Meteor Crater Helps Unlock Planetary History
A single large impact (the primary) could launch up to a million pieces of ejecta ( the secondaries) that fall back down, forming additional craters. E-mail: [email protected] Contact: surfaces by determining impact craters density in an area. . out till date to understand the actual form of the SFD of. Massive Impact Crater Found Hidden Under Greenland Ice formed, but the research team is still working on the precise dating,” he said.
The age of the East Clearwater Lake crater is much more difficult to determine. In previous work performed by other scientists, a different isotope method was used to measure the age of the crater.
The rubidium Rb to strontium Sr ratio suggested that this crater is also around million years old, roughly the same age as the West crater. However, this method of dating is ratherunreliable when it comes to dating impact craters. This makes it more difficult to determine an accurateargon age, but suggests a maximum age of around million years, which would be far older than the dating of the West Clearwater Lake crater. Inresearchers initially calculated a million-year age for the East Lake, but then assumed it to be incorrect out of suspicion that excess argon was contaminating the sample and mimicking an older age for the crater.
However, Schmieder and colleagues also determined an argon age of to million years for the East crater. They consider it highly unlikely that four different rock samples that were collected at different locations and depths at the impact melt layer inside the crater would all yield the same false age.
Further evidence Another point in favor of the older age of the East crater comes from studying the magnetization of rocks. The magnetic poles of the Earth are not fixed, and pole reversals have occurred many times in the past. This superchron, known as the Permo-Carboniferous Reversed Superchron, lasted from to million years ago, which agrees with the age found by the argon dating.
The rocks from the East Lake tell a different story. They have a number of different magnetic polarizations, which indicate viscous remnant magnetization. This is magnetization that is acquired slowly over a long period of time.
Crater Count Dating: Self-Secondaries Reduce Age Estimates
The more complex magnetic history points to the rocks being much older than the West Lake, as they have had more time to be altered. The argon-argon age of to million years for the East crater suggests that this impact occurred in the Ordovician time period in a near-coastal environment, when large parts of eastern Canada were occupied by a shallow ocean.
There are geological clues that point towards an impact in a shallow marine or coastalenvironment at the East crater.
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The rocks from the East crater have more chlorine in them than the West crater, which might be indicative of the presence of sea water. There is also some evidence of the increased movement of hot fluids after the East impact, which altered the rocks. The West crater was formed during the Permian, when the asteroid would have struck the Pangaea landmass.
Despite the fact that it is statistically very unlikely for the two craters to have been formed in two separate impact eventsthe new evidence unearthed by Schmieder and his team shows that in this case the more unlikely scenario is true. In our view, there is a whole line of geologic evidence that argues against the double impact.
Asteroids in Deep Space ] The impact on life Impacts that leave behind a kilometer In fact, impacts can even increase biodiversity.
For example, the Great Ordovician Biodiversification Eventwhich saw an explosion in the number of animal species around million years ago, has been linked to frequent impact events at the time. This is possibly due to the fact that an impact could disrupt local life just enough to let another species dominate, or because slowly cooling craters can provide habitats for life.
Even if the Clearwater Lakes impacts were caused by a double impact, the extra energy released by two bodies smashing into the Earth simultaneously would have had no significant effect on life.
According to David H. LevyGene "saw the craters on the Moon as logical impact sites that were formed not gradually, in eonsbut explosively, in seconds. Shoemaker noted Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at the Nevada Test Sitenotably Jangle U in and Teapot Ess in InEdward C. Chao and Shoemaker identified coesite at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures.
Bythey had tentatively identified more than Although their work was controversial, the American Apollo Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon.
Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters. Play media A laboratory simulation of an impact event and crater formation Impact cratering involves high velocity collisions between solid objects, typically much greater than the velocity of sound in those objects.
Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. Extremely large bodies abouttonnes are not slowed by the atmosphere at all, and impact with their initial cosmic velocity if no prior disintegration occurs. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater.
Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives.
Since craters are caused by explosionsthey are nearly always circular — only very low-angle impacts cause significantly elliptical craters.
Impacts on porous surfaces, such as that of Hyperionmay produce internal compression without ejecta, punching a hole in the surface without filling in nearby craters. This may explain the 'sponge-like' appearance of that moon. In practice, there is overlap between the three processes with, for example, the excavation of the crater continuing in some regions while modification and collapse is already underway in others.Barringer Meteor Crater in Arizona Documentary
Contact and compression[ edit ] Nested Craters on Mars, These nested craters are probably caused by changes in the strength of the target material. This usually happens when a weaker material overlies a stronger material.
This contact accelerates the target and decelerates the impactor.
Because the impactor is moving so rapidly, the rear of the object moves a significant distance during the short-but-finite time taken for the deceleration to propagate across the impactor. As a result, the impactor is compressed, its density rises, and the pressure within it increases dramatically.
Peak pressures in large impacts exceed 1 T Pa to reach values more usually found deep in the interiors of planets, or generated artificially in nuclear explosions. In physical terms, a shock wave originates from the point of contact. As this shock wave expands, it decelerates and compresses the impactor, and it accelerates and compresses the target.
Surprise! Canadian Double Crater Formed by 2 Separate Impact Events
Stress levels within the shock wave far exceed the strength of solid materials; consequently, both the impactor and the target close to the impact site are irreversibly damaged. Many crystalline minerals can be transformed into higher-density phases by shock waves; for example, the common mineral quartz can be transformed into the higher-pressure forms coesite and stishovite.
Many other shock-related changes take place within both impactor and target as the shock wave passes through, and some of these changes can be used as diagnostic tools to determine whether particular geological features were produced by impact cratering.
The damage produced by the shock wave raises the temperature of the material. In all but the smallest impacts this increase in temperature is sufficient to melt the impactor, and in larger impacts to vaporize most of it and to melt large volumes of the target.
As well as being heated, the target near the impact is accelerated by the shock wave, and it continues moving away from the impact behind the decaying shock wave. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely subsonic.
During excavation, the crater grows as the accelerated target material moves away from the point of impact. The target's motion is initially downwards and outwards, but it becomes outwards and upwards.
Impact crater - Wikipedia
The flow initially produces an approximately hemispherical cavity that 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.
Ejecta thrown out of the crater do not include material excavated from the full depth of the transient cavity; typically the depth of maximum excavation is only about a third of the total depth.
As a result, about one third of the volume of the transient crater is formed by the ejection of material, and the remaining two thirds is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials, a significant crater volume may also be formed by the permanent compaction of the pore space. Such compaction craters may be important on many asteroids, comets and small moons. In large impacts, as well as material displaced and ejected to form the crater, significant volumes of target material may be melted and vaporized together with the original impactor.
Some of this impact melt rock may be ejected, but most of it remains within the transient crater, initially forming a layer of impact melt coating the interior of the transient cavity. In contrast, the hot dense vaporized material expands rapidly out of the growing cavity, carrying some solid and molten material within it as it does so.
As this hot vapor cloud expands, it rises and cools much like the archetypal mushroom cloud generated by large nuclear explosions. In large impacts, the expanding vapor cloud may rise to many times the scale height of the atmosphere, effectively expanding into free space.
Most material ejected from the crater is deposited within a few crater radii, but a small fraction may travel large distances at high velocity, and in large impacts it may exceed escape velocity and leave the impacted planet or moon entirely. The majority of the fastest material is ejected from close to the center of impact, and the slowest material is ejected close to the rim at low velocities to form an overturned coherent flap of ejecta immediately outside the rim.
As ejecta escapes from the growing crater, it forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to be largely ballistic.
Small volumes of un-melted and relatively un-shocked material may be spalled at very high relative velocities from the surface of the target and from the rear of the impactor.