There are countless opinions about how to cook the best turkey. Some suggest slow roasting it for hours, while others prefer cooking at high heat. Some even dare to deep fry it in peanut oil. Of course all of these cooking suggestions are Earth-based, which is a bit limiting. Suppose you wanted to have a Thanksgiving dinner elsewhere in the solar system, such as Venus?

While Mars is a perennial favorite for human exploration, Venus is a strong second. It is similar in size to Earth, and has a thick atmosphere. Plus, at an altitude of about 50 kilometers above its surface, the temperature and atmospheric pressure is similar to that of Earth. So in many ways it would be much more welcoming to Earth explorers than Mars. Since Venus has the highest surface temperature of any planet in the solar system, it also would provide the opportunity to cook our Thanksgiving turkeys with natural heat.

Unfortunately, we couldn’t simply roast our turkey on the Venusian surface. With a surface temperature of 460 ^oC ( 860 ^oF) and an atmospheric pressure more than 90 times that of Earth at sea level, our turkey would be blackened to a crisp while the stuffing is still cold. Not even a foil tent can prevent that tragedy. So we would have to be a bit more creative.

Atmospheric temperature of Venus as a function of depth. Credit: Wikipedia

Given that a crewed mission to Venus is already a huge engineering challenge, it’s safe to assume that any colonists living in Venus’ upper atmosphere could send airships deeper into the atmosphere with relative ease. As you move closer to the Surface of Venus, the temperature and atmospheric pressure increases, so it would really be a matter of getting your turkey to a particular atmospheric depth for a certain amount of time. For example, Alton Brown suggests the high-temperature method for turkey, roasting it at about 500 ^oF  (260 ^oC) for about 2.5 hours. So you would just need to put your turkey into a probe and send it to a height of about 25 kilometers for a couple hours. While you’re at it, why not put all your food in different probes. Your pumpkin pie would need to go to an altitude of 35 km for about 40 minutes. Send your corn bread to about 35 km for 30 minutes, and so on.

If you timed your probes right, all your dishes could be cooked and piping hot at the same time. All you’d need to do is pour a glass of wine and brace yourself for the inevitable political arguments over dinner. Such as whether those colonists on Europa should be allowed to form an autonomous government.

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The Japanese spacecraft Akatsuki has made thrust corrections necessary to put itself in orbit around Venus. It was launched in 2010, and was scheduled to enter Venus orbit that year, however it’s main thruster failed. Instead it made a flyby of the planet, and orbited the Sun for the next five years. With adjustments to its course Akatsuki once again had a chance to reach orbit, this time using a long burn of its attitude control thrusters. 

Placing a spacecraft in orbit around Venus is a challenge because of the large amount of thrust necessary (known as delta-v), and reaching Venus and Mercury can be difficult because they are closer to the Sun than Earth.  Akatsuki is only the 8th spacecraft to orbit Venus. The goal of Akatsuki is to study the atmosphere of Venus, as well as look for evidence of volcanic activity on the surface.

Reaching a successful orbit after a second try is an excellent demonstration of how we can recover from failures. Space exploration is difficult, but with a bit of ingenuity we can still move forward to find a new dawn of scientific understanding.

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Different wavelengths of light interact with matter in different ways. Our atmosphere, for example, is largely transparent to visible wavelengths, but absorbs much of the ultraviolet. Dust surrounding the center of our galaxy absorbs visible light, but is more transparent to radio waves. As a result it is useful to observe objects at a range of wavelengths. Sometimes this light is reflected from natural sources or emitted by the objects themselves, but sometimes we actively shine light on an object to get an image. 

A radio map of Venus. Credit: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo

One way to do this is through radio waves. The radio telescope at Arecibo not only detects radio waves, it also has a large transmitter capable of sending radio signals into space. While it was once used to send a message to potential aliens, the transmitter’s main use it to reflect radio signals off solar system bodies. The timing of these signals can be used to measure the distance to planets more precisely, but with high resolution imaging we can also make detailed maps. Recently maps were made of the Moon (seen above) and Venus by beaming a radio signal from Arecibo and observing its reflection with the 100-meter radio telescope at Green Bank.

Reflected radio images such as these are useful because radio waves are not only transparent to the atmospheres of Earth and Venus, but they are also largely transparent to fine dust. For Venus this means we can get a high resolution image of the surface obscured by thick atmosphere. For the Moon this means we can get a map of any surface features that may be obscured by dust on the Moon’s surface.

What’s amazing about these images is how high their resolution is. While they look like regular telescope images, they are actually radio images. The shadows you see are radio shadows from the transmitted beam, similar to the way a rough surface can cast shadows from the beam of a flashlight. The resolution of these images are a good demonstration of just how sophisticated radio astronomy can be.

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When Venus is in the western sky after sunset, it is known as the evening star. It is a time when Venus is most prominently seen. For most of 2015 Venus will be quite visible in the evening as it approaches greatest elongation in May.

Because Venus and Mercury orbit closer to the Sun than Earth, we never see them far from the Sun in the sky. They are either in the evening sky just after sunset, or in the morning sky just before sunrise. The angular distance of a planet from the Sun is known as elongation, and the largest angle a planet reaches before moving closer to the Sun (in the sky) is known as greatest elongation. The greatest elongation of Venus is pretty consistent, only varying between 45 and 47 degrees. Because of its more eccentric orbit, Mercury’s greatest elongation can vary between 18 and 28 degrees.

With a much smaller elongation, Mercury can be particularly difficult to observe. Even when Mercury is at greatest elongation it can be close to the horizon. That’s because it also depends upon the time of year it occurs. The planets tend to orbit the Sun in a similar plane, known as the ecliptic. The seasons occur because of the axial tilt of the Earth relative to the ecliptic. Near the winter or summer solstice, the ecliptic has about a 20 degree tilt relative to the horizon. Near the fall or spring equinox, the ecliptic is nearly horizontal with the horizon. So the best time to view Mercury is when it is at greatest elongation near one of the solstices.

Over the next few days, Mercury has fairly favorable viewing conditions. It’s elongation is relatively good, and it’s still close to the winter solstice. More importantly, it is near Venus in the sky as well. Venus is much easier to find, being brighter, so if you find what appears to be the brightest star in the sky just after sunset, then you’ve found Venus. If you look a bit to the right of Venus over the next few days, you’ll find what appears to be a dimmer star, and that will be Mercury. You can see the exact positions day by day in the video.

If you happen to have a clear evening, definitely track down Venus and Mercury. Venus is always a wonderful sight, and if you find Mercury, you’ll have found a planet most people haven’t seen in the sky.

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Yesterday I mentioned that after discovering the moons of Jupiter, Galileo went on to observe the phases of Venus, which further reinforced the idea that the Earth moved about the Sun. So just how do phases of a planet prove it revolves around the Sun?

The phase of any planet or moon depends upon its position relative to the Sun and your position relative to it. This is because planets and moons don’t produce light themselves (at least not much in the visible spectrum), so only the side that is facing the Sun is illuminated. Basically that means that about half of any planet/moon is illuminated at any given time. But this doesn’t mean you always see half of it lit up.

A good example of this is seen in the phases of the Moon. The Moon orbits the Earth, and as it does we see different amounts of the Moon illuminated. A full moon occurs when the moon is positioned in the opposite direction of the Sun. With the Earth between the Sun and Moon, we see the fully lit moon, hence it appears full. Because of their orbits, the Sun, Earth and Moon aren’t in a perfectly straight line during most full moons, which is why lunar eclipses don’t happen every month.

When the Moon is aligned perpendicular to Earth and Sun, we see half the moon illuminated. These are called quarter moons because they occur at the first or third quarter of the lunar cycle. Quarter moons can be seen either in afternoon and early night (first quarter) or late night and early morning (third quarter).

When the Moon is between the Sun and Earth, then the illuminated side is away from us, so we don’t see it. It is known as the new moon. Again, since the Sun, Earth and Moon aren’t usually perfectly aligned, so solar eclipses are rare.

With Venus things are a bit different. You can see Galileo’s observations in the figure here. The phases of Venus are compared to the apparent sizes of Saturn, Jupiter and Mars. You’ll notice two interesting things about Venus. First, it has phases ranging from crescent to nearly full. Second, it changes dramatically in size, being large as a crescent and small when nearly full. Together these observations demonstrate that Venus orbits the Sun.

It was long known that Venus was never seen far from the Sun. It would appear in the morning sky before sunrise, or in the evening sky after sunset, but you’d never see Venus in the middle of the night. In the Earth-centered model it was thought that motion Venus was on an epicycle of a celestial sphere rotating about Earth in sync with the daily motion of the Sun. Since the epicycle of Venus was always aligned with the Sun, Venus never wandered far from the Sun. But this also meant that Venus would always be closer to the Earth than the Sun.

If that were the case, then Venus could never appear in a nearly full phase. If Venus is always between the Earth and Sun, then at most we would would only see half of it illuminated. When the Moon is in full phase it is farther from the Sun than Earth. So the fact that Galileo observed a nearly full phase of Venus meant it must have been further from the Earth than the Sun is from Earth. In other words, it must almost be behind the Sun.

When Venus is in its crescent phase it must be nearly in front of the Sun, since most of the illuminated side is away from us. This is where the change in size becomes important. Since Venus can be seen both as a crescent and as nearly full, it must sometimes be in front of the Sun and sometimes behind.

Now if you still support the Earth-centered model, you could argue that the Sun and Venus both lie on an epicycle, thus rotating about each other, but the epicycle still rotates about the Earth. But if that were the case, then either both the Sun and Venus would appear larger and smaller as they move closer and farther from the Earth. But only Venus changes its apparent size, and it does so exactly as you would expect for a planet orbiting the Sun.

So it must be that Venus orbits the Sun, not the Earth. The discovery of moons orbiting Jupiter just reinforces the fact that the Earth-centered model is wrong.

You could say the geocentric model was just a phase.

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Venus and Earth are quite similar in many ways. The diameter of Venus is about 95% of Earth’s, its mass is about 80% of Earth’s, it has a similar geological make up, and surface gravity. Where they differ greatly is in their surface temperature. Venus has a surface temperature of over 800 degrees Fahrenheit, while Earth’s average temperature is around 57 degrees (460 C vs 14 C for you science types). They also differ in the amount of water they have. Venus has almost no water, while Earth is a watery world.

Given their temperature, you might not think that’s surprising. After all, when your surface temperature is hot enough to melt lead you wouldn’t expect water to hang around. But since Earth and Venus are geologically similar, we would expect them to form in similar ways. We know from its geology that Earth was watery from early on, and we know from the levels of hydrogen and deuterium in Venus’ atmosphere that it too had a wet past. But somehow Venus and Earth diverged.

It has generally been thought that Venus started as a wet world. Thus, Earth and Venus were both wet during their first hundred million years or so, then diverged later. Earth is not only farther from the Sun, it also has a strong magnetic field which Venus lacks. Being more cool and magnetically protected, Earth was able to retain its water, while Venus gradually lost its water due to a higher temperature and solar wind.

But this month in Nature a different model was proposed. In this model it is only the distance from the Sun that matters, not the magnetic field or atmospheric temperature. If this model is right it would have major consequences for habitable exoplanets.

According to the model, during the formation of a planet, when its surface is still molten, water vapor from the magma ocean is released into the atmosphere, which saturates the atmosphere. Since water vapor is a greenhouse gas, it holds in much of the heat from the planet. You can calculate the effect of the water vapor, and it turns out that a saturated atmosphere would only allow about 300 watts of heat per square meter to be released from the planet. In other words, there is a fixed rate at which the planet could cool, depending on its size.

This assumes that the planet isn’t gaining any heat from the Sun. However if a planet is closer than a particular distance, the heat from the Sun would heat the planet by more than 300 watts per square meter. This means that as long as the planet’s atmosphere is saturated with water vapor, there is no way for the planet to cool, since it would be taking in as much or more heat than it could possibly lose. As a result, the planet can’t cool until the water in its atmosphere is baked off.

So there is a critical distance from a star which can be easily calculated. If a planet is closer than this distance, its water will largely bake off before it cools, and if it is farther than this distance the planet cools before losing its water. In our solar system Earth cooled quickly, forming a crust and then liquid oceans, while Venus remained hot until it lost its water, then cooled to form a dry world.

It is an interesting model, but at the moment we don’t know enough about Venus’ geological history to determine if Venus lost its water early on as this model proposes, or from some later process. If this model is true, then Earth-mass exoplanets that lie beyond that critical distance are likely to be wet planets. And if they exist in the star’s habitable zone they would likely have liquid oceans similar to ours.

If this model is right, there may be quite a few water worlds in our galaxy.

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Yesterday I wrote about how astronomers can use the principle of parallax to measure the distance to stars. However this principle was first applied to the moon and the planets. More than 2,000 years ago Hipparchus used the parallax of the moon to measure its distance. He found that the distance to the moon was about 60 times the radius of the Earth, or about 237,000 miles. (The actual average distance is 239,000 miles.)   Earlier, Aristarchus measured the angle between the Moon and the Sun when the Moon was in its quarter phase (when half of it is illuminated from the Sun), and used trigonometry to determine that the Sun was about 20 times further away than the Moon, or about 4,740,000 miles. This distance is much lower than its actual value, but was the accepted value until at least the 1400s.

Despite their success with the Moon and the Sun, the distances to the planets were essentially unknown. It was known, for example that Mercury and Venus must be closer than the Sun, since they occasionally passed in front of (or transitted) the Sun. The other known planets, (Mars, Jupiter and Saturn) never transited the Sun, and so they must be farther away. But at the time astronomers didn’t have the necessary tools to measure planetary distances accurately. This was further complicated by the fact that until the 1500s it was generally thought that the Sun, Moon and planets moved around the Earth, which was fixed at the center of the universe. It was not until Copernicus published his heliocentric (sun-centered) view of the universe that astronomers could finally determine the size of our solar system.

By the early 1600s, Johannes Kepler was able to demonstrate how the distances of the planets were directly related to the distance from the Earth to the Sun. Using precise measurements made by Tycho Brahe he devised three laws of planetary motion, which are now known as Kepler’s Laws. One of these laws stated that the period of a planet’s motion (how long it takes to orbit the Sun) is related to the semi-major axis of its orbit (essentially its average distance from the Sun). By determining a planet’s orbital period in Earth years, astronomers could easily calculate distance from the Sun as it relates the distance from the Earth to the Sun. This meant astronomers could define the distance from the Earth to the Sun as one astronomical unit (1 au). Thus, Venus was 0.72 au from the Sun, Jupiter was 5.2 au, Saturn was 9.5 au, and so on.

But this only gave the proportional distance. To find the actual distances of the planets, astronomers needed an accurate measurement of 1 au. In Kepler’s time no one had such a measurement. Kepler thought the distance was at least three times greater than the distance measured by Aristarchus, but wasn’t able to make an accurate measurement. Then in 1769, astronomers finally had the opportunity to undertake an accurate measurement. In June of that year Venus would transit the Sun, and astronomers now had clocks and telescopes of sufficient accuracy.

Several teams of astronomers were sent across the globe, including one to Tahiti by James Cook. The astronomer William Hirst wrote of his measurements of the transit in a letter to the Royal Society. Hisrt’s observations were like those of many other astronomers, in that he measured the times at which the transit of Venus began and ended as seen from his location. By comparing his measurements with those of other astronomers, it was possible to determine the parallax of Venus, and therefore determine its distance.

The exact timing of the transit was hampered by the fact that Venus would appear somewhat distorted when it was near the edge of the sun, which is known as the black drop effect.  You can see this in Hirst’s sketches seen below.  Still, by 1771 several different measurements were collected and analyzed. The result was that the distance Sun was 24,000 Earth radii, or about 95 million miles. This is slightly larger than the actual value of 93 million miles, but is significantly more accurate that of Aristarchus. Astronomers now had a reasonable measurement of the solar system.

Sketches of the transit of Venus. Credit: W. Hirst

Planetary distances can now be measured with radio telemetry to an accuracy of 3 parts in a billion. It is accurate enough to observe it changing over time. The Sun gradually loses mass as it radiates energy, and so the orbits of the planets (including Earth) are slowly getting larger. Thus the astronomical unit is increasing by several meters per century.  In 2012, the International Astronomical Union defined the astronomical unit as a fixed standard, which means the Earth’s distance is slowly getting larger than an astronomical unit.

But thanks to a rare alignment of planets 250 years ago, we were able to learn for the first time just how large our solar system really is.

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The image below shows the orbital path of Venus.  If it looks a little odd, that’s because it doesn’t show the path of Venus relative to the Sun, but rather relative to Earth.  It is taken from a 1799 book by James Fergeson entitled “Astronomy Explained Upon Sir Isaac Newton’s Principles”.  This was an astronomer’s handbook, so it gave the motion of Venus and other planets as we see them on Earth.

Motion of Venus as seen from Earth. Credit: James Fergeson

You might notice that the path of Venus seems to follow a kind of five-fold spirograph path.  This is because Venus orbits the sun once every 224.7 days, while the Earth 365.25 days.  The ratio of these two periods is 0.615, or about 8 to 13.  Since the difference between 8 and 13 is 5, the path of Venus has a five-fold symmetry relative to Earth.  This pattern is sometimes called the Rose of Venus.

Ratios such as the 8 to 13 ratio between Earth and Venus are relatively common.    They happen through a process of orbital resonance, where the small gravitational interactions between planets reinforce their stable orbits.  For planets, these ratios are only approximate, but they are stronger for smaller bodies.  For example, three of the moons of Jupiter (Io, Europa and Ganymede) have orbital ratios of 1 to 2 to 4.  In asteroids there is a related effect where certain resonances between Jupiter and the asteroids are orbitally unstable.  This means any asteroid that happens to have such a resonance is pushed a bit either closer or further from the Sun.  The result is a series of gaps in the asteroid belt known as Kirkwood gaps.

When you look at patterns such as this, you can see how difficult it was to accurately describe the motions of the planets.  Imagine starting with a pattern like this and proving the planets are actually moving around the Sun.  Often what seems obvious now is hard to figure out at first.

For those who like April Fools, I actually lied about the title of Fergeson’s book.  The title was actually “Astronomy explained upon Sir Isaac Newton’s Principles, and made easy to those who have not studied Mathematics. To which are added, a Plain Method of finding the Distances of all the Planets from the Sun, by the Transit of Venus over the Sun’s Disc, in the Year 1761. An Account of Mr. Horrox’s Observation of the Transit of Venus in the Year 1639: and, of the Distances of all the Planets from the Sun, as deduced from Observations of the Transit in the Year 1761”

…tenth edition.

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Venus is the second planet from the Sun, and the most like Earth in terms of its mass (80% Earth’s) and size (95% Earth’s).  In almost every other aspect it is radically different.  It has a thick, carbon dioxide atmosphere, little water, a weak magnetic field, and a surface temperature of 740 K (860 F, 460 C). Despite this radical difference, early Venus was a wet world much like early Earth. We know from the levels of hydrogen and deuterium in Venus’ atmosphere that it too had a wet past. But somehow Venus and Earth diverged.

Surface of Venus taken by Venera 13.
Credit: Soviet Space Agency.

Venus was a watery world for its first hundred million years or so.  Then, because of its proximity to the Sun, warmer temperatures caused more water vapor to enter the atmosphere.  Because Venus’ weak magnetic field couldn’t protect the atmosphere from the solar wind, the water vapor was gradually  photodissociated into hydrogen and oxygen.  The hydrogen escaped the atmosphere, leaving the oxygen to bond with carbon to form carbon dioxide.  This led to a runaway greenhouse effect, where rising carbon dioxide trapped more heat, causing more water vapor to enter the upper atmosphere, in turn leading to more carbon dioxide.

Perhaps the most unusual aspect of Venus is the fact that it rotates backwards on its axis compared to the other planets (what we call retrograde rotation).  On Venus you would see the Sun rise in the west and set in the East.  It’s rotation is also very slow, taking about 243 Earth days to make a complete turn. However because of its retrograde rotation a day on Venus is only about 117 Earth days long.  That means a year on Venus is a bit less than two days long.

We’re not exactly sure why Venus rotates in a retrograde fashion, but one idea is that the planet was impacted by two large bodies about 10,000 years apart.  Such a double impact could have given the planet its backward rotation while ensuring that any moons either escaped the planet or spiraled into it.

The runaway greenhouse effect of early Venus is sometimes held up as a cautionary tale for the global warming of our own planet.  Even Carl Sagan made such a comparison in his Cosmos series.  But it is important to recognize that early Venus and modern Earth are very different.  Global warming is a very real effect on Earth, and the evidence for anthropogenic greenhouse gases as a driving force of climate change is well documented.  But there’s no indication of such a runaway effect occurring here on Earth.

Up next: Earth

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