.

  • Written by Katarina Miljkovic, ARC DECRA fellow, Curtin University
Look at the circular patterns on the Moon's surface, as seen from Earth. Flickr/Bob Familiar, CC BY

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.

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.

Around 1960, the American astrogeologist Gene Shoemaker, 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.


Read more: Five ethical questions for how we choose to use the Moon


Images from Apollo

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 Apollo missions confirmed that impact cratering is a common geological process outside Earth.

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. NASA

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.

It does not mean Earth – even Australia – has not been battered. We should have been hit by more rocks from space than the Moon has, simply because our planet is larger.

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.

The big ones

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.

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. NASA/GSFC/University of Arizona

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 150-250km asteroid hurtling into the Moon at 15-20km per second!

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.

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.

My favourite is the Orientale impact basin, the youngest of the large impact craters on the Moon, but still estimated to have formed “only” about 3.7 billion years ago.

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. NASA/GSFC/Arizona State University

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 wiped out the dinosaurs on Earth 66 million years ago was only about 10-15km in size and left a crater larger than 150km in size, which was substantial enough to cause a mass extinction.)

As seen from Earth

With a small telescope, or fancy binoculars, you can check out some of the best-preserved complex craters on the Moon, such as the Tycho or Copernicus craters.

Tycho Crater is one of the most prominent craters on the Moon. NASA/Goddard/Arizona State University

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.

The formation of these craters excavated underlying material that was brighter than the actual surface. This is because lunar surface is subjected to space weathering, which causes surface rocks to darken.

Still a target for impacts

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 (ALSEP). During one year of operations, more than 1,000 seismic events were recorded, of which 10% were associated with meteoroids impacts.

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.


Read more: Target Earth: how asteroids made an impact on Australia


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 extraterrestrial seismometer would not be placed on a planetary surface (Mars) until 40 years later.

Nowadays, from Earth, using a small telescope (and armed with a little patience), you can see so-called “impact flashes”, which are small meteorite impacts on the lunar surface that is facing us.

You need to be quick to see the flashes – watch for the green boxes.

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.

Katarina Miljkovic receives funding from the Australian Research Council. She is a member of the National Committee for Space and Radio Science.

Authors: Katarina Miljkovic, ARC DECRA fellow, Curtin University

Read more http://theconversation.com/why-the-moon-is-such-a-cratered-place-118842

Your Airbnb guest could be a tenant. Until the law is cleared up, hosts are in limbo

A Victorian court decision that an Airbnb agreement had the status of a lease has profound implications for guests and hosts. Daniel Krason/ShutterstockWith summer holidays around the corner, many Vic...

Bill John Swannie, Lecturer in College of Law and Justice, Victoria University - avatar Bill John Swannie, Lecturer in College of Law and Justice, Victoria University

5 reasons I always get children picture books for Christmas

Children who love being read to are more likely to find learning to read easier. from shutterstock.comChristmas is just around the corner. If you’re wondering what to get your child, your friend...

Kym Simoncini, Associate professor Early Childhood and Primary Education, University of Canberra - avatar Kym Simoncini, Associate professor Early Childhood and Primary Education, University of Canberra

Tough nuts: why peanuts trigger such powerful allergic reactions

The humble peanut. Tasty for most, treacherous for some. Dr Dwan Price, Author providedFood allergens are the scourge of the modern school lunchbox. Many foods contain proteins that can set off an ove...

Dwan Price, Molecular Biologist and Postdoc @ Deakin AIRwatch pollen monitoring system., Deakin University - avatar Dwan Price, Molecular Biologist and Postdoc @ Deakin AIRwatch pollen monitoring system., Deakin University

Must end soon! But not too soon! The catch in time-limited sales tactics

Time-limited offers leverage risk-aversion. That is, the more you dislike risk, the more likely it is you will take the bait and buy now. www.shutterstock.comAs Christmas shopping ramps up, you may be...

Daniel Zizzo, Professor and Academic Dean of the School of Economics, The University of Queensland - avatar Daniel Zizzo, Professor and Academic Dean of the School of Economics, The University of Queensland

Refugees without secure visas have poorer mental health – but the news isn't all bad

Refugees without permanent visas can experience a prolonged sense of insecurity and displacement. From shutterstock.comThere are more than 29.4 million forcibly displaced asylum seekers and refugees a...

Yulisha Byrow, Postdoctoral Research Fellow, UNSW - avatar Yulisha Byrow, Postdoctoral Research Fellow, UNSW

God made the rainbow: why the Bible welcomes every colour in the gender spectrum

The Bible affirms, in various ways, the inclusion of those who diverge from male-female gender norms. ShutterstockThis article is part of a series exploring gender and Christianity “God made ...

Robyn J. Whitaker, Senior Lecturer in New Testament, Pilgrim Theological College, University of Divinity - avatar Robyn J. Whitaker, Senior Lecturer in New Testament, Pilgrim Theological College, University of Divinity

Jojo Rabbit: Hitler humour and a child's eye view of war make for dark satire

Jojo's allegiances in the film are split between an imagined friends and a real hideaway. Fox SearchlightJojo Rabbit is not Disney Studios’ first foray into Hitler parody. In 1943, it produced ...

Benjamin Nickl, Lecturer in International Comparative Literature and Translation Studies, University of Sydney - avatar Benjamin Nickl, Lecturer in International Comparative Literature and Translation Studies, University of Sydney

Boosting Your Child’s Learning through Play

Being a parent is probably the highest responsibility one has in a lifetime. Once it happens, it becomes ongoing and keeps posing new challenges to us. Of course, it may seem easy from some perspe...

Diana Smith - avatar Diana Smith

Different ideas of Christmas Gifts to give to your beloved ones

"CHRISTMAS GIFTS – one of the best festival gifts to give to your closed ones. Find Christmas presents for everybody on your checklist. Christmas is now here, and if that you haven't got a hop on ...

News Company - avatar News Company

The Ultimate Guide to Sneakers and Sneaker Brands

When it comes to finding the perfect pair of sneakers, it can be hard to know where to start. Finding the right pair for any kind of occasion can be difficult, so brushing up on your knowledge of sn...

News Company - avatar News Company

Conservative landslide at UK's Brexit election; Trump's ratings rise on strong US economy

Led by Boris Johnson, the Conservatives won 56% of the vote and will have an 80-seat majority. AAP/EPA/ VIckie FloresAt the December 12 UK election, the Conservatives won 365 of the 650 House of Commo...

Adrian Beaumont, Honorary Associate, School of Mathematics and Statistics, University of Melbourne - avatar Adrian Beaumont, Honorary Associate, School of Mathematics and Statistics, University of Melbourne

Johnson's thumping win an electoral lesson in not just having policies, but knowing how to sell them

With Johnson's crushing win, Brexit will now happen. But this may also be the start of the break-up of the UK. AAP/EPA/Vickie FloresSo for all the talk of narrowing polls, tactical voting, and possib...

Simon Tormey, Professor of Politics, University of Bristol - avatar Simon Tormey, Professor of Politics, University of Bristol

View from The Hill: Morrison won't have a bar of public service intrusions on government's power

Scott Morrison has rejected or sidelined a number of recommendations from the long-awaited Thodey review. AAP/Paul BravenScott Morrison has rejected or sidelined a number of recommendations from the l...

Michelle Grattan, Professorial Fellow, University of Canberra - avatar Michelle Grattan, Professorial Fellow, University of Canberra

VIDEO: Michelle Grattan reflects on the year in politics

For their last video for the year, University of Canberra Vice-Chancellor Deep Saini and Michelle Grattan look backwards to the big issues which have shaped political discourse. They discuss the surpr...

Michelle Grattan, Professorial Fellow, University of Canberra - avatar Michelle Grattan, Professorial Fellow, University of Canberra

Choosing a Heavy Weighted Bathmats

Decorating your bathroom doesn’t finish with the paint choice or tile colours. You can change the whole room with the change of your towel set. There are no rules that say that you need to have a ...

News Company - avatar News Company

God as man, man as God: no wonder many Christian men today are having a masculinity crisis

How men saw God shaped how they saw themselves, and in turn, how they saw women. WikimediaThis article is part of our Gender and Christianity series. To understand contemporary Christian ideas about...

William Loader, Emeritus Professor of New Testament at Murdoch University, Murdoch University - avatar William Loader, Emeritus Professor of New Testament at Murdoch University, Murdoch University

Australia needs a national crisis plan, and not just for bushfires

Bushfires aren't the only catastrophic emergency Australia is likely to see. AAP Image/Mick TsikasCalls are growing for a national bushfire plan, including from former prime minister Malcolm Turnbull...

Andrew Gissing, General Manager, Risk Frontiers, Adjunct Fellow, Macquarie University - avatar Andrew Gissing, General Manager, Risk Frontiers, Adjunct Fellow, Macquarie University

Your Christmas shopping could harm or help the planet. Which will it be?

Many Australian consumers are concerned at the environmental impact of their shopping habits, especially at Christmas. AAPAustralian shoppers are set to spend $52.7 billion this Christmas. In the word...

Louise Grimmer, Senior Lecturer in Marketing, Tasmanian School of Business and Economics, University of Tasmania - avatar Louise Grimmer, Senior Lecturer in Marketing, Tasmanian School of Business and Economics, University of Tasmania

Right-swipes and red flags – how young people negotiate sex and safety on dating apps

For many young people, app dating is just part of regular dating life. freestocks.org/UnsplashPopular commentary on dating apps often associates their use with “risky” sex, harassment and ...

Kath Albury, Professor of Media and Communication, Faculty of Health, Arts and Design, Swinburne University of Technology - avatar Kath Albury, Professor of Media and Communication, Faculty of Health, Arts and Design, Swinburne University of Technology

Sick and Tired of Your Dead End Job? Try Teaching!

Tired of the same old grind at the office? Want an opportunity to impact lives both in your community and around the world? Do you love to travel and have new experiences? Teaching English is the perfect job for you! All you need is a willingness to ...

News Company - avatar News Company

The Impact of an Aging Population in Australia

There’s an issue on the horizon that Australia needs to prepare for. The portion of elderly citizens that make up the country’s overall population is increasing, and we might not have the infrastructure in place to support this. Australians h...

News Company - avatar News Company

LifeStyle

Boosting Your Child’s Learning through Play

Being a parent is probably the highest responsibility one has in a lifetime. Once it happens, it...

Different ideas of Christmas Gifts to give to your beloved ones

"CHRISTMAS GIFTS – one of the best festival gifts to give to your closed ones. Find Christmas pr...

How to Have a Peaceful Retirement

Retirement is the time to treat yourself after a lifetime of working, to complete your bucket list...

Latest Wednesday Lotto Results

Wednesday Lotto draw 3917 Lucky numbers for this draw were 43 followed by 25. The rest of the...