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Proceedings Paper

Accretionary lapilli, tektites, or concretions: the ubiquitous spherules of Meridiani Planum
Author(s): Barry E. DiGregorio
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Paper Abstract

One of the most enigmatic discoveries made by the NASA Mars Exploration Rover Opportunity (MER-B) at the Meridiani Planum landing site are the ubiquitous spherules referred to as "blueberries" by the science team. They cover the entire landing area and can be seen in every direction within view of the rover cameras. Subsequent analysis of a small grouping of the spherules laying on top of a rock outcrop by Mossbauer spectroscopy showed an intense hematite signature not found on the rock or in the surrounding basaltic soils. Spherules were also found attached to and embedded within sedimentary sulfate rock outcrops found at the landing area that have been determined by the MER science team as having been formed in an acidic liquid water environment. The appearance of most of the Meridiani spherules is strikingly similar to the morphology and size of terrestrial accretionary lapilli and show similarities to terrestrial tektites. Accretionary lapilli are spherical balls and fragments with a concentric layered structure that are formed by a variety of mechanisms including hydrovolcanic eruptions, geysers and large meteorite impacts in water. Tektites are glassy impact spherules that form as a result of large meteorite impacts and also seem apparent in some of the rover images. Tektites can be perfectly spherical or have teardrop and dumbbell shapes. A lack of a visible volcanic source capable of producing high volumes of accretionary lapilli as seen in the MER-B images, in combination with the strong spectral signature of hematite, that some of the spherules display, led the MER science team to favor a concretion hypothesis thus far. All of these types of spherules involve interaction of with surface water or ice to form. Problems exist in explaining how the Martian “concretions”, if that is indeed what they are, are of such uniform size and have such a wide distribution. Evidence from Martian orbit and on the surface indicate that the Meridiani Planum landing ellipse is located within an ancient 800 km diameter impact structure with another 140 km crater under the site. Estimated hydrothermal output from this size of an impact would be equivalent to 38 times Yellowstone over a 15,000 year time period. Life as we understand it is dependent on a source of liquid water, energy, and nutrients. Hydrothermal energy can originate from either internal volcanic sources or through the action of large bolide impact. A 25 km diameter fluidized ejecta (rampart) crater named Victoria is located 50 km to the southwest of the Opportunity rover landing site and might explain how the Meridiani Planum region is covered with such an enormous abundance of spherules.

Paper Details

Date Published: 1 November 2004
PDF: 14 pages
Proc. SPIE 5555, Instruments, Methods, and Missions for Astrobiology VIII, (1 November 2004); doi: 10.1117/12.563673
Show Author Affiliations
Barry E. DiGregorio, Cardiff Ctr. for Astrobiology (United Kingdom)


Published in SPIE Proceedings Vol. 5555:
Instruments, Methods, and Missions for Astrobiology VIII
Richard B. Hoover; Gilbert V. Levin; Alexei Y. Rozanov, Editor(s)

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