What is it? Strange melt areas on an ancient volcano in the Hellas impact basin (NASA/HiRISE/Univ. of Arizona)
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This image is probably more suited to Nancy’s “Where In The Universe” series, but judging by the resolution and surrounding landscape, it may be fairly easy to distinguish which planet and what instrument took the shot. Of course, this is Mars and the image was snapped by the astounding HiRISE instrument on board the Mars Reconnaissance Orbiter (MRO). Still… what is it? Apart from looking like a particularly large coffee stain, the answer might not be very obvious. However, once we realise this is an image of an ancient volcano covered with ice, the big question is, why has the ice melted in discrete patches when the rest of the landscape looks like a winter wonderland?
On January 16th, the MRO dashed above the southern hemisphere of Mars, over the famous Hellas impact basin. This large crater is very interesting for many reasons, particularly as the altitude distance from the crater rim to the deepest part of the crater bottom is 9 km. This means there is a 89% increase in atmospheric pressure at the bottom of the crater when compared to the planet average. The pressure is therefore high enough to entertain the thought that liquid water may be a reality in this region (if the temperature gets higher than 0°C that is).
There are also ancient volcanoes in the region, of particular note is the group of volcanoes called Malea Patera (as captured in the HiRISE image above). As Hellas is so close to the southern arctic (antarctic?) region, it is currently entering spring time, surface ice is beginning to melt as the Sun creeps higher above the Martian horizon. However, there appears to be areas of ice that are melting faster than others, and a pattern is emerging.
Detail of the melting ice on Malea Patera (NASA/HiRISE/Univ. of Arizona)At first, I looked at the images and thought that there may be some heat being released from thermal vents in the volcanic region. However, HiRISE scientists have another explanation for the dalmatian spots that have appeared. On Earth, we will often find dark rocks that appear to have melted the snow from around them during a sunny day. This is because the sunlight will penetrate the snow and heat up the darker rocks quicker than the lighter rocks. Dark rocks will absorb solar energy faster than the more reflective light rock, dark rocks heat up faster, snow surrounding dark rocks melts quicker.
This basic ice melting mechanism is being singled out for what HiRISE is seeing on this ancient volcanic region. There are patches of dark rock melting the snow faster than the rest of the region as the Sun gradually heats the southern hemisphere. What is very interesting is the patches and shape of the melt region. Could it be an ancient lava outflow from a volcano? Are the patches sand dunes peppered with volcanic material? Or is there some other explanation? HiRISE scientists hope to take more images of Malea Patera as the seasons roll on to see how the ice continues to melt. It will be interesting to see what HiRISE finds under the ice during the summer…
Spirit is taking the long way around a low plateau called “Home Plate,” after loose soil at the edge blocked the shortest route south for the upcoming Martian summer and following winter. The rover has begun a trek skirting at least partway around the plateau instead of directly over it.
NASA officials say even a circuitous route to the destinations chosen for Spirit will be much shorter than the overland expedition the rover’s twin, Opportunity, is making on the opposite side of Mars. And they’re pointing out that Spirit has gotten a jump on its summer science plans, examining a silica-rich outcrop that adds information about a long-ago environment that had hot water or steam.
The view from "Home Plate" Plateau, where Spirit spent the winter.
Both of NASA’s Mars Exploration Rovers landed on Mars in 2004 for what were originally planned as three-month missions.
Spirit spent 2008 on the northern edge of Home Plate, a flat-topped deposit about the size of a baseball field, composed of hardened ash and rising about 1.5 meters (5 feet) above the ground around it. There, the north-facing tilt positioned Spirit’s solar arrays to catch enough sunshine for the rover to survive the six-month-long Martian winter.
The scientists and engineers who operate the rovers chose as 2009 destinations a steep mound called “Von Braun” and an irregular, 45-meter-wide (150-foot-wide) bowl called “Goddard.” These side-by-side features offer a promising area to examine while energy is adequate during the Martian summer. They’ll also provide the next north-facing winter haven beginning in late 2009. Von Braun and Goddard intrigue scientists as sites where Spirit may find more evidence about an explosive mix of water and volcanism in the area’s distant past. They are side-by-side, about 200 meters, or yards, south of where Spirit is now.
It’s mid-spring now in the southern hemisphere of Mars. The Sun has climbed higher in the sky over Spirit in recent weeks.
The rover team tried to drive Spirit onto Home Plate, heading south toward Von Braun and Goddard. They tried this first from partway up the slope where the rover had spent the winter. Only five of the six wheels on Spirit have been able to rotate since the right-front wheel stopped working in 2006. With five-wheel drive, Spirit couldn’t climb the slope. In January and February, Spirit descended from Home Plate and drove eastward about 15 meters (about 50 feet) toward a less steep on-ramp. Spinning wheels in loose soil led the rover team to choose another option.
“Spirit could not make progress in the last two attempts to get up onto Home Plate,” said rover project manager John Callas of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Alternatively, we are driving Spirit around Home Plate to the east. Spirit will have to go around a couple of small ridges that extend to the northeast, and then see whether a route east of Home Plate looks traversable. If that route proves not to be traversable, a route around the west side of Home Plate is still an option.”
During the drive eastward just north of Home Plate in January, Spirit stopped to use tools on its robotic arm to examine a nodular, heavily eroded outcrop dubbed “Stapledon,” which had caught the eye of rover-team scientist Steve Ruff when he looked at images and infrared spectra Spirit took from its winter position.
“It looked like the material east of Home Plate that we found to be rich in silica,” said Ruff, of Arizona State University in Tempe. “The silica story around Home Plate is the most important finding of the Spirit mission so far with regard to habitability. Silica this concentrated forms around hot springs or steam vents, and both of those are favorable environments for life on Earth.”
Sure enough, Spirit’s alpha particle X-ray spectrometer found Stapledon to be rich in silica, too. Researchers plan to use Spirit’s thermal emission spectrometer and panoramic camera to check for more silica-rich outcrops on the route to Von Braun and Goddard. However, the team has set a priority to make good progress toward those destinations. Winds cleaned some dust off Spirit’s solar panels on Feb. 6 and Feb. 14, resulting in a combined increase of about 20 percent in the amount of power available to the rover.
Oppy, meanwhile, shows signs of increased friction in its right-front wheel. The team is driving the rover backwards for a few sols, a technique that has helped in similar situations in the past, apparently by redistributing lubricant in the wheel. Opportunity’s major destination is Endeavour Crater, about 22 kilometers (14 miles) in diameter and still about 12 kilometers (7 miles) away to the southeast. Opportunity has been driving south instead of directly toward Endurance, to swing around an area where loose soil appears deep enough to potentially entrap the rover.
Rice University professors Patrick McGovern and Julia Morgan are proposing that pockets of water could be trapped under Olympus Mons on Mars -- and could support life. Credit: Rice University
Rice University professors Patrick McGovern and Julia Morgan are proposing that pockets of water could be trapped under Olympus Mons on Mars -- and could support life. Credit: Rice University
Olympus Mons is the latest hotspot in the hunt for habitable zones on Mars.
The Martian volcano is about three times the height of Mount Everest, but it’s the small details that matter to Rice University professors Patrick McGovern and Julia Morgan. After studying computer models of Olympus Mons’ formation, McGovern and Morgan are proposing that pockets of ancient water could still be trapped under the mountain. Their research is published in February’s issue of the journal Geology.
Olympus Mons is tall, standing almost 15 miles (24 km) high, and slopes gently from the foothills to the caldera, a distance of more than 150 miles (241 km). That shallow slope is a clue to what lies beneath, say the researchers. They suspect if they were able to stand on the northwest side of Olympus Mons and start digging, they’d eventually find clay sediment deposited there billions of years ago, before the mountain was even a molehill.
In modeling the formation of Olympus Mons with an algorithm known as particle dynamics simulation, McGovern and Morgan determined that only the presence of ancient clay sediments can account for the volcano’s asymmetric shape. The presence of sediment indicates water was or is involved.
The European Space Agency’s Mars Express spacecraft has in recent years found abundant evidence of clay on Mars. This supports a previous theory that where Olympus Mons now stands, a layer of sediment once rested that may have been hundreds of meters thick.
Morgan and McGovern show in their computer models that volcanic material was able to spread to Olympus-sized proportions because of the clay’s friction-reducing effect, a phenomenon also seen at volcanoes in Hawaii.
Credit: Rice University
But fluids embedded in an impermeable, pressurized layer of clay sediment would allow the kind of slipping motion that would account for Olympus Mons’ spread-out northeast flank – and they may still be there. And because NASA’s Phoenix lander found ice underneath the Martian surface last year, Morgan and McGovern believe it’s reasonable to suspect water could be trapped in the sediment underneath the mountain.
“This deep reservoir, warmed by geothermal gradients and magmatic heat and protected from adverse surface conditions, would be a favored environment for the development and maintenance of thermophilic organisms,” they wrote. On Earth, such primal life forms exist along deep geothermal vents on the ocean floor.
Finding a source of heat will be a challenge, Morgan and McGovern admit. “We’d love to have the answer to that question,” said McGovern. He noted that evidence of methane on Mars is considered by some to be another marker for life.
LEAD IMAGE CAPTION: Rice University professors Patrick McGovern and Julia Morgan are proposing that pockets of water could be trapped under Olympus Mons on Mars — and could support life. Credit: Rice University
Mars Exploration Rover Opportunity trundles over the dunes (NASA/HiRISE/Univ. of Arizona)
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Its pictures like these that put the Mars Program into perspective for me. We have two operational rovers that have rolled across the Martian landscape for five years (when they were designed to last only three months), and we have three satellites orbiting Mars carrying out a variety of key scientific studies. For one of the instruments orbiting over 250 km (155 miles) above the Red Planet on board the Mars Reconnaissance Orbiter (MRO), it is fulfilling the “reconnaissance” duties of the MRO rather nicely. The High Resolution Imaging Science Experiment (HiRISE) is helping out its roving buddy, Opportunity, to plot the best route through the undulating sandy dunes of Meridiani Planum. Robots helping other robots on Mars…
We’ve seen shots like this before taken by the high resolution camera used by HiRISE. From spotting the Phoenix Mars Lander repeatedly throughout 2008 to keeping a watchful eye on the progress of both rovers, the instrument has been an invaluable tool for NASA scientists to see what the landscape is like around the tough wheeled robots.
As another sol rolls on, MER Opportunity clocks up some more distance on its epic two year journey toward Endeavour, a crater 20 times larger than Opportunity’s previous crater subject, Victoria (now a feature shrinking in the rover’s rear view mirror). The rover has a long way to go, but should Opportunity survive the trip, it will be a momentous achievement. After all, the rover will be seven years old at that point.
A close-up of Opportunity, plus wheel tracks (NASA/HiRISE/Univ. of Arizona)For now, HiRISE is aiding the planning of Opportunity’s drive through the open Mars desert. As can be seen in the HiRISE image to the left (detail from the main image, top), 1783 sols into its mission, the rover is still going strong. The day before this image, Opportunity had driven 130 metres over the sand dunes. Generally, these dunes are mere ripples in the regolith, but some can be too big for Opportunity to traverse. However, HiRISE will spot any hazard well in advance, and NASA can plan Opportunity’s route accordingly.
So Opportunity roves on toward the southeast target of the Endeavour crater, about 17 km away. But HiRISE will be watching…
Launch abort system mock-up for Orion hits the road. Credit: NASA
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Imagine driving along in your car, minding your own business and getting passed by this. It could happen to you this week. This is a 13.7 meter (45 feet) -long full-scale mock-up that’s part of the rocket assembly for the launch abort system for the new Orion crew exploration vehicle. The system hit the road on Tuesday, March 3, 2009, and is traveling from NASA’s Langley Research Center in Hampton, Va., to White Sands Missile Range in New Mexico to undergo the first flight tests of the system. The launch abort system will allow the astronaut crew to safely escape in the event of an emergency during launch.
The mock-up, also known as the LAS pathfinder, represents the size, outer shape and specific mass characteristics of Orion’s abort system. Artist's rendering of a Launch Abort System (LAS) in operation. Credit: Orbital
The real system will be composed of solid rocket motors, separation mechanisms, and an adapter structure to provide escape capability for the Orion crew from pad operations through ascent. The new design, built by Orbital Sciences Corp. is key in vastly improving the safety of the flight crew as compared to what the shuttle has.
In case you’re wondering, in the background are large, white vacuum spheres used at the hypersonic wind tunnel complex at Langley.
It’s Wednesday, so that means its time for another Where In The Universe Challenge. Your mission, should you choose to accept is to name where in the Universe this image is from. Give yourself extra points if you can name the spacecraft responsible for this image. Check back on Thursday so find the answer. Good luck!
UPDATE: The answer has now been posted below. Don’t peek before you make your guess!
First of all, I owe everyone an apology, because a spacecraft didn’t take this image, it was an Earth-based telescope, the WIYN 3.5 meter telescope at Kitt Peak National Observatory near Tucson, AZ.
Hubble did take an image of the same object (which can be seen here) but its not the image above.
And what is this image? It shows a deep Hydrogen-alpha image of the brightest X-ray source in the sky, NGC 1275.
No one really knows exactly why or how these filaments emanating from the galaxy are produced, but they likely are the result of an interaction between the black hole in the center of the galaxy and the intracluster medium surrounding it. (The glowing background objects in this image are galaxies in that same galaxy cluster.)
At a distance of about 230 million light-years, this is the nearest example to Earth of such vast structures, which are seen surrounding the most massive galaxies throughout the Universe.
Its a very nifty image, that’s for sure , and yes, the Flying Spaghetti Monster does come to mind when looking at this! A few of you did say NGC 1275, and Jon Hanford actually got everything correct by saying the correct telescope, but I hope the rest of you didn’t get thrown off too much by my asking for the spaceraft– sorry, I got mixed up on which image I ended up using.
Thanks for playing, and check back again next week for another Where In The Universe Challenge.
Galileo’s first telescope was basically a tube containing two lenses, and was a three-power instrument. His next effort magnified objects approximately nine times. Now, you can have a Galileo-like experience, and view the things he saw looking through the “Galileoscope.” But the view will be much better. The Galileoscope, now on sale for the great price of $15 each USD (or less — see below), is a cornerstone project of the International Year of Astronomy, aiming to promote astronomical observing. These scopes are high quality, easy-to-assemble and easy-to-use. Order one or a ton at the Galileoscope website.
Galileoscopes are available for US $15 per kit. Discounts are available for group purchases of 100 or more, bringing the price down to US$12.50 each, reducing costs for schools, colleges, astronomical societies, or even parties of interested individuals.
Remember the first time you looked through a telescope? Consider sharing that experience by donating Galileoscopes to less-advantaged schools or organizations. Donating increases the project’s global impact and gives people who might otherwise never have the opportunity to look through a telescope the chance to join millions of skywatchers worldwide in a shared experience of astronomical discovery. Find out more about donating at the Galileoscope website.
The Galileoscope is a professionally endorsed scientific instrument, developed by astronomers, optical engineers and science educators to make the wonders of the night sky more accessible to everyone. Orders can now be placed through www.galileoscope.org for delivery beginning in late April.
The Galileoscope is a high quality 50-mm f/10 telescope, with a glass doublet achromatic objective. A 0-mm Plossl-like eyepiece with twin plastic doublet achromatic lens gives a magnification of 25x across a 1.5-degree field, and a 2x Barlow lens (also a plastic doublet achromat) gives a magnification of 50x. The Barlow lens can also be used as a Galilean eyepiece to give a magnification of 17x and a very narrow field of view to simulate the “Galileo experience”. The standard 1.25-inch focuser accepts commercial accessories.
Artist's conception of the binary supermassive black hole system. Credit P. Marenfeld, NOAO
Artist's conception of the binary supermassive black hole system. Credit P. Marenfeld, NOAO
Paired black holes are theorized to be common, but have escaped detection — until now.
Astronomers Todd Boroson and Tod Lauer, from the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, have found what looks like two massive black holes orbiting each other in the center of one galaxy. Their discovery appears in this week’s issue of Nature.
Astronomers have long suspected that most large galaxies harbor black holes at their center, and that most galaxies have undergone some kind of merger in their lifetime. But while binary black hole systems should be common, they have proved hard to find. Boroson and Lauer believe they’ve found a galaxy that contains two black holes, which orbit each other every 100 years or so. They appear to be separated by only 1/10 of a parsec, a tenth of the distance from Earth to the nearest star.
After a galaxy forms, it is likely that a massive black hole can also form at its center. Since many galaxies are found in cluster of galaxies, individual galaxies can collide with each other as they orbit in the cluster. The mystery is what happens to these central black holes when galaxies collide and ultimately merge together. Theory predicts that they will orbit each other and eventually merge into an even larger black hole.
“Previous work has identified potential examples of black holes on their way to merging, but the case presented by Boroson and Lauer is special because the pairing is tighter and the evidence much stronger,” wrote Jon Miller, a University of Michigan astronomer, in an accompanying editorial.
The material falling into a black hole emits light in narrow wavelength regions, forming emission lines which can be seen when the light is dispersed into a spectrum. The emission lines carry the information about the speed and direction of the black hole and the material falling into it. If two black holes are present, they would orbit each other before merging and would have a characteristic dual signature in their emission lines. This signature has now been found.
The smaller black hole has a mass 20 million times that of the sun; the larger one is 50 times bigger, as determined by the their orbital velocities.
Boroson and Lauer used data from the Sloan Digital Sky Survey, a 2.5-meter (8-foot) diameter telescope at Apache Point in southern New Mexico to look for this characteristic dual black hole signature among 17,500 quasars.
Quasars are the most luminous versions of the general class of objects known as active galaxies, which can be a hundred times brighter than our Milky Way galaxy, and powered by the accretion of material into supermassive black holes in their nuclei. Astronomers have found more than 100,000 quasars.
Boroson and Lauer had to eliminate the possibility that they were seeing two galaxies, each with its own black hole, superimposed on each other. To try to eliminate this superposition possibility, they determined that the quasars were at the same red-shift determined distance and that there was a signature of only one host galaxy.
“The double set of broad emission lines is pretty conclusive evidence of two black holes,” Boroson said. “If in fact this were a chance superposition, one of the objects must be quite peculiar.One nice thing about this binary black hole system is that we predict that we will see observable velocity changes within a few years at most.We can test our explanation that the binary black hole system is embedded in a galaxy that is itself the result of a merger of two smaller galaxies, each of which contained one of the two black holes.”
LEAD IMAGE CAPTION (more): Artist’s conception of the binary supermassive black hole system. Each black hole is surrounded by a disk of material gradually spiraling into its grasp, releasing radiation from x-rays to radio waves.The two black holes complete an orbit around their center of mass every 100 years, traveling with a relative velocity of 6000 kilometers (3,728 miles) per second. (Credit P. Marenfeld, NOAO)
SN 2009ab as seen by the AlbaNova Telescope in Stockholm, Sweden. Credit: Magnus Persson, Robert Cumming and Genoveva Micheva/Stockholm University
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SN 2009ab as seen by the AlbaNova Telescope in Stockholm, Sweden. Credit: Magnus Persson, Robert Cumming and Genoveva Micheva/Stockholm University
Have you ever discovered a supernova? Well, I haven’t, but I can only imagine finding a star that has blown itself to smithereens must be pretty exciting. At least that’s what I thought, anyway….
Seemingly, a fair amount of folks must be out there who have found supernovae. In 2008 alone, 278 supernovae were found, one by a 14-year old girl. But 2008 was a really slow year in the supernova department. In 2007, 584 were discovered – a record number – and in 2006, 557 supernovae were spied by astronomers, both professional and amateur. 40 have been found so far in 2009. But even with those fairly big numbers, I still gotta believe that finding a supernova must be absolutely incredible. So when someone I knew, Robert Cumming from Stockholm University in Sweden, recently played a part in finding a supernova, I emailed him my congratulations. Imagine my surprise when he replied, “It’s no big deal, really.”
But Robert, it’s a SUPERNOVA!
I had heard of Scandinavian stoicism, but this was off the charts! Besides that, I knew Robert is not originally from Sweden.
So, I begged him to tell me all about it.
“Well, since you ask,” he said with a smile. Okay, maybe, just maybe he was more excited than he was letting on. Robert Cumming.
Here’s the story of how Supernova 2009ab was discovered:
“I’ve observed a few supernovae before and I’ve had my name on the odd IAU circular, but this is the first time I’ve been one of the first to actually confirm one,” said Robert, with just a hint of excitement in his voice.
On February 8, the Katzman Automatic Imaging Telescope (KAIT), a 30-inch fully robotic telescope at the Lick Observatory on Mt. Hamilton in California discovered a bright spot not seen before in the outskirts of the spiral galaxy UGC 2998, 150 million light years away. Astronomers from KAIT wanted to make a second observation to verify, but bad weather made it impossible for them to confirm that the new object was not an asteroid or instrumental error. So, the KAIT astronomers requested observations from other telescopes around the world.
Magnus Persson, also from the Stockholm University was getting ready to do some observations using the University’s AlbaNova Telescope, when Robert received an email from KAIT about needing confirmation observations. AlbaNova Telescope. Credit: Teresa Riehm/Stockholm University.
“I knew Magnus was going to be observing – he was planning to take some pictures of the Crab nebula for a colleague,” said Robert. “And I had this mail from the KAIT in California.”
So, the two set to work in an effort to locate the possible supernova.
Robert and Persson used different filters and took a few images of galaxy UGC 2998. “The supernova was right there on our first 45-second exposure – we were kind of amazed!” he said.
The two astronomers from Sweden were able to establish that the new light source showed all the signs of being a supernova. The supernova shines in a blue color, in contrast to the stars in the galaxy which are generally old and red, and the other stars in the image which lie in our galaxy. Shortly after the explosion, such a supernova emits as much energy as the entire host galaxy.
“We did the observations properly, and then I picked the best data to make very rough photometry, got comparison magnitudes from Gregor Dusczanowicz, Sweden’s amateur supernova discoverer, talked to a colleague to check we hadn’t forgotten anything important, and mailed off the measurements to the Central Bureau for Astronomical Telegrams.”
Other telescopes have now observed SN 2009ab, but the AlbaNova telescope was the first to successfully take images and confirm it as a new supernova. The following day, astronomers on the Canary Islands took a spectrum using the considerably bigger Telescopio Nationale Galileo and were able to determine the supernova was of type Ia, that is a white dwarf star which had exploded in a binary system. As Magnus’ and Robert’s confirmation was published in an astronomical telegram, the new supernova was named SN 2009ab, this year’s 28th supernova.
It’s also the story of a new telescope in an unlikely location being used to make new and exciting — yes exciting –discoveries. The Stockholm University Department of Astronomy uses the AlbaNova telescope, a 1-meter reflector, mainly for education and instrumental development. Robert said the plan is to use the telescope to do environmental monitoring, using LIDAR to monitor ozone and particle pollution in the city.
But Robert said the discovery of the supernova shows it is also possible to do scientifically interesting astronomical observations with the telescope, despite the limitations from Stockholm’s bad weather and light pollution. The AlbaNova Observatory in Stockholm. Credit: Magnus Näslund/Stockholm University
“Our site is right in the city, so our sky brightness is scary. So far we haven’t measured just how bad it is, so it was a really nice surprise to get something out of it,” he said.
“The telescope is still pretty new, and with the Stockholm weather lately the experience of observing at all is pretty exciting,” Robert said. “And it is exciting that the telescope is now in full use. If we can do observations like these, we can do much more.”
So finally, I got Robert to admit he was excited. But the Scandinavian modesty and stoicism quickly returned.
This sequence of three images, obtained by NASA's Cassini spacecraft over the course of about 10 minutes, shows the path of a newly found moonlet in a bright arc of Saturn's faint G ring. Image credit: NASA/JPL/Space Science Institute
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Saturn’s G ring has been the ring without a moon, until now. In trying to understand the mysterious G ring, Cassini scientists have taken every opportunity they can to take a closer look at what could be creating the ring. In 2007, scientists identified a possible source of the G ring as relatively large, icy particles that resided within a bright arc on the ring’s inner edge. But the researchers thought there had to be more than just these particles “shepherding” the ring, and concluded that there had to be larger, yet-unseen bodies hiding in the arc. Their persistence has now paid off, as a small moonlet has been found within the ring. “Before Cassini, the G ring was the only dusty ring that was not clearly associated with a known moon, which made it odd,” said Matthew Hedman, a Cassini imaging team associate at Cornell University in Ithaca, N.Y. “The discovery of this moonlet, together with other Cassini data, should help us make sense of this previously mysterious ring.” The sequence of three images above, obtained by NASA’s Cassini spacecraft over the course of about 10 minutes, shows the path of the moonlet in a bright arc of Saturn’s faint G ring.
Cassini imaging scientists analyzing all the images acquired over the course of about 600 days found the tiny moonlet, about a half a kilometer (about a third of a mile) across, embedded within a partial ring, or ring arc, previously found by Cassini in Saturn’s tenuous G ring.
Scientists imaged the moonlet on Aug. 15, 2008, and then they confirmed its presence by finding it in two earlier images. They have since seen the moonlet on multiple occasions, most recently on Feb. 20, 2009. The moonlet is too small to be resolved by Cassini’s cameras, so its size cannot be measured directly. However, Cassini scientists estimated the moonlet’s size by comparing its brightness to another small Saturnian moon, Pallene.
Hedman and his collaborators also have found that the moonlet’s orbit is being disturbed by the larger, nearby moon Mimas, which is responsible for keeping the ring arc together.
This brings the number of Saturnian ring arcs with embedded moonlets found by Cassini to three. The new moonlet may not be alone in the G ring arc. Previous measurements with other Cassini instruments implied the existence of a population of particles, possibly ranging in size from 1 to 100 meters (about three to several hundred feet) across. “Meteoroid impacts into, and collisions among, these bodies and the moonlet could liberate dust to form the arc,” said Hedman.
Saturn’s rings were named in the order they were discovered. Working outward they are: D, C, B, A, F, G and E. The G ring is one of the outer diffuse rings. Within the faint G ring there is a relatively bright and narrow, 250-kilometer-wide (150-miles) arc of ring material, which extends 150,000 kilometers (90,000 miles), or one-sixth of the way around the ring’s circumference. The moonlet moves within this ring arc. Previous Cassini plasma and dust measurements indicated that this partial ring may be produced from relatively large, icy particles embedded within the arc, such as this moonlet.
Carl Murray, a Cassini imaging team member and professor at Queen Mary, University of London, said, “The moon’s discovery and the disturbance of its trajectory by the neighboring moon Mimas highlight the close association between moons and rings that we see throughout the Saturn system. Hopefully, we will learn in the future more about how such arcs form and interact with their parent bodies.”
Early next year, Cassini’s camera will take a closer look at the arc and the moonlet. The Cassini Equinox mission, an extension of the original four-year mission, is expected to continue until fall of 2010.
