TH-Lab5 (3)

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GEOL-490

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Dec 6, 2023

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GEOL104: Exploring the Planets LAB 4: IMPACT CRATERING Name: OBJECTIVES : I. Learn about impact crater morphology and the mechanisms of impact crater formation. II. Learn how impact craters are used as age-dating tools III. Investigate a scientific problem using the scientific methods BACKGROUND : Impact craters are formed when two bodies collide. Impact craters are produced on all bodies in the Solar System that have a solid surface, and have been observed on the four terrestrial planets, many moons, asteroids, and comets. The larger of the planetary bodies, usually the “survivor” of the impact, is called the target , while the smaller body which will likely be destroyed is called the impactor or projectile. Typical impactors are meteoroids (called meteorites after touchdown) and comets. These objects vary greatly in size, but impactor size has generally been reduced over time. A crater is not formed by an impactor physically pushing material aside: it is formed by an explosion similar to a nuclear bomb! Earth-bound impactors travel at speeds of 10 – 70 kilometers per second . These hypersonic speeds create a shock wave, deforming the target material and melting or vaporizing the impactor. This shock wave fractures the rock and excavates a large cavity (much larger than the impactor). This activity occurs within the first tenths of the first second of the impact event. The most cratered bodies in the solar system are rocky, airless bodies such as the Earth’s Moon and Mercury, where surface processes such as erosion by wind, water, ice, and tectonic activity are not present. However, all solar system bodies are capable of being scarred by impacts, even gaseous planets. In 1994, Jupiter was struck by the comet Shoemaker-Levy 9. The comet was broken into several pieces by Jupiter’s gravity before finally penetrating into the planet’s clouds. The Galileo spacecraft and the Hubble Space Telescope witnessed the comet’s break-up and collision, and showed scarring in the clouds at the impact site ( Figure 0 ). These scars have since disappeared. Figure 0: Scars from collision of comet Shoemaker-Levy 9 with Jupiter as photographed by the Hubble Space Telescope. Image credit: Hubble Space Telescope Comet Team and NASA. INTRODUCTION: History of Impact Crater Studies Page 1

GEOL104: Exploring the Planets In earlier studies of planetary surfaces, scientists were uncertain how the large circular pits, seen on many solar system bodies formed. In the early 1600s, Galileo Galilei recognized the existence of these landforms on the Earth’s moon. He called these features “spots” and described them as being circular depressions surrounded by a raised rim. Galileo did not suggest an origin for these “spots”. In the mid 1600’s Robert Hooke initially considered that these features may have formed from impacts, but he could not think of a source for the impactors. He later suggested that these features were volcanic calderas. This interpretation was accepted by scientists for nearly 300 years. In 1893 G. K. Gilbert proposed that these lunar features occurred from impacts . Crater Anatomy Not all craters are created equal. Although sharing a common process, craters vary in their size, shape, and overall appearance. The most basic distinguishing features of an impact crater are the crater rim , crater walls , crater floors , and ejecta blankets . A variety of landforms can also be present, including central peaks , multiple rings , peak rings , terraces , and concentric rings/rays surrounding the crater. Craters can be grouped into two major categories according to their morphology: simple craters and complex craters. Simple craters are small bowl-shaped, smooth-walled carters and are generally small (diameters up to several kilometers across). Two examples of simple craters on Earth are Nördlinger, Germany ( Figure 1) and Barringer Crater (aka; Meteor Crater) in Arizona ( Figure 2 ). Complex craters are larger (diameters up to a few tens up to a few hundred kilometers), typically have flatter floors and a central peak, and can possess one or more of the landforms listed above. Complex craters form from impacts of larger objects traveling at greater speeds, which means more explosive energy. Most craters are circular with radial, roughly symmetric ejecta, but there are also elongate/oblique craters ( Figure 7 ) as well as craters with lobate ejecta, called “rampart” or “splat” craters. In addition to these major categories, impact craters can also be grouped into microcraters or pits (subcentimeter craters caused by impacts of micrometeoroids or high-velocity cosmic dust), and multiring basins (large impact craters covering much larger areas than complex craters characterized by multiple concentric “rings”). Figure 3 shows an example of a rampart crater, Yuty Crater, on Mars. Yuty Crater is a complex crater and features the basic rim, floor, walls, and ejecta blanket, but also has a central peak. Other features of complex craters include ejecta rays, as seen extending from Tycho Crater on the Moon ( Figure 4 ), concentric rings, as seen in Callisto’s Valhalla impact basin ( Figure 5 ), and secondary impact craters produced by excavated debris, as seen surrounding Warhol Crater on Mercury ( Figure 6 ). The walls of complex craters can also become terraced after the impact event ( Figure 6, 8, and 9 ). Because they excavate the target material largely by vaporizing it, impact craters almost always produce circular structures. Elongated impact craters ( Figure 7 ) form only at very low impact angles of about <10-15 degrees above the surface. Earth has experienced impacts and formation of complex craters, but over time erosion has muted their appearance. The Earth is so good at eroding material and covering up the damage of impacts that some have unknowingly made their home inside craters. The city and farmland of Nördlinger, Germany are completely located inside of an ancient crater ( Figure 1 ). If you look at the previous image you can see a ring of mountains encircling the town and farms. These mountains are what is left of the impact crater rims. Other features of this complex crater have been eroded away. Page 2

GEOL104: Exploring the Planets Figure 1: The city of Nördlinger, Germany, located within a heavily eroded complex crater. Image credit: Created with NASA WorldWind by User: Vesta using Landsat 7 (Visible Color) satellite image., Public Domain, https://commons.wikimedia.org/w/index.php?curid=496218 Figure 2: Barringer Crater (aka Meteor Crater), Arizona, an example of a simple crater . This simple crater was created when a 50-meter-wide iron-rich meteoroid struck Earth’s surface about 50,000 year ago. The crater is about 1.2 km wide and 200 m deep. Interestingly, due to its relatively young age, the ejecta blanket beyond its rim is well preserved. Image credit: National Map Seamless Server - NASA Earth Observatory, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7549781 . Page 3

GEOL104: Exploring the Planets Figure 3: Yuty Crater, a complex rampart crater. This diagram shows the major distinguishing features of a typical complex crater. Rampart craters form when in impact hits a surface that contains a lot of water (or ice), causing the ejecta blanket to produce a distinctive pattern resembling a splash or mudflow. Rampart craters exist in many places on Mars. Image credit: Labels: Calvin Hamilton, Image: NASA/JPL/Arizona State University. Page 4 Figure 4: Tycho Crater, a complex crater on the Moon, displaying prominent ejecta rays . Image credit: Howard Mooers, East-View Observatory. Figure 5: Multi-Ring Basin Valhalla of Callisto, a moon of Jupiter. Image credit: NASA/JPL/California Institute of Technology

GEOL104: Exploring the Planets Figure 6: Warhol Crater, a crater on Mercury, surrounded by secondary impact craters . Secondary craters are produced when the debris ejected at high velocities during the initial shock wave strikes the ground and produces a secondary impact, analogous to the formation of a primary crater. Image: NASA/ MESSENGER. Figure 7. Orcus Patera, an elongate crater in Tyrrhena region on Mars formed by a very oblique (low angle) impact. Note the range of crater sizes visible in the image. Image credit: NASA/JPL/Arizona State University. Figure 8: Cross-section view through a complex crater showing central peak and terraced crater walls. Page 5 Secondary crater rays

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