NASA MESSENGER Captures Images of Ice on Mercury

Nasa's MESSENGER spacecraft

NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has provided the first optical images of ice and other frozen volatile materials within permanently shadowed craters near Mercury's north pole.

The images not only reveal the morphology of the frozen volatiles, but they also provide insight into when the ices were trapped and how they've evolved, according to an article published in the journal, Geology.

Two decades ago, Earth-based radar images of Mercury revealed the polar deposits, postulated to consist of water ice.

Prokofiev, named in August 2012 for the Russian composer, is the largest crater in Mercury’s north polar region to host radar-bright material.

Credit: NASA /Johns Hopkins University Applied Physics Lab /Carnegie Iinstitution of Washington

That hypothesis was later confirmed by MESSENGER through 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," said lead author Nancy Chabot, the Instrument Scientist for MESSENGER's Mercury Dual Imaging System (MDIS) and a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Beginning with MESSENGER's first extended mission in 2012, scientists launched an imaging campaign with the broadband clear filter of MDIS's wide-angle camera (WAC).

Mercury Dual Imaging System (MDIS)

Although the polar deposits are in permanent shadow, through many refinements in the imaging, the WAC was able to obtain images of the surfaces of the deposits by leveraging very low levels of light scattered from illuminated crater walls. "It worked in spectacular fashion," said Chabot.

The team zeroed in on Prokofiev, the largest crater in Mercury's north polar region found to host radar-bright material.

"Those images show extensive regions with distinctive reflectance properties," Chabot said.

"A location interpreted as hosting widespread surface water ice exhibits a cratered texture indicating that the ice was emplaced more recently than any of the underlying craters."

In other areas, water ice is present, she said, "but it is covered by a thin layer of dark material inferred to consist of frozen organic-rich compounds." In the 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, relative to the time scale for lateral mixing by impacts," said Chabot.

"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?" Chabot said.

"Understanding the age of these deposits has implications for understanding the delivery of water to all the terrestrial planets, including Earth."

Overall, the images indicate that Mercury's polar deposits either were delivered to the planet recently or are regularly restored at the surface through an 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 polar regions of Mercury show extensive areas that host water ice, but the Moon's polar regions, which also have areas of permanent shadows and are actually colder, look different," Chabot said.

"One explanation for differences between the Moon and Mercury could be that the volatile polar deposits on Mercury were recently emplaced," according to the paper.

"If Mercury's currently substantial polar volatile inventory is the product of the most recent portion of a longer process, then a considerable mass of volatiles may have been delivered to the inner Solar System throughout its history."

"That's a key question," Chabot said. "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."

NASA Cassini Image: Saturn's Southern Winter Approaches

Credit: NASA /JPL-Caltech /Space Science Institute

The shadows of Saturn's rings slide farther southward as Saturn approaches southern winter (or northern summer). 

Saturn now lies almost exactly halfway between its equinox (August 2009) and southern winter solstice (in May 2017). 

This view centers on an area at 22 degrees south latitude on Saturn. 

Cassini spacecraft took the image with its wide-angle camera on May 6, 2013.

ESA Rosetta Re-examines Asteroid Steins' Hidden Gems

ESA’s Rosetta spacecraft snapped images of asteroid Steins as it flew by the diamond-shaped asteroid on 5 September 2008, revealing a tiny world with a big history of collisions.

The images were taken by Rosetta’s Wide Angle Camera as the spacecraft flew within 800 km of the 5 km-wide asteroid, and later processed by amateur astronomer Ted Stryk to produce the image presented here.

By emphasising the shadows, the processing reveals high-contrast features, such as bright crater rims against their dark, shadowed interiors.

However, this technique can also create some artifacts, such as the illusion of boulders protruding from the surface, that are not present in the raw data.

One striking feature is the gaping hole at the south pole (top in this image) of Steins – its largest impact crater at about 2 km wide and nearly 300 m deep.

This crater is named ‘Diamond’ following the gem nomenclature chosen after the asteroid’s overall shape earned it the nickname ‘diamond in the sky’.

The circular crater in the centre of the asteroid as seen in this viewing angle is called Topaz and is about 650 m wide and 80 m deep.

A chain of several craters can be seen stretching from the asteroid’s north pole (bottom in this image) right up to Diamond crater.

So far, around 40 craters have been found, but the imposing Diamond crater likely influenced the appearance of the entire asteroid because the debris thrown out from this dramatic collision would have scattered over the surface, imprinting over other, older craters.

Rosetta has since passed by asteroid Lutetia in July 2010 and is now in deep-space hibernation. It will wake up on 20 January 2014 and rendezvous with comet 67P/Churyumov–Gerasimenko in May 2014. A few months later, its Philae probe will separate to make the first controlled landing on a comet.

Comets are considered to be the primitive building blocks of the Solar System, and Rosetta will help scientists to learn more about their role in the evolution of our local cosmic neighbourhood.