On 9 November 2005, 10 years ago today, ESA’s Venus Express spacecraft left Earth and began its 153-day journey to Venus. The craft then spent eight years studying the planet in detail before the mission came to an end in December 2014.

One of the mission aims was to observe the planet’s atmosphere continuously over long periods in a bid to understand its dynamic behaviour.

The atmosphere is the densest of all the terrestrial planets, and is composed almost entirely of carbon dioxide. The planet is also wrapped in a thick layer of cloud made mostly of sulphuric acid. This combination of greenhouse gas and perennial cloud layer led to an enormous greenhouse warming, leaving Venus’ surface extremely hot – just over 450ºC – and hidden from our eyes.

Although winds on the planet’s surface move very slowly, at a few kilometres per hour, the atmospheric density at this altitude is so great that they exert greater force than much faster winds would on Earth.

Winds at the 65 km-high cloud-tops, however, are a different story altogether. The higher-altitude winds whizz around at up to 400 km/h, some 60 times faster than the rotation of the planet itself. This causes some especially dynamic and fast-moving effects in the planet’s upper atmosphere, one of the most prominent being its ‘polar vortices’.

The polar vortices arise because there is more sunlight at lower latitudes. As gas at low latitudes heats it rises, and moves towards the poles, where cooler air sinks. The air converging on the pole accelerates sideways and spirals downwards, like water swirling around a plug hole.

In the centre of the polar vortex, sinking air pushes the clouds lower down by several kilometres, to altitudes where the atmospheric temperature is higher. The central ‘eye of the vortex’ can therefore be clearly seen by mapping thermal-infrared light, which shows the cloud-top temperature: the clouds at the core of the vortex are at a higher temperature, indicated by yellow tones, than the surrounding region, and therefore stand out clearly in these images.

Venus Express has shown that the polar vortices of Venus are among the most variable in the Solar System. This series of images of Venus’ south pole was taken with the VIRTIS instrument from February 2007 (top left) to April 2008 (bottom right).

The shape of this vortex core, which typically measures 2000–3000 km across, changes dramatically as it is buffeted by turbulent winds. It can resemble an ‘S’, a figure-of-eight, a spiral, an eye, and more, quickly morphing from one day to the next.

Each of the images in this frame is roughly 4000 km across.

Credit: ESA/VIRTIS-Venus Express/INAF-IAPS/LESIA-Obs. Paris/G. Piccioni

Tags:   Venus Venus Express atmosphere planet vortex

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The Cassini spacecraft captured this view of Saturn’s icy moon Enceladus as it approached for its closest-ever flyby of the moon's active south polar region.

The spacecraft flew about 49 km above the surface through the towering plumes of ice, water vapour and organic molecules spraying from that region. Previous flybys have sampled the plume but the low altitude of this close encounter was devised partly to provide greater sensitivity to heavier, more massive molecules, including organics.

Studies with Cassini have shown that beneath the moon’s icy exterior lies a global ocean heated in part by tidal forces from Saturn and its moon Dione.

Scientists will use the new information gathered during this dive through the plume to gain insights about how habitable the ocean environment may be for simple forms of life, and to study the chemistry and composition of the plume.

In this view of the moon, the heavily cratered northern latitudes at the top transition to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon’s active south polar region – Cassini's destination for this flyby – is visible at the bottom, where it disappears into wintry darkness.

This image of the Saturn-facing side of the moon was taken with the narrow-angle camera on 28 October 2015 when Cassini was at a distance of some 96 000 km from Enceladus. The image scale is 578 m per pixel.

More images from the ‘plume dive’ can be viewed on the JPL website.

The Cassini–Huygens mission is a cooperative project of NASA, ESA and ASI, the Italian space agency. NASA’s Jet Propulsion Laboratory manages the mission for NASA.

Credit: NASA/JPL-Caltech/Space Science Institute

Tags:   Cassini Enceladus Saturn Space Science Cassini-Huygens

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Eerie sheets and ripples of green hang above a deserted rocky landscape in this spooky Space Science Image of the Week. Spikes of neon and emerald seem to form the ominous form of a ghostly celestial eagle, with a sharp beak, bright head and majestic outstretched wings.

While this photograph may resemble paranormal happenings or alien activity, the dramatic skyscape shown here is actually due to a much more common astronomical event known as a coronal mass ejection, or CME.

This scene was captured on 24 January 2012 above Grotfjord, Norway, by photographer Bjørn Jørgensen. The day before, the Sun flung a burst of high-speed charged particles – electrons, protons and other ions – out into space. Large CMEs can contain up to a billion tonnes of matter, all streaming through space at speeds of up to 2000 km/s.

These particles sped towards Earth and some of them became trapped within our planet’s magnetosphere, a region of space in which charged particles are contained by Earth’s magnetic field.

These particles then began to rain down into our atmosphere, smashing into atoms and molecules of oxygen and nitrogen in the process. These collisions release large amounts of energy in the form of light, painting distinctive colours in the sky.

The colour depends on the particle hit. The most common colours are the reddish-blue of nitrogen and the red or greenish-yellow hues of atomic and molecular oxygen (as seen here). These colours can mix to produce striking shades of orange, yellow, pink and purple.

Because of their speed and particle density, CMEs often trigger stunning auroral displays. When the Sun is particularly active it can produce several CMEs per day, dropping to roughly one every five days at lower activity levels. On average, between one and four CMEs hit Earth each month; these are called “Halo CMEs”.

A flotilla of spacecraft, including the ESA-led SOHO, Proba-2 and Cluster missions, monitor the Sun and its effects on our home planet.

Credit: B. Jørgensen (www.arcticphoto.no/)

Tags:   aurora sky Norway

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Saturn has many, varied moons – over 60 have been discovered so far. One of the larger ones, Dione, is shown here in this image from the Cassini orbiter, pictured as it moved across the face of its parent planet.

Dione orbits Saturn at around 377 400 km, roughly the same distance as that between Earth and the Moon. It has a diameter of around a third that of the Moon, and is just under 1.5 times as dense as liquid water. While the moon is thought to be mostly water ice, this higher density indicates that it must have a core of dense material – most likely silicates, the same kind of rock that makes up Earth’s mantle.

In this image, Dione is seen passing across the face of Saturn, a phenomenon known as a transit. The dark line cutting across the middle of the frame marks Saturn’s rings – these are not illuminated from this perspective, which was about 0.3º below the ring plane.

Transits occur when one celestial body passes in front of another. They are seen most often when moons pass in front of their parent planets, or planets (or even moons) in front of their parent stars. Transits are important events in astronomy, allowing observers to investigate the transiting body’s atmosphere and orbit in greater detail.

We can see a number of transits from Earth, such as when Mercury and Venus pass between the Sun and Earth, and show up clearly as black dots moving across the Sun’s bright disc.

Using powerful telescopes, we can also study more distant planets in other star systems as they pass in front of their stars. Scientists rely heavily on transits in their hunt for and study of exoplanets.

Dione orbits within Saturn’s magnetosphere, a region of space surrounding the planet that is filled with highly energetic atomic particles. These particles rain down on Dione and smash into its surface. Dione is too small with too little gravity to hold on to an atmosphere of its own, but this continuous high-energy bombardment releases molecules from the moon’s surface that form a thin, atmosphere-like layer. This tenuous atmosphere was discovered during two of Cassini’s close flybys of Dione, on 11 October 2005 and 7 April 2010.

Cassini acquired this visible-light view using its narrow-angle camera when it was 2.3 million km from Saturn on 21 May 2015. The image scale is 14 km per pixel.

Credit: NASA/JPL-Caltech/Space Science Institute

Tags:   Saturn Dione Cassini moon

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BepiColombo is Europe’s first mission to Mercury. It will set off in 2017 on a journey to the smallest and least explored terrestrial planet in our Solar System, following in the footsteps of Mariner 10 and Messenger.

On 4 October, ESA’s ESTEC technical centre opened its doors to the public. On display that day were the two BepiColombo orbiters and the electric propulsion module.

This photograph shows BepiColombo literally in the spotlight. On the left, lit in blue, is ESA’s Mercury Planetary Orbiter, which will study the surface and internal composition of the planet. In the centre, under the red spotlight, is the Mercury Transfer Module, also provided by ESA, which will carry the two orbiters to their final destination. Completing the trio is Japan’s Mercury Magnetospheric Orbiter, glowing green, which will study the planet’s magnetic field.

All three were displayed behind glass within the cleanroom conditions of the Test Centre, where they are being tested.

Credit: ESA–J. Benkhoff

Tags:   BepiColombo ESTEC OpenESTEC test centre

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