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Don Paul: 'Mercurial' perfectly describes weather on Mercury

How can a planet with virtually no air have fascinating weather? Bear with me, and we shall see.

Back in the early 1980s when I was working in Detroit, I spent some time with University of Michigan scholar and planetary scientist Thomas Donahue, going through some basics about meteorology on the other planets in our solar system. I recall his touching upon Mercury as being similar to our moon in lacking a terrestrial atmosphere and having very little in common with other planets, in terms of weather. But there is a lot more to Mercury’s weather than either I knew or remembered.

A July article in Universe Today points out Mercury is airless at the surface, but it possesses a “tenuous exosphere” well above the surface made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor. These gas molecules were most likely captured from the solar wind, with some volcanic outgassing as well.

Tenuous? Try an atmosphere with one-quadrillionth of Earth’s atmospheric pressure. It’s rather rarified, I’d say.

Yet with that incredibly skimpy set of gases, combined with other planetary circumstances, Mercury has weather and climate zones. It is the planet closest to the sun, so you’d expect it to be the hottest planet. And hot it is, but not all the time in all places.

Nor is Mercury as hot as Venus, even though Venus is farther out from the sun. Venus has a very dense atmosphere, 96 percent of which is carbon dioxide. The enormous greenhouse effect on that planet gives the surface an average temperature of about 861 degrees Fahrenheit. Lead melts at that temperature.

A global view of the surface of Venus, where the average surface temperature is 861 degrees Fahrenheit. (NASA/JPL)

Airless Mercury’s sunny side can reach about 800 degrees Fahrenheit.

Mercury has the most eccentric orbit of the planets, meaning the orbit is anything but “circular.” At its closest point to the sun (perihelion), Mercury comes within 29 million miles of the star. At its farthest point (aphelion), it’s 41 million miles away from the sun. For the side of the planet in shadow, the surface temperature is -290 degrees Fahrenheit. But because of the highly eccentric orbit and the rotational rate of Mercury, things get much more interesting.

At its equator, Earth rotates at more than 1,000 mph. Mercury rotates at about 6.8 mph. Its solar day takes more than 58 of our solar days to inish. Without going into the math to determine the ratio of time for a solar year for Mercury versus a solar day (that ratio is about three rotations for every two orbits), a single day on Mercury is twice as long as a solar year. Got that?

Mercury has the lowest tilt of its axis of any planet. That very low tilt keeps the polar regions constantly in shadow. So, the polar regions are COLD. And they are icy. The floors of craters in polar regions are never exposed to sunlight. The presence of ice and organic molecules have been confirmed on Mercury’s surface in these locations. Considering the planet’s location and lack of surface atmosphere, I would rate the estimated amount of ice as shocking. That estimate is within a range of 1.1 to 11 billion U.S. tons of ice.

The ice may be covered by a thin mineral coating, which prevents the conversion of ice to gaseous water vapor, a process called sublimation. Planetary scientists speculate that the source of the water for this ice may be from volcanic outgassing and comet impacts.

Still, weather on Mercury is tied to shadow versus sun: incredibly cold shadow versus searing sunlight. Because of the orbital eccentricity and the slow rotational speed, the scorching side stays that way for two years – a two-year midnight sun – and the opposite is true for the frozen side. I’d venture a guess that Earthlings will not be settling on Mercury, no matter what things go wrong here.

If you’d like to learn more about this amazingly hostile but fascinating place, NASA is happy to help you out.

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