I’ll Have Ice With My Full Moon, Please

The nearly full moon rises over Lake Superior near Duluth last night March 15, 2014. Anglers fish for lake trout (left). Credit: Bob King

I took off for Lake Superior yesterday evening and walked out as far as I dared on its frozen surface. The ice was thick, maybe a foot or more, but the rumbling and booming coming from deep below gave me pause. The surface felt as firm and substantial as an airport tarmac, but it subtly flexed and heaved as if somehow alive.

As the sky darkened, moonlight left a trail of light on Lake Superior’s ice last night. Credit: Bob King

Above it all rose the moon shining pale orange against the dusky band of Earth’s shadow. This part of the planet would soon be plunged into night but not without a bright beacon to help us find our way in the dark.

The frozen “fingertip” of Lake Superior photographed yesterday March 15, 2014 by NASA’s Terra satellite. “X” marks the approximate location where the moon photos were taken. Credit: NASA

Glowing pale orange, the moon looked anything but icy, but we know that ice is hidden like buried treasure deep within craters in both its polar regions.

NASA’s orbiting Lunar Prospector, which could detect water up to 1.5 feet below the surface, found water signatures in permanently shadowed craters at both poles.

Lunar mosaic of ~1500 Clementine images of the south polar region of the moon where more area remains in permanent shadow than any other area of the moon. Temperatures in these craters remains steady around -280 F. Credit: NASA

The moon’s orbit is tilted 5.14 degrees to Earth’s and its axis just 1.54 degrees for a total possible inclination of about 6.7 degrees, a tilt small enough that sunlight only skirts the uppermost rims of craters in the north and south polar regions.

The interiors of some craters, particularly in the south polar region, are sunk so deeply in the lunar crust (up to 7.5 miles or 12 km), that no sunbeam has ever broken the darkness.

An image of debris ejected from Cabeus crater in the moon’s south polar region and into the sunlight about 20 seconds after the impact of NASA’s LCROSS probe on Oct. 9, 2009. Scientists detected a variety of compounds in the plume including pure water ice. The inset shows a close-up with the direction of the sun and the Earth. Click for more on the LCROSS mission. Credit: Science/AAAS

Water from ice-rich meteoroids and comets that have smacked the poles over the aeons has hidden out here for billions of years trapped in rocks or existing as solid ice in some cases. Water released by impacts elsewhere on the moon would quickly vaporize away in the sunlight except for here and there molecules that migrated to the polar regions. Bombardment of oxygen-rich rocks by protons (hydrogen plus oxygen = H2O) in the solar winds creates small amounts of water as well.

While not wet by any stretch, our satellite’s no stranger to the substance our lives depend upon.  As twilight deepened and the lake shuddered I sensed that ancient icy connection across a space of 240,000 miles.

2 Responses

  1. Sean

    Hey Bob. Sorry, but having a touch of trouble wrapping my mind around something here. U mention that the moon’s orbit is tilted to ours about 5 degrees and that its axis is tilted a further 1.5ish. Is the second figure its axial tilt compared to the plane of the celestial equator or compared to the ecliptical plane? When it comes to sunlight hitting craters the only thing that would matter would be its tilt compared to the sun right? So it would be compared to the ecliptic right?

    1. astrobob

      I used to just consider the 5 degree orbital tilt, but when researching the blog article I realized that the moon’s axial tilt to the ecliptic of 1.5 degrees plays a role. Even if the moon’s orbit had no inclination and were co-planar with Earth’s, that 1.5 degree axial tilt to would cause the sun to “nod” up and down by that amount as the moon revolved around Earth.

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