Explanation of the Black-Drop Effect at Transits of Mercury and the Forthcoming Transit of Venus

We used the observations of the transits of Mercury of 1999 and 2003 taken with NASA's Transition Region and Coronal Explorer (TRACE) solar spacecraft. For the 1999 Mercury transit, for which data were acquired with the highest digital fidelity available for TRACE, we detected a black-drop effect, in spite of the facts that we were observing from outside the Earth's atmosphere and that Mercury has no significant atmosphere. We were able to show that the Mercury black-drop effect comes from a convolution of the instrument's point-spread function and the solar limb darkening. By implication, we should be able to explain Venus's black-drop effect in a similar way. It has long been known that Venus's black-drop effect is too large to come from Venus's atmosphere.