Meteorites delivered gold to Earth - BBC News
made by such meteorites in the present geological period. the surface environments of the planets. Moreover, if the rate .. the asteroid belt. The dating of lunar rocks has led to an .. The moon was intensely bombarded early in its history when the larger Distributed to the book trade by Random House.,ear of service. Containing the most up-to-date information on Mars, this book is an important reference Meteorites: A Petrologic, Chemical and Isotopic Synthesis The Early Heavy Bombardment Period includes the period from planet formation that mid-latitudes can experience these trade winds during the winter, and this is. utterly dominates the survival-times of planetary surface rocks and that erosion akin to . of this process in comparison with meteorite bombardment was .. parison of these two objects is thus difficult as detailed trade-offs one of our velocity predictions to date can be approximated within. 10% by the.
The new work details results of analyses of agglutinates returned from several lunar landing sites. Their study measured both the amounts of hydroxyl present and its isotopic composition. A normal atom of hydrogen is a single proton and an electron. But in a rare form of hydrogen, called deuteriumthe nucleus contains both a proton and a neutron. The authors of this study suggest that the hydroxyl found on the Moon was created when a small impact flash heated the soil, releasing the adsorbed hydrogen and chemically reducing the metallic oxides in the soil into native metal found as extremely tiny grains on the surfaces of the agglutinates and hydroxyl molecules.
Multiplied by billions, such a process could account for the generation of water on the lunar surface. Subsequent migration of these molecules toward cooler-than-average areas of the Moon i. In the view of the authors of this study, lunar water comes mostly but not entirely from the Sun. This constant process, occurring on the sunlit hemisphere of the Moon, could create an enormous reservoir of hydroxyl molecules in motion due to their thermal instabilityslowly but constantly moving toward the poles.
If such a process occurs on the Moon, one might expect the accumulation of water in every location where water is stable i.Dwarf Planet Ceres
But it appears that ice at the poles is not uniformly distributed, occurring in high concentration in some areas while absent in others. This pattern suggests that the source of polar water might be controlled by a non-equillibrium process, such as episodic bombardment by asteroids and comets.
Such a measurement does not require the return of a polar ice sample to the Earth. It could be made remotely in situ on the Moon with a properly instrumented robotic spacecraft. It is important to emphasize that although the quantities of water generated by this process are potentially very large, the hydroxyl in agglutinate glass should not be considered an economic resource.
These molecules occur globally but at very low levels of concentration tens of ppm. Even if this water is the primary and ultimate source reservoir of lunar water, the migration of the molecules and their subsequent collection by the cold traps near the poles serve as a concentrating mechanism, where ice accumulates in large quantities, confined within small areas — the classic definition of an ore body.
What a change has occured in the mindset the lunar science community in the past few years! From a bone-dry lump of rock in space to a complex, still mysterious body with a dynamic hydrological cycle. Also, the Southern Hemisphere summer features more dust devils and dust storms than the Northern Hemiosphere summer, meaning the Southern Hemisphere summer is dustier and the surface is slightly shadier, also meaning the summer is cooler.
This means that, even in the relatively short Southern Hemisphere summer, temperatures are not going to get above K for long enough to sublimate away all of the carbon dioxide ice. The permanent carbon dioxide ice remains about 8 m thick through the summer.
Sublimation pits have long been observed on the South Polar Cap, where carbon dioxide sublimates explosively in geysers, sometimes pulling dust up with it. These steep-sided pits consistently show flat floors about 8 m below the surface ice.
These floors evidence water ice. So, the South Polar Cap has a residual carbon dioxide cover about 8 m thick on top of a permanent water ice core. This water ice core probably saw some basal melting in the past, as seen in imagery of stream channels emerging from below the ice. This creates at least some plausibility for the Argyre to Ares fluvial system, or Chryse Trough system proposed by Timothy Parker.
The South Pole Cap dominates the large air pressure swings in the atmosphere. At the Viking 1 landing site in Chryse Planitia, air pressure varied annually over a range from 6. The same thing would happen in the North Polar Cap's spring and summer, but the effect was smaller.
So, the southern cap has a stronger effect on the semi-annual march of air pressures on Mars, because the CO2 ice is more extensive than on the northern cap, and the winter there is longer and colder than the northern cap due to the exaggerated ellipticity of the planet's orbit interacting with the marked tilt in the axis. Variations in crater density The third order of relief includes regions smaller in extent than most of the second order features, though some are very large, as large or larger than many second order features already described.
As mentioned earlier, they do not "nest" within second order features though they do within the first orderas I reserved the second order as the level of really conspicuous large features of the planet.
Third order features are broad regions, but they are not visually conspicuous in the way of, say, Syrtis Major or the seasonal polar ice caps. They are all named as: Terra "extensive land mass" Planum "a plateau or high plain" Planitia "a lowland or low-lying plain" It is at this order that we can clearly see the variations in crater density, size, and condition, which are used to establish relative dating on the martian surface.
In discussing the third order of relief, then, I'll first cover the crater-counting system of relative aging and then the epochs of martian geology. Each epoch will be used to frame the third order landscape features. Crater-counting The idea here is that the longer a planetary surface has been around, the more "opportunity" it has to be the target of solar system debris.
This debris consists of the small dust grains to planet-sized objects that have accreted, largely through gravitational attraction, out of the planetary gas and dust nebula and disk that surrounds the proto-sun and sun. There is a magnitude-frequency relationship here, similar to what we see with many other hazards: The smaller impact events are vastly more common than the larger ones.
Doing this as a log-log chart, the association, ideally, forms a straight line, with slope b. The older the surface is, the higher a will be. The curve for an older surface will have the same slope but its height on the chart will be greater. Past a certain point, though, you reach saturation, a level of bombardment so severe, a landscape so old, that there is literally no more room for a new crater: Each new crater necessarily obliterates traces of older craters. Once saturation is reached, it is no longer possible to say that one saturated landscape is older or younger than another saturated landscape.
Once saturation is reached, all you can say is that surface is crazy-old, on Mars, over 4 billion years old. To do a crater count study, you need to calculate the area of your study area and normalize it so that counts can be scaled to a common areal base: A common system Hartmann and Neukum uses a square kilometer.
Then, you identify every crater on your image, recording its diameter in meters or kilometers.
Meteorites delivered gold to Earth
Then, you establish size bins: The common standard is an X axis with each bin's upper boundary equal to the lower boundary times the square root of 2.
The next one would be 1. After you have your size bins, you compare each of your crater diameter measurements to your bins and count up the craters that fall within each of the bins and then convert the counts so that they are proportional to 1 km2, instead of the original size of your actual study area. So, if your study area were km2, you'd divide your counts by and, yes, it seems weird to count the number of 5 km wide craters in a 1 km2 standardized area. That done, you plot the adjusted number of craters in each bin on the Hartmann-Neukum "isochron" graph, available at http: You'll find that the pattern of dots you plot at the intersection of the middle of the bins and the number of craters per square kilometer will align roughly with one of the dotted or solid lines on the isochron plot.
This can be very roughly: Typically, the rightmost dots, especially, are more widely divergent from the isochrons. The counts in the larger bins are smaller and smaller, so you get statistical small-sample effects that allow the dots to range pretty far afield. The long, straight solid line is saturation somewhere past 4 Gy. The longer of the two short solid lines represents the boundary between the Noachian Epoch and the Hesperian and the shorter, lower of the two short lines represents the boundary between the Hesperian and the Amazonian about which, later.
By looking at the height of the line your craters align with, you can estimate the relative age of your study area Noachian, Hesperian, or Amazonian and put some constraints on the absolute age of that surface, based on an elaborate adjustment of lunar cratering rates with corrections for Mars location in the solar system, its greater gravity, and its atmosphere.
There are a few "plot complications" with the use of the crater magnitude and frequency distribution for the estimation of absolute ages on Mars. If you look very closely at the dotted isochrons, you will see that they do not form completely straight lines: They turn down somewhere around 64 km. This reflects the drop in the supply of humongous potential impactors after about 3.
The LHB is a point of some controversy: Did it simply mark the end of the era of accretion and the removal of available big impactors by their making themselves unavailable by, well, impacting into something in the solar system? Was there a tumultuous and dramatic increase in the number of big items stirred up in the solar system about 4.
The exact meaning of the LHB is controversial but its existence is not: Things really quieted down in the inner solar system after about 3. If you look at the other end of the X axis, you'll see a much steeper turn upward at roughly and variably 1 km in crater diamter.
This has really been controversial. Some argue that there really is a break in the size of potential impactors, because there really is a qualitative break in the numbers of smaller objects. Others suspect that the upward break in the curves reflects secondary impacts: Ejecta that lands at various distances from the primary crater, creating craters of their own.
There's a whole cottage industry in trying to figure out ways of differentiating secondary craters from primary ones just to get a handle on how many of them there are and how their presence may distort estimated ages of a surface. They may have different shapes or different depth to diameter relations than primaries because they would be coming in at less than supersonic speeds but that is true mainly for the secondaries that fall close in; those that get tossed out a far way may well attain very high velocities coming back to ground.
They seem to have a propensity for falling in distinct lines or rays. Fresh craters generate rays of finer ejected materials interspersed with bigger objects. The rays may erode away on Mars but the alignments of the secondary craters may preserve that rayed appearance this is the subject of my own research on Mars, using statistical techniques to pick out potential alignments of craters that might identify secondaries.
If you look still farther to the left of the X axis, you'll notice yet another inflection point in the isochrons around m, where the lines curve back down a bit.
This probably reflects one or more of the following: The power law seems to work with a slope of Outside that range, b would be larger and of different magnitudes at either end of the X scale about Neukum tried to get around this by using higher order polynomial modelling, but he and Hartmann reconciled their different approaches to develop that isochron chart linked above.
So, there's now a more or less standardized approach to calculating relative ages and constraining absolute ages, but there remain all kinds of controversies over secondary cratering. So, variations in crater density and size distributions is converted into a periodization scheme for Mars. Unfortunately, the scheme most commonly used maddeningly departs from the system developed for geological time on Earth.
Here's a quick overview of geological time and rock units on Earth. A distinction is made between geological time and geological rock units: At the coarsest level is the eon time unit, which is associated with eonothem rock units.
On Earth, there are four of these: Some periods are subdivided even further into ages or the corresponding rock stages e.
This was a time of progressive loss of surface waters and most of the atmosphere after the collapse of the planetary magnetic field.
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Water photodissociated in the atmosphere, freeing its hydrogen to scoot off into space from the exosphere and drawing the heavier oxygen to bind with iron-bearing minerals "rust" in dry conditions. In what follows, we'll use the traditional crater-counting periodization but with attention paid to the geochemical issues at the heart of the Bibring et al. The oldest From the earliest formation of the planet through the gravitational accretion, collision, and consolidation of planetesimals, asteroids, comets, meteoroids, and dust.
Some people are dividing the traditionally understood Noachian into the "pre-Noachian" and the Noachian proper, with the pre-Noachian reserved for the time of planetary accretion, differentiation, and development of the planetary magnetic field. These folks would end the pre-Noachian at the point where crater saturation doesn't allow you to discern really old surfaces, a time by which the dynamo had clearly shut down the time of the Hellas and other huge impacts.
Traditionally, though, the whole period from the time of the planet's origins to the end of the Late Heavy Bombardment is referred to as the Noachian. So, the Noachian includes: The kinetic, compressional, and radioactive heating of the accreted materials Differentiation begins with melting of these materials and the "iron event," when iron, melting first, began to drift in blobs toward the center of the planet, pulling some siderophiles with it particularly nickel.
Formation of the mantle magma ocean. Formation of a crust on top of the magma ocean, in Mars' case, apparently quite a thick one, for reasons unknown. Mantle overturn because of the gravitational instability created when magnesium-rich olivine cumulates that crystallized out first at the hottest temperatures were overlain by denser iron-rich olivine cumulates that crystallized out later at a somewhat cooler temperature.
Initiation of the planetary magnetic field through motion in the outer, liquid iron-dominated core. The sustained bombardment of the differentiated planets as the solar sys once part of igneous and metamorphic rocks.
Zircon contains some uranium, thorium, and lead, the ratios among which has allowed them to be radiometrically dated to as old as 4. There's been a controversy more recently about the age of actual mafic rocks in Canada that might be as old as these zircons: These have been dated to 4. If you're on campus or logged into the library from home, you can view Barlow's map of martian age distibutions here: So, while Mars is geologically active, it's nowhere near the level of activity seen on Earth with its plate tectonism, and that has allowed the preservation of ancient surfaces on Mars and their obliteration on Earth except for those zircons and maybe the Canadian greenstones The constraint on the Noanchian timeframe is based on analysis and dating of Moon rocks from similarly cratered surfaces brought back to Earth by Apollo.
This is a fairly elaborate reasoning process. Rocks were taken back to Earth from the Moon by the Apollo astronauts from regions that had been previously relative-dated by crater-counting techniques. The returned rocks, then, allowed for an absolute date to be assigned to surfaces of previously described as of particular relative dates. Then, the size-frequency curve for the Moon had to be calibrated for use on martian surfaces, factoring in Mars' atmosphere which would both destroy more of the smaller objects and slightly reduce their incoming velocityMars' location closer to the putative source of orbiting debris in the solar system closer to the asteroid belt and to Jupiter, the gravity of which dislodges objects and puts them on new orbits, including orbits that intersect the inner solar system bodies.
You can get an overview of the Moon to Mars isochron correction system optional link for the curious: Characteristics of Noachian surfaces Noachian surfaces on Mars are intensely cratered: You do not see that on the Moon, which lacks such familiar geological activities as wind and water erosion, transport, and deposition. There was quite a bit of this geological work back in Noachian times: Valley networks are found almost exclusively on Noachian surfaces, showing fluvial action by what is more and more accepted as water, even precipitation-fed channelization.
Later in the Noachian, volcanic activity became increasingly concentrated in the two great volcanic rises, Tharsis and Elysium, which built up at this time.
The viscosity of lavas associated with the later volcanism allowed the construction of very tall shield edifices and, in some cases, ashy eruptions were part of the mix, which allowed the construction of steep sided tholi. The Late Noachian saw such extensive and massive volcanism that global geochemistry was drastically changed. Late Noachian and Early Hesperian geochemistry shows a strong sulfate signal, as volcanoes spewed out massive amounts of sulfuric acid, carbon dioxide, and water and created a strongly acidic aqueous chemistry.
This would explain the near lack of calcium carbonate on Mars: The presence of sulfate SO and sulfur dioxide SO2 prevents the formation of calcium carbonate and favors the formation of hydrated calcium sulfite CaSO3 - H2O instead, which can oxidize to create sulfates, iron oxides, and more acidity. Most of the arguments about possible oceans on Mars place it in the Noachian time frame, and, given the previous argument about sulfate chemistry, if those oceans were strongly acidified, the lack of calcium carbonate on the putative ocean floors becomes more comprehensible.
Noachis Terra, the prototype, is a large region to the west of Hellas and east and north of Argyre. This is a contender for the greatest crater density on Mars prize. Mariner 4 got images of Noachis Terra during its flyby, which created the then rather shocking image of Mars as a dead, dry planet much like the Moon. Subsequent closer looks showed it to be a lot more interesting: The craters themselves turned out to be pretty strange They often have softened rims and flattened floors, including some "ghost craters" that are so softened and infilled that they have practically vanished.
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Softened craters turned out not to be the result of standard-issue erosion and deposition mechanisms: It's as though entire landscapes of old craters sagged, spread out, and flattened, but new craters haven't. This suggests that there was a lot of soil moisture and ice back then, which could flow, deform, and relax, softening the look of the ancient craters.
Pedestal and rampart craters were found here, too. These look like impacts into surfaces loaded with ice, which vaporized and liquefied on impact, creating that "wet splat" look.
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The ejecta blankets appear to have solidified as a particularly resistant material, which functioned kind of like a cap rock of resistant material. Erosive agents attacked the surrounding landscape, but the area under the ejecta blankets was protected from whatever the regionally dominant erosive agent was, leaving the crater and its ramparts of ejecta perched high above the worn-down landscape, kind of like mesas with holes punched in the top.
Drainage networks that looked like fluvial systems on Earth showed that water or some other similar fluid ran over martian landscapes and eroded them. Long networks featuring several tributaries, most of them fairly short with few of their own tributaries, such as Nirgal Vallis Several smaller drainage basins with relatively long tributaries and drainage densities larger than the Nirgal Vallis system's but smaller than typical for Earth catchments and with nowhere near the degree of interfluve dissection common on Earth The origins of such valley networks have long been contentious.
Some authors argue for a precipitation-fed runoff history and the evidence their existence gives to arguments that Noachian Mars had higher atmospheric density and warmer temperatures, allowing at least for snow to fall and liquid water to exist long enough to flow overland into drainage channels e.
This would pertain to the dendritic drainages. Others have pointed out that most such networks have fewer, shorter tributaries than most Earth valley networks and that many of the short tributaries originate in alcoves or theater-shaped headwalls most akin to the slope morphologies of groundwater sapping-fed networks in arid Earth environments e.
Aonia Terra southwest of Noachis Terra and Argyre Planitia Its central areas are classic Noachian landscapes, highland cratered units with many small dendritic valley networks. Much of the Aonian cratered landscape shows signs of being subdued in contrasts, very akin to the crater softening and flattening seen in the discussion of Noachis Terra.