Water Ice on Mercury: NASA Findings Change How We Look For Life
A panel of NASA scientists announced today that the MESSENGER ( an acronym for Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft has detected the presence of water ice on Mercury, an astonishing find given the planet's proximity to the sun and its reputation as one of the hottest in the solar system. In addition to the discovery of water ice, NASA scientists also believe that there are highly-concentrated deposits of organic material similar to the kind that gave rise to life on Earth.
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"Mercury is now a planet of astrobiological interest. In terms of the book of life there are some early chapters, and Mercury may inform us of what's in those chapters," said Sean Solomon, head of MESSENGER's mission, during today's press event. They estimate there to be between 100 billion to 1 trillion metric tons of water ice on Mercury. David Lawrence, a NASA scientist who is part of the MESSENGER team, put it in factoid terms.
"The new data indicate the water ice in Mercury's polar regions, if spread over an area the size of Washington, D.C., would be more than 2 miles thick," he explained.
The panel doesn't expect that there is life on Mercury, but today's water ice findings give NASA scientists more insight into how water originated on Earth. Conventional wisdom suggest that the water formed here, but the MESSENGER findings are giving scientists reason to believe that the water "arrived" on Earth in the form of frozen comets and asteroids.
"A lot of water originated at a different part of the solar system," Solomon said. "The history of water on our planet and the history of water ice on Mercury are very much also the history of interactions of comets and asteroids with the inner solar system."
Jim Green, head of NASA's planetary division, summed it up. "The more we examine the solar system the more we realize it's a soggy place," he said.
The water ice findings were confirmed by a series of three different tests, according to the NASA panel. The first test involved detecting the activity of neutrons shooting off the planet's surface. The neutrons are activated by cosmic rays that bombard the planet, and MESSENGER measured the regions where neutron activity was low. Neutrons stop when they run into water ice, so regions of low neutron activity likely contained water ice.
"It's billiard ball physics," said NASA's David Lawrence during the event. "Whenever the cue ball dies after hitting another ball, that's how neutrons react when they hit hydrogen."
The regions with decreased activity matched findings from the 70s that theorized the water ice could exist inside impact crater on Mercury's surface. Portions of the craters that don't receive sunlight maintain a low enough temperature to support water ice.
The second test involved the implementation of a Mercury Laser Altimeter, or MLA, that is attached to the MESSENGER probe. The MLA bounces signals off the planet's surface and uses the data to form topographical maps. Among the other data collected by the MLA scans, signs of high reflectivity were detected in the craters present in the low-neutron areas.
"Yes we are seeing something that is reflective as water ice, and we're seeing it just exactly were the neutron spectrometers and radars have confirmed the presence of hydrogen and reflective material," said NASA's Gregory Neumann.
The third test involved the data gathered by temperature reading instruments on MESSENGER. Despite it's proximity to the sun, Mercury is capable of very cold temperatures and has the greatest range of any planet in the solar system.
"The temperatures on Mercury range from 700 kelvin all the way to 50 kelvin, which is 50 C from absolute zero. It's as cold as it gets," said NASA's David Paige, the scientist who presented the temperature findings. Paige said the temperature data was a "perfect match" when compared with the other data on where water ice could exist on Mercury.
When it comes to whether or not the data from each test matches the prevailing theories on water ice on other planets, Solomon came to one conclusion: absolutely.
"Does the neutron spectrometry signal match? Yes it does. Does if have near infrared reflectants where you'd expect them? Yes it does. Does it match detailed thermal models? Yes it does," said Solomon. "My own punctuation mark on the story today would be an exclamation point."
"It's an absolute exclamation point," he said.
Here is the official press release from NASA regarding today's findings:
NASA Spacecraft Finds New Evidence for Water Ice on Mercury
WASHINGTON -- A NASA spacecraft studying Mercury has provided compelling support for the long-held hypothesis the planet harbors abundant water ice and other frozen volatile materials within its permanently shadowed polar craters.
The new information comes from NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Its onboard instruments have been studying Mercury in unprecedented detail since its historic arrival there in March 2011. Scientists are seeing clearly for the first time a chapter in the story of how the inner planets, including Earth, acquired their water and some of the chemical building blocks for life.
"The new data indicate the water ice in Mercury's polar regions, if spread over an area the size of Washington, D.C., would be more than 2 miles thick," said David Lawrence, a MESSENGER participating scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of one of three papers describing the findings. The papers were published online in Thursday's edition of Science Express.
Spacecraft instruments completed the first measurements of excess hydrogen at Mercury's north pole, made the first measurements of the reflectivity of Mercury's polar deposits at near-infrared wavelengths, and enabled the first detailed models of the surface and near-surface temperatures of Mercury's north polar regions.
Given its proximity to the sun, Mercury would seem to be an unlikely place to find ice. However, the tilt of Mercury's rotational axis is less than 1 degree, and as a result, there are pockets at the planet's poles that never see sunlight.
Scientists suggested decades ago there might be water ice and other frozen volatiles trapped at Mercury's poles. The idea received a boost in 1991 when the Arecibo radio telescope in Puerto Rico detected radar-bright patches at Mercury's poles. Many of these patches corresponded to the locations of large impact craters mapped by NASA's Mariner 10 spacecraft in the 1970s. However, because Mariner saw less than 50 percent of the planet, planetary scientists lacked a complete diagram of the poles to compare with the radar images.
Images from the spacecraft taken in 2011 and earlier this year confirmed all radar-bright features at Mercury's north and south poles lie within shadowed regions on the planet's surface. These findings are consistent with the water ice hypothesis.
The new observations from MESSENGER support the idea that ice is the major constituent of Mercury's north polar deposits. These measurements also reveal ice is exposed at the surface in the coldest of those deposits, but buried beneath unusually dark material across most of the deposits. In the areas where ice is buried, temperatures at the surface are slightly too warm for ice to be stable.
MESSENGER's neutron spectrometer provides a measure of average hydrogen concentrations within Mercury's radar-bright regions. Water ice concentrations are derived from the hydrogen measurements.
"We estimate from our neutron measurements the water ice lies beneath a layer that has much less hydrogen. The surface layer is between 10 and 20 centimeters [4-8 inches] thick," Lawrence said.
Additional data from detailed topography maps compiled by the spacecraft corroborate the radar results and neutron measurements of Mercury's polar region. In a second paper by Gregory Neumann of NASA's Goddard Flight Center in Greenbelt, Md., measurements of the shadowed north polar regions reveal irregular dark and bright deposits at near-infrared wavelength near Mercury's north pole.
"Nobody had seen these dark regions on Mercury before, so they were mysterious at first," Neumann said.
The spacecraft recorded dark patches with diminished reflectance, consistent with the theory that ice in those areas is covered by a thermally insulating layer. Neumann suggests impacts of comets or volatile-rich asteroids could have provided both the dark and bright deposits, a finding corroborated in a third paper led by David Paige of the University of California at Los Angeles.
"The dark material is likely a mix of complex organic compounds delivered to Mercury by the impacts of comets and volatile-rich asteroids, the same objects that likely delivered water to the innermost planet," Paige said.
This dark insulating material is a new wrinkle to the story, according to MESSENGER principal investigator Sean Solomon of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.
"For more than 20 years, the jury has been deliberating whether the planet closest to the sun hosts abundant water ice in its permanently shadowed polar regions," Solomon said. "MESSENGER now has supplied a unanimous affirmative verdict."
MESSENGER was designed and built by APL. The lab manages and operates the mission for NASA's Science Mission Directorate in Washington. The mission is part of NASA's Discovery Program, managed for the directorate by the agency's Marshall Space Flight Center in Huntsville, Ala.
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