April 2015: Opposites

In the show this time, we talk to Dr. Cormac Purcell about The Gum Nebula, Indy rounds up the latest news, and we find out what we can see in the April night sky from Ian Morison and Claire Bretherton.

The News

In this month's news, we bring you updates from two planets at opposite ends of the solar system: Mercury, and the dwarf planet Ceres.

• Nasa's Messenger probe, launched in 2004, is fast approaching the end of its almost 11 years in the solar system. The spacecraft's mission was to provide the first orbital study of Mercury, and it has fulfilled its goals with great success. On its way to Mercury it flew by Venus twice, in 2006 and 2007, and then performed three mercury flybys in 2008 before being inserted into orbit around the planet in 2011. Messenger has sent back hundreds of thousands of pictures of the surface of Mercury, and over 10 terabytes of data from its seven instruments, including a gamma-ray and neutron spectrometer, an x-ray spectrometer, a magnetometer and a laser altimeter. Among the important scientific results derived from 4000 orbits' worth of data, scientists have found that Mercury's magnetic field lines converge differently at the north and south poles, that its atmospheric composition depends on its distance from the Sun, and they managed to confirm the presence of up to 1 trillion tons of water ice on Mercury. Within the next 30 days, however, Messenger will end its mission, and crash into the surface of the planet on the 30th of April. Messenger is currently at an altitude of about 15 km, the closest it's ever been to the surface of Mercury, and has been sending back the best pictures yet of the surface. Scientists have been able to make out the presence of permanently frozen water at the bottom of craters. This seems incredible so close to the Sun - but the ice down there never gets exposed to direct sunlight. Some photographs suggest that the ice is overlaid with a dark, carbon-rich material. One hypothesis is that the impact of a comet or asteroid could have deposited the water and the overlaid material. The new pictures also reveal small versions of Mercury's famous Scarps, ridges up to hundreds of kilometers long that run across the surface of the planet. Messenger has photographed 99.9% of the surface of Mercury, and scientists will doubtless be analysing the data it sent back for a few years to come. At the moment, it has enough fuel for 5 remaining burns to give it a little more altitude before it finally crashes into the planet it has told us so much about. Its task will be taken up by the next probe to get to Mercury, ESA's BepiColombo, in 2024.

• Going to the other side of the solar system, the dwarf planet Ceres is slowly revealing its secrets to Nasa's Dawn spacecraft. We mentioned in the March news that Dawn had managed to resolve a "bright spot" on Ceres, previously spotted by astronomers at the bottom of a crater, into two separate bright spots. However speculation was rife as to what these spots actually were. New images taken by Dawn during the month of March have now narrowed down the possibilities somewhat. The feature, known simply as "feature number 5" has been photographed at different angles as Ceres rotates, and the resulting images show that the bright features are visible even when the crater is viewed at an extreme angle, almost edge-on. This would suggest that the spots are located quite high above the bottom of the crater, as the sides of the crater render the bottom invisible at this angle. Furthermore, the brightness of the feature changes with time: it is bright at dawn, but fades somewhat until dusk. Taken with the fact that Ceres is thought to be around 25% ice, scientists believe that a good explanation for the bright spots would be the cryovolcano theory: ice residing just beneath the thin dusty surface of the dwarf planet could get blasted into the air due to internal pressure of the planet, aided by the sun's heat. The plumes of ice thus ejected could theoretically rise quite high above the surface, thus giving rise to the bright spots that Dawn is now seeing. In any case, the answer may be just around the corner, as Dawn is continuously getting closer to the surface of the planet, looking for regions of activity. The mission operators expect that the best images they will get will show details as small as 30m across, which would finally enable scientists to figure out what the source of these bright spots actually is - which will hopefully be the subject of a future News segment.

• In other news, a new crew of astronauts launched from the Baikonur Cosmodrome to commence a one-year stay on the International Space Station. The stay, the longest so far on the ISS, will be undertaken by US astronaut Scott Kelly and russian cosmonaut Mikhail Kornienko. The aim of Expedition 43 is to study the long-term effets microgravity has on the human body, in view of launching a mission to Mars. Kelly and Kornienko will undergo a variety of tests and studies over the course of their year-long stay. In an interesting twist, Kelly's twin brother Mark - himself a former astronaut - will be staying on the ground and serve as a "control" of sorts, to help gauge the effects of a year in space.

Interview with Dr Cormac Purcell

Dr Cormac Purcell from the University of Sydney has been researching the properties of the Gum Nebula. He talks to the Jodcast about magnetic fields in the Milky Way, why they are interesting, and how we can measure them. He explains why it is so difficult to measure our own galaxy's magnetic fields in comparison to those of distant galaxies, and how stars blow giant bubbles which may help solve this dilemma.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during April 2015.

Orion is sinking in the west as darkness falls, followed by Gemini and its bright stars Castor and Pollux. Further towards the south is the faint constellation of Cancer. It contains the Beehive Cluster, an open star cluster visible in binoculars, and currently plays host to the planet Jupiter as well. Leo is due south in the evening, with its bright star Regulus. Nearby, in Virgo and Coma Bernices, is an area called the Realm of the Galaxies. In this region, an 8" telescope can pick out a number of galaxies that are part of the Virgo Cluster, the largest cluster of galaxies in our local universe and itself part of a much bigger supercluster. Higher up is Boötes, with its bright star Arcturus, and overhead is Ursa Major. Later in the night, Lyra and its bright star Vega rise in the north-east.

The Planets

• Jupiter, two months past opposition (when it was opposite the Sun in the sky), is still high in the south-west in the evening. During April, it dims from magnitude -2.3 to -2.1 and shrinks from 41.5 to 38" across. As it switches from westward (retrograde) to eastward motion this month, it remains in Cancer and moves very little relative to the stars. A small telescope can reveal the equatorial bands, Great Red Spot (at certain times) and Galilean moons.
• Saturn rises in the evening, a little earlier each night, and lies close to the left-hand star of the fan of Scorpius. It brightens from magnitude +0.3 to +0.1 and grows from 17.8 to 18.4" in diameter during the month. It reaches 22° elevation when due south in the early hours of the morning, and the ring system is inclined at 25° to our line of sight.
• Mercury reaches superior conjunction (behind the Sun in the sky) on the 10th, and remains invisible until around the 19th, when it appears low in the west-north-west about 45 minutes after sunset. Shining at magnitude -1.4, it climbs higher each evening on its way to eastern elongation (its furthest from the Sun in the sky) on the 7th of May.
• Mars, ever-present in the evening sky for many months, is finally disappearing into the Sun's glare. Lying close to Mercury from around the 19th to 24th, it has an angular size of 4" and so reveals no surface details to us here on Earth.
• Venus blazes at magnitude -4, rising higher in the evening western sky as the month progresses. It moves from Aries into Taurus on the 7th, aproaching the Pleiades Cluster around the 13th. Its angular size increases from 14 to 16" during April, as its phase wanes from 78 to 68%.

Highlights

• Saturn is near a waning gibbous Moon before dawn on the 9th.
• Venus can be seen near the Pleaides Cluster about 45 minutes after sunset from the 11th to 13th.
• Mars, Mercury and a wafer-thin crescent Moon form a near-equilateral triangle just after sunset on the 19th, with about 4-5° between each of them. With the Moon under a day old and only 13.5° from the Sun, it is only just visible and may require binoculars or a wide-field telescope to be seen. Look up and a little to the left of where the Sun has set, but not until the Sun really has gone down.
• Venus is close to a waxing crescent Moon, just above the red star Aldebaran and the Hyades Cluster in Taurus, in the western sky on the evening of the 21st.
• Jupiter lies about 6° above a first-quarter Moon in Cancer on the 26th.

Southern Hemisphere

Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during April 2015.

The night of the 4th-5th sees the first lunar eclipse of the year, and also the backward move of the clocks in parts of the southern hemisphere. In Wellington, New Zealand, the penumbral phase begins at 22:03 NZDT (New Zealand Daylight Time, 13 hours ahead of Universal Time) as the Earth begins to obscure sunlight from the Moon's surface. The umbral phase, when sunlight is fully blocked from part of the Moon, starts at 23:17. The Moon is cast into total shadow for 7 minutes, from 00:57 to 01:04. The umbral phase ends at 02:44, while the penumbral phase finishes at 02:58 NZST (New Zealand Standard Time, 12 hours ahead of Universal Time).

The Moon is in Virgo during the eclipse, about halfway between Jupiter in the north-west and Saturn in the east. Venus appears brightly in the evening, and sets 2 hours after the Sun by the end of the month. Saturn is in Scorpius, a little below the red star Antares. It rises around 22:00 NZDT at the beginning of the month and 19:00 NZST at the end.

The constellation of Centaurus is high in the east after dark, with its two brightest stars, Alpha and Beta Centauri, pointing towards Crux, the Southern Cross. Centaurus is one of the largest constellations and contains many bright stars, clusters and nebulae. The globular cluster Omega Centauri, the largest and brightest in the Milky Way Galaxy, appears similar in size to the full Moon when seen with the naked eye at magnitude +3.7. Binoculars reveal individual stars and a dense core. With a population of stars that are around 12 billion years old, it is a relic of the early Universe and may be the remnant of a dwarf galaxy that merged with the Milky Way. NGC 3766 and NGC 5460 are two open star clusters in Centaurus, both just visible to the naked eye. The planetary nebula NGC 3918, the 'Blue Planetary', is also located there, and, at magnitude +8, its blue oval shape can be seen with a small telescope. Centaurus hosts one of the closest galaxy clusters to Earth. Separately, it is home to NGC 5128, the galaxy known as Centaurus A. Centaurus A is elliptical, but has a dark dust lane across the middle, and the supermassive black hole at its heart is thought to be consuming a spiral galaxy with which it has merged. As a result, it emits relativistic jets that can be detected at radio and X-ray wavelengths. Located less than 5° from Omega Centauri, Centaurus A is the fifth-brightest galaxy in our sky (excluding the Milky Way) and is easily visible in binoculars. The bright central bulge and dark lane may be viewed with larger binoculars, while a telescope reveals more of the galaxy's structure.

Odds and Ends

NASA's Mars Exploration Rover Opportunity has completed the very first Marathon distance of any human-made device on another planet. The Rover completed the 26.219 mile distance in 11 years and 2 months.

Astronomers studying the properties of dark matter have found that the mysterious stuff doesn't interact with itself. Studying cluster mergers using Hubble and Chandra X-ray observations, the scientists found that when two clumps of dark matter collide, they simply pass right through each other. The other significant result is a statistical detection of dark matter with a very high probability: it is more than 99.99999999999% likely that dark matter is present in the clusters studied. What it actually is, remains to be seen.

The discovery of an aurora and a mysteriously high altitude dust cloud in the Martian atmosphere by NASA's MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft has surprised planetary scientists. Neither of these phenomena have been predicted by current models of the Martian atmosphere or by any of the known processes which occur on Mars. The aurora was much deeper than any seen on Earth, implying the particles responsible must be very energetic, most probably originating from the Sun. Mars is much more vulnerable to solar particles than Earth as it lost its protective magnetic field billions of years ago. This allows the energetic solar particles to strike through the atmosphere creating aurora closer to the surface. The origin of the dust cloud is much more uncertain. It is possible it has risen from the atmosphere; been transferred by Mars' moons Phobos and Deimos or come from solar winds blowing dust from passing comets. The full story can be found here.

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