The search for life on Mars is an important goal of NASA and other space agencies. It is not known if chemical evolution on Mars produced the same or similar types of life as on Earth. If not, what would non-Earth biosignatures look like? If life has left its footprint on Mars, what chemical signatures can we recognize, and how can we prevent missing novel life signatures? Alternatively, chemical evolution on Mars may have produced complex chemical systems, which, however, did not lead to life. How can such systems be identified? We use as a model a complex inorganic-organic-biotic system on Earth, commonly called desert or rock varnish, which has been known to Darwin, and which is now also indicated on Mars. We describe unique complex chemical markers that are preserved in rock varnish on Earth. An intricate interaction between minerals, metals, and organic compounds is responsible for their preservation. We suggest some important types of organic compounds to look for in the Martian varnish, should it exist.
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