Week 6: 1st May - 8th May
Planetary science: Crater abundances
Marine science: Video transect data analysis
Week 6: Planetary Image Analysis population comparisons
Is Crater abundance higher in the lunar highlands than in the lunar seas?
The numbers of images from planetary bodies (extreme ecosystems) has been greatly increasing as a result of imaging, computing, power and data transfer technological improvements.
As in the deep sea, it is desirable to generate numerical data from these image sets for the use in statistically assessing certain research hypotheses.
Is Crater abundance higher in the lunar highlands than in the lunar seas?
The numbers of images from planetary bodies (extreme ecosystems) has been greatly increasing as a result of imaging, computing, power and data transfer technological improvements.
As in the deep sea, it is desirable to generate numerical data from these image sets for the use in statistically assessing certain research hypotheses.
A glance at the moon shows it to be heavily cratered. To the naked eye it would appear that the craters are more abundant in the highlands (the whiter areas) than the lowlands (the darker areas).
Is this actually the case?
Is this actually the case?
Impact Craters as Key Features
Collisions happen frequently at all bodies in our solar system. Those impacts are violent events due to the impact velocity (e.g. on Earth impact velocities are between 10-40 km/s). An impact is a depression in a target body that is often surrounded by an ejecta blanket. Ejecta is formed by material, which was excavated by the impact event.
The composition, size, and speed of the impactor and target body are influencing the crater dimensions.
Crater formation is divided into three stages:
1. Contact and Compression Stage
The collision occurs and kinetic energy is transferred to the target body in form of shock waves.
Due to high pressures rocks in the impact area vaporize. The propagation of shock waves highly depends on the target material.
2. Ejection/Excavation Stage
After the collision a plume, consistent of vaporized material, expands up and outwards. At the same time the shock wave gets gradually weaker by further expansion through the target body. Rarefaction waves behind the shock wave decompress the target and initiate the excavation of material. Material down to 1/3 of the transient cavity (maximum depth while crater formation) is excavated over several minutes or pushed upwards to form the future crater walls and rim. Underlying material is being highly compressed. Large craters have usually a small crater depth, whereby small craters have a relatively deep crater depth. Craters at the end of the excavation stage are called transient craters. The shape of a transient crater is only depended on impactor properties (velocity, composition, speed, size, impact angle) and target properties (gravity, composition, surface structure).
3. Collapse and Crater Modification Stage
Transient craters are being modified by various geological processes over time. Erosion done by wind, water, ice, volcanoes, and other impacts will smoothen the crater morphology until it is completely erased of the surface.
Craters can be classified in different crater types based on their morphology.
1. Simple Craters
They are bowl shaped depressions in the ground up to 7-12km in diameter. The depth is usually about 1/5 its diameter.
2. Complex Craters
These craters are above 7-12km in diameter, but have usually sized up to hundreds of km. They have a flat floor and a central peak. Sometimes terraces are located on the interior rim sides, due to collapse of the crater walls.
3. Multiring Basin Craters
Basins occur only on the largest craters in the solar system. They do not have one crater rim, but rather a concentric ring system surrounding the crater.
Often impact craters are also classified into primary and secondary craters. Primary craters are the direct result of an impact. Whereby, secondary craters are formed by excavated material ejected from a primary crater. Secondary craters can be oriented in crater fields or chains. Sometimes bright linear features that radiate outwards an impact crater can be identified (especially on the Moon). Those features are called Rays and originate from lighter subsurface material, which was ejected by the impact. The unit consisted of excavated/ejected material around the crater is called ejecta blanket and based on its shape, color, layering, and size it can provide important information about the subsurface material. The unit inside the crater is called breccia and usually consists out of high temperature and pressure minerals, due to the impact itself. Find grained material on a body’s surface is called Regolith. It is through impact cratering ground down material, which can form several meters of a surface.
Impact craters are of high importance for planetary scientists, because they represent the most dominant landform on planetary bodies. If you observe the Moon through a telescope, or remember your mapping project, you will recognize the highly cratered surface of our neighbor.
A lot of research has been done on impact cratering to better understand the impact mechanics and gain also additional information about the target history and age.
Crater shape tasks:
1. Classify the nine craters in the image below into the three different crater types (or Week6_planetary_task1.pdf)
2. Fill out the blank fields in the schematic illustration of impact crater formation (download Week6_planetary_task2.pdf for this.)
3. Read the Article from “G. G. Michael & G. Neukum (2010): Planetary surface dating from crater size-frequency distribution measurements: Partial resurfacing events and statistical age uncertainty” until next week.
Collisions happen frequently at all bodies in our solar system. Those impacts are violent events due to the impact velocity (e.g. on Earth impact velocities are between 10-40 km/s). An impact is a depression in a target body that is often surrounded by an ejecta blanket. Ejecta is formed by material, which was excavated by the impact event.
The composition, size, and speed of the impactor and target body are influencing the crater dimensions.
Crater formation is divided into three stages:
1. Contact and Compression Stage
The collision occurs and kinetic energy is transferred to the target body in form of shock waves.
Due to high pressures rocks in the impact area vaporize. The propagation of shock waves highly depends on the target material.
2. Ejection/Excavation Stage
After the collision a plume, consistent of vaporized material, expands up and outwards. At the same time the shock wave gets gradually weaker by further expansion through the target body. Rarefaction waves behind the shock wave decompress the target and initiate the excavation of material. Material down to 1/3 of the transient cavity (maximum depth while crater formation) is excavated over several minutes or pushed upwards to form the future crater walls and rim. Underlying material is being highly compressed. Large craters have usually a small crater depth, whereby small craters have a relatively deep crater depth. Craters at the end of the excavation stage are called transient craters. The shape of a transient crater is only depended on impactor properties (velocity, composition, speed, size, impact angle) and target properties (gravity, composition, surface structure).
3. Collapse and Crater Modification Stage
Transient craters are being modified by various geological processes over time. Erosion done by wind, water, ice, volcanoes, and other impacts will smoothen the crater morphology until it is completely erased of the surface.
Craters can be classified in different crater types based on their morphology.
1. Simple Craters
They are bowl shaped depressions in the ground up to 7-12km in diameter. The depth is usually about 1/5 its diameter.
2. Complex Craters
These craters are above 7-12km in diameter, but have usually sized up to hundreds of km. They have a flat floor and a central peak. Sometimes terraces are located on the interior rim sides, due to collapse of the crater walls.
3. Multiring Basin Craters
Basins occur only on the largest craters in the solar system. They do not have one crater rim, but rather a concentric ring system surrounding the crater.
Often impact craters are also classified into primary and secondary craters. Primary craters are the direct result of an impact. Whereby, secondary craters are formed by excavated material ejected from a primary crater. Secondary craters can be oriented in crater fields or chains. Sometimes bright linear features that radiate outwards an impact crater can be identified (especially on the Moon). Those features are called Rays and originate from lighter subsurface material, which was ejected by the impact. The unit consisted of excavated/ejected material around the crater is called ejecta blanket and based on its shape, color, layering, and size it can provide important information about the subsurface material. The unit inside the crater is called breccia and usually consists out of high temperature and pressure minerals, due to the impact itself. Find grained material on a body’s surface is called Regolith. It is through impact cratering ground down material, which can form several meters of a surface.
Impact craters are of high importance for planetary scientists, because they represent the most dominant landform on planetary bodies. If you observe the Moon through a telescope, or remember your mapping project, you will recognize the highly cratered surface of our neighbor.
A lot of research has been done on impact cratering to better understand the impact mechanics and gain also additional information about the target history and age.
Crater shape tasks:
1. Classify the nine craters in the image below into the three different crater types (or Week6_planetary_task1.pdf)
2. Fill out the blank fields in the schematic illustration of impact crater formation (download Week6_planetary_task2.pdf for this.)
3. Read the Article from “G. G. Michael & G. Neukum (2010): Planetary surface dating from crater size-frequency distribution measurements: Partial resurfacing events and statistical age uncertainty” until next week.
Crater Counting for age determination
After learning about impact craters in the last chapter let us focus on the scientific meaning of this important surface feature. As already mentioned, a lot of research has been done on impact cratering to gain additional information about the target history and age.
The spatial density of craters (number of craters per unit area) and the crater size frequency distribution (number of craters per unit area as a function of crater size) give us important information about the target and differs a lot throughout the solar system. This is the result of the cratering and crater removal rate.
Cratering Rate: Scientists assume that the cratering rate is inconsistent within the last 4 billion years. In the early times of our solar system the bodies were not in order, the orbits not defined, and the planets not fully grown. A lot of large sized impacts occurred during that era, which is also named the Heavy Bombardment. The change of the cratering rate over time is not fully understood and still a topic for discussion. While the solar system came in order impacts happened less likely. So the cratering rate dropped off rapidly within the first billion years to reach a roughly constant cratering flux during the last 3 billion years. That means it is less likely that planetary bodies are being hit by impacts, the rate of large sized impacts decreased dramatically as well.
How can we link the impacts we see on a planetary surface to a specific age? This is one of the achievement of the lunar landings. Astronauts brought samples from different impact craters back to Earth. Nine missions provided in total 382 kg of lunar rocks and soil, which were analyzed and dated using radioisotope dating. By knowing from which region the samples came, the obtained ages could be linked to the region and craters itself, providing absolute ages of the lunar crust. Based on that, isochrones are calculated throughout the last billion years for the Moon. Those help to interpret the results of your crater counting. There is ongoing research in the topic of surface ages and dating as well.
Crater Removal: The shape of the impact craters is first of all depend on the target and impactor properties. Additionally, there is the crater modification stage, in which various processes flatten the crater rim and fill up the crater interior. The modification of the crater will reach its maximum if the crater is no longer visible on the planetary surface, at that time we consider this crater removal. Endogenic and exogenous processes are responsible for crater removal. Endogenic sums up all processes that originate within the planetary body (e.g. volcanic activity, tectonics, wind erosion, water and ice erosion, gravity related erosion). Lava blankets can fill up impact craters until they are not recognizable anymore. Wind and water erosion flatten the terrain until the rim and floor of a crater is not identifiable. Exogenous are processes that do not originate from the planetary body itself. Impact related ejecta and tectonic movements are considered exogenous processes.
Surface areas with more craters are older than areas with barely any or only small sized impact craters. Small craters will erode quickly, because their morphology is less dominant. Old craters will take longer to erode and eventually they do not have a sharp and distinct crater floor, rim, and ejecta. You will hardly see small old craters, as they are removed most easily from the surface over time. Crater removal rates do not only depend on the region on a planetary body, but also on the planetary body itself and its activity of its endogenic processes.
Crater counting tasks:
1. Which body in our solar system has the least amount of impact craters and why?
2. Obtain surface ages by conducting a manual Hartmann plot.
a) Use the provided table with crater diameters. Perform a Hartmann binning and input the results into the plot (craters in each bin/km2 to diameter) to obtain the surface age for area 1. The shown isochrones are 200 Ma, 500 Ma, 1 Ga, 2 Ga, 3 Ga, and 4 Ga.
b) Conduct the same study with area 2. How does the change of area relate to the surface age?
3. Perform crater counting to obtain surface ages of your mapping area.
a) Perform a crater counting within JMARS in three different regions.
b) Use the software crater stats to input your size distributions.
c) Adapt the curve to obtain a surface age.
d) What is the surface age of your three chosen areas?
e) What does that mean?
Crater counting and ANOVA:
1. If you have yet to do so, download and read the 'iass_guidetoimagej.docx' from the resources page.
2. Download a number of images via the NASA web portals (links below and on the weekly document) from the lunar highlands and the lunar seas. These images should have reasonably similar spatial coverage areas, and download the same number of images from each category of region.
3. Use ImageJ to count the numbers of craters in the images from each area.
4. Use an ANOVA test to compare abundances between the two regions.
5. Comment on the regional differences you observe in the course forum or on an email to '[email protected]'. Please indicate which data was analysed.
6. Comment in the forum on the suitability of the methodology in answering the research question – do you see any strengths or drawbacks with the experimental or analytical approaches?, on the statistical results given by the ANOVA and any other observations or opinions.
Week 6: Marine Science: ANOVA in the marine environment
ANOVA tests, and other numerical tests based on image counts of species and image features, have been becoming progressively more prevailent in Marine science over the last two decades. A typical application is to analyse how populations of animals may vary in two contrasting ecosystems (such as polluted and unpolluted areas). We have prepared some realworld data to allow you to run your own tests to determine if shrimp numbers vary on the Norwegian margin with seafloor habitat type. Please download 'marine_biology_ANOVA.pdf' for instructions. The datasets described in the document are provided below.
This weeks activities:
In addition to the tasks on the page above:
1. Download 'IASS_spring2016_Week6_mailout.pdf' from below and work through the documents tasks.
Resources:
planetary_week6.pdf | |
File Size: | 547 kb |
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planetary_week6_task1.pdf | |
File Size: | 753 kb |
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planetary_week6__task2.pdf | |
File Size: | 560 kb |
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planetary_week6_refs.pdf | |
File Size: | 351 kb |
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michael2010_planetary_surface_dating.pdf | |
File Size: | 723 kb |
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planetary_week6b.pdf | |
File Size: | 320 kb |
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cratercounting_craterstats.pdf | |
File Size: | 1023 kb |
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planetary_week6b_refs.pdf | |
File Size: | 358 kb |
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2.4_hartmann_plot.png | |
File Size: | 103 kb |
File Type: | png |
iass_guidetoonlineanova_basic.pdf | |
File Size: | 420 kb |
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marine_biology_anova.pdf | |
File Size: | 213 kb |
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data_sotbakken_shrimp.zip | |
File Size: | 1840 kb |
File Type: | zip |
data_rost_shrimp.zip | |
File Size: | 2067 kb |
File Type: | zip |