Why is Venus so hot?

Venus, the second planet from our Sun is a hellish place. Despite being further away than Mercury, the planet is several hundred degrees hotter. At 735K (that’s 462ºC or 863F in old money) the air is hot enough to melt lead and once you start looking at the planet a bit more closely, the reason becomes clear.

Venus's thick atmosphere

Venus’s thick atmosphere prevents astronomers from seeing the planet’s surface. Credit: NASA

Venus’s has one heck of an atmosphere. The surface pressure is around 92 times that found on Earth and it’s made out of some pretty nasty stuff: 96.5% carbon dioxide, 3.5 nitrogen, laced with sulfur dioxide,  sulphric acid and a few other trace elements.

It’s no great secret that carbon dioxide is a green house gas – it lets light and heat in, but not out again. Venus is an example of what can happen if you let carbon dioxide build up in your atmosphere to ridiculous levels.

But what I always wondered was why does Venus have such a thick atmosphere in the first place? In most respects the planet is much like Earth. It’s slightly smaller, with around 80% of Earth’s mass, meaning its gravity is pretty much the same. It’s at 0.7 AU, one AU being the distance between the Earth and the Sun, so it’s not that much closer and from most observations seems to be pretty similar to the Earth. At first glance its warmer atmosphere and lower gravitational pull should mean atmospheric molecules are more likely to escape. So what the hell happened?

The surface of Venus

Using radar, scientists have managed to get a good idea of what Venus’s surface looks like. Credit: Venus

The answer is all to do with that magical substance that has defined the hunt for life in the Galaxy: water. Venus doesn’t have any, or at least not much. It has trace amounts in the atmosphere, around 20 parts per million, but compared to the 40,000 parts per million of water found in our atmosphere it’s practically nothing.

But it used to. As I said, Venus is very similar to Earth and almost certainly formed in much the same way. The Earth has water, as did Mars in the beginning for that matter. It stands to reason that Venus had its fair share too, though we don’t currently have any evidence as no Venusian lander has lasted longer than two hours on the surface.

At some point though the temperature on the planet reached a tipping point and it began to lose its oceans. The Sun has been warming at a rate of a few percent every billion years. On Earth this hasn’t appeared to have effected us too much, but Venus receives twice the energy from the Sun. The change in temperature was too much, eventually the oceans began to evaporate and as that happened, things began to go oh so terribly wrong.

Water’s of Venus

The evaporating water saturated the atmosphere. In the high atmospheric levels the Sun’s radiation started to break apart the water into hydrogen and oxygen. The light hydrogen floated away into the deep dark depths of space while the heavier oxygen does what oxygen always does, and reacted with absa-bloody-everything, most notably any carbon hanging around to create carbon dioxide.

Not only that but as the water evaporated the planet’s mantel began to dry out. Plate tectonics require liquid water to absorb minerals, act as a lubricant and so on. Without it, everything just seizes up, and that’s exactly what happened on Venus. Volcanic activity is another way that carbon dioxide gets sucked up and stored in rocks, being re-released when volcanoes do their thing.

All of this lead to rising carbon dioxide levels in the atmosphere, which caused the planet to begin heating causing the oceans to evapourate and eventually boil. To make matters even worse, water vapour is itself a green house gas, insulating a planet. Once the water began to be lost, Venus never stood a chance. On Earth the interaction between water, rock and carbon dioxide help to regulate our nice, temperate climate [1]. With this thrown out of kilter, everything went awry. The end result is a planet no one ever wants to go on holiday to.

Venus surface by Venera 13

The Russian Venera programme has sent several probes to Venus, including several landers. The missions were the first to land on another planet, but none lasted longer than two hours. However they did manage to send back several images of the planet surface, including this image taken by Venera 13. Credit: Roscosmos

[1] The Earth is pretty good at regulating its temperature. It does this through the interactions of volcanism, carbon dioxide and water. Volcanic activity is constantly catching and releasing carbon dioxide in the Earth’s crust. If the global temperature drops, water freezes, stopping it from absorbing carbon dioxide, which creates a green house effect, which heats the planet and melts the ice. Global temperatures get too high, the ice caps melt, the atmosphere becomes more humid and all that lovely liquid water sucks up the carbon dioxide and temperatures drop again. It should also be noted that while the planet is doing this, there are colossal civilisation ending, mass extinction educing storms and weather pattern shifts, which is why ‘global warming’ does not mean ‘a bit sunnier in the Summer’.

 

Did you know…

1994 Solar eclipse

The light of the corona is usually only visible on Earth during a total eclipse. It can be seen with specialist equipment though. Like a space telescope. Credit: Luc Viatour

The Sun’s corona, this aura of plasma that surrounds the main star, is many hundreds of times hotter than the photosphere, the surface that we see. While the temperature of the Sun’s surface is only 6,000K the corona can reach up to 1,000,000K.

All hail hypnothread. ALL HAIL.

Simulation of a cross section of a thread of solar material. All hail hypnothread. ALL HAIL.
Credits: NAOJ/Patrick Antolin

No one is 100% sure why this is, though the current leading theory is that it’s probably magnets… or rather that magnetic waves generated by the motions of matter inside the star. These oscillate through the Sun and cause the plasma in the corona to move in a turbulent motion (queue mesmerising gif to left) and the friction heats up the corona.

The Harvard Computers

The birth of the photographic plate was one of the most revolutionary moments in the history of astronomy. Before, an astronomer would have to spend long hours in the middle of the night, sitting in the cold at an often precariously placed eyepiece. Taking observations often took many hours and careful notation in less than ideal conditions.

The invention of the photograph had two different ramifications for astronomy. Firstly you could leave the plate there for hours, exposed to all the light of the night sky for as long as you could keep it steady. The human eye refreshes every 10 to 12 times a second, and so we can only see the light that falls on the eye in this time. With a plate you can leave it for 30 seconds, 5 minutes or all night if you can track what you’re looking at across the sky, gathering all the light falling on it in this time. This increase in light collection means that you can observe objects that are much dimmer than what can be seen with the human eye.

Secondly, once you have your photo taken and developed you can then take it away and look at it in the comfort of your office or study, in the middle of the day while sitting by the fire, which was much more cosy.

Williamina Fleming

Williamina Fleming

However there was a problem. The young male astronomers, usually PhD students and the like, didn’t think that sitting inside all day looking at photos was real astronomy. At Harvard College Observatory the men working under director Edward Charles Pickering moaned so much and did such a bad job that in 1881 he fired the lot of them, allegedly claiming that ‘his maid could do a better job’. True to his words, Prof. Pickering hired his maid, Williamina Fleming.

However, it transpired that Mrs Fleming was not just any ordinary maid. She was a highly intelligent and educated woman fallen on hard times after her husband abandoned her just when she was ready to give birth to their son. Initially she did simple tasks; clerical duties, copying and ‘computing’ i.e. maths [1]. However over time she began to take on more scientific and complicated tasks, such as looking at and analysing the spectrum produced by shining a star’s light through a prism.

Pickering quickly realised that not only was hiring women considerably cheaper than their male counterparts, they actually did a much better. For the price of one male astronomer Pickering could hire a dozen women, and they were soon known by the unflattering title of Pickering’s Harem, or the Harvard Computers. Together these women catalogued every single star observed in the sky, carefully measuring their colour, temperature, spectra and many other important properties. This was a feat that many thought impossible but after decades of hard work they completed the task. One of the group’s leaders, Annie Jump Cannon, cataloged and categorized over 350,000 stars in her lifetime, finding more stars in just four years than every male astronomer in history to the point put together [2].

The Harvard Computers

The Harvard Computers taken on 13th May 1913 . Back row, left to right: Margaret Harwood, Mollie O’Reilly, Prof. Pickering, Edith Gill, Annie Jump Cannon, Evelyn Leland, Florence Cushman, Marion Whyte, Grace Brooks. Front row: Arville Walker, Johanna Mackie, Alta Carpenter, Mabel Gill, Ida Woods. For more information look here.

The women were often paid less than the secretaries employed by the university and working in such a career was often seen as an admission of spinsterhood. All these women took this job for the love of astronomy and understanding. Many of these women went on to publish astronomical papers in their own right and many were the first women to be granted into the ranks of institutions such as the Royal Astronomical Society and the American Astronomical Society, and many are remembered on the moon having craters named after them. Henrietta Swan Leavitt, was even considered for a Nobel Prize in Physics for her work on Cephid variables that allowed astronomers to measure the enormous distances of the universe. Unfortunately she died before she could be officially nominated [3].

The work of the Harvard Computers was a huge leap forward not just in terms of astronomy but also for women in science. Though they themselves remain relatively unknown their work is still used by countless astronomers, both professionally and amateur, and will be for years to come.

[1] – Women of Science: Righting the Record by Gabriele Kass-Simon

[2] – Ladies of the Laboratory 2 by Lewis D. Eigen

[3] – Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How To Measure The Universe by George Johnson

Did you know…?

The Square Kilometer Array (SKA), a radio telescope being built in both Australia and South Africa and expected to actually cover 5 square kilometres of ground, will produce enough raw data to fill 15 million 64GB iPods a day. That’s equivalent to every piece of information sent and received over the entire internet. Twice.

SKA antenna

An artists impression of what the SKA antenna will look like. Thousands of these 12m diameter dishes, split between the two sites, will cover the square kilometre.

But it’s alright. You don’t have to rush out and panic buy iPods. Most of that information doesn’t tell us anything and gets thrown out straight away. Working out what to lose and what to keep, however, is one of the most challenging aspects of any project as big as the SKA.

The Truth About Dust

When we astronomers talk about dust we don’t mean the type that you spend your life endlessly hoovering up (that’s mostly good old fashioned dirt). We mean cosmic dust, the stuff that is made in the space between stars and covers whole galaxies in huge clouds. It’s the stuff that gives birth to stars and planets. Every thing you ever touched or tasted or loved was once part of a massive cloud of dust floating out in space.

But what actually is this dust? I’ll be honest. We are not 100% sure but we’ve got a pretty good idea. The problem is we can’t just go out into space and grab a handful to look at in the lab. Voyager 1, the furthest man made object from the Earth, is only just leaving our solar system and it took 35 years to get there and won’t be coming back. Going on a jolly to pick up some dust isn’t looking likely. We can try and look at the dust that is in the solar system, and spacecraft like Ulysses and Cassini are doing just that, but it’s still not the same as going out and looking at all the lovely dust inbetween stars.

Sometimes we get lucky and we get hit by a meteorite. If you crack open a meteorite you might just get some stardust (and that is the technical term). It’s quite tricky to get out without destroying it, but we’ve managed to find that cosmic dust is mostly made up of stuff like graphite, silica carbide, aluminium oxide and other such fun things. When a star dies, either by going supernova or just wasting away to a white dwarf, it throws all the elements it made in its life out into the universe. All these atoms form something called the interstellar medium (astronomers call it the ISM, because we love our acronyms!). In the ISM the atoms come together to form dust grains, fractions of a millimeter long. How exactly they come together is still something we’re trying to work out.

These grains all hang out together in huge clouds that go right the way across entire galaxies. The problem is these clouds aren’t transparent, and this can cause a lot of issues if you’re an astronomer. Plonk a big cloud of dust in front of a star and it will soak up all the light from that star and block it from view. However, as it soaks up all that light the dust cloud is also soaking up all of the heat. Everything that has a temperature emits thermal radiation and you can see these clouds if you use giant, very fancy, very expensive heat cameras. With these cameras we can reclaim a lot of lost information as nearly half of all the light that shines from stars gets soaked up by dust clouds. If we didn’t look at the heat images of these clouds, all that information would be lost.

Andromeda Galaxy

The Andromeda galaxy is the closest galaxy that resembles the Milky Way and so has long fascinated astronomers. The top image shows the visible image, so if your eyes were giant telescopes this is what you would see. Throughout you can see dark bands, where dust lanes are blocking out the star light. The lower images is in the infrared, so shows the heat pattern, with the bands of dust glowing brightly. If you look carefully you can see that the dark patches in the optical match up with the bright lanes in the infrared. (Visible: Kitt Peak National Observatory, Infrared: Spitzer, Image: HubbleSite)

This dust isn’t just acting like a giant rain cloud blocking out the starlight. It’s really important to the growth and life cycle of stars and galaxies. Dust is made from dead stars but it’s in these huge clouds, or nebula, that new stars get born. Our own sun was born from the remains of other stars that died millions of years before, as were all the planets including the Earth and everything on it. As Carl Sagan said ‘we are all made of star stuff’. The next time you’re doing the hoovering spare a thought for the dust, because once that dust was cosmic and lived in the space between the stars.