2002 Annual Science Report
NASA Johnson Space Center Reporting | JUL 2001 – JUN 2002
Secondary Carbonate in Igneous Rocks
This project is a continuation of last year’s research project: Carbonate Globules in Igneous Rocks. Observations of carbonate globules from the Columbia River basalts (CRB) from Spokane, WA, and from Spitsbergen, Norway, were made and compared to carbonate globules found in Martian meteorite ALH84001. Microstratigraphic relationships in the CRB & Spitsbergen samples prove that some and possibly all of the silica formed after the carbonate was crystallized. The lack of alteration or decomposition of the siderite adjacent carbonate shows that the silica was emplaced at relatively low temperatures, likely as an aqueous deposit of a gel. The relationship between biogenically precipitated carbonate and related low temperature silica deposition is complex and requires more detailed studies.
Other secondary carbonate structures that fill fractures were found in Martian meteorites Nakhla and ALH84001. It is important to determine the relationship between the types of carbonate fill in the fractures of igneous rocks in both the terrestrial analogs and in ALH84001. The Nakhla carbonates have compositions that vary considerably over submicrometer distances.
These carbonates have generated two major controversies: (1) did the carbonates form at high temperatures, thereby excluding any possible biological role in their formation? (2) if the carbonates formed at low temperatures, did biology play any part? Samples from the Columbia River basalt and from Spitsbergen, Norway, both contain Fe-carbonate (siderite) as well as other compositions of carbonate. The CRB globules and the Spitsbergen globules are radially zoned in texture and composition, showing some similarity to the ALH84001 carbonates globules.
The glass or gel-like silica in the terrestrial samples both show a complex surface morphology. Work is under way to search for organic particulates or residue in these two terrestrial samples and to use the results to influence additional work on the ALH84001 samples. Even without organic residue or microfossil morphology, the presence of secondary carbonates in igneous rocks has preserved a history of likely hydrothermal and/or low temperature alteration, which adds a new dimension to the preserved volcanic history of the rocks; the rocks have recorded information about the hydrosphere and atmosphere. Preliminary literature search indicates that secondary carbonate globules in igneous rocks on Earth may be much more common than realized. For example, carbonates are often found in vesicles in suboceanic basalts. Carbonates readily form in cavities in desert surface rocks, including at least one documented meteorite. With one example already from Mars and with multiple terrestrial examples, the possibility exists that many igneous rocks on Mars (mainly basalts and andesites) may contain secondary carbonate globules formed at relatively low temperatures. Such carbonates may record a history of near surface aqueous activity on Mars. The carbonates may also be potential locations of fossils or even extant life. Consequently they constitute a possible new environment for life in the solar system, particularly for bodies that are dominated by igneous rocks.
PROJECT MEMBERS:David McKay
RELATED OBJECTIVES:Objective 6.0
Define how ecophysiological processes structure microbial communities, influence their adaptation and evolution, and affect their detection on other planets.
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Define climatological and geological effects upon the limits of habitable zones around the Sun and other stars to help define the frequency of habitable planets in the universe.
Refine planetary protection guidelines and develop protection technology for human and robotic missions.