April 2012: Fooling No-one

In the show this month, Megan rounds up the latest news and we find out what's in the April night sky from Ian Morison and John Field. This show is shorter than usual because all the Jodcasters were kept so busy by the National Astronomy Meeting (NAM) at the end of March, so if your appetite for interviews is not sated then please listen to our daily reports from the first, second, third and fourth days of NAM.

The News

In the news this month:

• Compared to the other terrestrial planets, surprisingly little is known about Mercury. The closest planet to the Sun, its daytime surface temperature can reach some 700 Kelvin, while temperatures on the night side of the planet can drop below 100 Kelvin. Observations from ground-based telescopes on the Earth reveal very little about Mercury, but since 2008 the MESSENGER spacecraft has been mapping the planet in detail, and March 21st saw the publication of two studies using some of the data collected. MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) is the first spacecraft to visit Mercury since Mariner 10 in the mid 1970s, and, after three flybys of the planet, the craft entered orbit on March 17th 2011, making it the first spacecraft ever to actually orbit Mercury. Among the many scientific goals of the mission, MESSENGER carries instruments to investigate the planet's surface composition, its geological history, and the structure of its magnetic field.

  On March 21st 2012, two papers were published in Science Express detailing new results from some of MESSENGER's sensors. The first describes a new topographic model of the northern hemisphere of the planet, constructed using a laser-altimeter to measure the changing height of the surface features as the spacecraft passed overhead on its orbital path. Mariner 10 photographed less than 50% of the surface during its three flybys more than thirty years ago, but MESSENGER is mapping the entire surface for the first time, and in much greater detail. Using the new maps from the laser altimeter, the research team has identified features which indicate that Mercury was tectonically active on a large scale for much of its history. Mercury has a large, dense core, much larger (relative to the diameter of the planet) than that of the Earth, and does not currently experience active plate tectonics in the way the Earth does. The new topographic maps, however, show evidence of large-scale movements of the planet's crust relatively recently. In the early life of the solar system there were large numbers of small bodies and asteroids which would frequently collide with planets, creating some of the impressive craters we see today. One such crater on Mercury is the Caloris basin, a giant impact feature 1500-km in diameter. The basin was somewhat filled in with fresh lava after the impact, creating a flat floor within the circular crater walls. The new topographic data from MESSENGER show that there is a large-scale slope across much of the northern hemisphere of the planet, and in some places the flat lava plain of the Caloris basin floor has been raised by this slope so that it is actually higher than parts of the older surrounding crater rim. Since lava settles at a constant height above the centre of gravity, much like water in a glass, this height difference across the basin implies that the large-scale geological event which caused the hemisphere-wide slope happened after the early period of heavy bombardment in the solar system.

  The second paper describes a new model of Mercury's magnetic field, calculated using spacecraft tracking data from NASA's network of ground-station facilities, a series of satellite dishes used for tracking and communication with spacecraft in the solar system. By regularly tracking MESSENGER during the first few weeks after it entered orbit around Mercury in March 2011, the team has been able to build up a detailed model of the gravitational anomalies in Mercury's crust. These anomalies are places where there are variations in the density of the rock below the surface which affect the orbit of the spacecraft as it passes overhead, and they can be used to build up a picture of the internal structure of the planet. Based on these new observations, the model proposed by the researchers includes a solid silicate crust and mantle overlying a dense silicate-rich layer and an iron-rich liquid outer core and, perhaps, a solid inner core. This new model is somewhat different to that expected from our knowledge of the other terrestrial planets, and the researchers point out that these new results will mean changes for models of Mercury's evolution.

• Many hundreds of extrasolar planets are now known, with more being discovered all the time, but so far we know very little about them individually. One thing of particular interest in the search for life elsewhere in the Universe is determining the contents of exoplanetary atmospheres. This can be attempted by observing the light reflected from a particular planet and looking for absorption features in the spectrum of the light due to chemicals such as methane, water and oxygen. Such studies are difficult, however, because the light reflected from a planet is much, much fainter than the light from its parent star. But now, in a study published in Nature on Feb 29th, a team of researchers has tested a way to search for the signatures of life on other worlds, by using observations of our own planet.

  The team, led by Michael Sterzik of the European Southern Observatory in Santiago, Chile, used the 8.2-metre Very Large Telescope (VLT) to observe earthshine, sunlight which has been reflected from the daylit side of the Earth up to the Moon and back again. Using a telescope the size of the VLT to observe the Moon may sound like overkill, but this is the first time that an observational technique has been able to detect evidence of vegetation on a planet. The trick to the technique lies in measuring the polarisation of the light, the orientation of the oscillations of the light waves, and how this polarisation changes with wavelength across a spectrum. Light transmitted directly from a star is much brighter than that reflected from a planet, but the starlight itself is unpolarised. Light reflected from a planet, on the other hand, is partially polarised, and the two signals can be separated. The form of the polarisation depends on the nature of whatever surface reflected the light and the characteristics of the planet's atmosphere. By using these observations of earthshine the researchers were able to determine the fraction of the Earth's surface which was covered by clouds, oceans and vegetation.

  The researchers made two observations, the first on April 25th 2011, the second on June 10th 2011. The alignment of the Earth-Moon-Sun system was different during the two observations, with different parts of the Earth's surface responsible for reflecting light up to the Moon. The data collected during the observing sessions show significant differences. In the results from April there is a strong feature due to vegetation, but this feature is not present in the data from June, suggesting that very little of the reflected light came from cloud-free forested regions of the Earth's surface.

  The observations provide a valuable means of testing theoretical models of Earth-like exoplanets, in preparation for future telescopes which may be able to carry out similar, but far more tricky, observations for other planetary systems. Finding evidence of life is one thing, but proving the existence of intelligent life is, of course, quite another.

• And finally: Among the exciting results presented at the UK's National Astronomy Meeting in March, held this year in Manchester, was the discovery of a vast amount of gas and dust around a black hole in the very early Universe. In a talk on March 28th, team leader Bram Venemans of the Max Planck Institute for Astronomy in Heidelberg presented the new results. The European team used the IRAM interferometer, an array of radio telescopes which detect emission at millimetre wavelengths, to observe a galaxy known as J1120+0641, a galaxy so far away that the light we detect on the Earth today left the galaxy when the Universe was a mere 740 million years old, 1/18 of its current age. The observations showed a surprising amount of carbon, an element created mainly in the nuclear fusion reactions which power stars. Since carbon is created in stars and distributed throughout galaxies in violent supernova explosions, there was not expected to be large amounts of carbon this early in the history of the Universe. The presence of such large quantities of dust and carbon-rich gas this early in the Universe's history adds to evidence that there was a massive burst of star formation in the relatively short time between the Big Bang and the time the light from this distant galaxy was emitted. The amount of dust detected shows that this particular galaxy was forming stars at a rate one hundred times higher than the star formation rate in our own Milky Way today.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during April 2012.

The constellation of Orion is in the west after sunset, below Taurus, which contains the Hyades and Pleiades Clusters. Above is the bright yellow star Capella in the constellation of Auriga, which sits beside open star clusters along the plane of the Milky Way. Castor and Pollux, the Twins, are to the left in Gemini. Leo is fairly high in the south later in the evening, with its bright star Regulus. Virgo is down and to the left, with its bright star Spica. Between Spica and the back of Leo is the Realm of the Galaxies, where a telescope will show a number of galaxies on a dark night, including M84 and M87. The bright star Arcturus is nearby, in Boötes, while the arc of Corona Borealis, the Northern Crown, is to the left. Ursa Major, the Great Bear, is almost overhead in the north, and contains the asterism of the Plough. The middle star of the Plough's handle is a double, called Alcor and Mizar, which can be distinguished with the naked eye under a dark sky, or with binoculars. The brighter of the two, Mizar, can itself be seen to be a double when using a small telescope. Hercules rises in the east late in the evening, near to Lyra, with its bright star Vega. Hercules contains the globular cluster M13, which is on the right of the Keystone, an asterism of its four brightest stars.

The Planets

• Jupiter is nearing the end of its current apparition, but can still be seen below Venus, low to the west after sunset. At the beginning of the month it is still up for two and a half hours after sunset, but by the end this is down to three quarters of an hour. It has a magnitude of -2 and an angular size of 34", and a small telescope will show its equatorial bands as well as the four Galilean moons.
• Saturn rises about an hour after sunset at the beginning of the month, and attains a maximum elevation of around 32°. It reaches opposition (opposite to the Sun in the sky) on the 15th, putting it roughly due south at midnight UT (Universal time, one hour behind British Summer Time). With a magnitude of +0.3, it is in Virgo, 5° up and to the left of the first-magnitude star Spica. It will not get very high in the northern hemisphere sky for some time, as it is heading to the southerly part of the ecliptic. Its disc is 19" across and its rings have opened to 14° to the line of sight by the beginning of the month, but this drops to 13° by month's end and continues to decline thereafter.
• Mercury reaches its greatest eastern elongation (furthest distance from the Sun in the sky) on the 18th, but the shallow angle between the ecliptic and the horizon at dawn means its elevation remains low. At magnitude +0.2, it is visible through binoculars half an hour before sunrise.
• Mars reached opposition on the 3rd, so it gets high in the sky, and is 5° to the left of the star Regulus. Its retrograde (westward) motion ends on the 15th, so it barely moves against the starry background this month. Its brightness fades from magnitude -0.7 to -0.1 during the month, while its angular diameters decreases from about 12 to 10". However, a medium-sized telescope should still reveal markings on its surface.
• Venus is prominent in the south-west after sunset at the beginning of the month, and is almost as high as it ever gets at about 41° above the horizon. It only reaches this high elevation once every eight years. Its angular size increases from 25" to 36" over the month, but the illuminated fraction falls from 48 to 37% during the same period, keeping the brightness between magnitude -4.5 and -4.7. A deep blue filter may allow cloud structure to be seen on the planet.

Highlights

• Venus passes close to the Pleiades Cluster on the 3rd. It is below and left of the Pleiades, half a degree from the star Alcyone.
• Saturn is at its best this month. The A and B rings and Cassini's Division can be discerned using a small telescope, with Encke's Division and the C ring also visible with a medium-sized telescope. Some of Saturn's smaller moons may be spotted alongside its largest, Titan. The northern hemisphere is now more visible, and bands may be seen on the planet's surface.
• The Lyrid meteor shower peaks on the nights of the 22nd and 23rd, conveniently close to New Moon. It typically provides around 15 meteors per hour, appearing to originate from the constellation of Lyra, the Lyre. It is best viewed around 1am BST. Dust from Comet Thatcher causes this meteor shower.
• Saturn's moons group together on the 19th.
• Mars is near the 9-day-old Moon on the 20th, below Leo.
• A thin cresent Moon hangs below Venus on the 24th, the sliver of the Moon allowing earthshine to be seen, which is light reflected from the Earth illuminating the dark part of the Moon.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during April 2012.

Venus is low in the north-west at dusk, shining brilliantly until it sets about two hours after the Sun. A telescopes will show it to be a crescent, and it is now moving towards inferior conjunction (in front of the Sun as seen from Earth). Jupiter is even lower in the west, and is briefly visible at twilight as it approaches conjunction (behind the Sun as seen from Earth). Mars gets midway up in the north, shining orangey-red and near to Leo's brightest star, Regulus. It is now moving away from us, but a telescope will still show its disc. Saturn appears yellowy, midway up the eastern sky at dusk and below Virgo's brightest star, first-magnitude Spica. It gets high in the north by midnight. With a telescope, Saturn's rings, its surface bands and its orangey largest moon, Titan, may be seen.

Far from the Milky Way, Virgo is a good constellation in which to spot galaxies. 10° west of Spica is the Sombrero Galaxy, visible through binoculars at magnitude +9 and with discernible features when using a medium-sized telescope. Above Spica is a small, kite-shaped constellation called Corvus, the Crow. In Greek mythology, the Crow, or Raven, was banished to the sky by the Sun god Apollo for failing to bring him a cup of water. Its two brightest stars are around magnitude +2.6, while Delta Corvi is a wide double star. Nearby is Hydra, the Water Snake, the largest of the 88 modern constellations. It has five stars forming its head and a long path of stars making up its body. M83, the Southern Pinwheel Galaxy, is one of the few objects to be found within it. It has a magnitude of +7.5, allowing it to be seen through binoculars, while a telescope will reveal its spiral structure.

The winter constellation of Scorpius rises in the eastern sky in the evening. Its brightest star is the red giant Antares, meaning Rival of Mars. Named for its hue, it can currently be compared to Mars as they appear in the sky at the same time. Since New Zealand does not contain scorpions, to the Māori Scorpius was Te Matau a Māui, the Hook of Māui. The mythological figure of Māui used this Hook to fish the North Island of New Zealand out of the ocean. The Hook was the jaw bone of one of his ancestors, and Antares, or Rehua, represents his blood which he placed on the Hook. When a giant fish was pulled up and became the North Island, the Hook flew into the sky. The North Island was then named Te Ika a Māui, the Fish of Māui.

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