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The second class of channels is known as outflow channels. These formed later during Martian history, but still billions of years ago. Most of them start at a fault zone or the breached rim of a crater or valley. Several of the largest outflow channels arise at the eastern end of Valles Marineris. Outflow channels are reminiscent of wide channels in Earth's channeled scablands, which were carved when water catastrophically burst from a large glacial lake. Many scientists think Mars' outflow channels formed similarly, when large lakes burst their shorelines or when groundwater pressure caused an aquifer to erupt violently to the surface. Others think that these channels were cut by glaciers.
The third, and most controversial and exciting, class of channels is called gullies. These occur mostly at mid-latitudes. They are very young, geologically speaking: they lack craters, and some gullies even cut across recently formed sand dunes. Gullies' origins are very controversial. Some scientists think they are stream channels formed where groundwater seeped to the surface as springs. Others think they were formed by the melting of snow that accumulated thousands of years ago when Mars' climate was different. Still others think that gullies formed without flowing water, by a process called mass wasting.
Mars' Atmosphere, Weather, and Frozen VolatilesMars' atmosphere today is a wisp compared to Earth's. At the lowest point on the planet, atmospheric pressure is only 1/80th of that on Earth. Over more than half the surface, it's less than 1/150th. At the low pressure on Mars' surface, water can't exist as a liquid. As soon as it melts from ice, the water evaporates. A glass of ice water poured onto the surface would boil away as violently as a glass of water on Earth thrown into a hot oven.Mars' air, if you call it that, is nothing like Earth's air. It's 95% carbon dioxide, whereas Earth's air is 78% nitrogen and 21% oxygen. Most of the remaining 5% of Mars' air is nitrogen and argon, with traces of carbon monoxide and oxygen. Mars' air is also nearly bone-dry. In the most humid regions of the planet, all of the water vapor would, if condensed out, form a layer thinner than a sheet of paper (3/1000th of an inch thick). Despite the thinness of the atmosphere, the most dynamic aspect of Mars is its weather. Most obviously in the composite image below, taken from Mars Global Surveyor, is that Mars has clouds. Water ice clouds are most common:
Mars also has clouds of dust. They
may occur at any season but are most prevalent during southern spring
and summer. At that time local dust storms are common in the southern
hemisphere, and in some years they explode in proportion and become
globe-encircling events. Famous global dust storms occurred in 1971,
the year Mariner 9 – the first Mars orbiter – arrived at the planet, and in 2001. Dust
storms like the one below also occur around each pole in spring as the
seasonal polar cap dissipates.
Each polar region has a seasonal cap of frozen carbon dioxide that either snows out of the atmosphere or condenses directly on the surface. On average the seasonal cap is a fews tens of centimeters – or about a foot – in thickness. It contains a trace of water ice, which lingers longer on the surface in spring after the frozen carbon dioxide evaporates. (On Mars, neither carbon dioxide ice nor water ice melts. It goes directly to gas because the atmospheric pressure is so low.) Earth has a seasonal polar cap as well – winter snow, made of water ice. Earth's snow forms from a minor constituent of the atmosphere, water vapor. In contrast Mars' carbon dioxide ice seasonal cap forms from the freezing of the main atmospheric gas. Such a large part of the atmosphere gets tied up in the seasonal cap that, in mid-winter, atmospheric pressure drops by one-fourth. Most of each polar cap that's left after disappearance of the seasonal cap polar cap – the "residual cap" – is made of water ice. The residual cap consists of layers of cleaner and dirty ice with a total thickness of more than a kilometer. The northern residual cap is nearly all water ice. The southern residual cap also has a surface layer of carbon dioxide ice a few tens of meters (about one hundred feet) thick. The MGS/Mars Orbiter Camera has been imaging the southern residual cap for years and, amazingly, the carbon dioxide ice layer is disappearing! This is a dramatic change in Mars' surface, happening right now while spacecraft are watching.
One of the most important questions about Mars is, was its atmosphere ever thicker? Mars' channels suggest to some scientists that at one time Mars had a denser carbon dioxide atmosphere that warmed the planet by the greenhouse effect allowing liquid water to flow. (Other scientists think that Mars was never globally warmer, and that valley networks formed by geothermal heating or from local rains after impact craters melted ground ice.) If there ever was a denser atmosphere, where did it go? There is not enough carbon dioxide in the polar caps to explain a warmer climate in the past, if that carbon dioxide was once in the atmosphere. Perhaps the ancient atmosphere was eroded away by the solar wind. Or perhaps carbon dioxide reacted with crustal rocks and was trapped as carbonate minerals. Finding massive deposits of ancient carbonates would help us to understand how Mars' channels formed. Seeing Mars' GeologyMars' surface, like Earth's, is mostly covered by loose, unconsolidated material. On Earth, that is mostly soil, sediment that has been deposited by glaciers, rivers, or wind, and even dead vegetation. On Mars, the unconsolidated material is mostly windblown sand, or very fine windblown dust. The sand tends to be dark fragments of parent rock, whose composition has been only minimally altered chemically by interactions with atmospheric gases and water – "weathering." In contrast, the dust is brighter and very red, and represents rocks that have been oxidized by exposure to the atmosphere and possibly water. Martian dust is sticky and tends to adhere to exposed surfaces.For the most part, Mars' surface layer of unconsolidated material is thought to be quite thin, so that it does not hide the planet's geomorphology. Thus, imaging that shows features as small as a few meters (about 10 feet) in size is useful to understand geologic processes that shaped the surface. However, at the wavelengths of light measured by CRISM, it takes only a few tens of micrometers (about 1/1000th inch) of adhering dust to hide the spectral properties of underlying rock. This hides the evidence for its composition. So, for CRISM, it's very important to observe areas that have a minimal coating of windblown dust.
The Question of Life on MarsAstronomers in the 1800s and early 1900s misinterpreted some of Mars' features and gave rise to an idea that life existed on the surface of the planet. For example, the darkening of sandy regions when the winds scoured them of dust was misinterpreted as the greening of plants when they received moisture from melting polar caps in spring. (The polar ice doesn't really melt, because the air is too thin for liquid to exist over most of the surface; it just evaporates instead.) Some few astronomers got fooled by randomly placed streaks and blotches and imagined them to be irrigation ditches, or canals, constructed by a dying race of intelligent beings.Later studies of Mars showed that, in fact, the planet's surface is hostile to life. Mariner 4 – the first spacecraft to visit the vicinity of Mars – returned pictures in 1965 of a barren, cratered surface lacking in vegetation, canals, or any sign of life. But the question of life on Mars is a valid one. Mars is the only planet besides Earth to show evidence for liquid water on its surface, and liquid water is a key ingredient of life. Even if there is no life on Mars now, remnants of it may have been fossilized – preserved inside mineral deposits – in the distant past. The 21st century hunt for Martian life has become the hunt for minerals fhat formed in water enivironments that might preserve evidence for ancient life. That's where CRISM comes in. Links on Mars GeologyAlthough a large number of sites provides information on the geology of Mars, the following provide some of the most interesting images and unique information:
Geological Measurements that CRISM will MakeCRISM's measurements of Mars focus on three major questions being investigated by MRO:
The Search for Aqueous DepositsBackground. Finding aqueous minerals, that is, minerals formed in water, is important to understanding Mars in two ways. First, aqueous minerals show where liquid water existed long enough to react chemically with rock and soil. If water did exist long enough for chemical reactions to occur, it wouldn't have provided the right conditions for life. Second, many aqueous minerals could have fossilized evidence for past Martian life. Today, fossils are our only way to know if life existed in Mars' distant past. The environments most likely to preserve a Martian fossil record include soils, ancient lakes or ancient seas, and springs. In these environments, biologic materials are rapidly encased by iron oxides, phyllosilicates, zeolites, carbonates, sulfates, or opal. The mineralogy of iron oxides, phyllosilicates, zeolites, carbonates, and sulfates, in particular, are indicators of redox conditions and salinity at the time of their deposition. One promising environment for a fossil record is hot springs: these environments combine an energy source (heat) with rapid fossilization, and are thought to have preserved the earliest life on Earth. Evidence for the occurrence of aqueous mineralogies comes from both remotely sensed and on-site measurements. OMEGA data show evidence for two distinct types of aqueous deposits. The first type, characterized by sulfates including gypsum and kieserite, is correlated spatially with layered deposits of Hesperian age (Martian middle age). The second type, rich in several different kinds of phyllosilicates, is more limited areally and correlates with exposures of Noachian (very ancient) material. The different ages and chemistries of the two types of deposits are suggestive of different water environments. The main evidence from landed measurements comes from MER. At the sites of both MER-Opportunity and MER-Spirit, there are non-basaltic materials enriched in oxidized iron and salts, especially sulfates. Significantly, only the MER-Opportunity deposits exhibit a distinctive spectral signature in orbital data from TES and OMEGA. MER-Spirit's discovery of comparable deposits – despite lack of orbital spectral evidence for their existence – strongly suggests that CRISM's one to two orders of magnitude improvement in spatial resolution over TES and OMEGA will reveal additional sites of interest besides those summarized above. CRISM's Questions and Measurement Objectives. OMEGA's global mapping has identified several regions that contain outcrops of aqueous deposits. CRISM provides increased capabilities to map spatial structure of these materials due to its higher spatial resolution, and to resolve mineralogy due to its 2x increase in spectral resolution (6.55 nm spectral sampling. vs. 13 nm for OMEGA at comparable wavelengths). Thus CRISM's measurement strategy focuses on finding small-scale deposits, characterizing their mineralogic variations, and improving the accuracy of mineralogic determinations.
 Spectral evidence from OMEGA for hydrated mineralogies in the layered deposits of Valles Marineris. Map of depth of 1.9-micron absorption due to sulfates, overlain on a map-projected digital image mosaic. Green areas have 1.9-micron absorption depths greater than 2.5%.
Understanding the Composition of Mars' CrustBackground. The mineralogic composition of the Martian surface tells us about the processes that formed Mars' crust. It has been studied using remotely sensed data, meteorites that originated on Mars, and landed observations. Remotely sensed and landed measurements imply that the crust is dominantly igneous rock, specifically basalt composed mostly of feldspar and pyroxene. The upper crust is composed of hundreds of thinner layers accumulated to kilometers in thickness. Several classes of volcanoes occur, ranging from shallowly sloped shield volcanoes to steeper-sloped highlands volcanoes composed of layered erodible and resistant units. Resistant units are interpreted as flows, whereas erodible units are considered to be volcanic ash. Some of the highlands volcanoes resemble composite volcanoes or stratovolcanoes on Earth, although their origin is probably quite different and doesn't involve plate tectonics. Two major divisions in crustal mineralogic composition are recognized on the basis of their thermal infrared spectral signatures in MGS/Thermal Emission Spectrometer data. Type I material, predominantly in the highlands, is interpreted as basalt – in agreement with earlier interpretations based on telescopic spectra. Type II, found predominantly in the northern plains, was first interpreted to be andesite or basaltic andesite. This would be an important discovery because andesite is a more "evolved" composition than basalt. But other scientists interpret type II materials as just more weathered type I basalt. Some regions exhibit high concentrations of olivine or unusually low-calcium pyroxene (LCP). Granite, an even more evolved igneous rock than andesite, is found in the northwestern part of the Syrtis Major shield volcano. OMEGA's higher spatial resolution
than TES provides additional information on the geologic context of
olivine and LCP enriched units. In addition, OMEGA's (and CRISM's)
wavelength range is sensitive to different minerals and therefore
provides complementary information to TES. OMEGA showed that olivine occurs on
surfaces of all ages, so olivine-forming magma compositions must have
occurred throughout Martian history. In contrast, high concentrations of
LCP are found only in the oldest parts of the highlands. They seem to
represent the oldest volcanic materials.
CRISM's Questions and Measurement Objectives. CRISM's investigation of Martian crustal mineralogy focuses on questions addressed through improved spatial resolution, primarily vertical crustal structure, and the sensitivity of CRISM's wavelength range to altered phases.
Volatiles, the Polar Caps, and AtmosphereMars' climate is like Earth's in that the planet has seasons, clouds, and seasonal and permanent polar caps. But it is unlike Earth's in that the climate is unaffected by oceans, or the cycling of water between clouds, rain, groundwater, and seas. Understanding Mars' climate provides basic insights into how atmospheres work, and it helps us to understand how a planet that once had flowing water ended up being a frigid, hostile desert. Background: Water and Dust in the Atmosphere. The current Martian climate exhibits annual cycles in the abundance of water vapor, carbon monoxide, water ice clouds and hazes, and dust. The perihelion southern summer season exhibits the greatest and most variable atmospheric dust abundance. Regional dust storms show surprising repeatability Mars-year to Mars-year. Every decade or so they grow into global-scale dust storms. In contrast, the aphelion northern summer season is characterized by lower temperatures, an equatorial water-ice cloud belt, and minimal atmospheric dust. Atmospheric water vapor varies in abundance seasonally, with greatest abundance in summertime at high latitudes. Carbon monoxide abundance should also vary at high winter latitudes and provide a tracer for atmospheric circulation. Dust and water ice can contribute as much as 20–30% of reflected Martian light measured from space over much of CRISM's spectral range. So, accurately measuring surface mineralogy also requires measurement and characterization of the atmosphere. These effects of atmospheric dust and ice are summarized below, where total light from the surface, atmospheric gases, and dust or ice are ratioed to the light that would come from an airless surface. Dust and ice hazes have a wavelength-dependent effects, and these differences allow them to be distinguished.
Background: Volatiles at the Surface. From ground-based telescopic spectra of the 3-micron water absorption, molecular water is known to be present in the regolith, either adsorbed on soil particles or chemically bound to minerals. OMEGA data show two types of concentrations of water, evidenced by an increased 1.9-micron and/or 3-micron absorption. In the first type, an increased 1.9-micron absorption is associated with specific geologic formations, and additional spectral features attribute it to sulfates or phyllosilicates. In the second type, an increased 1.9-micron absorption is observed without accompanying mineral bands, or increases in the 3-micron absorption are observed without a 1.9-micron absorption. This occurs most prominently around the polar region, in soils from which the seasonal polar cap recently evaporated. Strength of the 1.9-micron absorption increases poleward from mid-latitudes, but there are no accompanying features to suggest sulfates or phyllosilicates. This trend suggests that the water exists in adsorbed form. Adsorbed water is expected to vary in abundance seasonally, but it is not yet clear from OMEGA data where and when the abundance of water in the regolith varies. Water and carbon dioxide also exist as seasonal and residual polar caps. The residual caps were long thought to consist mostly of water ice at the north pole and carbon dioxide ice at the south pole. OMEGA reflectance spectra show that the southern cap's carbon dioxide ice is widespread, but only a veneer no more than a few 10's of meters in thickness. The underlying cap consists of mostly water ice, mixed with a small amount of carbon dioxide ice and dust. The seasonal caps are mostly carbon dioxide ice, in both fine- and coarse-grained forms. At least in the northern seasonal cap there is also a significant water ice component. The fine-grained water ice remains after evaporation of the carbon dioxide ice, and then itself evaporates leaving just coarse-grained water ice. No large reservoirs of carbon dioxide other than the polar ices and atmosphere have been recognized. In particular, OMEGA has not detected any kilometer-scale outcrops of carbonate minerals, despite its sensitivity to percent-level abundances. The lack of large carbonate reservoirs provides no evidence for retention of an earlier, thicker, carbon dioxide-rich atmosphere that was combined with crustal rocks.Key Volatiles Questions and Measurement Approach. OMEGA is providing a baseline on kilometer-scale spatial variations in water content of the surface, and on polar ice composition and seasonal variation. CRISM's surface volatile measurement strategy emphasizes temporal variations, small-scale deposits, and accurate mineralogic determinations.
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