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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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What’s up? October’s night sky

Diagram of October 10's night sky

The thin crescent Moon will be near Venus and Mars in the western sky around sunset on October 10, 2010.

This month is a pretty bare one for planet watchers, with both Mercury and Saturn too close (in angle) to the Sun to be visible. Venus, which has been shining brightly and prominently in the western sky during and after sunset, will drop below the horizon in the second half of the month. This means that the only naked-eye planets to be seen all month long will be Jupiter and Mars.

October 1

The Moon will be at Last Quarter today, at 1:52pm Sydney time (or 03:52 Universal Time).

October 2

The crescent Moon will be near the stars Castor and Pollux in the constellation Gemini.

October 5

If you’re up early, you’ll see the crescent Moon near the star Regulus, which is the brightest star in the constellation Leo.

October 6

The Moon is at perigee today—the point in its elliptical orbit when it is closest to Earth—at a distance of 359,452 kilometres.

October 8

It’s New Moon today, at 5:44am Sydney time (or 18:44 Universal Time on October 7). In the western sky, Mars and Venus will be close together

October 10

The Moon will appear near to the planets Venus and Mars in the western sky. It’s easy to tell which planet is which—Venus appears bigger and brighter with a whitish colour, while Mars is dimmer and has a ruddy orangey-red colour. Venus is dropping down lower toward the horizon, and will be gone from view in the second half of October.

October 11

Watch for the Moon near another well-known star; this time it is Antares, the brightest star in the constellation Scorpius. Compare the colour of Antares with the planet Mars, and you’ll see they look very much alike. The name Antares means “rival of Mars”.

October 15

The Moon will be at First Quarter today, at 8:27am Sydney time (or 21:27 Universal Time on October 14).

October 19

The Moon is at apogee today—the point in its elliptical orbit when it is furthest from Earth—at a distance of 405,432 kilometres.

October 20

The Moon will appear near the planet Jupiter tonight.

October 20-24

The Orionid meteor shower will be on display, with perhaps up to 20-30 meteors visible per hour from a dark location. As their name suggests, the Orionids appear to emanate from the constellation Orion, which is currently in the north-eastern sky (for Southern Hemisphere observers) in the hours between midnight and dawn. The problem this year, however, is that the Moon will be big and bright in the sky, and the wash from its light tends to drown out faint meteors. Still, give it a go and you might see some Orionids.

October 23

It’s Full Moon today, which occurs at 12:36pm Sydney time (or 01:36 Universal Time).

October 25

Last month, the Moon twice came quite close to a small star cluster called the Pleiades, or Seven Sisters, in the constellation Taurus. Well, tonight they get together again. If you have a pair of binoculars or a small telescope, take a look at the Pleiades—they’re a beautiful sight to behold.

October 30

The Moon will be at Last Quarter today, at 10:46pm Sydney time (or 12:46 Universal Time). Have a look tonight and you’ll see that, just like on the 2nd, the Moon will be near the stars Castor and Pollux.

If you have any questions or comments on the night sky, please use the feedback form below. Happy stargazing!

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