Home Plate, Mars

Introduction

In January of 2006, NASA successfully landed the rovers Spirit and Opportunity on opposite sides of Mars. The Mars Exploration Rover mission objective is to find evidence of past water. While Opportunity's landing site showed clear evidence in the water-lain sediments of the bedrock, Spirit has operated for more than two years before finding promising evidence for water, oddly at a volcanic site. After traversing the Columbia Hills, Spirit approached the light-toned circular feature (~80m in diameter) known as "Home Plate".

Objective

Test whether Home Plate was formed by a hydrovolcanic eruption as a single event, or by eruption and subsequent aeolian reworking, by comparing the Mars deposits with various hydrovolcanic sites in New Mexico.

Zuni Salt Lake, New Mexico

Methods

Observations were made during one week of field work (June 11-June 16, 2006) completed in New Mexico at sites analogous to Home Plate. The goal of the field work was to document deposits of terrestrial hydrovolcanic maar volcanoes. Sites visited include Valles Caldera, White Rock Canyon, Montoso Maar, and most importantly Zuni Salt Lake. At Zuni Salt Lake a stratigraphic section, at a comparable scale to that of Home Plate (~2m), was measured and described. Samples were collected at these various sites for further analysis and later reference. Photographic images were taken along with descriptive field notes in order to document the features that were observed. These images were then compared to the images of Home Plate.

Similarities

• Low angle cross-bedding - result of the fragmental material spreading turbulently outwards from the volcanic vent
• Accretionary lapilli - grains formed during the eruption by the accumulation of fragmented lava onto an initial bomb, usually up to 64mm in diameter
• Bomb sag - sagging layers created by impact of a volcanic bomb into soft beds
• Vesicles - voids in the rock caused by degassing of the lava during deposition
• Tephra beds - deposits of scoria, ash, and pumice
• Fine grained, finely layered units
• Vesicular basalt lag deposit cap
• Alternating layers of coarse and fine clasts or tephra

Bomb sag, Zuni Salt Lake, Earth

Bomb sag, Home Plate, Mars

Differences

• Collapse features - present at Zuni Salt Lake; absent at Home Plate
• Accidental (bedrock) material - abundant at Zuni Salt Lake; appears rare at Home Plate, but incompletely documented
• Palagonite - abundant at Zuni Salt Lake; appears absent at Home Plate, but incompletely documented

Conclusion

Each of the New Mexico sites demonstrates similar features to those seen at Home Plate; however, Zuni Salt Lake provides the most accurate terrestrial analog based on lateral extent and overall scale. Zuni Salt Lake is classified as a maar on the basis that slumping around the crater rim is evident and a large amount of accidental (bedrock) material is present. Lack of slumping and accidental material indicates that Home Plate is probably not a maar, but a tuff ring dominated by magma material. Low angle cross beds are found in both types of deposits; therefore, features found at Home Plate can result from a base surge produced by a hydrovolcanic eruption. The hydrovolcanic tuff ring model allows for all features observed at Home Plate, including the cross-bedding, to be accounted for within a single volcanic event, without requiring a later stage of wind-reworking. This style of eruption indicates that there once was water at or below the surface of Mars.

References

Cabrol, N.A., 2004, Spirit: Stratigraphy and Sedimentology Investigation on Barnhill at A761, Geo Memo.

Cas, R.A.F., and Wright, J.V., 1987, Volcanic Successions. Allen & Unwin, London, 528 p.

Francis, P., 1994, Volcanoes: A Planetary Perspective. Oxford University Press, New York, 443 p.

"Mars Exploration Rover Mission." July 2006. http://marsrovers.nasa.gov/gallery/all/spirit.html.

"Pancam." July 2006. http://marswatch.astro.cornell.edu/pancam_instrument/index.html.

Acknowledgements

This research was conducted at the Smithsonian National Museum of Natural History through the Research Training Program, which is supported by the NMNH Office of the Director. Field work was supported by MER science project.