Molybdenum is a transition metal with the atomic symbol Mo, atomic number 42, and atomic weight 95.94. The pure metal is silvery white in color, fairly soft, and has one of the highest melting points of all pure elements. Physiologically, it exists as an ion in the body. It is an essential trace element, being a component of the enzymes xanthine oxidase, aldehyde oxidase, and nitrate reductase. There is a trace requirement for molybdenum in plants, and soils can be barren due to molybdenum deficiencies. Plants and animals generally have molybdenum present in amounts of a few parts per million. In animals molybdenum is a cofactor of the enzyme xanthine oxidase which is involved in the pathways of purine degradation and formation of uric acid. In some animals, adding a small amount of dietary molybdenum enhances growth. Francis Crick suggested that since molybdenum is an essential trace element that plays an important role in many enzymatic reactions, despite being less abundant than the more common elements, such as chromium and nickel, that perhaps this fact is indicative of "Panspermia." Crick theorized that if it could be shown that the elements represented in terrestrial living organisms correlate closely with those that are abundant in some class of star - molybdenum stars, for example, that this would provide evidence of such Directed Panspermia. In small quantities, molybdenum is effective at hardening steel. Molybdenum is important in plant nutrition, and is found in certain enzymes, including xanthine oxidase. Molybdenum is used to this day in high-strength alloys and in high-temperature steels. Special molybdenum-containing alloys, such as the Hastelloys, are notably heat-resistant and corrosion-resistant. Molybdenum is used in oil pipelines, aircraft and missile parts, and in filaments. Molybdenum finds use as a catalyst in the petroleum industry, especially in catalysts for removing organic sulfurs from petroleum products. It is used to form the anode in some x-ray tubes, particularly in mammography applications. And is found in some electronic applications as the conductive metal layers in thin-film transistors (TFTs). Molybdenum disulfide is a good lubricant, especially at high temperatures. And Mo-99 is used in the nuclear isotope industry. Molybdenum pigments range from red-yellow to a bright red orange and are used in paints, inks, plastics, and rubber compounds.