My Science School

          The moon experiences temperatures both hotter and colder than those on Earth. When the Sun is directly over-head, the temperature on the Moon’s surface is higher than the boiling point of water — 100°C (212°F). However, at night, the Moon becomes very cold, with temperatures dropping to —173°C (-280°F). Earth and the Moon are approximately the same distance from the Sun, and therefore receive the same amount of heat. But the lack of an atmosphere on the Moon means that its temperature range is much more extreme. The Sun’s radiation is not filtered out by gases in the atmosphere, and there are no clouds to stop heat escaping at night.

          The moon rotates on its axis in about 27 days. Daytime on one side of the moon lasts about 13 and half days, followed by 13 and a half nights of darkness. When sunlight hits the moon's surface, the temperature can reach 260 degrees Fahrenheit (127 degrees Celsius). When the sun goes down, temperatures can dip to minus 280 f (minus 173 c). Temperatures change all across the moon, as both the near and far side experience sunlight every lunar year, or terrestrial month, due to lunar rotation.

          The moon tilts on its axis about 1.54 degrees — much less than Earth's 23.44 degrees. This means the moon does not have seasons like Earth does. However, because of the tilt, there are places at the lunar poles that never see daylight.

          The Diviner instrument on NASA’s Lunar Reconnaissance Orbiter measured temperatures of minus 396 F (minus 238 C) in craters at the southern pole and minus 413 F (minus 247 C) in a crater at the northern pole. 

          “These super-cold brightness temperatures are, to our knowledge, among the lowest that have been measured anywhere in the solar system, including the surface of Pluto,” David Paige, Diviner's principal investigator and a UCLA professor of planetary science, said in a 2009 statement. Since then, NASA’s New Horizons mission set Pluto’s temperature range at a comparable minus 400 to minus 360 F (minus 240 to minus 217 C).

          Scientists suspected that water ice could exist in the moon's dark craters that are in permanent shadow. In 2010, a NASA radar aboard India's Chandrayaan-1 spacecraft detected water ice in more than 40 small craters at the moon's North Pole. They hypothesized that over 1.3 trillion lbs. of water ice hid among the permanently darkened craters.

          The Moon is Earth’s closest neighbour in space. Its orbit around Earth is elliptical, rather than circular, which means that its distance from us varies. At its closest point to Earth (its perigee), the Moon is 384,400km (240,000 miles) away. Incredibly, the Moon’s orbit is slowly carrying it away from Earth at a rate of around 5cm (2in) a year.

          The distance between London, where I live, and Oxford, where I used to live, is about 100 km (or 60 miles). It takes about 90 minutes by car and about 120 minutes by bus. I can easily make sense of that distance.

          Harder to consider: the distance between the Earth and the moon, which is 384,400 km (240,000  miles). It’s a fact we’ve likely all learned in high school. Unlike the distance between London and Oxford, however, it’s not easy to comprehend what 384,400 km means in real terms.

          Luckily, you don’t have to think too hard. A NASA spacecraft has solved that problem for us. In October, OSIRIS-REX, a spacecraft that’s bound to intersect an asteroid in August this year, took the photo above from about 5 million km (3 million miles) away from the Earth. NASA posted the picture on Jan. 2, providing the public with a unique view of our planet and its moon. The angle is great to get a grasp of what the distance between the two celestial bodies really looks like, but it’s not perfect.

          Here’s a back-of-the-envelope calculation to explain why. For ease, we’re going to use round figures. The Earth’s diameter is about 13,000 km (8,000 miles). That means, in the 390,000 km distance between the Earth and the moon.

          Although the moon’s interior structure is difficult to study, scientists believe that it has a small iron core. Surrounding this is a partially molten zone called the lower mantle. Above this lies the mantle, which is made up of dense rock, and the crust, which is also made of rock. Together, the mantle and the crust form the lithosphere, which can be up to 800km (500 miles) thick. There are only two basic regions on the Moon’s surface — dark plains called mania and lighter highlands. These heavily cratered highlands are the oldest parts of the Moon’s crust, dating back over four billion years. The darker plains are craters that were filled with lava.

          The composition of the Moon is a bit of a mystery. Although we know a lot about what the surface of the Moon is made of, scientists can only guess at what the internal composition of the Moon is. Here’s what we think the Moon is made of.

          Like the Earth, the Moon has layers. The innermost layer is the lunar core. It only accounts for about 20% of the diameter of the Moon. Scientists think that the lunar core is made of metallic iron, with small amounts of sulfur and nickel. Astronomers know that the core of the Moon is probably at least partly molten.

          Outside the core is the largest region of the Moon, called the mantle. The lunar mantle extends up to a distance of only 50 km below the surface of the Moon. Scientists believe that the mantle of the Moon is largely composed of the minerals olivine, orthopyroxene and clinopyroxene. It’s also believed to be more iron-rich than the Earth’s mantle.

          The outermost layer of the Moon is called the crust, which extends down to a depth of 50 km. This is the layer of the Moon that scientists have gathered the most information about. The crust of the Moon is composed mostly of oxygen, silicon, magnesium, iron, calcium, and aluminum. There are also trace elements like titanium, uranium, thorium, potassium and hydrogen.

          Venus is covered by hundreds of thousands of volcanoes. This is because the surface of the planet is a thin skin floating on hot molten rock. This lava is vented wherever possible, meaning that, unlike Earth, Venus has volcanoes everywhere. Most of these volcanoes are around 3km (2 miles) wide and 90m (395ft) high, but there are over 160 much larger than this. Some volcanoes on Venus are over 100km (60 miles) in diameter! The volcanic activity on Venus means that the surface of the planet is always changing.

          The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System. It has a surface that is 90% basalt, and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide, with a density that is 90 times greater than Earth's atmosphere.

          Even though there are over 1,600 major volcanoes on Venus, none are known to be erupting at present and most are probably long extinct. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus’s highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank. Although many lines of evidence suggest that Venus is likely to be volcanically active, present-day eruptions at Maat Mons have not been confirmed.

          Less than 20% of sunlight falling on Venus breaks through the clouds. Despite this, Venus has the hottest surface temperature of any planet in the Solar System. This is because infrared radiation (heat) released from the planet cannot escape back into space. The atmosphere traps heat inside, like the glass in a green-house, meaning that the temperature is over 400°C (750°F), greater than it would he if Venus had no atmosphere.

          Greenhouse involving carbon dioxide and water vapor may have occurred on Venus. In this scenario, early Venus may have had a global ocean if the outgoing thermal radiation was below the Simpson-Nakajima limit but above the moist greenhouse limit. As the brightness of the early Sun increased, the amount of water vapor in the atmosphere increased, increasing the temperature and consequently increasing the evaporation of the ocean, leading eventually to the situation in which the oceans boiled, and all of the water vapor entered the atmosphere. This scenario helps to explain why there is little water vapor in the atmosphere of Venus today. If Venus initially formed with water, the greenhouse would have hydrated Venus’ stratosphere, and the water would have escaped to space. Some evidence for this scenario comes from the extremely high deuterium to hydrogen ratio in Venus' atmosphere, roughly 150 times that of Earth, since light hydrogen would escape from the atmosphere more readily than its heavier isotope, deuterium. Venus is sufficiently strongly heated by the Sun that water vapor can rise much higher in the atmosphere and be split into hydrogen and oxygen by ultraviolet light. The hydrogen can then escape from the atmosphere while the oxygen recombines or bonds to iron on the planet’s surface. The deficit of water on Venus due to the runaway greenhouse effect is thought to explain why Venus does not exhibit surface features consistent with plate tectonics, meaning it would be a stagnant lid planet. Carbon dioxide, the dominant greenhouse gas in the current Venusian atmosphere, owes its larger concentration to the weakness of carbon recycling as compared to Earth, where the carbon dioxide emitted from volcanoes is efficiently sub ducted into the Earth by plate tectonics on geologic time scales through the carbonate-silicate cycle, which requires precipitation to function.

          Radar works in the same way as an echo. When you shout loudly at a distant wall, you will hear the echo of your voice a few seconds later. This is because the sound waves hit the solid wall and bounce back towards you. Radar uses high-frequency waves that travel much faster and much further. The radar sends out a short burst of radio waves and then listens for an echo, which tells it how far away the target is, and what it is made of.

          Airplane pilots get around this difficulty using radar, a way of "seeing" that uses high-frequency radio waves. Radar was originally developed to detect enemy aircraft during World War II, but it is now widely used in everything from police speed-detector guns to weather forecasting. Let's take a closer look at how it works!

          We can see objects in the world around us because light (usually from the Sun) reflects off them into our eyes. If you want to walk at night, you can shine a torch in front to see where you're going. The light beam travels out from the torch, reflects off objects in front of you, and bounces back into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don't trip over things.

          Radar works in much the same way. The word "radar" stands for radio detection and ranging—and that gives a pretty big clue as to what it does and how it works. Imagine an airplane flying at night through thick fog. The pilots can't see where they're going, so they use the radar to help them.

          An airplane's radar is a bit like a torch that uses radio waves instead of light. The plane transmits an intermittent radar beam (so it sends a signal only part of the time) and, for the rest of the time, "listens" out for any reflections of that beam from nearby objects. If reflections are detected, the plane knows something is nearby—and it can use the time taken for the reflections to arrive to figure out how far away it is. In other words, radar is a bit like the echolocation system that "blind" bats use to see and fly in the dark.

          The most detailed information about Venus was acquired by a space probe called Magellan. Launched in 1989, Magellan travelled to Earth's neighbour and spent three years building a complete map of the planet. Flying as low as 294km (183 miles) above the surface, Magellan bounced radar pulses off the solid ground beneath and sent the data back to Earth to he analyzed. It measured strips of land 24km (14 miles) wide and 10,000km (6000 miles) long each time it circled the planet, while its altimeter measured its height above the surface.

          The Magellan spacecraft was the first planetary explorer to be launched by a space shuttle when it was carried aloft by the shuttle Atlantis from Kennedy Space Center in Florida on May 4, 1989. Atlantis took Magellan into low Earth orbit, where it was released from the shuttle's cargo bay and fired by a solid-fuel motor called the Inertial Upper Stage (IUS) on its way to Venus. Magellan looped around the Sun one-and-a-half times before arriving at Venus on August 10, 1990. A solid-fuel motor on the spacecraft then fired, placing Magellan into a near-polar elliptical orbit around Venus.

          Spacecraft carried a sophisticated imaging radar, which was used to make the most highly detailed map of Venus ever captured during its four years in orbit around Venus from 1990 to 1994. After concluding its radar mapping, Magellan also made global maps of Venus's gravity field. Flight controllers then tested a new maneuvering technique called aero braking, which uses a planet's atmosphere to slow or steer a spacecraft. The spacecraft made a dramatic plunge into the thick, hot Venusian atmosphere on October 12, 1994, and was crushed by the pressure of Venus's atmosphere. Magellan's signal was lost at 10:02 Universal Time (3:02 a.m. Pacific Daylight Time) that day.

          The Magellan mission was divided up into "cycles" with each cycle lasting 243 days (the time necessary for Venus to rotate once under the Magellan orbit). The mission proceeded as follows:

Magellan Assembly

          On May 4, 1989, the Magellan spacecraft was deployed from the shuttle. The spacecraft is topped by a 3.7-meter (12-foot) diameter dish-shaped antenna that was a spare part left over from the Voyager program. The long, white, horn-shaped antenna, attached just to the left of the dish antenna, is the altimeter antenna that gathers data concerning the surface height of features on Venus. Most of the spacecraft is wrapped in reflective white thermal blankets that protect its sensitive instruments from solar radiation. 

Deployment

          The Magellan spacecraft's deployment from the shuttle Atlantis' cargo bay was captured by an astronaut with a hand-held camera pointed through the shuttle's aft flight deck windows. Deployment occurred in the early evening of May 4, 1989, after Atlantis had carried Magellan and its Inertial Upper Stage (IUS) booster rocket, into low Earth orbit. Once the shuttle was safely away from the spacecraft, the IUS ignited and placed Magellan on course for its 15-month journey to Venus.

Magellan Orbiting Venus

          On August 10, 1990, Magellan entered into orbit about Venus, as depicted in this artist's view. During its 243-day primary mission, referred to as Cycle 1, the spacecraft mapped well over 80 percent of the planet with its high-resolution Synthetic Aperture Radar (SAR). The spacecraft returned more digital data in the first cycle than all previous U.S. planetary missions combined.

          Venus’ atmosphere is formed from clouds of carbon dioxide, nitrogen and sulphuric acid. This heavy layer of clouds is over 30km (18 miles) deep in some places, meaning that no part of the planet’s surface can be seen with the naked eye. Only since the 1970s have scientists been able to “look” past these clouds to see the solid ground beneath. This has mainly been done with equipment mounted on space probes. Radar technology allows probes to record the geography of the planet, and to produce a map of surface features.

          The clouds of Venus are its defining characteristic. We can see the surface of Mars and Mercury, but the surface of Venus is shrouded by thick clouds. For most of history, astronomers had no idea what was beneath those clouds, and they imagined a tropical world with overgrown vegetation and constant rainfall. They couldn’t have been more wrong.

          The climate of Venus isn’t tropical at all; it’s hellish. Temperatures on the surface of Venus approach 475°C and the atmospheric pressure is 93 times what you experience here on Earth. To experience that kind of pressure, you would need to swim down 1 km beneath the surface of the ocean. Venus’ atmosphere is made almost entirely of carbon dioxide, and not the oxygen/nitrogen mix we have here on Earth.

          The clouds we see on Venus are made up of sulfur dioxide and drops of sulfuric acid. They reflect about 75% of the sunlight that falls on them, and are completely opaque. It’s these clouds that block our view to the surface of Venus. Beneath these clouds, only a fraction of sunlight reaches the surface. If you could stand on the surface of Venus, everything would look dimly lit, with a maximum visibility of about 3 km.

          The upper cloud deck of Venus is between 60-70 km altitudes. This is the part of Venus that we see in telescopes and visible light photographs of the planet. The clouds on Venus rain sulfuric acid. This rain never reaches the ground, however. The high temperatures evaporate the sulfuric acid drops, causing them to rise up again into the clouds again.

          Venus spacecraft have detected lightning on Venus, coming out of the clouds with a similar process to what we have on Earth. The first bursts of lightning were detected by the Soviet Venera probes and later confirmed by ESA’s Venus Express spacecraft.

          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.

          Despite being the closest planet to the Sun, often orbiting less than 60 million kilometres away from the star, temperatures on Mercury can drop below —180°C (-290°F). This is because Mercury is too hot and too small to be able to hold on to much gas. With no clouds to stop heat from escaping into space at night, temperatures on Mercury plummet.

          Mercury is the planet in our solar system that sits closest to the sun. The distance between Mercury and the sun ranges from 46 million kilometers to 69.8 million kilometers. The earth sits at a comfy 150 million kilometers. This is one reason why it gets so hot on Mercury during the day.

          The other reason is that Mercury has a very thin and unstable atmosphere. At a size about a third of the earth and with a mass (what we on earth see as ‘weight’) that is 0.05 times as much as the earth, Mercury just doesn’t have the gravity to keep gases trapped around it, creating an atmosphere. Due to the high temperature, solar winds, and the low gravity (about a third of earth’s gravity), gases keep escaping the planet, quite literally just blowing away.

          Atmospheres can trap heat, that’s why it can still be nice and warm at night here on earth. Mercury’s atmosphere is too thin, unstable and close to the sun to make any notable difference in the temperature.

          Space is cold. Space is very cold. So cold in fact, that it can almost reach absolute zero, the point where molecules stop moving (and they always move). In space, the coldest temperature you can get is 2.7 Kelvin, about -270 degrees Celsius.

          Sunlight reflected from other planets and moons, gases that move through space, the very thin atmosphere and the surface of Mercury itself are the main reasons that temperatures on Mercury don’t get lower than about -180 °C at night.