The lecture notes show the spectrum in reverse order. These are the absorbed wavelengths. If we can identify the absorbed wavelengths, we can identify the substance. This is called spectroscopy . The wavelengths missing from that light tell us the composition of the surface.
Spectroscopy works very well, as proven by tests on Earth, so it's a great tool.
If the object has a higher temperature, it means the atoms are moving faster than in an object with a cooler temperature. Look at the graph of thermal radiation from the lecture notes. Hotter objects have peaks at shorter wavelengths, and colder objects have peaks at longer wavelengths. If we know where the peak is, we can determine the exact temperature of the object, and vice versa. There are four groups of elements, listed here in order of decreasing abundance.