Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars - Ruff, S.; Farmer, J.D.; Calvin, W.; Herkenhoff, Kenneth; Johnson, J.; Morris, R.V.; Rice, M.S.; Arvidson, R. E.; Bell III, J.F.; Christensen, P.; Squyres, S. (Journal of Geophysical Research E: Planets, 2011)