4. what is a scanning electron microscope? what are its advantages?

A scanning electron microscope is a type of specialized electron microscope, which is used to study the surface of a smaller, solid sample, such as fossils, sediments, in a raster type of pattern, with a beam of electrons. This electron beam is comparatively low energy. The source of electrons and the type of electromagnetic lenses used in this microscope are similar to the one that is used in the transmission electron microscope.

It is very widely used and has various advantages. It provides the topography of the solid surface of the sample, the morphology of the sample and composition of the taken sample. These are key information used in a variety of both industrial applications as well as in the science laboratories.

thymine. in rna, thymine is replace by uracil

this age may represent the age of the earth's accretion, of core formation, or of the material from which the earth formed. this dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.