Category The Earth, Earth Science, Planet Earth

          Early astronomers claimed that Venus was Earth’s sister planet. They believed that the light and dark areas they saw on the planet through their telescopes were oceans and continents. Modern astronomy has proved that nothing could be further from the truth! The light and dark areas are Venus’ suffocating atmosphere — a layer of clouds containing sulphuric acid released by volcanic eruptions. The temperature on Venus can rise to 464°C (867°F), and the heavy layers of cloud make the air pressure on the surface over 100 times that of Earth.

          Venus is the second planet from the Sun and our closest planetary neighbor. Similar in structure and size to Earth, Venus spins slowly in the opposite direction from most planets. Its thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead. Glimpses below the clouds reveal volcanoes and deformed mountains.

          Had Venus drawn slightly luckier tickets in the cosmic lottery, our solar system could host two habitable planets today, according to recent simulations from a group of NASA researchers. Instead, our neighbor is a desolate place—and might give us a terrifying glimpse of our own future.

          Planetary scientists have traditionally viewed Venus’s hellish temperatures, carbon dioxide-saturated atmosphere, and congealed crust as the inevitable outcome of its place in the solar system. Sitting too close to the sun, the hapless planet was doomed from birth to be burnt to a crisp. In recent years, however, an alternative possibility has thrown some shade at this simple story. Given the right starting conditions, cloud cover could have protected Venus from the barrage of sunlight and kept it balmy and wet for billions of years, according to simulations presented this week at a planetary science conference in Switzerland. In this scenario, Venus may have actually been the solar system’s first habitable planet… until some unknown catastrophe smothered it in carbon dioxide. While our carbon emissions probably couldn’t completely fry the Earth in quite the same way, the transformation of Venus may still hold an important moral for humanity.

          “If there was life on Venus, they only had one home,” says Colin Goldblatt, a planetary scientist at the University of Victoria in Canada, “and that home isn’t very good anymore.”

          Craters are the most widespread landforms in the solar system. Craters are found on all of the terrestrial planets—Mercury, Venus, Earth and Mars. The surfaces of asteroids and the rocky, ice covered moons of the outer gas planets are cratered as well. The craters left by impacting objects can reveal information about the age of a planet’s surface and the nature and composition of the planet’s surface at the time the crater was formed.

MERCURY AND THE MOON

          Impact craters dominate the surfaces of Mercury and the Earth’s Moon. Both bodies lack liquid water on their surfaces that would erode impact craters over time. They also lack an atmosphere which, on planets like the Earth and Venus, could disintegrate meteoroids before they impact the surface. However, old craters can be eroded by new impact events. Mercury and the Moon have very old surfaces. One of the youngest large craters on the Moon is Tycho, which was formed about 109 million years ago.

EARTH

          Liquid water, wind and other erosional forces erase impact craters on the Earth. There are still many craters on Earth which are visible from space. Some craters in areas of low rainfall (i.e. where little erosion occurs) are relatively intact, such as this crater – Meteor Crater in Arizona, U.S.A.

MARS

          Mars has experienced significant bombardment. The southern hemisphere is more heavily cratered than the northern hemisphere. Winds are the main erosional force on Mars and windblown dust and soil erode craters over time. The structure of some Martian impact craters, such as the one pictured here at left, provide evidence that suggests the presence of water or ice in the surface at the time the impact occurred.

ASTEROIDS

          Asteroids are rocky and usually heavily cratered due to a long history of impacts with other asteroids and possibly comets. Old impact craters on asteroids have beem deformed and erased by newer impact craters. Alternatively, impact events can disintegrate asteroids into smaller pieces. This asteroid, Mathilde, is interesting because of the large size of the impact craters on its surface. Despite the obvious intensity of the impacts, the asteroid was not destroyed. Scientists believe the asteroid must be  uncommonly dense to have withstood such bombardment.

PLANETARY SATELLITES

          The outer gas planets do not have solid surfaces, but their moons do. Most of these moons are rocky, icy worlds with a variety of surface features and compositions. Most of them are cratered, such as Europa, one of the Galilean satellites of Jupiter. Europa’s surface is thought to consist of a thick layer of ice overlaying a liquid water ocean.

IMPACT WITH JUPITER

          Terrestrial planets aren’t the only ones that are hit by meteors, comets and asteroids. The planets known as gas giants, such as Jupiter, don’t have a solid surface to keep a record of impacts. However, the impact of comet Shoemaker-Levy in 1993 left visible holes in the cloud tops of Jupiter. The effects of these holes began to fade after only a few months, but it was the first time humans observed a major collision between two objects in our solar system.

          Mercury is the innermost planet in the solar system. Since it is the closest to the Sun, Mercury is the most difficult planet to see because it is always seen quite near to the Sun in the sky and the Sun’s glare or the bright sky usually overwhelms the planet’s light.

          The only chance to see it is as a faint “star” in the morning or evening sky near the horizon, shortly before sunrise in the dawn or just after sunset in the dusk. So it has always been almost impossible to get any information about the surface of the planet by means of ground-based observations. The first, detailed images were obtained with the NASA Mariner 10 spacecraft which also procured most of our present information about Mercury’s surface.

          Like our Moon, Mercury is small and its surface is scarred by craters that were formed by impacting rocks and asteroids, soon after the birth of the solar system. They smashed into the planet and blasted the material away from the surface. Mercury also has real cliffs, or scarps which formed when the young cooling planet shrunk like an old apple, with wrinkles on its surface.

          Mercury has the largest day-to-night temperature variation of all planets. The days are burning hot (about 400 °C) and the nights are freezing cold (about -200 °C). This is because it only has a very thin atmosphere.

         Mercury is one of the most heavily scarred objects in the Solar System. Thousands of meteor craters cover the planet, including the largest — the Calories Basin. This was formed when a piece of rock 100km (60 miles) wide collided with Mercury 3.6 billion years ago. Mercury is also shaped by wrinkles and cracks that formed when the surface of the planet cooled and shrank.

          The orbit of Mercury is the most eccentric of the planets in our Solar System. The planet has an orbital period of 87.969 Earth days. At perihelion it is 46,001,200 km from the Sun and at aphelion it is 69,816,900 km, a difference of 23,815,700 km giving it an eccentricity of 0.21. Mercury’s orbit is inclined by 7 degrees to Earth’s ecliptic. Mercury can only be seen crossing the face of the Sun when the planet is crossing the plane of the ecliptic and is between the sun and Earth. This happens about once every seven years.

Source: Orbit of Mercury – Universe Today

          A more precise value of the eccentricity of Mercury’s orbit is 0.205 630. By comparison, the eccentricity of Earth’s orbit is 0.0167086, and the eccentricity of the orbit of Venus is 0.006772.

          Mercury is locked in a 3:2 spin-orbit resonance making three rotations about its spin axis every two orbits about the sun. Because of this, if you were on the surface of Mercury, the Sun would pass overhead once every two orbits around the Sun, or 176 Earth days. In other words, one day on Mercury (sunrise to sunrise) takes two Mercury years. A Mercury year takes 88 Earth days, the length of time to orbit the Sun.

Source: Mercury’s Orbit

          So one solar day on Mercury is about 176 Earth days, and one “Mercury day” (a sidereal day or the period of rotation of Mercury around itself) is equal to approximately 58.7 Earth days.

         And there is also the precession of the perihelion of Mercury. The closest distance of Mercury from the Sun doesn’t happen at the same place but moves slowly around the Sun. The other planets of the solar system have perihelion shifts, but classical mechanics did not give an accurate value of Mercury’s perihelion precession. The General theory of Relativity was able to show and predict that Mercury’s orbit shifts by about 43 seconds of arc per century.

          The planet Mercury is often cited as the most difficult of the five brightest naked-eye planets to see. Because it’s the planet closest to the Sun, it never strays too far from the Sun’s vicinity in our sky. It is often referred to as “the elusive planet.” And there’s even a rumor that Copernicus, never saw it, yet it’s not really hard to see. You simply must know when and where to look, and find a clear horizon. And for those living in the Northern Hemisphere, a great “window of opportunity” for viewing Mercury in the evening sky is about to open up.

          Mercury is called an “inferior planet” because its orbit is nearer to the Sun than the Earth’s. Therefore, it always appears from our vantage point to be in the same general direction as the Sun. In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the Sun and then the other.

          Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning. It is said that Pythagoras, about the fifth century B.C., pointed out that they were one and the same.

          Because of its proximity to the Sun, Mercury is a very difficult planet to explore. It is normally obscured by the Sun’s glare, which prevents even observatories such as the Hubble Space Telescope from peering at it because of the risk to light-sensitive equipment. Mariner 10 is the only probe to have visited Mercury, but it too could only photograph half the planet.

          Mercury’s diameter is 3,030 miles (4,878 km), comparable to the size of the continental United States. This makes it about two-fifths the size of Earth. It is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan.

          But it’s not going to stay that size; the tiny planet is shrinking. When NASA’s Mariner 10 spacecraft visited the planet in the 1970s, it identified unusual features known as scarps that suggest the world is shriveling. As the hot interior of the planet cools, the surface draws together. Since the planet boasts only a single rocky layer, rather than the myriad tectonic plates found on Earth, it pushes on itself to create scarps.

          A 2014 study of nearly 6,000 scarps taken by NASA’s MESSENGER spacecraft suggest that Mercury contracted radially as much as 4.4 miles (7 kilometers) since its birth 4.5 billion years ago. The discovery helped balance models of the planet’s interior evolution with observations at its surface.

          “These new results resolved a decades-old paradox between thermal history models and estimates of Mercury’s contraction,” Paul Byrne, a planetary geologist and MESSENGER visiting investigator at Carnegie’s Department of Terrestrial Magnetism, said in a statement. “Now the history of heat production and loss and global contraction are consistent.”

          The planet has a mean radius of 1,516 miles (2,440 km), and its circumference at the equator is 9,525 miles (15,329 km). Some planets, such as Earth, bulge slightly at the equator due to their rapid rotation. However, Mercury turns so slowly on its axis that astronomers once thought that the planet was tidally locked, with one side constantly facing the nearby sun. In fact, the planet spins on its axis once every 58.65 Earth days. Mercury orbits once every 87.97 Earth days, so it rotates only three times every two Mercury years. The slow spin keeps the planet’s radius at the poles and the equator equal.

          Although mercury is the second smallest planet in the Solar System, it is heavier than Mars, and almost as heavy as Earth. The reason for this is that Mercury has an enormous core of iron —almost 3600km (2237 miles) in diameter.