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New images of an icy world

Cassini image of Rhea

NASA's Cassini spacecraft took this raw, unprocessed image of Saturn's moon Rhea on March 10, 2012. The camera was pointing toward Rhea from a distance of approximately 41,873 kilometres.

THESE RAW, UNPROCESSED IMAGES of Saturn’s second largest moon, Rhea, were taken on March 10, 2012, by NASA’s Cassini spacecraft. This was a relatively distant flyby with a close-approach distance of 42,000 kilometres, well suited for global geologic mapping.

At 1,530 kilometres diameter, Rhea is the ninth-largest moon in the Solar System.

During the flyby, Cassini captured these views of the moon’s cratered surface, creating a 30-frame mosaic of Rhea’s leading hemisphere and the side of the moon that faces away from Saturn.

The observations included the large Mamaldi (480 kilometres across) and Tirawa (360 kilometres across) impact basins and the 47-kilometre-wide “ray crater”Inktomi, one of the youngest surface features on Rhea.

Cassini image of Rhea

This second raw, unprocessed Cassini image of Rhea was taken from a distance of approximately 42,258 kilometres, and shows the moon's icy, cratered surface. The streaks on the right are an artefact of the imaging.

Cassini image of Rhea

Shadows help to give a 3D effect to Rhea's craters in this raw, unprocessed Cassini shot taken from a distance of approximately 42,096 kilometres.

Cassini image of Rhea

This raw, unprocessed shot was taken from much further away, approximately 115,060 kilometres, and shows Rhea's "terminator"—the dividing line between day and night.

Cassini has been investigating Saturn and its moons since 2004. This included dropping a probe called Huygens onto the surface of Saturn’s largest moon, Titan, in 2005. Launched in 1997, Cassini-Huygens mission is a co-operative project of NASA, the European Space Agency and the Italian Space Agency.

See all of Cassini’s raw images at NASA’s Saturn page.

Adapted from information issued by NASA / JPL-Caltech / SSI.

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Latest Dawn image of Vesta

Dawn image of the surface of the asteroid Vesta

Craters large and small litter the surface of the 530-kilometre-wide asteroid Vesta. This image was taken by the Dawn's spacecraft's framing camera. Dawn will spend the next 12 months in orbit around Vesta.

NASA’S DAWN SPACECRAFT obtained this image of Vesta with its framing camera on July 31, 2011. It was taken from a distance of about 3,700 kilometres from the giant asteroid.

Vesta is one of the largest of the asteroids, and orbits the Sun in the main asteroid belt between Mars and Jupiter.

The smallest visible detail, which is about 2 pixels, corresponds to roughly 70 metres.

Dawn arrived at Vesta on July 16 after a journey of almost four years. It is now in orbit around the 530-kilometre-wide asteroid and will spend the next 12 months investigating it.

At the end of those 12 months, it will depart Vesta and voyage to the even-larger main belt body, the dwarf planet Ceres, and spend a further 12 months studying that object.

The aim of the Dawn mission is to shed light on the origins of the Sun and the planets by examining these two asteroids, which are thought to be amongst the oldest surviving bodies in the Solar System.

More information:

First close-up images from Vesta

NASA’s Dawn mission pages

Adapted from information issued by NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

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Earth from Space — Outback crater

The Shoemaker Impact Structure in Western Australia, seen from orbit.

The Shoemaker Impact Structure in Western Australia, seen from orbit.

THE SHOEMAKER (FORMERLY TEAGUE) IMPACT STRUCTURE—located in Western Australia in a drainage basin south of the Waldburg Range—presents an other-worldly appearance in this photograph taken by an astronaut aboard the International Space Station.

The Shoemaker impact structure is approximately 30 kilometres in diameter and clearly defined by concentric ring structures formed in sedimentary rocks (brown to dark brown, image centre).

The structure is thought to have formed following the impact of a large meteoroid approximately 1.63 billion years ago, although some age-dating analyses of rocks at the core of the structure have called this age into question.

Several saline and ephemeral lakes—Nabberu, Teague, Shoemaker, and numerous smaller ponds—are found between the ring structures. Differences in colour result from both water depth and from suspended sediments, with some bright salt crusts visible around the edges of smaller ponds (image centre).

The Teague Impact Structure was renamed Shoemaker in honour of Dr Eugene M. Shoemaker (1928-1997), a pioneer in impact crater studies and planetary geology, as well as the founder of the Astrogeology Branch of the US Geological Survey. Dr Shoemaker (and his wife, Carolyn) made many trips to Australia to scout out impact craters. He died tragically in a head-on car collision during one of those expeditions.

Astronaut photograph provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Centre. Text adapted from information issued by William L. Stefanov, Jacobs/ESCG at NASA-JSC.

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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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Scars on Mars

Elongated crater on Mars

This elongated crater on Mars is about 78km in length and reaches a depth of 2km. It was probably formed by the impact of a train of projectiles.

A NEW IMAGE OF AN ELONGATED impact crater in the southern hemisphere of Mars hints at a violent origin. Scientists think it could have been carved out by a train of meteoroid projectiles striking the planet at a shallow angle.

Image of a region of Mars including Huygens crater

The elongated crater (centre) is located near the 450km-wide Huygens crater.

The image above was captured by the European Space Agency’s (ESA) Mars Express spacecraft on 4 August 2010, and the smallest objects distinguishable by the camera are about 15m across.

The unnamed crater sits just to the south of the much larger Huygens basin (see image at right). It is about 78km in length, opens from just under 10km wide at one end to 25km at the other, and reaches a depth of 2km.

Impact craters are generally round because the projectiles that create them push into the ground before the shockwave of the impact can explode outwards. So why is this one elongated?

The clue comes from the surrounding smattering of material, thrown out in the initial impact. This ‘ejecta blanket’ is shaped like a butterfly’s wings, with two distinct lobes. It hints that two projectiles, possibly halves of a once-intact body, slammed into the surface here.

And the formation of this sort of elongated feature is not finished. In a few tens of millions of years, the Martian moon Phobos will plough into the planet, breaking up in the process, and likely creating new crater chains across the surface.

3D view of the crater

A striking perspective view of the crater.

Adapted from information issued by ESA / DLR / FU Berlin (G. Neukum) / NASA / MGS / MOLA Science Team.

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Crater on Mars

Part of the Schiaparelli Basin on Mars

This image shows a small part of Mars' huge Schiaparelli Basin (flat area on the left-hand side of the image) with a 42km-wide crater embedded in its rim (bottom right).

SCHIAPARELLI IS A LARGE IMPACT BASIN about 460 km across, located in the eastern Terra Meridiani region of Mars’ equatorial region.

The image here shows just a tiny part of the basin’s northwestern rim, cutting diagonally across the image from top left to bottom right (ie. the left-hand side of the image is part of Schiaparelli; everything else is outside the basin). The prominent crater at the bottom of the image is 42km wide and is embedded in Schiaparelli’s rim.

The image was taken on 15 July 2010 by the High-Resolution Stereo Camera (HRSC) on ESA’s Mars Express spacecraft. Ground resolution of the image is about 19 metres per pixel.

See the full-size (1.2MB) image here. Don’t forget to zoom in!

The image below shows a radar altimeter map of the entire Schiaparelli Basin, made by the Mars Orbital Laser Altimeter aboard NASA’s Mars Global Surveyor spacecraft. The large crater shown in the image above, is visible in this radar map on the rim of the basin at the 10 o’clock position (the orientation of the photo and map are turned 90 degrees to each other).

Radar map of the Schiaparelli Basin

Radar map of the Schiaparelli Basin. Colours indicate altitude in metres, as per the scale at top.

Adapted from information issued by ESA / DLR / FU Berlin (G. Neukum) / NASA.

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Moon’s face is a history book

Artist's impression of LRO

Artist's impression of the NASA's Lunar Reconnaissance Orbiter spacecraft, which has been mapping the Moon with unprecedented precision.

  • Moon’s craters came from bombardment by impactors
  • Now, first comprehensive map of craters has been made
  • Shows the bombardment changed dramatically at a certain stage

Take a cursory look at the Moon, and it can resemble a pockmarked golf ball. The dimples and divots on its surface are testament that our satellite has withstood a barrage of impacts from comets, asteroids and other space matter throughout much of its history.

Because the geological record of that pummelling remains largely intact, scientists have used the Moon to help reconstruct the history of the chaotic early days of the inner Solar System.

Now a team led by Brown University planetary geologists has produced the first uniform, comprehensive catalogue of large lunar craters…which could help shed light on the full-scale, planetary bombardment suffered by the planets of the inner Solar System more than 4 billion years ago.

In a paper appearing in the journal Science, the team used data from the Lunar Orbiter Laser Altimeter (LOLA), one of a suite of instruments aboard NASA’s Lunar Reconnaissance Orbiter, to identify and map 5,185 craters that are 20 kilometres in diameter or larger.

Map of craters on the Moon

A research team mapped nearly 5,200 craters on the Moon, the first global catalogue of large craters on the lunar surface. The analysis could shed light on planetary bombardment in the inner Solar System more than 4 billion years ago.

From the crater count and analysis, the team (which includes scientists from the Massachusetts Institute of Technology and the NASA Goddard Space Flight Centre) determined the Moon’s oldest regions are the southern near side and the north-central far side.

The group also confirmed that the South Pole–Aitken Basin is the oldest basin, meaning that any samples from there could be invaluable to further understanding the Moon and other bodies of the inner Solar System.

Rethinking the Moon’s bombardment

A major finding deals with the stream of projectiles pinballing throughout the inner Solar System in its earliest days.

For years, the prevailing wisdom was that the Moon was buffeted by a volley of space matter that held a steady ratio between larger and smaller objects, which planetary scientists refer to as “size-frequency distribution.”

The bombardment activity has never been questioned. But in 2005, the size-frequency distribution part of it was challenged. In a paper in Science, a group led by University of Arizona geologist Robert Strom hypothesised that the ratio of larger and smaller objects striking the Moon had differed during its first billion years of existence.

The Brown-led team’s crater analysis lends added credence to that hypothesis.

The researchers studied impact craters formed early in the Moon’s history (when major basins were created by large projectiles striking the surface) and compared them with those they knew were formed later (when objects struck lava flows that had covered these basins).

Craters on the Moon

Scientists have found differences in the size range of craters on the lunar highlands compared to the lowlands.

They found that the oldest surfaces (located in the lunar highlands) bore crater markings indicating a greater ratio of larger impactors. The group looked in particular at Orientale Basin, formed by a massive impactor about 3.8 billion years ago, and determined that this is approximately when the era of larger projectiles versus smaller projectiles ended.

Much more to do

The finding opens a set of intriguing questions for what was going on in the inner Solar System leading up to roughly the time that Orientale Basin was formed.

“We know the asteroid belt has been spinning off projectiles at a relatively constant rate for three and a half billion years,” said Caleb Fassett, a postdoctoral researcher at Brown. “But now we go back earlier in the Solar System’s history, and suddenly things are completely different.”

The scientists think the change may have been caused by the gravitational pull on the asteroid belt exerted by larger planets such as Jupiter and Saturn as they settled into their orbits…or it could have been a temporary abundance of comets, an unexplained change in the size of impactors emanating from the asteroid belt, or something else.

The Lunar Orbiter Laser Altimeter measures the Moon’s surface topography with a vertical precision of 10 centimetres using laser pulses bounced off the lunar surface just 25 metres apart.

In all, the findings “are telling us something about the infancy of the Solar System,” said James W. Head III, a planetary geologist at Brown and the paper’s lead author. “It is clear we can find out and learn so much more from future missions, robotic or otherwise. There is so much to do.”

Adapted from information issued by NASA / LRO / LOLA / GSFC / MIT / Brown.

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Which moon is this?

The surface of Saturn's moon Dione

Craters and fracture lines cover the surface of Saturn's moon Dione in this image taken by NASA's Cassini spacecraft.

  • Saturn’s heavily cratered moon Dione
  • Always keeps the same face toward Saturn
  • Seems to have been spun around in the past

At first glance it looks a bit like Earth’s Moon, but it’s actually Saturn’s moon Dione.

NASA’s Cassini spacecraft took this close-up view of the cratered, fractured surface on January 27, 2010. Cassini came within about 45,000 kilometres (28,000 miles) of the moon during the flyby, and this image was acquired at a distance of approximately 46,000 kilometres (29,000 miles). It shows detail down to about 270 metres (886 feet) per pixel.

Dione (pronounced Die-OH-nee) is a small moon of 1,118 kilometres (695 miles) diameter that orbits Saturn every 2.7 days at a distance of 377,400 kilometres (234,000 miles), which is roughly the same distance that the Moon orbits around the Earth.

Its features include heavily cratered terrain with craters as large as 100 kilometres (62 miles) across, plus other moderately cratered plains, lightly cratered plains, and fractured areas.

The heavily cratered areas are most common on the trailing hemisphere. Logically, a moon’s leading hemisphere should be the more heavily cratered—just like your car’s windscreen collides with more insects that its back window—so it has been suggested that an impact with another body spun Dione around. It has been calculated that bodies as small as those that made 35-kilometre (22-mile) craters could have spun Dione on its axis.

However, the fact that Dione seems to have spun exactly 180 degrees is a mystery.

A wispy, icy moon

Fractured areas, seen in Voyager spacecraft images as bright thin wispy lines, have lengths of tens to hundreds of kilometres, often cutting through plains and craters. Cassini fly-bys starting in 2005 showed “the wisps” as bright canyon ice walls (some of them several hundred metres high), probably caused by subsidence cracking. The walls are bright because darker material falls off them, exposing the bright water ice underneath. These fracture cliffs suggest Dione experienced tectonic activity in its past. They could be a mature phase of the so-called “tiger stripes” on one of Saturn’s other moons, Enceladus.

Wispy terrain on Dione

Wispy terrain stretches across the trailing hemisphere of Saturn's moon Dione. The wisps are caused by bright ice lining canyon walls.

Very fine ice powder (equivalent to cigarette smoke) from Saturn’s E-ring constantly bombards Dione. The dust in the E-ring originally comes from Enceladus, which has prominent geyser activity.

Dione’s density is 1.48 times that of liquid water, suggesting that about a third of the moon is a dense core (probably silicate rock) and the rest is ice. At Dione’s extremely cold average temperature, ice is very hard and behaves like rock.

As with Earth’s Moon, Dione is “phase locked” with its parent, which means the same side always faces toward Saturn. Likewise, Dione has gravitationally locked two much smaller moons—Helene orbits Saturn 60 degrees ahead of Dione, and Polydeuces orbits Saturn 60 degrees behind Dione.

Dione is in “resonance” with two nearby moons, Mimas and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which helps keep them locked in their positions. Dione keeps Enceladus locked at a period exactly one half of the Dione orbit.

Dione’s discovery

The Italian astronomer Giovanni Cassini discovered Dione in 1684. The English astronomer John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.

The name Dione comes from the Greek goddess (or titan) Dione, who by some accounts was the daughter of Tethys and Oceanus and who Homer described as the mother of Aphrodite.

Cassini referred to Dione as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Iapetus, Tethys, and Rhea). Other astronomers named the moons of Saturn by number in terms of distance from the planet. Thus, Dione was Saturn IV.

The International Astronomical Union controls naming of astronomical bodies. Geological features on Dione generally are given names from people and places in Virgil’s Aeneid.

Adapted from information issued by NASA / JPL / Space Science Institute.

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Saturn’s icy moon Dione

One side of Saturn's moon, Dione

Icy terrain with wispy patterns covers one side of Saturn's moon Dione.

Wispy terrain stretches across the trailing hemisphere of Saturn’s moon Dione in this view taken by NASA’s Cassini spacecraft during its January 27, 2010, non-targeted flyby.

Cassini came within about 45,000 kilometres of the moon during this flyby, but this image was acquired at a distance of approximately 137,000 kilometres from Dione. This view looks toward the side of the moon that was facing away from Saturn, and in particular its trailing hemisphere (the half of Dione that faces “backwards” as the moon orbits the planet).

Dione (pronounced dy-OH-nee) is 1,123 kilometres wide, and this image shows detail down to 819 metres per pixel.

Dione is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.

Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.

Contrary to what scientists had expected upon studying this fascinating moon, much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This anomaly suggests that during the period of heavy meteors bombardment, Dione was ‘tidally locked’ to Saturn in the opposite orientation. (A moon is tidally locked when it keeps the same face to its parent planet at all times.)

Because Dione is relatively small, an impact big enough to cause a 35-kilometre-diameter crater could have spun the moon. Since there are many craters larger than 35-kilometres, Dione could easily have been spun more than once. The moon has probably been tidally locked in its current orientation for the past several billion years.

Adapted from information issued by NASA / JPL / Space Science Institute.