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The amount of sodium in Mercury's exosphere depends on the planet's position in its orbit around the sun. In this image, the large whitish-reddish mass is the sodium in exosphere around Mercury (dark blue circle). In the upper left corner, Mercury's position (purple ball) in its orbit (light blue circle) around the sun (yellow ball) is shown. (TAA stands for true anomaly angle—the angle between Mercury, the sun, and the closest point to the sun along the orbit, which is known as the perihelion point.) As Mercury orbits, the distance between the planet and the sun changes and so does the speed at which Mercury is moving relative to the sun. These changes, in turn, affect both the brightness of the sodium emission and the amount of radiation pressure a sodium atom experiences. These factors yield the greatest amount of sodium in the exosphere when Mercury is at a middle distance from the sun. This still image was taken from an animation showing the seasonal effects of sodium in Mercury's exosphere. (For the ani
