On July 4, 1997, the Mars Pathfinder made a spectacular
landing on Mars carrying with it a small robot, the Sojourner rover, which was to make history by
measuring rocks in the vicinity of the landing site with its Alpha Proton X-Ray Spectrometer.
Before communications between the Jet Propulsion Laboratory and the Mars Pathfinder and Sojourner were lost and the mission was officially winding down on November 4, 1997, 16,000 pictures of the planet had been received from the Mars Pathfinder and Sojourner.
The mission was declared a phenomenal success: all the equipment continued functioning well beyond their anticipated life, and gave scientists much more information than was expected.
The only mission that was not completed was the so-called Super-Pan, a 360-degree view of the Martian landscape around the Dr. Carl Sagan Memorial Station, the name conferred to the Mars Pathfinder in honor of the astronomer who had brought the study of the universe to millions of television viewers and readers.
In the above panorama, Twin Peaks are visible on the horizon, and Sojourner is next to the rock called Yogi, on which it was to begin a series of measurements.
Nothing epitomizes the saga of explorations of the Mars Pathfinder mission more aptly than the indefatigable Sojourner navigating, on command from Earth, to one rock after another to study the chemical composition of its target.
Mars Global Surveyor
On September 11, 1997, the Mars Global Surveyor came to Mars in an elliptical orbit, which over the next few months was brought to a circular trajectory through aerobraking. Once this orbit is achieved, the Mars Global Surveyor began a two-year photography mission of the Martian surface.
The Global Surveyor has studied the entire Martian surface, atmosphere, and interior, and has returned more data about Mars than all other Mars missions combined. Observations by the spacecraft are expanding our understanding of the Martian climate and may indicate the climate is changing significantly even today. Among key science findings, Surveyor has taken pictures of gullies and debris flow features that suggest there may be current sources of liquid water, similar to an aquifer, at or near the surface of the planet.
TES is both an instrument and a technique. The Thermal Emission Spectrometer (TES) is a scientific instrument that first flew aboard the Mars Observer spacecraft. Following the loss of that spacecraft, TES was rebuilt and launched along with five of the original seven Mars Observer instruments aboard the new Mars Global Surveyor spacecraft. The purpose of TES is to measure the thermal infrared energy (heat) emitted from Mars. This technique, called thermal emission spectroscopy, can tell us much about the geology and atmosphere of Mars. TES data will provide the first detailed look at the composition of Mars.
TES science results generally fall into one of four categories: surface mineralogy, polar processes, atmospheric processes, and thermophysical properties of the surface.
Key surface mineralogy results include:
The mineralogy of volcanic materials varies from basaltic, composed of plagioclase, feldspar, clinopyroxene, olivine, plus/minus sheet silicates, to andesitic, dominated by plagioclase, feldspar and high-silica volcanic glass. The basalts occur primarily in the ancient, southern hemisphere highlands, and the andesites occur primarily in the younger northern plains.
The spectra from dark regions closely match both the spectral shape and contrast of particulate samples of terrestrial rocks.
No unusual particle size or other environmental effects are observed, nor are required, to account for the spectra observed for Mars.
Aqueous mineralization has occurred in limited regions under ambient or hydrothermal conditions. Gray, crystalline hematite is found in three locations that are interpreted to be in-place sedimentary rock formations. These units provide evidence for the long-term stability of liquid water near the surface of Mars.
No evidence for carbonates has been found. Arguments can be made for the failure to detect these minerals, but we can conclude that large-scale (10's of km), coarse-grained (>50 micron) deposits of >~10% carbonates are not currently exposed at the Martian surface. This lack of detection is consistent with many models for early Mars in which large volumes of carbonates never formed.
Olivine has been identified and mapped in specific locations in the basaltic terrains at abundances up to 15-20%.
Unweathered volcanic minerals (pyroxene, feldspar, and minor olivine) dominate the spectral properties of Martian dark regions. Conversely, no evidence has been found for weathering products above the TES detection limit. This lack of evidence for chemical weathering of the Martian surface indicates a geologic history dominated by a cold, dry climate in which mechanical weathering was the dominant form of erosion.
The composition of "White Rock" appears to match that of typical Martian dust. Many other unique surfaces remain to be investigated.
Key polar conclusions include:
CO2 condensation occurs in three forms, fine-grained, coarse grained, and slab ice; the form can change in a few days. Most condensation occurs at the surface, not in the atmosphere. Slab ice is the prevalent form in the outer regions of the forming cap, and persists until shortly after seasonal sunrise.
The interiors of the seasonal caps are characterized by spatially nonuniform behavior, with several small, unique regions. Comparisons with Viking observations indicate little difference in the seasonal cycle 12 Martian years later. The observed radiation balance indicates CO2 sublimation budgets of up to 1250 kg m-².
For most of the seasonal cap, while kinetic temperatures remain near the CO2 frost point, albedos increase slowly with the rise of the Sun, then drop rapidly as the frost becomes patchy and disappears over a period of ~20 days.
A "Cryptic" region in the south cap remains dark and mottled throughout its cold period. TES spectra indicate that the Cryptic region has much larger grained solid CO2 than the rest of the cap and that the solid CO2 here may be in the form of a slab. Although CO2 grain size may be the major difference between different regions, incorporated dust is also required to match the observations.
The Mountains of Mitchel remain cold and bright well after other areas at comparable latitude, apparently as a result of unusually small-sized CO2 frost grains.
Regional atmospheric dust is common; localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm.
Key atmospheric science results include:
The life cycle of five regional dust storms has been observed. These storms have significant impact on the atmospheric temperature structure, increasing the temperature by up to 15 K to several scale heights.
Direct heating of the atmosphere in one hemisphere can lead to an intensification of the Hadley cell circulation and produce a similar-scale heating of the atmosphere in the opposite hemisphere almost instantaneously.
The occurrence of water-ice clouds is highly sensitive to atmospheric temperatures, and heating by dust virtually removes water-ice clouds from a large portion of the planet for months.
Water-ice clouds have a seasonal cycle as distinctive as the dust seasonal cycle. In aphelion (northern summer) season, an equatorial cloud belt is observed between 10 degrees south and 30 degrees north, where upward motion of the Hadley circulation is expected. at all seasons (except during regional dust storms) clouds are common near large topography (Tharsis, Alba Patera, and Elysium).
The thermal structure of the atmosphere is observed to warm and cool according to season and distance from the Sun. Maximum atmospheric temperatures are found at the south pole at southern hemisphere solstice.
The Hadley circulation changes from a (nearly) symmetrical two-cell configuration at equinox to one cross-equatorial cell at solstice.
At solstice the steep temperature gradient between the descending branch of the hadley cell and the polar night produces a strong eastward jet of winds or polar vortext with velocities approaching 160 m s-¹. waves are common throughout the atmosphere and are especially strong in the winter mid-latitudes. zonal wavenumber 2 dominates at lower altitudes while zonal wavenumber 1 becomes stronger at higher altitudes.
Key surface physical property results include:
A third inertia-albedo mode, corresponding to intermediate inertia and albedo values, has been identified using high-resolution albedo and temperature TES data. This distinct unit is separate from the low-inertia/bright, and high-inertia/dark regions discovered previously. It may consist of a bonded, duricrust unit.
Localized regions of high inertia (greater than 800 J-m²-K¹-1-s-1/2) are identified in TES data. These low-lying surfaces, e.g., channel and crater floors, may have formed by a combination of aeolian, fluvial, or erosional processes, or may be exposed bedrock.
2001 Mars Odyssey
On April 7, 2001, the Mars Odyssey was launched. It achieved Mars Orbit Insertion (MOI) on October 23, 2001. After completion of aerobraking in January 2002, the Odyssey began its mapping and relay mission for 917 Earth days, from February 2002 to August 2004. Odyssey has four goals: (1) determine whether life had ever existed on Mars. (2) characterize Mars' climate. (3) characterize the geology of Mars, and (4) prepare for human exploration. The Odyssey spacecraft also serves as a communications relay for U.S. and international spacecraft scheduled to arrive at Mars in 2003 and 2004. To help carry out its important mission, Odyssey is equipped with an array of instruments.
Odyssey's camera system and gamma ray spectrometer suite are continuing to collect data and are working well. The camera's infrared and visible image data are providing "new eyes" to see the makeup of Martian surface materials. Current targets for the camera include the candidate landing sites for the twin 2003 Mars exploration rovers. The neutron detectors in the gamma ray spectrometer suite are refining the detail in maps of near-surface hydrogen and are tracking changes in the surface as the Martian northern winter comes to an end.
Odyssey's gamma sensor head, which is part of the gamma ray spectrometer suite, sits at the end of a boom to minimize interference from any gamma rays coming from the spacecraft itself. The two other gamma ray spectrometer instruments, the neutron spectrometer and the high-energy neutron detector, are mounted on the main spacecraft structure.
The Gamma Ray Spectrometer (GRS) measures the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon. Knowing what elements are at or near the surface will give detailed information about how Mars has changed over time. To determine the elemental makeup of the Martian surface, the experiment uses gamma ray spectrometer and two neutron detectors.
By measuring neutrons, it is possible to calculate the abundance of hydrogen on Mars, thus inferring the presence of water. The neutron detectors on the GRS are sensitive to concentrations of hydrogen in the upper meter of the surface. Like a virtual shovel the spectrometer allows scientists to peer into this shallow subsurface of Mars, and measure the amount of hydrogen there. Since hydrogen is most likely present in the form of water ice, the spectrometer will be able to measure directly the amount of permanent ground ice and how it changes with the seasons.
Onboard Thermal Emission Imaging System (THEMIS) measures radiation from the Martian surface to discover the planet's mineral composition. During the Martian day, the sun heats the surface. Surface minerals radiate this heat back to space in characteristic ways that can be identified and mapped by the instrument. At night, since it maps heat, the imager will search for active thermal spots and may discover "hot springs" on Mars. In the infrared spectrum, the instrument uses 9 spectral bands to help detect minerals within the Martian terrain. These spectral bands, similar to ranges of colors, can obtain the signatures (spectral "fingerprints") of particular types of geological materials. Minerals, such as carbonates, silicates, hydroxides, sulfates, hydrothermal silica, oxides and phosphates, all show up as different colors in the infrared spectrum. This multi-spectral method allows researchers to detect in particular the presence of minerals that form in water and to understand those minerals in their proper geological context. THEMIS' infrared capabilities significantly improve the data from Thermal Emission Spectrometer (TES), a similar instrument on Mars Global Surveyor.
Since space radiation presents an extreme hazard to crews of interplanetary missions, the Martian Radiation Environment Experiment (MARIE) on Odyssey attempts to predict anticipated cosmic rays doses emitted by stars, that would be experienced by future astronauts and help determine possible effects of Martian radiation on human beings.
Since February 2002 Odyssey has gathered tantalizing information on Mars climate and the presence of ice.
Odyssey's neutron and gamma-ray sensors have tracked seasonal changes as layers of "dry ice" (carbon-dioxide frost or snow) accumulate during northern Mars' winter and then dissipate in the spring, exposing a soil layer rich in water ice--the Martian counterpart to permafrost. Researchers used measurements of Martian neutrons combined with height measurements from the laser altimeter on Mars Global Surveyor to monitor the amount of dry ice during the northern winter and spring seasons. Mars Odyssey's instruments, called the gamma-ray spectrometer suite, can identify elements in the top meter (3 feet) or so of Mars' surface. Mars Global Surveyor's laser altimeter is precise enough to monitor meter-scale changes in the thickness of the seasonal frost, which can accumulate to depths greater than a meter. The new findings show a correlation in the springtime between Odyssey's detection of dissipating carbon dioxide in latitudes poleward of 65 degrees north and Global Surveyor's measurement of the thinning of the frost layer in prior years.
Another report combines measurements from Odyssey and Global Surveyor to provide indications of how densely the winter layer of carbon-dioxide frost or snow is packed at northern latitudes greater than 85 degrees. The Odyssey data are used to estimate the mass of the deposit, which can then be compared with the thickness to obtain a density. The dry-ice layer appears to have a fluffy texture, like freshly fallen snow, according to the report by Dr. William Feldman of Los Alamos National Laboratory, N.M., and 11 co-authors. The study also found that once the dry ice disappears, the remaining surface near the pole is composed almost entirely of water ice.
New maps of Mars combine images from the Mars Orbiter Laser Altimeter (MOLA) on the Mars Global Surveyor with Mars Odyssey spectrometer data through more than half a Martian year of 687 Earth days. From about 55 degrees latitude to the poles, Mars boasts extensive deposits of soils that are rich in water-ice, bearing an average of 50 percent water by mass. A typical pound of soil scooped up in those polar regions would yield an average of half a pound of water if it were heated in an oven.
The tell-tale traces of hydrogen, and therefore the presence of hydrated minerals, also are found in lower concentrations closer to Mars' equator, ranging from two to 10 percent water by mass. Surprisingly, two large areas, one within Arabia Terra, the 1,900-mile-wide Martian desert, and another on the opposite side of the planet, show indications of relatively large concentrations of sub-surface hydrogen.
Scientists advanced two possible theories of how all that water got into the Martian soils and rocks.
First the vast water icecaps at the poles may be the source. The thickness of the icecaps themselves may be enough to bottle up geothermal heat from below, increasing the temperature at the bottom and melting the bottom layer of the icecaps, which then could feed a global water table.
The second hypothesis is based on evidence that about a million years or so ago, Mars' axis was tilted about 35 degrees, which might have caused the polar icecaps to evaporate and briefly create enough water in the atmosphere to make ice stable planet-wide. The resultant thick layer of frost may then have combined chemically with hydrogen-hungry soils and rocks.
No one can yet precisely describe the abundance and stratigraphy of these deposits, but the neutron spectrometer shows water ice close to the surface in many locations, and buried elsewhere beneath several inches of dry soils, giving rise to theories predict these deposits may extend a half mile or more beneath the surface. If so, their total water content may be sufficient to account for the missing water of Mars.
A team of Los Alamos scientists has begun a research project to interpret the Mars Odyssey data and their ramifications for the history of Mars' climate.
Los Alamos' neutron spectrometer, a more sensitive version of the instrument that found water ice on the moon five years ago, is one component of the gamma-ray spectrometer suite of instruments aboard Odyssey. W.T. Boynton of the University of Arizona leads the gamma-ray spectrometer team.
The neutron spectrometer looks for neutrons generated when cosmic rays slam into the nuclei of atoms on the planet's surface, ejecting neutrons skyward with enough energy to reach the Odyssey spacecraft 250 miles above the surface.
Elements create their own unique distribution of neutron energy - fast, thermal or epithermal - and these neutron flux signatures are shaped by the elements that make up the soil and how they are distributed. Thermal neutrons are low-energy neutrons in thermal contact with the soil; epithermal neutrons are intermediate, scattering down in energy after bouncing off soil material; and fast neutrons are the highest-energy neutrons produced in the interaction between high-energy galactic cosmic rays and the soil.
By looking for a decrease in epithermal neutron flux, researchers can locate hydrogen. Hydrogen in the soil efficiently absorbs the energy from neutrons, reducing their flux in the surface and also the flux that escapes the surface to space where it is detected by the spectrometer. Since hydrogen is likely in the form of water-ice at high latitudes, the spectrometer can measure directly, a yard or so deep into the Martian surface, the amount of ice and how it changes with the seasons.
The Mars Exploration Rover Project
The Mars Exploration Rover Project proceeds apace. The Mars Global Surveyor orbiter has identified deposits at Meridiani Planum of a type of mineral that usually forms in wet environments. The two recent rovers Spirit and Opportunity will function as robotic geologists, examining rocks and soil for clues about whether past environments at their landing sites may have been hospitable to life.
On June 10, 2003, the Spirit spacecraft was sent on its way to Mars, followed on July 7, 2003 by its twin the Opportunity, each carrying a Mars Exploration Rover equipped with a battery of 10 instruments: three science cameras and seven engineering cameras, including three spectrometers. The three science instruments include the Pancam color panoramic cameras and the Microscopic Imagers. Spirit is scheduled to arrive at Mars January 3, 2004, and Opportunity is expected to arrive evening of January 24, Eastern and Pacific times . After arrival, the rovers examine their landing areas for geological evidence about the history of water on Mars.
The seven engineering cameras include a stereo navigation camera pair, stereo hazard-avoidance camera pairs on the front and back of the rover, and a downward-pointing descent camera on the lander to aid a system for reducing horizontal motion just before impact.
Each rover sports a spectrometer for identifying minerals from a distance, called the miniature thermal emission spectrometer, or mini-TES. Two other spectrometers - an alpha particle X-ray spectrometer and a Mössbauer spectrometer - are mounted on an extendable arm for close-up examination of the composition of rocks and soil. The alpha particle X-ray spectrometer provides information about the elements in a rock. The Mössbauer spectrometer gives information about the arrangement of iron atoms in the crystalline mineral structure within a rock.
NASA Associate Administrator for Space Science Dr. Ed Weiler said, "Opportunity joins Spirit and other Mars-bound missions from the European Space Agency, Japan and the United Kingdom, which together mark the most extensive exploration of another planet in history. This ambitious undertaking is an amazing feat for Planet Earth and the human spirit of exploration."
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