tag:theconversation.com,2011:/us/topics/meteorite-crater-61083/articlesMeteorite crater – The Conversation2022-09-19T20:14:45Ztag:theconversation.com,2011:article/1907552022-09-19T20:14:45Z2022-09-19T20:14:45ZFor the first time, robots on Mars found meteorite impact craters by sensing seismic shock waves<figure><img src="https://images.theconversation.com/files/485209/original/file-20220919-60301-kjcm6t.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2000%2C1994&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://mars.nasa.gov/insight/multimedia/raw-images/?order=sol+desc%2Cdate_taken+desc&per_page=50&page=2&mission=insight">NASA / JPL-Caltech</a></span></figcaption></figure><p>Since 2018, NASA’s <a href="https://mars.nasa.gov/insight/mission/overview/">InSight mission</a> to Mars has recorded seismic waves from more than <a href="https://www.essoar.org/doi/10.1002/essoar.10512017.1">1,300 marsquakes</a> in its quest to probe the internal structure of the red planet. The solar panels of the car-sized robotic lander have become caked with Martian dust, and NASA scientists <a href="https://mars.nasa.gov/news/9191/nasas-insight-still-hunting-marsquakes-as-power-levels-diminish/?site=insight">expect</a> it will completely power down by the end of 2022.</p>
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Read more:
<a href="https://theconversation.com/first-recorded-marsquakes-reveal-the-red-planets-rumbling-guts-132091">First recorded 'marsquakes' reveal the red planet's rumbling guts</a>
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<p>But the internal rumblings of our planetary neighbour aren’t the only things that InSight’s seismometers detect: they also pick up the thuds of space rocks crashing into the Martian soil.</p>
<p>In <a href="https://www.nature.com/articles/s41561-022-01014-0">new research</a> published in Nature Geoscience, we used data from InSight to detect and locate four high-speed meteoroid collisions, and then tracked down the resulting craters in satellite images from NASA’s Mars Reconnaissance Orbiter.</p>
<h2>Rocks from space</h2>
<p>The Solar System is full of relatively small rocks called meteoroids, and it’s common for them to collide with planets. When a meteoroid encounters a planet with an atmosphere, it heats up due to friction – and may burn up entirely before reaching the ground.</p>
<p>On Earth, we know these incoming meteoroids as shooting stars, or meteors: beautiful events to observe in the night sky. Sometimes a meteoroid explodes when it reaches the thicker atmosphere closer to the ground, creating a spectacular airburst.</p>
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Read more:
<a href="https://theconversation.com/where-do-meteorites-come-from-we-tracked-hundreds-of-fireballs-streaking-through-the-sky-to-find-out-160096">Where do meteorites come from? We tracked hundreds of fireballs streaking through the sky to find out</a>
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<p>Occasionally, a space rock survives its fiery path through the air and drops to the ground, where it is known as a meteorite. </p>
<p>A few of these meteorites hit the surface at such speed they blast a hole in the ground called an impact crater. Compared to a human lifetime, these events are very rare on Earth. </p>
<h2>Recording space rock impacts</h2>
<p>Scientists have detected the vibrations from meteoroid airbursts using seismic detectors numerous times, including <a href="https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/abs/statistical-analysis-of-fireballs-seismic-signature-survey/1683309FE1240CFCE1460AD3A11776BC">a recent survey</a> of bright meteors above Australia. </p>
<p>However, only once has a high-speed space rock crashing into the ground been observed both visually and with modern seismic equipment. This was an impact crater that <a href="https://en.wikipedia.org/wiki/2007_Carancas_impact_event">formed in 2007</a> near the village of Carancas in Peru. </p>
<p>Numerous impacts were detected on the Moon by the network of seismic sensors set up during the US Apollo missions of the 1960s and ’70s. However, there was no recording of a natural impact associated with visual detection of a new crater. </p>
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Read more:
<a href="https://theconversation.com/the-moon-is-still-geologically-active-study-suggests-116768">The moon is still geologically active, study suggests</a>
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<p>The closest things to such an observation were artificial impacts: the crash-landings of the booster rockets of the ascent modules that lifted Apollo astronauts off the Moon. </p>
<p>These human-made impacts on the Moon were recorded both in seismic data and visual imagery from orbit. These data were <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021EA001887">recently used</a> to test simulations of how impacts produce seismic waves.</p>
<h2>Martian meteorites</h2>
<p>Incoming meteoroids make waves in the atmosphere and also the ground. The atmosphere of Mars is equivalent to 1% of the Earth’s, and has a different chemical composition. This means meteor events on Mars take a different form.</p>
<p>For meteor events large enough to drop a meteorite, the fate of the meteorite and any resulting crater is <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021JE007149">different</a> from what we have come to expect on our home planet. </p>
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<a href="https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=445&fit=crop&dpr=1 600w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=445&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=445&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=560&fit=crop&dpr=1 754w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=560&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=560&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Many craters on Mars come in clusters, because meteoroids often explode into fragments not long before they hit the surface.</span>
<span class="attribution"><a class="source" href="https://www.uahirise.org/ESP_028444_2040">MRO / HiRISE / NASA / JPL-Caltech / UArizona</a></span>
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<p>Here on Earth, or on the Moon, single craters are the norm. On Mars, however, about half the time a high-speed space rock will burst in the atmosphere shortly before impact, resulting in a tightly grouped cluster of craters.</p>
<p>The separation of these individual fragments remains close at ground level, forming a <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2021JE007145">cluster of small impacts</a>.</p>
<h2>From vibrations to craters</h2>
<p>Recently, the InSight mission has observed acoustic and seismic waves from four meteoroid impact events. These waves travel at different speeds, and comparing their different arrival times and other properties allowed us to estimate the location of the impacts.</p>
<p>These impact locations were then confirmed with satellite imaging from the Mars Reconnaissance Orbiter.</p>
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<img alt="" src="https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=538&fit=crop&dpr=1 600w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=538&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=538&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=676&fit=crop&dpr=1 754w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=676&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=676&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">A sketch of how an incoming space rock makes waves that InSight can detect and interpret.</span>
<span class="attribution"><a class="source" href="https://www.nature.com/articles/s41561-022-01014-0">Garcia et al. / Nature Geoscience</a></span>
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<p>Knowing the size and exact location of these impact craters helps us calculate the size and speed of the incoming space rock and how much energy the impact released.</p>
<p>Once we are confident we know something about the impact that created the seismic waves we detected, we can use the waves to learn about the interior of Mars. What’s more, when we compare seismic observations on Mars with observations from Earth and the Moon, we can learn more about how the planets formed and how the Solar System evolved.</p><img src="https://counter.theconversation.com/content/190755/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr Katarina Miljkovic works for Curtin University and is fully funded by the Australian Research Council. She is a science collaborator for the NASA InSight mission.</span></em></p>In an extraterrestrial first, scientists have linked seismic waves on Mars to meteorite impact craters spotted via satellite.Katarina Miljkovic, ARC Future Fellow, School of Earth and Planetary Sciences, Curtin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1188422019-07-17T02:06:34Z2019-07-17T02:06:34ZWhy the Moon is such a cratered place<figure><img src="https://images.theconversation.com/files/283444/original/file-20190710-44497-s6v8v7.jpg?ixlib=rb-1.1.0&rect=91%2C133%2C2527%2C1893&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Look at the circular patterns on the Moon's surface, as seen from Earth.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/bobfamiliar/6245070828/">Flickr/Bob Familiar</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Look up on a clear night and you can see some circular formations on the face of our lunar neighbour. These are impact craters, circular depressions found on planetary surfaces. </p>
<p>About a century ago, they were suspected to exist on Earth but the cosmic origin was often met with suspicion and most geologists believed that craters were of volcanic origin.</p>
<p>Around 1960, the American astrogeologist <a href="https://www.britannica.com/biography/Gene-Shoemaker">Gene Shoemaker</a>, one of the founders of planetary science, studied the dynamics of crater formation on Earth and planetary surfaces. He investigated why they – including our Moon – are so cratered. </p>
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Read more:
<a href="https://theconversation.com/five-ethical-questions-for-how-we-choose-to-use-the-moon-116801">Five ethical questions for how we choose to use the Moon</a>
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<h2>Images from Apollo</h2>
<p>By 1970, there were more than 50 craters discovered on Earth but that work was still considered controversial, until pictures of the lunar surface brought by the <a href="https://www.nasa.gov/mission_pages/apollo/missions/index.html">Apollo missions</a> confirmed that impact cratering is a common geological process outside Earth.</p>
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<a href="https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=587&fit=crop&dpr=1 600w, https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=587&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=587&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=737&fit=crop&dpr=1 754w, https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=737&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/283450/original/file-20190710-44448-3ck3pf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=737&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The crater Daedalus on the far side of the Moon as seen from the Apollo 11 spacecraft in lunar orbit. Daedalus has a diameter of about 80km.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/multimedia/imagegallery/image_feature_25.html">NASA</a></span>
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<p>Unlike Earth’s surface, the lunar surface is covered with craters. This is because Earth is a dynamic planet, and tectonics, volcanism, seismicity, wind and oceans all play against the preservation of impact craters on Earth.</p>
<p>It does not mean Earth – <a href="https://theconversation.com/target-earth-how-asteroids-made-an-impact-on-australia-92836">even Australia</a> – has not been battered. We should have been hit by more rocks from space than the Moon has, simply because <a href="https://solarsystem.nasa.gov/moons/earths-moon/by-the-numbers/">our planet is larger</a>.</p>
<p>In contrast to Earth, our Moon has been inactive over long geological timescales and has no atmosphere, which has allowed the persistent impact cratering to remain over eons. The lunar cratering record spans its entire bombardment history - from the Moon’s very origins to today. </p>
<h2>The big ones</h2>
<p>The largest and oldest impact crater in the Solar system is believed to be on the Moon, and it is called the South Pole-Aitken basin, but we cannot see it from Earth because it is on the far side of the Moon. The Moon is tidally locked to Earth’s rotation and the same side always faces toward us.</p>
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<span class="caption">The South Pole-Aitken Basin shown here in the elevation data (not natural colours) with the low center in dark blue and purple and mountains on its edge, remnants of outer rings, in red and yellow.</span>
<span class="attribution"><a class="source" href="https://moon.nasa.gov/resources/38/south-pole-aitken-basin/">NASA/GSFC/University of Arizona</a></span>
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<p>But this crater, more than 2,000km across, is thought to predate any other large impact bombardment that occurred during lunar evolution. Impact simulations suggested it was formed by a <a href="https://www.lpi.usra.edu/exploration/training/resources/impact_cratering/">150-250km asteroid</a> hurtling into the Moon at 15-20km per second!</p>
<p>From Earth, the human eye can observe areas of different shades of grey on the surface of the Moon facing us. The dark areas are called maria, and can be up to more than 1,000km across.</p>
<p>They are volcanic deposits that flooded depressions created by the formation of the large impact basins on the Moon. These volcanic eruptions were active for millions of years after these impacts occurred.</p>
<p>My favourite is the <a href="https://svs.gsfc.nasa.gov/4499">Orientale</a> impact basin, the youngest of the large impact craters on the Moon, but still estimated to have formed “only” about <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JE005038" title="Subsurface morphology and scaling of lunar impact basins">3.7 billion years ago</a>.</p>
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<span class="caption">Orientale basin is about 930km wide and has three distinct rings, which form a bullseye-like pattern. This view is a mosaic of images from NASA’s Lunar Reconnaissance Orbiter.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/news/news.php?feature=6662">NASA/GSFC/Arizona State University</a></span>
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<p>No other large impact event has occurred on the Moon since then. This is a good sign, because it implies there were no very large impacts occurring on Earth either after this time in evolutionary history. (The asteroid that <a href="https://www.nhm.ac.uk/discover/how-an-asteroid-caused-extinction-of-dinosaurs.html">wiped out the dinosaurs</a> on Earth 66 million years ago was only about <a href="https://www.sciencedirect.com/science/article/pii/S0012821X0800188X?via%3Dihub">10-15km</a> in size and left a crater larger than 150km in size, which was substantial enough to cause a mass extinction.)</p>
<h2>As seen from Earth</h2>
<p>With a small telescope, or fancy binoculars, you can check out some of the best-preserved complex craters on the Moon, such as the <a href="https://www.lpi.usra.edu/education/timeline/gallery/slide_61.html">Tycho</a> or <a href="https://moon.nasa.gov/resources/66/copernicus-crater/">Copernicus</a> <a href="https://ase.tufts.edu/cosmos/view_picture.asp?id=383">craters</a>.</p>
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<a href="https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/283459/original/file-20190710-44472-1nuibkm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Tycho Crater is one of the most prominent craters on the Moon.</span>
<span class="attribution"><span class="source">NASA/Goddard/Arizona State University</span></span>
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<p>They are called complex craters because they are not entirely bowl-shaped, but are a bit shallower and include a peak in the centre of the crater as a consequence of the material collapsing into the hole made during impact. Tycho and Copernicus are both 80-100km across but have spectacular central peaks and prominent “ejecta rays” – areas where material was ejected across the lunar surface after an impact.</p>
<p>The formation of these craters excavated underlying material that was brighter than the actual surface. This is because lunar surface is subjected to <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2016JE005128">space weathering</a>, which causes surface rocks to darken.</p>
<h2>Still a target for impacts</h2>
<p>The Apollo 12, 14, 15, and 16 missions placed several seismic stations on the Moon between 1969 and 1972, creating the first extraterrestrial seismic network (<a href="https://www.hq.nasa.gov/alsj/HamishALSEP.html">ALSEP</a>). During one year of operations, more than <a href="https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/JB079i029p04365" title="Meteoroid impacts recorded by the short‐period component of Apollo 14 Lunar Passive Seismic Station">1,000 seismic events were recorded</a>, of which 10% were associated with meteoroids impacts.</p>
<p>So the Moon is still being hit by objects, albeit mostly tiny ones. But as there is no atmosphere on the Moon, there is no gas to help burn up these rocks from space and stop them smashing into the Moon.</p>
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Read more:
<a href="https://theconversation.com/target-earth-how-asteroids-made-an-impact-on-australia-92836">Target Earth: how asteroids made an impact on Australia</a>
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<p>The seismic network was functional until it was switched off in 1977, in preparation for new space missions. No one expected that the next fully operational <a href="https://theconversation.com/launching-in-may-the-insight-mission-will-measure-marsquakes-to-explore-the-interior-of-mars-91080">extraterrestrial seismometer</a> would not be placed on a planetary surface (Mars) until 40 years later. </p>
<p>Nowadays, from Earth, using a small telescope (and armed with a little patience), you can see so-called “<a href="https://science.nasa.gov/science-news/science-at-nasa/2013/16may_lunarimpact/" title="Bright Explosion on the Moon">impact flashes</a>”, which are small meteorite impacts on the lunar surface that is facing us.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/bBMxgyCpKnc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">You need to be quick to see the flashes – watch for the green boxes.</span></figcaption>
</figure>
<p>Thanks to the atmosphere on Earth, similar-sized rocks from space cannot make an impact here because they tend to predominantly burn up, but on the Moon they crash into the soil and release its kinetic energy of the impact via bright thermal emission.</p><img src="https://counter.theconversation.com/content/118842/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Katarina Miljkovic receives funding from the Australian Research Council. She is a member of the National Committee for Space and Radio Science.</span></em></p>The Apollo missions to the Moon helped our understanding of the cosmic origin of craters on our lunar neighbour, and here on Earth.Katarina Miljkovic, ARC DECRA fellow, Curtin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1051292018-10-18T06:02:50Z2018-10-18T06:02:50ZHow rare minerals form when meteorites slam into Earth<figure><img src="https://images.theconversation.com/files/241184/original/file-20181018-41122-qw3ops.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Scientists working at the central peak of Gosses Bluff meteorite crater in Northern Territory. </span> <span class="attribution"><span class="source">Nick Timms </span>, <span class="license">Author provided</span></span></figcaption></figure><p>The discovery of a rare mineral (reidite) at the Woodleigh meteorite impact structure in Western Australia was <a href="https://doi.org/10.1130/G45127.1">published</a> this week by Curtin University honours student Morgan Cox and colleagues. </p>
<p>Reidite – and other minerals – are sometimes formed when meteorites crash into Earth. </p>
<p>This takes a particular set of circumstances. Only six prior discoveries of reidite had ever been reported. </p>
<p>Here’s what happens when a meteorite slams into Earth. </p>
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Read more:
<a href="https://theconversation.com/target-earth-how-asteroids-made-an-impact-on-australia-92836">Target Earth: how asteroids made an impact on Australia</a>
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<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=464&fit=crop&dpr=1 600w, https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=464&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=464&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=582&fit=crop&dpr=1 754w, https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=582&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/241397/original/file-20181019-105770-7wuxcp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=582&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Electron diffraction image of a shocked zircon from the Woodleigh central uplift that was partially transformed to reidite. Colours show damage caused by impact. Red lines are deformation twins; reidite is shown in purple (see upper right).</span>
<span class="attribution"><span class="source">Morgan A. Cox.</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Rocks from space</h2>
<p>Our planet is continuously bombarded by meteorites – rocks from space – and has been since its formation about four and half billion years ago. </p>
<p>These objects include <a href="https://pubs.geoscienceworld.org/gsa/geology/article/45/9/819/208120/collisional-history-of-asteroid-itokawa">rocky</a> and <a href="https://solarsystem.nasa.gov/small-bodies/asteroids/16-psyche/in-depth/">metallic</a> asteroids, comets and other debris left over after the <a href="https://www.nature.com/articles/nature17678">formation of the solar system</a>, rock fragments ejected off planet surfaces from <a href="https://www.nature.com/articles/nature12764">impact events</a> and potentially even rare visitors that have travelled from <a href="https://www.nasa.gov/feature/solar-system-s-first-interstellar-visitor-dazzles-scientists">outside our Solar system</a>. </p>
<p>Space objects range in size from tiny particles to huge asteroids. They are usually travelling towards us at velocities of many kilometres per second – so-called hypervelocities. </p>
<p>Fortunately for us, though, <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2006.tb00485.x">small rocks are the most common</a>, and Earth’s atmosphere simultaneously slows them down, burns them away and breaks them up. We can often see this happening as fireballs and meteor showers. Any surviving chunks of rock free fall to the Earth’s surface to be collected as meteorites. </p>
<p>The <a href="http://fireballsinthesky.com.au/">Fireballs in the Sky</a> team at Curtin University has an amazing network of cameras to track incoming fireballs and predict the final land location of meteorites – and work out where in the Solar System they came from. They’ve made some <a href="https://www.abc.net.au/news/2016-01-06/professor-phil-bland-celebrates-lake-eyremeteorite-find/7072084">great meteorite discoveries</a> in this way. </p>
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<em>
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Read more:
<a href="https://theconversation.com/how-to-find-a-meteorite-thats-fallen-to-earth-52906">How to find a meteorite that's fallen to Earth</a>
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<p>Some rocks never make the final landing. Some may also produce an airburst – an atmospheric pressure wave that can cause damage like at <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/maps.12511">Chelyabinsk</a> in Russia in 2013. Here, an asteroid approximately 20 metres across and travelling at 19 km per second exploded about 30 km above the ground, causing a blast that was strong enough blow out windows of buildings in six nearby cities. </p>
<h2>Too big to slow down</h2>
<p>Some incoming rocks are too big for our atmosphere to slow down, and these are much more rare. </p>
<p>These slam into the Earth at hypervelocities, which imparts a huge amount of energy and causes impact craters. The size of an impact crater mainly depends on the dimensions, density and speed of the meteorite. </p>
<p>There are many known impact craters in Australia, such as <a href="https://parks.dpaw.wa.gov.au/park/wolfe-creek-crater">Wolfe Creek</a> in the Kimberly, and <a href="https://nt.gov.au/leisure/parks-reserves/find-a-park-to-visit/tnorala-gosse-bluff-conservation-reserve">Gosses Bluff</a> near Alice Springs. We also know of craters that are now buried under layers of recent sedimentary rocks, such as <a href="https://www.revolvy.com/page/Woodleigh-crater">Woodleigh</a>, Western Australia. </p>
<p>Globally, around 190 impact craters (or their eroded remnants) have been <a href="http://www.passc.net/EarthImpactDatabase/">discovered on Earth</a> – a lot fewer than scientists predict should have formed over all of Earth’s history. </p>
<p>This is because the Earth’s surface is quite a dynamic place, and processes of erosion and plate tectonics act to erase evidence of impact craters over time. </p>
<p>The known craters range in diameter from a few metres to a few hundred kilometres across, and range in age from a few thousand years to approximately two billion years. </p>
<p>No impact craters have formed in recent history, so scientists rely on studying ancient craters in combination with laboratory experiments and computer simulations to figure out what happens during such catastrophic events.</p>
<h2>Speed and pressure</h2>
<p>A hypervelocity impact event puts the impactor (that is, the rock arriving from space) and “ground zero” target rocks under immense pressure, which propagates through the Earth as a shock wave faster than the speed of sound. </p>
<p>It is not uncommon for the rocks to reach pressures in the tens or even hundreds of gigapascals – equivalent to a hundred billion times the pressure of Earth’s atmosphere. Even in the fractions of a second that rocks spend at these pressures, some minerals transform into new “high-pressure” minerals. </p>
<p>For example, <a href="https://pubs.geoscienceworld.org/gsa/geology/article/25/11/967/206583/diamonds-from-the-popigai-impact-structure-russia">graphite can form diamonds</a>, and the mineral <a href="https://www.sciencedirect.com/science/article/pii/S0012825216302331">zircon can turn into reidite</a> – as described in the <a href="https://pubs.geoscienceworld.org/gsa/geology/article-abstract/34/4/257/129509/crystal-plastic-deformation-of-zircon-a-defect-in?redirectedFrom=fulltext">new paper</a>. </p>
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<strong>
Read more:
<a href="https://theconversation.com/a-disappointing-earring-and-the-worlds-hottest-rock-zirconia-97084">A disappointing earring, and the world's hottest rock: zirconia</a>
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<p>As the shock wave passes, heat energy is produced by the release from high pressure. This can heat the rocks <a href="https://www.newscientist.com/article/mg23531433-600-found-the-hottest-place-ever-found-on-earths-surface/">enough to melt</a>, and in many cases even completely vaporise the meteorite and rocks at ground zero.</p>
<p>Shock waves also cause a lot of damage to rocks. They may break into fragments and get ejected high into the atmosphere and even into space, leaving behind a bowl-shaped crater. </p>
<p>Eventually the shock wave loses energy, so it slows down and becomes less destructive, and ripples through the Earth as seismic waves similar to those emitted during an earthquake. </p>
<h2>Earth changed forever</h2>
<p>In huge impact events – like the one that caused extinction of the dinosaurs and the 180 km across Chicxulub Crater in the gulf of Mexico – the centre of the crater is pushed upwards to form a <a href="https://www.sciencemag.org/news/2016/11/updated-drilling-dinosaur-killing-impact-crater-explains-buried-circular-hills">central peak or peak ring</a>. </p>
<p>It is quite alarming to think that all of these things occur within seconds to minutes of an impact, and can leave long-lasting scars on Earth’s surface, cause significant environmental effects, and even result in mass extinctions. </p>
<p>Impact craters are relics of truly catastrophic events on Earth. Rare mineral formation is just one of the possible outcomes when rocks arrive from space.</p><img src="https://counter.theconversation.com/content/105129/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nick Timms received funding from UK IODP NERC grant to study the drill core recovered from the Chicxulub Crater in the gulf of Mexico during IODP/ICDP Expedition 364 in 2016.</span></em></p>A meteorite hitting Earth at many kilometres per second puts ‘ground zero’ target rocks under immense pressure. A shock wave faster than the speed of sound can result – and new materials created.Nick Timms, Senior Lecturer, Curtin UniversityLicensed as Creative Commons – attribution, no derivatives.