Pluto Has a Mass About 1/500 That of Earth

Pluto has a mass about 1/500 that of Earth. How can it hold an atmosphere while Earth's moon cannot? Use the graph below to help in your explanation. The graph plots the escape velocity of several solar system objects along the vertical axis and the surface temperature along the horizontal. The lines plotted in the figure are the average speeds of gas particles as a function of temperature for various gases. The escape velocity of Earth's moon is 2.38 km/s and its sunlit surface temperature is 400 K. Plot a point on the graph representing these two values to represent Earth's moon. Compare the position relative to the gas thermal velocity curves for Pluto and Earth's moon.