carrying cameras and spectrometers to measure the temperature of Mars' surface and the composition of its atmosphere. Their photos again showed no canals or other signs of life, but did reveal a volcano, plains without impact craters, and areas of chaotic hills. Mars' mass and density were calculated from spacecraft tracking. The spectrometers showed that Mars was very cold (-123°C at the south pole), and that Mars' thin atmosphere was almost all carbon dioxide. At the time, Apollo 11's landing on the Moon overshadowed the successes of Mariners 6 and 7.

Exciting as they were, the early Mariner spacecraft only spent a short time near Mars as they flew past; more time was needed, and that meant going into orbit. So in 1971, Mariner 9 arrived at Mars and became the first artificial object ever to orbit another planet. More than twice as big as its predecessors, Mariner 9 carried color cameras and new instruments tailored to investigating Mars' surface and atmosphere. An unsung part of the spacecraft was its computer system, which allowed Mariner 9 to wait until Mars' atmosphere cleared of a planetwide dust storm. Mariner 9 operated for almost a year, mapped 85% of Mars' surface in more than 7000 images, analyzed Mars' gravity field, measured surface temperatures and dust abundances, and measured temperatures and humidity of its atmosphere.

Mariner 9's view of Mars was the first detailed global view of another planet; it revealed a "New Mars," unlike any earlier concept. The earlier Mariner spacecraft saw land typical of the southern hemisphere: craters and more craters. Mariner 9 saw what they missed such as the Valles Marineris, a canyon up to 100 kilometers wide and 10 kilometers deep that would reach from Los Angeles to New York! Giant valleys extend from the Valles and elsewhere and are mute testimony to devastating floods in Mars' distant past. Most of the valleys end in the northern plains, a vast lowland encompassing almost a third of the planet. There, the floodwaters ponded into huge lakes or perhaps even an ocean. Signs of water appear in the southern highlands too, for the most part as small valleys draining away from the largest craters and uplands. Despite these signs of ancient water, Mars now is too cold and its atmosphere too thin for liquid water to remain.

Ceraunius Tholus voldano on the Tharsis Rise

Mariner 9 also was the first to see Mars' volcanos, the biggest in the solar system. The biggest of all, Olympus Mons, is 600 kilometers across at its base and 25 kilometers tall. Smaller volcanos and lava flows appear all over Mars, especially on the Tharsis Rise, a huge bulge distorting Mars' spherical shape.

Looking toward space, Mariner 9 took the first close-up images of Mars' moons, Phobos and Deimos. They are little more than large potato-shaped rocks, about 10 kilometers long, and appear similar to asteroids.

Mariner 9's global perspective and spectacular images of water-carved landscapes inspired further exploration of Mars to focus on the search for life. After extensive development, the twin spacecraft Viking 1 and 2 were launched in 1975 and entered Mars orbit in 1976. Each Viking was actually two spacecraft: an orbiter and a lander. Each orbiter had a pair of cameras and instruments for mapping surface temperature and atmospheric humidity. Each lander included a weather station, a seismometer for detecting "marsquakes," instruments for analyzing soil, and a stereo TV camera.

The Viking 1 lander touched down gently on July 20, 1976, on Chryse Planitia in the northern lowlands. Its robot eyes took the first photos of the martian surface: a rolling desolation of dark rounded rocks and brick-red dust under a pink sky. The rocks are probably volcanic, pitted and smoothed by eons of blowing sands. On landing, the winds were light, at most 30 kilometers per hour. Viking 1 sits at a latitude comparable to the Sahara Desert on Earth, but its daytime temperatures climbed to a high of -10°C (14°F), and dropped to a numbing -90°C (-130°F) before sunrise.

The Viking 2 lander touched down two months later on Utopia Planitia, closer to Mars' north pole, a latitude comparable to Maine or Mongolia on Earth. The plains of Utopia are rockier than the Viking 1 site in Chryse; one of Viking 2's legs stands on a rock. The landscape at Utopia is nearly flat; only a few low crater hills appear on the distant horizon. In the summer, Utopia was no warmer than Chryse, but its winter night temperatures plunged to -120°C (-184°F). In winter, a thin layer of water frost was present for several months.

The Viking landers saw nothing alive, and recorded no movement except blowing sand, shifting dunes, and their own robot arms. The arms pushed and scraped the martian soil, and scooped some for analysis. The landers' soil instruments were designed to detect Earthlike life. The instruments cooked soil, soaked it, and fed it nutrient broth. Although the soil contained no organic material, a few experiments seemed to indicate living organisms. After years of debate, almost all scientists now agree that the life signs came from unusual minerals in the soil, and that Mars' surface is lifeless.

Meanwhile, the two Viking orbiters sailed overhead, recording the martian landscape. Instruments measured water abundances in the atmosphere and both day and night temperatures on the surface. The orbiters took more than 52,000 images, giving complete coverage of Mars in great detail. These images have fueled years of intense study of Mars, and are still yielding new insights into its volcanos, water, and ancient history.

From the end of the Viking program in 1982 until 1996, there were no successful spacecraft missions to Mars. Between 1988 and 1996, four spacecraft, one from the United States and three from Russia, were unsuccessful. Of these, only the Russian Phobos 2 spacecraft returned even a limited amount of useful data. These setbacks were unfortunate and show that space exploration remains a difficult and challenging endeavor. Beginning in late 1996, however, the United States successfully resumed robotic exploration of Mars.

Without spacecraft at Mars, the Hubble Space Telescope was one of the few highlights of Mars exploration. Hubble cannot see surface details smaller than about 25 kilometers, but can see Mars well enough to map clouds, dust storms, and seasonal changes in the polar caps. Measurements from Hubble show that Mars' atmosphere is now colder and much less dusty than during the Viking missions. Clouds of water ice are more abundant now than during the Viking missions, and show how water moves from pole to pole as Mars' seasons change.

Another highlight in Mars exploration was the discovery that a few meteorites on Earth came originally from Mars! These meteorites contain traces of gas identical to the martian atmosphere as analyzed by the Viking landers. Asteroid impacts ejected the meteorites off Mars into orbit around the Sun; after millions of years, they landed on Earth. The martian meteorites are all volcanic rocks, most are young (erupted only 180 million years ago), and almost all have reacted with martian groundwater. These meteorites have revolutionized thinking about Mars' atmosphere and its water, and are "ground truth" for interpreting images of the distant geology of Mars. The martian meteorites are almost like sample return missions, except we don't know where on Mars they formed. Some scientists believe that one martian meteorite includes traces of ancient martian life - fossilized martian bacteria. However, many scientists remain unconvinced by this claim, which is now a topic of intense investigation.

WHY CONTINUE?
Is there any reason to continue exploring Mars? Haven't we learned everything already? Telescope and spacecraft exploration have taught us a lot, but many important questions remain unanswered.

For instance, why is Mars' surface (with many craters and huge volcanos, and no continents) different from the Earth's surface (with continents and chains of smaller volcanos, but few craters)? The answer seems to lie deep within the planets, where hot rock flows slowly upward toward the surface. This motion is called mantle convection, and it seems to take different forms on Earth and Mars. On Earth, mantle convection moves large pieces of the surface, the geologic plates, and most volcanos, earthquakes, and mountains form at plate boundaries. On Mars, however, the upward flow of mantle rock bows up the surface but doesn't break it into pieces. The upward flow is centered at Tharsis, a bulge or high plateau about 4000 kilometers across and up to 10 kilometers high. Tharsis is covered by volcanos that reach even higher; Olympus Mons is 25 kilometers tall. It appears that the volcanos on Tharsis have erupted for almost the entire history of Mars. The Tharsis volcanos might still be active but dormant - no volcano eruptions have ever been seen. Around Tharsis are many long cracks (including the Valles Marineris), showing that the martian crust was stretched and broken as Tharsis swelled. The high elevations, volcanos, and cracks were all caused by mantle convection. But compare this stable pattern with Earth, where mantle convection produces chains of volcanos and long mountain ranges that come and go through time.

Another question: Why doesn't Mars have oceans like the Earth does? Mars' atmosphere is now too thin and its temperature too cold to allow liquid water. But the important questions are about water itself - how much water does Mars have, and where is it? Mars certainly had surface water and groundwater once; only liquid water could have shaped the valley networks in the highlands and the huge flood channels that cut from the highlands to the northern lowlands. But how much water was there? Estimates range from the equivalent of an ocean 10 meters deep covering the entire surface to the equivalent of a layer kilometers deep. The first is not much water at all, and the second is a lot of water! However much water there was, it is not now on the surface, except for a bit in the polar ice caps. Where did the water go? It could be underground in pools of groundwater, either small or huge depending on how much water Mars started with. Or it could have escaped to space and been lost completely - the hydrogen from water can escape easily through Mars' low gravity and small magnetic field.

And finally, we don't know if there is or was life on Mars. There are no canals or ancient cities, and no clear signs of any life on Mars' inhospitable surface. But Mars' climate was mild once, with a thicker atmosphere, flowing water, open lakes, and perhaps even an ocean. Life on Earth may have started under similar conditions, possibly at underwater hot springs. With its volcanos and lava flows, Mars probably also had hot springs - if Mars had oceans or lakes, could life have also started on Mars? We know about the origins and history of life on Earth from fossils - how and where would we look for fossils on Mars? And why confine our search to Mars' surface? On Earth, many kinds of bacteria live deep inside rocks, and die when exposed to light and fresh air. Could organisms like these be alive and prospering in groundwater far beneath the surface of Mars? And do we now have fossils of these bacteria, preserved for eons in the martian meteorites?

EXPLORATION NOW
The United States has resumed an active program of robotic spacecraft exploration of Mars. These spacecraft may provide answers for some of these important questions during the next several years.

Mars Pathfinder, the first successful space probe to Mars in 20 years, landed on July 4, 1997, near the mouth of Ares Vallis. Viewed from orbit, Ares Vallis looks like a giant flood channel that formed long ago. Photographs taken by Pathfinder reenforce this view, showing an undulating landscape with many rocks, some lined up in the direction that the flood waters flowed. Pathfinder carried a miniature, six-wheeled rover called Sojourner, which explored the region around the lander. Chemical measurements made by Sojourner suggest that the rocks in this region are andesites, a type of volcanic rock that is common in some places on Earth but was not expected to occur on Mars. Based on Viking results and studies of the martian meteorites, it was expected that the Pathfinder site would consist of basalt, which is the most common type of volcanic rock on Earth (for example, the Hawai'ian volcanos produce basalt).

If additional study verifies the presence of andesite, it will alter our understanding of how Mars has evolved. During its three months of activity, Pathfinder also measured temperatures and wind speeds on Mars and even recorded the passing of several "dust devils" (swirling dust clouds) over the landing site. Radio tracking of Pathfinder provided new information about the rotation of Mars, in particular about how it slowly "wobbles." All planets wobble slightly as they rotate (like a top), and this wobbling depends on how material is distributed inside the planet. The data for Mars indicate that its central core is composed mainly of iron and fills about half the planet.

Mars Global Surveyor entered orbit around Mars in September 1997. Originally, this orbit was very elongated, but by repeatedly allowing the spacecraft to drag through the upper reaches of the atmosphere (a technique called aerobraking), the orbit was gradually changed to a low, circular orbit that allows close-up study of Mars. Some important scientific observations were made during this time, but most of Global Surveyor's observations will be made during 1999. A camera will obtain images that show features as small as 2 meters (7 feet) in size. These images are 10 to 100 times as detailed as previous images of Mars and will allow a much better understanding of the processes that have shaped the surface of Mars. An altimeter uses a laser beam to measure the topography of Mars, such as the heights of its volcanos and the depths of its canyons and craters. An infrared spectrometer measures the composition of rocks and dust on the surface. Variations in Mars' gravity will be mapped from changes in the spacecraft's orbital speed. This subtle measurement allows us to "see" into the interior of Mars and locate regions of heavy and light rock. A magnetometer has already shown that Mars does not have magnetic poles (north and south) like the Earth; a compass would not be very useful on Mars. Several of these instruments are also measuring weather on Mars, such as wind speeds, cloud heights, and temperatures at the surface and in the atmosphere. UPDATE

As Earth and Mars orbit about the Sun, the opportunities for the easiest trips from one planet to the other occur about every 26 months. Current NASA plans are to send both orbiter and lander spacecraft to Mars at each of the next several launch opportunities. The Mars Climate Orbiter was launched in December 1998. It will enter orbit around Mars in September 1999 and perform a two-year study of weather on Mars. A digital camera will take color photographs of cloud patterns and an infrared light detector will measure the temperature, water vapor, and dust in different parts of the atmosphere. UPDATE

Mars Global Surveyor image of Nanedi Valles

The Mars Polar Lander was launched in January 1999 and is scheduled to land near the south pole in December 1999. It will be the first spacecraft to land near either pole of Mars. A camera, robotic arm, and chemical analyzer will be used to study the polar soil. Other instruments will measure the temperature, pressure, and water vapor of the atmosphere and the speed of the wind. A small laser will measure dust and ice in the atmosphere. A microphone will record wind noises. UPDATE

The orbiter mission scheduled for launch in April 2001 will carry two instruments to map the types of rocks present in different regions of Mars. The lander mission scheduled for 2001 will use a robotic arm, a rover similar to Sojourner, and several different types of chemical analyzers to study the composition of rocks and soil at the landing site. Other experiments will study properties of the soil that might be hazardous to future human explorers. UPDATE

Additional missions are being planned for 2003 and 2005. These may include larger rovers that will be able to travel for many kilometers away from the landing site. These rovers will collect small rock samples for possible return to Earth. Designing a rocket that is able to return such samples to Earth remains an important engineering challenge, but it is hoped that such samples can be returned to Earth as early as 2008. Studying Mars rocks in Earth laboratories will provide a far more detailed understanding of Mars than is possible from unmanned spacecraft alone. Further in the future, a lander network of seismometers to measure "marsquakes" could provide information about the internal structure of Mars. Meteorology instruments on these landers could add to our understanding of weather on Mars. An orbiting radar could be used to look for underground water. Some of these missions may be carried out in collaboration with other countries. UPDATE

When will humans explore Mars? No space agency has concrete plans for human landings on Mars in the near future; landings before 2020 are unlikely. But someday, people will descend from a spacecraft, stand on red soil, and see for themselves the canyons, volcanos, and dried lakebeds of Mars.

U.S. MARS MISSIONS - SUCCESSFUL AND ONGOING
Mission	Launch	Arrival	Highlights
Mariner 4 Flyby	Nov. 28, 1964	July 14, 1965	22 black-and-white images of desolate, cratered southern hemisphere. No canals or signs of life. Water frost seen. Proof that Mars' atmosphere is very thin.
Mariner 6 and 7 Flybys	Feb. 24 and Mar 27, 1969	July 31 and Aug. 5, 1969	75 and 126 black-and-white images of equatorial region, southern hemisphere, and south polar ice. Measured Mars' mass and density.
Mariner 9 Orbiter	May 30, 1971	Orbit: Nov. 14, 1971	7329 images, many in color. First views of huge volcanos of Tharsis, chasms of Valles Marineris, water-cut channels, Mars' moons.
Viking 1 Orbiter, Lander	Aug. 20, 1975	Orbit: June 19, 1976 Landing: July 20, 1976	Orbiter takes >30,0000 images of surface, many in color. Global maps of temperature, atmosphere water content, surface properties. Lander gives first images from Mars' surface: dark rocks, red dust, pink sky. Tests soil for life and finds none. Records Mars' weather.
Viking 2 Orbiter, Lander	Sept. 9, 1975	Orbit: Aug. 7, 1976 Landing: Sept. 3, 1976	Like Viking 1, Orbiter takes >20,000 images of surface. Lander finds no life; again dark rocks, red dust, pink sky. Records Mars' weather.
Mars Global Surveyor	Nov. 7, 1996	Orbit: Sept. 11, 1997	High-resolution images, maps of surface elevation and mineral distribution.
Mars Pathfinder Lander	Dec. 4, 1996	Landing: July 4, 1997	Landing in Ares Vallis; rover tests, imaging and chemical investigations.
Mars Climate Orbiter	Dec. 11, 1998	Failed to achieve orbit: Sept. 23, 1999	Global imagery, atmospheric temperature profiles.
Mars Polar Lander	Jan. 3, 1999	Failed to land: Dec. 3, 1999	First landing attempt near martian south pole.

FOR THE CLASSROOM

FOR MORE INFORMATION

If you have problems or comments concerning LPI on the Web,
please send e-mail to the following address:
webmaster@lpi.usra.edu

FOR THE CLASSROOM

FOR MORE INFORMATION

If you have problems or comments concerning LPI on the Web, please send e-mail to the following address: webmaster@lpi.usra.edu

If you have problems or comments concerning LPI on the Web,
please send e-mail to the following address:
webmaster@lpi.usra.edu