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Our damaged, wrinkly Moon

Surface of the Moon

The ups and downs of the Moon's battered surface hint at the processes that have shaped it for eons.

WRITTEN ON THE MOON’S WEARY FACE are signs of the damage it has endured for the past 4.5 billion years. From impact craters to the dark plains or ‘maria’ left behind by volcanic eruptions, the scars are all that remain to tell the tale of the past.

But these features only hint at the processes that once acted—and act today—to shape the surface.

To get more insight, Meg Rosenburg and her colleagues at the California Institute of Technology have put together the first comprehensive set of maps revealing the slopes and roughness of the Moon’s surface, based on detailed data collected by the Lunar Orbiter Laser Altimeter (LOLA) on NASA’s Lunar Reconnaissance Orbiter.

Like wrinkles on skin, the roughness of craters and other features on the Moon’s surface can reveal their age.

“The key is to look at the roughness at both long and short scales,” says Rosenburg, who is the first author on the paper describing the results, published in the Journal of Geophysical Research earlier this year.

The roughness depends on the subtle ups and downs of the landscape, a quality that the researchers get at by measuring the slope at locations all over the surface.

A lunar maria

The lunar maria are smooth regions of solidified lava.

To put together a complete picture, the researchers looked at roughness at a range of different scales—the distances between two points—from 17 metres to as much as 2.7 kilometres.

“Old and young craters have different roughness properties—they are rougher on some scales and smoother on others,” says Rosenburg. That’s because the older craters have been pummelled for eons by meteorites that pit and mar the site of the original crater, changing its shape.

“Because this softening of the terrain hasn’t happened at the new impact sites, the youngest craters immediately stand out,” says Gregory Neumann, a co-investigator on LOLA at NASA’s Goddard Space Flight Centre.

By looking at where and how the roughness changes, the researchers can get important clues about the processes that shaped the Moon.

A roughness map of the material surrounding Orientale basin, for example, reveals subtle differences in the ejecta, or debris, that was thrown out when the crater was formed by a giant object slamming into the Moon.

That information can be combined with a contour map that shows where the high and low points are.

“By looking at both together, we can say that one part of Orientale is not just higher or lower, it’s also differently rough,” Rosenburg says. “That gives us some clues about the impact process that launched the ejecta and also about the surface processes that later acted to modify it.”

The smooth plains of the maria, which were created by volcanic activity, have a different roughness “signature” from the Moon’s highlands, reflecting the different origins of the two terrains. Maria is Latin for “seas,” and they got that name from early astronomers who mistook them for actual seas.

Adapted from information issued by Elizabeth Zubritsky, NASA’s Goddard Space Flight Centre.

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Wrinkles on the Moon

LROC image of Brisbane Z crater

A 'wrinkle ridge' splits the crater known as Brisbane Z, located in the Mare Australe region of the Moon. Image width is 100 kilometres. The region within the white box is shown in detail in the image below.

A WRINKLE RIDGE SEEMS TO DIVIDE the crater Brisbane Z in half. Brisbane Z is a mare-flooded crater within the Mare Australe region of the Moon.

Wrinkle ridges are one of several styles of tectonic deformation present on the Moon, and occur primarily in the maria, or lunar ‘seas’.

Wrinkle ridges are the result of contractional forces, and in the maria, these forces are believed to be from the weight of the basalts poured onto the surface by volcanic activity billions of years ago.

The same reasoning explains why wrinkle ridges are sometimes found in magma-flooded craters, where similar contractional forces are present at a smaller scale.

Close-up view of Brisbane Z's wrinkle ridge

A Lunar Reconnaissance Orbiter Camera close-up image of the terrain on Brisbane Z's wrinkle ridge. Image width is 500 metres.

Adapted from information issued by LRO Team / NASA / GSFC / Arizona State University.

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Lunar rock and roll

Crater Tycho

The 85-kilometre-wide crater Tycho on the Moon. The arrow marks the location of the huge boulder shown in detail below.

THE 85-KILOMETRE-WIDE CRATER Tycho is one of the standout features of the side of the Moon that faces Earth.

Named after the 16th century Danish astronomer Tycho Brahe, it is thought to have formed 108 million years ago when an asteroid smashed into the lunar surface.

Southern region of the Moon

Like the spokes of a wheel, rays of lighter-coloured ejected rock stretch away from the crater Tycho (the prominent crater near centre, with the central peak). Photo by Joe Huber, sourced from Wikipedia.

The impact formed the crater, and also flung out huge amounts of molten rock—called ejecta— in all directions.

That ejecta can still be seen today, in the form of bright ‘rays’ stretching away from Tycho.

Astronauts of the final Apollo mission, Apollo 17, managed to collect samples of Tycho ejecta from their landing site in the Taurus-Littrow valley, thousands of kilometres away.

Like many craters, it has a central peak that rises high above the crater floor. This peak was formed as the molten rock splashed back upwards immediately after the impact…just as a central splash back occurs when an object is dropped into a glass of milk.

Tycho’s central peak rises 1.6 kilometres above the floor.

Along with the rest of the lunar surface, Tycho has been imaged in detail by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft, currently still operating.

Large boulder in crater Tycho

A 320-metre-long block of ejected rock sits near the rim of Tycho. It has a smooth top, thought to be a veneer of solidified molten rock droplets.

One of LRO’s most startling images of Tycho shows a huge boulder, 320 metres long, perched near the crater’s rim.

This boulder is thought to have been blasted out of the lunar surface when the impactor that formed the crater, struck.

It has a smooth top, which scientists think is the result of a rain of molten rock droplets settling on it and solidifying.

Images courtesy of NASA / GFSC / Arizona State University.

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Moon’s backside is bulging!

Apollo image of Earthrise over the limb of the Moon

Apollo image of Earthrise over the limb of the farside of the Moon. The lunar farside has regions of much thicker crust than other parts of the Moon.

  • Moon’s crust is thin in some places and thick in others
  • Lunar farside has a particularly thick patch of crust
  • Earth’s tides could have caused it when the Moon was young

A bulge of elevated crust on the farside of the Moon—known as the lunar farside highlands—has defied explanation for decades. But a new study led by researchers at the University of California, Santa Cruz, shows that the highlands may be the result of tidal forces acting early in the Moon’s history when its solid outer crust floated on an ocean of liquid rock.

Ian Garrick-Bethell, an assistant professor of Earth and planetary sciences at UC Santa Cruz, found that the shape of the Moon’s bulge can be described by a surprisingly simple mathematical function.

“What’s interesting is that the form of the mathematical function implies that tides had something to do with the formation of that terrain,” said Garrick-Bethell, who is the first author of a paper on the new findings published in the November 11 issue of Science.

Image of the lunar farside

This false-colour image of the lunar farside from NASA's Lunar Reconnaissance Orbiter shows the highest elevations (above 20,000 feet) in red and the lowest areas in blue.

The paper describes a process for formation of the lunar highlands that involves tidal heating of the Moon’s crust about 4.4 billion years ago. At that time, not long after the Moon’s formation, the crust was separated from the mantle below it by an intervening ocean of magma.

As a result, the gravitational pull of the Earth caused tidal flexing and heating of the crust. At the polar regions, where the flexing and heating was greatest, the crust became thinner, while the thickest crust would have formed in the regions in line with the Earth.

This process still does not explain why the bulge is now found only on the farside of the Moon.

“You would expect to see a bulge on both sides, because tides have a symmetrical effect,” Garrick-Bethell said. “It may be that volcanic activity or other geological processes over the past 4.4 billion years have changed the expression of the bulge on the nearside.”

Lunar gravity maps

The researchers analysed topographical data from NASA’s Lunar Reconnaissance Orbiter and gravitational data from Japan’s Kaguya orbiter.

A map of crustal thickness based on the gravity data showed that an especially thick region of the Moon’s crust underlies the lunar farside highlands.

The variations in crustal thickness on the Moon are similar to effects seen on Jupiter’s Moon Europa, which has a shell of ice over an ocean of liquid water. Nimmo has studied the effects of tidal heating on the structure of Europa, and the researchers applied the same analytical approach to the Moon.

Tsiolkovskiy crater on the farside of the Moon.

Tsiolkovskiy crater on the farside of the Moon.

“Europa is a completely different satellite from our Moon, but it gave us the idea to look at the process of tidal flexing of the crust over a liquid ocean,” Garrick-Bethell said.

The mathematical function that describes the shape of the Moon’s bulge can account for about one-fourth of the Moon’s shape, he said. Although mysteries still remain, such as what made the nearside so different, the new study provides a mathematical framework for further investigations into the shape of the Moon.

“It’s still not completely clear yet, but we’re starting to chip away at the problem,” Garrick-Bethell said.

The paper’s co-authors include Francis Nimmo, associate professor of Earth and planetary sciences at UCSC, and Mark Wieczorek, a planetary geophysicist at the Institut de Physique du Globe in Paris.

Adapted from information issued by UCSC / NASA / Goddard.

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