Jarvis323 said:
I read the wikipedia page on the spirit rover and Gusev crater, and I skimmed this paper.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2002JE002026
There is a section about specific hypotheses:
4. Testable Hypotheses for MER
[35] Spirit's suite of sensors and imaging systems are anticipated to provide insight into the depositional and soil formation processes that have occurred in Gusev Crater [
Squyres et al., 2003]. Integrated observations that will be key for assessing the nature of past and present geological processes and environments include geochemistry, mineralogy and microtextural information obtained from rocks and soils. While lacustrine processes may have supplied significant material to Gusev, volcanic, fluvial [
Kuzmin et al., 2000], aeolian [
Greeley et al., 2003], and glacial processes [
Grin and Cabrol, 1997] are likely to have contributed material too. The basic rock and soil characteristics that may be addressed with the Athena instrument payload in testing the various depositional hypotheses are reviewed in the following section.
4.1. Main Hypotheses
[36] Data from the Viking, MGS, and MO missions have allowed several main hypotheses (
Table 2) to emerge that attempt to explain the spectral and albedo characteristics of surface material and the geomorphology observed in Gusev. Each main hypothesis carries a set of subhypotheses that require in situ measurements for either accepting or rejecting the subhypotheses. The variations of each subhypotheses are not mutually exclusive and in some cases overlap each other.
Table 2. Main and Subhypotheses on the Origin of the Sediments and Material in Gusev Landing Ellipse
a
| Main Hypotheses for Sediments Origin | Subhypotheses | Origin |
|---|
| 1. Lacustrine | 1.1. perennial lake 1.2. episodic lake | 1.1.1/1.2.1. precipitation 1.1.2/1.2.2. groundwater 1.1.3/1.2.3. hydrothermal water 1.1.4/1.2.4. glacial meltwater 1.1.5/1.2.5. combination of two or more of the above sources due to changing conditions through time |
| 2. Fluvial | 2.1. runoff 2.2. outflow from intravalley lake | 2.1.1/2.2.1. precipitation 2.1.2/2.2.2. groundwater 2.1.3/2.2.3. hydrothermal water 2.1.4/2.2.4. glacial meltwater 2.1.5/2.2.5. combination of two or more of the above sources due to changing conditions through time |
| 3. Glacial | 3.1. glacier | 3.1.1. local snow and ice packs 3.1.2. regional glaciation |
| 3.2. ice‐covered stream | 3.2.1. free water underneath the ice until the water supply ceased 3.2.2. progressive complete freezing down of the water | |
| 4. Volcanic | 4.1. plastic flowb | 4.1.1. hyperfluid lava 4.1.2. viscuous lava 4.1.3. pyroclasts and ashes are filling Gusev basin |
| 5. “Exotic” fluid | 5.1. CO2 flowc | 5.1.1. liquid CO2reservoir |
| 5.2. clathrate flowc | 5.2.1. clathrate reservoir | |
| 6. Aeolian | 6.1. regional to local windsd | 6.1.1. wind regimes |
| 6.2. global air falle | 6.2.1. global atmosphere circulation. | |
| 7. Subsurface hydrothermal | 7.1. hydrothermal minerals | 7.1.1. impact‐generated 7.1.2. crustal magma sources |
- a The subhypotheses shown reflect discussions about processes that have been presented in the literature over the years, either for the formation of the Ma'adim Vallis/Gusev crater system or for channels in general on Mars.
- b Hyperfluid lava carved Ma'adim Vallis and deposited material in Gusev. Viscuous lava generated a landform that mimics a delta at the outlet of Ma'adim. Pyroclasts and ashes are filling Gusev basin.
- c Obliquity changes provided temperature conditions for CO2 or clathrates release at the latitude of Ma'adim and Gusev. The surface pressure is still problematic.
- d Wind regimes following climate changes have driven the deposition and exhumation of material in Gusev.
- e Sediments in Gusev are made of material extracted over the planet and deposited in the basin by global atmosphere circulation.
4.2. Soil Formation and Sedimentary Processes
[37] The main hypotheses can be associated with specific soil and/or sediment types that may be detected by the Athena instrument payload which include: (1) global soil; (2) soils formed in a nonaqueous environment; (3) soils formed in an aqueous environment; (4) volcanic materials; (5) lacustrine sediments; (6) fluvial sediments; (7) aeolian sediments; and (8) glacial sediments. Soil and sediment profiles may be observed in the form of ejecta blocks from impact events, outcrops, and aeolian exposures (e.g., yardangs, see
Figure 10). Excavation by spinning a rover wheel while the rover remains stationary is also anticipated to access soil and sediment 5–10 cm below the surface.
Figure 10
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[38] This discussion cannot touch on all possible signatures of all sediment and soil formation processes that could be detected by the Athena instruments. Moreover, we do not presume to have identified every possible type of soil or sediment that could conceivably occur within Gusev. This is meant as a guide, and attempts to demonstrate what the Athena instruments may detect and what hypotheses could be identified if presented with certain evidence of a geologic or pedogenic process.
Table 3 summarizes what results from the Athena instruments would suggest a particular geologic or pedogenic process listed above and is supported by the following discussion.
Table 3. Possible Results From Athena Instruments of Hypothetical Soils and Sediments in Gusev Crater
a
| Hypothesis | Panoramic Camera | Microscopic Imager | Alpha Particle X Ray Spectrometer | Mini‐Thermal Emission Spectrometer | Mössbauer Spectrometer |
|---|
| Global soil | Similar multispectral characteristics as MPF soil. | No uniquely identifiable characteristics. | Similar elemental chemistry as MPF and VL 1, 2 soil. | No uniquely identifiable characteristics. | No uniquely identifiable characteristics. |
| Soils from physical weathering of local rock | No uniquely identifiable characteristics. | Angular soil grain morphology. | Soil elemental chemistry similar to local rock chemistry. | Soil spectra similar to local rock spectra. No secondary mineralogy. | Soil Fe mineralogy similar to local rock Fe mineralogy. No secondary Fe‐oxyhydroxides. |
| Volcanic ash | Sedimentary deposit that conforms to topography. | Glass shards cupsate, blocky, platy <250 μm. | Any range of Si content. Chemistry may be different than local rock. | Poorly crystalline to crystalline material (e.g., plagioclase, pyroxene, hornblende). | Ilmenite, titanomagnetite, titanomaghemite, magnetite, Fe‐pyroxene. |
| Maar base surge deposit | Massive, planar sedimentary deposits. Soft sediment deformations, vesicles, bedding sags. | Glass shards, blocky. Fine‐grained material <1 mm. | Any range of Si content. | Poorly crystalline to crystalline material (e.g., plagioclase, pyroxene, hornblende). | Ilmenite, titanomagnetite, titanomaghemite, magnetite, Fe‐pyroxene. |
| Soil from aqueous weathering (e.g., melting snow) | Soil structure. Columns, wedge, blocky, platy. Vesicular porosity near soil surface. | Vesicular porosity near soil surface. | Loss or accumulation of Ca, Mg, K, Na relative to local surface rock. | Phyllosilicates, carbonates, sulfates, secondary Fe‐oxyhydroxides. | Secondary Fe‐oxyhydroxides. |
| Fluvial deposit | Conglomerate facies;sheet, tabular cross stratified, lateral, channel fill, rounded/subrounded clasts up to 30 cm. | No visible grains. | No uniquely identifiable characteristics. | Primary minerals with cementing mineralogy; Fe‐oxyhydroxide, carbonate, or phyllosilicates. | Detect mineralogy of Fe‐cementing mineral if present. Possible siderite (FeCO3) Fe2+‐smectite if outer oxidized layer on sedimentary rock is removed by the RAT. |
| Sandstone facies; tabular and trough cross bed and ripple bed. | | | | |
| Shale facies; planar bed. | | | | |
| Lacustrine deposit | Alternating planar layers of light‐colored layers with darker layers. Layer thickness few cm to 10s cm. | Sand and gravel grains at lake's margin; clay/silt grains toward lake's center. | High levels of Ca, Mg, K, Na, S, Cl, N in basin. | Mineralogy variation from lake margin to lake center (e.g., carbonate → sulfate); phyllosilicates. | Possible siderite (FeCO3) Fe2+‐smectite if outer oxidized layer on sedimentary rock is removed by the RAT. |
| Lake's margin: sandstone facies; possibly similar to fluvial facies. | Rounded sand grains. | | | |
| Lake's middle: shale facies; planar layers of silt/clay. | No visible grains. | | | |
| Aeolian deposit | No particles larger than can be moved by creeping. | Grain size <4 mm. | Sediment chemistry differing from local rock chemistry. | Comparisons of sediment and local rock spectra suggest differing mineralogies. | Comparisons of sediment and local rock Mössbauer spectra suggest differing Fe mineralogies. |
| Sandstone facies: planar, laminar cross bedding or ripple bedding. No trough cross bedding. | | | | |
| Presence of global soil (see above). | | | | |
| >20 m thick deposits with little stratification (loess). | Grain size not easily discernable (loess). | | | |
| Glacial deposit (glacial till/moraine) | Poorly sorted material; cm to large boulders, striated rocks, gravel, boulders. Flattened rocks and gravel. | Striated rocks and gravels. | No uniquely identifiable characteristics. | Primary minerals. | Primary Fe minerals. |
| Glacial lake | Varves, rain‐out debris in planar layered sediments. | No visible grains. | High levels of Ca, Mg, K, Na, S, Cl, N | Phyllosicates. | Possible siderite (FeCO3) Fe2+‐smectite if outer oxidized layer on sedimentary rock is removed by the RAT. |
- a Bold text of instrument analytical results suggest a positive identification of an individual hypothesis.
4.2.1. Global Soil
[39] The Viking and Mars Pathfinder (MPF) sites show widely similar bulk soil elemental compositions, suggesting a soil that has been globally distributed by aeolian activity [
Rieder et al., 1997]. The presence of global soil in Gusev will be indicated by alpha particle X‐ray spectrometer (APXS) bulk chemical analyses reporting elemental concentrations similar to the Viking and MPF sites. Further support for global soil in Gusev may be established if Pancam multispectral imaging shows spectra similar to what was obtained by MPF [
Bell et al., 2000].
4.2.2. Soil Formation in a Nonaqueous Environment
[40] The APXS data of soils derived from local rocks will be expected to have total elemental composition similar to the local rock. Furthermore, Mini‐TES and Mössbauer spectrometer (MB) spectra would show that the rocks and soils have similar spectral properties. Soils that show no evidence of secondary mineralogy (e.g., clay minerals, iron‐oxyhydroxides, carbonates, and sulfates) would suggest that the soils were not affected by postdepositional aqueous activity. The Microscopic Imager (MI) data may show soil particles with angular morphology suggesting that they were derived locally.
4.2.3. Volcanic Ash/Maar Base Surge Deposits
[41] Apollinaris Patera is 250 km north [
Robinson et al., 1993] and may have deposited volcanic ash and pyroclasts in Gusev [
Kuzmin et al., 2000]. The detection of blocky, platy, and/or cupsate glass shards by MI will support the idea that volcanic ash is component of the soil [
Orton, 1996]. Mini‐TES spectra of an ash deposit may show a basaltic or andesitic signature if the ash has significant lithic component. Ash deposits with a significant vitric component would tend to show a poorly crystalline Mini‐TES spectra. If the volcanic ash is not altered by water, the MB may detect Fe phases such as ilmenite, titanomagnetite, titanomaghemite, and magnetite and Fe containing pyroxene [
Fischer and Schmincke, 1984;
Gunnlaugsson et al., 2002].
[42] While craters in Gusev are presumed to be of impact origin, it is conceivable that some craters could be maars or tuff rings (
Figure 11). Maar or tuff ring volcanoes can produce base surge deposits that resemble fluvial deposits (e.g., planar to wavy layering) as shown in
Figure 12. However, unlike fluvial deposits, base surge deposits may contain soft sediment deformations (i.e., folded layering between undeformed layer), vesicles (gas bubbles) and bedding sags that may be observable with Pancam [
Cas and Wright, 1987;
Fischer and Schmincke, 1984].
Figure 11
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Figure 12
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Caption
4.2.4. Soil Formation in Aqueous Environment
[43] The observation of platy, blocky, prismatic, and/or columnar, soil structures in an exposed soil profile by the Pancam would suggest that the soil may have been affected by water. Extensive leeching of base cations from the soil profile would have occurred regardless of whether Mars had a reducing or an oxidizing environment. Soil levels of Ca, Mg, K, and Na as detected by the APXS may be lower relative to local rocks. Mini‐TES may detect clay minerals (e.g., kaolinite, vermiculite, chlorite, and smectite), gibbsite, Fe‐oxyhydroxides [e.g., ferrihydrite (5Fe2O3·9H2O), goethite (FeOOH), or hematite (Fe2O3)], and calcite (CaCO3). MB could detect the presence of any Fe‐oxyhydroxides.
[44] Arid soils experiencing low or episodic water activity can develop structure as discussed above. Vesicular porosity is prevalent near the soil surface in arid soils [
Dunkerley and Brown, 1997] and may be observable by the Pancam and MI. Arid aqueous activity would be indicated by the APXS detecting elevated levels of Ca, Mg, K, Na, S, Cl, and/or N in the soil relative to local rock. Mini‐TES may detect calcite (CaCO3), gypsum (CaSO4· 2H2O), annhydrite (CaSO4), and possibly nitratite (NaNO3) [
Eriksen, 1981;
Clark et al., 1982;
Amit and Yaalon, 1996;
Li et al., 1996;
Böhlke et al., 1997]. Smectite is usually the prevalent clay mineral formed in arid environments [
Allen and Hajek, 1992] and could be detected by Mini‐TES.
[45] The above discussion of aqueous weathering assumed Earth oxidizing conditions. Reducing conditions could well have prevailed during aqueous mineral weathering on early Mars [
Catling, 1999]. Siderite (FeCO3) may have formed in greater abundance than calcite [
Catling, 1999]. Under reducing conditions magnetite (Fe3O4), siderite, and pyrite (FeS2) would have the potential to precipitate rather than goethite, ferrihydrite, or hematite. The prevalence of the apparent oxidizing conditions on Mars today may obscure any evidence of reducing mineralogy. The abrasion of a sedimentary rock or indurated soil blocks with the RAT to analyze material not directly exposed to the past and/or present Martian oxidizing conditions should allow MB and Mini‐TES to test the existence of reducing mineralogy.
4.2.5. Lacustrine Sediments
[46] Pancam could detect soil profiles of a lake deposit showing alternating layers of light colored salts and darker colored clay/silt layers [
Li et al., 1996]. However, complex mixtures of evaporites and clay mineralogies can occur in lacustrine soils and may not be discernable by Pancam. Soil structure in lacustrine soils may also be observable by the Pancam. Evaporite and clay minerals may also be detected by Mini‐TES. The APXS may show elevated levels of Ca, Mg, K, Na, S, Cl and possible N relative to local rock. Pancam and MI could detect a lake's margin because of the presence of beach sands and gravels relative to clays and silts that occur toward the center of the lake. Some of the precipitation sequence of (carbonates → sulfate) → halite could be detected as Mini‐TES and APXS sample from the outer reaches of the lake and moves toward the center of the lake [
Eugster and Kelts, 1983;
Shaw and Thomas, 1997]. Within a 600 m traverse, the spectrometers and cameras onboard the rover are likely to observe chemical transition. Soils with abundant evaporite minerals could be indurated, and clay mineral deposits may become shale‐like. Any shale‐like material with planer layering or indurated evaporites may occur as ejecta blocks large enough to be examined with the Athena instruments.
4.2.6. Fluvial Sediments
[47] Evidence of past fluvial activity in Gusev crater may occur as ejecta blocks or outcrops of conglomerate, sandstone, or shale at the surface. Any material exhibiting layered morphology (e.g., cross bedding, ripple bedding and trough cross bedding) [
Collinson, 1996] observed by Pancam are candidates for fluvial deposition. Rounded soil grains observed by MI would suggest fluvial activity. Mini‐TES would detect the primary mineralogy of the sand or conglomerate particles and may detect their cementing agents (e.g., silica, carbonate, clay, iron oxyhydroxides) [
Klein and Hurlbut, 1993]. The color of the sandstone may reflect the cementing agent with silica and carbonate agents producing a light color and the iron oxyhydroxides producing a red to reddish brown color. If an iron‐oxyhydroxide or siderite is the primary cementing agent, then MB may indicate the iron mineralogy of the cementing agent. PanCam images of a rock or outcrop material showing planar layering with no MI identifiable sand‐sized grains would suggest shale‐like material. Mini‐TES would produce spectra with primary mineralogies and clay mineralogies. Shale‐like material containing a significant vitric component derived from volcanic ash would tend to produce poorly crystalline Mini‐TES spectra.
4.2.7. Aeolian Sediments
[48] Detection of the global soil would suggest aeolian deposition of soil. MI will detect aeolian sedimentary deposits that show planar cross bedding and rippled bedding if the aeolian grains are large (>30 μm. However, trough cross bedding that occurs in fluvial environments typically does not occur in aeolian environments. Furthermore, layering of materials coarser than 4mm would suggest only fluvial and not aeolian activity [
Greeley et al., 1992].
[49] Loess deposits on Earth tend to be 20–30 m thick but have been known to be as thick as 60 m and usually are derived from fluvial and glacial sediments [
Pye, 1987;
Dunkerley and Brown, 1997]. Loess particle sizes range from 10 to 50 μm and are usually deposited in weakly stratified accumulations. Any Pancam and MI observations of deposits that appear to have little or no stratification with particle sizes barely or not visible by the MI may indicate loess.
4.2.8. Glacial Sediments
[50] Soil profiles or outcrops containing glacial till or material deposited at the terminus (moraine material) of a glacier will be poorly sorted and contain all grain sizes ranging from clay‐sized grains to meter‐sized boulders. The layering observed with glacially deposited material could look similar to fluvial material. However, closer examination of rock fragments (>2 mm) in glacial till could show indications of striations resulting from the abrasion of the rock, which is characteristic of the grinding action of glaciers on rock against another rock or bedrock surface. Some striated pebbles or rock may be elongated or flattened and would lie in the direction of glacial movement. “Rain‐out” debris from rafted ice that is deposited in the lake's sediment may indicate a glacial lake [
Bennett and Glasser, 1996]. Varves are usually indicative of glacial activity and, if so, consist of alternating layers of clay and silt/sand in the lake's sediment. Varving also occurs in temperate climates with seasonal fluctuation in precipitation. All of the above indicators of glacial activity may be observed with Pancam or MI.
[51] It is important to note that the key indicators for each of the described hypotheses could all be identified in situ within the range of the rover traverse as they are strongly based on the mineralogy of sedimentary exposures and grains and their morphology. In situ observations from the rover will be then complemented by larger‐scale orbital data surveys (MGS and MO) during the mission in order to fully understand the significance of the observation and the results of the measurements.
