The Beauty and Mystery of the Planet Venus

Venus is the second planet from the Sun and is the sixth largest. It is the brightest object in the night sky except for the Moon. Venus orbits the Sun once every 224.7 Earth days and gets as close to the Sun as 107.476 million kilometers and as far away from the Sun as 108.942 million kilometers. This makes the orbit of Venus less elliptical and more circular than any other planets. The temperature on the surface of Venus can reach as high as 740 degrees Kelvin. This is due to a phenomenon called the greenhouse effect whereby carbon dioxide in the atmosphere of Venus traps the Suns heat inside. This makes Venus the hottest planet in the Solar System. Venus is even hotter than Mercury despite being farther away from the Sun.

Venus is 12,100 kilometers in diameter and has a mass of 4.869e+24 kilograms. This makes Venus similar to the Earth and has often been called Earths sister planet. But the similarities end there. One major difference between the Earth and Venus is that Venus rotates on its axis from east to west, which means if you lived on Venus you would see the Sun rise in the west and set in the east. The atmosphere on Venus is mostly carbon dioxide choked with sulfuric acid and has a pressure at the surface more than 92 times the pressure at sea level on Earth. Unlike the Earth, Venus does not have a magnetic field generated by its iron core. This may be the result of how slowly Venus rotates on its axis. The only magnetic field Venus has is very weak and is produced by the interaction of the solar wind and the ionosphere of Venus.

The surface of Venus is difficult to see through the thick, dense clouds and the first crude images of the surface were obtained using ground based radar. More detailed images were obtained by the Magellan spacecraft which was launched to Venus on May 4, 1989 and spent four and a half years radar mapping 98 percent of the surface of Venus. Later, the European Space Agency launched the Venus Express on November 9, 2005 and on April 11, 2006 it slipped into a polar orbit around Venus. These probes have now provided us with an accurate map of Venus.

Most of the surface of Venus is relatively flat plains created by giant pools of lava. Venus has thousands of small volcanoes and hundreds of large volcanoes many of which are over 100 kilometers in diameter. There are fairly large craters scattered at random all over the surface of Venus. These craters are more than 2 kilometers wide and smaller craters do not exist because smaller meteors burn up in the thick atmosphere of Venus. The map of Venus is dominated by two large highland areas, the Ishtar Terra, where the Maxwell Montes, the highest mountain can be found and the Aphrodite Terra highlands.

More missions to Venus are planned for the future and NASAs MESSENGER spacecraft just completed two flybys of Venus in October 2006 and June 2007 while on its way to Mercury. A spacecraft called BepiColombo, which was launched by the European Space Agency, will also perform two flybys of Venus on its way to Mercury. Japan is planning to launch the Planet-C Venus climate orbiter in 2010 and NASA has proposed a spacecraft called VISE the Venus In-Situ Explorer which will actually land on Venus. Once on the surface the Venus In-Situ Explorer will take a core sample and examine it. These mission to Venus will tell us more about the chemical composition and climate on Earths sister planet.

The Star We Call The Sun

The Sun is called Sol in Latin and that is where the term Solar System comes from. It is a typical main-sequence star and is by far the largest and most massive object in our Solar System. The Sun contains 99.8 percent of all the matter in the Solar System with the planet Jupiter taking up most of the rest. The Sun is a population one, GV2 class star and is sometimes referred to as a typical star and that is true in many respects. However the Sun is actually larger than most of the other stars in the same class as the Sun.

The Sun is composed of 74 percent hydrogen, 25 percent helium and traces of other elements. The temperature at the Suns core which is considered the inner 20 percent, is approximately 15.6 million degrees Kelvin, the pressure is 250 billion atmospheres and the mass density is more than 150 times that of water. Under such extreme conditions nuclear fusion takes place where by hydrogen atoms are combined to from helium atoms. This reaction releases massive amounts of energy in the form of gamma rays and is responsible for the Suns 386 billion billion megawatt power output. During the course of their journey out to the surface these gamma rays are repeatedly absorbed and re-emitted at lower and lower temperatures. By the time the energy reaches the surface is has been reduced to mostly visible light and is carried through the last part of the way more by convection than radiation.

The convection zone is the Suns outer layer down to about 70 percent of the Suns radius. It is an area where thermal convection takes place in the form of great thermal columns. These thermal columns are heated by nuclear fusion taking place in the Suns core and rise up to the Suns surface where they release their energy out into space in the form of sunlight and particles. As the thermal columns discharge their energy they cool and sink back down in the Suns interior where they are reheated and back up to surface again in a great cycle. The tops of these great thermal columns can be seen on the surface of the Sun in the form of what is called solar granulation and supergranulation.

The surface layer of the Sun that we can see is called the photosphere and has a temperature of about 5800 degrees Kelvin. Above the photosphere are five layers that compose the Suns atmosphere. They are the temperature minimum, the chromosphere, the transition region, the corona and the heliosphere. The temperature minimum region extends from the photosphere up to 2000 kilometers and has a temperature of about 4000 degrees Kelvin. This is cool enough for molecules such as water and carbon monoxide to exist and they can be detected by their absorption spectra.

The chromosphere extends from the top of the temperature minimum region up another 2000 kilometers and is named for the Greek word chromo which means color. The chromosphere can be seen as a flash of color right at the start and the end of a total solar eclipse of the Sun. Strangely enough, the temperature of the chromosphere gradually increases with altitude up to about 100,000 degrees Kelvin at the top.

Above the chromosphere is a transition region where the temperature rises rapidly to about one million degrees Kelvin. This temperature increase is caused by what is known as a phase transition of the element helium present in the transition region. The transition region does not have a well defined altitude and is in constant motion. The transition region is not easily seen from Earth but can be observed by space based instruments operating in the far ultraviolet region of the spectrum.

After the chromosphere is the corona which is much larger than the previous layers of the Suns atmosphere and extends far out into space. The corona is characterized by solar prominences which are immense clouds of super heated glowing gas that has erupted from the upper chromosphere. The corona can be clearly seen during total eclipses of the Sun and is very spectacular to see. The corona is composed of charged particles that become what we call the solar wind as they radiate outward from the Sun at 450 kilometers per second and are responsible for the aurora borealis.

Beyond the corona is the heliosphere which is also know as the magnetosphere. The heliosphere is immensely strong and extends far beyond the orbit of the dwarf planet Pluto. The solar wind travels outward along the heliosphere until it collides with the helipads about 50 astronomical units from the Sun.

When observed with the proper filters we can see sunspots on the surface of the Sun. These spots have a lower temperature than the surrounding area and therefore appear dark. Sunspots are areas of intense magnetic power where thermal convection from the interior of the Sun has been inhibited. Sunspots usually form pairs with opposite magnetic polarity and are responsible for solar flares. The number of sunspots varies over the course of an eleven year solar cycle.

The Sun has been active for about four and a half billion years and has used up about half of the hydrogen fuel it started with. The Sun will continue to burn for about another five billion years after which it will start to force helium to under go nuclear fusion into heavier elements. This will cause the Sun to swell up in size to the point of consuming the Earth and more as it becomes what is called a red giant. A billion years after becoming a red giant our Sun will finally collapse into a white dwarf. Incredibly, it could then take as much as one trillion years to cool off.

The Origins of The Solar System

We live in a universe of almost unimaginable size. To give you an idea of the scale, we can only use optical and radio methods to observe objects out to about 13,000 million light years away and nobody knows what lies beyond that. Closer to us we can see clusters of galaxies at distances out to about 750 million light years. The other galaxies of what is know as the Local Group of galaxies are all within 2.5 million light years of us. These galaxies are composed of millions and billions of stars.

Our own galaxy, the Milky Way, is a typical spiral galaxy. This means is looks like a great swirling whirlpool with spiral arms extending out from the center. The Milky Way galaxy is 100,000 light years in diameter and our own star, the Sun, lies on the edge of one of the spiral arms. The Sun is about 30,000 light years from the center of the Milky Way galaxy and takes 225 million years to complete one orbit around the galactic center.

According to modern scientific theory the star we call the Sun was formed about 4,500 million years ago. The Solar System originated when gravitational forces caused what is called the solar nebula to collapse and coalesce into a spinning disc with the central mass forming the Sun. The planets formed from large clouds of gas and dust which gradually built up over million of years as they orbited the central mass of the disc.

The central mass continued to build up in size and as it did the gravitational field it exerted grew stronger. As the gravitational field grew stronger the central mass pulled in more gas and dust and grew even larger.

Eventually, the gravitational force became so powerful that the temperature and pressure at the center of the mass rose high enough to cause hydrogen atoms to undergo fusion into helium atoms. This fusion reaction releases enormous amounts of energy and is the source of the Suns power.

Now illuminated by the Suns rays, the planets of our solar system continued to form and transform. All of the planets have undergone a great deal of change since they first accreted. These changes were brought about in a variety of ways, including violent collisions which resulted in craters that can still be seen today. The planets also went through changes caused by volcanism, melting, structural deformation and the release of gases from deep within.

The planets of our Solar System are generally divided into two groups. The inner planets, which include Mercury, Venus, Earth and Mars and the outer planets, Jupiter, Saturn, Uranus, Neptune and until recently the Dwarf Planet Pluto. The inner planets are all characterized by having solid surfaces and are relatively small compared to the outer planets. The outer planets are all characterized by not having a solid surface and are often referred to as gas giants. The inner and outer planets are also separated by a band of asteroids orbiting between Mars and Jupiter.

Comets, meteors and asteroids can also be found orbiting the sun in large numbers from the outer reaches of the Solar System to very close to the Sun. Comets are balls of frozen liquids and dust which often follow eccentric orbits for outside the orbit of Dwarf Planet Pluto and only display their characteristic tails when their orbit brings them closer to the Sun. Asteroids and meteors are both made mainly of rock. The difference between asteroids and meteors is their size, meteors can be as small a grain of dust, asteroids can be miles in diameter and are sometimes called planetoids.

At the outer fringes of the Solar System beyond the orbit of Neptune is the Kuiper belt where at least three dwarf planets orbit. They are Ceres, Pluto and Eris and they are accompanied by millions of smaller objects about which little is known. At this time a variety of spacecraft are either on their way to the far reaches of the Solar System or are planned for the near future. It will be fascinating to see what wonders these space probes will tell us about the Solar System we call home.

Orbital Mechanics And Your Daily Life

January 3rd was an interesting date in orbital mechanics; it represented the day when the Earth was closest to the sun (perihelion) and the moon was farthest from the Earth (apogee). The first only happens once per year; the second happens every lunar orbit - every 29 days. These two effects have a lot of star watching history.

The Earth orbits the sun, with one orbit being one year. While it orbits the sun, it spins on its axis, and each spin on the axis makes for a day. While the Earth rotates around the sun at a bit over 30 kilometers per second (108,000 km per hour!), and is spinning like a top (at around 0.45 km/second) you or I don't feel this motion - though it is noticeable because the combination of these two motions sets the yearly procession of the constellations and the day-night cycle...and is why some telescopes have clock drives to keep them pointed at a target once it's been set.

What do perihelion and lunar apogee mean to you? Well, when it comes to observation, the difference is a function of the eccentricity of the orbits. Eccentricity, if it's been a while since your algebra class, is the ratio that defines the distance between two focal points of an ellipse. For an orbit, eccentricity will always be between 0 and 1, anything of 1 or more isn't an orbit - it's a object flying through the solar system on a hyperbolic path.

Earth's orbit has an eccentricity of 0.0167, which means that the difference between closest approach to the sun and farthest approach to the sun is 0.0167% of the mean distance from the Earth to the Sun. The Moon's orbit is somewhat more eccentric, at 0.05.

From an observational standpoint, the difference in visual size between perihelion and aphelion (the farthest distance from the Earth to the Sun) is minimal; the average difference between the Earth and the Sun is 500 light seconds, or about 150,000,000 km. 0.0167% of that is about 2.5 million km of difference below the average, and aphelion is about 2.5 million km farther away. Because the sun is so far away, that variation in distance, as far as it seems to us, makes almost no difference in the visual size of the sun. For the moon, the difference translates into something that would be a noticeable difference in angular size - if the events didn't happen 15 days apart in a different phase of the moon. If you can catch a photograph of the full moon at perigee and apogee on different occasions, you'll notice that the moon appears almost 10% larger - about 4.1 arc seconds larger at perigee than apogee.

The secondary effect of the lunar apogee/perigee combination is a difference in lunar tides, as tidal attraction works on the cube of the difference in forces. This is the variation in orbital mechanics that has the greatest impact on human society.

From a standpoint of living on Earth, one of the oddities of Earth's perihelion is when it occurs. It occurs near the height of the summer for us (January 3rd), which means it's in the depth of the winter for the Northern hemisphere. There is a very small difference in total solar radiation (called instellation) of about 4% between January and July - though it does mean that seasons at aphelion are about 4 days longer due to Keplerian mechanics.

That may not sound like much, but it's significant due to the time of the year those differences come in compared to the seasons, and how this changes over time. Right now, the current cycle of perihelion and aphelion moderate Northern hemisphere summers and winters; over a 100,000 year cycle, the seasons that perihelion and aphelion occur in shift; these shifts are called Milankovic cycles, and the last time aphelion occurred during the Northern hemisphere's winter, we were in an ice age.. Some of the hottest climates in geological records appear to have occurred when the Earth's orbit was a bit more eccentric (variations in orbital eccentricity are also called Milankovic cycles), and perihelion and aphelion happened closer to the equinoxes.

This isn't to say that there aren't other contributing causes to variation in the Earth's climate, nor should we discount the effect of CO2 in the atmosphere. It is pointing out that there are LOTS of contributing causes, and some of them (the Earth's orbit) are well beyond our control.

So is the earth hotter in the southern hemisphere or the winters milder in the northern hemisphere due to Perihelion falling in January ? The answer is yes, but no more so than in your grandparents day or their great great great ... grandparents!