MUST READ : Order Of the Planets From The Sun

Order Of the Planets From The Sun:

Planets and other objects in our Solar System. Credit: NASA.

Remembering the order of the planets can be a tricky task. With eight celestial bodies, and all the names taken from classical nomenclature, getting them mixed up is a common mistake. First the quick facts: Our Solar System has eight “official” planets which orbit the Sun. Here are the planets listed in order of their distance from the Sun:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. An easy mnemonic for remembering the order is “My Very Educated Mother Just Served Us Noodles.”

If you add in the dwarf planets, Ceres is located in the asteroid belt between Mars and Jupiter, while the remaining dwarf planets are in the outer Solar System and in order from the Sun are Pluto, Haumea, Makemake, and Eris. There is, as yet, a bit of indecision about the Trans-Neptunian Objects known as Orcus, Quaoar, 2007 O10, and Sedna and their inclusion in the dwarf planet category.

A mnemonic for this list would be “My Very Educated Mother Could Just Serve Us Noodles, Pie, Ham, Muffins, and Eggs” (and Steak, if Sedna is included.) Now, let’s look at a few details including the definition of a planet and a dwarf planet, as well as details about each of the planets in our Solar System.

Artistic impression of the Solar System, with all known terrestrial planets, as giants, and dwarf planets. Credit: NASA

What is a planet?
In 2006, the International Astronomical Union (IAU) decided on the definition of a planet. The definition states that in our Solar System, a planet is a celestial body which:

• is in orbit around the Sun,
• has sufficient mass to assume hydrostatic equilibrium (a nearly round shape),
• has “cleared the neighborhood” around its orbit.
• is not a moon.

This means that Pluto, which was considered to be the farthest planet since its discovery in 1930, now is classified as a dwarf planet. The change in the definition came after the discovery three bodies that were all similar to Pluto in terms of size and orbit, (Quaoar in 2002, Sedna in 2003, and Eris in 2005).

With advances in equipment and techniques, astronomers knew that more objects like Pluto would very likely be discovered, and so the number of planets in our Solar System would start growing quickly. It soon became clear that either they all had to be called planets or Pluto and bodies like it would have to be reclassified.



With much controversy then and since, Pluto was reclassified as a dwarf planet in 2006. This also reclassified the asteroid Ceres as a dwarf planet, too, and so the first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens more dwarf planets awaiting discovery.

Later, in 2008, the IAU announced the subcategory of dwarf planets with trans-Neptunian orbits would be known as “plutoids.” Said the IAU, “Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit.”

This subcategory includes Ceres, Pluto, Haumea, Makemake, and Eris.

The Planets in our Solar System:
Having covered the basics of definition and classification, let’s get talking about those celestial bodies in our Solar System that are still classified as planets (sorry Pluto!). Here is a brief look at the eight planets in our Solar System. Included are quick facts and links so you can find out more about each planet.

Mercury:Mercury is the closest planet to our Sun, at just 58 million km (36 million miles) or 0.39 Astronomical Unit (AU) out. But despite its reputation for being sun-baked and molten, it is not the hottest planet in our Solar System (scroll down to find out who that dubious honor goes go!)

Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is also the smallest planet in our Solar System, and is also smaller than its largest moon (Ganymede, which orbits Jupiter). And being equivalent in size to 0.38 Earths, it is just slightly larger than the Earth’s own Moon. But this may have something to do with its incredible density, being composed primarily of rock and iron ore. Here are the planetary facts:

• Diameter: 4,879 km (3,032 miles)
• Mass: 3.3011 x 10²³ kg (0.055 Earths)
• Length of Year (Orbit): 87.97 Earth days
• Length of Day: 59 Earth days.
• Mercury is a rocky planet, one of the four “terrestrial planets” in our Solar System. Mercury has a solid, cratered surface, and looks much like Earth’s moon.
• If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mercury.
• Mercury does not have any moons.
• Temperatures on Mercury range between -173 to 427 degrees Celcius (-279 to 801 degrees Fahrenheit)
• Just two spacecraft have visited Mercury: Mariner 10 in 1974-75 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011 and ended its mission by impacting the surface of Mercury on April 30, 2015. MESSENGER has changed our understanding of this planet, and scientists are still studying the data.
• Find more details about Mercury at this article on Universe Today, and this page from NASA.

Venus:
Venus is the second closest planet to our Sun, orbiting at an average distance of 108 million km (67 million miles) or 0.72 AU. Venus is often called Earth’s “sister planet,” as it is just a little smaller than Earth. Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume. The surface gravity, which is 8.87 m/s², is equivalent to 0.904 g – roughly 90% of the Earth standard.

A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL

And due to its thick atmosphere and proximity to the Sun, it is the Solar Systems hottest planet, with temperatures reaching up to a scorching 735 K (462 °C). To put that in perspective, that’s over four and a half times the amount of heat needed to evaporate water, and about twice as much needed to turn tin into molten metal (231.9 °C)!

• Diameter: 7,521 miles (12,104 km)
• Mass: 4.867 x 10²⁴ kg (0.815 Earth mass)
• Length of Year (Orbit): 225 days
• Length of day: 243 Earth days
• Surface temperature: 462 degrees C (864 degrees F)
• Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
• Venus has no moons.
• Venus spins backwards (retrograde rotation), compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
• If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Venus.
• Venus is also known and the “morning star” or “evening star” because it is often brighter than any other object in the sky and is usually seen either at dawn or at dusk. Since it is so bright, it has often been mistaken for a UFO!
• More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface. Find out more about all the missions here.
• Find out more about Venus on this article from Universe Today, and this page from NASA.

Earth:Our home, and the only planet in our Solar System (that we know of) that actively supports life. Our planet is the third from the our Sun, orbiting it at an average distance of 150 million km (93 million miles) from the Sun, or one AU. Given the fact that Earth is where we originated, and has all the necessary prerequisites for supporting life, it should come as no surprise that it is the metric on which all others planets are judged.

Earth, pictured by the crew of the Apollo 17 mission. Credit: NASA

Whether it is gravity (g), distance (measured in AUs), diameter, mass, density or volume, the units are either expressed in terms of Earth’s own values (with Earth having a value of 1) or in terms of equivalencies – i.e. 0.89 times the size of Earth. Here’s a rundown of the kinds of

• Diameter: 12,760 km (7,926 miles)
• Mass: 5.97 x 10²⁴ kg
• Length of Year (Orbit): 365 days
• Length of day: 24 hours (more precisely, 23 hours, 56 minutes and 4 seconds.)
• Surface temperature: Average is about 14 C, (57 F), with ranges from -88 to 58 (min/max) C (-126 to 136 F).
• Earth is another terrestrial planet with an ever-changing surface, and 70 percent of the Earth’s surface is covered in oceans.
• Earth has one moon.
• Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% various other gases.
• Earth is the only world known to harbor life.
• Find out more about Earth at a series of articles found here on Universe Today, and on this webpage from NASA.

Mars:Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU. It is also known as “the Red Planet” because of its reddish hue, which is due to the prevalence of iron oxide on its surface. In many ways, Mars is similar to Earth, which can be seen from its similar rotational period and tilt, which in turn produce seasonal cycles that are comparable to our own.

Global image of the planet Mars. Credit: NASA

The same holds true for surface features. Like Earth, Mars has many familiar surface features, which include volcanoes, valleys, deserts, and polar ice caps. But beyond these, Mars and Earth have little in common. The Martian atmosphere is too thin and the planet too far from our Sun to sustain warm temperatures, which average 210 K (-63 ºC) and fluctuate considerably.

• Diameter: 6,787 km, (4,217 miles)
• Mass: 6.4171 x 10²³ kg (0.107 Earths)
• Length of Year (Orbit): 687 Earth days.
• Length of day: 24 hours 37 minutes.
• Surface temperature: Average is about -55 C (-67 F), with ranges of -153 to +20 °C (-225 to +70 °F)
• Mars is the fourth terrestrial planet in our Solar System. Its rocky surface has been altered by volcanoes, impacts, and atmospheric effects such as dust storms.
• Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mars.
• Mars has two small moons, Phobos and Deimos.
• Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil to look red.
• More than 40 spacecraft have been launched to Mars. You can find out more about missions to Mars here.Find out more about Mars at this series of articles on Universe Today, and at this NASA webpage.

Jupiter:Jupiter is the fifth planet from the Sun, at a distance of about 778 million km (484 million miles) or 5.2 AU. Jupiter is also the most massive planet in our Solar System, being 317 times the mass of Earth, and two and half times larger than all the other planets combined. It is a gas giant, meaning that it is primarily composed of hydrogen and helium, with swirling clouds and other trace gases.

Io and Jupiter as seen by New Horizons during its 2008 flyby. (Credit: NASA/Johns Hopkins University APL/SWRI).

Jupiter’s atmosphere is the most intense in the Solar System. In fact, the combination of incredibly high pressure and coriolis forces produces the most violent storms ever witnessed. Wind speeds of 100 m/s (360 km/h) are common and can reach as high as 620 km/h (385 mph). In addition, Jupiter experiences auroras that are both more intense than Earth’s, and which never stop.

• Diameter: 428,400 km (88,730 miles)
• Mass: 1.8986 × 10²⁷ kg (317.8 Earths)
• Length of Year (Orbit): 11.9 Earth years
• Length of day: 9.8 Earth hours
• Temperature: -148 C, (-234 F)
• Jupiter has 67 known moons, with an additional 17 moons awaiting confirmation of their discovery – for a total of 67 moons. Jupiter is almost like a mini solar system!
• Jupiter has a faint ring system, discovered in 1979 by the Voyager 1 mission.
• If you weigh 45 kg (100 pounds) on Earth, you would weigh 115 kg (253) pounds on Jupiter.
• Jupiter’s Great Red Spot is a gigantic storm (bigger than Earth) that has been raging for hundreds of years. However, it appears to be shrinking in recent years.
• Many missions have visited Jupiter and its system of moons, with the latest being the Juno mission will arrive at Jupiter in 2016. You can find out more about missions to Jupiter here.
• Find out more about Jupiter at this series of articles on Universe Today and on this webpage from NASA.

Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)

Saturn:Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU. Like Jupiter, it is a gas giant, with layers of gaseous material surrounding a solid core. Saturn is most famous and most easily recognized for its spectacular ring system, which is made of seven rings with several gaps and divisions between them.

• Diameter: 120,500 km (74,900 miles)
• Mass: 5.6836 x 10²⁶ kg (95.159 Earths)
• Length of Year (Orbit): 29.5 Earth years
• Length of day: 10.7 Earth hours
• Temperature: -178 C (-288 F)
• Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
• If you weigh 45 kg (100 pounds) on Earth, you would weigh about 48 kg (107 pounds) on Saturn
• Saturn has 53 known moons with an additional 9 moons awaiting confirmation.
• Five missions have gone to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings. You can out more about missions to Saturn here.
• Find out more about Saturn at this series of articles on Universe Today and at this webpage from NASA.

Uranus:Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU. Though it is classified as a “gas giant”, it is often referred to as an “ice giant” as well, owing to the presence of ammonia, methane, water and hydrocarbons in ice form. The presence of methane ice is also what gives it its bluish appearance.

Uranus as seen by NASA’s Voyager 2 space probe. Credit: NASA/JPL

Uranus is also the coldest planet in our Solar System, making the term “ice” seem very appropriate! What’s more, its system of moons experience a very odd seasonal cycle, owing to the fact that they orbit Neptune’s equator, and Neptune orbits with its north pole facing directly towards the Sun. This causes all of its moons to experience 42 year periods of day and night.

• Diameter: 51,120 km (31,763 miles)
• Mass:
• Length of Year (Orbit): 84 Earth years
• Length of day: 18 Earth hours
• Temperature: -216 C (-357 F)
• Most of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.
• Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4). The methane gives Uranus a blue-green tint.
• If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Uranus.
• Uranus has 27 moons.
• Uranus has faint rings; the inner rings are narrow and dark and the outer rings are brightly colored.
• Voyager 2 is the only spacecraft to have visited Uranus. Find out more about this mission here.
• You can find out more about Uranus at this series of articles on Universe Today and this webpage from NASA.

Neptune: Neptune is the eighth and farthest planet from the Sun, at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU. Like Jupiter, Saturn and Uranus, it is technically a gas giant, though it is more properly classified as an “ice giant” with Uranus.

Neptune photographed by the Voyager 2 space probe. Credit: NASA/JPL

Due to its extreme distance from our Sun, Neptune cannot be seen with the naked eye, and only one mission has ever flown close enough to get detailed images of it. Nevertheless, what we know about it indicates that it is similar in many respects to Uranus, consisting of gases, ices, methane ice (which gives its color), and has a series of moons and faint rings.

• Diameter: 49,530 km (30,775 miles)
• Mass: 1.0243 x 10²⁶ kg (17 Earths)
• Length of Year (Orbit): 165 Earth years
• Length of day: 16 Earth hours
• Temperature: -214 C (-353 F)
• Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
• Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
• Neptune has 13 confirmed moons and 1 more awaiting official confirmation.
• Neptune has six rings.
• If you weigh 45 kg (100 pounds) on Earth, you would weigh 52 kg (114 pounds) on Neptune.
  Neptune was the first planet to be predicted to exist by using math.
• Voyager 2 is the only spacecraft to have visited Neptune. You can find out more about this mission here.
• Find out more about Neptune at this series of articles on Universe Today and this NASA webpage. We have written many articles about the planets for Universe Today. Here are some facts about planets, and here’s an article about the names of the planets.If you’d like more info on the Solar System planets, dwarf planets, asteroids and more, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.Venus is the second planet from the Sun, and it is the hottest planet in the Solar System due to its thick, toxic atmosphere which has been described as having a “runaway greenhouse effect” on the planet.

Now you know! And if you find yourself unable to remember all the planets in their proper order, just repeat the words, “My Very Educated Mother Just Served Us Noodles.” Of course, the Pie, Ham, Muffins and Eggs are optional, as are any additional courses that might be added in the coming years!

We have many great articles on the Solar System and the planets here at Universe Today. Here is a rundown of the Inner Planets, the Outer Planets, a description of Terrestrial Planets, the Dwarf Planets, and Why Pluto is no Longer a Planet?.

Astronomy Cast also has some cool episodes about the Solar System. Here’s Episode 68: Pluto and the Icy Outer Planets, Episode 306: Accretion Discs, and Episode 159: Planet X.

About Matt Williams

Matt Williams is the Curator of the Guide to Space for Universe Today, a a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC.

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How Long is a Day on Pluto?

How Long is a Day on Pluto?:

On approach in July 2015, the cameras on NASA’s New Horizons spacecraft captured Pluto rotating over the course of a full “Pluto day.” The best available images of each side of Pluto taken during approach have been combined to create this view of a full rotation. Credit: NASA/JHUAPL/SwRI.

Pluto takes 6.4 Earth days (6 days 9 hours and 36 minutes) to complete one rotation, so this is how long a day is on Pluto.

When the New Horizons spacecraft flew by Pluto and its moons in July of 2015, it took hundreds of images. The montage above shows Pluto rotating over the course of a full day. It provides our first close-up look at what a day on Pluto might be like.


What Makes a Day?

To clarify, one day on any planet is the time it takes for the planet to completely spin around and make one full rotation about its axis. Here on Earth that takes 24 hours, but each planet has a different rotational speed. Since Pluto rotates more slowly than Earth, its day is longer.

This artist’s concept of the frosty surface of Pluto with Charon and our sun as backdrops illustrates that while sunlight is much weaker than it is here on Earth, it isn’t as dark as you might expect. In fact, you could read a book on the surface of Pluto. Credit: NASA.

What is a Day on Pluto Like?

Since Pluto is so much farther from the Sun, the amount of sunlight that reaches Pluto is much less that what we receive on Earth. It has been estimated that the Sun would appear about 1,000 times dimmer than it appears on Earth. NASA has said that instead of a big yellow disc, the Sun would look more like other stars, although the Sun would be the brightest object in the sky.

However, it isn’t completely dark on Pluto. Since Pluto has a thin atmosphere, that atmosphere would scatter the light, but probably not enough to make a bright sky like we see on Earth or Mars. NASA says that at a certain time near dawn and dusk each day, the illumination on Earth matches that of high noon on Pluto. NASA has a “Pluto Time” website where you can plug in your location and find out what time of day you could experience the same amount of light (on a clear day) that Pluto is receiving.

A graphic depicting the Pluto system’s orbital orientation. Credit: NASA.

However, seasonal variations of daylight on Pluto can be extreme. Pluto’s year is 248 Earth years long, and so the seasons are very long. Plus, compared to most of the planets and their moons, the whole Pluto-Charon system is tipped on its side. Therefore, Pluto rotates on its “side” in its orbital plane, with an axial tilt of 122 degrees – very similar to the “sideways” planet Uranus. So at its solstices, one-fourth of Pluto’s surface is in continuous daylight, while another fourth is in continuous darkness.

Take a look at the Solar System from above, and you can see that the planets make nice circular orbits around the Sun. But dwarf planet’s Pluto’s orbit is very different. It’s highly elliptical, traveling around the Sun in a squashed circle. And Pluto’s orbit is highly inclined, traveling at an angle of 17-degrees. This strange orbit gives Pluto some unusual characteristics, sometimes bringing it within the orbit of Neptune. Credit: NASA

Also, Pluto travels around the Sun in a very elliptical orbit. At its closest point, or perihelion, Pluto gets as close as 4.4 billion km from the Sun. At its most distant point, or aphelion, Pluto is 7.4 billion km from the Sun. Therefore, the amount of sunlight varies throughout Pluto’s long year depending on how close or far it is to the Sun.

A portrait from the final approach of the New Horizons spacecraft to the Pluto system on July 11, 2015. Pluto and Charon display striking color and brightness contrast in this composite image. Credit: NASA-JHUAPL-SWRI.

One interesting note is that Pluto and Charon are a binary planet system, and the two worlds are in orbit around each other. Also, Pluto’s moon Charon is tidally locked around Pluto. This means that Charon takes 6 days and 9 hours to orbit around Pluto – the same amount of time it takes for a day on Pluto. This means that Charon is always at the same place in the sky when seen from Pluto.

You would have the same view from Charon as well. From some vantage points on Charon, Pluto would always hang at the same spot in the sky, and for other parts, you wouldn’t be able to see Pluto at all.

New Horizons also captured a full day rotation for Charon, too, which you can see below:

On approach to the Pluto system in July 2015, the cameras on NASA’s New Horizons spacecraft captured images of the largest of Pluto’s five moons, Charon, rotating over the course of a full day. The best currently available images of each side of Charon taken during approach have been combined to create this view of a full rotation of the moon. Credit: NASA/JHUAPL/SwRI.

The images used in the Pluto and Charon “day” montages were taken by the Long Range Reconnaissance Imager (LORRI) and the Ralph/Multispectral Visible Imaging Camera as the New Horizons spacecraft zoomed toward the Pluto system, and in the various images the distance between New Horizons and Pluto decreased from 5 million miles (8 million kilometers) on July 7 to 400,000 miles (about 645,000 kilometers) on July 13, 2015. You can read more about these images here on Universe Today, and here on the New Horizons website.

About Nancy Atkinson

Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

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Pre-Order “Treasures of the Universe” Astrophotography Book Through Kickstarter

Pre-Order “Treasures of the Universe” Astrophotography Book Through Kickstarter:

Treasures of the Universe by André van der Hoeven

We’ve featured the photography of André van der Hoeven here many times, and all of his photos are wonderful. Well, now you can get them all in one big book, titled Treasures of the Universe.


This 150+ page book contains photos of most of the major objects in the Solar System as well as deep sky objects, like galaxies, star clusters and nebulae. van der Hoeven provides many of the pictures in the book, and then fills out the rest with the highest quality photos from the Hubble Space Telescope, Spitzer, Subaru and many of the top observatories around the world. There are also great photos from rovers and spacecraft sent to distant worlds (including the latest pictures of Pluto from New Horizons). If you want a coffee table book with great images of space, it’s a great choice.

Treasures of the Universe by André van der Hoeven

The book is currently being run as a Kickstarter, but unlike most campaigns, this book is complete and ready to go to the printers, so you’re really just deciding if you want a copy or not – a printed, signed copy or an electronic PDF.

At the time I’m writing this, there are just 5 days left in the Kickstarter, which is already fully funded. This project is already happening, but you can help André reach the stretch goal of 25,000 Euros.

The Kickstarter ends on Monday, November 30th at 3:00pm Pacific Time.

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Watch SETI-Seeking Radio Dishes Dance Across the Universe

Watch SETI-Seeking Radio Dishes Dance Across the Universe:

A radio dish at Owens Valley Observatory in Owens Valley California. Credit and copyright: Credit and copyright: Harun Mehmedinovic and Gavin Heffernan.

Radio dishes always evoke wonder, as these giants search for invisible (to our eyes, anyway) radio signals from objects like distant quasars, pulsars, masers and more, including potential signals from extraterrestrials. This new timelapse from Harun Mehmedinovic and Gavin Heffernan of Sunchaser Pictures was shot at several different radio astronomy facilities — the Very Large Array (VLA) Observatory in New Mexico, Owens Valley Observatory in Owens Valley California, and Green Bank Observatory in West Virginia. All three of these facilities have been or are still being partly used by the SETI (Search for the Extraterrestrial Intelligence) program.

Watch the dishes dance in their search across the Universe!


The huge meteorite streaking across the sky above Very Large Array (2:40) is from the Aquarids meteor shower. The large radio telescope at Green Bank is where scientists first attempted to “listen” to presence of extraterrestrials in the galaxy. The Very Large Array was featured in the movie CONTACT (1997) while Owens Observatory was featured in THE ARRIVAL (1996).

This video was created for SkyGlowProject.com, a crowdfunded educational project that explores the effects and dangers of urban light pollution in contrast with some of the most incredible Dark Sky Preserves in North America.

The music is by Tom Boddy, and titled “Thoughtful Reflections.”

Thanks to Gavin Heffernan for sharing this video.

Screenshot from the DishDance timelapse. Credit and copyright: Harun Mehmedinovic and Gavin Heffernan.

SKYGLOW: DISHDANCE from Sunchaser Pictures on Vimeo.

About Nancy Atkinson

Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

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The Solar Heliospheric Observatory at 20

The Solar Heliospheric Observatory at 20:

Solar cycle #23 as seen via SOHO, and an artist’s conception of the observatory in space. All images credit of NASA/ESA/SOHO

Flashback to 1995: Clinton was in the White House, Star Trek Voyager premiered, we all carried pagers in the pre-mobile phone era, and Windows 95 and the Internet itself was shiny and new to most of us. It was also on this day in late 1995 when our premier eyes on the Sun—The SOlar Heliospheric Observatory (SOHO)—was launched. A joint mission between NASA and the European Space Agency, SOHO lit up the pre-dawn sky over the Florida Space Coast as it headed space-ward atop an Atlas IIAS rocket at 3:08 AM EST from launch complex 39B at Cape Canaveral Air Force Station.



Envisioning SOHO

SOHO on Earth

There aren’t a whole lot of 20^th century spacecraft still in operation; SOHO joins the ranks of Hubble and the twin Voyager spacecraft as platforms from another era that have long exceeded their operational lives. Seriously, think back to what YOU were doing in 1995, and what sort of technology graced your desktop. Heck, just thinking of how many iterations of mobile phones spanned the last 20 years is a bit mind-bending. A generation of solar astronomers have grown up with SOHO, and the space-based observatory has consistently came through for researchers and scientists, delivering more bang for the buck.

“SOHO has been truly extraordinary and revolutionary in countless ways,” says  astrophysicist Karl Battams at the Naval Research Laboratory in Washington D.C. “SOHO has completely changed our way of thinking about the Sun, solar active regions, eruptive events, and so much more. I honestly can’t think of a more broadly influential space mission than SOHO.”



SOHO has monitored the Sun now for the complete solar cycle #23 and well into the ongoing solar cycle #24. SOHO is a veritable Swiss Army Knife for solar astrophysics, not only monitoring the Sun across optical and ultraviolet wavelengths, but also employing the Michelson Doppler Imager to record magnetogram data and the Large Angle Spectrometric Coronograph (LASCO) able to create an artificial solar eclipse and monitor the pearly white corona of the Sun.

Peering into the solar interior.

SOHO observes the Sun from its perch one million miles sunward located at the L1 Sun-Earth point. It actually circles this point in space in what is known as a lissajous, or ‘halo’ orbit.

SOHO has revolutionized solar physics and the way we perceive our host star. We nearly lost SOHO early on in its career in 1998, when gyroscope failures caused the spacecraft to lose a lock on the Sun, sending it into a lazy one revolution per minute spin. Quick thinking by engineers led to SOHO using its reaction wheels as a virtual gyroscope, the first spacecraft to do so. SOHO has used this ad hoc method to point sunward ever since. SOHO was also on hand to document the 2003 Halloween flares, the demise of comet ISON on U.S. Thanksgiving Day 2013, and the deep and strangely profound solar minimum that marked the transition from solar cycle 23 to 24.

What was your favorite SOHO moment?

A massive sunspot witnessed by SOHO in 2000, compared to the Earth.

SOHO is also a champion comet hunter, recently topping an amazing 3000 comets and counting. Though it wasn’t designed to hunt for sungrazers, SOHO routinely sees ’em via its LASCO C2 and C3 cameras, as well as planets and background stars near the Sun. The effort to hunt for sungrazing comets crossing the field of view of SOHO’s LASCO C3 and C2 cameras represents one of the earliest crowd-sourced efforts to do volunteer science online. SOHO has discovered enough comets to characterize and classify the Kreutz family of sungrazers, and much of this effort is volunteer-based. SOHO grew up with the internet, and the images and data made publicly available are an invaluable resource that we now often take for granted.

A ‘neat’ image…  Comet NEAT photobombs the view of SOHO’s LASCO C3 camera.

NASA/ESA has extended SOHO’s current mission out to the end of 2016. With any luck, SOHO will complete solar cycle 24, and take us into cycle 25 to boot.

“Right now, it (SOHO) is operating in a minimally funded mode, with the bulk of its telemetry dedicated solely to the LASCO coronagraph,” Battams told Universe Today. “Many of its instruments have now been superseded by instruments on other missions. As of today it remains healthy, and I think that’s a testament to the amazing collaboration between ESA and NASA. Together, they’ve kept a spacecraft designed for a two-year mission operating for twenty years.”

Today, missions such as the Solar Dynamics Observatory, Hinode, and Proba-2 have joined SOHO in watching the Sun around the clock. The solar occulting disk capabilities of SOHO’s LASCO C2 and C3 camera remains unique, though ESA’s Proba-3 mission launching in 2018 will feature a free-flying solar occulting disk.

Happy 20^th SOHO… you’ve taught us lots about our often tempestuous host star.

-It’s also not too late to vote for your favorite SOHO image.

About David Dickinson

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe from Tampa Bay, Florida.

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Astro-Challenge: Watch the Moon Occult Venus in the Daytime

Astro-Challenge: Watch the Moon Occult Venus in the Daytime:

The Moon meets Venus on February 26th, 2014. Image credit and copyright: Konstantinos Spanos

The year 2015 saved one of the best astronomical events for last, as the waning crescent Moon occults (passes in front of) the planet Venus as seen from North America on Monday, December 7^th.

This is the final of seven naked eye occultations of planets by the Moon in 2015, three of which involve Venus. It’s also the best of the year, well positioned for North America. The event will transpire in daytime skies for a majority of the continent, but don’t despair; Venus is easily visible to the unaided eye near the limb of the daytime Moon. Lucky residents in southern Alaska will get to see the event occur under twilight dawn skies before sunrise, while western North America will have a favorable view after sunrise, with the Moon and Venus high in the sky. Eastern North America will have the toughest challenge, with the pair sandwiched between the high noonday Sun and the western horizon.



On the morning of December 7^th, the Moon is a 13% illuminated waning crescent and just four days from New. Venus shines at magnitude -4.2, is 69% illuminated, and 16” in size. The event occurs 42 degrees west of the Sun, and Venus will ingress along the Moon’s daytime side and emerge along its dark nighttime limb.

The viewing area for the December 7th occultation of Venus by the Moon. Solid lines denote the region within which the event occurs before sunrise. Image credit: The IOTA/Occult 4.2

The International Occultation and Timing Association (IOTA) maintains a great page including event times for northwestern North America. Here’s a selection of events for daytime sites:

Los Angeles, CA: (Moon elevation 44 degrees)

Ingress: 8:06 PST

Egress: 9:54 PST

Salt Lake City, UT (Moon elevation 39 degrees)

Ingress: 9:19 MST

Egress: 10:55 MST

Winnipeg, Canada (Moon elevation 27 degrees)

Ingress: 10:58 CST

Egress: 11:57 CST

Dallas, TX (Moon elevation 41 degrees)

Ingress: 11:05 AM CST

Egress: 12:42 PM CST

Toronto, Canada (Moon elevation 20 degrees)

Ingress: 12:31 PM EST

Egress: 1:35 PM EST

Miami, FL (Moon elevation 27 degrees)

Ingress: 12:54 PM EST

Egress: 2:15 PM EST

Boston, MA (Moon elevation 14 degrees)

Ingress: 12:45 PM EST

Egress: 1:44 PM EST

Note: Moon the elevation value quoted is for the start (ingress) of each event.

Venus near the daytime Moon as seen from Tampa, Florida at noon on December 7th. Image credit: Stellarium

Unfortunately, the gibbous phase of Venus means we probably won’t get a chance to replicate the hunt for the elusive and spurious ‘ashen light of Venus’ watched for by David and Joan Dunham from the Australian Outback earlier this year on October 8^th, as Venus emerged from behind the dark limb of the Moon:

The planet Venus is surprisingly bright against the blue daytime sky, and its cloud tops present a much higher albedo at 90%. This is the highest of any planet in the solar system, compared to the Moon’s paltry average albedo of 13%. We easily managed to catch Venus near the daytime Moon low to the horizon during an occultation seen from northern Maine on June 18^th, 2007.



Reports of Venus in the daytime are also legend… one medieval UFO report of a ‘star’ burning near the daytime crescent Moon spied on June 13^th, 1589 by residents of the French village of Saint-Denis was actually the planet Venus. You can wonder just who might have spotted the silvery daytime ember first, and how fear spread through the village, at least enough that someone thought to chronicle the curious sighting…

Westward across the international dateline on morning of December 8^th, skywatchers in Australia and the Far East we be treated to a close pairing of the crescent Moon and Venus reminiscent of the Islamic crescent and star symbol that graces many national flags. Watch for Earthshine on the dark side of the Moon, the double reflection of sunlight of the gibbous Earth coming back for one more pass.

But wait, there’s more: Comet C/2013 US10 Catalina shines around magnitude +5.5 just 5 degrees from the Moon-Venus pair on the mornings of the 7^th and 8th. If you haven’t seen this binocular comet yet, early next week is a great time to try.

Venus, the waning crescent Moon, and Comet US10 Catalina as seen from latitude 30 degrees north at 5 AM local. Image credit: Starry Night Pro 7

The Moon sits just 0.0027 AU or 1.25 light seconds away on the morning of December 7^th, while Venus is 1.004 AU (8+ light minutes) and Comet US10 Catalina is currently 1.41 AU and just under 12 light minutes distant. Watch that comet, as it is headed northward and will put on its best show in early 2016.

Venus (arrowed) near the daytime Moon. Image credit: Dave Dickinson

And speaking of 2016, we are indeed hammering away like Santa’s elves at our annual ‘Top 101 Astronomical Events for 2016’ post; expect it out on the buffer week between Christmas and New Years Day. Thanks to all who’ve inquired, we always like to refer to it as the ‘blog post it takes us six months to write!’

About David Dickinson

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe from Tampa Bay, Florida.

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What are the Earth’s Layers?

What are the Earth’s Layers?:

There is more to the Earth than what we can see on the surface. In fact, if you were able to hold the Earth in your hand and slice it in half, you’d see that it has multiple layers. But of course, the interior of our world continues to hold some mysteries for us. Even as we intrepidly explore other worlds and deploy satellites into orbit, the inner recesses of our planet remains off limit from us.

However, advances in seismology have allowed us to learn a great deal about the Earth and the many layers that make it up. Each layer has its own properties, composition, and characteristics that affects many of the key processes of our planet. They are, in order from the exterior to the interior – the crust, the mantle, the outer core, and the inner core. Let’s take a look at them and see what they have going on.

Like all terrestrial planets, the Earth’s interior is differentiated. This means that its internal structure consists of layers, arranged like the skin of an onion. Peel back one, and you find another, distinguished from the last by its chemical and geological properties, as well as vast differences in temperature and pressure.

Our modern, scientific understanding of the Earth’s interior structure is based on inferences made with the help of seismic monitoring. In essence, this involves measuring sound waves generated by earthquakes, and examining how passing through the different layers of the Earth causes them to slow down. The changes in seismic velocity cause refraction which is calculated (in accordance with Snell’s Law) to determine differences in density.

These are used, along with measurements of the gravitational and magnetic fields of the Earth and experiments with crystalline solids at pressures and temperatures characteristic of the Earth’s deep interior, to determine what Earth’s layers looks like. In addition, it is understood that the differences in temperature and pressure are due to leftover heat from the planet’s initial formation, the decay of radioactive elements, and the freezing of the inner core due to intense pressure.

Model of a flat Earth, with the continents modeled in a disk-shape and Antarctica as an ice wall. Credit: Wikipedia Commons

History of Study:
Since ancient times, human beings have sought to understand the formation and composition of the Earth. The earliest known cases were unscientific in nature – taking the form of creation myths or religious fables involving the gods. However, between classical antiquity and the medieval period, several theories emerged about the origin of the Earth and its proper makeup.

Most of the ancient theories about Earth tended towards the “Flat-Earth” view of our planet’s physical form. This was the view in Mesopotamian culture, where the world was portrayed as a flat disk afloat in an ocean. To the Mayans, the world was flat, and at it corners, four jaguars (known as bacabs) held up the sky. The ancient Persians speculated that the Earth was a seven-layered ziggurat (or cosmic mountain), while the Chinese viewed it as a four-side cube.

By the 6th century BCE, Greek philosophers began to speculate that the Earth was in fact round, and by the 3rd century BCE, the idea of a spherical Earth began to become articulated as a scientific matter. During the same period, the development of a geological view of the Earth also began to emerge, with philosophers understanding that it consisted of minerals, metals, and that it was subject to a very slow process of change.

However, it was not until the 16th and 17th centuries that a scientific understanding of planet Earth and its structure truly began to advance. In 1692, Edmond Halley (discoverer of Halley’s Comet) proposed what is now known as the “Hollow-Earth” theory. In a paper submitted to Philosophical Transactions of Royal Society of London, he put forth the idea of Earth consisting of a hollow shell about 800 km thick (~500 miles).

Illustration of Edmond Halley’s model of a Hallow Earth, one that was made up of concentric spheres. Credit: Wikipedia Commons/Rick Manning

Between this and an inner sphere, he reasoned there was an air gap of the same distance. To avoid collision, he claimed that the inner sphere was held in place by the force of gravity. The model included two inner concentric shells around an innermost core, corresponding to the diameters of the planets Mercury, Venus, and Mars respectively.

Halley’s construct was a method of accounting for the values of the relative density of Earth and the Moon that had been given by Sir Isaac Newton, in his Philosophiæ Naturalis Principia Mathematica (1687) – which were later shown to be inaccurate. However, his work was instrumental to the development of geography and theories about the interior of the Earth during the 17th and 18th centuries.

Another important factor was the debate during the 17th and 18th centuries about the authenticity of the Bible and the Deluge myth. This propelled scientists and theologians to debate the true age of the Earth, and compelled the search for evidence that the Great Flood had in fact happened. Combined with fossil evidence, which was found within the layers of the Earth, a systematic basis for identifying and dating the Earth’s strata began to emerge.

The development of modern mining techniques and growing attention to the importance of minerals and their natural distribution also helped to spur the development of modern geology. In 1774, German geologist Abraham Gottlob Werner published Von den äusserlichen Kennzeichen der Fossilien (On the External Characters of Minerals) which presented a detailed system for identifying specific minerals based on external characteristics.

The growing importance of mining in the 17th and 18th centuries, particularly for precious metals, led to further developments in geology and Earth sciences. Credit: minerals.usgs.gov

In 1741, the National Museum of Natural History in France created the first teaching position designated specifically for geology. This was an important step in further promoting knowledge of geology as a science and in recognizing the value of widely disseminating such knowledge. And by 1751, with the publication of the Encyclopédie by Denis Diderot, the term “geology” became an accepted term.

By the 1770s, chemistry was starting to play a pivotal role in the theoretical foundation of geology, and theories began to emerge about how the Earth’s layers were formed. One popular idea had it that liquid inundation, like the Biblical Deluge, was responsible for creating all the geological strata. Those who accepted this theory became known popularly as the Diluvianists or Neptunists.

Another thesis slowly gained currency from the 1780s forward, which stated that instead of water, strata had been formed through heat (or fire). Those who followed this theory during the early 19th century referred to this view as Plutonism, which held that the Earth formed gradually through the solidification of molten masses at a slow rate. These theories together led to the conclusion that the Earth was immeasurably older than suggested by the Bible.

In the early 19th century, the mining industry and Industrial Revolution stimulated the rapid development of the concept of the stratigraphic column – that rock formations were arranged according to their order of formation in time. Concurrently, geologists and natural scientists began to understand that the age of fossils could be determined geologically (i.e. that the deeper the layer they were found in was from the surface, the older they were).

HMS Beagle in the Galapagos Islands, painted by John Chancellor. Credit: hmsbeagleproject.otg

During the imperial period of the 19th century, European scientists also had the opportunity to conduct research in distant lands. One such individual was Charles Darwin, who had been recruited by Captain FitzRoy of the HMS Beagle to study the coastal land of South America and give geological advice.

Darwin’s discovery of giant fossils during the voyage helped to establish his reputation as a geologist, and his theorizing about the causes of their extinction led to his theory of evolution by natural selection, published in On the Origin of Species in 1859.

During the 19th century, the governments of several countries including Canada, Australia, Great Britain and the United States funded geological surveying that would produce geological maps of vast areas of the countries. By this time, the scientific consensus established the age of the Earth in terms of millions of years, and the increase in funding and the development of improved methods and technology helped geology to move farther away from dogmatic notions of the Earth’s age and structure.

By the early 20th century, the development of radiometric dating (which is used to determine the age of minerals and rocks), provided the necessary the data to begin getting a sense of the Earth’s true age. By the turn of the century, geologists now believed the Earth to be 2 billion years old, which opened doors for theories of continental movement during this vast amount of time.

The Earth’s Tectonic Plates. Credit: msnucleus.org

In 1912, Alfred Wegener proposed the theory of Continental Drift, which suggested that the continents were joined together at a certain time in the past and formed a single landmass known as Pangaea. In accordance with this theory, the shapes of continents and matching coastline geology between some continents indicated they were once attached together.

Research into the ocean floor also led directly to the theory of Plate Tectonics, which provided the mechanism for Continental Drift. Geophysical evidence suggested lateral motion of continents and that oceanic crust is younger than continental crust. This geophysical evidence also spurred the hypothesis of paleomagnetism, the record of the orientation of the Earth’s magnetic field recorded in magnetic minerals.

Then there was the development of seismology, the study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies, in the early 20th century. By measuring the time of travel of refracted and reflected seismic waves, scientists were able to gradually infer how the Earth was layered and what lay deeper at its core.

For example, in 1910, Harry Fielding Ried put forward the “elastic rebound theory”, based on his studies of the 1906 San Fransisco earthquake. This theory, which stated that earthquakes occur when accumulated energy is released along a fault line, was the first scientific explanation for why earthquakes happen, and remains the foundation for modern tectonic studies.

Earth viewed from the Moon by the Apollo 11 spacecraft. Credit: NASA

Then in 1926, English scientist Harold Jeffreys claimed that below the crust, the core of the Earth is liquid, based on his study of earthquake waves. And then in 1937, Danish seismologist Inge Lehmann went a step further and determined that within the earth’s liquid outer core, there is a solid inner core.

By the latter half of the 20th century, scientists developed a comprehensive theory of the Earth’s structure and dynamics had formed. As the century played out, perspectives shifted to a more integrative approach, where geology and Earth sciences began to include the study of the Earth’s internal structure, atmosphere, biosphere and hydrosphere into one.

This was assisted by the development of space flight, which allowed for Earth’s atmosphere to be studied in detail, as well as photographs taken of Earth from space. In 1972, the Landsat Program, a series of satellite missions jointly managed by NASA and the U.S. Geological Survey, began supplying satellite images that provided geologically detailed maps, and have been used to predict natural disasters and plate shifts.

Layers:The Earth can be divided into one of two ways – mechanically or chemically. Mechanically – or rheologically, meaning the study of liquid states – it can be divided into the lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. But chemically, which is the more popular of the two, it can be divided into the crust, the mantle (which can be subdivided into the upper and lower mantle), and the core – which can also be subdivided into the outer core, and inner core.



The inner core is solid, the outer core is liquid, and the mantle is solid/plastic. This is due to the relative melting points of the different layers (nickel–iron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases. At the surface, the nickel-iron alloys and silicates are cool enough to be solid. In the upper mantle, the silicates are generally solid but localized regions of melt exist, leading to limited viscosity.

In contrast, the lower mantle is under tremendous pressure and therefore has a lower viscosity than the upper mantle. The metallic nickel–iron outer core is liquid because of the high temperature. However, the intense pressure, which increases towards the inner core, dramatically changes the melting point of the nickel–iron, making it solid.

The differentiation between these layers is due to processes that took place during the early stages of Earth’s formation (ca. 4.5 billion years ago). At this time, melting would have caused denser substances to sink toward the center while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron, along with nickel and some lighter elements, whereas less dense elements migrated to the surface along with silicate rock.

Crust:The crust is the outermost layer of the planet, the cooled and hardened part of the Earth that ranges in depth from approximately 5-70 km (~3-44 miles). This layer makes up only 1% of the entire volume of the Earth, though it makes up the entire surface (the continents and the ocean floor).

The Earth’s layers (strata) shown to scale. Credit: pubs.usgs.gov

The thinner parts are the oceanic crust, which underlies the ocean basins at a depth of 5-10 km (~3-6 miles), while the thicker crust is the continental crust. Whereas the oceanic crust is composed of dense material such as iron magnesium silicate igneous rocks (like basalt), the continental crust is less dense and composed of sodium potassium aluminum silicate rocks, like granite.

The uppermost section of the mantle (see below), together with the crust, constitutes the lithosphere – an irregular layer with a maximum thickness of perhaps 200 km (120 mi). Many rocks now making up Earth’s crust formed less than 100 million (1×10⁸) years ago. However, the oldest known mineral grains are 4.4 billion (4.4×10⁹) years old, indicating that Earth has had a solid crust for at least that long.

Upper Mantle:The mantle, which makes up about 84% of Earth’s volume, is predominantly solid, but behaves as a very viscous fluid in geological time. The upper mantle, which starts at the “Mohorovicic Discontinuity” (aka. the “Moho” – the base of the crust) extends from a depth of 7 to 35 km (4.3 to 21.7 mi) downwards to a depth of 410 km (250 mi). The uppermost mantle and the overlying crust form the lithosphere, which is relatively rigid at the top but becomes noticeably more plastic beneath.

Compared to other strata, much is known about the upper mantle, thanks to seismic studies and direct investigations using mineralogical and geological surveys. Movement in the mantle (i.e. convection) is expressed at the surface through the motions of tectonic plates. Driven by heat from deeper in the interior, this process is responsible for Continental Drift, earthquakes, the formation of mountain chains, and a number of other geological processes.

Computer simulation of the Earth’s field in a period of normal polarity between reversals.  Credit: science.nasa.gov

The mantle is also chemically distinct from the crust, in addition to being different in terms of rock types and seismic characteristics. This is due in large part to the fact that the crust is made up of solidified products derived from the mantle, where the mantle material is partially melted and viscous. This causes incompatible elements to separate from the mantle, with less dense material floating upward and solidifying at the surface.


The crystallized melt products near the surface, upon which we live, are typically known to have a lower magnesium to iron ratio and a higher proportion of silicon and aluminum. These changes in mineralogy may influence mantle convection, as they result in density changes and as they may absorb or release latent heat as well.

In the upper mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F). Between the upper and lower mantle, there is also what is known as the transition zone, which ranges in depth from 410-660 km (250-410 miles).

Lower Mantle:The lower mantle lies between 660-2,891 km (410-1,796 miles) in depth. Temperatures in this region of the planet can reach over 4,000 °C (7,230 °F) at the boundary with the core, vastly exceeding the melting points of mantle rocks. However, due to the enormous pressure exerted on the mantle, viscosity and melting are very limited compared to the upper mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous.

The internal structure of Earth. Credit: Wikipedia Commons/Kelvinsong

Outer Core:The outer core, which has been confirmed to be liquid (based on seismic investigations), is 2300 km thick, extending to a radius of ~3,400 km. In this region, the density is estimated to be much higher than the mantle or crust, ranging between 9,900 and 12,200 kg/m³. The outer core is believed to be composed of 80% iron, along with nickel and some other lighter elements.

Denser elements, like lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. The temperature of the outer core ranges from 4,300 K (4,030 °C; 7,280 °F) in the outer regions to 6,000 K (5,730 °C; 10,340 °F) closest to the inner core.

Because of its high temperature, the outer core exists in a low viscosity fluid-state that undergoes turbulent convection and rotates faster than the rest of the planet. This causes eddy currents to form in the fluid core, which in turn creates a dynamo effect that is believed to influence Earth’s magnetic field. The average magnetic field strength in Earth’s outer core is estimated to be 25 Gauss (2.5 mT), which is 50 times the strength of the magnetic field measured on Earth’s surface.

Inner Core:Like the outer core, the inner core is composed primarily of iron and nickel and has a radius of ~1,220 km. Density in the core ranges between 12,600-13,000 kg/m³, which suggests that there must also be a great deal of heavy elements there as well – such as gold, platinum, palladium, silver and tungsten.

Artist’s illustration of Earth’s core, inner core, and inner-inner core. Credit: Huff Post Science

The temperature of the inner core is estimated to be about 5,700 K (~5,400 °C; 9,800 °F). The only reason why iron and other heavy metals can be solid at such high temperatures is because their melting temperatures dramatically increase at the pressures present there, which ranges from about 330 to 360 gigapascals.

Because the inner core is not rigidly connected to the Earth’s solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been considered. By observing changes in seismic waves as they passed through the core over the course of many decades, scientists estimate that the inner core rotates at a rate of one degree faster than the surface. More recent geophysical estimates place the rate of rotation between 0.3 to 0.5 degrees per year relative to the surface.

Recent discoveries also suggest that the solid inner core itself is composed of layers, separated by a transition zone about 250 to 400 km thick. This new view of the inner core, which contains an inner-inner core, posits that the innermost layer of the core measures 1,180 km (733 miles) in diameter, making it less than half the size of the inner core. It has been further speculated that while the core is composed of iron, it may be in a different crystalline structure that the rest of the inner core.

What’s more, recent studies have led geologists to conjecture that the dynamics of deep interior is driving the Earth’s inner core to expand at the rate of about 1 millimeter a year. This occurs mostly because the inner core cannot dissolve the same amount of light elements as the outer core.



The freezing of liquid iron into crystalline form at the inner core boundary produces residual liquid that contains more light elements than the overlying liquid. This in turn is believed to cause the liquid elements to become buoyant, helping to drive convection in the outer core.

This growth is therefore likely to play an important role in the generation of Earth’s magnetic field by dynamo action in the liquid outer core. It also means that the Earth’s inner core, and the processes that drive it, are far more complex than previously thought!

Yes indeed, the Earth is a strange and mysteries place, titanic in scale as well as the amount of heat and energy that went into making it many billions of years ago. And like all bodies in our universe, the Earth is not a finished product, but a dynamic entity that is subject to constant change. And what we know about our world is still subject to theory and guesswork, given that we can’t examine its interior up close.

As the Earth’s tectonic plates continue to drift and collide, its interior continues to undergo convection, and its core continues to grow, who knows what it will look like eons from now? After all, the Earth was here long before we were, and will likely continue to be long after we are gone.

We have written many articles about Earth for Universe Today. Here’s are some Interesting Facts about Earth, and here’s one about the Earth’s inner inner core, and another about how minerals stop transferring heat at the core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Earth. Listen here, Episode 51: Earth.

About Matt Williams

Matt Williams is the Curator of the Guide to Space for Universe Today, a a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC.

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