THE POTENTIAL OF DESERT VARNISH IN PRESERVING BIOSIGNATURES

Mars Global Surveyor Thermal Emission Spectrometer data indicate regions with significant levels of hematite (alpha-Fe2O3). Iron oxides, like hematite, can form as aqueous mineral precipitates and as such may preserve microscopic fossils or other biosignatures. This process is often biologically-mediated. One possible terrestrial analogue is desert varnish, which is composed of iron oxides, manganese oxides and clays. We are currently using a high resolution scanning electron microscope and electron dispersive spectroscopy analysis to study desert varnish samples on rocks from Pilbara, Australia and Hoover Dam, Nevada to assess the ability of desert varnish to preserve biosignatures.

Preliminary investigation has demonstrated that the surface of varnish layers consists of a thin coat of iron oxides over clays. While iron oxides are present throughout the varnish, the quantity of manganese oxides increases with depth in the varnish and appears in dense layers or in loosely associated small nodules. Significant gaps, probably created by chemical weathering, exist between the varnish and the substrate. This interface is characterized by the presence of iron oxides cementing both detrital grains and clasts of weathered substrate. The varnish has a heavy fungal presence on the surface and in the top layers. Most fungal structures, including biofilm, are strongly encrusted with iron oxides and clays. Over time these structures are incorporated within the varnish. Small spore-size microfossils are identifiable at greater depths within the varnish. Biofilms created by the fungi and possibly bacteria may be essential in creating the structure of the varnish. For example, rising from the surface of the varnish are “tree-like” varnish structures created by complex weaving of fungal films. These structures may increase the potential for trapping airborne detritus.