MarsMars is the fourth planet from the sun and orbits the sun at a distance of about141 million mi. Mars is named for the Roman god of war because it appears fieryred in the earth’s night sky.
Mars is a small planet that has about half thediameter of Earth and about one-tenth Earth’s mass. The force of gravity onthe surface of Mars is about one-third of that on Earth. Mars has twice thediameter and twice the surface gravity of Earth’s moon. The surface area ofMars is almost exactly the same as the surface area of the dry land on Earth. The Martian day, or the time it takes Mars to rotate once on its axis, is abouta half an hour longer than an Earth day. Its year, or the time it takes torevolve once around the sun, is about two Earth years long.
Mars has two moons,Phobos and Deimos. THE INTERIOR OF MARS Scientists believe that Mars’sinterior consists of a crust, mantle, and core like Earth’s interior, but theydo not know the relative sizes of these components. Because no spacecraft hasever brought instruments that can study Mars’s interior to the planet, theonly real data that scientists have about the planet’s structure are its mass,size, and the structure of the gravity field. Compared to Earth, Mars probablyhas a relatively thick crust.
Beneath the surface is an area of volcanicactivity in the northern hemisphere, it may be as thick as 80 mi. Beneath thelanding site of the United States spacecraft Viking 2, it may be as thin as 9mi. The core is probably consists of mostly iron, with a small amount of nickel. Other light elements, mainly sulfur, could exist in the core also. If so, thecore may be quite large. Mars does not have a significant magnetic field, soscientists believe that Mars’s core is probably solid.
Mars does not, andprobably did not ever, have active plate tectonics. Because Mars is so muchsmaller than Earth, it must cooled quickly after formation and the crustthickened, forming one solid piece and eliminating any possibility of platetectonics as it was on and still is on Earth. Though the Martian crust is notbroken into separate plates, Mars’s liquid mantle has sculpted the planet’ssurface. The molten rock has broken through the crust to form volcanoes and itsmotion has cracked the crust to form large rifts. THE SURFACE OF MARS Thesurface of Mars would be a harsh place for humans, but it is more like thesurface of Earth than any other planet. The temperature on Mars does not getmuch cooler than the temperature at Antarctica.
At the surface it ranges fromabout -140? C to 15? C (about -225? F to 60? F). During most of the yearwind speeds are normally low around 4. 5 mph, but during dust storms they canapproach 40 to 50 mph. These winds often originate in large basins in thesouthern hemisphere and carry large volumes of dust from the basins to otherregions, sometimes covering the entire planet in the storm. The dust is notsandy, as in a sandstorm on the earth, but has the consistency of flour.
Thenorthern and southern hemispheres of Mars have different characteristics. Thesouthern hemisphere has many impact craters and has a generally much higherelevation than the northern hemisphere. The southern highlands are probably theoldest ground on Mars. The northern hemisphere of Mars contains a much widervariety of geologic features, including large volcanoes, a great rift valley,and a variety of channels. The northern hemisphere also contains large expansesof relatively featureless plains. Mars has the largest volcano in the solarsystem, Olympus Mons.
It is 16 mi high (almost twice as high as the earth’sMount Everest) and covers an area comparable to the state of Arizona. Near it,three other volcanoes almost as large-Arsia Mons, Pavonis Mons, and AscraeusMons-form a line running from southwest to northeast. These four volcanoes arethe most noticeable features of a large bulge in the surface of Mars, calledTharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge, but isquite different in appearance. It is probably less than 4 mi high, but has adiameter of 1000 mi.
None of Mars’s volcanoes appear to be active. The Tharsisbulge has had a large effect on the appearance of the surface of Mars. TheTharsis bulge includes many smaller volcanoes and stress fractures, in additionto the large volcanoes. Its presence affects the weather on Mars and may havechanged the climate by changing the rotation of the planet. Valles Marineris(named for the U. S.
Mariner spacecraft that discovered it) is the most notablestress feature associated with the Tharsis bulge. It is a great rift valleyextending from the Tharsis region away to the east-southeast. It is about thesame length as the distance from New York to California. This canyon systemreaches widths of 440 mi and depths of 4 mi. Three types of channels on Marswere probably formed by the action of water. These channels are unrelated to the”canals” thought to be seen in early telescopic views of Mars.
Channelnetworks are similar in appearance to streambeds on the earth and occur in thesouthern highlands. These channels may date from a time early in Mars’shistory when the atmosphere was thicker and liquid water could flow on thesurface. Outflow channels, which giant floods may have formed, occur on theboundary between the southern highlands and the northern plains regions. AresVallis, where the Mars Pathfinder spacecraft landed, is one of these outflowchannels. Landslides and other erosion probably formed fretted channels byenlarging preexisting channels. The Mars Pathfinder spacecraft found minerals inAres Vallis that are similar to minerals that form near water on Earth,supporting the theory that Mars had liquid water at some point in its history.
Mars has small, permanent ice caps at its north and south poles. The capsincrease in size in the winter of each hemisphere. The caps in the north andsouth are quite different from one another. The northern permanent cap iscomposed of water ice and is about 620 miles across.
A seasonal cap of frozencarbon dioxide adds to the northern ice cap in the northern winter. The southernpermanent cap is one-third the diameter of the northern cap because summer inthe southern hemisphere is warmer than in the north. The southern seasonal capis larger than the northern cap because more carbon dioxide is frozen out in thesouth than the north because Mars is farthest from the sun, and thereforecoldest, in the southern winter. Carbon dioxide may also make up the southernpermanent cap. Regions of striped-looking terrain, probably formed of layers ofdust and ice, occur at the edges of both polar caps.
Climate cycles almost likethe ice ages on the earth may have caused this layering. THE ATMOSPHERE OF MARSThe atmosphere of Mars is 95 percent carbon dioxide, nearly 3 percent nitrogen,and nearly 2 percent argon with tiny amounts of oxygen, carbon monoxide, watervapor, and other gases. The earth’s atmosphere is mostly nitrogen and oxygen,with only 0. 03 percent carbon dioxide. The pressure of Mars’s atmospherevaries with the season, ranging from 6 to 10 millibars (1 millibar is almost oneone-thousandth of the air pressure at the surface of Earth).
The variation inpressure is caused by carbon dioxide freezing out at the poles of the planet infall and winter. The pressure also varies with altitude and is about a factor often less on the top of Olympus Mons than on the floor of Hellas Planitia. Theatmosphere of Mars contains very little water vapor. The level of water vaporaverages about 0.
016 percent, compared to the earth’s average level of about 2percent. The water content of the atmosphere on Mars varies seasonally and bylocation and can form clouds and even frost. Six major types of clouds form inMars’s atmosphere. The polar hood is a haze of water and perhaps carbondioxide ice that forms over the polar regions in the fall and can cover much ofthe northern plains. Wave clouds form on the sheltered side of large obstacles,such as craters, and have very distinct ridges. Convective clouds form in highareas at midday.
Orographic clouds form when air lifts over large-scale objectslike Olympus Mons, and are most common in spring and summer when the water vaporcontent of the air is highest. Ground hazes occur in low areas at dawn and duskand probably consist of water ice. Wispy high-altitude clouds sometimes occurjust at dawn and dusk. The Viking 2 lander recorded images of water-ice frostduring the winter. Past Space Stations One past space station is Mir.
Mir was aRussian space station designed to provide long-term accomodations forcrewmembers while they orbit the earth. Mir was launched on Febraury 19, 1986. Crewmwmbers reached Mir aboard Soyuz spacecraft and, more more recently thtroughan American space program aboard a spaceshuttle. Mir was the first space stationdesigned for expansion and was originally only a single module.
Now Mir consistsof seven modules. Mir replaced the Salyut series of space stations as the mostimportant part of the Russian manned space program. The Salyut series of spacestations were smaller and simpler stations that helped develop most of thetechnology needed to build Mir. The Mir space station is composed of sevenmodules that together weigh about 109,000 kg and are about 19 m long without anyvisiting spacecraft. The Mir core module is the control center and livingquarters for the Mir station. The 20-ton module measures about 4.
18 m in widthand about 13 m in length. At each end of the main part is a hatch fitted toconnect with other spacecraft called a docking port. The rear port leads througha tunnel into the living compartment, which contains a kitchen, exerciseequipment, two sleeping compartments that are smaller than phone booths, and atoilet stall. Mir’s first crew was Salyut 7 veterans Leonid Kizim and VladimirSolovyov. They flew to the Mir core module in the Soyuz-T 15 spacecraft in March1986 to activate and check Mir’s systems.
They undocked and flew to theabandoned Salyut 7 station to salvage scientific equipment and dropped off therecovered equipment at Mir. They returned to earth in July 1986. Mir flewunmanned until February 1987. Except for two short periods from July 1986 toFebruary 1987 and from March 1989 to September 1989, Mir has been staffedwithout interruption.
Normally, teams of two or three cosmonauts work on boardin six-month shifts. There are, however, occasional exceptions. For example,medical doctor Valeri Polyakov set a new world space-endurance record by livingon Mir for 438 days-long enough for a spacecraft to travel to Mars. During thattime, Polyakov studied his body’s reactions to prolonged weightlessness. Hereturned to earth aboard Soyuz-TM 20 in March 1995. With him was YelenaKondakova, the first woman to complete a long-duration stay in space.
She livedaboard Mir for 168 days. Also in March 1995, U. S. astronaut Norman Thagard begana 114-day Mir flight, breaking the U. S. 84-day space-endurance record set onSkylab in 1974.
Thagard reached Mir on Soyuz-TM 21 with cosmonauts VladimirDezhurov and Gennadi Strekalov. He returned to earth with his Russian crewmateson the space shuttle Atlantis, which docked with Mir for the first time on June29, 1995. Since Thagard’s visit, six other U. S. astronauts have lived on Mir.
German astronaut Thomas Reiter arrived at Mir aboard Soyuz-TM 22 in September1995. He returned to earth in February 1996, after 179 days in space, havingcompleted two space walks to install European instruments outside of thestation. Mir was over a decade old when its career was nearing an end. In 1997the station experienced a small fire, failure of the oxygen generation system, atemperature-control failure that made the living quarters uncomfortably warm,failures of Mir’s main computer and navigation system, and a collision with asupply ship. None of the onboard cosmonauts and astronauts were hurt, but theincidents caused crew members and engineers to monitor the station’s conditionmore closely.
Just as scientific equipment from Salyut 7 was transferred to Mir,equipment from Mir will be transferred to Mir’s planned follower ship, theInternational Space Station (ISS), at the end of Mir’s career. Space shuttlemissions to Mir ended in mid-1998 and the first component of ISS was scheduledfor launch in late 1998. ISS was assembled in orbit from U. S. , Russian,European, Japanese, and Canadian parts.
