Author: Jerry Coffey

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The density of the Earth is 5.513 g/cm³. This is an average of all of the material on the planet. Water is much less dense than iron, hence an average is needed for ease of use. Earth is the most dense planet in the Solar System; however, if gravitational compression where factored out, the second most dense planet, Mercury, would be more dense. The density of Earth is calculated by dividing the planet’s mass by its volume, then simplifying from kg/km to g/cm cubed.

Here is the density of the other planets in our Solar System so you can compare to Earth’s.

Mercury	5.43 g/cm3
Venus	5.243 g/cm3
Mars	3.934 g/cm3
Jupiter	1.326 g/cm3
Saturn	0.687 g/cm3
Uranus	1.270 g/cm3
Neptune	1.638 g/cm3
The Sun	1.408 g/cm3

Just knowing the density of a planet is not much information. It sort of only gives a partial picture. Here are a few more interesting facts about the Earth that may help you understand our planet a little more.

The Moon is thought to have been formed when a large asteroid or a planetesimal impacted Earth. The Moon is the portion that was thrown back into space and the particles that accreted to it. Scientist think that other planets may have obtained some of their moons in a similar manner. The Earth is the only planet with a single Moon, but has two quasi-satellites 3753 Cruithne and 2002 AA₂₉.

The Sun is constantly evolving. In a few billion years it will begin to heat up on its way to the red giant phase of a star’s life. Along the way it will become hot enough to destroy life on Earth. The question will become how will humans survive. Colonizing other celestial objects is one option. Some scientists have developed a theoretical way to move the entire planet. It would require finding an asteroid large enough to perturb Earth’s orbit and push away from the Sun. Colonizing another planet could actually be easier.

Despite a lot of internet hype, there is no credible threat to the Earth that will coincide with the end of the Mayan calendar. The Mayan calender does not even end, 2012 marks the end of the current long-count period. 2013 marks the beginning of another.

The density of Earth is one of thousands of interesting facts that you find about your home planet. Here at Universe Today, we hope that you want to find many more and continue to research the world around you.

We have written many articles about density for Universe Today. Here’s an article about the density of the Sun, and here’s one about the density of Mars.

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

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
NASA
Physics Forums

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The size of Earth, like the size of all of the celestial bodies, is measured in a number of parameters including mass, volume, density, surface area, and equatorial/polar/mean diameter. While we live on this planet, very few people can quote you the figures for these parameters. Below is a table with many of the pieces of the data used to measure the size of the Earth.

Mass	5.9736×1024kg
Volume	1.083×1012 km3
Mean diameter	12,742 km
Surface area	510,072,000 km2
Density	5.515 g/cm3
Circumference	40,041 km

Those numbers tell you the size of the Earth, but what about its other statistics? The atmospheric pressure at the surface is 101.325 kPa, average temperature is 14°C, the axial tilt is approximately 23°, and it has an orbital speed of 29.78 km/s. Earth orbits with a perihelion of 147,098,290 km, and an aphelion of 152,098,232 km, making for a semi-major axis of 149,598,261 km. Even though we need oxygen to survive, it is the second most abundant component of Earth’s atmosphere. Nitrogen accounts for 78% of the gases in the atmosphere and oxygen occupies 21%.

The Earth only has one moon. That is pretty uncommon in our Solar System. There are currently 166 recognized moon in our system. There is one asteroid that has a quasi relationship with Earth. 3753 Cruithne has a 1:1 orbital resonance with the Earth. It is a periodic inclusion planetoid that has a horseshoe orbit. It was discovered in 1986.

Since we occupy this planet, it is understandably the most extensively studied body in space. We have sent scientist to most of the corners of our world. Yet, we find dozens of new species each year and there are areas that have rarely seen a human’s footprints. There are aspects of our world that we do not understand and have theories too inadequate to explain. Science is light years ahead of where it was just 50 years ago. These advancements are exciting enough to make the possibilities of the near future seem boundless.

Now that you know the size of the Earth, you could look for information on extremophiles, the Mariana Trench, and the Tunguska event. Earth bound events are often taken for granted since we live here, but, with a little research, you may find much more excitement outside of your back door than you ever expected.

We have written many articles about the Solar System for Universe Today. Here’s an article about the size of Mars, and here’s one about the size of the Moon.

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

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
NASA Earth Facts
NASA Solar System Guide on Earth
NASA Solar System Orbit Diagram

Frequently, readers send us questions here on Universe Today. One very good question is ”why does Jupiter have the Great Red Spot?” The short answer is that the Great Red Spot is a storm that has been raging since the 1600s, but a short answer does not tell the whole story. Read on for a much more detailed accounting.

The Great Red Spot (GRS) is an anti-cyclonic(rotates counter-clockwise) storm that is located 22° south of Jupiter’s equator. The storm has lasted an estimated 346 years, but many scientists believe that it is much older. The storm is known to have been larger than 40,000 km in diameter at one time and can be easily seen with large backyard telescopes. Currently it measures approximately 24–40,000 km east–to–west and 12–14,000 km north–to–south. The GRS is large enough to envelope two to three Earths. Despite the GRS’s enormous size, it is shrinking. In 2004, it had about half the longitudinal size that it had a century ago. Some scientist estimate that if it continues to shrink at its current rate, it could become circular by 2040. A study by scientists at the University of California, Berkeley showed that the GRS lost 15 percent of its diameter along its major axis between 1996 and 2006. Xylar Asay-Davis, a team member on the study, noted that the spot is not disappearing because ”[v]elocity is a more robust measurement because the clouds associated with the Red Spot are also strongly influenced by numerous other phenomena in the surrounding atmosphere.”

Infrared data indicates that the GRS is colder and located at a higher altitude than most of Jupiter’s other clouds. The cloudtops of the GRS are about 8 km above the surrounding clouds. The storm is held in place by an eastward jetstream to its south and a very strong westward jetstream to its north. Winds around the edge of the GRS peak at 432 km/h, but winds within the storm seem to nearly none existent, with little inflow or outflow. In 2010, astronomers imaged the GRS in the far infrared and found that its central(reddest) region is warmer than its surroundings by about 4 K. The warm airmass is located in the upper troposphere. This warm central spot slowly rotates in the opposite direction of the remainder of the storm and could be a remnant of air flow in the center.

Alright, so why is the storm red? The exact cause of the coloring has not been proven, but…lab experiments support the theory that the color is caused by complex organic molecules, red phosphorus, or another sulfur compound that are pulled from deeper within Jupiter. The color of the GRS varies. At times it is brick-red, fading to a pale salmon, and even white. The spot occasionally disappears from the visible spectrum and can only be seen as the Red Spot Hollow; its niche in the South Equatorial Belt(SEB). The visibility of GRS is apparently coupled to the appearance of the SEB. If the SEB is bright white, the spot tends to be dark. When it is dark, the GRS is usually light. The periods that the color changes last and occur on an unpredictable schedule.

As you can see, the answer to ”why does Jupiter have the Great Red Spot?” has been well researched by NASA and other space agencies. While the answer is not crystal clear at this time, future missions to the planet are designed to better study the atmosphere; hopefully, rendering the answers scientists seek.

We have written many articles about Jupiter for Universe Today. Here are some interesting facts about Jupiter, and here’s an article about the color of Jupiter.

If you’d like more information on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Sources:
http://www.nasa.gov/multimedia/imagegallery/image_feature_413.html
http://www.nasa.gov/centers/goddard/news/topstory/2006/little_red_spot.html

[/caption]The size of Mars can not be given in one set of numbers. Scientists describe a planet by many factors. First there is radius, for Mars that is 3,389.5 km. Its circumference is 21,344 km. Next is volume, which is 1.63116 X 10¹¹ km³. Last is Mar’s mass at 6.4169 x 10²³ kg.

For comparison, Mars has 53% of the diameter of Earth. It has about 38% of the surface area of Earth. That sounds small, but that is equal to the total dry land here on Earth. The volume of Mars is equal to 15% of Earth’s and the Red Planet’s mass is 11% of Earth’s. As you can see, Mars is a small world, the second smallest in the Solar System.

Despite its small size, Mars has many interesting features that would seem larger than life. Olympus Mons is the tallest mountain in the Solar System and Valles Marineris is the deepest valley. Mars is home to hundreds of thousands of impact craters. Northern Polar Basin-Borealis Basin is largest at 10,500 km and Hellas Basin at 2,100 km is the third largest.

In addition to the extremes in topography, Mars is a world of weather extremes. Overall, it is a very cold world with an average surface temperature of about -47°C. During the summer, near the equator the temperatures can reach nearly 20°C during the day, but drop to -90°C at night. That 110° change in temperature can drive winds that reach tornado speeds. Once these winds start, they pick up the iron oxide dust that covers the planet, turning into a dust storm. There have been dust storms on Mars that have gotten large enough to engulf the entire planet for days at a time.

Scientists believe that Mars was a larger planet early in the history of the Solar System. The impact that created the Northern Polar Basin-Borealis Basin would have been large enough to eject a portion of the planet into space and beyond its gravitational pull; thus, the planet may have lost part of itself from the crash.

As you can see, the size of Mars is a minor fact amongst all of the interesting facts that you can discover about the Red Planet, but, hopefully, it is enough to get you to do more research.

Want information on other planets? Here’s an article about the size of Jupiter, and here’s one on the size of Saturn.

If you’d like more info on Mars, check out Hubblesite’s News Releases about Mars, and here’s a link to the NASA Mars Exploration home page.

We have recorded several podcasts just about Mars. Including Episode 52: Mars and Episode 92: Missions to Mars, Part 1.

Source:
NASA

The atmosphere of Mars is less than 1% of Earth’s, so it does not protect the planet from the Sun’s radiation nor does it do much to retain heat at the surface. It consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon, and the remainder is trace amounts of oxygen, water vapor, and other gases. Also, it is constantly filled with small particles of dust(mainly iron oxide), which give Mars its reddish hue.

Scientist believe that the atmosphere of Mars is so negligible because the planet lost its magnetosphere about 4 billion years ago. A magnetosphere would channel the solar wind around the planet. Without one, the solar wind interacts directly with the ionosphere stripping away atoms, lowering the density of the atmosphere. These ionized particles have been detected by multiple spacecraft as they trial off into space behind Mars.

This leads the surface atmospheric pressure to be as low as 30 Pa(Pascal) with an average of 600 Pa compared to Earth’s average of 101,300 Pa. The atmosphere extends to about 10.8 km, about 4 km farther than Earth’s. This is possible because the planet’s gravity is slighter and does not hold the atmosphere as tightly.

A relatively large amount of methane has been found in the atmosphere of Mars. This unexpected find occurs at a rate of 30 ppb. The methane occurs in large plumes in different areas of the planet, which suggests that it was released in those general areas. Data seems to suggest that there are two main sources for the methane: one appears to be centered near 30° N, 260° W, with the second near 0°, 310° W.

It is estimated that Mars produces 270 ton/year of methane. Under the conditions on Mars, methane breaks down as quickly as 6 months(Earth time). In order for the methane to exist in the detected quantities, there must be a very active source under the surface. Volcanic activity, comet impacts, and serpentinization are the most probable causes. Methanogenic microbial life is a very remote alternative source.

The atmosphere of Mars will cause a great number of obstacles for human exploration of the planet. It prevents liquid water on the surface, allows radiation levels that humans can barely tolerate, and would make it difficult to grow food even in a greenhouse. NASA and other space agencies are confident that they will be able to engineer solutions for the problem within the next 30 years, though. Good luck to them.

Of course, we have written many articles about Mars’ atmosphere. Here’s an article about how the planet once held enough moisture for drizzle or dew. And here’s an article about the Mars methane mystery.

If you’d like more info on Mars, check out Hubblesite’s News Releases about Mars, and here’s a link to the NASA Mars Exploration home page.

We have recorded several podcasts just about Mars. Including Episode 52: Mars and Episode 92: Missions to Mars, Part 1.

Sources:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mars&Display=OverviewLong
http://quest.nasa.gov/aero/planetary/mars.html
http://www.nasa.gov/home/hqnews/2009/jan/HQ_09-006_Mars_Methane.html

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The size of a planet is measured in many ways: mass, volume, equatorial diameter, and surface area are the most common. In this article we are going to explore each of these ways to express the size of Venus and a few interesting facts about our close neighbor.

Venus has a diameter that is about 95% of Earth’s. It is 12,100 km across. The Venusian surface area is around 90% of our own at 4.6×10⁸ km². The planet has a volume of 9.38×10¹¹ km³. That puts it a little over 85% of Earth’s volume. One final way to measure the size of Venus is to consider its mass. It has a mass of 4.868 x 10²⁴ kg., just over 81% of Earth’s. These physical characteristics have led many scientists to call Venus the sister planet of Earth.

Size characteristics are the only things that Earth and Venus have in common. At 462°C, Venus has an average temperature that is 410°C higher than the hottest deserts on Earth. The temperature on the Venusian surface can melt lead. You have to be 50 km from the surface to find temperatures that are anything like here on Earth.

Temperature is not the only extreme on Venus. The atmosphere would prevent any life as we know from surviving. To start with, the atmospheric pressure is 92 times that of Earth. It is choked with volcanic ash, sulfuric acid clouds, and is made of 95% carbon dioxide. There are constant hurricane force winds churning the atmosphere. Sustained winds in excess of 360 kph are always present. The conditions on the planet are so extreme that probes can only last a few hours.

The surface shows over 1000 volcanoes or volcanic remnants that are over 20 km in diameter. There are no small impact crater, because the atmosphere is too thick to allow small objects to penetrate to the surface. Scientists believe that the entire surface of the planet was been replaced by volcanic activity 300 to 500 million years ago.

Venus has been visited by spacecraft several times. NASA sent Mariner 2 in 1962. It was the first spacecraft to send information from another planet. The Soviet space program landed Venera 7 in 1970. It was the first spacecraft to land on another planet. NASA’s Magellan mapped 98% of the surface in the early 1990s and the European Space Agency currently has the Venus Express in orbit studying the planet’s atmosphere. In all, we have gained a great deal of information about this inhospitable planet in the last 30 years.

Want to know the size of other planets in the Solar System. Here’s an article about the size of Jupiter, and here’s an article about the size of Saturn.

Want more information on Venus? Here’s a link to Hubblesite’s News Releases about Venus, and here’s NASA’s Solar System Exploration Guide to Venus.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Venus. Listen to it here, Episode 50: Venus.

References:
http://www.nasa.gov/audience/forstudents/5-8/features/F_The_Planet_Venus_5-8.html
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus&Display=OverviewLong
http://en.wikipedia.org/wiki/Venus

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The mass of Venus is 4.868×10²⁴ kg. That is about 82% of the mass of Earth. Alright, end of story and thank you for reading. Okay, we would never do that to you here at Universe Today. There are far too many interesting facts about Venus to leave you hanging like that.

Here are a few other physical characteristics of the second rock from the Sun:

Diameter	12,100 km
Surface Gravity	8.87m/s2
Surface Area	460,000,000 km2
Volume	9.38×1011km3
Surface Atmospheric Pressure	92 times that of Earth
Average Surface Temperature	462 degrees Celsius
Rotation	Retrograde
Density	5.204 g/cm3

Scientists believe that the high mass and density of Venus can be accounted for by its high concentration of rock and metals. They believe that the planet has a liquid metallic core that is surrounded by a molten rock mantle. Actual proof of this is nearly impossible since the reflective nature of the planet’s atmosphere makes many types of observation impossible.

Venus was once thought to be a dead planet. There is no life on the surface for many reasons, but recent study of the surface has revealed that there may be active volcanoes on the planet. That means that it is alive, geologically speaking. Previously, scientists had known that the planet had been resurfaced by volcanic activity 300 to 500 million years ago, but had thought that the activity died out during that same time frame.

There have been many missions sent to Venus. The Soviet space program started the race to Venus with the Venera program. It is hard to tell exactly how many Soviet missions to Venus were launched since the program would not announce a probe that failed, but more than a dozen were successful. NASA launched several mission of its own. Early missions from both programs failed because neither was prepared for the extreme pressure within the Venusian atmosphere. Even those that were able to transmit from the surface could only survive for less than one hour.

The Venus Express is currently in orbit around Venus. BepiColumbo is set to launch in 2014. It is hoped that the Akatsuki probe can reawakened to gather information when it arrives in the area in 2016 and the Venus In-Situ Explorer is in the planning stages. Scientists are determined to unravel the planet’s mysteries. Like you, they want to know more than the mass of Venus.

Here’s an article about the mass of Mercury, and here’s an article about the mass of the Earth.

Want more information on Venus? Here’s a link to Hubblesite’s News Releases about Venus, and here’s NASA’s Solar System Exploration Guide to Venus.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Venus. Listen to it here, Episode 50: Venus.

References:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus&Display=Facts
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus&Display=OverviewLonghttp://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus&Display=Educ

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Mercurian world is one of extremes. 700 Kelvin on the side exposed to the Sun, yet some areas are never exposed to sunlight and are as cold as deep space. Scientists do not believe there has ever been life on Mercury. The atmosphere on Mercury is almost non-existant. It doesn’t protect the planet from the harsh radiation of the Sun or radiation from space, nor does it trap heat and provide a breathable atmosphere. Mercury is inhospitable and sterile.

In order for life (as we know it) to exist, Mercury would need to have temperatures that allow liquid water to remain on its surface for long periods of time. But the temperatures on Mercury extend from just above absolute zero when the surface is shadowed to 700 Kelvin when its in sunlight. Liquid water just can’t exist in that kind of environment.

Any ancient life on Mercury would have faced many extinction events. Here on Earth many past life forms have been destroyed by asteroid impacts. The dinosaurs are a classic example. Images of Mercury’s surface returned by the Mariner 10 and MESSENGER spacecraft have shown that the surface has suffered many large impacts. In fact, it was heavily bombarded during the Late Heavy Bombardment that occurred about 3.9 billion years ago. Any one of those impacts could have destroyed any life on the planet. Many scientists believe that a great deal of the planet’s surface was stripped away by one impact. If the impact removed a large portion of the surface, surely it would have taken any life that existed at the time with it.

All evidence that science has do date indicates that there has never been life on Mercury and never will be. The harsh conditions on the planet’s surface and the tenuous atmosphere make it impossible for any life form known to man to exist.

But there are other planets in the Solar System. Here’s an article about life on Pluto, and here’s one about life on Mars.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

Vida sobre el mercurio

References:
NASA Solar System Exploration: Mercury
Wikipedia
JAXA: Mercury Quantities
NASA MESSENGER Mission
NASA Multimedia

[/caption]Mercury, the planet closest to the Sun, is a study in extremes and offers many surprises. The extremes of the planet have made it an understudied body in our Solar System, though the MESSENGER mission is trying to change that as you are reading this article.

In addition to being the planet closest to the Sun, Mercury is also the smallest by mass. If you ignore the former planet Pluto, it is also the smallest by surface area, as well. The planet has the most eccentric orbit: at perihelion it is 46,001,200 km from the Sun and at aphelion it is 69,816,900 km. The planet’s short orbital period(87.969 Earth days) and slight axial tilt combine to make the day on Mercury(116 Earth days) longer than the year.

The average temperature on the planet is 442.5°K. Because of the planet’s thin atmosphere there is a wide temperature range, 100°K to 700°K. The temperature at the equator can be as much as 300°K more than the temperature at the poles. Despite its proximity to our central star, the poles of the planet are thought to have water ice hidden within impact craters. Claims for water ice are substantiated by observations by the 70 m Goldstone telescope and the Very Large Array. There are areas of very high radar reflection at the pole areas so, since water is highly reflective of radar, astronomers believe that water ice is the most likely cause of this reflection.

Due to its size and average temperatures, the planet’s gravity can not retain a significant atmosphere over a long period. It does have a negligible surface-bounded exosphere that is dominated by hydrogen, helium, oxygen, sodium, calcium, and potassium. Atoms are continuously being lost and replenished from this exosphere. Hydrogen and helium atoms are thought to derive from the solar wind that buffets the planet. These elements diffuse into Mercury’s magnetosphere before escaping back into space. Radioactive decay within the crust is a source of helium, sodium, and potassium.

Mercury has been explored by two mission: Mariner 10 and MESSENGER. Mariner 10 was able to map 40-45% of Mercury’s surface through more than 2,800 photos. It revealed a more or less moon-like surface, a slight atmosphere, a magnetic field, and a large iron rich core. MESSENGER was launched in August of 2004. After a 31/2 year flight, it made its first flyby in January 2008 and arrived in orbit on March 18, 2011. So far, the probe has discovered large amounts of water in the exosphere, evidence of past volcanic activity, and evidence of a liquid planetary core.

As the MESSENGER mission continues, the closest planet to the Sun should continue to reveal more surprises for the scientists at NASA. It appears a new age of discovery has begun for Mercury.

We have an extensive section just on Mercury on Universe Today. And did you know there’s a spacecraft visiting Mercury called MESSENGER? You can read news about this mission here.

Here’s a link to NASA’s Solar System Exploration Guide on Mercury.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

References:
Wikipedia: Mercury
NASA Solar System
NASA: Messenger Mission