Monday, December 3, 2012

Reflecting on the ice of Mercury and the Moon

Composite image of the north pole of Mercury. Red are the areas of permanent shadow; yellow delineates radar bright deposits mapped from Earth. Data are plotted on a photomosaic of MESSENGER images [NASA].
Paul D. Spudis
Smithsonian Air & Space

Mercury – the planet, not the element – was in the news this past week.  For some time, we had suspected that the poles of Mercury might harbor deposits of water ice.  This – on a planet so close to the Sun that the surface temperature at the equator is hot enough to melt lead!

Yet like the Moon, Mercury’s spin axis is perpendicular to the plane in which it orbits the Sun.  This means that large craters near Mercury’s poles lie in permanent shadow (“shivering” around -170° C), unaffected by the Sun’s searing heat (equivalent to more than eleven times the solar flux we get on Earth).  As on the Moon, these permanently shadowed areas get heat from only two sources – the 3 K background heat of space, created during the Big Bang some 15 billion years ago, and whatever heat is being generated now from the deep interior (a quantity that geophysicists call the heat flow of a planet).

Large planets (like Earth) generate heat mostly from the decay of radioactive elements deep inside them.  This heat is lost largely through the phenomenon of volcanism, in which melted rock from the interior is erupted onto a planet’s surface as lava and ash.  Smaller planets and moons likewise experience this heating and volcanism, but because they are have lower overall contents of heat-producing elements, their volcanic episodes occurred in the distant past.  Much of the heat of these smaller planets has been largely dissipated.  Thus, on Mercury, we suspect that the overall heat flow is very low, resulting in extremely cold temperatures on the floors of its permanently shaded polar craters.

For many years, astronomers have studied Mercury with radio telescopes from Earth (using radar to make images of its surface).  Because the orbital inclination of Mercury is relatively high (about 7°), we can get a fairly good look into the interiors of the polar craters.  Interestingly, even though Mercury is much farther away than the Moon, we can see more of the mercurian polar areas because of this relatively high orbital inclination (the Moon’s orbital plane is inclined only 5°).  These radar pictures showed an amazing and unexpected feature – the dark areas are filled with material that is highly reflective at radio frequencies, properties similar to the surfaces of the icy moons of Jupiter (Europa, Ganymede and Callisto).

These results were so unexpected and startling that debate raged for many years whether these deposits really were what they appeared to be: water ice.  Facts are stubborn things and few materials have radio properties similar to ice.  Some suggested that sulfur might be an alternative explanation, but provided little evidence for such behavior.  Moreover, another moon of Jupiter, Io, which has a surface largely composed of sulfur, does not show the radar brightness or “glint” seen on the other, ice-rich Jovian moons.

The debate on the nature of the Mercury polar deposits has now been settled with the release of new data from the MESSENGER mission.  Launched on August 3, 2004, with insertion into obit around the planet on March 18, 2011, the spacecraft has been taking pictures and making measurements of Mercury for the last two years.  We have mapped the extent of darkness near the poles, measured the temperatures of the surface inside these regions, and detected the presence of significant amounts of hydrogen there.  All of these results are strongly supportive of the water ice interpretation.

The existence of ice near the poles of Mercury supports the case for water ice on our own Moon, although there are some significant differences between the two occurrences.  Like Mercury, the Moon’s spin axis is nearly perpendicular to the plane of its orbit around the Sun.  The similarity of the terrain of both bodies results in deep holes that hide large expanses of terrain from the glare and heat of the Sun.  Both objects have been volcanically active in the past, but not today, meaning that the average rates of heat flow on both are low.  These properties result in the creation of polar “cold traps” in which any entering volatile substance (such as water molecules) cannot escape.

The solid bodies of the inner Solar System are constantly hit by debris from comets and asteroids.  This material contains water, both in free form and bound within hydrous minerals.  On smaller objects (like the Moon and Mercury), most of this water is lost to space, but we suspected that some of it might be retained within these dark cold traps near the poles.  Now we know that such a process does occur.

Differences between the Moon and Mercury result in differing amounts and settings for their polar deposits.  Being much closer to the Sun, one might expect Mercury to contain less water ice, but a variety of evidence suggests that the opposite is the case.  The polar ice of Mercury appears to be greater in extent and thickness than comparable deposits on the Moon.  This probably results from two factors.  First, Mercury is a bigger object, with a surface gravity about twice that of the Moon.  Thus, it is more difficult for water to “escape” from Mercury.  Second, the closeness of Mercury to the Sun (the edge of biggest gravity well of the Solar System) results in a higher flux of cometary impacts there than experienced in the Earth-Moon system.  So more water is being added to Mercury, where it is more easily retained.


A planetary “two-fer.”  Let’s get on with it.


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