Exploring Mars: Update 2002
This document is an update to the Exploring Mars Educational Brief. The material in this update is current as of January 2002.
Since February 1999, the Mars Global Surveyor spacecraft has been making detailed observations of the geology and topography of Mars. These measurements address many important questions about the structure and evolution of Mars. Did Mars once have an ocean? Are any of the volcanos still active? How has the climate of Mars changed with time? Analysis of these observations will keep planetary scientists busy for years to come.
NASA is also studying Mars with other spacecraft. Mars Odyssey entered orbit around Mars in October 2001. Its studies of the chemical composition of Mars will continue until 2004. Two robotic rovers are under development for launch to Mars in 2003. The Hubble Space Telescope has also continued making observations of Mars.
A More Detailed View of the Martian Surface Topography and Gravity of Mars
A More Detailed View of the Martian Surface
This image shows part of the Hrad Vallis channel system. The prominent scouring in this image was formed during an immense flood at some time in the past. A small impact crater is located in the lower center part of the image. This image is about 5 kilometers across and shows features as small as a few meters across. Image by the Mars Global Surveyor Camera team.
During the late 1970s, the Viking 1 and 2 spacecrafts obtained images of virtually all of Mars. These images typically showed features as small as 100 meters across, roughly the size of a football field, although finer details are shown in a small number of images. Our knowledge of the geologic history of Mars comes primarily from studies of these images.
However, geologists have long desired an even more detailed view of the surface of Mars. The Mars Orbital Camera on Mars Global Surveyor is providing this improved view. Its ultra-detailed images show features as small as 2 to 3 meters across. These new images serve many purposes. For example, Mars is now a dry world but the many channels on Mars record a time when Mars had liquid water at its surface and even catastrophic floods. Details visible in the new images will help us better understand the formation of these channels and the changes that have occurred in the climate of Mars. Other images reveal details of layering in the rocks. Understanding the origin of these layers will help geologists decipher the history of Mars.
Craters are produced by the impact of meteors and comets with the surface of Mars. The number of craters in a region is a clue to its age, with older features having larger numbers of craters. By allowing scientists to see very small craters, the new images are providing improved knowledge of the ages of some geologic events on Mars. Some of the images reveal lava flows that are less than 30 million years old and possibly even less than 10 million years old. Volcanism seems to be an on-going process on Mars, although no active volcanic eruption has ever been observed there. The new images also reveal the surface textures of individual lava flows, which may provide clues to the chemical composition of the lava and the rate at which it erupted.
Topography and Gravity of Mars
The topography of the western hemisphere of Mars. The highest structures, shown in white, are a group of volcanos on the left side of the image. Olympus Mons is at the upper left of this group of volcanos. Brown and red are also high, yellow and green are intermediate in elevation, and low regions are blue. The Valles Marineris canyon system is the low region at lower right. The northern plains are generally low, and the north polar cap is shown in green at the top of the image. Image by the Mars Global Surveyor Laser Altimeter team.
Since the early 1970s, NASA scientists have known that Mars has the largest volcanos and the longest canyon system in the Solar System. However, the heights and depths of these features were not precisely known prior to 1999. Mars Global Surveyor is using a laser altimeter to accurately measure how high the spacecraft is above the surface of Mars. This information can then be used to determine the elevations of features such as volcanos, channels, and craters, providing a three dimensional view of Mars. Based on these measurements, our knowledge of the topography of Mars is now better than our knowledge of the topography for much of the Earth's ocean floor. These measurements show that Olympus Mons, the largest volcano in the Solar System, is more than 21 kilometers high, somewhat lower than previously believed. Parts of the canyon walls in Valles Marineris are nearly 11 kilometers high. The combination of images and topography measurements will provide scientists with new insights into the forces that shaped the dramatic landscape of Mars.
On Mars, as on Earth, the strength of gravity varies slightly from place to place. These variations in gravity pull on the Mars Global Surveyor spacecraft and modify its orbit. The changes in the spacecraft's orbit can be measured by using radio waves to track the spacecraft's motion. In this way, Mars Global Surveyor has created a new, more precise map of the gravity field of Mars. Regions where the gravity is stronger than average can occur because the topography is high or because the rocks are denser than average. On the other hand, regions where the gravity is weaker than average can occur because the topography is low or because the rocks are less dense than average. Because we have also measured the topography, the gravity measurements in essence provide a way to look inside the planet and "see" how the density of rocks varies from place to place. For example, the crust of Mars (its outermost, rocky layer) has a different chemical composition than the deeper parts of the planet. By combining the gravity and topography measurements, we can map out how the thickness of the crust varies from place to place. These measurements can also be used to study the forces that support the giant volcanos of Mars.
An Ocean on Ancient Mars?
An artistic rendering by Michael Carroll of what an ancient ocean might have looked like on Mars. Image copyright Michael Carroll, all rights reserved.
The many channels on Mars indicate that liquid water once flowed across the surface of Mars. If these channels were all active at the same time, the flow of water into the low-lying northern plains could have formed large lakes or perhaps even an ocean. In the late 1980s, some geologists thought they saw evidence for shoreline features in photographs of Mars taken by the Viking spacecrafts. If correct, such features would be strong evidence for the existence of an ancient ocean on Mars. However, many other scientists remained unconvinced at that time.
Recent observations from Mars Global Surveyor have reinvigorated the debate about ancient oceans on Mars. Measurements by the laser altimeter show that one of the proposed shorelines is quite uniform in height over a distance of thousands of kilometers, just as one would expect for a shoreline. In addition, the area inside the proposed ocean is much smoother than other regions on Mars. This could occur if sediments deposited on the ocean floor had covered earlier, rougher terrain. On the other hand, study of high-resolution images of the proposed shoreline have not yet revealed clear evidence for features such as sand bars and beach terraces. Such features should have formed in some places if the proposed shoreline is a real feature. One possibility is that such small-scale features did form but are no longer visible because of several billion years of erosion. High-resolution images exist for only a very small fraction of the proposed ocean shoreline. More conclusive evidence may be found as additional images are obtained. Although a final answer is not yet available, this issue provides an excellent example of science in action, with researchers on both sides of the debate continuing to examine the evidence.
Magnetic Stripes on Mars
Details of the magnetic field of part of the southern hemisphere of Mars. Red and blue represent regions where the magnetic field points in opposite directions. Green and yellow regions have little or no magnetic field, and gray regions have not been measured. Image by the Mars Global Surveyor Magnetometer team.
Vigorous fluid motions of molten iron in Earth's core create a magnetic force. In essence, the entire Earth behaves like a giant magnet. This magnetic field is a global feature -- it can be detected anywhere on the surface of Earth, although its strength varies from place to place.
Until recently, information about the magnetic field of Mars was contradictory. Detailed measurements by Mars Global Surveyor have now shown that the magnetic properties of Mars are quite different from Earth. Mars does not have a global magnetic field. Surprisingly, Mars Global Surveyor did observe some local regions of magnetic activity on Mars. These regions have the appearance of stripes on the surface of Mars (the red and blue features in the image). These magnetic stripes are only observed in ancient, heavily cratered regions of Mars but not in younger regions of the planet. Evidently, vigorous fluid motions in the core of Mars were possible for a few hundred million years after Mars formed. As Mars cooled, the vigor of these fluid motions decreased and were no longer able to generate magnetism. Because Earth is larger than Mars, its interior has remained hotter and more active, so magnetic activity continues to be generated inside Earth.
Similar but much weaker stripes occur in the Earth's magnetic field and were a crucial piece of evidence in the discovery of plate tectonics on Earth. Some scientists propose that the magnetic stripes on Mars are a sign of plate tectonics in the early history of Mars. Other scientists point out that there is no geological evidence in photographs of Mars that plate tectonics ever occurred there. The strength and widths of the magnetic stripes on Mars are also quite different from those that occur on Earth. The magnetic stripes are probably the single most unexpected observation made by Mars Global Surveyor. It is likely to be some time before these structures are well understood.
Hubble Space Telescope Observes Mars
This Hubble Space Telescope image shows Mars in late April 1999. At this time, it was summer in the northern hemisphere of Mars and the north polar cap at the top of the image was near its minimum size. The polar cap is surrounded by a ring of dark sand dunes. An unusual storm system, about 1500 kilometers across, is visible to the lower left of the north polar cap.
Although the Hubble Space Telescope is mostly used to study distant objects such as galaxies, it is also used to study planets. Images of Mars taken with Hubble sometimes reveal details as small as 16 kilometers across. Although not as detailed as images from Mars Global Surveyor, the global perspective of the Hubble images is useful in studying weather on Mars, including clouds, dust storms, and seasonal changes in the polar caps. In addition, Hubble can study Mars at wavelengths of light (or "colors") that cannot be measured by Mars Global Surveyor. This information may be useful in mapping the chemical composition of minerals in different regions of Mars.
NASA attempted two new missions to Mars during 1999. The Mars Climate Orbiter was scheduled to enter orbit around Mars in September 1999. During a planned two-year mission, it would have studied clouds, water vapor, dust, and temperatures in the martian atmosphere. Unfortunately, there was a mistake made in plotting the spacecraft's course as it approached Mars. The mistake occurred because one team of controllers used English units (pounds) to measure forces acting on the spacecraft, while a second control team used metric units (Newtons) in planning corrections to the spacecraft's course. Because of the discrepancy, the spacecraft approached too close to Mars and was destroyed in the atmosphere.
The Mars Polar Lander was supposed to land near the south pole of Mars in December 1999. It would have taken detailed images of the landing site, measured the temperature and wind speed, and used a robotic arm to study the properties of the polar soil. Two miniature probes would have provided additional information about the soil in nearby regions. Unfortunately, radio contact could not be made with any of these spacecraft after the scheduled landing times. Because Mars Polar Lander was not in radio contact with Earth at the time of the failure, the precise cause of the failure will never be known. However, a NASA review panel concluded that the most likely failure was a premature shut down of the spacecraft's rocket engine, leading to a high speed crash into the martian surface.
New Spacecraft Missions to Mars
The Mars Odyssey spacecraft.
NASA launched Mars Odyssey on April 7, 2001. It entered orbit around Mars on October 23, 2001. Its study of the chemical composition of the surface of Mars is scheduled to continue until July 2004.
Mars Odyssey carries three scientific instruments. THEMIS is an infrared spectrometer that will detect the presence of different minerals in the martian soil. A similar instrument on Mars Global Surveyor has detected the presence of hematite, an oxidized (or "rusty") form of iron, which probably formed at a time when Mars had liquid water at its surface. THEMIS will provide a much sharper view of these features, because it can see details that are 30 times smaller than is possible with the spectrometer on Mars Global Surveyor. THEMIS will also look for other types of minerals, such as clays and carbonates, that will provide clues about the early climate on Mars.
The Gamma Ray Spectrometer will measure the abundances of chemical elements such as iron, aluminum and silicon in the rocks of Mars. Chemical measurements have been made previously at three locations by the Viking and Mars Pathfinder landers. Mars Odyssey will extend these measurements to the rest of Mars, helping geologists to unravel the geological history of the planet. The Gamma Ray Spectrometer will also measure the presence of hydrogen and hence the distribution of water on the surface of Mars. The third instrument, MARIE, will measure radiation around Mars that might be hazardous to future human explorers.
An artist's conception of NASA's 2003 rover mission to Mars. The rover is powered by solar cells on its top surface. At left and center are two communication antennas. The mast at right carries two science instruments, the panoramic camera and an infrared spectrometer.
The lander mission that NASA originally planned for 2001 was cancelled. Many of the instruments that were being developed for that mission will be used in 2003, when NASA plans to launch two robotic rovers to Mars. These Mars Exploration Rovers will be considerable larger and more capable than the Sojourner rover used in 1997 by Mars Pathfinder. They will have some ability to navigate independently across the surface of Mars, without requiring detailed directions from the mission control center on Earth. Each rover is expected to travel up to 1 kilometer (about 0.6 miles) from its landing site during its 90 day mission. The rovers will take panoramic photographs of the geology as well as close-up views of rock textures. Several instruments will study the chemical composition of the martian rocks and soil. Taken together, these measurements will help geologists understand how these rocks formed and how they may have later been altered by weathering and other processes in the Mars environment. Possible landing sites for these missions are now under consideration.
Mars is also a target for exploration by other countries. The European Space Agency's Mars Express mission will be launched in 2003. It will study the geology of Mars from orbit and also carries a small lander, Beagle 2. Japan's Nozomi spacecraft is already enroute to Mars. When it arrives in December 2003, it will study the upper atmosphere of Mars and its interaction with the solar wind.
In later years, additional missions are being planned. In 2005, NASA plans to launch the Mars Reconnaissance Orbiter. Its high resolution imaging system will be able to detect features as small as 0.3 meters (1 foot) across, contributing to the selection of safe landing sites for future missions. A radar will be used to look below the surface of Mars, particularly for evidence of liquid water or ice deposits. Missions in 2007 and later years are also being considered and are likely to be collaborations between NASA and various international partners. The ultimate goal of this exploration program is the return of rock and soil samples from Mars for study in Earth laboratories. However, because of the technical difficulties of such missions, a sample return mission probably will not be attempted until 2014 or later. No time-table has been set for sending astronauts to Mars.
Author: Walter S. Kiefer, Lunar and Planetary Institute
Document URL: http://www.lpi.usra.edu/expmars/edbrief/update.html
Document Updated: January 2002