When we look at light reflected from places like the polar i…

When we look at light reflected from places like the polar ice caps of Mars, we look for absorption “features” like the three shown here in the red enclosures for two different sizes of CO2 ice granules:   What is the physical meaning of the large dips in the data at 2.7 microns (μm), 2.8 microns and 3.3 microns, all infrared wavelengths?