The satellite giveth, and the satellite taketh away. While some space-borne probes seem to confirm that earthly ice caps are shrinking,
others indicate that billion-year-old ice deposits lurk in Mercury’s
deep arctic shadows—which remain at a cool -173 °C despite the planet’s
proximity to the sun, which can drive summer noon temperatures up to 627
°C.
Are Mercurians swiping Earth’s ice to chill their martinis? Hardly. For
one thing, it’s too hard to get good vermouth up there. More important,
it appears that the ice (and possibly some organic matter, which may
serve as insulation, like sawdust in an old icehouse) was delivered by
accommodating comets.
Two instrument packages on the Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft (Messenger)
provide complementary evidence confirming that the deep eternal shadows
of the planet’s steep-walled polar craters harbor massive plates of
ice.
Two Science papers--describing Messenger’s Mercury Laser Altimeter (MLA) and Neutron Spectrometer (NS) experiments—provide the up-close confirmation that, yes, those really are caves of ice in that sunniest of pleasure domes.
The MLA, built at NASA’s Goddard Space Flight Center, lights up the
Mercurial surface with a 1064-nanometer (deep infrared)
chromium/neodymium yttrium-aluminum garnet laser. Like orbiting laser
altimeters everywhere, it flashes light off the ground below (in this
case, lighting up spots about 50 meters in diameter at 400-meter
intervals) and gauges distance by measuring the time until the reflected
light returns. The MLA gathers additional information about the surface
by alternating high- and low-power flashes. By comparing the strengths
of the returning signals, it can provide more accurate estimates of the
reflectance of the ground below, possibly revealing something about the
surface's composition.
The result is a topographic map of Mercury based on more than 4 million
individual elevation measurements—half of them including information on
the nature of the ground.
The Messenger researchers were already primed to look for ice.
Earth-based radar probes had previously indicated that the craters
might hide deposits of frozen water—or some other captured volatile
substance such as sulfur—in radar-bright areas hidden deep in the
perpetual shadows of arctic craters.
The scientists found that these radar-bright areas fall into two
categories. Those at the highest latitudes—where the crater-wall shadows
are longest and the valleys coldest—reflect both radar and 1064-nm
laser light strongly. These, the team says, are consistent with large
areas of exposed ice, in layers at least several meters thick.
In slightly lower latitudes, the researchers found that the radar
bright areas are often enveloped or overlaid by larger
laser-altimeter-dark regions (areas that reflect little infrared).
Indeed, “all craters with [radar bright] deposits and sufficient
altimeter sampling show at least some [laser-altimeter dark] features in
their poleward facing portions.” This suggests, say the researchers,
that in these warmer craters, a thin layer of something—regolith or even
comet-deposited organic compounds—may shield deeper strata of ice,
protecting them from sublimation.
The altimeter findings are supported by the Johns Hopkins–built Neutron
Spectrometer, which detects neutrons thrown off of atoms on Mercury’s
surface as the atoms are struck by gamma rays. The emitted neutrons fall
into three energy ranges: fast (energy greater than 0.5 megaelectron
volt), epithermal (0.5 electron volt to 0.5 MeV), and thermal (less than
0.5 eV). Because hydrogen atoms and neutrons have such similar masses,
they transfer momentum very efficiently, and the hydrogen absorbs
momentum from the epithermal neutrons.
The neutron spectrometer counts the number of incoming fast,
epithermal, and thermal neutrons (correcting for changes in altitude)
and extrapolates the data to infer the amount of hydrogen in the surface
materials. The relative proportion of epithermal neutrons among the
outbound particles reveals the amount of hydrogen present. A drop of
even 4 percent in the rate of incoming fast and epithermal neutrons
indicates that the spacecraft is passing over hydrogen rich water.
The neutron data suggest that only half of the radar-bright regions are
actually water at the surface. At the same time, the overall picture
shows as much as 1000 cubic kilometers of water icebound at Mercury's
poles, lying in layers “tens of centimeters thick.” Many of these layers
are insulated below a superficial covering (much poorer in hydrogen
compounds) 10 to 20 centimeters deep.
Image: NASA/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington/National Astronomy and
Ionosphere Center, Arecibo Observatory.
