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## Jets of water found coming from the moon Europa

Earlier this week it was announced that NASA’s Hubble Space Telescope had observed evidence for water geysers shooting from the surface of Europa, one of Jupiter’s larger moons. Here is a link to NASA’s press release. I was on BBC TV talking briefly about this on Tuesday (27 September), the day after NASA’s announcement.

NASA has announced that the Hubble Space Telescope has observed water geysers emanating from the south pole of Jupiter’s moon Europa.

In fact, this announcement was additional evidence to add to a finding which had first been announced in 2013. In December 2012, astronomers used a spectroscope on Hubble to look in ultraviolet wavelengths at Europa. They found auroral activity near the moon’s south pole, and upon analysis of the spectrum of the UV emission from this auroral activity they found the spectral signatures of hydrogen and oxygen, the constituents of water.

Those 2012 observations have since been followed up using a different method. This time astronomers have observed how the Sun’s light, which is reflected from Jupiter, is affected as it passes Europa. As Europa transited in front of its parent planet, astronomers looked for signs of absorption of this light near the limb of the moon, which would be due to gases associated with Europa. Such a technique can, for example, be used to find and study the atmosphere of an extra-solar planet as it passes in front of its parent star.

Whilst not finding any evidence that Europa has an atmosphere, what the team found was that absorption features were seen near the moon’s south pole. When they calculated the amount and extent of material required to produce these absorption features they found that their results were consistent with the 2012 finding. They calculate that water jets are spewing out from the surface of Europa and erupting to a height of about 160 km from the moon’s surface.

We have had evidence since the Voyager mission in the 1980s that Europa has an ocean of water below its icy surface. This evidence was further enhanced during the Galileo mission in the 1990s. Where there is water there may be life, so it is possible that Europa’s ocean is teeming with microbial life. To find out, we need to directly study the water in this sub-surface ocean.

Unfortunately, due to the thickness of the icy crust covering its ocean, studying this water directly poses a huge challenge. We currently don’t have the capability to drill through such a large thickness of ice, although it is certainly something we would hope to do in the future. This discovery of water jets provides a much easier way to sample the water directly, and so it is quite feasible that NASA and/or ESA could send a probe to fly through the jet, take a sample of the water, and analyse it to see whether there are any signs of microbial life. This is very exciting, and is why this discovery of water geysers erupting on Europa is so important.

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## More on NASA’s Juno space probe

I have done a few interviews on the BBC in the last week about NASA’s Juno space probe; it is great to see the mission getting such press coverage. You can listen to my BBC Radio Cymru interview here, and my BBC Radio Wales interview here. With all the press coverage there have inevitably been a few misunderstandings, so I thought I would try and explain as clearly as I can what Juno hopes to accomplish and how it will do it.

Artist’s impression of the Juno spacecraft. Juno is the first space probe sent to such a large distance in the Solar System (5 AUs) to be powered entirely by its solar panels.

## Some background on Jupiter

Jupiter is by far the largest planet in the Solar System. All the other planets together would fit into it, and the Earth would fit into it over 1,300 times! Because it is the largest planet in  the Solar System, we believe that it would have dominated the formation of the planets. Once the gas in the central part of the solar nebula (the cloud of gas and dust from which the Sun and Solar System formed) had collapsed to form a nascent star, the disk of material around the still-forming Sun would have started clumping together under gravity and collisions to form the planets.

Because Jupiter is the largest planet, it sucked up most of the material in the disk of the solar nebula. It is mainly hydrogen and helium, as that is what the Sun and most of the Universe is made up of; about 75% hydrogen and 24% helium. But, the details of Jupiter’s composition are mainly based on theory rather than any hard observations.

## What are Juno’s (main) scientific goals?

According to NASA’s Juno webpage (click here to go to it), the main objectives of Juno are

• Determine how much water is in Jupiter’s atmosphere
• Look deep into Jupiter’s atmosphere to measure composition, temperature, cloud motions and other properties
• Map Jupiter’s magnetic and gravity fields, which will reveal the deep structure of the planet
• Explore and study Jupiter’s magnetosphere near the planet’s poles, especially the aurorae, and provide new insights into how the planet’s enormous magnetic field is generated and how it affects the planet’s atmosphere

I will blog about each of these four points over the next few weeks, so let me start with the determination of how much water is in Jupiter’s atmosphere.

## How much water is there in Jupiter’s atmosphere?

The reason this is an important question is that the two most popular theories for how Jupiter formed predict different amounts of water. Jupiter is thought to have either formed (i) from the collapse of a massive fragment of the Solar nebula, or (ii) from the build-up of planetesimals. In the first theory, the amount of water would be less than in the second theory, as the rocky planetesimals in the second theory would have been been coated in water-ice and ammonia-ice.

If you look at an astronomy textbook the interior model of Jupiter shows a solid core, but we have never actually observed this core.

A model of the interior of Jupiter. We believe that it has a rocky core, with a region of hydrogen under such extreme pressure that it takes on metallic properties and can conduct electricity. But, we have no direct observations of the interior.

Therefore, measuring the amount of both water and ammonia should help us decide which theory is closer to the truth. Water, ammonia, carbon dioxide and methane are examples of what we call ‘ices’ in astronomy, as in the environment of the Solar System all of these compounds can exist as gases but also as solids.

The water and ammonia will be measured by a microwave radiometer. This instrument consists of six antennae measuring the radiation at 600 MHz, 1.2, 2.4, 4.8, 9.6 and 22 GHz. These are the only microwave frequencies which are able to pass through the thick Jovian atmosphere. These radiometers will measure the abundance of water and ammonia down to a pressure of 200 bar, which corresponds to a depth below the cloud tops of 500 to 600 km. This is a small fraction of the radius of Jupiter, which is about 70,000 km, but it is still further below the cloud-tops than we have so far been able to study.

In the next blogpost on Juno, I will talk about how it will measure the gravitational and magnetic fields of Jupiter.

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## NASA’s Juno arrives at Jupiter

Later this morning (Monday 4 July) I will be on BBC radio talking about NASA’s Juno mission to the planet Jupiter. This is the latest space probe to be sent to study the largest planet in the Solar System, and follows on the highly successful Galileo spacecraft which studied Jupiter in the 1990s.

Juno left Earth in August 2011 and is due to arrive at Jupiter today. But, in order to go into orbit about the planet a rocket needs to be fired to slow the spacecraft down and put it into orbit. This is due to happen tomorrow (Tuesday 5 July). The rocket engine which will do this was built in England. If the ‘burn’ fails, the mission will fail, as the space probe will just hurtle past Jupiter and continue on into the outer Solar System.

NASA’s Juno satellite was launched in August 2011 and arrives at Jupiter this week. It will be put into a polar orbit about the planet, but with a highly elliptical orbit which will take it out beyond Callisto’s orbit. Each orbit will take 14 days.

## What are Juno’s scientific objectives?

In addition to studying Jupiter, the Galileo spacecraft spent a great deal of time studying her four large moons; Io, Europa, Ganymede and Callisto. Galileo was in an equatorial orbit. Juno, on the other hand, will be put into a polar orbit. Its main objective is to study Jupiter, rather than its moons.

Jupiter is what is known as a gas giant. It is mainly hydrogen, and contains more mass than all the other planets in the Solar System put together. In fact, it is a failed star; if it were some 10 times more massive it would have had enough mass to ignite hydrogen fusion in its core. Even though it is not burning hydrogen, it is still leaking heat left over form its collapse into a planet 4.5 billion years ago.

In the last 20 years we have discovered many Jupiter-like planets orbiting other stars. Most of these are much closer to their parent star than Jupiter is to the Sun, and this has raised questions about how gas giants can be so close to their parent star, and how is Jupiter where it is in our Solar System? Jupiter is about five times further away from the Sun than the Earth is, and much further away than the Jupiter-like planets we have found around other stars. Did Jupiter start off closer to the Sun and get kicked further out, or did it migrate inwards from further away? We don’t know.

Some of the things Jupiter hopes to determine are

• the ratio of oxygen to hydrogen in Jupiter’s atmosphere. By determining this ratio it will effectively be measuring the amount of water, which will help distinguish between competing theories of how Jupiter formed.
• the mass of the solid core believed to lie at the planet’s centre, deep below the very thick and extensive atmosphere. This also has implications for its origin.
• the internal structure of Jupiter – it will do this by precisely mapping the distribution of Jupiter’s gravitational field.
• its magnetic field to better understand its origin and how deep inside Jupiter the magnetic field is created.
• the variation of atmospheric composition and temperature at all latitudes to pressures greater than 100 bars (100 times the atmospheric pressure at sea level on the Earth).

Juno has a funded operational lifetime of about 18 months. In order to better study the interior of Jupiter, the spacecraft will plunge into the planet’s atmosphere in February of 2018, making measurements as it does so.

## ++UPDATE++

Juno’ rocket successfully fired at about 3:20 UT today (Tuesday 5 May) and is now in orbit about Jupiter. It will complete two large 53-day orbits before being inserted into its 14-day orbit for science operations. This 14-day orbit is highly elliptical, and at its closest the probe will come to within 4,300 km of the cloud tops.

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## JUICE mission to Jupiter moves a step closer

The European Space Agency’s JUICE mission to Jupiter moved a step closer recently with the signing of an important contract between ESA and Airbus. JUICE stands for JUpiter ICy moon Explorer, and is an ESA mission to send a probe to explore Jupiter and her moons, with a launch date of 2022 and an arrival at Jupiter in 2029. The contract signed with Airbus will see them lead the development and construction of this satellite. There will also be some involvement from NASA and the Japanese space agency JAXA.

Upon arrival at Jupiter, JUICE will manoeuvre to achieve close passes of its moons Callisto and Europa, before settling into orbit about its largest moon Ganymede. Ganymede, together with Europa and possibly Callisto, is believed to have a liquid ocean beneath an icy crust.

Airbus have recently signed a contract with the European Space Agency (ESA) to lead the construction of JUICE, a probe which will be sent in 2022 to study Jupiter’s moons.

The main focus of the JUICE mission will be to see how habitable Ganymede is for microbial life. With liquid water, and heating from the tides caused by Jupiter’s tides, Ganymede, Europa and Callisto are believed to be amongst the most likely places in our solar system for life to have developed. Longer term plans are to build a probe which will be able to burrow through the icy crust of one of these moons and actually look directly for life in their oceans.

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## The planets this winter (2015/16)

With winter (in the Northern Hemisphere) approaching, I thought it was about time I gave a summary of which planets are visible over the next few months. The longer nights, enabling easier viewing of the night-time sky, is one of the few pluses about this time of year as far as I am concerned. So, which planets are visible this winter (2015/16)?

The times I will give for various planets rising or setting are for Cardiff, where I live. So, if you are living elsewhere the times will almost certainly be different. Obviously, if you are living in the southern hemisphere you are about to move into summer not winter. But, although times may vary depending on your location; whether a planet is visible or not, and whether it is visible in the evening after sunset or in the morning before sunrise will not be different.

Of the 5 naked-eye planets, all but Saturn are visible this winter. Here is more detail about each.

## Mercury

Mercury is currently in Sagittarius, rising before the Sun and thus setting after the Sun. So, it is currently an evening object. It reaches maximum elongation on the 29 December when it will be $25^{\circ}$ to the East of the Sun, and on this day it will set in Cardiff at 17:43. The Sun sets on this day at 16:11 in Cardiff, giving some 1.5 hours after sunset to see Mercury. Although these setting times will vary depending on your location, what will not vary is the time between sunset and Mercury setting, which will be about 1.5 hours no matter where you live.

1.5 hours between sunset and Mercury setting it very good. Mercury is rarely this far from the Sun; so for those of you who have never seen Mercury, this month of December provides a very good chance. Find a view to the western horizon which is uninterrupted and away from city lights, and use the chart below to find Mercury. It will be reasonably bright, at a magnitude of -0.5.

Mercury just after sunset as seen from Cardiff on 29 December 2015. This month is a good month to see Mercury, as its maximum eastern elongation (the maximum angle between it and the Sun) is nearly as large as it can ever be. There are no bright stars near Mercury at the end of December.

Mercury will reach inferior conjunction on 14 January, whereupon it will reappear as a morning object later in January and February.

## Venus

Venus is currently in Libra. It is a morning object, very bright before sunrise. At a magnitude of -4.1 you cannot fail to see it. It will reach maximum western elongation on 12 January. You can see it in the diagram below of the sky before sunrise, which also shows where to find Mars and Jupiter. Venus will be visible as a morning object throughout this winter and into the spring.

## Mars

Mars is currently in Virgo. It is rising at the end of December just after 2am, so is a morning object. In fact, it can be seen in the morning sky along with Venus and Jupiter throughout much of the winter, as the diagram below shows. At the end of December it has a magnitude of +1.3, fainter than nearby Spica, which is at +1.05. Mars will reach opposition on 22 May, by which time it will have brightened to -2.1, so some 23 times brighter; making April, May and June by far the best time to see this planet.

The morning sky at the end of December as seen from Cardiff. Venus, Mars and Jupiter are all visible in the morning sky this winter. Jupiter and Venus are easy to find as they are so bright. Mars is a little trickier, but will brighten as it approaches opposition in the spring

## Jupiter

Jupiter is in Leo, and is also currently a morning object. At the end of December it rises just before 11pm. It will be at opposition in early March (8 March), and so in late winter and spring it will be an evening object, but for most of this winter it is better seen in the morning before sunrise.

I like it when one can see Jupiter and Venus at the same time, as it allows one to see how much brighter Venus is than Jupiter. Normally Jupiter is the brightest point-like object in the sky, but when Venus is visible it outshines Jupiter by a factor of 6 or so.

## Saturn

Saturn is currently in Ophiuchus, and this winter is not the time to see Saturn. It will reach opposition in early June (3 June), so spring and summer are the best times to see Saturn in 2016 and over the next few years. It will not become a winter object again for another 14 years or so.

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## The sky in June (2015)

In two days’ time, Venus will be as far to the East of the Sun in the sky as it can be (what is called “maximum Eastern elongation”), so I thought I would take that event as an excuse to summarise what is happening in the sky this month of June.

For anyone who has been paying even the most cursory attention to the evening sky over the last several months, Venus has been almost impossible to miss as it has dominated the Western sky after sunset. The only objects which can outshine Venus in the sky are the Moon and the Sun; so when Venus is visible it is the first object to appear as the sky darkens after sunset, or the last object to disappear as the sky lightens before dawn. For the last several months, Venus has been shining with a magnitude of about $-4$ (it varies because its distance from the Earth is changing and also its phase is changing). This is some eleven times brighter than Sirius, the brightest star in the sky (which has a magnitude of $-1.44$). It has even outshone Jupiter, the object which has dominated the later evening sky throughout most of the last several months, by a factor of about six.

This Saturday (the 6th), it will get as far to the East of the Sun as it can get as seen from Earth, and when it is at greatest Eastern elongation, the angle between looking towards the Sun and towards Venus is about $45^{\circ}$. This means that Venus will set some three hours after the Sun. For those who wish to be precise, this particular maximum eastern elongation will occur at 19:13 UT on the 6th, and the angle between the Sun and Venus will be $45^{\circ} 24^{\prime}$.

The figure below shows the orbit of Venus (in red) and that of the Earth (in blue). Both planets orbit the Sun in an anti-clockwise direction if one were to look down on the Solar System from above the Earth’s North pole (which is the convention used, sorry Southern Hemisphere people!). Venus is currently heading towards inferior conjunction (the time when Venus and the Sun lie in a straight line as seen from Earth, and so it is not visible). When it is heading towards inferior conjuction it is to the East of the Sun, and hence sets after the Sun and is seen as an “evening star”. The upcoming inferior conjunction happens on the 15th of August, so just a couple of months away.

I diagram of Venus’ and Earth’s orbits. Venus’ orbit is shown in red, the Earth’s orbit in blue. Currently, Venus is to the East of the Sun, and will reach maximum Eastern elongation on Saturday (the 6th), at 19:23 (UT)

After passing inferior conjunction, Venus will lie to the West of the Sun as seen from Earth, and so will slowly re-appear as a “morning star”, but you will have to wait for a few weeks after inferior conjunction for this, as initially it will be too close to the Sun and so lost in the glow of dawn.

However, long before it reaches inferior conjunction, there is a celestial highlight to look out for, which happens towards the end of June. As June progresses, Venus and Jupiter will appear to get closer and closer together in the sky, and by month’s end there will be a spectacular conjunction of the two brightest planets, something not to be missed. The two diagrams below show Venus and Jupiter on the evening of maximum Eastern elongation (the 6th), and then again at the end of the month.

Venus and Jupiter at 21:30 BST (20:30 UT) as seen from London on the 6th of June. On this evening, Venus will be at “maximum Eastern elongation”.

Venus and Jupiter as seen at 21:30 BST (20:30 UT) from London on the 28th of June. The two planets will get even closer over the following few nights, producing a spectacular conjunction of the two brightest planets.

The other planet worth looking out for this month is Saturn. Saturn is currently in Libra, but moving into Scorpio. In the middle of June it will be rising at just before 7pm and transiting at just before 11:30pm, so this month is a good time to see it.

Saturn in the middle of June, at 21:30 BST (20:30 UT) as seen from London. Saturn is currently in Libra, heading into Scorpio.

Saturn is not particularly bright at the moment, but you can use the bright stars Antares (in Scorpio) and Spica (in Virgo) to find it; just look at the diagram above.

The other highlight of June is, of course, the summer solstice (or winter solstice to people in the Southern Hemisphere). This is, of course, the longest day of the year for people living in the Northern Hemisphere, the moment when the Sun reaches its most northernly point in the sky. This year’s solstice will happen on the 21st of June at 16:38 UT, so at that moment the sky will be directly overhead for a person at the correct longitude on the Tropic of Cancer (for anyone on the Tropic of Cancer the Sun will effectively cast no shadow at midday on that day). Here is south Wales, the days around the Summer Solstice are really long, with the Sun rising at about 4:30am and not setting until nearly 9:30pm. It is my favourite time of the year!

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## The sky from Buenos Aires

One of the lectures I will be giving on my cruise from Buenos Aires to Santiago is how the sky as seen from the southern parts of South America will look considerably different to the skies that Europeans and people from North America are used to seeing.﻿ Let me explain some of the obvious differences. First of all, although the Sun rises in the East and sets in the West in both the northern and southern hemisphere, if you are as far south as the southern parts of South America you need to look north to see the Sun. This means that you are facing north, and the Sun will appear to move from right to left across the sky, not from left to right as we northerners are used to seeing it. This can be quite disorientating.

Jupiter, and all the other planets, are visible from the Southern Hemisphere but again, one needs to look north to see them, not south. Just as in the Northern Hemisphere, Jupiter will dominate the evening sky for the next several months, and is in the constellation Cancer, slowly moving eastwards into Leo over the next 6-12 months.

The evening sky (7:15pm) from Buenos Aires on the 5th of March 2014. Jupiter is clearly visible, and will dominate the evening sky for the next several months. Just as with the Sun, from this location you need to look north to see Jupiter, not south as in the Northern Hemisphere.

This next diagram below shows Orion and Sirius, two very well known things in the winter sky, but as you can see from the Southern Hemisphere everything looks upside down! We are used to seeing Orion with Betelgeuse in the top left and Rigel in the bottom right, but from Buenos Aires this is flipped; Betelgeuse is in the bottom right, and Rigel in the top left (just imagine looking at Orion from the Northern Hemisphere but standing on your head to do so!). Just as confusingly, we are used to seeing Sirius (the brightest star in the sky) below Orion, closer to the horizon, because it is to the south of Orion. But, from Buenos Aires, it is above it, further away from the horizon. Very confusing!

This shows how confusing the southern skies can be to someone from the Northern Hemisphere. Orion is upside down, and Sirius is above (further south) Orion, not below as we see it in the Northern Hemisphere.

During the cruise, the other very bright object that people cannot miss is Venus, which is dominating the early evening sky. Venus will be at greatest eastern elongation on﻿ the 6th of June, which means that between now and then it will be moving further and further to the east of the Sun as seen from Earth (remember both we and Venus are moving in orbit about the Sun as this is going on), and as it moves further and further east the time between sunset and Venus setting gets bigger and bigger. On the 5th of March the Sun sets at 7:25pm from Buenos Aires, and Venus will set at 8:46pm. This gives a good hour to see Venus after sunset.
By early June, from the same location, the Sun sets at 5:50pm and Venus will set at 9:04pm, giving about three hours.

Venus is the evening sky as seen from Buenos Aires on the 5th of March at 7:14pm. At the moment Venus and Mars are close, and Uranus is near them too.

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