NASA’s Messenger spacecraft has taken the first pictures of ice frozen in dark spots on Mercury , the planet closest to the sun .
The probe , which has orbited Mercury since 2011, snapped shots of water ice and other frozen volatile materials in permanently shadowed craters near the planet’s north pole .
The images provide clues as to when ice formed on Mercury and how it has evolved , according to a study published in the journal Geology . That, in turn, is expected to help scientists figure out more about how water came to other planetary bodies, including Earth and the moon .
“One of the big questions we’ve been grappling with is ‘When did Mercury’s water ice deposits show up ?’ Are they billions of years old, or were they emplaced only recently?” says lead author Nancy Chabot , instrument scientist for MESSENGER’s Mercury Dual Imaging System and a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
“Understanding the age of these deposits has implications for understanding the delivery of water to all the terrestrial planets, including Earth .”
Mercury is not the hottest planet in the solar system—that honor goes to Venus—but temperatures on the “day” side facing the sun do rise to about 800 degrees Fahrenheit .
But with scarcely any atmosphere to trap heat , the planet plunges to around minus-300 degrees at night . Because the planet rotates around an axis that is not tilted like Earth’s , the sun is never high in the sky at the poles, and some parts of craters never see sunlight at all .
Two decades ago , images from Earth-based radar first revealed the frozen polar deposits, believed to be water ice . That was later confirmed by Messenger—even before the new pictures—with a combination of neutron spectrometry , thermal modeling, and infrared reflectometry .
“But along with confirming the earlier idea , there is a lot new to be learned by seeing the deposits” in the new visible light photos , Chabot says .
THE LARGEST CRATER
Beginning in 2012, scientists began looking for ice with the broadband clear filter of MDIS’s wide-angle camera . Although the polar ice deposits are in permanent shadow in the depths of craters , the camera was able to obtain images using very low levels of light scattered from illuminated crater walls. “It worked in spectacular fashion ,” Chabot says .
The team zeroed in on Prokofiev, the largest crater in Mercury’s north polar region known to contain radar-bright material .
“Those images show extensive regions with distinctive reflectance properties ,” Chabot says . Analysis revealed a texture indicating that the ice is younger than underlying craters .
Water ice is also present elsewhere near the pole, covered by a thin layer of dark material believed to be frozen organic-rich compounds . In images of those areas, the dark deposits display sharp boundaries .
“This result was a little surprising , because sharp boundaries indicate that the volatile deposits at Mercury’s poles are geologically young ,” Chabot says .
WHY IS IT DIFFERENT?
Overall , the images indicate that Mercury’s polar deposits either were first delivered to the planet relatively recently or are regularly restored at the surface through some ongoing process . The images also reveal a noteworthy distinction between the moon and Mercury , one that may shed additional light on the age of the frozen deposits .
“The moon’s polar regions—which also have areas of permanent shadows and are actually colder—look different ,” Chabot says .
One explanation could be that the ice on Mercury is newer. If that newer ice comes from the most recent action in a longer ongoing process , then , the published study says , “a considerable mass of volatiles may have been delivered to the inner solar system throughout its history .”
“That’s a key question ,” Chabot says . “Because if you can understand why one body looks one way and another looks different , you gain insight into the process that’s behind it , which in turn is tied to the age and distribution of water ice in the solar system . This will be a very interesting line of inquiry going forward .”
Messenger—an acronym for Mercury Surface , Space Environment , Geochemistry and Ranging—is the first space probe to orbit the planet closest to the sun . It was launched in August 2004 and entered Mercury orbit on March 18, 2011 .
The Johns Hopkins University Applied Physics Laboratory built Messenger and manages its mission for NASA .
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