Curiosity on Mars
[The Landing: Another Small Step for Man - A Giant Leap for Robot Kind]    [The Landing Site: Geological Jackpot at Gale Crater]   
[Curiosity: The Escalade of Mars Rovers]    [The Science Payload]    [References and Further Reading]

The Landing Site
The Geological Jackpot at Gale Crater
Written and edited by Bob Keller

The Science Goals and Mission Objectives

Mars Science Laboratory's scientific goals include determining whether Mars could ever have supported life, studying the geology and climate of Mars, and collecting data for a manned mission to Mars. To contribute towards these goals, Curiosity has four primary mission objectives:

1: Assess the biological potential of at least one target environment.
     a: Determine the nature and inventory of organic carbon compounds.
     b: Inventory the chemical building blocks of life (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur).
     c: Identify features that may represent the effects of biological processes.

2: Characterize the geology and geochemistry of the landing region at all appropriate spatial scales.
     a: Investigate the chemical, isotopic, and mineralogical composition of Martian surface and near-surface geological materials.
     b: Interpret the processes that have formed and modified rocks and regolith.

3: Investigate planetary processes of relevance to past habitability, including the role of water.
     a: Assess long-timescale (4-billion-year) atmospheric evolution processes.
     b: Determine present state, distribution, and cycling of water and carbon dioxide.

4: Characterize the broad spectrum of surface radiation, including galactic cosmic radiation, solar proton events, and secondary neutrons.

As part of its exploration, Curiosity also continuously measured the high energy radiation exposure in the interior of the spacecraft as it traveled to Mars, using its RAD radiometer, one of Curiosity's ten on-board scientific instruments. Measurements taken during three solar storms in February, March and May 2012 have provided scientists with valuable data providing insights into the radiation environment in the Solar System. Curiosity is continuing radiation measurements as it explores the surface of Mars, collecting important data for a future manned mission.

Mars Science Laboratory is intended to be a discovery-driven mission, with the science operations team retaining flexibility in how and when the various capabilities of the rover and payload are used to accomplish the overall scientific objectives. There exists an enormous variety of ways in which the mission may unfold, because of the unknown nature of the discoveries, the flexibility of the scientific payload, and the capabilities of the rover.

Historic and Candidate Landing Sites on Mars

The Great Debate

Mission sites upon or near rover accessible geological evidence for past aqueous activity such as fluvial systems, lake beds and depositional fans and deltas correlated with mineralogical evidence including phyllosilicate (clay mineral) and sulfate bearing deposits are most relevant to Curiosity's science goals and objectives. During the five year process of selecting Curiosity's mission site, the scientific potentials of more than fifty candidate sites were examined, evaluated and weighed against engineering constraints and safety margins. The science community broadly participated in the selection process via five open workshops through which the candidate sites were successively winnowed down to four finalist mission sites.

Important science criteria encompassed the ability to assess past habitable environments, including diversity, context, and the preservation of biosignatures, including organics. All of the final four sites have layered sedimentary rocks with spectral evidence for phyllosilicates that clearly address the science objectives of the mission. Important engineering constraints included latitude, elevation, relief, slope, rock abundance, and a radar-reflective, load-bearing, and trafficable landing surface near go to sites accessible within the lifetime of the mission. Sites were evaluated in fine detail using targeted data from instruments on all active orbiters, and especially the Mars Reconnaissance Orbiter.

The highly scrutinized and passionately debated candidate site finalists for the Mars Science Laboratory mission included Eberswalde Crater, Gale Crater, Holden Crater, and Mawrth Vallis.

Holden and Eberswalde Craters

Inverted Delta Exhumed within Eberswalde Crater 40-mile diameter Eberswalde Crater (24°S 33°W) is a partially buried impact crater lying along the northern perimeter of Holden Crater. Both Eberswalde and Holden Craters are located in Margaritifer Terra, an ancient, heavily cratered region of Mars.

Eberswalde Crater predates Holden Crater and was partially buried by ejecta from Holden Crater's formation. Eberswalde contains an exhumed, inverted relief delta with phyllosilicates, a potentially habitable environment that is particularly favorable to the preservation of organic materials.

After its deposition, additional sediments were deposited on top of the Eberswalde delta, burying it. The buried deposits in the channels formed sedimentary rock. As the surrounding softer sediments were eroded away, the delta was exhumed, but inverted. The surface are of the delta is approximately 45 square miles. A series of valleys leading into the Eberswalde delta drain a "watershed" of approximately 1500 square miles.

Alluvial Fan in Holden Crater 90-mile diameter Holden Crater (26.4°S 34.0°W) is an old crater containing numerous smaller craters, many of which are filled with sediment. Holden Crater's central mountain is also obscured by sediment. Holden contains finely layered phyllosilicates suggesting deposition in quiet fluvial or lacustrine setting with a well understood context. Holden's rim is cut with gullies, and at the mouths of some of them are alluvial fans.

Holden crater is notable for many features believed to have been created by flowing water, including Uzboi Vallis, an outlet channel that begins on the northern rim of the Argyre basin and cuts through several craters before ending at Holden Crater.

It is believed that layered deposits formed on the floor of Uzboi Vallis when drainage through it was blocked by the impact that formed Holden Crater, forming a lake. Eventually the lake went high enough to overtop the rim of Holden. The water then eroded the deposits to expose the layers as they are today. The sediments that make up the layers appear to be coarse in size, suggesting they were probably formed by a rapid flow.

Mawrth Vallis Descending from the Highlands of Arabia Terra

Mawrth Vallis (22.3°N, 343.5°E) is an ancient water outflow channel carved by catastrophic floods in the region between the southern highlands and northern lowlands of Mars. It was formed in and subsequently covered by layered rocks, from beneath which it is now being exhumed. The Mawrth Vallis region holds special interest because of the presence of phyllosilicate minerals which form only if water is available. Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer for Mars has identified aluminium-rich and iron-rich clays, each with a unique distribution. Some of the clays recently discovered by the Mars Reconnaissance Orbiter include montmorillonite, kaolinite, and nontronite. Since some clays seem to drape over high and low areas, it is possible that volcanic ash landed in an open body of water. On Earth such clays occur in weathered volcanic rocks and hydrothermal systems, where volcanic activity and water interact.

On July 22, 2011, NASA announced the selection of Gale Crater for Curiosity's mission site. In the final selection, Gale crater was favored over Eberswalde based on its greater diversity and potential habitability. Gale was also a candidate mission site for the Spirit and Opportunity rovers preceding Curiosity.

The Geological Jackpot at Gale Crater

Curiosity's Landing Site in Gale Crater Gale Crater (5.4°S 137.8°E), named after Australian astronomer Walter F. Gale, spans 96 miles in diameter and holds a central peak rising 18,000 ft above the northern crater floor and 15,000 ft above the southern crater floor - higher than any mountain in the lower 48 states and slightly taller than the southern rim of the crater itself. Based on its size and state of preservation, scientists estimate the impact event forming Gale occurred 3.8 to 3.5 billion years ago.

An unusual feature of Gale Crater is the enormous mound of debris around its central peak. Informally named "Mount Sharp" by mission scientists, in May 2012 the International Astronomical Union gave the mountain the official name "Aeolis Mons", and the name "Aeolis Palus" to the crater floor plain between the northern wall of Gale Crater and the northern foothills of the mountain.

Curiosity Landing Site and the Central Mound in Gale Crater

Layering in the crater's central mound suggests it is the surviving remnant of an extensive sequence of deposits offering geologic attractions similar to Mars' spectacular Valles Marineris, the largest canyon in the solar system. The southerly view above looks toward the crater's central peak in the left background. The larger. lighter ellipse represents the pre-launch calculation of a 99% probability ellipse for Curiosity's landing. The smaller, darker ellipse represents NASA's June 2012 revision to the landing ellipse after a more refined in-flight analysis - reducing it from 12.4 miles x 15.5 miles to 12 miles x 4 miles, and moving the center of the ellipse closer to the foothills of Mount Sharp holding the rock strata that are Curiosity's prime destination. The red X marks the spot where Curiosity actually landed, less than 1.5 miles away from dead center of the revised ellipse.

Geologic Features of Interest at Gale Crater

Curiosity has landed in a portion of the Gale Crater floor plain with an alluvial fan of debris deposited at the mouth of an erosion channel descending the crater wall. Similar canyons and channels are common on the crater rim and central mound, indicating that Gale was probably a closed watershed in the past, possibly a lake. Much like the Grand Canyon in Arizona, Gale's canyons and channels expose layers of rock representing tens or hundreds of millions of years of environmental change.

A thick stack of stratified, sedimentary rocks composing the lower portion of the mound inside Gale Crater awaits exploration by the Curiosity rover. Variations in texture and mineralogy among the strata suggest that the materials contain a geologic record of major climatic changes in Mars' history.

Clay Mineral and Sulfate Salt Deposits

An area of top scientific interest is at the base of Mount Sharp, near the edge of the landing ellipse and beyond a dark dune field. Here at the mouth of a canyon, orbiting instruments have detected signatures of clay minerals and sulfate salts, both known to form in water. Scientists studying Mars have several important hypotheses about how these minerals reflect changes in the Martian environment, particularly changes in the amount of water on the surface of Mars. Scientists will use Curiosity's instrument suite to study these minerals and how they formed to provide insights into those ancient Martian environments. These rocks are also a prime target in checking for organic molecules, since these environments may have been habitable and able to support microbial life.

Geologic Features in Gale Crater

The false-color image above combines a daytime photo taken at visible wavelengths with a nighttime infrared image taken by the Thermal Emission Imaging System (THEMIS), a multi-wavelength camera aboard NASA's Mars Odyssey orbiter. This overhead view encompasses features of Gale Crater's central mound and the canyon that MSL mission planners intend to explore with Curiosity, The bluish-white tones indicate areas where fine-grain sand and dust thickly cover the surface, while redder tones indicate exposures of harder, rockier material.

The prominent canyon just left of center in the above image cuts down into and exposes faces of the lower rock layers composing the mound. High-resolution images captured by the Mars Orbiter Camera aboard NASA's Mars Global Surveyor suggest the lower layers of Gale's mound includes an unconformity, a gap in the geologic record caused by intervening erosion. Curiosity may be able to provide further data and evidence regarding the unconformity and insights into the time span it represents.

The nameless crater in the extreme lower left of the above image is just over a mile in diameter. It and the two smaller craters flanking it are filled to their rims with sediments. Many small craters visible on the mound resemble these craters, indicating that Gale has undergone numerous cycles of erosion and deposition. For scientists, the probability of water playing an important role forms a big part of Gale's attraction. If water was ever abundant here, then life - or its traces - might be found.

Curiosity was very intentionally landed on the smooth crater floor and will drive towards this canyon in the central mound of Gale. Bedrock lies exposed across most of Gale's northern floor, an object of geologic interest in itself.

The canyon cuts into the mound at an average grade that is readily negotiable by Curiosity. However, there is no guarantee of access into the canyon, as outcropping ledges could easily present insurmountable obstacles.

Dozens, if not hundreds of individual rock layers make up the mound, each archiving a geological chapter of the Martian past. Scientists do not know with certainty how much history is recorded in Gale's layers - an educated guess says the layered material of the Gale Crater mound may have been laid down over a period of approximately 2 billion years. The origin of this mound is also not known with certainty, but observations suggest it is the eroded remnant of sedimentary layers that once filled the entire crater completely to its rim and perhaps beyond. Observations of possible cross-bedded strata on the upper mound suggest aeolian processes at play during their formation.

Geologic Map of Features in Gale Crater

The topographic image of Gale Crater above provides context for the geological map below. The map details structural units within correspondingly colored areas on the context image, which encompass geological features along Curiosity's planned route into the canyon ascending lower strata of the mound surrounding Mount Sharp. The map was constructed by combining data from narrow-angle image M03-01521 acquired by the Mars Orbital Camera aboard the Mars Global Surveyor with a topographic profile acquired by the Mars Orbiter Laser Altimeter.

Planimetric configuration, relief, brightness, tone, texture, pattern and context were interpreted to distinguish 11 lithographic units within a 1.4 mile thick vertical sequence - nearly twice as thick as the Paleozoic formation exposed in Grand Canyon, Arizona. The lower units of the sequence are horizontally bedded with many layers exposed in stair-step fashion. The upper units are massive and exhibit lighter tones relative to lower units.

The most significant finding is an erosional unconformity between the Unit 6, the uppermost layered unit, and Units 7 and 8, the lowermost massive units. The unconformity is indicated by impact craters and an old channel - all presently being exhumed- on the upper surfaces of the layered unit. Their presence indicates that an as yet undetermined but considerable amount of time passed between the deposition of the lower layered and the upper massive units.

Geologic Map of Features in Gale Crater

Unit 1 - Intermediate-to-dark-toned layered unit; thin layers evident in upper portion near contact with Unit 2; rugged surface; thickness unknown because it is at the bottom of the sequence.

Unit 2 - Light to intermediate-toned, layered-to massive unit between Units 3 and 1; approximately 40 meters thick.

Unit 3 - Intermediate-to-dark-toned; possibly layered; apparently overlain by Unit 6 and unconformably overlain by Units 4 and 5; surface somewhat rugged with sharp-topped mounds at the decameter scale; thickness uncertain, might be approximately 400 meters.

Unit 4 - Intermediate-toned channel-fill and ridge/mound-forming unit; occupies channel cut into Unit 6; thickness probably a few tens of meters at most.

Unit 5 - Thin, intermediate-toned, nearly-flat unit immediately adjacent to and embaying ridge material of Unit 4; upper surface exhibits faint rippled pattern and is about 5-10% covered by nearly-circular, dark floored pits; thickness likely greater than 30 meters.

Unit 6 - Thick, intermediate-toned, layered unit; most layers have similar thickness, tone, and smooth upper surfaces; some layers expressed as narrow ledges, on others as cliff-bench; beds horizontal; upper surface cratered, some craters only partly-emergent from beneath Units 7 and 8; Unit 6 also cut by channel largely filled by Unit 4; thickness approximately 600 - 950 meters.

Unit 7 - Layered-to-massive, intermediate-toned, relatively thin unit above unconformity at contact with Unit 6. Extent beneath Unit 8 is unknown, might not be continuous; surface expression smooth at meter to decameter scale; mounds characterize surface erosional expression; thickness may be approximately 20-30 meters or less.

Unit 8 - Massive, light-toned; surface dominated by large, sharply-tapered ridges and intervening furrows; ends of ridges point up and down slope with sharper end down slope; ridge orientation varies from NE-SW to NW-SE in less than 3 kilometers lateral distance; contact with Unit 7 may be sharp and/or unconformable; thickness approximately 150 - 500 meters.

Unit 9 - Massive, intermediate-toned; surface includes sharp-edged, tapered and approximately parallel ridges separated laterally by tens to approximately 200 meters of flatter, smoother, darker-surface terrain; ridge orientation approximately N-S and somewhat up-down slope; thickness may be up to approximately 400 meters.

Unit 10 - Uppermost unit; massive with intermediate to light tone surface with poorly-organized pattern of sharp-edged, low ridges and scarps separated laterally by hundreds of meters of smoother terrain; ridges trend updown slope; several craters occur on or in Unit 10 but are severely degraded; thickness indeterminate because top of section is probably not present.

Unit 11 - Light-to-intermediate-toned thin, possibly mesa-forming unit (because contact with Unit 1 is along a low scarp approximately less than 10 meters high); lies above upper surface of Unit 1; contact with Unit 1 might be unconformable.

Context of Geologic Section of Lower Formation Members

The image above details layering in the formation members at the base of Unit 1 and provides context for the geologic section below.

Five kilometers of strata are exposed in the Gale mound, but a key record is found in rock layers contained within the lowest 600 feet. The sedimentary record of the lower mound captures a significant transition on Mars, from a relatively wet phyllosilicate-forming environment to a drier sulfate-forming environment. These sediments are believed to contain a relatively pristine and decipherable record of ancient Martian environments, presenting an opportunity to frame the question of habitability in a stratigraphic context.

The lower rock layers composing the mound in Gale Crater are believed to date from Mars' Noachian period. The Noachian is a geologic system and early time period on Mars characterized by high rates of meteorite and asteroid impacts and the presence of abundant surface water. The absolute age of the Noachian period is uncertain but probably corresponds to the lunar Pre-Nectarian to Early Imbrian periods of 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment. Many of the large impact basins on the Moon and Mars formed at this time. The Noachian Period is roughly equivalent to the Earth’s Hadean and early Archean eons when the first life forms likely arose.

Noachian-aged terrains on Mars are prime spacecraft landing sites to search for fossil evidence of life. During the Noachian, the atmosphere of Mars was denser than it is today, and the climate possibly warm enough to allow rainfall. Large lakes and rivers were present in the southern hemisphere, and an ocean may have covered the low-lying northern plains. Extensive volcanism occurred in the Tharsis region, building up enormous masses of volcanic material and releasing large quantities of gases into the atmosphere. Weathering of surface rocks produced a diversity of phyllosilicate clay minerals that formed under chemical conditions conducive to microbial life.

Hydrous mineralogy of these lower members includes smectite (nontronite) and magnesium sulfates (kieserite and polyhydrated). Smectite-forming environments present significant hope for finding biosignatures of organic processes. One of the challenges at Gale Crater will be to find and explore the least compromised of smectite rich strata. These are likely to be those deepest in the mound section, just beyond Curiosity's landing ellipse. Characterizing the pristinity of the smectite strata as well as any mineralogical overprinting is a significant scientific goal of the mission. Curiosity's mobile laboratory capabilities of are up to this challenge.

Geologic Features in Gale Crater

The image above is Curiosity's first 360-degree color panorama taken of the Gale Crater landing site. Click it to open a high resolution view. Gale's central mound is visible in the top portion of the panorama. Distant northern crater walls are visible in the upper right and left portions of the panorama. Greyish colored blast marks from the descent stage rocket engines are also documented in this image. Once rolling, Curiosity will make its way towards sedimentary layers in the base of the mound at the location outlined in red.

Curiosity will go beyond the "follow-the-water" strategy of recent Mars exploration. The rover's science payload can identify other ingredients of life, such as organic compounds, the carbon-based building blocks of biology. However, Mars Science Laboratory is not a life detection mission designed to detect extant vital processes revealing present-day microbial life. Nor does Curiosity have the ability to image microorganisms or their fossil remains. Curiosity does have the capability to detect complex organic molecules in rocks and soils. If present, these might be of biological origin, but could also reflect the influx of carbonaceous meteorites.

More indirectly, Curiosity has the analytical capability to probe for other less unique biosignatures including the isotopic composition of inorganic and organic carbon in rocks and soils, particular elemental and mineralogical concentrations and abundances, and the attributes of unusual rock textures. The main challenge in establishing a biosignature is finding patterns, either chemical or textural, that are not easily explained by physical processes. Curiosity will also be able to evaluate the concentration and isotopic composition of potentially biogenic atmospheric gases such as methane, which has recently been detected in the modern atmosphere.

Curiosity will be able to do chemistry better than any other preceding spacecraft. Its scientific instruments will determine, element by element, what is there. Curiosity will not only determine what elements are there, but what chemical isotopes and minerals they formed into, revealing the temperature, the amount of water around, and the environmental conditions in which those minerals formed.

[The Landing: Another Small Step for Man - A Giant Leap for Robot Kind]    [The Landing Site: Geological Jackpot at Gale Crater]   
[Curiosity: The Escalade of Mars Rovers]    [The Science Payload]    [References and Further Reading]

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Bob Keller