In the news this month: stormy Saturn days.
For many astronomers, one of the most impressive sights in the night sky will undoubtedly be the rings of Saturn. The rings can claim this accolade because of how readily visible they are in night sky even when using amateur telescopes. The rings appear from the Earth to be made of two main components that glow with a slightly brownish light and are separated by a narrow dark stripe. However, it was not until space probes like Voyager and the later, more specialised Cassini mission actually travelled to Saturn, that the incredible nature of the rings could be understood. Instead of being made up of just two concentric rings, Saturn's rings appear as hundreds of concentric circular filaments made up mostly of billions of fragments of water ice. This month's news is dedicated to the science of Saturn, both in its rings and its atmosphere.
Viewed edge-on, Saturn's rings all but disappear. This is because, even though Saturn's rings stretch up thousands of kilometres above the planet's equator, they have a thickness, when unperturbed by passing moons, of merely ten metres. This is equivalent to the Earth being enclosed by a ring only a single atom thick. It may be assumed, then, that observing Saturn's rings edge-on may yield no useful information, and for much of the time this is true. But every fourteen-and-a-half years the entire scene changes, when the Saturnian equinox occurs. The Saturnian equinox is the moment when, from the perspective of Saturn, the Sun passes from one side of the rings to the other. During this transition there are a few days where the Sun's light shines through the disc. It is at this point that it becomes easy to see walls of cloud that protrude up out of the plane of the disc. These clouds are made up of fragments of ice that have been ejected after a meteor impact.
A team of astronomers working with images taken by the Cassini orbiter has been investigating the ejecta clouds caused by meteors striking Saturn's rings. The meteors in question are thought to be only a few centimetres up to about one metre in radius. This sort of meteor is far too small to be able to observe directly, which means that the clouds ejected after an impact with Saturn's rings may be the only way for astronomers to know they existed. Although it is unlikely that Cassini will observe an impact as it occurs, this does not matter as the impact time can be estimated by measuring the length of the cloud. Initially the impact cloud will be a spherical ball of icy material thrown in all directions, but, as Saturn's rings rotate, the ball is stretched out into a line that could be up to thousands of kilometres long. What the astronomers hope to understand from this investigation of small meteor impacts on Saturn's rings is whether or not there are enough impacts to replenish the ring system with fresh material faster than it is lost to the atmosphere of Saturn. This has large implications for the age of Saturn's rings, as there is currently debate as to whether the rings are a young 100 million years old or formed not long after Saturn several billion years ago.
In August, 1980, the Voyager I space probe began its one and only visit to Saturn, eventually passing as close as just a few million kilometres above its gassy atmosphere. The images taken by Voyager I became the best pictures of Saturn since NASA's previous visit with Pioneer 11. However, there were unusual features found in some of the images taken. These features were two or three large dark shadows banded around the surface of Saturn; the cause of these shadows at the time was unknown. Some scientists had a theory that perhaps this effect could be caused by rain falling onto Saturn's upper atmosphere, originating from Saturn's own rings. Unfortunately there was no way of being sure what the cause of the shadows was as Voyager 1 was bound on a course that would leave Saturn far behind it. How would water be transported from the rings and down onto Saturn? The answer to this is related to the interactions between the smallest fragments of water ice in the rings and Saturn's magnetic field. The smallest icy particles are able to acquire static charges from interactions with light or meteor strikes on the rings. The motion of these charged ice particles then becomes dominated by Saturn's magnetic field and they soon find themselves spiralling along the magnetic field lines, falling out of the ring until they eventually crash into Saturn's upper atmosphere as rain.
With the goal of proving or disproving the possibility of rain falling from Saturn's rings, some astronomers set to work using the Keck Observartory's near-infrared spectroscopic telescope. The aim was to try to measure emission from ionised molecular hydrogen high up in the ionosphere of Saturn, the reason being that, if water molecules were falling onto the ionosphere from the rings, then the amount of ionised molecular hydrogen would be depleted. This is exactly what was observed and, slightly unexpectedly, it seemed that a much larger area of Saturn was experiencing rainfall than had been predicted. This mechanism for removing water ice from the rings is known as electromagnetic erosion and acts to destroy the rings. Electromagnetic erosion, like the meteor impacts in the previous story, is another clue in estimating the age of Saturn's rings.
And finally: The Cassini orbiter science team last month released a new set of processed images of a gigantic hurricane forever blowing around the north pole of Saturn. They have been observing the north polar regions of Saturn since the beginning of the Cassini mission because of an unusual hexagon-shaped feature that could be seen. However, the core of the feature could not be observed because Saturn's pole was tilted away from the Sun. This changed after the 2009 Saturnian equinox as the north pole became illuminated, revealing the eye of Saturn's perpetual hurricane. This Saturnian hurricane is different to terrestrial hurricanes, not because of its size or tremendous wind speeds of 330 miles per hour but because it is locked, unmoving, onto the north pole. Earth-based hurricanes drift gradually towards their nearest polar region because of the interaction between the rotation of the spiralling winds and the rotation of the Earth. However, a Saturnian hurricane has nowhere left to go and so remains stationary. Polar hurricanes may not be something we see on the Earth but elsewhere in the Solar System they are certainly not unusual. Saturn itself has a matching hurricane on its south pole, and there may be one on Neptune and even one on Titan, one of Saturn's moons.
Interview with Dr Rubina Kotak
Dr Rubina Kotak, from Queen's University Belfast, talks to us about her research into supernovae arising from massive stars. She explains that more and more objects are being found that defy the current supernova classification system, from very bright, distant objects to very dim ones, and tells us what is being done to try explain them and to fit them into our framework for understanding supernovae.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during May 2013.
The constellation of Taurus is setting in the west at dusk, with Leo moving across the southern sky. To the south-east are Virgo and Coma Berenices, between which the 'Realm of the Galaxies' provides a rich area for telescope observations. The planet Saturn is near to Virgo's brightest star, Spica. Boötes, with its bright star Arcturus, lies to the east, above Hercules. The four brightest stars in Hercules make a trapezium known as the Keystone, on one side of which is the largest and brightest globular cluster in the northern hemisphere, M13. Lyra, Cygnus and Aquila rise later in the evening, with their bright stars Vega, Deneb and Altair forming the 'Summer Triangle'. Near to Vega, in Lyra, is what appears through binoculars to be a double star. A telescope reveals that each component is itself a double star, lending the system the name of the 'Double Double'. Jodrell Bank's Astronomical A-list gives details of some the night sky's nicest objects.
- Jupiter is low in the west at sunset, shining at magnitude -2 at the beginning of May, lying between the horns of Taurus the Bull, 9° above the star Aldebaran. Its angular diameter drops from 33.6 to 32.4" during the month, but a small telescope will show the planet's atmospheric bands are four largest moons.
- Saturn reached opposition (on the opposite side of the sky to the Sun) on the 28th of April, and so is visible all night at the beginning of the month, in the constellation of Libra. It moves into Virgo in the middle of the month, travelling westwards (retrograde motion) as the Earth overtakes it in its smaller orbit around the Sun. The rings are inclined at 18° to the line of sight and extend to a diameter of 43". We are seeing Saturn's southern hemisphere, as the northern half is largely covered by the rings. Its largest moon, Titan, can be seen with a small telescope. It reaches a maximum elevation of 28°.
- Mercury passes behind the Sun (superior conjunction) on the 11th, becoming visible again around the 19th, when it joins Jupiter and Venus in the evening sky. It then has a magnitude of -1.4, but binoculars will be needed to see it just after sunset.
- Mars reached superior conjunction on the 18th of April and will not reappear into the pre-dawn sky for several months.
- Venus emerges from behind the Sun in the second week of May, shining at magnitude -3.9. Its disc spans 10", with its phase (illuminated fraction) dropping slightly from almost 100% to 96% by month's end.
- Saturn is again prominent this month. It dims slightly from magnitude +0.1 to +0.3 and its angluar size declines from 18.8 to 18.5". The gap between its A and B rings, called Cassini's Division, is easily visible using a small telescope, but a larger telescope shows the fainter inner C ring. The rings will continue to become more visible for the next two years as their angle to our line of sight increases.
- The Eta Aquariid meteor shower peaks before dawn on the 6th, with around 10 meteors per hour potentially visible. Its radiant is low to the east-south-east, near to a thin crescent Moon which should not hamper visibility much. The shower results from dust particles released by Comet Halley as it approached the Sun around 4,000 years ago.
- Two comets can be seen this month. C/2011 L4 (PANSTARRS) is now circumpolar, passing through Cassiopeia and visible throughout the night. It is visible in binoculars or a telescope and shows a nice tail. C/2012 F6 (Lemmon) arrives in the northern hemisphere, moving up the outside of the Square of Pegasus and visible before dawn. Binoculars will be needed to see it as it dims from magnitude +6.5 to around +8 during the month.
- The Moon is closer than usual this month, and therefore larger and brighter when it is full on the night of the 24th to 25th.
- Mercury, Venus and Jupiter will form a tight group just above the western horizon after sunset from the 24th to the 28th. You may need binoculars to spot Mercury, but don't use them until after the Sun has set.
John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during May 2013.
May sees Orion the Hunter in the western sky in the evening. Above it is Sirius, the brightest night-time star, which often twinkles in different colours as shifting air currents refract its light. Sirius is known as the Dog Star because it is part of Canis Major, one of Orion's two dogs. To Māori it is Takarua, the Winter Star, while to Egyptians it is Sothis, whose dawn rising heralds the annual floods of the river Nile. Orion is on his side, with a line of three stars forming his Belt. These three are part of the Bird Snare, Te Manu Rore, in Māori star lore. A fainter line of stars above the Belt marks the Orion's Sword. The Sword and Belt together are nicknamed the Pot or the Saucepan.
On the opposite side of the sky, Scorpius and Sagittarius rise in the east after sunset, containing a wealth of night-sky objects. Crux and the Pointer Stars are high overhead in the south. Near to the star Beta Crucis is a star cluster called the Jewel Box, visible as a hazy glow to the naked eye. Individual stars can be seen with binoculars, while a telescope reveals more detail. The constellations of Carina, the Keel, and Vela, the Sails, run along the Milky Way between Crux and Sirius, and host many bright stars, clusters and nebulae. The brightest nebula, the Eta Carina Nebula, covers a larger area of sky than the Orion Nebula, and binoculars show star clusters nestling among glowing clouds of gas that are intertwined with dark lanes. The star Eta Carinae is there, and has interested astronomers by brightening from magnitude +8 in 1940 to +4.6 today.
Now is a good time to observe the Aurora Australis, or Southern Lights. The phenomenon is caused by the interaction of the solar wind with the Earth's magnetic field and atmosphere, and its colourful glow is sometimes visible from southern parts of New Zealand, Australia and South America. It is governed by the Sun's activity, which is currently increasing towards a probable peak in the spring. You can check a number of websites for an aurora forecast.
- Saturn is visible throughout the night, appearing as a yellow star in the east during Twilight.
- Jupiter sets shortly after the Sun.
- Venus becomes visible later in the month, climbing higher as the month progresses. On the 28th, it is just over 1° below Jupiter in the sky.
- Mercury completes the planetary trio by appearing in the west at sunset, producing a conjunction that can be seen for about an hour after sunset late in the month. Binoculars may be required to see Mercury, 3° to the right of the other two planets, but don't use them until after the Sun has set.
- Observers in northern Australia have the chance to see an annular eclipse of the Sun for a number of hours around midday on the 10th. Observers nearby will see a partial eclipse. In Wellington, less than 1% of the Sun's disc will be covered, while North Cape will see 13L coverage.
- Comet C/2012 S1 (ISON) has reached 17th magnitude this month as it brightens on its approach to the Sun. The Hubble Space Telescope has imaged a very active nucleus, and the current prediction is that the comet will be visible in binoculars from August and with the unaided eye in October - although this is subject to change!
Odds and Ends
As of last week, Ireland has a new mini LOFAR station. It consists of four low-band dipole antennae, as compared to the 96 in a full LOFAR station. The I-LOFAR group took four 'unrepairable' antennae from the UK LOFAR station in Chilbolton, damaged in storms last year, and repaired them. They have set them up in the grounds of Birr Castle, once the site of the world's largest telescope. Birr Castle is located in the centre of the country and has extremely low levels of radio frequency interference, making it ideal for radio astronomy. Already the group has used the station to observe radio bursts from the Sun and plan to do solar, planetary and pulsar observations in the future.
Up to 95% of the water currently in Jupiter's stratosphere may have been deposited there by Comet Shoemaker-Levy 9, according to observations made with the Herschel infrared telescope. The comet provided a unique sight to astronomers in 1994 when it crashed into Jupiter, and now water can be seen in the areas where its fragments struck. With water known to be relatively abundant in the Solar System, this appears to be a novel mechanism by which it can be deposited onto a planet.
The Zooniverse is a collection of online citizen science projects, including the hugely popular Galaxy Zoo. Galaxy Zoo users can now explore their classifications in more detail using the Navigator tool, which allows you to make plots and see how your classifications compare to everyone else's. Another site, Zooteach, contains lesson plans to bring Zooniverse projects into the classroom.
|Interview:||Dr Rubina Kotak and Indy Leclercq|
|Night sky:||Ian Morison and John Field|
|Presenters:||Jen Gupta, Evan Keane and Mark Purver|
|Editors:||Indy Leclercq, George Bendo, Claire Bretherton and Mark Purver|
|Segment Voice:||Mike Peel|
|Website:||Mark Purver and Stuart Lowe|
|Cover art:||A map of water in Jupiter's stratosphere, produced using infrared spectrometry with the Herschel Space Observatory. CREDIT: ESA/Herschel/T. Cavalié et al.|