Author: Fraser Cain

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When you look at the Moon and Mercury, their surfaces are pounded with impact craters. Mars has many craters, and even Earth has its share. But what about Venus, are there craters on Venus?

There are craters on Venus, but not many. The Solar System is relatively empty now, but less than a billion years after the formation of the Solar System, there were still many objects left over. These crashed into planets and moon, during a time scientists call the late period of heavy bombardment. Many of the craters on Mercury and the Moon were formed during that time.

Strangely, Venus shows no record of the heavy bombardment period. Either it didn’t get struck, which is unlikely, or some process resurfaced the planet, removing all traces of the impact craters. The resurfacing process stopped at some time in Venus’ more recent history. And so, all the craters that scientists do see on the surface of Venus are relatively young.

Craters on Venus are different from craters on other planets. The planet’s thick atmosphere stops the smaller objects from even reaching the surface of Venus; they just burn up in the atmosphere. There are about 1000 craters identified on the surface of Venus.

Crater Mead is the largest known crater on Venus, named after the American anthropologist, Margaret Mead. It measures 280 km in diameter, and contains several concentric rings.

We have written many articles about Venus on Universe Today. Here’s an article about the evolution of Venus’ surface, and here’s a “Where in the Universe” challenge featuring a crater on Venus.

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.

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Like all the planets, Venus formed approximately 4.6 billion years ago when the Sun and the Solar System came out of the solar nebula. So, the age of Venus is 4.6 billion years old.

Before the Solar System, there was just a large cloud of hydrogen gas in a giant nebula. Some event, like a nearby supernova explosion put a shock into the cloud, and caused it to begin collapsing. Many stars, large and small, formed in this nebula, and one of these went on to be the Sun. As the material condensed together, conservation of momentum caused it to spin up and flatten out.

A protoplanetary disk of material formed around the newborn Sun, and it was here that the planets formed. Dust clumped together to form rocks, rocks smashed together into boulders, and mountain-sized objects became protoplanets. In the first few hundred million years of the age of Venus, it’s likely that the planet was smashed many times by these large asteroid and protoplanets. But eventually, Venus became the dominant object in the region, sucking in everything with its gravity.

We know that Venus was probably the victim of a large collision because it rotates in the opposite direction from the rest of the planets in the Solar System. A large collision could have turned its rotation backwards.

How do we know Venus’ age? We can’t measure the age of Venus directly, because of the intense heat and pressure on the surface of Venus. Instead, scientists measure the age of meteorites that have fallen to Earth. After analyzing hundreds of objects, scientists have found that they all formed at approximately the same time. These meteorites are the leftover pieces from the formation of the Solar System, and help prove that all the objects in the Solar System formed at the same time.

And so we know that the age of Venus is 4.6 billion years old.

We have written many articles about Venus. Here’s one about how life on Venus could be blown to Earth, and here’s an article about how you might keep a Venus rover cool.

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.

Reference:
NASA Solar System’s Big Bang

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Ancient people have known about the planets since we were able to look up. Some stars were brighter than the rest, and seemed to move across the sky from night to night. These moving stars were known as planets, and there were 5: Mercury, Venus, Mars, Jupiter and Saturn. But the movements of the planets were puzzling to ancient astronomers. Some times the planets would slow down, go backwards, and then go forwards again. When a planet goes backwards, it’s called retrograde, and one of the best planets for this is Mercury. Let’s examine Mercury retrograde.

In ancient times, people thought the Earth was the center of the Universe, and all objects in the night sky orbited around us. One complication of this model was the planets which took these very predictable retrograde paths in their orbit. If the planets were orbiting the Earth, why would they go backwards? Why would Mercury go retrograde? They developed elaborate models where the planets followed a spiraling path around the Earth to account for this retrograde motion.

It wasn’t until Nicolaus Copernicus developed his Sun-centered model of the Solar System that the bizarre retrograde motion of Mercury and the rest of the planets finally made sense. The Earth is just another planet, and they’re all orbiting the Sun together. The retrograde motion of Mercury and the other planets is due to our relative positions in orbit.

So let’s understand retrograde motion, and look at what is Mercury retro in particular. The motion of the planets around the Sun follow the right-hand rule. Hold your right hand out, make a fist and stick the thumb up. The direction of the thumb points in the direction of the Sun’s northern pole. The curve of the fingers indicates the direction all the planets orbit around the Sun.

Mercury moves faster than the Earth as it travels around the Sun; however, Mercury has a highly elliptical orbit, so the speed of its orbit changes. When Mercury is furthest from the Sun, it’s at the slowest point in its orbit, and this gives the Earth a chance to “catch up”. Imagine you’re driving next to a car in the freeway which is speeding up and slowing down. It’s still going down the highway at a high speed, but it seems to be going back and forth compared to you. When this happens, astronomers say that Mercury is in retrograde.

Astrologers seem to think that retrograde motion is an unlucky or bad situation because it goes against a planet’s natural movements. Of course, the planet isn’t really changing its motions at all, it’s only our perspective of the planet that’s changing. Furthermore, at any one time, 40% of the outer planets are in retrograde motion anyway. Something’s almost always in retrograde.

Is Mercury in retrograde right now? It depends on the date. Check the list of dates below to check.

So when is Mercury going to be in retrograde? Here are some Mercury retrograde dates for the next few years.

Mercury Retrograde 2009

• January 11-31
• May 6-20
• September 6-29
• December 26-January 15, 2010

Mercury Retrograde 2010

• April 17-May 11
• August 20 – September 12
• December 10-December 29

Mercury Retrograde 2011

• March 30-April 23
• August 2 – August 26
• November 23 – December 13

Mercury Retrograde 2012

• March 11-April 4
• July 14 – August 7
• November 6 – November 26

Here’s a link to the 2009 mercury retrograde dates.

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.

Mercurio retrógrada

References:
NASA: Planetary Motion
NASA Astronomy Picture of the Day
NASA: Mercury

Neptune is the most distant planet from the Sun, with temperatures that plunge down to 55 Kelvin, or -218 degrees Celsius. You would think that a planet that cold would be frozen and locked down, with very little weather. But you’d be very wrong. In fact, the weather on Neptune is some of the most violent weather in the Solar System.

Just like Jupiter and Saturn, Neptune has bands of storms that circle the planet. While the wind speeds on Jupiter can reach 550 km/hour – twice the speed of powerful hurricanes on Earth, that’s nothing compared to Neptune. Astronomers have clocked winds on Neptune traveling at 2,100 km/hour.

So why can the winds on Neptune reach such huge speeds? Astronomers think that the cold temperatures on Neptune might have something to do with that after all. The cold temperatures might decrease the friction in the system, so that winds can get going fast on Neptune.

During its 1989 flyby, NASA’s Voyager 2 spacecraft discovered the Great Dark Spot on Neptune. Similar to Jupiter’s Great Red Spot, this is an anti-cyclonic storm measuring 13,000 km x 6,600 km across. A few years later, however, the Hubble Space Telescope failed to see the Great Dark Spot, but it did see different storms. This might mean that storms on Neptune don’t last as long as they do on Jupiter or even Saturn.

The more active weather on Neptune might be due, in part, to its higher internal heat. Although Neptune is much more distant than Uranus from the Sun, receiving 40% less sunlight, temperatures on the surface of the two planets are roughly similar. In fact, Neptune radiates 2.61 times as much energy as it receives from the Sun. This is enough heat to help drive the fastest winds in the Solar System.

We have written many articles about Neptune for Universe Today. Here’s an article about how Neptune’s south pole is the warmest part of the planet, and here’s more information about the atmosphere on Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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To really understand how big Neptune really is, you need some kind of comparison. Let’s see how Neptune compares to Earth in every way.

First, let’s just look at pure size. The diameter of Neptune is approximately 49,500 km. This makes Neptune the 4th largest planet in the Solar System. And compared to Earth? Neptune is 3.9 times bigger.

Now mass. The mass of Neptune is 1.02 x 10²⁶ kg. If you wanted to write it out, it would be 102,000,000,000,000,000,000,000,000 kg. Neptune has 17 times as much mass compared to the Earth.

How about volume? The volume of Neptune is 6.3 x 10¹³ km³. You could fit 57 Earths inside Neptune and still have room to spare.

A day on Earth is 24 hours, but a day on Neptune is 16 hours and 6 minutes. A year on Earth is, um, 1 year obviously, while a year on Neptune is 164.79 years.

Here’s one element that’s actually pretty close. The surface gravity on Neptune (if it actually had a surface that you could stand on) is only 14% stronger than the pull of gravity on Earth. You would have a difficult time noticing if you were standing on the surface of Neptune compared to the surface of Earth.

We have written many articles about Neptune for Universe Today. Here’s an article about three new trojan asteroids found in Neptune’s orbit, and a possible mission to Neptune under study.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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Neptune orbits the Sun at an average distance of 4.5 billion km.

That’s the simple answer to the question, what is the orbit of Neptune. However, things are actually a little more complicated than that. Like all the planets in the Solar System, Neptune follows an elliptical path around the Sun, varying its distance to the Sun at different points along its orbit.

At its closest point in its orbit, which astronomers call perihelion, Neptune gets within 4.45 billion km, or 29.77 astronomical units (1 astronomical unit or AU is the average distance of the Earth to the Sun).

At its most distant point in its orbit, called aphelion, Neptune reaches a distance of 4.55 billion km, or 30.44 astronomical units.

One interesting feature about the orbit of Neptune is the fact that Pluto’s very elliptical orbit sometimes brings it closer to the Sun. Back in the days when Pluto was still a planet, it would spend a few decades every orbit closer to the Sun. So Neptune was actually the most distant planet, and Pluto was closer. The last time this happened started in 1979, and ended in 1999. Of course, Pluto isn’t a planet any more, so Neptune’s orbit makes it the most distant planet.

We have written many article about Neptune on Universe Today. Here’s an article with images of Neptune captured by the Hubble Space Telescope. And here’s another discussing the planet’s relatively warm south pole.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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The equatorial radius of Neptune is 24,764 km.

That’s the quick answer. But you need to understand that things are a little more complicated. Like all of the planets in the Solar System, Neptune is spinning rapidly, completing a rotation in 16 hours and 6 minutes. This rapid rotation causes the planet to flatten out, so that the radius across the equator is bigger than the radius to the poles.

So here’s the more precise answer. The radius of Neptune, measured from the center to the equator is 24,764 km. And the radius of Neptune, measured from the center to either pole is 24,341 km. I’ll do the math for you. That means that the points on the equator are 423 km further away from the center of Neptune than either pole.

Need some comparison? Neptune’s radius is 3.9 times the radius of Earth. In other words, you could line up almost 4 Earths side by side to match the width of Neptune.

We have written many articles about Neptune for Universe Today. Here’s an article about the potential for liquid water deep down within Neptune. And here’s an article about how Neptune’s largest moon Triton might have been captured by Neptune’s gravity.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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Neptune is so dim and distant that you can only see it with a moderately powerful telescope. But Neptune is the 4th largest planet in the Solar System. Let’s take a look at how big Neptune is, and compare it to some other objects that you might be familiar with.

Neptune is the 4th largest planet in the Solar System, after Jupiter, Saturn, and Uranus. It’s much larger than the terrestrial planets: Mercury, Venus, Earth and Mars.

The diameter of Neptune is 49,500 km. Need some comparison? That’s approximately 3.9 times the diameter of Earth. In other words, you could put almost 4 Earths side to side to match the diameter of Neptune.

The volume of Neptune is 6.25 x 10¹³ km³. That’s an enormous number, so once again, for comparison, that’s 57.7 times the volume of Earth. You could fit 57 Earths inside Neptune with room to spare.

The surface area of Neptune is 7.64 x 10⁹ km². That’s 15 times as much surface area as Earth; of course, Neptune doesn’t have a solid surface, so you wouldn’t want to live there.

The mass of Neptune is 1.02 x 10²⁶ kg. Again, for comparison, that’s the equivalent of 17.1 Earths.

So now, when you look through a telescope and see that tiny blue-green dot, you can get a better sense of the size of Neptune.

We have written many articles about Neptune on Universe Today. Here’s an article about a minor planet found near Neptune. And an article about how Neptune’s south pole is the hottest place on the planet.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.