Mercury’s Vapors Form a Thin Layer on Surface

Some interesting information was reported in the fall of 1989 concerning Mercury’s atmosphere by Ann L. Sprague of the University of Arizona.

Mercury’s atmosphere? That seems strange because it is well known that Mercury doesn’t have an atmosphere. Mercury is very close to the sun and very hot. It is also quite small and has a low gravitational pull. Hot gases are harder to hold than cool gases are, and Mercury’s weak pull isn’t much of a holder anyway. Therefore--no atmosphere.

It depends, though, on what you call an atmosphere. The moon, for instance, has no atmosphere in an Earthly sense. There is a vacuum immediately above its surface. However, there are considerably more gas molecules in a cubic foot of space near the moon’s surface than in a cubic foot of space far away from any planet. The moon, therefore, might be viewed as having a very thin atmosphere, one that is only about a billionth as dense as Earth’s. That’s not much, but it’s something and it’s there.

In the same way, there is a very thin layer of gas in the immediate vicinity of Mercury’s surface. Two elements have been detected in that gas, because they happen to be easily detectable. They are sodium and potassium. These are metallic elements that melt into liquid at comparatively low temperatures. Mercury is not hot enough to boil these liquids, but it is hot enough to keep some of them in vapor form. (Thus, Earth is not hot enough to boil water, but it is hot enough to keep some water in vapor form in the atmosphere.)

Mercury can’t hold on to such vapors, however. Any sodium or potassium vapor that existed in the past should have disappeared long ago. Since the vapors are still there, they must be manufactured, somehow, at the same rate that they disappear.

One possibility is that small meteors are constantly hitting Mercury’s surface and that these bring new supplies of sodium and potassium, which heat into vapor. Another is that charged particles from the sun (the “solar wind”) slam into the surface of Mercury and knock sodium and potassium out of its rocks.

As it happens, though, there is a huge crater system on Mercury called Caloris (from the Latin word for “hot,” because it faces the sun when Mercury is closest to that body).

Caloris was undoubtedly formed by a colossal meteoric slam in the early days of the solar system. It must have cracked and fractured Mercury’s crust, and the crust has probably remained cracked and fractured ever since, because Mercury is very likely geologically dead, so that its crust remains in whatever form it has been pushed into.

Sprague’s findings involved Caloris. When Caloris is in view from Earth, about 10 times as much potassium can be detected above Mercury’s surface, than when Caloris is out of view. The natural conclusion is that although any sodium or potassium on Mercury’s surface would have been lost long ago, there are still large quantities of it below the surface. The sub-surface supply is heated by Mercury’s sun, and small quantities leak upward through cracks and fissures in Mercury’s crust. It would do so most readily where the surface was really smashed, as at Caloris, and that is why more potassium is detected when that feature is in view.

Studies such as this may give us better ideas concerning the interiors of worlds--interiors that we can’t study directly.

For instance, the moon also has sodium and potassium atoms present in its very thin atmosphere. There is more sodium present than potassium, which is not surprising. Sodium has a smaller atom than potassium and, in the universe generally, smaller atoms are more common than larger ones. In fact, there is five times as much sodium than potassium in the moon’s atmosphere.

It is not surprising, then, that there is more sodium than potassium in Mercury’s atmosphere, too. However, sodium is 15 times as common as potassium in Mercury’s atmosphere.

One ready explanation for this is that potassium is more easily converted into vapor than sodium, because potassium has a lower boiling point. This means that on Mercury, which is much hotter than the moon, subsurface supplies of sodium and potassium have been vaporized at a greater rate than on the moon, and this is especially true of potassium. For that reason, the layers beneath Mercury’s crust must be much more nearly stripped of their potassium than is true for the moon. As a result, it is not so much that Mercury has more sodium in its interior, but that it has less potassium.