Starship Asterisk*

clothesliner wrote:Suggesting that the smoothness of some craters implies volcanism seems like a stretch. The surface of Mercury looks like it is continually being bombarded and the smoothness could be caused by dust.

clothesliner wrote:Have any volcanic cones ever been found on Mercury?

http://tinyurl.com/ocoaxu wrote:
Date Acquired: January 14, 2008
Image Mission Elapsed Time (MET): Mosaic of 108826812 and 108826877
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Spacecraft Altitude: 10,500 kilometers (6,500 miles)

Of Interest: MESSENGER Science Team members are busy studying in detail the newly discovered volcanoes on Mercury. This figure, recently published in Science magazine, shows a NAC mosaic of the largest volcano currently identified on Mercury and a geologic sketch map of the major features in the surrounding area. The “irregularly-shaped depressions” are believed to correspond to volcanic vents, and the “margin of the dome-like feature” marks the outer limits of lava flows from the vents that are thought to have covered up the underlying surface of “hummocky plains.” The unlabeled double line outlines bright material associated with the volcano, believed to be pyroclastic deposits ejected during volcanic eruptions at the vents. A “highly-embayed impact crater” also appears to have had lava flow up to its rim, while a slightly more distant impact crater is “relatively fresh” and unchanged by any lava. The volcano is located just inside the rim of the Caloris impact basin, labeled as “Caloris basin rim units” on this map. Maps such as this are aiding scientists as they work to understand the history of volcanism on Mercury.

• Craters dot the surface of a lunar mare, or basalt sea, on the dark side of the moon.
  
  By counting craters, scientists have determined that volcanoes were active
  on the dark side of the moon far later than previously thought.
  The fewer craters on a surface, the younger it is.

anewc2 wrote:What leaps out at me from this one is the three structures that appear to be concentric craters,
with the outer about twice the diameter of the inner. What's up with that?

http://en.wikipedia.org/wiki/Impact_crater wrote:
<<Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for a large impact. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely sub-sonic. During excavation, the crater grows as the accelerated target material moves away from the impact point. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity. The cavity continues to grow, eventually producing a paraboloid (bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has reached its maximum size, it is called the transient cavity.

In most circumstances, the transient cavity is not stable: it collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is some limited collapse of the crater rim coupled with debris sliding down the crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains bowl-shaped and superficially similar to the transient crater. In simple craters, the original excavation cavity is overlain by a lens of collapse breccia, ejecta and melt rock, and a portion of the central crater floor may sometimes be flat.

Above a certain threshold size, which varies with planetary gravity, the collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a complex crater. The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound which is a process in which a material with elastic strength attempts to return to its original geometry; rather the collapse is a process in which a material with little or no strength attempts to return to a state of gravitational equilibrium.

Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls. At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an impact basin rather than an impact crater. Complex-crater morphology on rocky planets appears to follow a regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for example Tycho; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called peak-ring craters, for example Schrödinger; and the largest craters contain multiple concentric topographic rings, and are called multi-ringed basins, for example Orientale. On icy as opposed to rocky bodies, other morphological forms appear which may have central pits rather than central peaks, and at the largest sizes may contain very many concentric rings – Valhalla on Callisto is the type example of the latter.>>

orin stepanek wrote:What I first noticed was all the rubble laying around. To me that suggests volcanic activity. I would tend to think the heat from the sun would cause these to crumble into dust after a while. So maybe this is fairly recent; say maybe just a few million years.

• Maximum surface temperature on Mercury: 700° K
  
  The temperature of earth lava runs about 1000° to 1,500° K.

neufer wrote:

orin stepanek wrote:What I first noticed was all the rubble laying around. To me that suggests volcanic activity. I would tend to think the heat from the sun would cause these to crumble into dust after a while. So maybe this is fairly recent; say maybe just a few million years.

Well...there is no water to evaporate and dry things out
or (on the night side) to freeze and crack apart rocks.

And there is no wind for sandblasting like on Mars.

So about all that can happen is melting:

• Maximum surface temperature on Mercury: 700° K
  
  The temperature of earth lava runs about 1000° to 1,500° K.

orin stepanek wrote:I'm not quite convinced Art. You may be right; but look at the Moon; a lot of dust.

orin stepanek wrote:True it's very old; but eventually the rock will deteriorate. Maybe my time line is off; but I think a few million years ought to do it.

http://en.wikipedia.org/wiki/Moon_rock wrote:
<<In general, the rocks collected from the Moon are extremely old compared to rocks found on Earth. They range in age from about 3.16 billion years old for the basaltic samples derived from the lunar maria, up to about 4.5 billion years old for rocks derived from the highlands.>>

orin stepanek wrote:I'm not a scientist; so it is just speculation on my part. True there is very little atmosphere; so most of the wind is probably solar. It probably also depends on the type of rock. Did you ever build a fire on a rock bed, and after the fire was out notice some of the rocks were cracked? No water, no wind, just temperature change.

• Nope.

apodman wrote:The Space Mission Acronym List and Hyperlink Guide lists many horrible contrived acronyms for space probes but does not include MESSENGER. Maybe it was too horrible and contrived even to be listed among its peers. Since "messenger" is a word in common usage, they could drop most of the capital letters, but apparently the originators are so proud of "MErcury Surface, Space ENvironment, GEochemistry, and Ranging" that they have to dominate every line of text where it appears to rub it in.

orin stepanek wrote: http://www.youtube.com/watch?v=Ufs0mnE4Ocs

Looks like dust to me.

neufer wrote:

orin stepanek wrote: http://www.youtube.com/watch?v=Ufs0mnE4Ocs

Looks like dust to me.

orin stepanek wrote: http://www.youtube.com/watch?v=Ufs0mnE4Ocs

Looks like dust to me.

http://en.wikipedia.org/wiki/Lunar_soil wrote:
<<Lunar soil is the fine regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. It is essentially devoid of moisture and air, two important components found in soil on Earth. The term lunar soil is often used interchangeably with "lunar regolith" but typically refers to the finer fraction of regolith, that which is composed of grains one centimeter in diameter or less. Some have argued that the term "soil" is not correct in reference to the Moon because soil is defined as having organic content, whereas the Moon has none. However, standard usage among lunar scientists is to ignore that distinction. Lunar dust generally connotes even finer materials than lunar soil, the fraction which is less than 30 micrometres in diameter.
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The major solar weathering processes involved in the formation of lunar soil are:

* Comminution: breaking of rocks and minerals into smaller particles;

* Agglutination: welding of mineral and rock fragments together by micrometeorite-impact-produced glass;

* Solar wind spallatation and implantation: sputtering caused by impacts of high energy particles; and

* Fire fountaining: deposition of dark-mantled (DM) deposits, such as the shorty crater orange soil.
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The significance of acquiring appropriate knowledge of lunar soil properties is great. The potential for construction of structures, ground transportation networks, and waste disposal systems, to name a few examples, will depend on real-world experimental data obtained from testing of lunar soil samples. The load-carrying capability of the soil is an important parameter in the design of such structures on Earth.

Due to a myriad of meteorite impacts (with velocities in the range of 20 km/s), the lunar surface is covered with a thin layer of dust. The dust is electrically charged and sticks to any surface it comes in contact with. Soil is commonly said to become very dense beneath the top layer of regolith.

Other factors which may affect the properties of lunar soil include large temperature differentials, the presence of a hard vacuum, and the absence of a significant lunar magnetic field (thereby allowing charged solar wind particles to continuously hit the surface of the moon). A weaker gravitational force and the absence of an atmospheric pressure are additional factors which will affect the design of structures on the surface of the Moon.
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. Moon fountains and electrostatic levitation

The Moon appears to have a tenuous atmosphere of moving dust particles constantly leaping up from and falling back to the Moon's surface, giving rise to a "dust atmosphere" that looks static but is composed of dust particles in constant motion. The term "Moon fountain" has been used to describe this effect by analogy with the stream of molecules of water in a fountain following a ballistic trajectory but appearing static due to the constancy of the stream. According to the model recently proposed by Timothy J. Stubbs, Richard R. Vondrak, and William M. Farrell of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center, this is caused by electrostatic levitation. On the daylit side of the Moon, solar ultraviolet and X-ray radiation is so energetic that it knocks electrons out of atoms and molecules in the lunar soil. Positive charges build up until the tiniest particles of lunar dust (measuring 1 micrometre and smaller) are repelled from the surface and lofted anywhere from meters to kilometers high, with the smallest particles reaching the highest altitudes. Eventually they fall back toward the surface where the process is repeated over and over again. On the night side the dust is negatively charged by electrons in the solar wind. Indeed, the fountain model suggests that the night side would charge up to higher voltages than the day side, possibly launching dust particles to higher velocities and altitudes. This effect could be further enhanced during the portion of the Moon's orbit where it passes through Earth's magnetotail. On the terminator there could be significant horizontal electric fields forming between the day and night areas, resulting in horizontal dust transport - a form of "moon storm".

This effect was also predicted in 1956 by science fiction author Hal Clement in his short story "Dust Rag" published in Astounding Science Fiction. Also in 1956, the American scientist Thomas Townsend Brown appears to have predicted a similar lofting-falling cycle of photoelectrically excited lunar dust.

There is some evidence for this effect. In the early 1960s before Apollo 11, Surveyor 7 and several subsequent Surveyor spacecraft that soft-landed on the Moon returned photographs showing an unmistakable twilight glow low over the lunar horizon persisting after the Sun had set. Moreover, the distant horizon between land and sky did not look razor-sharp, as would have been expected in a vacuum where there was no atmospheric haze. Apollo 17 astronauts orbiting the Moon in 1972 repeatedly saw and sketched what they variously called "bands," "streamers" or "twilight rays" for about 10 seconds before lunar sunrise or lunar sunset. Such rays were also reported by astronauts aboard Apollo 8, 10, and 15. These may have been similar to crepuscular rays on Earth.

• lunar "twilight rays" sketched by Apollo 17 astronauts
  

Apollo 17 also placed an experiment on the Moon's surface called LEAM, short for Lunar Ejecta and Meteorites. It was designed to look for dust kicked up by small meteoroids hitting the Moon's surface. It had three sensors that could record the speed, energy, and direction of tiny particles: one each pointing up, east, and west. LEAM saw a large number of particles every morning, mostly coming from the east or west—rather than above or below—and mostly slower than speeds expected for lunar ejecta. Also, a few hours after every lunar sunrise, the experiment's temperature rocketed so high—near that of boiling water—that LEAM had to be turned off because it was overheating. It is speculated that this could have been a result of electrically-charged moondust sticking to LEAM, darkening its surface so the experiment package absorbed rather than reflected sunlight.

It's even possible that these storms have been spotted from Earth: For centuries, there have been reports of strange glowing lights on the Moon, known as "Transient lunar phenomenon" or TLPs. Some TLPs have been observed as momentary flashes—now generally accepted to be visible evidence of meteoroids impacting the lunar surface. But others have appeared as amorphous reddish or whitish glows or even as dusky hazy regions that change shape or disappear over seconds or minutes. These may have been a result of sunlight reflecting off of suspended lunar dust.>>