Astronomy: How A Comet Interacts with Solar Wind

uly 30, 2015: Rosetta is making good progress in one of its key investigations, which concerns the interaction between the comet and the solar wind.  Credit: European Space Agency (ESA)

July 30, 2015: Rosetta is making good progress in one of its key investigations, which concerns the interaction between the comet and the solar wind.
Screenshot from a simulation of plasma interactions between Comet 67P/C-G and the solar wind around perihelion. Image Credit: Modelling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin, European Space Agency (ESA)

The solar wind is the constant stream of electrically charged particles that flows from the Sun, carrying its magnetic field out into the Solar System. Like all comets, 67P/Churyumov–Gerasimenko must navigate this flow in its orbit around the Sun.

It is the constant battle fought between the comet and the solar wind that helps to sculpt the comet’s ion tail. Rosetta’s instruments are monitoring the fine detail of this process.

Using the Rosetta Plasma Consortium Ion Composition Analyzer, Hans Nilsson from the Swedish Institute of Space Physics and his colleagues have been studying the gradual evolution of the comet’s ion environment. They have seen that the number of water ions— molecules of water that have been stripped of one electron— accelerated away from the comet increased hugely as 67P/C-G moved between 3.6AU (about 538 million km) and 2.0AU (about 300 million km) from the Sun. Although the day-to-day acceleration is highly variable, the average 24-hour rate has increased by a factor of 10,000 during the study, which covered the period August 2014 to March 2015.

The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. Once in gaseous form, the collision of extreme ultraviolet light displaces electrons from the molecules, turning them into ions. Colliding particles from the solar wind can do this as well. Once stripped of some of their electrons, the water ions can then be accelerated by the electrical properties of the solar wind.

Not all of the ions are accelerated outwards, some will happen to strike the comet’s surface. Solar wind particles will also find their way through the coma to hit home. When this happens, they cause a process called sputtering, in which they displace atoms from material on the surface—these are then ‘liberated’ into space.

Peter Wurz from the University of Bern, Switzerland, and colleagues have studied these sputtered atoms with Rosetta’s Double Focussing Mass Spectrometer (DFMS), which is part of the ROSINA experiment.

They have so far discovered sodium, potassium, silicon and calcium, which are all present in a rare form of meteorites called carbonaceous chondrites. There are differences in the amounts of these atoms at the comet and in these meteorites, however. While the abundance of sodium appears the same, 67P/C-G shows an excess of potassium and a depletion of calcium.

Most of the sputtered atoms come from the winter side of the comet. Although this is the hemisphere that is mostly facing away from the Sun at present, solar wind particles can end up striking the surface because they are deflected during interactions with ions in the comet’s coma. This can be a significant process so long as the density of the coma ions is not too large. But at some point the comet’s atmosphere becomes dense enough to be a major defence, protecting the icy surface.

As the comet gets closer to the Sun, the sputtering will eventually stop because the comet will release more gas and the coma will become impenetrable. When this happens, the solar wind ions will always collide with atoms in this atmosphere or be deflected away before striking the surface.

The first evidence that this deflection is taking place at 67P/C-G has been measured with the Rosetta Plasma Consortium Ion and Electron Sensor, by Thomas Broiles of the Southwest Research Institute (SwRI) in San Antonio, Texas, and colleagues.

Their observations began on August 6, 2014 when Rosetta arrived at the comet, and have been almost continuous since. The instrument has been measuring the flow of the solar wind as Rosetta orbits 67P/C-G, showing that the solar wind can be deflected by up to 45° away from the anti-solar direction.

The deflection is largest for the lighter ions, such as protons, and not so much for the heavier ions derived from helium atoms. For all ions the deflection is set to increase as the comet gets closer to the Sun and the coma becomes ever denser.

As all this happens, Rosetta will be there to continue monitoring and measuring the changes. This was the raison d’être for the rendezvous with this comet. Previous missions have taken snapshots during all too brief fly-bys but Rosetta is showing us truly how a comet behaves as it approaches the Sun.

This blog post is based on the papers “Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: Observations between 3.6 and 2.0 AU ” by H. Nilsson et al.; “Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko” by T.W. Broiles et al.; and “Solar Wind Sputtering of Dust on the Surface of 67P/Churyumov-Gerasimenko ” by Peter Wurz et al., which have all been accepted for publication in Astronomy and Astrophysics, and “Dynamical features and spatial structures of the plasma interaction region of 67P/Churyumov–Gerasimenko and the solar wind” by C. Koenders et al, which is published in Planetary and Space Science.

On the Web: SIMULATION OF PLASMA INTERACTIONS BETWEEN COMET 67P/C-G AND THE SOLAR WIND AROUND PERIHELION

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Astronomy: Saturn’s Moon Titan Not So Titanic

Credit: NASA/JPL-Caltech/Space Science Institute

Credit: NASA/JPL-Caltech/Space Science Institute

Although Titan (3200 miles or 5150 kilometers across) is the second-largest moon in the solar system, Saturn is still much bigger, with a diameter almost 23 times larger than Titan’s. This disparity between planet and moon is the norm in the solar system.

Earth’s diameter is “only” 3.7 times our moon’s diameter, making our natural satellite something of an oddity. (Another exception to the rule: dwarf planet Pluto’s diameter is just under two times that of its moon.) So the question isn’t why is Titan so small (relatively speaking), but why is Earth’s moon so big?

This view looks toward the anti-Saturn hemisphere of Titan. North on Titan is up. The image was taken with the Cassini spacecraft wide-angle camera on April 18, 2015 using a near-infrared spectral filter with a passband centered at 752 nanometers.

The view was acquired at a distance of approximately 930,000 miles (1.5 million kilometers) from Titan. Image scale is 56 miles (90 kilometers) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

On the Web:

For more information about the Cassini-Huygens mission 

The Cassini imaging team homepage

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Astronomy: Venus-Mass Planet Orbiting Brown Dwarf

Venus Mass Planet Orbiting Brown Dwarf

An international team of Polish, Korean, American, Israeli, and Italian astronomers have announced the unusual discovery of a Venus mass planet OGLE-2013-BLG-0723LB/Bb, orbiting a cool brown dwarf star.

Both the planet and it’s brown dwarf host, are in a wide orbit around a larger stellar companion OGLE-2013-BLG-0723LA, with perhaps another, (as yet unconfirmed) much larger third stellar companion at a much larger separation distance than the two confirmed binary stellar objects.

The discovery was made using the technique of microlensing which gives astronomers reliable information about the mass of the planet : 0.69 ± 0.06 M⊕ (Earth) and it’s orbital distance : 0.34 ± 0.03 AU or 439993738 km. This distance places the planet in an orbit very similar to that of Mercury (0.38 AU) but our Sun is far hotter than this cool brown dwarf.

The microlensing event OGLE-2013-BLG-0723 was first discovered by the Optical Gravitational Lensing Experiment (OGLE-IV) in one of the starfields towards the Galactic bulge that OGLE astronomers Udalski et al. observed on May 12th 2013, using the 1.3 meter Warsaw Telescope at the Las Campanas Observatory in Chile.

The planetary system is estimated to lie some 0.49 ± 0.04 kilo parsecs towards the Galactic Center, having been identified by it’s lensing effect on a background star that is a further 6.51 kilo parsecs from Earth.

This new planetary find may prove to be very important. OGLE-2013-BLG-0723LBb is a missing link between planets and moons. This is because its brown dwarf host OGLE-2013-BLG-723LB  is intermediate between stars and planets, in both size and hierarchical position.

The scaled mass and host-companion separation of this Venus-mass planet and brown dwarf host are in many ways similar to planets and moons in the solar system. That is, a Venus-mass planet orbiting a brown dwarf, may be viewed either as a scaled down version of a planet and star, or as a scaled up version of a moon and planet, orbiting a star.

So this system is an intermediate between Neptune-Triton or Jupiter-Callisto planet-moon systems, and the Sun-Mercury or the Sun-Venus star-planet systems.

It suggests that in all cases, planets and moons are formed in an accretion disk. Planets form around all types and size of star, and moons are formed in an accretion disk around planets. The process is the same, regardless of the size or scale of the individual objects.

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Astronomy Reader: Distant Black Hole Wave Twists Like Giant Whip

This artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Caltech manages JPL for NASA Credit: NASA/JPL-Caltech

This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun.
Caltech manages JPL for NASA
Credit: NASA/JPL-Caltech

Fast-moving magnetic waves emanating from a distant supermassive black hole undulate like a whip whose handle is being shaken by a giant hand, according to a new study using data from the National Radio Astronomy Observatory’s Very Long Baseline Array. Scientists used this instrument to explore the galaxy/black hole system known as BL Lacertae (BL Lac) in high resolution.

Fast Facts:
› Black hole jets set magnetic waves in motion like whips being jerked from side to side.
› The findings help researchers understand how black holes produce jets.

“The waves are excited by a shaking motion of the jet at its base,” said David Meier, a now-retired astrophysicist from NASA’s Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena.

The team’s findings, detailed in the April 10 issue of The Astrophysical Journal, mark the first time so-called Alfven (pronounced Alf-vain) waves have been identified in a black hole system.

Alfven waves are generated when magnetic field lines, such as those coming from the sun or a disk around a black hole, interact with charged particles, or ions, and become twisted or coiled into a helical shape. In the case of BL Lac, the ions are in the form of particle jets that are flung from opposite sides of the black hole at near light speed.

“Imagine running a water hose through a slinky that has been stretched taut,” said first author Marshall Cohen, an astronomer at Caltech. “A sideways disturbance at one end of the slinky will create a wave that travels to the other end, and if the slinky sways to and fro, the hose running through its center has no choice but to move with it.”

A similar thing is happening in BL Lac, Cohen said. The Alfven waves are analogous to the propagating sideways motions of the slinky, and as the waves propagate along the magnetic field lines, they can cause the field lines — and the particle jets encompassed by the field lines — to move as well.

It’s common for black hole particle jets to bend — and some even swing back and forth. But those movements typically take place on timescales of thousands or millions of years. “What we see is happening on a timescale of weeks,” Cohen said. “We’re taking pictures once a month, and the position of the waves is different each month.”

“By analyzing these waves, we are able to determine the internal properties of the jet, and this will help us ultimately understand how jets are produced by black holes,” said Meier.

Interestingly, from the vantage of astronomers on Earth, the Alfven waves emanating from BL Lac appear to be traveling about five times faster than the speed of light, but it’s only an optical illusion. The illusion is difficult to visualize but has to do with the fact that the waves are traveling slightly off our line of sight at nearly the speed of light. At these high speeds, time slows down, which can throw off the perception of how fast the waves are actually moving.

Other Caltech authors on the paper include Talvikki Hovatta, a former Caltech postdoctoral scholar. Scientists from the University of Cologne and the Max Planck Institute for Radioastronomy in Germany; the Isaac Newton Institute of Chile; Aalto University in Finland; the Astro Space Center of Lebedev Physical Institute, the Pulkovo Observatory, and the Crimean Astrophysical Observatory in Russia; Purdue University in Indiana and Denison University in Granville, Ohio.

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Archaeologists find Bronze Age food at prehistoric settlement “comparable to the Mary Rose”

Ancientfoods


Archaeologists found food from between 800-1000 BC in a set of pots, textiles and other material at a Cambridgeshire settlement destroyed by fire during the Bronze Age© Cambridge Archaeological Unit

An “extraordinary testimony” to the lives of prosperous people in Bronze Age Britain could lie under the soil of a 1,100-square metre site destroyed in a fire 3,000 years ago, say archaeologists who are about to start digging within a brick pit near Peterborough.
Must Farm – part of the Flag Fen Basin, and the site where nine pristine log boats were famously unearthed in 2011 – was protected by a ring of wooden posts before a dramatic fire at the end of the Bronze Age caused the dwelling to collapse into the river.
Its submergence preserved its contents, creating what experts are describing as a “time capsule” of “exceptional” decorated tiles made from lime tree bark.
Rare small pots…

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