Messenger Spies a Spider in Caloris on Mercury. Image:NASA, Johns Hopkins, American Taxpayer

Greek "Hermes" in the morning, Greek "Apollo" in the evening, Roman "Mercury," on either side of the Sun, has been hammered.

Hammered by Sun, by Wind (solar), by Rain (iron and silicates) for billions of years.

Violence in its past? You bet. It is a small target, but over time, things heading in to or toward the Sun pummel it.

 www.solarviews.com/cap/vss/VSS00092.htm

NASA Text: "The Narrow Angle Camera of the Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft obtained high-resolution images of the floor of the Caloris basin on January 14, 2008. Near the center of the basin, an area unseen by Mariner 10, this remarkable feature – nicknamed “the spider” by the science team – was revealed.

 A set of troughs radiates outward in a geometry unlike anything seen by Mariner 10. The radial troughs are interpreted to be the result of extension (breaking apart) of the floor materials that filled the Caloris basin after its formation. Other troughs near the center form a polygonal pattern. This type of polygonal pattern of troughs is also seen along the interior margin of the Caloris basin. An impact crater about 40 km (~25 miles) in diameter appears to be centered on “the spider.” The straight-line segments of the crater walls may have been influenced by preexisting extensional troughs, but some of the troughs may have formed at the time that the crater was excavated." -- NASA

From NASA- "The tiny spacecraft discovered a unique feature that the scientists dubbed, “The Spider.” This type of formation has never been seen on Mercury before, and nothing like it has been observed on the Moon."

[Or anywhere else (yet), for that matter. I keep thinking really hot dry rock -- on a really round world. Like, Mercury is round -- you know where Earth is "flattened" by rotation from spinning once in 24 hours[0.0033] Mercury is round to 0.0000 (four places) Les Porter]

NASA: "It is in the middle of the Caloris basin and consists of over a hundred narrow, flat-floored troughs (called graben) radiating from a complex central region. “The Spider” has a crater near its center, but whether that crater is related to the original formation or came later is not clear at this time." -- NASA

Go to this website, below. [Or www.solarviews.com] Download this image of the two "Caloris Basin" (ideas), click on it and ENLARGE the image and move around inside this "basin", an area about the size of Texas and Alaska combined. The area of Caloris depends on where you think the splash ripple of mountain/crater rings ends. Caloris Basin is Big, possibly with a diameter of 1550 km or so. (Will the real Caloris Basin stand up, Please?)

"The Spider" is easy to spot on the enlarged image, a little left and above the center of either yellow or blue dashed ring. I agree -- it isn't "clear" yet -- if the formations of the "small" (40 km) crater and the "Spider" system of troughs are related in formation. (I do not think so, but there are always will be surprises with higher image resolution.)

Here are some questions that might get an answer with Messenger. Was Mercury always there, near the Sun so close? Say two, three billion years ago? And why is it so "smooth"? Lots of stuff could/should have fallen into and past Mercury when it was forming -- being so close to the Sun. Why is it so smooth? Was it impact-hot (molten) for a long time, especially after a hit like Caloris? If Mercury were further out from the Sun, then migrated inward, where was it? It could not have been much further than where it is, or it would have left some echoes or resonances in the orbital motions of Earth and Venus. Eventually we will know.

photojournal.jpl.nasa.gov/targetFamily/Mercury    Visit this site for some images, but not the "spider." [I do not know why all the images are so hard ot locate...these are yours and mines Tax dollars...]

Even though it is "dense" and weighs a lot more than Earth's Moon, how long would it take to dampen out the movement of a small planet through a populated inner system of planets? Most scientist assume Mercury has resided where it is for billions of years. With it's density -- it probably has. This close to the Sun when the Sun was gravitationally contracting, before fusion ignition, the denser material objects in the disk of materials from which the Sun formed would have solidified and coalesced into the terrestrial worlds. With lot of iron. Mercury needs to show its iron directly, or a fraction thereof. Or some indication of it's dense heavy stuff. Did its old volcanoes die young? Did it "cool" quickly? Did all the dense heavy stuff just sink into Mercury's core? Where is the iron? Would some of the iron containing dense minerals come back to the surface with volcanic activity?

We assume Mercury has the usual Iron Core but we do not really know. Still, It can hardly be otherwise, from the density. . . The formation of the Spider. No one knows, yet. To me, it looks like the small crater atop the spider is much newer and is superimposed upon the spider. The real question is how, and how long after the Caloris Impact did the troughs form? As to the troughs, these graben also appear to have shifted the walls of the crater, making the crater earlier than the final troughing of the Spider. [but the images aren't that clear so likely not, but. . .]

In close examination of the existing images as released, it appears the crater walls have been shifted somewhat by whatever formed the radial troughs. I think only better imagery will show that the troughs are much older than the superimposed crater. I think the disturbed walls might be a result of the impact and some rebound. (Or NOT.) But better images might render the answer completely. (Okay, okay. There appear to be physical distortions in the crater walls related to the graben. Perhaps the graben are deep enough to affect the crater wall formation -- deforming what would be a nicely shaped crater wall into a distorted-looking crater wall, as the material excavated by the impact is distributed away from the central location of the impact.) Seeing all those craters "in the troughs," and "over" them, makes the troughs/graben look old and they could have been a the final result of whatever formed Caloris Basin, since the troughs are extensive in the basin area. The great number of similar sized craters in the whole ediface is important too. The troughs shape and decay could mean they are "remnants" of the original impact. Maybe a few hundred million years later than the Caloris-Basin-forming impact, but formed as some kind of rebound rising from the deep semi-liquid of a molten mantle pierced by a "light material" impactor -- trying to be spit out to the surface where the lighter floaters end up. Especially if it was slow enough to pierce the mantle and enter the denser but lower viscosity of the upper molten core. (think of a silicate goo over a nearly liquid ball of hot iron and siderophiles.) The overall fluidity of the rock surface when the Caloris impact occured must be factored in, but just shooting off the hip, the troughs look like they appeared in a solid skin lifted from within, from beneath, by something trying to rise to the surface -- that almost makes it. Like the rising bubble still driving lightweight India into Asia, wherein it is like a "surfer" sub-continent still riding the same wave that set Gondwanaland assunder. But in this case on Mercury, poked up, spread the trough regime, than sank back to equilibrium. Caloris itself has to be really, really old. Caloris has to be much, much older then the troughs -- but what ever formed Caloris might have formed the Spider as it tried to ooze itself out of the differentiating core, and rising molten through the mantle.. So Caloris' Spiders could be the result of a long delayed internal rebound. What do you think?

This is an old Mariner 10 image: NASA--" Mercury, in some differences from the Moon, displays occasional structural features indicative of compression. One example is this fault, interpreted to be a thrust in nature, that forms a scarp 3 km high:"

 NASA TEXT "As the MESSENGER team continues to study the high-resolution images taken during the Mercury flyby encounter on January 14, 2008, scarps (cliffs) that extend for long distances are discovered. This frame, taken by the Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS), shows a region of Mercury's surface previously unseen by spacecraft and a large scarp crossing vertically through the scene, on the far right of the image. This scarp is the northern continuation of the one seen in the NAC image released on January 16. The width of this image is about 200 kilometers (about 125 miles), showing that these scarps can be hundreds of kilometers long on Mercury" -- End NASA  excerpt..

“In sum, the history of Mercury is twofold: volcanic events that produced plains units and impact cratering that have greatly modified the terrains dominated by flows.” --NASA Image: NASA, Goddard Space Flight Center

Since Mercury is so close to the Sun -- How HOT does it get?

Mercury's Solar Constant. Rough Computation.

At the distance of Earth, the "Solar Constant" (sunlight intensity at the top of earths atmosphere, the threshold of "Space" is about 1367 Watts/second/Meter2.

At Mercury, we would be much closer to the Sun. So what is the "Wattage" the Sun provides to the surface of Mercury?

The Sun's luminosity is 3.854x10^33 ergs/second. 1 watt / second = 10,000,000 ergs / second so. . . To convert the value to watts, we knock down seven zeros and change the name from ergs to watts.

The solar output is 3.854 x 10^26 watts/second, from a spherical surface with a radius of 695,980 km -- and an area of 6,087,001,144,794.05 km (6 trillion km2). Area of a sphere's surface = 4 x pi x radius squared. (6.087 trillion square kilometers.)

Each square kilometer covers a million square meters. Therefore the number of square meters on the Sun’s surface is given by multiplication: 6,087,001,144,794.05 x 1,000,000 = 6,087,001,144,794,050,000 square meters. To find how much a single square meter of “Sun” emits, we divide . . .

3.854 x 10^26 watts/second / 6,087,001,144,794,050,000 m2 = 63,315,250.126 watts/second. So each "square meter" of the Sun's apparent surface transmits to space 63,315,250.126 watts/second. That is 63.3 million watts from each of a lot of square meters.

So, at the distance of Mercury’s orbit, what is Mercury's "Solar Constant?" Mercury's perihelion is 0.307 A.U. and aphelion is 0.467 A.U.and we can average those values to get an average distance of 0.387 A.U. or ~ 58,000,000 km from the sun.

(Rs)Radius of Sun = 695,980 km
(Rm)Radius of Mercury's orbit = 58,000,000 km

Solar Constant = 63,315,250.126 Watts/second x ((Rs) /(Rm))^2
Solar Constant = 63,315,250.126 x (695,980km / 58,000,000km)^2
Solar Constant = 63,315,250.126 x (0.01199965517)^2
Solar Constant = 63,315,250.126 x (1.439917242 x 10^-4)
Solar Constant at Mercury  = 9117 watts/second/m^2

9117 / 1367= 6.7

Compared to Earth, 6.7 times as much sunlight/unit area. 6.7 times the solar energy. Intense.

With this ~ 9117 watts, Mercury is heated directly by the Sun to a mean (average daylight side) surface temperature of 180° C (356°F Oven-y) (on its sunny side), but Mercury’s sunny side can become as high as 425° C., (800°F.). and yet on the nightside, it can drop to -170° C (-274° F.).

MERCURY RESONATES! Mathematical Elegance in the spin of Mercury.

Mercury "spins." But it spins really, exceedingly, quite very slowly. About 59.6 Earth days will equal 1 Mercury-day. (Actually. . . 58.6461 +/- 0.005 days, in excellent agreement with the period required for a precise 2/3 resonance with its orbital period (58.6462 days) --a value likely to be improved via observational methods when Messenger orbits Mercury in 2011. http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html

Imagine the Sun subtends an angle 3 times larger in the sky than it does on Earth! Here it is half a degree, on Mercury it is about a degree and a half. You think the Sun beats down on you here?

Mercury’s Atmosphere

Image: NASA, Johns Hopkins, The sodium emission is at 5890 Angstroms (589nm, in the visible part of the spectrum and the same wavelength, or color, as in sodium vapor lamps and street lights on Earth). Because sodium atoms have intense emission, they are easy to detect, and this makes sodium a good tracer for other volatile elements in Mercury’s atmospheric envelope (exosphere.)

Mercury's Atmosphere: Yeah. This is a kind of a joke. Although there are bits of Hydrogen and Oxygen they don't together weigh a metric ton, it might take a long time to assemble enough for a drop of water.

The Solar Wind is harsh upon Mercury’s surface. Imagine a 4.567 billion-year wind of nuclei, protons mostly, Hydrogen, and neutrons too, Helium -- with or without electrons -- a torrent of charged or neutral particles hurled from the Sun; an endless spalling spray of kinetically energetic protons pinging and blasting away at the molecules on the surface of Mercury. As below O2 and H2 could make water if kept in the shade, but it would not be much.

The metals Sodium (Na) and Potassium (K) are chipped off to become part of a vacuous collection of particles blasted "free" by the Sun and blown from sunny side to dark side and into nearby space, and then blasted away. We also see some spallation from magnetic field effects and re-direction of the solar wind onto the dark side by flux transfer events.  We had known there was a strong sodium tail emanating from Mercury. Which is how we had had early estimates of quantities and percentages and over all mass. The image above is what it looks like via intensity for specific emission wavelengths, though I do not know how well the vertical scale of the image compares to the horizontal. (I think the vertical is exaggerated, and that it should be a larger representation of disk of Mercury)

The spall of atoms blown off by the by the Solar Wind, mixes with the Solar Winds constituents, but streaming like a small comet’s tail. Not a real atmosphere at all. Just a toned down G-star long-time ablation. (The blast of particle wind energy from an O-star or a B-star placed where the Sun is relevant to Mercury would have turned Mercury into a blob of molten silicates and iron and heavier atomic species -- and there would have been the case of Mercury just torched away in a few million years at most. Because of the gentle G type star, this gentle wind ablation can continue for billions of years, at least until the Sun's Red Giant phase, where Mercury is likely swallowed and slowed by the outer layers of the Sun and its orbit starts to be slowed and caused to spiral into the Sun. . .

Hey this ablation might let us get closer to the rare valuable platinum group metals on Mercury -- maybe a space miner's heaven. Hopefully the orbital phase of Messenger mission will hint or directly point at possible valuable quantities our species can mine in centuries ahead.)

Surface pressure: ~10-15 bar (0.001 picobar)
Average temperature: 440 K (167 C) (590-725 K, sunward side) Total mass of atmosphere: <~1000 kg (Well,The entire planetary atmosphere is less than the weight of a cubic meter of water.
Atmospheric composition: 42% Oxygen (O2), 29% Sodium (Na),
22% Hydrogen (H2), 6% Helium (He), 0.5% Potassium (K),
possible trace amounts of Argon (Ar), Carbon Dioxide (CO2),
Water (H2O), Nitrogen (N2), Xenon (Xe), Krypton (Kr), Neon (Ne) "(The atmosphere of Mercury is essentially a vacuum. Compositional values are not well constrained, values from "Mercury"; Vilas, Chapman, and Matthews, eds., University of Arizona Press, 1988)"

Magnetic field drawing image: Credit: Image produced by NASA/Goddard Space Flight Center/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS.

This close to the Sun, the effects of the solar wind and its particles magnetic properties are a dominant mechanism in energy to loft particles from the surface of Mercury.  The line that indicates Messenger's path and the surrounding characterization of Mercury's Magnetic Field is a diagram of the October 6, 2008, MESSENGER flyby that revealed there were "magnetic tornadoes" forming in the magnetic field of Mercury. These "tornadoes" are twisted vortices of magnetic field lines and plasma; corkscrew-shaped bundles of similarly magnetically aligned particle aligned with the field lines.  The pinkish area in the drawing indicates  the boundary of Mercury's magnetic field, determined by spacecraft measurements, and called the magnetopause. [because of Earth's magnetic field, Earth also has a tear-drop shaped magnetosphere and magnetopause.]  The tornadoes are  known as "flux transfer events" (twisted magnetic field lines) when they form at the magnetopause and "plasmoids" (yellow areas) when they form in the long magnetic "tail" extending opposite the day-side of Mercury. The large magnetic field leakage through the magnetopause and the flux transfer events acts as open channels through which the solar wind can flow down to the surface of the planet and sputter neutral atoms into Mercury’s atmosphere, and this appears to be possible on both the day-side and the otherwise shaded "safe" from the Sun, night-side. 

Mercury's North Pole A NOTE:  When Messenger starts orbiting Mercury in March 2011, Messenger obaervations and measurements will help Earth-based scientists more accurately determine the orientation of Mercury's Spin Axis. Right now you can go everywhere, even NASA records, and find how our idea's of the spin axis' orientation has evolved from our thinking it to be 90 degrees (normal) to the orbital plane -- to what JPL uses now, which is what I used below. . .

According to NASA, JPL's analysis:

Mercury's North Pole of Rotation points to a celestial position in the Constellation Draco, the Dragon. Draco the Dragon winds itself quite a ways around -- "our" North Celestial Pole in such a way that I can only describe Mecury's North Pole in the Sky -- by coordinates, and very roughly at that. The point in the sky Mercury's axis of rotation points toward is: Right Ascension: 281.01 - 0.003T (RA 18H 44' and change)
Declination : 61.45 - 0.005T (Dec +61° 28')
Reference Date : 12:00 UT 1 Jan 2000 (JD 2451545.0)
T = Julian centuries from reference date

The North Star for Mercury is a lot fainter than Polaris, the North Star of Earth (present) -- fainter than you can see without big binoculars, or easily, say, with a three-inch objective lens/mirror. The star I find in the sky nearest that position is from the Tycho Hipparcos Catelog, and is called: Tycho 4215-996-1, with a magnitude of 10.32 and a distance of 193 light years. The absolute Magnitude of Mercury's North Pole Star is +6.46, which makes it much fainter than our Sun, and it is more orange in color, a spectral K-type star. Tycho 4215-996-1 is the star now that makes a little circle in the sky around Mercury's North Celestial Pole -- a circle smaller than the almoste  minute of arc  diameter circle Polaris makes around Earth's North Celestial Pole.  If it spins in a perterbed gravitational field, it precesses. We know that the very orbit of Mercury precesses since it is buried more deeply in the Sun's gravity well than the other terrestrial planets.  It may take Mercury surface-based equipment  to really refine such a precession speculation, but the images may give it a stong analysis from the Messenger orbiter.

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Living on Mercury -- the Science of Science Fiction?

You need water. Messanger is designed to be able to detect the possible presence of water in the form of ice on the surface of Mercury. For an abode, Mercury is "better" than Venus. A lot better. Mankind might someday land a spaceship on Mercury, and work on the surface of Mercury. The spalling of the surface by solar wind, even on the night-side will have to be countered by shielding, but perhaps armored suits could be designed for particular areas of spalling intensity relative to the protection offered by a planetary magnetic field.  It will be an advanture.

Mankind likely could inhabit (infest?) selected places near the surface -- providing he brings along sufficient good radiation protection. In the Polar regions of Mercury, there may be places where the subsurface temperature is reasonable for the construction of some habition. It would be under the surface, beneath it, both for radiation protection and to stay cool. If man can work on the surface of the Moon or near it-- he can work on the surface of Mercury, or near it, living in some thermal equilibrium subsurface construction that has had 4.567 billion years to adjust to the increasing output of the Sun while staying cool.

A Mercury colony is a possiblity that could exist for a long time, since there are cool places on Mercury. This posting deals with the Sun’s future, and the eventual evaporation of Mercury. . .

http://www.xomba.com/the_suns_future_a_brief_outline

Could you live on Venus: No . . .Think about it. Man will never be able to do much more than several hours of work on the surface of Venus, at any one time. A “worker” would have to “leave the worksite” to dissipate the heat -- since there is no place to do so on the surface of Venus. How could something of human scale wrapped in a suit to cool him --work in 900° F. temperatures. You haul the heat away from him, away from his body and put it . . . where? Eventually Earth will be come as Venus is now. The poles of Venus are basically at the same, much hotter than an oven temperature every where. (sure it is a little cooler at the poles -- even if 300° F cooler (at 600° F), it would take some real engineering with a nuclear powered something to keep a human from cooking in seconds to minutes. (How do you like your human? Seared, scorched, or charcoaled completely? Crispy black? If you overcook by a few seconds. . .you could burn it. . .up.)

Mercury is a nicer place than Venus. Mercury is closer to the sun, rotates at 2/3 of the time it takes to orbit the Sun -- and does not have gravity enough to hold a significant atmosphere to it's surface. Unlike Venus, or Earth, Mercury's surface is not dominated by air. Venus has really heavy air, dominated by CO2, and is hotter than Mercury. The density of Venusian air, and the consequent pressure of the clear CO2 on its surface is 90 times the Earth's air pressure, and density. That is equivalent to the pressure [at 33 feet x 90 atmospheres = 2970 feet] nearly 3000 feet deep in the Ocean's of Earth. (1 atmosphere's worth of Earthly pressue = the pressure of a column of water 33 feet high. . .so . . .90 atmospheres . . . )

IMAGEs: NASA, JPL The 1991 and 1994 radar echo said it "looks like water. . .on Mercury."

From NASA: “Water ice on the surface of Mercury is exposed directly to vacuum, and will rapidly sublime and escape into space unless it is kept cold at all times. This implies that the ice can never be exposed to direct sunlight. The only locations on the surface of Mercury where this is possible would seem to be near the poles, where the floors of some craters might be deep enough to afford permanent shading. Whether such permanently shadowed craters exist on Mercury is still problematic.

Especially in light of the tilt of the axis of rotation to the plane of orbit. The discovery of ice on the Earth's moon can only serve to strengthen the arguments for ice on Mercury. The only close-up images we have of Mercury before Messenger images were those taken by the Mariner 10 on three close passes in 1974 and 1975. The same hemisphere of Mercury was sunlit on each of these passes, so nearly half the planet has never been imaged, and no determination can be made of what polar areas, if any, are permanently shadowed. However, theoretical studies assuming typical crater dimensions show that craters near the poles should have areas which never rise above about 102 K, and that even flat surfaces at the poles would not exceed about 167 K. Other studies also indicate that water ice in polar craters on Mercury could be stable over the age of the solar system. Image NASA http://www331.jpl.nasa.gov/group8/mercvla.html So how about Water. Is the water there? Personally, I think water is just a "hope" on Mercury. But, of course, I "hope" it is more than a "hope," and I hope there is water -- and like the Moon -- if there is water, it could only be at the "poles," in the shade. Surely, if we can build habitat on the Moon we can do the same on Mercury--but water is a problem until we get out to the icy worlds, so it is likely we would have to pack or manufacture water to "camp" on Mercury. That is what the miners would have to do.

IMAGE: National Radio Astronomical Observatories, Education

It is not water alone that is precious. But for life as we, it is. If you can "prospect," with a lot of possibly available water, it makes it easier to do it up close.

The excitement of prospecting for precious useful metals might go beyond the scientific "stimulus" to understand and know -- but many scientists are prospectors at heart. Some strike it rich. (You could call that Scientific "Stimulust.")

Being closer to the Sun when it formed, perhaps, there ought to be siderophile materials and metals (and others) aplenty on Mercury. Maybe close enough to the surface for easy mining, maybe deeper. . . Writer Jerry Pournelle, I think, but it could have been Larry Niven -- years ago had his science fictional people's society build Solar Collectors on Mercury that operated powerful launching lasers for use within the solar system, or to propel lightweight packages toward the stars. Mining on Mercury using solar energy makes sense. Messenger has some instrument packages upon it that in orbit might help us figure where to look for easy pickings! Messenger may tell us answers to some of these question during the next few years. And It will raise more and more questions.

Image: NASA, JPL      REMBRANDT IMPACT BASIN

http://photojournal.jpl.nasa.gov/jpeg/PIA12049.jpg

This is a late add "Comparing the Rembrandt impact basin on Mercury with the Northeastern United States.

And finally, Shadows from a Wider Sun

Vivaldi at sunrise.   Notice the fuzziness of the long shadows from the craters peaks. They hint the diameter of the Sun to be illuminating them much larger than shadows we are used to seeing shadows on the Moon. so they appear broader and their edges softer.