Which one of the following gases is found in highest quantity in Exosphere?

The exosphere is the outermost layer of Earth's atmosphere, beginning above 480–500 km [2]. In this region, the atmosphere is extremely rarefied, and gases are distributed by gravity in the heterosphere, with lighter elements rising to the top [1]. While the lower exosphere (near the exobase) contains traces of heavier atoms like oxygen and nitrogen, these concentrations drop rapidly with altitude [2]. Above approximately 1,500 km, hydrogen becomes the most dominant species, followed by helium [2]. Hydrogen is the lightest gas and, due to its low molecular mass, it can achieve escape velocity more easily than heavier gases, forming the geocorona [2]. Consequently, hydrogen is found in the highest quantity in the uppermost reaches of the exosphere, as it is the primary constituent of the transition between the atmosphere and interplanetary space [3].

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Dirunion of Atmosphere > p. 6
2. [2] https://ntrs.nasa.gov/api/citations/20220016573/downloads/White%20Paper(4).pdf
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Exosphere (700 to 1,000 km) > p. 279