My Science School

          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.

          The images revealed bright deposits on the floors of some craters — a discovery shrouded in mystery without higher-resolution images –are actually clusters of rimless pits surrounded by halos of reflective material.

          “The etched appearance of these landforms is unlike anything we’ve seen before on Mercury or the moon,” said Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory, in a prepared statement. “We are still debating their origin, but they appear to be relatively young and may suggest a more abundant than expected volatile component in Mercury’s crust.”

          In other words, Mercury’s surface might look a lot like the moon. But evidence of recent volcanic history suggests the planet has more going on than scientists thought.

          Planets are born from the countless collisions of rocks and space debris that were part of the early Solar System. The heat from these impacts remains deep within the core of the planet, released through volcanic eruption. Mercury's cratered appearance shows that there has been no volcanic activity on the planet for billions of years. This makes Mercury a dead planet.

          Mercury is the closest planet to the Sun, and as a result is a dry, barren planet scorched by solar heat. Parts of Mercury's surface often exceed 450 °C (840 °F) when the planet is closest to the Sun. However, at night, temperatures can drop by over 600 °C (1,100 °F) and some scientists believe that there is actually ice in deep craters that never see the Sun. Radar imaging of the planet has revealed areas of high reflectivity near the planet's poles. This may be frozen water carried to Mercury by meteorites.

          This orthographic projection view provides a look at Mercury's North Polar Region. The yellow regions in many of the craters mark locations that show evidence for water ice, as detected by Earth-based radar observations from Arecibo Observatory in Puerto Rico. MESSENGER has collected compelling new evidence that the deposits are indeed water ice, including imaging within the permanently shaded interiors of some of the craters, such as Prokofiev and Fuller. The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System's innermost planet. In the mission's more than four years of orbital operations, messenger has acquired over 250,000 images and extensive other data sets. messenger's highly successful orbital mission is about to come to an end, as the spacecraft runs out of propellant and the force of solar gravity causes it to impact the surface of Mercury in April 2015.

Earth is the only planet in the Solar System that has a surface split into geological plates. These plates are constantly moving, carried on oceans of rocky mantle no faster than two centimetres each year. 250 million years ago all of the plates on Earth were compressed together in a giant super-continent called Pangaea. Over millions of years this land mass was pulled apart as forces caused the plates to move away from each other.

The earth has not always looked the way it looks today. In other words, the United States one billion years ago was in a totally different location than it is today!! How does this happen? And why does this happen? Let's take a look. In order for us to some understand of how the earth has changed over time, we first need to understand some of the things that took place, and are still taking place, in the earth.

What about the internal structure of the Earth? Our best clues about the interior come from waves that pass through the Earth's material. When earthquakes shake and shatter rock within the Earth, they create seismic waves which travel outward from the location of the quake through the body of the Earth. Seismic waves are disturbances inside the Earth that slightly compress rock or cause it to vibrate up and down. The velocity and characteristics of the waves depend on the type of rock or molten material they traverse.

Studies of seismic waves have revealed two important types of layering in the Earth: chemical and physical. Compositional layering refers to layers of different composition. Physical layering refers to layers of different mechanical properties, such as rigid layers verses "plastic" or fluid layers. 

Compositional layering was the first type of layering recognized. Seismic and other data indicate that the Earth contains a central core of nickel-iron metal. The core is surrounded by a layer of dense rock, called the mantle, that extends most of the way from the core to the surface. Near the surface, the densities of the rocks are typically lower. The crust is a thin outer layer of lower density rock about 3 miles thick under the oceans and about 18.5 miles thick under the continents.

Human beings are late arrivals on planet Earth. Humankind's earliest ancestor —Australopithecus afarensis — appeared over two million years ago. Neanderthals had evolved by 400,000 years ago, and Homo sapiens, modern humans, only existed around 100,000 years ago. Just how short a time this is can be seen when we look at the history of the Earth as a clock, with 12 o’clock midnight being the time that Earth was formed 4.6 billion years ago. Each hour on the clock represents 383 million years.

Millions of years ago, “humans” may have walked on two legs like us, but they were very different from us. They had to hunt and gather food and they had to brave the environment in order to survive. The structure and anatomy of early humans are much different than humans now. Currently, we humans are much lighter than our ancestors. We have large brains with a skull that has high and thin walls. We have thinner jaws and smaller teeth. Our ancestors millions did not have these features, but the features we see now slowly evolved as time passed.

When we think of humans in the past, we need to think of humans that have some of the same general characteristics as us, but they do not look or act like us.

We are still learning about our ancestors, but we guess that the first humans existed between five and seven million years ago: the median time is six million years ago. These humans walked upright on two legs, just like us. Around 90,000 years ago, these humans started making tools to catch fish. Then, around 12,000 years ago, humans began to grow food and change their surroundings in order to survive and eat. As food became more sustainable, and living became easier, humans began to produce more.

As humans developed and grew, their bodies changed. Their brains became bigger, which helped them to develop new tools, including language. They changed the world around them to better survive harsh and changeable weather. Over time, these humans created civilizations and became what we know as humans now.

It may seem like humans have been around for a while, because six million years seems like a long time; in the overall timeline of the Earth, however, six million years is not very long. The Earth itself is 4.5 billion years old. Nonetheless, the six million years humans have been on Earth has allowed them to evolve, build tools, create civilizations, adapt to their environment, and become the humans we are today.

Nobody knows WHAT conditions are needed for life to begin. Some scientists have suggested that living cells may have been brought to Earth by a comet. When the Giotto probe investigated Halley’s Comet in 1986, it found molecules that were similar to living cells. If a comet like this collided with Earth at the right time, then life may have taken hold. Another theory is that powerful lightning bolts flashing through Earth’s early atmosphere may have caused chemical reactions, which created living cells.

One of the first ideas, popularised by biochemist Sidney Fox in the wake of the Miller-Urey experiment, was that amino acids assembled into simple proteins. In modern organisms, proteins perform a huge range of functions, including acting as enzymes that speed up essential chemical reactions. However, this proteins-first hypothesis has largely fallen out of favour.

A much more popular notion is that life began with RNA, a close cousin of DNA, in an “RNA World”. RNA can carry genes and copy itself just like DNA, but it can also fold up and act as an enzyme, just like a protein. The idea was that organisms based solely on RNA arose first, and only later developed DNA and protein.

The RNA World has amassed a lot of supporting evidence, but it is not clear that RNA alone was enough. In recent years, some researchers have suggested that RNA only really reaches its potential when it is paired with proteins – and that both must have existed for life to get started.

A third school of thought is that the first organisms were simple blobs or bubbles. These “protocells” would have resembled modern cells in one key attribute: they acted as containers for all the other components of life. More advanced protocells developed by the Nobel Prize winning biologist Jack Szostak also contain self-replicating RNA.

The final hypothesis is that life began with a series of chemical reactions that extracted energy from the environment and used that energy to build the molecules of life. This “metabolism-first” idea was championed in the late 1980s by Günter Wachtershauser, a German chemist turned patent lawyer. Wachtershauser envisioned a series of chemical reactions taking place on crystals of iron pyrite (“fool’s gold”), a scheme he dubbed the “Iron-Sulphur World”. However, nowadays this idea has been supplanted by Michael Russell’s suggestion that the first life was powered by currents of electrically-charged protons within alkaline vents on the sea bed.

While we cannot know for sure which of these scenarios played out on our planet, successfully creating life from chemicals in the lab would at least tell us which of the proposed mechanisms actually works. 

The ecosphere is a narrow band around the Sun where the temperature is neither too hot nor too cold for life to exist. Earth is the only planet in this zone, and is therefore the only planet in the Solar System able to support life. Mercury and Venus are too close to the Sun for water to exist in liquid form. The remaining planets lie well beyond the ecosphere, where it is too cold for life. The temperature on Pluto can reach as low as —223 °C (-370°F)!

An ecosphere is a planetary closed ecological system. In this global ecosystem, the various forms of energy and matter that constitute a given planet interact on a continual basis. The forces of the four Fundamental interactions cause the various forms of matter to settle into identifiable layers. These layers are referred to as component spheres with the type and extent of each component sphere varying significantly from one particular ecosphere to another. Component spheres that represent a significant portion of an ecosphere are referred to as a primary component spheres. For instance, Earth’s ecosphere consists of five primary component spheres which are the Geosphere, Hydrosphere, Biosphere, Atmosphere, and Magnetosphere.

Like all stars, our Sun will eventually die. In around five billion years its supply of hydrogen will run out, and it will become a red giant, expanding to well over thirty times its current size. As it grows, the Sun will engulf all the inner planets, making them far too hot for life to survive.

There’s nothing we can do to prevent this cataclysm. Yet according to scientists who study the far future, including Yale University astronomer Gregory Laughlin, the prospect for life is, oddly, rather bright. Given technological advances and the continuing evolution of our species, humans should be able to survive — in some form — long after Earth has ceased to exist.

But our distant descendants are going to have to do some planet-hopping. The first major cosmic crisis will strike in about 1.5 billion years. At that point, according to projections by environmental scientist Andrew J. Rushby at the University of East Anglia in England, the brightening sun will set off what might be termed “super-global” warming. Earth will be heated until the oceans boil.

By then, though, will we care? We already have the technology to establish bases on the moon and Mars. So a billion and a half years from now, we’ll likely have colonized the whole solar system — and perhaps other star systems in our Milky Way galaxy.

As the sun grows hotter, other planets will become more appealing. Just as Earth becomes too toasty to sustain life, Mars will reach a temperature that makes it habitable. Cornell University astronomer Lisa Kaltenegger has run models showing that the Red Planet could then stay pleasant for another 5 billion years.

About 7.5 billion years from now, the sun will exhaust its hydrogen fuel and switch to helium. That will cause it to balloon into an enormous red giant. Mars as well as Earth will be fried. On the other hand, the once icy moons of Jupiter and Saturn will have become tropical water worlds — prime real estate for human colonies. We could live there for a few hundred million years.

About 8 billion years from now, the flaring sun will make conditions intolerably hot all the way out past Pluto. “The exact dates depend on how much mass you estimate the sun will lose and how much planets will move,” Kaltenegger says. But the message is clear: Life will be impossible in our solar system.

For much of its early history, Earth was a bubbling, volcanic ball — far too hot to sustain life. Over millions of years, the surface of the planet began to cool and harden, releasing enormous clouds of steam and gas. The moisture in these clouds eventually became rain, forming the seas. Scientists believe that the first life-forms originated in shallow pools of water, where different chemicals were concentrated to form single-celled organisms. These gradually evolved into more complex life-forms. All living creatures on Earth are still evolving.

Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea–floor hydrother­mal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight. Similar environments may be present elsewhere in the solar system.

Under­standing the processes that lead to life, however, is complicated by the actions of biology itself. Earth’s atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed; it has been nearly reconstituted by the bacteria, vegetation, and other life forms that have acted upon it over the eons. Fortunately, the solar system has preserved for us an array of natural laboratories in which we can study life’s raw ingredients — volatiles and organics — as well as their delivery mechanisms and the prebiotic chemical processes that lead to life. We can also find on Earth direct evidence of the interactions of life with its environments, and the dramatic changes that life has undergone as the planet evolved. This can tell us much about the adaptability of life and the prospects that it might survive upheavals on other planets.

Earth is the only place in the Solar System on which scientists have encountered life. Conditions on our planet are perfect for sustaining life — the surface temperature averages around 15°C (59°F) , allowing water to exist in liquid form. Water is a vital ingredient for life, and its presence on Earth has enabled an incredible variety of creatures to live on every part of the planet. Also, Earth is large enough to contain a protective atmosphere, but not big enough to become a suffocating gas planet like Jupiter or Saturn.

Although the exact process by which life formed on Earth is not well understood, the origin of life requires the presence of carbon-based molecules, liquid water and an energy source. Because some Near-Earth Objects contain carbon-based molecules and water ice, collisions of these objects with Earth have significant agents of biologic as well as geologic change.

For the first billion years of Earth’s existence, the formation of life was prevented by a fusillade of comet and asteroid impacts that rendered the Earth’s surface too hot to allow the existence of sufficient quantities of water and carbon-based molecules. Life on Earth began at the end of this period called the late heavy bombardment, some 3.8 billion years ago. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier - just at the end of the period of late heavy bombardment. So the window when life began was very short. As soon as life could have formed on our planet, it did. But if life formed so quickly on Earth and there was little in the way of water and carbon-based molecules on the Earth’s surface, then how were these building blocks of life delivered to the Earth’s surface so quickly? The answer may involve the collision of comets and asteroids with the Earth, since these objects contain abundant supplies of both water and carbon-based molecules.

Once the early rain of comets and asteroids upon the Earth subsided somewhat, subsequent impacts may well have delivered the water and carbon-based molecules to the Earth’s surface - thus providing the building blocks of life itself. It seems possible that the origin of life on the Earth’s surface could have been first prevented by an enormous flux of impacting comets and asteroids, then a much less intense rain of comets may have deposited the very materials that allowed life to form some 3.5 - 3.8 billion years ago.

Comets have this peculiar duality whereby they first brought the building blocks of life to Earth some 3.8 billion years ago and subsequent commentary collisions may have wiped out many of the developing life forms, allowing only the most adaptable species to evolve further. It now seems likely that a comet or asteroid struck near the Yucatan peninsula in Mexico some 65 million years ago and caused a massive extinction of more than 75% of the Earth’s living organisms, including the dinosaurs. At the time, the mammals were small burrowing creatures that seemed to survive the catastrophic impact without too much difficulty. Because many of their larger competitors were destroyed, these mammals flourished. Since we humans evolved from these primitive mammals, we may owe our current preeminence atop Earth’s food chain to collisions of comets and asteroids with the Earth.

When the planets were forming, they were in a molten state. In any object, gravity pulls from the centre, and parts of the object at the same distance from the centre are pulled inward with equal force, creating a sphere. This will only happen to objects with sufficient mass, such as planets and stars. Smaller objects, such as asteroids, have a weaker gravitational force, so they cannot pull themselves into a spherical shape. Gravity is also responsible for denser materials being pulled to the centre of a star or planet.

Planets are round because of its gravitational field. As a planet gets massive enough, internal heating takes over and the planet behaves like a fluid. Gravity then pulls all of the material towards the center of mass (or core). Because all points on the surface of a sphere are an equal distance from the center of mass, planets eventually settle on a spherical shape. For major planets, one of the requirements is that it’s large enough for its gravity to pull it into a sphere. Though, even for small asteroids and such, it’s not uncommon for these bodies to be “roundish” (though, they are often oval shaped).

It is interesting to note though that, because planets rotate, they aren’t perfect spheres and actually bulge out at the equator.

In the case of a cube, the corners are further away from the center of mass than the rest of the cube. Especially for objects as massive as a planet or a star, the corners would collapse under their own weight and the object would take on a spherical shape. As cool as a cubical planet would be, they simply can’t exist. Well, correction, a cubic planet could probably be engineered by a civilization bent on assimilating all life in the galaxy, but the point is a square planet won’t form without outside help.

All of the planets in the Solar System formed from the same cloud of debris. The inner planets have solid cores of iron, surrounded by rocky mantles, topped with a very thin silicate crust. The Gas Giants have solid cores of rock and ice, but these are much smaller in proportion to those of the inner planets. Jupiter and Saturn are made of hydrogen and helium, which becomes denser towards their centres. Uranus and Neptune both have mantles of icy water, methane and ammonia.

Astronomers think the giants first formed as rocky and icy planets similar to terrestrial planets. However, the size of the cores allowed these planets (particularly Jupiter and Saturn) to grab hydrogen and helium out of the gas cloud from which the sun was condensing, before the sun formed and blew most of the gas away. 

Since Uranus and Neptune are smaller and have bigger orbits, it was harder for them to collect hydrogen and helium as efficiently as Jupiter and Saturn. This likely explains why they are smaller than those two planets. On a percentage basis, their atmospheres are more "polluted" with heavier elements such as methane and ammonia because they are so much smaller.

Scientists have discovered thousands of exoplanets. Many of these happen to be "hot Jupiters," or massive gas giants that are extremely close to their parent stars. (Rocky worlds are more abundant in the universe, according to estimates from Kepler.) Scientists speculate that large planets may have moved back and forth in their orbits before settling into their current configuration. But how much they moved is still a subject of debate.

There are dozens of moons around the giant planets. Many formed at the same time as their parent planets, which is implied if the planets rotate in the same direction as the planet close to the equator (such as the huge Jovian moons Io, Europa, Ganymede and Callisto.) But there are exceptions. 

One moon of Neptune, Triton, orbits the planet opposite to the direction Neptune spins — implying that Triton was captured, perhaps by Neptune's once larger atmosphere, as it passed by. And there are many tiny moons in the solar system that rotate far from the equator of their planets, implying that they were also snagged by the immense gravitational pull.

Our solar system formed from the force of an exploding star. When some stars reach the end of their lives, they can explode into a supernova, sending shockwaves of energy deep into space. Roughly 4.5 billion years ago, a shock-wave from a supernova, travelling at 30 million kilometres (19 million miles) per hour, hit a cloud of ice, dust and gas. The force of the impact caused the cloud to flatten and rotate. From this spinning disc, our Solar System began to form.

The most widely accepted scientific explanation for the formation of the Solar System is called the Solar Nebular Model. According to this model, the entire Solar System formed around 4.5 billion years ago from the gravitational collapse of a small fraction of a giant molecular cloud, also known as a nebula. 

A disturbance, most likely a nearby supernova, caused a giant cloud of gas and dust floating in space to contract and begin to collapse on itself. Most of the gas collected in the center to form a gaseous sphere that would eventually become the Sun. As more gas was drawn inward by the force of gravity, friction and pressure caused this sphere, called a protostar, to become hot and start to glow. 

As the nebula continued to contract, conservation of angular momentum caused it to spin faster. It flattened out into a protoplanetary disk, with the hot, dense protostar in the center. Over millions of years, all eight planets formed by accretion from this disk. In other words, gravity pulled the disk into many clumps of gas and dust. These clumps stuck together and grew larger and larger, turning into planetesimals. The planetesimals further coalesced to eventually form planets, with comets and asteroids being the leftovers. Gravitational interaction with the planets caused them to be grouped into distinct regions such as the asteroid belt and Kuiper belt. 

Due to their higher boiling points only metals and silicates could exist in the warm inner solar system, and these would form the rocky planets of Mercury, Venus, Earth, and Mars. Since metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. It is thought that as many as 100 small protoplanets used to exist in the inner solar system, but they eventually collided and merged to create the four inner planets we know today. 

The gas giants (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line where icy compounds can remain solid. The gas and ice that formed the Jovian planets was more abundant within the protoplanetary disk, allowing them to become massive enough to gain large atmospheres of hydrogen and helium and grow to mammoth proportions. Uranus and Neptune are thought to have formed closer to the Sun, and then migrated out to their current orbits. 

Throughout all this, the infant Sun continued to grow hotter. Once the temperature and pressure at the core was high enough, thermonuclear fusion of hydrogen began, and the Sun became a fully-fledged main-sequence star. Solar wind swept away the remaining gas and dust leftover from the protoplanetary disk into interstellar space, ending the growth of the planets. This entire process of solar system formation happened within several hundred million years and was finished by around 4.5 billion years ago. 

Planets and moons are just a few of the objects orbiting the Sun. Astronomers already know of thousands of large rocky bodies called asteroids (shown right), and icy objects called comets. Millions of smaller rocks, called meteoroids, also orbit the Sun.

More than 150 moons orbit worlds in our solar system. Known as natural satellites, they orbit planets, dwarf planets, asteroids, and other debris. Among the planets, moons are more common in the outer reaches of the solar system. Mercury and Venus are moon-free, Mars has two small moons, and Earth has just one. Meanwhile, Jupiter and Saturn have dozens, and Uranus and Neptune each have more than 10. Even though it’s relatively small, Pluto has five moons, one of which is so close to Pluto in size that some astronomers argue Pluto and this moon, Charon, are a binary system.

Too small to be called planets, asteroids are rocky chunks that also orbit our sun along with the space rocks known as meteoroids. Tens of thousands of asteroids are gathered in the belt that lies between the orbits of Mars and Jupiter. Comets, on the other hand, live inside the Kuiper Belt and even farther out in our solar system in a distant region called the Oort cloud.

The solar system is enveloped by a huge bubble called the heliosphere. Made of charged particles generated by the sun, the heliosphere shields planets and other objects from high-speed interstellar particles known as cosmic rays. Within the heliosphere, some of the planets are wrapped in their own bubbles—called magnetospheres—that protect them from the most harmful forms of solar radiation. Earth has a very strong magnetosphere, while Mars and Venus have none at all.

Most of the major planets also have atmospheres. Earth’s is composed mainly of nitrogen and oxygen—key for sustaining life. The atmospheres on terrestrial Venus and Mars are mostly carbon dioxide, while the thick atmospheres of Jupiter, Saturn, Uranus, and Neptune are made primarily of hydrogen and helium. Mercury doesn’t have an atmosphere at all. Instead scientists refer to its extremely thin covering of oxygen, hydrogen, sodium, helium, and potassium as an exosphere.

Moons can have atmospheres, too, but Saturn’s largest moon, Titan, is the only one known to have a thick atmosphere, which is made mostly of nitrogen.