Lunar and Planetary Institute







Video Simulations of Impact Cratering Processes

Introduction by
Ross W. K. Potter and David A. Kring

Impact cratering is a fundamental geologic process that has affected all bodies within the Solar System. Craters on solid planetary surfaces are divided into three groups based on their morphology, which varies as a function of diameter: simple craters, complex craters, and impact basins. The formation process for each crater type is illustrated below using computer models. The effects of a number of impact parameters, including impactor size, surface gravity and target temperature, on the cratering process are also highlighted.

SIMPLE CRATERS

Simple craters

The 2.2 kilometer lunar simple crater Linné [LROC image (NASA/GSFC/Arizona State University); color coded shaded relief map (NASA/GSFC/Arizona State University); Apollo 15 image (AS15-9348)].

A simple crater: The canonical formation
Impact details:
Simple crater (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 100 m
Impact velocity: 15 km/s
Gravity: 1.61 m/s2
Target material: Lunar crust (2700 kg/m3 density)

Simple craters are characterized by a straightforward bowl-shaped cavity. The following video illustrates the formation of a simple crater on the Moon. The impactor has a density suitable for a number of different meteorite groups (~3300 kg/m3). The target has a density suitable for the lunar crust (~2700 kg/m3). Overlaid on the target is a grid of cells to highlight material displacement during the crater-forming process. On impact, a cavity forms as material is displaced and excavated from the impact site. At the edge of the expanding cavity, material is overturned at the fold hinge creating an ejecta curtain that rises over one kilometer above the surface. The cavity reaches its maximum depth of ~600 meters 7.5 seconds after impact. The cavity, however, continues to grow laterally reaching its maximum volume 23.5 seconds after impact. At maximum volume, the cavity is referred to as the transient crater.  After reaching its maximum volume, fractured and displaced material on the crater walls slumps under gravity into the crater center creating a breccia lens. At the same time, material in the ejecta curtain lands back on the target surface forming the crater rim, which is elevated above the target surface, and an ejecta blanket. In this scenario, a crater ~2.4 kilometers in diameter with a depth of 600 meters, similar to the dimensions of Linné crater, is formed within ~60 seconds of the initial impact. 

Simple Moon video

The canonical simple crater formation (above). Single click on the image to open the video.

Other sites of interest:


Simple craters: Altering impact parameters

What happens to the crater formation process described above if the impactor size, impactor density, and impact velocity are changed? The following videos illustrate the effect of these parameters on the formation process and the ultimate size of the crater.

A simple crater: Altering impactor size
Impact details:
Simple crater (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 200 m
Impact velocity: 15 km/s
Gravity: 1.61m/s2
Target material: Lunar crust (2700 kg/m3 density)

The following videos highlight how altering impactor size affects the cratering process. Here, the impactor is twice the diameter of the canonical scenario above (200 meters compared to 100 meters); all other parameters are kept constant. The impactor, therefore, has 8 times the volume and 8 times the energy of the canonical impact. In this scenario, the cavity created on impact is much deeper and wider; the cavity reaches a maximum depth of 1.32 kilometers after 23 seconds and its maximum volume after 30 seconds. Collapse of crater wall material into the crater center is more pronounced in this scenario and reduces the crater's depth by ~300 meters. Here, the final crater is ~4.6 kilometers in diameter, similar in size to the lunar crater Armstrong and roughly twice that of the canonical impact above, and is formed in ~150 seconds.

Simple Moon Double DiamImpactor Simple Moon

Impact of the 200 meter diameter object (left) and the canonical 100 meter diameter object (right). Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

Crater Armstrong

4.6 kilometer lunar simple crater Armstrong (Lunar Orbiter V Frame 5074)


A simple crater: Altering impactor velocity I (a slower impactor)

Impact details:
Simple crater (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 100 m
Impact velocity: 10 km/s
Gravity: 1.61 m/s2
Target material: Lunar crust (2700 kg/m3 density)

The following videos highlight how altering impactor velocity affects the cratering process. Here, the impact velocity is two-thirds that of the canonical scenario (10 km/s instead of 15 km/s); all other parameters are kept constant. This impact, therefore, has ~44% of the kinetic energy of the canonical scenario. On impact, the cavity created is shallower and smaller than the canonical impact. The cavity reaches a maximum depth of 480 meters after 8 seconds; the maximum volume is reached after 21 seconds. Collapse of crater wall material into the crater center shallows the crater floor by ~30 meters. The produced crater is 1.8 kilometers in diameter, ~70% the diameter of the canonical scenario.

Simple Moon Velocity Simple Moon

10 km/s impactor velocity (left) and the canonical 15 km/s impactor velocity (right). Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

The crater produced by this type of impact is similar in size to the lunar crater St. George, which was at the Apollo 15 landing site.

Other sites of interest:


A simple crater: Altering impactor velocity II (a faster impactor)

Impact details:
Simple crater (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 100 m
Impact velocity: 35 km/s
Gravity: 1.61 m/s2
Target material: Lunar crust (2700 kg/m3 density)

The following videos highlight how a faster velocity (35 km/s), suitable for a short-period comet, affects the cratering process. Here, the impact velocity is over twice that of the canonical scenario; all other parameters are kept constant. This impact, therefore, has ~5.5 times the kinetic energy of the canonical scenario. On impact, the cavity created is deeper and larger than the canonical impact. The cavity reaches a maximum depth of 830 meters after 9.5 seconds; the maximum volume is reached after 25 seconds. Collapse of crater wall material into the crater center begins after ~45 seconds, shallowing the crater floor by ~80 meters. The produced crater is 3.3 kilometers in diameter, almost 40% greater in diameter than the canonical scenario.

35 km/s impactor velocity 15 km/s impactor velocity

35 km/s impactor velocity (left) and the canonical 15 km/s impactor velocity (right). Single click on the images to open the files.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.


A simple crater: Altering impactor density

Impact details:
Simple crater (Moon)
Impactor material: Iron (7800 kg/m3 density)
Impactor diameter: 100 m
Impact velocity: 15 km/s
Gravity: 1.61 m/s2
Target material: Lunar crust (2700 kg/m3 density)

The following videos highlight how altering impactor density affects the cratering process. Here, the impactor material is iron, instead of dunite (7800 kg/m3 density compared to 3300 kg/m3); all other parameters are kept constant. This impact, therefore, has ~2.4x more energy and mass than the canonical impact. In this scenario, the cavity reaches a maximum depth of 790 meters after 7.5 seconds; the maximum volume is reached after 23.5 seconds. The collapse of crater wall material into the crater center is completed ~120 seconds after impact. The final crater is 2.8 kilometers in diameter, less than 20% greater in diameter than the canonical scenario.

Simple Moon Iron Impactor Simple Moon

Iron impactor (left) and the canonical dunite impactor (right). Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

Geological (field) studies of craters as well as cratering experiments have shown that simple craters have a crater depth to diameter ratio of 0.2-0.25. The relationship between crater depth and diameter for the simple craters modeled here is shown below. These results are consistent with the field studies and cratering experiments.

Crater Depth

Crater depth against crater diameter for the 5 simple crater model simulations described above.


COMPLEX CRATERS

Complex craters

Lunar central peak complex craters (from left to right): 28 kilometer diameter Euler (AS17-2923); 86 kilometer diameter Tycho (NASA/GSFC/Arizona State University); 93 kilometer diameter Copernicus (AS17-151-23260).

Is there any difference in the impact process between Solar System bodies, such as planets? This can be investigated by altering the surface gravity in computer simulations. The following video illustrates the impact of the 200 m diameter impactor scenario described above using Earth’s gravity (9.81 m/s2) instead of lunar gravity (1.61 m/s2).

Complex craters: Altering surface gravity
Impact details:
Complex crater (Earth)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 200 m
Impact velocity: 15 km/s
Gravity: 9.81 m/s2
Target material: Earth crust (2700 kg/m3 density)

On impact, the cavity that forms is smaller than its counterpart on the Moon; it reaches a maximum depth of 990 meters after 8.5 seconds and a maximum volume after only 10 seconds. After 10 seconds the crater floor begins to rise, forming a central uplift that decreases the depth of the crater center by ~600 meters.  Thus, the same size projectile produces a central peak complex crater on Earth, while forming a simple crater on the Moon.  The cratering process is complete within about 50 seconds, which is much faster than the comparable impact on the Moon. The scenario using Earth gravity forms a central peak complex crater 3.6 kilometers in diameter that is similar in size to Steinheim, Germany and Flynn Creek, USA, craters on Earth.

Complex Earth Simple Moon Double Diam Impactor

200 meter diameter impactor hitting Earth (gravity = 9.81 m/s2) creating a complex crater (left) and hitting the Moon (gravity = 1.61 m/s2)
creating a simple crater (right). Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

Why does this set of impact conditions form a complex crater on Earth, but a simple crater on the Moon? This is due to the difference in gravity between the two bodies — Earth's gravity is ~6 times greater than that of the Moon. This means the impact energy has a greater gravitational force to overcome, hence the shallower transient cavity depth and final crater diameter on Earth. The central uplift is created by a strength threshold being exceeded beneath the crater, whereby gravity (rather than strength) becomes the dominant force uplifting the crater floor. The transition from simple to complex craters is inversely proportional to surface gravity. On Earth (9.81 m/s2 gravity) the transition is 2–4 kilometers, on Mars (3.7 m/s2 gravity) 5–10 kilometers, and on the Moon 15–20 km


Complex craters: Peak-ring craters

A peak-ring crater: Chicxulub
Impact details:
Peak-ring crater (Earth)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 14.4 km
Impact velocity: 12 km/s
Gravity: 9.81m/s2
Target material:  Carbonate platform sediments (2600 kg/m3 density) over granitic crust (2700 kg/m3 density) and mantle (3300 kg/m3)

As impact size increases, central peaks are replaced by a ring of massifs (mountains) referred to as a peak ring. The peak ring diameter is approximately half the crater diameter. The following video illustrates the formation process for the Chicxulub crater, Mexico — the best-preserved large-scale crater with a peak ring on Earth. The model setup follows that of the Chicxulub target site — a 2.8 kilometer thick calcite layer (limestone) on top of a 30 kilometer thick granite layer (collectively, these two layers comprise the crust), overlaying a dunite mantle.

On impact, the transient cavity opens up reaching its maximum depth of 32.4 kilometers 20 seconds after impact. Following this, the crater floor begins to rise; maximum cavity volume is reached after 55 seconds while the crater floor continues to rise. The central uplift surpasses the pre-impact target surface 2 minutes after impact and reaches a maximum height of ~15 kilometers at ~3 minutes. As the central uplift collapses, granitic material is spread out over the surface burying the calcite later.  After ~8 minutes the crater formation process is complete. The final crater has a rim-to-rim diameter of ~160 kilometers; the peak ring, formed by the collapsing central uplift, has a diameter of ~90 kilometers. At depth, the crater collapse process has uplifted the crust/mantle boundary by ~2 kilometers beneath the crater center and created a slight thickening (~1 kilometer) of crustal material 35 kilometers out from the center.

Chicxulub

Chicxulub-sized impact on Earth. Single click on the image to open the video.
 

The Chicxulub impactor has a mass and kinetic energy approximately 250,000 times (equivalent to 5 orders of magnitude) greater than the impactor that produced the central peak complex crater illustrated above. The greater energy results in a far larger transient crater and a more pronounced uplift of crater floor material, which ultimately collapses back into the target helping to form the peak ring. The magnitude of this impact event means a greater volume of target material is affected by the impact, including the crust/mantle boundary at a depth of 30 kilometers.

The Chicxulub impact occurred 65 million years ago and is famous for leading to the extinction of the dinosaurs.

Other sites of interest:


A peak-ring crater: Chicxulub on the Moon

Impact details:
Peak-ring crater (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 14.4 km
Impact velocity: 12 km/s
Gravity: 1.61 m/s2

How would the Chicxulub impact differ if the event occurred on the Moon instead of the Earth? The following video illustrates the crater-forming process for a Chicxulub-sized impactor on the Moon. Under the Moon's weaker gravity, the transient cavity reaches a greater depth (40.5 kilometers) and reaches its (greater) maximum volume after a longer time period (170 seconds).  The central uplift also reaches a greater height. The peak ring is ~90 kilometers in diameter with the final crater ~210 kilometers in diameter, greater in size than the crater formed on Earth. On the Moon, the main impact processes for this scenario are complete after 20 minutes.

Chicxulub-sized impact on the Moon (left) and the Earth (right). Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

This impact scenario on the Moon produces a crater similar in size to the lunar crater Schwarzschild which has a diameter of 207 kilometers and a peak ring diameter of 71 kilometers.

Schwarzschild

Schwarzschild crater on the Moon (NASA Clementine image)


IMPACT BASINS

Impact Basins

Lunar impact basins (from left to right): 320 kilometer diameter Schrödinger (Clementine Mosaic), 930 kilometer diameter Orientale (LRO WAC Mosaic and LOLA topography) and 2400 kilometer diameter South Pole-Aitken (LOLA topography)

As impact energy continues to increase, so too does the size of the crater. The largest impact structures are known as basins. On the Moon, these are impact structures greater than 300 kilometers in diameter. The best-preserved 'small' basin on the Moon is Schrödinger (320 kilometers diameter). Schrödinger is also the second youngest impact basin on the Moon.

Other sites of interest:


A multi-ring basin: Orientale

Impact details:
Multi-ring basin (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 50 km
Impact velocity: 15 km/s
Gravity: 1.61 m/s2
Target thermal gradient: 10 K/km

At the very largest scale, impact basins are characterized by multiple ring structures rather than just a single peak ring structure. The best-preserved multi-ring basin on the Moon is the 930 kilometer diameter Orientale, which also happens to be the youngest impact basin on the Moon — it formed 3.8 billion years ago. The following video illustrates the formation of the Orientale basin. The mass of this impactor is 40 times greater than that of the Chicxulub impactor; its kinetic energy is nearly 65 times greater. The mass and kinetic energy of this impactor is ~40 times and ~17 times, respectively, greater than that forming the Orientale basin.

On impact, a transient cavity forms as material is excavated or displaced from the impact site. The cavity expands vertically and laterally, before the floor of the cavity begins to rise after 3 to 4 minutes. The cavity reaches its maximum volume (the transient crater) 4 minutes after impact. Following transient crater formation the cavity begins to collapse. After 9 minutes, the rising crater floor has formed a central uplift that has risen above the original target surface. The uplift continues to rise until 16 minutes, when it begins to collapse back into the Moon. During the rise of the central uplift the ejecta curtain, formed by the excavated material, begins to drape over the lunar surface creating an ejecta blanket. The collapsing uplift also overturns crustal material onto itself helping to form a slight bulge in the crustal thickness at a distance of ~250 kilometers from the basin center. The uplift has fully collapsed by 27 minutes, and is followed by a secondary, far smaller uplift. Uplift rise and collapse phases cease around 50 minutes. The complete basin-forming process for Orientale is finished within two hours of the initial impact.

Orientale

Orientale-sized impact on the Moon. Single click on the image to open the video.

Other sites of interest:


Multi-ring basins: The effect of impact velocity

Impact details:
Multi-ring basin (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 60 km
Impact velocity: 10 km/s and 20 km/s
Gravity: 1.61 m/s2
Target thermal gradient: 10 K/km

The following videos show the effect of impact velocity on the cratering process for an Orientale-sized impact. The video on the left uses an impact velocity of 10 km/s. The video on the right uses an impact velocity of 20 km/s. These velocities are typical of those expected around the time of Orientale’s formation. The videos illustrate material on the right hand side (crust in beige, mantle in gray, with the grid of cells overlaid) and temperature on the left hand side (blues are low temperatures, reds are high temperatures).

In the 10 km/s case, the crater reaches a maximum depth of 162 kilometers after three minutes, whereas at 20 km/s, the crater reaches a maximum depth of 218 kilometers after 4 minutes.  The transient crater forms after 4 minutes for the impact at 10 km/s and 5 minutes for the impact at 20 km/s. After transient crater formation, the craters begin to collapse as the crater floor rises. The central uplift that is created via this process is far wider in the faster impact and also reaches a greater maximum height (~260 kilometers compared to ~160 kilometers). As the uplift collapses, overturn of crustal material is more prominent in the faster impact. Enough energy is also still present in the faster impact for a secondary uplift/collapse phase to occur.  Again the final basins are produced within two hours of the initial impact. The faster impact creates a larger central zone of mantle material (250 kilometer radius) than the slower impact (~150 kilometer radius). The location of the thickened annular bulge of crustal material is also further away from the basin center in the faster impact (>300 kilometers compared to ~250 kilometers). Finally, the post-impact thermal conditions noticeably differ between the two scenarios. Temperatures are hotter (light to dark reds) around the basin center, and extend to greater depths in the faster impact scenario. Overall, the difference in impact velocity between these two scenarios produces basins with very different features and dimensions. 

Orientale-sized impacts on the Moon at 10 km/s (left) and 20 km/s (right). Colors represent temperatures: blues are low temperatures and reds are high temperatures. Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

Multi-ring basins: Orientale on the Earth
Impact details:
Multi-ring basin (Earth)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 50 km
Impact velocity: 20 km/s
Gravity: 9.81 m/s2
Target thermal gradient: 10 K/km

The following videos illustrates the effect of surface gravity on the formation process for a multi-ring basin. Here, the Orientale-sized impactor traveling at 20 km/s is modeled as if it hit the Earth. All other parameters are kept constant. Again, the video illustrates material on the right hand side (crust in beige, mantle in gray, with the grid of cells overlaid) and temperature on the left hand side (blues are low temperatures, reds are high temperatures).

On impact, the cavity reaches a maximum depth of 156 km, with the transient crater forming after only 2 minutes (the transient crater volume is ~25% that of the impact into the Moon). During crater collapse, the central uplift reaches a lower maximum height than the equivalent impact on the Moon and also involves the overturn of less crustal material. Uplift and collapse phases are completed within 30 minutes compared to ~70 minutes for the impact on the Moon. The impact into Earth produces a far smaller crustal annular bulge radius (~100 kilometers). Less crustal material is excavated in this impact, resulting in a far thicker layer of hot crustal material around the basin center.

Orientale Orientale

Orientale-sized impacts on the Earth (left) and the Moon (right), both at 20 km/s. Colors represent temperatures: blues are low temperatures and reds are high temperatures. Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.


The largest lunar basin: South Pole-Aitken

South Pole-Aitken

The South Pole-Aitken (SPA) basin (LOLA topographic map). SPA is defined by the region of low (blues and greens) topography.

Impact details:
Multi-ring basin (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 170 km
Impact velocity: 10 km/s
Gravity: 1.61m/s2
Target thermal gradient: 50 K/km

The largest impact basin on the Moon is the South Pole-Aitken (SPA) basin. This impact structure, located on the far side of the Moon, is ~2400 kilometers across, which is greater than the radius of the Moon. The following video illustrates the formation of this basin.

On impact, a transient cavity forms as material is excavated or displaced from the impact site. The cavity expands vertically and laterally, before the floor of the cavity begins to rise after 7 minutes. Despite this uplift, the transient cavity does not reach its maximum volume until 9 minutes after impact. At 15 minutes, the rising crater floor has formed a central uplift that is higher than the original target surface. The uplift continues to rise until 43 minutes after impact, when it begins to collapse back into the Moon. (The uplift height may be artificially higher than that in the natural impact event because of the computer techniques employed.) During the rise of the central uplift the ejecta curtain, formed by the excavated material, begins to drape over the lunar surface creating an ejecta blanket. The uplift has fully collapsed by 65 minutes. The collapse sends shock waves across the lunar surface altering topography. By 180 minutes the basin-forming process is complete. The impact has completely removed crustal material within a radius of ~600 kilometers from the impact site, leaving lunar mantle material exposed at the surface. Further out, excavated crustal and mantle material has been draped over the lunar crust to a radial distance of ~1200 kilometers from the basin center. The final basin diameter is ~2400 kilometers.

SPA material

South Pole-Aitken (SPA) basin-sized impact on the Moon. Colors represent material: beige (crust), gray (mantle), brown (core).
Single click on the image to open the video.

The largest lunar basin: Internal effects
The following video shows the same SPA basin-forming event, but illustrating how pressure within the Moon varies as the impact process takes place (shades of blue are low pressures; shades of red are high pressures). On impact (1 minute) the projectile fully penetrates the surface and begins to excavate and displace material. The energy of the projectile creates a shock wave (red crescent-shaped area). The shock wave travels through the Moon; as it expands outwards its energy is dispersed over a wider area and the intensity of the shock wave decreases. At 4 minutes, the shock wave passes into the Moon's core. After 7 minutes, the shock wave has passed through the core and has dispersed the majority of its energy. A lower-pressure secondary shock (seen above the low pressure (blue) area approximately half way through the Moon), continues through the Moon, with the shock waves merging together at the impact antipode after 13 minutes. Similarly, another shock front reaches the antipode at 16 minutes creating a transient pressure increase at this location. Pressures within the Moon settle after about 25 minutes. The next shock wave is created as the central uplift collapses back into the target at 57 minutes, however this wave quickly dissipates (by 61 minutes). The collapse of the central uplift also sends surface waves around the Moon distorting the surface topography (the surface waves have reached a radial distance of ~1000 kilometers after 77 minutes). After 120 minutes, post-impact pressures within the Moon have once again stabilized.

South Pole-Aitken (SPA) basin-sized impact on the Moon. Colors represent pressures: blues are low pressure, reds are high pressure.
Single click on the image to open the video.

The largest lunar basin: The effect of internal temperature
Impact details:
South Pole-Aitken basin (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 150 km
Impact velocity: 10 km/s
Gravity: 1.61 m/s2
Target thermal gradient: 10 and 50 K/km

The following videos illustrate the effect of target temperature on a SPA-sized basin-forming impact into the Moon. The videos show temperature (left hand side) and material (right hand side).

On impact, the transient cavities formed in both targets are comparable in size. The transient crater is slightly larger (~10%) in the warmer target, however. During transient crater collapse, crustal material is not uplifted in the cooler target. It remains cool, strong and rigid. Uplifted mantle material is draped over this crust as it collapses back into the target. Further, though lesser magnitude, uplift and collapse phases drape more mantle material over the crust. In the warmer target, crustal material is involved at the edges of the central uplift. As the uplift collapses, this crustal material is pushed away from the basin center. The final structure of these basins is quite different. In the cooler Moon, mantle material is draped over the surface to a radial distance of 1000 km from the basin center. The crust is buried beneath this mantle material and forms a bulge in crustal thickness at a radial distance of 300 kilometers from the basin center. In the warmer Moon, a continuous zone of mantle extends out to a radial distance of 500 kilometers from the basin center. A bulge in crustal thickness is less noticeable. The post-impact Moon is also hotter over a greater extent (both radially and vertically) in the originally warmer Moon.

SPA material SPA cold moon

South Pole-Aitken (SPA) basin-sized impacts into 10 K/km thermal gradient Moon (left) and a 50 K/km thermal gradient Moon (right). Colors represent temperatures: blues are low temperatures, reds are high temperatures. Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

The difference in final basin structure is, therefore, heavily dependent on the pre-impact target temperature. Material that is originally cooler (blues are cooler temperatures) will be stronger, more rigid and require greater energy to flow and melt compared to originally warmer (reds are warmer temperatures) and, therefore, weaker material.

The largest lunar basin: Large vs small impactors
Impact details:
South Pole-Aitken basin (Moon)
Impactor material: Dunite (3300 kg/m3 density)
Impactor diameter: 150 and 250 km
Impact velocity: 10 km/s
Gravity: 1.61 m/s2
Target thermal gradient: 50 K/km

The following videos illustrate the effect of impactor size on a SPA-sized basin-forming event into the Moon. The impactors are 150 and 250 km in diameter.  The larger impactor has ~4.5 times the mass and energy of the smaller impactor. On impact, the transient cavity reaches a maximum depth of ~400 kilometers for the smaller impactor and >500 kilometers for the larger impactor. Consequently, the collapse of the transient crater is on a greater scale for the large impactor, with the central uplift reaching a far greater height before collapsing back into the Moon. The Moon's shape and topography is also far more distorted in the more energetic impact. The Moon has also been heated internally to greater temperatures over a larger area in the larger impact. The final structure of the basins is noticeably different. The zone of mantle exposure on the lunar surface for the larger impactor is approximately twice (~800 kilometers) that of the smaller impactor. Both impacts, however, have a similar crustal profile - tapering of the crustal thickness towards the center of the basin.

SPA 150km impactor SPA 250km impactor

South Pole-Aitken (SPA) basin-sized impacts with a 150 kilometer impactor (left) and a 250 kilometer impactor (right). Colors represent temperatures: blues are low temperatures, reds are high temperatures. Single click on the images to open the videos.

Both simulations can be viewed simultaneously, side-by-side, by single clicking on the image.

Other sites of Interest:

Impact video documentation 

The video simulations of impact cratering above were produced using the same methods published in the following research reports:

R. W. K. Potter, G. S. Collins, W. S. Kiefer, P. J. McGovern, and D. A. Kring (2012) Constraining the size of the South Pole-Aitken Basin impact.  Icarus 220, pp. 730-743, doi:10.1016/j.icarus.2012.05.032.

R. W. K. Potter, D. A. Kring, G. S. Collins, W. S. Kiefer, and P. J. McGovern (2012) Estimating transient crater size using the crustal annular bulge: Insights from numerical modeling of lunar basin-scale impacts.  Geophysical Research Letters 39, 5 p., doi:10.1029/2012GL052981.

R. W. K. Potter, D. A. Kring, G. S. Collins, W. S. Kiefer, and P. J. McGovern (2013) Numerical modeling of the formation and structure of the Orientale impact basin.  Journal of Geophysical Research: Planets 118, pp. 963–979, doi:10.1002/jgre.20080.

R. W. K. Potter, D. A. Kring, and G. S. Collins (2013) Quantifying the attenuation of structural uplift beneath large lunar craters. Geophysical Research Letters
40
, 5615–5620, doi:10.1002/2013GL057829.

These impact simulations were produced using the iSALE shock physics code. We thank Gareth Collins, Boris Ivanov, Jay Melosh, Kai Wünnemann and Dirk Elbeshausen for their work developing iSALE.

 

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