Monthly Archives: May 2012

Venus and Earth: worlds apart

At first glance, Venus appears much like the Earth: their size and mass are fairly similar and so are their densities. Like Earth, Venus’ rocks are likely mostly basaltic, created during intense periods of volcanic activity when the planets formed around four and a half billion years ago. Both planets have an atmosphere, too.

But it turns out that any similarities end there. Close up, they are worlds of extremes.

Earth and Venus – worlds apart. Credits: Earth: NASA; Venus: Magellan Project/NASA/JPL

The Venus greenhouse
Under a thick, choking atmosphere of carbon dioxide and sulphuric dioxide clouds the surface pressure is 90 times greater than on Earth – that would be like walking underwater at a depth of 900 metres in Earth’s oceans! Then there’s the oven-hot 465 degrees Celsius surface temperature to contend with – the hottest planetary surface in the Solar System.

The dominant component of the Venus atmosphere, carbon dioxide, is a so-called greenhouse gas. Just like a greenhouse in which you might grow tomatoes, carbon dioxide is very good at trapping heat.

On Earth, the natural greenhouse effect is dominated by water vapour, a very efficient greenhouse gas, with contributions from carbon dioxide, methane and ozone. Without this naturally warming greenhouse effect we’d be shivering at -18 degrees Celsius!

An inhospitable surface lies beneath Venus’ thick cloud cover. Credits: ESA/C.Carreau.

On Venus, only ten percent of the incoming solar radiation reaches the surface; most of it is reflected back into space by the planet’s dense cloud cover, making Venus appear to shine brightly in the night sky.

Despite the small amount of solar radiation reaching the surface, it is trapped effectively by gases in the lower atmosphere, giving rise to the high temperatures. Scientists think that Venus was once more like the Earth, but at some point it started heating up uncontrollably.  Any water that may have once existed in oceans on the surface evaporated into the atmosphere causing further runaway heating. It’s like an Earth gone wrong with a runaway greenhouse effect that some planetary scientists say should be a warning as to what may become of our home planet if human activities – such as burning fossil fuels and destroying forests, which intensifies the natural greenhouse effect – continue.

A volcanic pressure cooker
Venus’ surface is a land of gently rolling uplands and volcanoes interspersed with low-lying plains and dotted with large impact craters. There are few small impact craters thanks to the planet’s thick atmosphere that burns up small meteoritic debris before it can strike the surface.  The oldest craters are in fact very young, no more than 500 million years old, suggesting that perhaps the planet undergoes regular global eruptions of lava that covers the entire surface.

On Earth, the steady eruption of volcanoes and shifting plates of crust that give rise to earthquakes mean that energy is released gradually over long periods of time. Venus, however, is like a volcanic pressure cooker with a tightly sealed lid that is only occasionally ruptured, but with catastrophic consequences.

Artist’s view of ESA’s Venus Express flying over atmospheric storms. Credits: ESA – AOES Medialab.

A backwards planet
Venus rotates backwards with respect to Earth, therefore the Sun appears to rise from the west. It also rotates very slowly – one Venus day takes 243 Earth days, which is longer than its 225 day year! However, since the planet rotates backwards, the time between a day measured from noon to noon becomes 117 days. Despite these long days, the winds on Venus are so fierce that they race around the planet in just four days.

ESA’s Venus Express, currently the only spacecraft in orbit around Earth’s mysterious sister planet, is providing spectacular detail of the complexities of the atmosphere, right down to the surface. Studying Venus is extremely important to learn how Earth and Venus, despite forming in the same neighbourhood and with similar planetary processes occurring on both worlds, grew up to be so different.

Venus and Earth: vital statistics

  Venus Earth
Mass 4.87x1024 kilograms  5.98 x1024 kilograms 
Equatorial radius  6052 kilometres 6378 kilometres
Density (mean) 5250 kilograms per cubic metre 5520 kilograms per cubic metre
Average distance form the Sun 108 million kilometres (0.7 Astronomical Units) 150 million kilometres (1 Astronomical Unit)
Rotation period 243 Earth days (retrograde) 23 hours 56 minutes
Year length (orbital period) 224.7 (Earth) days 365.2 days
Surface temperature (mean) 465 oC 15 oC
Atmospheric pressure on the surface 90 bars 1 bar (sea level)
Visual albedo (reflectivity)  0.76 0.37
Highest point on the surface Maxwell Montes (17 kilometres) Mount Everest (8.8 kilometres)
Major atmospheric components 96% carbon dioxide, 3% nitrogen 78% nitrogen, 21%oxygen, 1%argon
Surface materials Basaltic rock and altered materials Basaltic and granite rock and altered materials
Orbit inclination 3.4 o 0 o (per definition)
Obliquity of rotation axis 178 o 23.5 o
Surface gravity (at equator) 8.9 metres per square second 9.8 metres per square second

Measuring the size of the Solar System – Parallax

By guest blogger Peter Bond

Edmond Halley’s method, which involved observations of a transit made from widely spaced places, was based on the principle of parallax. This uses the fact that objects appear to shift position against a fixed background if they are observed from two different places. The further the object, the smaller the parallax shift.

This principle can be shown by holding one finger in front of your face. Look at it with the left eye, then with the right eye. You will notice that the finger seems to shift position, even though it has not been physically moved.

If you move the finger further from your face and carry out the same experiment, you will notice that the amount of parallax shift is smaller. If the distance between your eyes is known, and the angle from each eye to the finger is measured, it is possible to use simple trigonometry to calculate the distance of the finger.

For the transit of Venus, observers on the Earth are separated by thousands of kilometres, so they will see the disc of Venus at slightly different locations on the Sun’s disc. By making observations from two widely spaced points on the Earth’s surface, and timing the start and end of the transit accurately at each place, you can work out the solar parallax, the apparent difference in the position of the Sun from those two locations.

By measuring the angular shift between the apparent locations of Venus across the Sun, and taking into account the baseline distance between the two observing sites, you can calculate the distance to Venus by using triangulation.

In practice, however, this is an extremely difficult measurement to make because the disc of Venus is so small (1/60 of a degree) and the parallax angles are very difficult to measure directly (1/120 of a degree). This is why astronomers use each entire path of Venus (the ‘chord’) across the Sun’s disc as a better way of determining the parallax angle.

An explanation of the calculation method used by Halley can be found here.

For your own transit of Venus parallax calculator, click here.

Today, distances in the Solar System are calculated with great precision through very different means, such as ground-based radar and time delays in radio signals from spacecraft.

Measuring the size of the Solar System – the ‘black drop’ problem

By guest blogger Peter Bond

Despite the best efforts by astronomers who voyaged to far flung reaches of the Earth to watch the transits, the results of the observations was not as conclusive or accurate as had been hoped. The observations were plagued by many technical difficulties, and by the slightly fuzzy outline of Venus, caused by its dense atmosphere. There was also an unforeseen problem with a phenomenon known as the ‘black drop’ effect.

One of the chief problems the observers faced was pinpointing the precise time of ‘second contact’, when the whole of Venus was first visible on the face of the Sun. They noticed that its black disc seemed to remain linked to the edge of the Sun for a short time by a dark ‘neck’, making it appear almost pear-shaped. The same happened in reverse when Venus began to leave the Sun.

Click for an animation of the black drop effect.

This so-called ‘black drop effect’ was one of the main reasons why timing the transits failed to produce consistent accurate results for the Sun-Earth distance. Halley expected second contact could be timed to within about a second. The black drop reduced the accuracy of timing to more like a minute.

The black drop effect is often mistakenly attributed to Venus's atmosphere, but modern research has suggested that it is due to a combination of two key effects. One is the image blurring that takes place when a telescope is used (described technically as ‘the point spread function’). The other is the way that the brightness of the Sun diminishes close to its visible ‘edge’ (known to astronomers as ‘limb darkening’). There may also be a small contribution from observing through Earth’s atmosphere, but observations of the black drop effect during Mercury’s 1999 transit across the Sun using NASA’s TRACE satellite confirmed that neither the planet’s nor Earth’s atmosphere is needed to produce the effect.

Despite the disappointments of the 18th century expeditions, optimistic astronomers tried again during the transits of 9 December 1874, and 6 December 1882. Once again the results were inconclusive, and scientists began to realise that the practical problems with Halley’s method were just too great to overcome. Nevertheless, the value of the Sun-Earth distance was known with much greater accuracy than ever before after the results of the 1882 transit were analysed.

Read more about Halley's method in part 3: Parallax

Measuring the size of the Solar System – transits through the ages

By guest blogger Peter Bond

The first person to predict a transit of Venus was the German mathematician and astronomer, Johannes Kepler, who calculated that one would take place on 6 December 1631. Unfortunately, the transit was not visible from Europe and there is no record of anyone seeing it.

Jeremiah Horrocks, a young English astronomer, studied Kepler’s planetary tables and discovered, only a month in advance, that a previously unrecognised transit of Venus would occur on 24 November 1639. (Note: two calendars were used at that time. According to the Gregorian calendar, which added 10 days to the older Julian calendar, the date of the 1639 transit took place on 4 December).

Horrocks observed part of the transit from his home at Much Hoole, near Preston. His friend, William Crabtree, also saw it from Manchester, having been alerted by Horrocks. As far as is known, they were the only people to witness the event.

By the mid-17th century, the relative distances of the planets from the Sun were well known. According to Kepler’s third law of planetary motion, if Earth was at one astronomical unit (1 AU), then Venus orbited the Sun at 0.72 AU, Mars at 1.52 AU, and so on.

However, the actual distance of the Sun and planets from Earth was not known with any degree of accuracy. The best estimate of the time was that the Sun-Earth distance was 137.7 million km. (The actual figure is about 150 million km.)

Edmond Halley (of comet fame) suggested that observations of transits of Venus could, in principle, be used to find out how far the Sun is from Earth.

He suggested that observers in widely separated locations would carefully measure the time that either side of Venus’s disc first ‘touched’ one edge of the Sun, and then exited on the opposite limb of the Sun. This sequence comprised four separate events, called contacts. Each of these had to be timed with split-second accuracy at the far-flung observing sites.

Once the precise contact times were determined, a complex mathematical calculation was performed to determine the observed path of Venus across the Sun from each location. The angular difference between these paths resulted from a parallax shift. Corrections had to be made for the slight differences in contact times caused by east-west differences in longitude.

Halley died in 1742, but hundreds of scientists travelled across the world to try out his method during the transits of 1761 and 1769. Captain James Cook’s expedition to Tahiti in 1769 is one of the most famous expeditions, part of a voyage in which he discovered the east coast of Australia.

One of the unluckiest expeditions was led by Guillaume le Gentil, who set out for Pondicherry, a French colony in India. As his ship was nearing India, he learned that the British had occupied Pondicherry, so he returned to Mauritius. Unable to make proper observations of the 1761 transit, he decided to stay on the island until the next transit, eight years later.

Unfortunately, when the long-awaited day arrived, the Sun was hidden behind a blanket of cloud. When he finally arrived back in Paris in October 1771, he had been declared legally dead, his wife had remarried and all of his possessions had been divided up among his relatives!

Continue reading in part 2: the 'black drop' problem...

Transits of Mercury

Since Mercury and Venus are the only planets that lie inside the Earth’s orbit they are the only planets that can pass between Earth and the Sun to produce a transit.

The orbital plane of Venus is not exactly aligned with that of Earth, such that transits occur very rarely, in pairs eight years apart but separated by more than a century. The last was in 2004, but after next week’s event there won’t be another until 2117 – so make the most of this twice-in-a-lifetime event!

There are more chances to see transits of Mercury, however, with an average of 13 transits of Mercury per century.

Mercury has a highly eccentric orbit, varying in distance from the Sun from 46-70 million kilometres. In addition, its orbit is inclined by 7 degrees to that of Earth.

Mercury's orbit crosses Earth's orbital plane in early May and early November each year, but only if it passes between Earth and the Sun will a transit be seen. Because of Mercury’s eccentric orbit, for transits occurring in May Mercury appears 158 times smaller than the diameter of the Sun, while for November transits it appears 194 times smaller. For comparison, Venus’ diameter is approximately 32 times smaller than that of the Sun, which is right at the limit of good human eyesight.

The small size means you need telescopic equipment to see a Mercury transit but remember – NEVER look directly at the Sun with unprotected eyes, and NEVER look through a telescope or binoculars at the Sun as this will cause permanent blindness.

Mercury’s eccentric and inclined orbit also means that transits do not occur every year. So, some dates for your diary for the remainder of this century:

09 May 2016
11 November 2019
13 November 2032
07 November 2039
07 May 2049
09 November 2052
10 May 2062
11 November 2065
14 November 2078
07 November 2085
08 May 2095
10 November 2098

In the meantime, don’t forget the last transit of Venus of the 21st century, occurring next week, on 5-6 June.

Venus exploration timeline

While ESA’s Venus Express is currently the only spacecraft in orbit around Venus, many spacecraft have returned data from the planet over the last half-century. As space- and ground-based telescopes prepare to watch Venus during the transit on 5-6 June, here's a look back at some of the highlights of Venus exploration:

1962 – Mariner 2 (US) – first successful flyby of Venus; confirmed high surface temperatures and pressures, a carbon dioxide rich atmosphere, continuous cloud cover, and the slow retrograde rotation of the planet.

1967 – Venera 4 (USSR) – first spacecraft to survive entry into another planet’s atmosphere. Returned atmospheric data, and deployed a parachute system to an altitude of 25km, becoming the first successful probe to perform in situ analysis of another planet’s environment.

Surface photo of Venus by Venera 13. Credits: NASA/NSSDC.

1967 – Mariner 5 (US)– successful flyby returning data on magnetic fields and plasmas, as well as UV emissions from the atmosphere.

1969 – Venera 5 and 6 (USSR) – atmospheric probes detected the presence of nitrogen and oxygen.

1970: Venera 7 (USSR) – first soft-landing on Venus and the first time data was returned from a manmade object after landing on another planet. It measured a surface temperature of 475°C and a surface pressure of 90 bar.

1972: Venera 8 (USSR) – landed on Venus and was the first to measure wind speeds as it descended through the atmosphere (from 100 metres/second above 48 kilometres to 1 metre/second below 10 kilometres).

1974: Mariner 10 (US) – Venus flyby en route to Mercury; tracked global atmospheric circulation with visible and ultraviolet imagery. It was the first spacecraft to have an imaging system.

1975: Venera 9 and 10 (USSR) – First spacecraft in orbit around Venus. They photographed the clouds and looked at the upper atmosphere, while the landers returned the first black and white panoramic images of the surface.

Venus by Magellan

Radar image of the surface of Venus by Magellan. Credits: Magellan Project/NASA/JPL

1978-1992: Pioneer Venus 1 and 2 – included a large entry probe and three smaller entry probes, providing vertical profiles of the atmosphere. Also the first orbiter to make radar mapping of the surface. Over a decade, it recorded a 90% decrease of sulphur dioxide, possibly indicating a large volcanic eruption just before the arrival.

1978 Venera 11 and 12 (USSR) – successful landing on the surface, and detection of lightning and thunder.

1982: Venera 13 and 14 (USSR) – the landers sent back the first colour pictures of the surface.

1983: Venera 15 and 16 (USSR) – the orbiters provided radar maps and atmospheric analyses.

1985: Vega 1 and 2 (USSR) – released landers and balloons at Venus en route to Halley's comet that recorded winds running at 240 kilometres/hour. Landers provided precise temperature profiles down to the surface and in situ measurements of cloud composition.

1990-94: Magellan (US) – mapped 98 per cent of the surface of Venus using synthetic aperture radar, at a resolution of 300 metres per pixel.

1990: Galileo (US) – flyby en route to Jupiter. First spacecraft observations of Venus' lower clouds and spectral imaging of the night side near infrared emissions.

1998, 1999: Cassini-Huygens (US, ESA) – flybys en route to Saturn.

Venus Express in orbit around Venus

ESA's Venus Express has been in orbit around Venus since 2006. Credits: ESA.

2006 – present: Venus Express (ESA)– the first European spacecraft to orbit Venus.

2006 and 2007: Messenger (US) – flybys en route to Mercury.

2010: Akatsuki (JAXA) – Japanese orbiter, failed orbit insertion but a second attempt will be made in 2015.

2015+: Bepi-Colombo (ESA) – with a scheduled launch in 2015, two flybys of Venus are planned en route to Mercury.

Note that the dates marked are those of operation at Venus, rather than launch dates.

ESA Euronews: Unveiling Venus

It can be called the morning or evening star, depending on where you are or what time it is, but it is anything but a star. In fact, it is one of our nearest planetary neighbours. Venus and Mars may be Earth's close cousins, but they are oh-so different. Only now are we starting to peer through Venus' clouds to reveal the burning planet's secrets.

Find out more about Venus and what we've learnt from Venus Express, and discover how ESA plans to observe the planet as it transits across the face of the Sun on 5-6 June.

Transit Terminology

Key phases during a transit of a planet across the face of the Sun are often referred to as 1st, 2nd, 3rd and 4th contact. Credit: Michael Zeiler, eclipse-maps.com

Key phases during a transit of a planet across the face of the Sun are often referred to as 1st, 2nd, 3rd and 4th contact. Credit: Michael Zeiler, eclipse-maps.com

Astronomers use different terms to describe the four main phases of a transit:

  1. Ingress, exterior (or first contact): the point at which Venus’ disc is just touching the outer edge of the Sun. Shortly after, the planet appears to make a small black indent on the solar disc.
  2. Ingress, interior (or second contact): the point at which the entire planet has moved onto the solar disc.
  3. Egress, interior (or third contact): the point at which the planet touches the opposite solar limb.
  4. Egress, exterior (or fourth contact): the point at which Venus is just outside the Sun’s disc, concluding the transit.

Here are some other useful transit terms:

Image showing the aureole observed during the 2004 egress of Venus with the Dutch Open Telescope in La Palma - Credit: Tanga et al. 2012

Image showing the aureole observed during the 2004 egress of Venus with the Dutch Open Telescope in La Palma - Credit: Tanga et al. 2012

Aureole: the bright arc seen around the circumference of Venus’ disc partially outside the solar limb during ingress and egress. It was first observed during the transit of 1761 and revealed that Venus has an atmosphere. The effect is caused by refraction of sunlight in the dense upper atmosphere of Venus.

Black drop effect: the small black teardrop shape that appears to connect Venus to the limb of the Sun as it fully enters the solar disc just after ingress, interior, and just before egress, interior as it begins to leave. It is thought to be an optical effect caused in part by the effect of observing through Earth’s atmosphere, combined with diffraction of light inside the telescope, and by the dimming of the intensity of the Sun’s surface just inside its apparent outer edge.

Greatest transit: the point at which Venus is in the middle of its path across the solar disc, marking the halfway point in the timing of the transit.

Can I see the transit?

For a first glance look, check this map to see if you are suitably located to observe the 2012 transit.

Visibility map for the 2012 transit of Venus Credit: Michael Zeiler, eclipse-maps.com

Visibility map for the 2012 transit of Venus Credit: Michael Zeiler, eclipse-maps.com

The 2012 transit will be visible in its entirety only from the western Pacific, eastern Asia, eastern Australia and high northern latitudes.

For much of Europe the Sun will rise on 6 June with the transit almost finished; for the US, the transit will begin in the afternoon of 5 June.

Example of the tool at transitofvenus.nl, set for observing conditions from Svalbard, where ESA will be reporting live during the transit

Example of the tool at transitofvenus.nl, set for observing conditions from Svalbard, where ESA will be reporting live during the transit

There’s also a handy tool at http://transitofvenus.nl/wp/where-when/local-transit-times/ that allows you to set your precise location to find out how much of the eclipse you will be able to see, along with the predicted path that Venus will trace across the Sun from your location.

How do I see the transit?

Whatever method you choose to observe this historical event, please be extremely cautious. NEVER look at the Sun with your naked eye or through ordinary sunglasses, and especially not through an unprotected telescope – this will cause permanent blindness. Instead, use one of these tried and tested methods:

Solar shades
For those with keen eyesight, the transit will be resolvable with a pair of ‘eclipse shades’, which have a special filter to permit safe, direct viewing of the Sun. However, subtle features such as the black drop effect will not be visible without magnification.  Do NOT use these glasses to filter sunlight through a telescope eyepiece – the intensity will be too strong to protect your eyes.

Pinhole projection
Projecting a magnified view of the Sun through a telescope or binoculars onto a piece of white card (taking care to avoid overheating of the instrument optics by giving them a break every now and then) will provide a safe and satisfying view of the transit and will allow a group of people to admire the transit at the same time.  Find out how to use pinhole projection with a pair of binoculars here.

Using the projection method to view the Sun, as described on <a href="http://www.exploratorium.edu/transit/how.html">http://www.exploratorium.edu/transit/how.html</a>

Using the projection method to view the Sun, as described on http://www.exploratorium.edu/transit/how.html

It is also possible to project an unmagnified view of the Sun without the need for a telescope or binoculars, although finer details of the transit will not be resolvable. Find out how to build a simple pinhole projector here:

Solar telescope
The transit is undoubtedly best viewed when magnified, either through a specially designed solar telescope or through a telescope fitted with a solar filter (although do not use filters that fit over the eyepiece – these can shatter under concentrated sunlight). Be sure to cap the finderscope, too; to safely find the Sun, orientate it such that the shadow of the telescope is at its smallest.

Live webcast
Many professional observatories around the world will be the streaming the event live across the Internet, providing an even safer way to share this once-in-a-lifetime event. ESA will be streaming the transit from both hemispheres of the planet: from Svalbard, Norway and from Canberra, Australia at the Venus Transit Monitor.