From Juan Ramón Vidal Romaní:

Re: Martian “Yellowstone” might have nourished life (May 22): I, as geologist, understand that this article refers to the discovery of opal associated to hot springs which is a normal environment in the Earth where it is frequent to find colonies of microorganisms (bacteria). However, though is it normal to see volcanoes, impact craters, dunes, channels excavated in rivers, slope slidings in the images coming from Mars, up to now no thermal fountains have been shown as the repeatedly mentioned ones of Wyoming’s Yellowstone National Park also near where the Spirit found the silica deposits.

In the Earth there also exist other environments not associated to hot springs where silica deposits exist containing great number of biological remains (bacteria, fungi, spores, pollen grains, dinoflagellates, protozoans, diatoms, oligochaetes, mites, etc.) (Vidal Romaní J. R. ; J. A. Bourne, C. R. Twidale & E. M. Campbell. Siliceous cylindrical speleothems in granitoids in warm semiarid and humid climate. Zeitschrift. für Geomorfologie. N. F. 2003, 47, 4, 417-437. ; C. R. Twidale and Vidal Romaní J. R. (2005, Lanforms and Geology of Granite Terrains, Ed. Balkema, Amsterdam, 351pp.). The volcanic and hydrothermal activity in Mars was not the only possible activity which could originate the formation of amorphous silica deposits (opal) but also there had been superficial water circulation (rivers, seas, etc.).

The silica deposits to which I refer are associated with cavities such as the volcanic tubes or the fissure systems of magmatic rocks. Both types of environments are visible in Mars so, if it is considered the possibility of life forms associated to hot springs, there could also be possible to find that type of deposits associated to cavities similar to the ones found in Mars.

Juan Ramón Vidal Romaní
Instituto Universitario de Geología
Campus de Elviña s/n
University of Corunna
15071 Corunna Spain