More “Hollowed Ground” on Mercury

by Jason Major on January 5, 2012

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MESSENGER's latest NAC image of curious "hollows" surrounding a crater peak

The latest featured image from NASA’s MESSENGER spacecraft, soon to complete its first year in orbit around Mercury, shows the central peak of the 78-mile (138-km) – wide crater Eminescu surrounded by more of those brightly-colored surface features dubbed “hollows”. Actually tinted a light blue color, hollows may be signs of an erosion process unique to Mercury because of its composition and close proximity to the Sun.

First noted in September of last year, hollows have now been identified in many areas across Mercury. They showed up in previous images as only bright spots, but once MESSENGER established orbit in March of 2011 and began high-resolution imaging of Mercury’s surface it became clear that these features were something totally new.

The lack of craters within hollows seems to indicate that they are relatively young features. In fact, they may be part of a process that continues even now.

“Analysis of the images and estimates of the rate at which the hollows may be growing led to the conclusion that they could be actively forming today,” said David Blewett of the Johns Hopkins University Applied Physics Laboratory (APL).

One hypothesis is that the hollows are formed by the sublimation of subsurface material exposed during the creation of craters, around which they are most commonly seen. Being so close to the Sun (29 million miles/46 million km at closest) and lacking a protective atmosphere like Earth’s, Mercury is constantly being scoured by the powerful solar wind. This relentless stream of charged particles may literally be “sandblasting” exposed volatile materials off the planet’s surface!

The image above shows an area approximately 41 miles (66 km) across. It has been rotated to enhance perspective; see the original image and caption here.

Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

Anonymous January 5, 2012 at 11:31 AM

For those not well informed about this recently discovered feature of the quicksilver planet:
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“Images taken from orbit reveal thousands of peculiar depressions at a variety of longitudes and latitudes, ranging in size from 60 feet to over a mile across and 60 to 120 feet deep. No one knows how they got there. ….

“Mars Reconnaissance Orbiter spotted similar depressions in the carbon dioxide ice at Mars’ south pole, giving that surface a ‘swiss cheese’ appearance. But on Mercury they’re found in rock and often have bright interiors and halos. [ A similar process, and/or mechanism, but in a different medium? ] ….

“As the planet closest to the Sun, Mercury is exposed to fierce heat and extreme space weather. Blewett believes these factors play a role.

“A key clue, he says, is that many of the hollows are associated with central mounds or mountains inside Mercury’s impact craters. These so-called ‘peak rings’ are thought to be made of material forced up from the depths by the impact that formed the crater. Excavated material could be unstable when it finds itself suddenly exposed at Mercury’s surface.” Vaporization of sulfur “and other volatiles”, for example. ….

I found this particularly interesting: “[ Mecury is ] the anchor at one end of the Solar System. Learning how Mercury formed will have major implications for the rest of the planets. And MESSENGER is showing that, up to now, we’ve been completely wrong about this little world in so many ways!”

NASA Science News:
http://science.nasa.gov/science-news/science-at-nasa/2011/24oct_sleepyhollows/
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The B&W image here is a bit reminiscent of the appearance of radiation burns seen on survivors of the nuclear detonations in Japan. A strange form of planetary radiation burn, perhaps.

Dennis Ray Wingo January 5, 2012 at 3:16 PM

This is interesting!

Torbjörn Larsson January 5, 2012 at 6:25 PM

To clear this up a bit, sublimation is a thermodynamical phase transition from thermal vaporization. There can be several processes at work here, and mass ejection by sputtering (say, in the solar wind) is not considered sublimation.

That said, the luminescence, if that is what it is, looks a lot like the sputter glow from sputtered material when you look into a sputter chamber or the air glow around atmospheres demonstrated in many ISS images. Even metals, and alkali metals are known to be emitted by Mercury, have efficient emission. (Hence the use of only minute amounts of mercury in luminescent lighting.)

Lights in the Dark January 5, 2012 at 9:56 PM

So you’re suggesting it may be a geo-fluorescence of some sort?

Torbjörn Larsson January 6, 2012 at 5:07 PM

Certainly fluorescence, “lights in the light”, could be part of what I suggest. But at a guess the likelihood for ionization is much higher, so I would assume the most part could be emission from electron recombination.

Note that the article claims only “tint”, and image processing with more or less artificially reproduced colors introduces artifacts that looks like emission and “bleeding over” effects. I was just wondering if for once there was something more going on, based on my physics intuition and no more.

I thought about it some more, triggered by your response, and considering the intensity of ambient light so close to the Sun, I realize a) secondary emission effects would be difficult to make themselves seen however efficient, and b) image processing is perhaps challenged more than usual. So I must lower my expectations on my hunch.

Torbjörn Larsson January 6, 2012 at 5:07 PM

Certainly fluorescence, “lights in the light”, could be part of what I suggest. But at a guess the likelihood for ionization is much higher, so I would assume the most part could be emission from electron recombination.

Note that the article claims only “tint”, and image processing with more or less artificially reproduced colors introduces artifacts that looks like emission and “bleeding over” effects. I was just wondering if for once there was something more going on, based on my physics intuition and no more.

I thought about it some more, triggered by your response, and considering the intensity of ambient light so close to the Sun, I realize a) secondary emission effects would be difficult to make themselves seen however efficient, and b) image processing is perhaps challenged more than usual. So I must lower my expectations on my hunch.

Torbjörn Larsson January 6, 2012 at 5:07 PM

Certainly fluorescence, “lights in the light”, could be part of what I suggest. But at a guess the likelihood for ionization is much higher, so I would assume the most part could be emission from electron recombination.

Note that the article claims only “tint”, and image processing with more or less artificially reproduced colors introduces artifacts that looks like emission and “bleeding over” effects. I was just wondering if for once there was something more going on, based on my physics intuition and no more.

I thought about it some more, triggered by your response, and considering the intensity of ambient light so close to the Sun, I realize a) secondary emission effects would be difficult to make themselves seen however efficient, and b) image processing is perhaps challenged more than usual. So I must lower my expectations on my hunch.

Torbjörn Larsson January 6, 2012 at 5:07 PM

Certainly fluorescence, “lights in the light”, could be part of what I suggest. But at a guess the likelihood for ionization is much higher, so I would assume the most part could be emission from electron recombination.

Note that the article claims only “tint”, and image processing with more or less artificially reproduced colors introduces artifacts that looks like emission and “bleeding over” effects. I was just wondering if for once there was something more going on, based on my physics intuition and no more.

I thought about it some more, triggered by your response, and considering the intensity of ambient light so close to the Sun, I realize a) secondary emission effects would be difficult to make themselves seen however efficient, and b) image processing is perhaps challenged more than usual. So I must lower my expectations on my hunch.

Torbjörn Larsson January 5, 2012 at 6:25 PM

To clear this up a bit, sublimation is a thermodynamical phase transition from thermal vaporization. There can be several processes at work here, and mass ejection by sputtering (say, in the solar wind) is not considered sublimation.

That said, the luminescence, if that is what it is, looks a lot like the sputter glow from sputtered material when you look into a sputter chamber or the air glow around atmospheres demonstrated in many ISS images. Even metals, and alkali metals are known to be emitted by Mercury, have efficient emission. (Hence the use of only minute amounts of mercury in luminescent lighting.)

Wayne January 6, 2012 at 2:36 AM

I’m going to go out on a limb here and make a prediction that these features are related to the trail of sodium ions that Mercury leaves in it’s wake…

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