Astronomy

Comparing Venus to Earth



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Venus is sometimes known as Earth’s “sister planet,” because it is the closest planet to Earth and also the most similar in size. Until the second half of the 20th century there was much speculation about conditions on Venus, and many people even believed it would be a likely place to find extraterrestrial life. Starting in the 1960s, though, a number of space probes have examined the planet and even landed on its surface, and a lot more is now known about it. While there are indeed many similarities to Earth there are also some huge differences.

Size and Structure

Venus is by far the closest planet to Earth in size. Its diameter at the equator is 7,513 miles, 94.9% of Earth’s, and it has 81% of Earth’s mass. Its density is 5.243g/cm3, very close to our own planet’s 5.515 g/cm3. Although it’s impossible to be certain without monitoring seismic activity on Venus, this similarity in density suggests that Venus has a similar internal structure, with a crust, a mantle and a core. As Venus is almost exactly the same size and age as Earth it can be calculated that at least part of its core is still molten. However Venus only has a very weak magnetic field, suggesting that unlike Earth its core doesn’t have the convection currents needed to create a planetary dynamo effect. This is probably because the surface of Venus traps heat more effectively, slowing the rate of cooling in the core. It is likely that Venus doesn’t have a solid inner core like Earth’s, due to both slower cooling and its slightly lower mass reducing internal pressure and thus the melting point of the core.

Geology

The geology of Venus is very different from Earth’s. The major factors in shaping Earth’s surface are erosion and tectonic plate movement, which constantly creates new crust at divergent boundaries like the Mid-Atlantic Ridge and recycles ocean floor back into the mantle at subduction zones. On Venus, though, erosion seems much more limited, probably due to the weather, and there are no tectonic plates. The crust of Venus doesn’t move around like Earth’s does, probably because it’s usually too solid to subduct without water to decrease its viscosity. There is no water on Venus, for reasons that will be discussed shortly. The surface of Venus was mapped in detail by NASA’s Magellan probe between 1990 and 1994, and its features have been carefully studied. It’s clear that Venus is very volcanic; over 1600 major volcanoes are currently known and many thousands of smaller ones. 167 of them are over 60 miles wide, larger than the Hawaiian shield, the largest volcano on Earth. The highest volcano on Venus is over 36,000 feet high, 7,000 feet higher than Mount Everest. 80% of Venus’ surface is covered in smooth plains of volcanic lava. Interestingly, study of impact craters shows that most of the surface isn’t that old; it seems to be between 300 and 600 million years old. That seems like a long time, but most of Earth’s continental crust is nearly 4 billion years old. Scientists now believe that heat builds up in the mantle of Venus until the surface rock starts to soften, at which point runaway subduction occurs and most of the planet is resurfaced in about 100 million years. Most astrogeologists think that Venus used to have oceans, but as temperatures rose these evaporated; over time the water vapour will have been broken down into hydrogen and oxygen, and the hydrogen will have escaped into space.

Atmosphere

Venus has the densest atmosphere of any of the rocky planets. It’s over 92 times as dense as Earth’s; this means that standing on the surface of Venus you would be experiencing the same pressure as you would under 3,000 feet of water on Earth. That’s about three times as deep as most Navy submarines can dive without being crushed. Earth’s atmosphere is about 78% nitrogen and most of the rest – 21% – is oxygen. Almost everything that remains is argon, with traces of carbon dioxide, neon, helium and other gases. This is because Earth has plant life, which expires oxygen, and an active carbon cycle which locks up carbon in living things and sedimentary rock. On Venus, though, there is no life and no carbon cycle; the result is that the atmosphere is now 95% carbon dioxide and 3.5% nitrogen. Carbon dioxide is a greenhouse gas, meaning it traps heat inside the atmosphere instead of allowing it to escape into space. Many climatologists are worried that increasing CO2 in Earth’s atmosphere from 0.035% to 0.04% could cause global warming, so it’s no surprise that Venus is very warm indeed. The average surface temperature is 864°F (462°C,) hot enough to melt lead. This creates a self-perpetuating cycle; the high temperature bakes carbonate out of the rock, keeping CO2 levels high and maintaining the greenhouse effect. The atmosphere also seems to contain gases from volcanic eruptions, including sulphur.

Climate

Weather on Earth is very variable, with everything from clear sunny days to rain and snow. Winds are usually mild but the most powerful storms sometimes produce gusts of 150mph. On Venus though, winds are very slow at the surface – only a few miles per hour – but because of the dense atmosphere they have a lot of force and blow dust and stones across the surface. At higher altitudes 200mph winds blow constantly. There are no seasons on Venus because its axis is nearly vertical, whereas Earth’s has a 23° tilt, and temperature hardly varies between the equator and the poles. It does get cooler with increasing altitude, though; the Magellan probe saw a substance that looks like snow on some of the highest mountains. Scientists believe that this is lead sulphide, which was released from volcanoes and then condensed at higher altitudes and fell just like snow on Earth. Higher up the temperature falls even lower and clouds form. Unlike Earth’s clouds of water vapour these are made up of concentrated sulphuric acid, and form a thick layer that covers the entire planet. It is these bright white clouds that make Venus so bright in our sky; they are highly reflective and give Venus an albedo – reflectivity – of .75, compared to Earth’s .3. Condensed acid from the clouds falls as rain, but the temperatures are so high that it evaporates again 15 miles above the ground and is recycled back into the clouds. There appears to be lightning on Venus; this was suspected since the Soviet Venera probes detected lightning-like signals in the 1960s, and confirmed when the European Space Agency’s Venus Express detected “whistler mode” waves in 1994. On Earth lightning is generated in rainclouds, but on Venus it happens much too low in the atmosphere for this and is probably caused by static electricity buildup in clouds of dust.

Orbit

Being closer to the Sun, Venus has a shorter orbital period than Earth. Earth orbits the Sun in 365.25 days, while on Venus a year lasts only 224.7 Earth days. That’s not surprising, but the planet’s rotation certainly is. Firstly, every other planet in the solar system rotates anti-clockwise when viewed from above its north pole. Venus, though, rotates clockwise. It also rotates very, very slowly. In fact it rotates once every 243 Earth days, so a local day on Venus is actually longer than a local year. Because of the planet is rotating in a different direction to the others, though, the “solar day” is different from the “sidereal day” given by its rotation rate; to an observer on Venus the Sun would appear to rise in the west, rather than the east as on Earth, every 116.75 Earth days.

Even more strangely, the rotation of Venus has slowed down by 6.5 minutes per local sidereal day since the Magellan probe arrived there. It’s likely that this is because of a tidal effect caused by the heavy atmosphere, and that Venus may eventually become “tidally locked” to the Sun, with the same side of the planet always facing it, just as our moon is tidally locked to Earth. Venus itself has no moon, which could also help slow its rotation. Some scientists think that it once had one, but it was lost in an impact with a rogue planet that also reversed the direction of Venus’ rotation.

On the scale of the universe Earth and Venus really are very similar – small, rocky planets with an atmosphere, orbiting close to a yellow star – but from a human point of view they’re very different indeed. For scientists this is extremely valuable, though, as it gives an idea of what conditions on other worlds can be like.

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