Draw an ellipse using the Ellipse Tool (L). Give it a light to medium dark green radial gradient using the Gradient Palette.
Duplicate and paste the ellipse on top of itself by pressing Command + C then Command + F. Command + F pastes directly on top of where the object was copied from, while Command + V pastes in the center of the artboard. Uniformly scale the ellipse down slightly by holding down Shift + Option while you drag a corner of the ellipse.
