My Science School

Earth is the only planet in our Solar System with oxygen in its atmosphere and lots of liquid water on its surface, allowing life, in its various forms, to exist. It is our beautiful blue planet, thriving with a multitude of living animals, plants and, of course, human beings!

In the solar system, the Earth is the third planet from the sun, and it is the only planet known to have life. According to different sources of evidence like radiometric dating, the Earth is believed to be more than 4.5 billion years old. Out of the four terrestrial planets, the Earth is the largest and densest planet. The lithosphere is made up of numerous tectonic plates that keep moving over millions of years. Water in the oceans cover about 71% of the total surface of the Earth, and the remaining 29% is covered by the continents and islands, which have rivers and lakes. The ability of the Earth to harbor life makes the Earth a unique planet in the solar system, and this stems from the fact that water in liquid form exists on the planet. Similarly, the existence of gaseous oxygen in the atmosphere of the Earth also supports life.

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Scientists think Earth was formed at roughly the same time as the Sun and other planets when the Solar System came together from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disc. Most of the material was pulled towards the centre to form the Sun. Gradually, the rest of this vast cloud began to cool and the gas condensed into trillions of droplets. These droplets were slowly pulled together by their own gravity and formed clumps. Leftover particles within the disc collided and stuck together to form other larger bodies, including Earth.

When Earth formed 4.5 billion years ago, it was a sterile ball of rock, slammed by meteorites and carpeted with erupting volcanoes. Within a billion years, it had become inhabited by microorganisms. Today, life covers every centimetre of the planet, from the highest mountains to the deepest sea. Yet, every other planet in the solar system seems lifeless.

Many ideas have been proposed to explain how life began. Most are based on the assumption that cells are too complex to have formed all at once, so life must have started with just one component that survived and somehow created the others around it. When put into practice in the lab, however, these ideas don’t produce anything particularly lifelike. It is, some researchers are starting to realise, like trying to build a car by making a chassis and hoping wheels and an engine will spontaneously appear.

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Venus – Earth’s “Twin”

Venus is nearly the same size as Earth, so it is often called Earth’s “twin”. But it is nothing at all likes our world. Venus’s atmosphere is full of poisonous gases. Its clouds contain a chemical strong enough to dissolve metal! And the clouds on Venus are so thick that cameras can’t see the planet’s surface.

High in Venus’s atmosphere, powerful windstorms are raging. Venus’s windstorms are much worse than storms on Earth. Lightning flashes in the sky as often as 20 times a minute.

Venus is the second closest planet to the sun. This makes it extremely hot and dry. As seen from Earth, Venus is brighter than all the other planets and stars. It is so bright that it can sometimes be seen in the daytime! A year on Venus is as long as 225 Earth days. Like Mercury, Venus has no moons.

Scientists once thought the planet Venus would be much like Earth, but warmer. They were wrong. Venus is extremely hot, and its atmosphere is very heavy. Scientists have used many space probes to study Venus, including the Soviet Vanera probes, the U.S. Mariner and Magellan probes, and a European Space Agency probe named Venus Express.

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Earth

Earth is a watery planet. More than two-thirds of Earth’s surface is covered with water. That’s good for all the living things in our world because animals and plants need water to live. Animals and plants live almost everywhere on Earth.

Earth is the third planet from the sun, and air surrounds it. The air is made up of gases, such as oxygen, nitrogen, and carbon dioxide. These gases are needed for almost all living things to survive. Human beings breathe in oxygen. Plants need carbon dioxide.

If you look at a globe, you will find the North Pole on one end and the South Pole on the other. Earth looks like a ball, but it is actually a little flatter at the North and South poles.

Earth travels 958 million kilometres on its journey around the sun. It takes about 365 days for Earth to orbit the sun once. That’s why there are about 365 days in an Earth year.

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Mercury, closest to the Sun

Mercury is the nearest planet to the sun. It is a bare rocky ball covered with craters, much like our moon. Also like our moon, Mercury has broad, flat plains and steep cliffs.

Mercury spins and has day and night, but it spins very slowly. One day on Mercury takes 59 Earth days.

Mercury is very hot during the long day. Temperatures there reach higher than 400 °C. At night, temperatures take a big dip, sometimes to as low as -170 °C!

Mercury has a bigger temperature change than any other planet. This is because it is closest to the sun, and because it has very long days.

Mercury is a small planet. It could fit inside Earth two and one-half times. There are hardly any gases surrounding Mercury, so it has very little atmosphere.

Mercury is much closer to the sun than Earth is. So if you were standing on Mercury, the sun would appear much bigger and brighter.

Of course, you could not stand on Mercury in the middle of the day or night because it is either too hot or too cold. But scientists have explored it with a spacecraft that had no people on board.

The U.S. Mariner 10 was the first spacecraft to reach Mercury. Mariner 10 was a space probe. A space probe is a machine that explores space and sends information and pictures back to Earth. Astronauts do not travel in space probes. Scientists on Earth use computers to control space probes. On March 29, 1974, Mariner 10 flew within 740 kilometres of Mercury. It swept past the planet again on Sept. 24, 1974, and on March 16, 1975. During these flights, the probe took photographs of parts of Mercury’s surface. The U.S. space probe Messenger was launched in 2004 and began orbiting Mercury in 2011. Messenger became the first space probe to orbit Mercury. Messenger began to map the planet’s surface and study its magnetic field.

Moons of other planets

Earth is not the only planet that has a moon. Other planets do, too!

Mars has two little moons that are just lumpy chunks of rock. The largest of these, Phobos, is only about 27 kilometres wide. Mars’s other moon, Deimos, is about 15 kilometres wide.

Just how big are Mars’s moons? On a map, find the Cape Cod Canal, which separates Cape Cod from the rest of Massachusetts, U.S.A. The canal is 27 kilometres long. So Mars’s larger moon, Phobos, is as wide as the canal is long! Deimos is a little more than half that size.

Jupiter has at least 63 moons. A few of the smallest moons are smaller than some of the mountains on Earth. But the biggest, Ganymede, is bigger than the planet Mercury.

Saturn has at least 62 moons. Like Jupiter’s moons, Saturn’s are very different in size. Some are smaller than 10 kilometres in width. Saturn’s biggest moon, Titan, is bigger than Earth’s moon. Titan is also bigger than Mercury.

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EARTH

Our own planet, Earth, is the largest of the four inner planets. Third in order of from the Sun, 71% of its surface is taken up by oceans. Water is also present as droplets or ice particles that make up the clouds, as vapour in the atmosphere and as ice in polar areas or on high mountains.

Liquid water is essential for the existence of life on Earth, the only body in the Solar System where life is known to be present. Earth’s distance from the Sun - neither too close nor too far - produces exactly the right temperature range. The atmosphere traps enough of the Sun’s energy to avoid temperature extremes. It also screens the harmful rays of the Sun and acts as a shield against bombardment by meteoroids.

Earth’s magnetic field is generated by electrical currents produced by the swirling motion of the liquid inner core. The magnetic field protects Earth from the solar wind.

Earth’s outer shell, made up of the rocky crust and partly-molten upper mantle, is divided into about 15 separate pieces, called tectonic plates. Volcanoes and earthquakes occur where plate edges meet.

            When Earth lies directly between the Sun and the Moon it casts its shadow on the Moon. This is called a lunar eclipse.

            In contrast to the barren landscapes of the other planets, much of Earth’s is covered by vegetation, including forest, scrub and grassland. Different climates determine the types of plants and animals that live in different places. Large areas show the important influence of humans: for example, farmland, roads and cities. Land areas are continually sculpted by the weather and moving water or ice.

Mars has one of the most dramatic surfaces of any planet in the Solar System. Enormous volcanoes dominate the landscape, the largest of which —Olympus Mons — is over 25 kilometres (15.5 miles) tall. This is three times larger than Mount Everest on Earth! The giant canyon Vales Mariners is long enough to stretch across the entire United States of America.

Mar’s surface is a dry, barren wasteland marked by old volcanoes and impact craters. The entire surface can be scoured by a single sand storm that hides it from observation for days at a time. Despite the formidable conditions, Mar’s surface is better understood by scientists than any other part of the Solar System, except our own planet, of course.

Mars is a small world. Its radius is half of the Earth’s and it has a mass that is less than one tenth. The Red Planet’s total surface area is about 28% of Earth. While that does not sound like a large world at all, it is nearly equivalent to all of the dry land on Earth. The surface is thought to be mostly basalt, covered by a fine layer of iron oxide dust that has the consistency of talcum powder. Iron oxide(rust as it is commonly called) gives the planet its characteristic red hue.

In the ancient past of the planet volcanoes were able to erupt for millions of years unabated. A single hotspot could dump molten rock on the surface for millenia because Mars lacks plate tectonics. The lack of tectonics means that the same rupture in the surface stayed open until there was no more pressure to force magma to the surface. Olympus Mons formed in this manner and is the largest mountain in the Solar System. It is three time taller than Mt. Everest. These runaway volcanic actions could also partially explain the deepest valley in the Solar System. Valles Marineris is thought to be the result of a collapse of the material between two hotspots and is also on Mars.

As it exists today, Mars is a planet hostile to life. Unlike Earth, Mars has no ozone layer to protect life from the Sun’s lethal ultraviolet radiation. There is no breathable oxygen in the air, and giant dust storms are common around the planet. The first astronauts to live on Mars will probably do so in large domes that can contain an artificial, Earth-like atmosphere.

Earth is the only place that we know for certain supports life. Many claims have been made by observers who thought they saw evidence of life on Mars, but we now know they were tricked by the very difficult measurements. From Earth, even with our most powerful telescopes, we just cannot see enough detail on Mars to answer this question. We need a close-up look at the planet.

While robotic spacecraft have given us wonderful views, no humans have ever tried to journey to Mars, and no such missions will be attempted for many years. In fact, whoever will turn out to be the first people on Mars may be your age today, and when you are an adult, perhaps you will watch -- or even participate!-- as people make the first voyage to that planet.

In the meantime, NASA is working hard now to discover whether there is life on Mars. The United States and other countries have been sending spacecraft to orbit or land there since the 1960s, and each mission teaches us more about this fascinating planet. We have learned that even though Mars is more similar to Earth than anywhere else in the solar system, and therefore is a good place to look for life, it is still different from Earth in many ways.

A compass point to the North Pole on Earth because our whole planet acts like a giant magnet, but Mars does not act this way. Besides turning a compass needle, Earth's magnetic field turns away dangerous particles of space radiation. Without a magnetic field on Mars and with much, much less air than on Earth, more harmful space radiation reaches its surface. Although some measurements tell us there probably is water on Mars, there is far less than on Earth. And it is so cold there that most of the water is probably not liquid but rather is ice. Overall, Mars would be a pretty uncomfortable place to try to live!

Even if there were no life on Mars, it would be exciting to know whether there used to be life there. So in addition to looking for living bacteria, NASA will be searching for tiny fossils that might indicate life got a start early in Mars' history but, unlike on our home planet, it did not survive and evolve into larger life forms.

Many of the studies of Mars will involve robots, like the ones that have gone there before, but getting more advanced with each flight. Someday a spacecraft may pick up samples from Mars and bring them back to Earth where they can be studied in our best laboratories. Eventually, humans may make the daring journey, but many important problems have to be solved before trying such an expensive, difficult, and exciting voyage.

When mars first formed it had a much thicker atmosphere than it does today. Because the planet’s gravity is not very strong, this atmosphere gradually escaped into space. The climate became increasingly cold, and all the water on Mars froze. Today, the water on Mars exists only as an icy, permafrost layer deep in the soil. Temperatures in Mars’ Polar Regions are so low that carbon dioxide in the atmosphere freezes, covering sheets of water ice with a layer of frosty crystals of dry ice.

Almost all water on Mars today exists as ice, though it also exists in small quantities as vapor in the atmosphere. What was thought to be low-volume liquid brines in shallow Martian soil, also called recurrent slope lineae, may be grains of flowing sand and dust slipping downhill to make dark streaks. The only place where water ice is visible at the surface is at the north polar ice cap. Abundant water ice is also present beneath the permanent carbon dioxide ice cap at the Martian South Pole and in the shallow subsurface at more temperate conditions. More than 21 million km3 of ice have been detected at or near the surface of Mars, enough to cover the whole planet to a depth of 35 meters (115 ft). Even more ice is likely to be locked away in the deep subsurface.

Some liquid water may occur transiently on the Martian surface today, but limited to traces of dissolved moisture from the atmosphere and thin films, which are challenging environments for known life. No large standing bodies of liquid water exist on the planet's surface, because the atmospheric pressure there averages just 600 pascals (0.087 psi), a figure slightly below the vapor pressure of water at its melting point; under average Martian conditions, pure water on the Martian surface would freeze or, if heated to above the melting point, would sublime to vapor. Before about 3.8 billion years ago, Mars may have had a denser atmosphere and higher surface temperatures, allowing vast amounts of liquid water on the surface, possibly including a large ocean that may have covered one-third of the planet. Water has also apparently flowed across the surface for short periods at various intervals more recently in Mars' history. On December 9, 2013, NASA reported that, based on evidence from the Curiosity rover studying Aeolis Palus, Gale Crater, contained an ancient freshwater lake that could have been a hospitable environment for microbial life.

Many lines of evidence indicate that water ice is abundant on Mars and it has played a significant role in the planet's geologic history. The present-day inventory of water on Mars can be estimated from spacecraft imagery, remote sensing techniques (spectroscopic measurements, radar, etc.), and surface investigations from landers and rovers. Geologic evidence of past water includes enormous outflow channels carved by floods, ancient river valley networks, deltas, and lakebeds; and the detection of rocks and minerals on the surface that could only have formed in liquid water. Numerous geomorphic features suggest the presence of ground ice (permafrost) and the movement of ice in glaciers, both in the recent past and present. Gullies and slope lineae along cliffs and crater walls suggest that flowing water continues to shape the surface of Mars, although to a far lesser degree than in the ancient past.

Although the surface of Mars was periodically wet and could have been hospitable to microbial life billions of years ago, the current environment at the surface is dry and subfreezing, probably presenting an insurmountable obstacle for living organisms. In addition, Mars lacks a thick atmosphere, ozone layer, and magnetic field, allowing solar and cosmic radiation to strike the surface unimpeded. The damaging effect of ionizing radiation on cellular structure is another one of the prime limiting factors on the survival of life on the surface. Therefore, the best potential locations for discovering life on Mars may be in subsurface environments. On November 22, 2016, NASA reported finding a large amount of underground ice on Mars; the volume of water detected is equivalent to the volume of water in Lake Superior. In July 2018, Italian scientists reported the discovery of a sub glacial lake on Mars, 1.5 km (0.93 mi) below the southern polar ice cap, and extending sideways about 20 km (12 mi), the first known stable body of water on the planet.

Terraforming is the process of changing the environment of a planet to make it more like Earth. Many scientists have proposed terraforming Mars as a way of dealing with over-crowding on Earth. Nobody knows exactly how terraforming would work, and whether it would have a damaging effect on Mars’ natural environment, but in theory, Mars could be transformed into a second Earth, where many forms of life could live naturally. The diagrams to the right show how it could be done.

Terraforming or terraformation (literally, “Earth-shaping”) of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to the environment of Earth to make it habitable by Earth-like life.

The concept of terraforming developed from both science fiction and actual science. The term was coined by Jack Williamson in a science-fiction short story (“Collision Orbit”) published during 1942 in Astounding Science Fiction, but the concept may pre-date this work.

Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods of altering the climate of Mars may fall within humanity's technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to it. The long timescales and practicality of terraforming is the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics, and methodology of altering the environment of an extraterrestrial world.

The most convincing evidence for life on the red planet comes from a Martian meteorite that landed on Earth around 13,000 years ago. This meteorite contained microscopic structures that could have been formed by living organisms.

The general consensus now is that the original rock formed 4 billion years ago on Mars. It was eventually catapulted into space by an impact and wandered the solar system for millions of years before landing on Earth 13,000 years ago.

Over 50 other meteorites have been identified as coming from Mars, but ALH84001 is by far the oldest, with the next in age being just 1.3 billion years old. "That alone makes ALH84001 a very important sample," says Allan Treiman of the Lunar and Planetary Institute. "It’s our only hope to understand what Mars was like at this time period."

The first thing that struck researchers examining the meteorite was the presence of 300-micron-wide carbonate globules that make up 1% of the rock. Dave McKay from NASA’s Johnson Space Center and his colleagues determined that the carbonate most likely formed in the presence of water.

Although evidence for a wet ancient Mars has accumulated in the subsequent years, the claim that ALH84001 once sat in water was pretty revolutionary at the time, says Kathie Thomas-Keprta, also from the Johnson Space Center.

Inside the ALH84001 carbonates, McKay spotted odd features that resembled very small worm-like fossils, so he asked Thomas-Keprta to look at them more closely with electron microscopy. "I kind of thought he was crazy," she says. "I thought I would join the group and straighten them out."

In the end, she helped the team characterize the biomorphic features, as well as unusual grains of the mineral magnetite found in the meteorite. In a 1996 Science paper, these two phenomena – along with the chemical distribution in the globules and the detection of large organic molecules – were taken collectively as signatures of biological activity occurring long ago on Mars.

Mars has been known as the red planet for thousands of years. The Ancient Romans named the planet Mars because it reminded them of their God of anger and war. Mars gets its striking colour from large amounts of iron oxide (rust) in its soil.

Even photos from spacecraft show that it’s a rusty red color. The hue comes from the fact that the surface is actually rusty, as in; it’s rich in iron oxide. Iron left out in the rain and will get covered with rust as the oxygen in the air and water reacts with the iron in the metal to create a film of iron oxide.

Mars’ iron oxide would have formed a long time ago, when the planet had more liquid water. This rusty material was transported around the planet in dust clouds, covering everything in a layer of rust. In fact, there are dust storms on Mars today that can rise up and consume the entire planet, obscuring the entire surface from our view. That dust really gets around.

But if you look closely at the surface of Mars, you’ll see that it can actually be many different colours. Some regions appear bright orange, while others look browner or even black. But if you average everything out, you get Mars’ familiar red colour.

If you dig down, like NASA’s Phoenix Lander did in 2008, you get below this oxidized layer to the rock and dirt beneath. You can see how the tracks from the Curiosity Rover get at this fresh material, just a few centimeters below the surface. It’s brown, not red.

And if you could stand on the surface of Mars and look around, what colour would the sky be? Fortunately, NASA’s Curiosity Rover is equipped with a full colour camera, and so we can see roughly what the human eye would see. The sky here is blue because of Raleigh scattering, where blue photons of light are scattered around by the atmosphere, so they appear to come from all directions. But on Mars, the opposite thing happens. The dust in the atmosphere scatters the red photons makes the sky appear red. We have something similar when there’s pollution or smoke in the air.

But here’s the strange part. On Mars, the sunsets appear blue. The dust absorbs and deflects the red light, so you see more of the blue photons streaming from the Sun. A sunset on Mars would be an amazing event to see with your own eyes. Let’s hope someone gets the chance to see it in the future.

When mars first formed it had a very thick atmosphere. However, the gases have long since disappeared into space due to the planet’s weak gravity. Mars’ atmosphere is now very thin, and made mainly of carbon dioxide.

The atmosphere of Mars is the layer of gas surrounding Mars. It is primarily composed of carbon dioxide (95.32%), molecular nitrogen (2.6%) and argon (1.9%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen and other noble gases. The atmosphere of Mars is much thinner than Earth’s. The surface pressure is only about 610 Pascal’s (0.088 psi) which is less than 1% of the Earth’s value. The currently thin Martian atmosphere prohibits the existence of liquid water at the surface of Mars, but many studies suggest that the Martian atmosphere was much thicker in the past. The highest atmospheric density on Mars is equal to the density found 35 km above the Earth’s surface. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today.

The atmosphere of Mars is colder than Earth’s. Owing to the larger distance from Sun, Mars receives less solar energy and has a lower effective temperature (about 210 K). The average surface emission temperature of Mars is just 215 K, which is comparable to inland Antarctica. The weaker greenhouse effect in the Martian atmosphere (5 °C, versus 33 °C on Earth) can be explained by the low abundance of other greenhouse gases. The daily range of temperature in the lower atmosphere is huge (can exceed 100 °C near the surface in some regions) due to the low thermal inertia. The temperature of the upper part of the Martian atmosphere is also significantly lower than Earth’s because of the absence of stratospheric ozone and the radiative cooling effect of carbon dioxide at higher altitudes.

Dust devils and dust storms are prevalent on Mars, which are sometimes observable by telescopes from Earth. Planet-encircling dust storms (global dust storms) occur on average every 5.5 earth years on Mars and can threaten the operation of Mars rovers. However, the mechanism responsible for the development of large dust storms is still not well understood.

The Martian atmosphere is an oxidizing atmosphere. The photochemical reactions in the atmosphere tend to oxidize the organic species and turn them into carbon dioxide or carbon monoxide. Although the most sensitive methane probe on the recently launched ExoMars Trace Gas Orbiter failed to find methane in the atmosphere over the whole Mars, several previous missions and ground-based telescope detected unexpected levels of methane in the Martian atmosphere, which may even be a bio signature for life on Mars. However, the interpretation of the measurements is still highly controversial and lacks a scientific consensus.

Of all the planets in the Solar System, Mars most resembles Earth. Its day is only slightly over 24 hours, and it is tilted at the same angle as our planet, meaning that seasons are very similar to ours. Early on in its history, Mars had water on its surface. Oceans formed, kept warm by volcanic activity, and primitive life may have started here. Today, freezing conditions on Mars, and the planet’s thin atmosphere, mean that life can no longer exist on the planet’s surface.

The search for life on Mars shouldn’t focus exclusively on the distant past, some researchers say. Four billion years ago, the Martian surface was apparently quite habitable, featuring rivers, lakes and even a deep ocean. Indeed, some astrobiologists view ancient Mars as an even better cradle for life than Earth was, and they suspect that life on our planet may have come here long ago aboard Mars rocks blasted into space by a powerful impact.

Things changed when Mars lost its global magnetic field. Charged particles streaming from the sun were then free to strip away the once-thick Martian atmosphere, and strip it they did. This process had transformed Mars into the cold, dry world we know today by about 3.7 billion years ago, observations by NASA's MAVEN orbiter suggest. (Earth still has its global magnetic field, explaining how our planet remains so livable.)

One of the most promising hiding places is the Martian underground. Though the Red Planet's surface has no liquid water these days — apart, possibly, from temporary flows on warm slopes now and again — there’s a likely lot of the wet stuff in buried aquifers. For example, observations by Europe’s Mars Express orbiter suggest that a big lake may lurk beneath the Red Planet’s South Pole.

Earth’s diverse residents advertise their presence in dramatic and obvious ways; an advanced alien civilization could probably figure out pretty quickly, just by scanning our atmosphere, that our planet is inhabited. 

We don’t see any such clear-cut evidence in the Martian air, but scientists have spotted some intriguing hints recently. For example, NASA's Curiosity rover has rolled through two plumes of methane inside the 96-mile-wide (154 kilometers) Gale Crater, which the six-wheeled robot has been exploring since its 2012 touchdown. The rover mission also determined that baseline methane concentrations in Gale's air go through cycles seasonally.

More than 90% of Earth's atmospheric methane is produced by microbes and other organisms, so it's possible the gas is a signature of modern Martian life.

But the jury is most definitely still out on that. Abiotic processes can generate methane, too; the reaction of hot water with certain types of rock is one example. And even if the Mars methane is biogenic, the creatures that created it could be long dead. Scientists think the Red Planet methane plumes leaked out from underground, and there's no telling how long the gas lay trapped down there before making its way to the surface.