In 2003, NASA will launch two lander missions to Mars. One of the landers is slated to touch down in a region marked by aqueous hematite deposition. If we are to maximize the scientific opportunities of this mission, we must first complete careful studies of analogous systems on Earth, where biological and physical processes can be tied directly to paleobiological and geochemical patterns in deposited iron-rich sediments. The Rio Tinto drainage area of southern Spain offers just such an opportunity. During the past funding year, we completed two field sessions in the Rio Tinto region with colleagues from the Spanish CAB. Using a combination of petrology, Moessbauer spctroscopy and XRD, we established that iron-sediments precipitated from Rio Tinto waters are primarily hydronium jarosite, with unusual Fe-sulfates such as copiapite and poorly ordered iron oxides forming late in the season, when water pools evaporate to dryness. Early in diagenesis, highly soluble jarosites are replaced by goethite, which has a high capacity for preserving cellular details of cells and tissues caught up in Rio Tinto sediments. In ca. 300,000 year old terraces, much goethite has, in turn, been replaced by hematite, some of which is coarse grained. Thus, through diagenesis, Rio Tinto sediments come to resemble hematites observed on the Martian surface. Comparison of modern and Pleistocene sediments also shows that aspects of physical and biological environment are encrypted in the textures of Fe-sediments, providing a basis for the interpretation of images returned from the Mars MER rover.