Carbonates on Mars
Encyclopedia
Evidence for carbonates on Mars
has remained elusive until recently. For example, most remote sensing instruments such as OMEGA
and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, had not suggested the presence of carbonate outcrops at 100 m or coarser spatial scales. Though ubiquitous, carbonates dominated by Magnesite
(MgCO3) in Martian dust had mass fractions less than 5% and could have formed under current atmospheric conditions. Furthermore, with the exception of the surface dust component, carbonates had not been detected by any in situ mission, even though mineralogic modeling does not preclude small amounts of calcium carbonate in Independence class rocks of Husband Hill
in Gusev crater
(note: An IAU
naming convention within Gusev is not yet established).
The first successful identification of a strong infrared spectral signature from surficial carbonate
minerals of local scale (< 10 km2) was made by the MRO
-CRISM
team. Their spectral modeling identified a key deposit in Nili Fossae
dominated by a single mineral phase that was spatially associated with olivine
outcrops. The dominant mineral appeared to be magnesite
, while morphology inferred with HiRISE
and thermal properties suggested that the deposit was lithic. Stratigraphically, this layer appeared between phyllosilicates below and mafic
cap rocks above, temporally between the Noachian
and Hesperian
eras. Even though infrared spectra are representative of minerals to less than
100 micrometre depths (in contrast to gamma spectra which are sensitive to tens of cm depths), stratigraphic, morphologic, and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds. Nevertheless, the morphology was distinct from typical terrestrial sedimentary carbonate layers suggesting formation from local aqueous alteration of olivine and other igneous minerals. However, key implications were that the alteration would have occurred under moderate pH
and that the resulting carbonates were not exposed to sustained low pH
aqueous conditions even as recently as the Hesperian
. This increased the likelihood of local and regional scale geologic conditions on Mars that were favorable to analogs of terrestrial biological activity over geologically significant intervals.
The absence of more extensive carbonate
deposits on Mars
was thought to be due to global dominance of low pH
aqueous
environments. Even the least soluble carbonate
, siderite
(FeCO3), precipitates
only at a pH
greater than 5
Evidence for significant quantities of carbonate deposits on the surface began to increase in 2008 when the TEGA and WCL experiments on the 2007 Phoenix Mars lander found between 3–5wt% calcite (CaCO3) and an alkaline soil. In 2010 analyses by the Mars Exploration Rover Spirit, identified outcrops rich in magnesium-iron carbonate (16–34 wt%) in the Columbia Hills of Gusev crater, most likely precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian era.
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
has remained elusive until recently. For example, most remote sensing instruments such as OMEGA
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...
and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, had not suggested the presence of carbonate outcrops at 100 m or coarser spatial scales. Though ubiquitous, carbonates dominated by Magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...
(MgCO3) in Martian dust had mass fractions less than 5% and could have formed under current atmospheric conditions. Furthermore, with the exception of the surface dust component, carbonates had not been detected by any in situ mission, even though mineralogic modeling does not preclude small amounts of calcium carbonate in Independence class rocks of Husband Hill
Husband Hill
Husband Hill is one of the Columbia Hills in Gusev crater, Mars, which are close to the landing site of NASA's Spirit rover. It was named in honor of Rick D...
in Gusev crater
Gusev crater
Gusev is a crater on the planet Mars and is located at . The crater is about 166 kilometers in diameter and formed approximately three to four billion years ago. It was named after Russian astronomer Matvei Gusev in 1976....
(note: An IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
naming convention within Gusev is not yet established).
The first successful identification of a strong infrared spectral signature from surficial carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
minerals of local scale (< 10 km2) was made by the MRO
MRO
-Operations:* maintenance, repair, and operations -Companies:...
-CRISM
CRISM
The Compact Reconnaissance Imaging Spectrometer for Mars is a visible-infrared spectrometer aboard the Mars Reconnaissance Orbiter searching for mineralogic indications of past water on Mars. The CRISM instrument team comprises scientists from over ten universities and led by principal...
team. Their spectral modeling identified a key deposit in Nili Fossae
Nili Fossae
Nili Fossae is a fracture in the surface of Mars that has been eroded and partly filled in by sediments and clay-rich ejecta from a nearby crater. It is located at approximately 22°N, 75°E, and has an elevation of...
dominated by a single mineral phase that was spatially associated with olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
outcrops. The dominant mineral appeared to be magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...
, while morphology inferred with HiRISE
HiRISE
High Resolution Imaging Science Experiment is a camera on board the Mars Reconnaissance Orbiter. The 65 kg , $40 million instrument was built under the direction of the University of Arizona's Lunar and Planetary Laboratory by Ball Aerospace & Technologies Corp....
and thermal properties suggested that the deposit was lithic. Stratigraphically, this layer appeared between phyllosilicates below and mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
cap rocks above, temporally between the Noachian
Noachian
The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the presence of abundant surface water...
and Hesperian
Hesperian
The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history...
eras. Even though infrared spectra are representative of minerals to less than
100 micrometre depths (in contrast to gamma spectra which are sensitive to tens of cm depths), stratigraphic, morphologic, and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds. Nevertheless, the morphology was distinct from typical terrestrial sedimentary carbonate layers suggesting formation from local aqueous alteration of olivine and other igneous minerals. However, key implications were that the alteration would have occurred under moderate pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
and that the resulting carbonates were not exposed to sustained low pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
aqueous conditions even as recently as the Hesperian
Hesperian
The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history...
. This increased the likelihood of local and regional scale geologic conditions on Mars that were favorable to analogs of terrestrial biological activity over geologically significant intervals.
The absence of more extensive carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
deposits on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
was thought to be due to global dominance of low pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
aqueous
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
environments. Even the least soluble carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
, siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
(FeCO3), precipitates
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
only at a pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
greater than 5
Evidence for significant quantities of carbonate deposits on the surface began to increase in 2008 when the TEGA and WCL experiments on the 2007 Phoenix Mars lander found between 3–5wt% calcite (CaCO3) and an alkaline soil. In 2010 analyses by the Mars Exploration Rover Spirit, identified outcrops rich in magnesium-iron carbonate (16–34 wt%) in the Columbia Hills of Gusev crater, most likely precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian era.