Update on Pluto

A while back, I posted an argument about why Pluto is not a planet. Recently, there has been a movement to reinstate Pluto as one of the planets in our solar system. Let's delve into this a bit further.


From the previous post, here are the IAU definitions of a planet and a dwarf planet.

• A planet is a spherical body that orbits the Sun and has cleared its orbit of other objects, i.e. it does not share an orbit with other bodies (not including moons).
• A dwarf planet is a spherical body that orbits the Sun but has not cleared its orbit of other objects. They may co-orbit with other bodies. Many of the Trans-Neptunian Objects, Kuiper Belt Bodies, Oort Cloud comets may have the same semi-major axis as other objects, therefore are not planets.

As to these current definitions, we can see that Pluto is not a planet. All the arguments I made are in the previously linked post. A nice thing about science is that new information can change our understanding of nature and the universe. Science is a fluid subject. Our perceptions can alter. So that is why it may be important to reinvestigate the idea of a planet.


If we were to redefine what makes a planet, we should be clear on what is and what is not a planet. Pluto is smaller than seven moons in our solar system, including our Moon. However, Mercury is also smaller than the two largest moons, Ganymede (orbiting Jupiter) and Titan (orbiting Saturn). We can all agree that Ganymede, Titan, the three other Galilean satellites, Triton (orbiting Neptune), and our Moon are NOT planet, they are moons. They orbit around planets which in turn orbit around the Sun. Pluto, only orbits the Sun (though it can be argued that it also orbits around the common center of mass of its system (Pluto, Charon, and its other orbital companions).


If the IAU does change the definition of a planet, it will have to get rid of the idea of co-orbiting bodies that are a significant fraction of the largest body's mass and radius. Remember, Charon is about 11.6% the mass of Pluto and has a radius about half that of Pluto. Our Moon is only 1.2% the mass of Earth and has a radius just over a quarter of the Earth. Looking at all the large satellites of the gas giant planets shows that all of them are significantly smaller in comparison to their parent planet than our Moon is to the Earth.


So if the definition is changed, what other objects in our solar system will have to be redesignated as a planet? Pluto is obviously the first. Eris will also have to redefined as it is a larger body than Pluto. After that, it depends on what the lower limit the IAU wants to use. Makemake may become a planet, Ceres may as well, though if Ceres does get redefined, than all the trans-Neptunian objects larger than Ceres will have to be classified as planets. Not only that, we will have to add a third type of planetary body along with terrestrial and Jovian. This will have to be something in between, though that type is not really a bridge between terrestrial and Jovian.


At the moment, I personally like the definition we have for planets. It's clear, concise, and makes a lot of sense. But as I said before, science can change and our understanding of what is going on can help us make more informed conclusions. Only if the IAU changes the definition of a planet, only then will Pluto, Eris, and some of the other dwarf planets/minor planets in our solar system become full-fledged planets.


We will learn more about Pluto once New Horizons reaches the Plutonian system in July of 2015. Then we will know more about Pluto and its sisters and may be able to make more informed conclusions about what they are.

Ganymede

Surface of Ganymede

Image Credit:

USGS Astrogeology Science Center/Wheaton/ASU/NASA/JPL-Caltech

 



Ganymede is the third closest Galilean Moon, and the largest of all Jupiter's moons. In fact, it is actually larger than Mercury and our Moon, and only three quarters the size of Mars. It has a density of only 1.9 g/cm³, about two times the density of water, implying that Ganymede is composed mostly of water and ice, though it does contain some rocky material and a small iron core. The iron core is inferred from Ganymede's size. It is large enough that in the past, radioactivity in the interior made the iron and rock molten, allowing for the heavier iron to sink to the center of the moon. Its ice crust is about 500 km thick, but it does have a 5-km thick liquid water layer about 170 km below the surface. Despite it being larger than Europa, its ice crust is too thick to allow life to exist, so we believe.

 

 Ganymede orbits at approximately 1.07 million km from Jupiter, giving it an orbital period of 7.15 days (twice that of Europa and four times that of Io). Its radius is 41.3% that of Earth, but its mass is only 0.025% of Earth, as Earth is mostly iron, nickel, and rocky material while Ganymede, as mentioned above, is mostly ice and water. Much like the Moon as it orbits the Earth, Ganymede is tidally locked to Jupiter. For that matter, so are Io, Europa, and Callisto. This means that for every orbit the Galilean moons make around Jupiter, each one rotates on its axis once. So much like the same face of the Moon is facing Earth, the same faces of Io, Europa, Ganymede, and Callisto are pointed to Jupiter.

 

Tidally locked moon orbiting a planet

Small line pointed to planet at all times

 

 

Ganymede has a surface that is not uniform. About a third of the surface is old, dark, and heavily cratered. We know that this makes the surface old as the early Solar System was heavily bombarded by asteroids and comets, though there are some impacts today, but not on the scale seen when the Sun was just being born. A way to look at this is to compare the surface of the Earth with that of the Moon. The Earth has a very active surface, from earthquakes, volcanoes, flowing water, and the weather, where the Moon is not active at all. A larger percentage of the surface of the Moon is cratered compared to the Earth. We know that we are not being impacted as much now as the Earth was in the beginning, as we do not see large meteors every day. However, Ganymede also has a large percentage of its surface that is much younger. Much like Io and Europa are affected by tidal forces from Jupiter and the other Galilean moons, Ganymede is as well. The tidal forces can cause fractures in the surface crust, allowing liquid water to come up to the surface and flow. This flowing water creates grooves on the surface, which flow through the craters. If the grooves formed before the craters, then the grooves would be broken, but we see the grooves as continuous.

"Old" surface vs. "New" surface

The left side of the image is the older surface of Ganymede, dark and cratered. The right side is younger, with less craters and grooves running along the surface

Image Credit:

NASA/JPL/DLR

 

The Satellites of Mars

Mars has two satellites, Phobos and Deimos. They are named after creatures summoned by Ares (the Greek equivalent of the Roman war god Mars) in the Iliad and their names mean Fear (Phobos - where the word Phobia comes from) and Fright (Deimos). Unlike the Moon, these satellites did not form in the same location in the solar nebula as Mars but rather formed elsewhere (likely, the asteroid belt between Mars and Jupiter) and wandered too close to Mars and were captured by its gravity.

AreaVoices.com

 

Phobos is an 11-km diameter, irregularly shaped object that only orbits 6000 km from the surface of Mars. Compare that to the Moon, which orbits 384,400 km from the surface of the Earth. If the Moon orbited only 6000 km from the Earth, not only would our tides be much higher (see post on tides) but the Moon would appear 64 times bigger in the sky making it about 32° across. At that apparent diameter, it would fill up a sixth of the sky!  Because Phobos is so close to Mars, it only takes about seven and a half hours to orbit Mars, which means that it crosses the Martian sky twice in one Martian day, taking only about four and a quarter hours to cross the sky.  It also orbits retrograde around Mars, meaning that it rises in the west and sets in the east.

 

Deimos is smaller than Phobos, being only 6.2 km in diameter, but orbits much farther away, at 23,500 km from the Martian surface.  At that distance, Deimos takes about 30.3 hours to complete one orbit around Mars, or about a Martian day and a quarter.

 

Both Phobos and Deimos were discovered in August of 1877 by Asaph Hall at the United States Naval Observatory in Washington, D.C. (Quick fact - the USNO is home to the official Master Clock for the US and is also the official residence of the Vice President.)  Despite being smaller, Deimos was actually discovered first on August 12th and Phobos was discovered on August 18th.  The names were suggested by Henry Madan from the Iliad.

 

The reason why Phobos and Deimos are believed to be captured asteroids is because they are similar in composition, albedo, densities of C- or D-type asteroids.

• C-type asteroids (carbonaceous asteroids)
• The most common type of asteroids (make up about 75% of all asteroids
• They have a low albedo which means they do not reflect a lot of light, almost appearing black
• Their compositions are similar to the early solar nebula except for the lack of volatile elements (gases, water, etc) but do contain hydrated minerals (water-containing minerals)
• D-type asteroids
• They have a lower albedo than C-type asteroids
• Their spectra are the strongest toward the red end of the electromagnetic spectrum
• They contain organic, carbon, and anhydrous (lacking-water) silicates
• However, they may have water ice cores

 

Tides

The tides on Earth are driven by gravity, but not Earth's gravity.  The main reason we have tides is from the Moon.  The Sun affects the tides to some extent, but the Moon is the chief driver of the tides.  The gravitational pull from the Moon on the Earth attracts the oceans in a way to create tides.

High tides occur when the Moon is directly overhead or overhead on the opposite side of the Earth. Low tides are when the Moon is on the horizon.  When the Moon is directly overhead, the Moon is pulling water towards it, creating a bulge.  This is called a sublunar tide.  When the Moon is at the other side, it is pulling the Earth away from the water.  We call this an antipodal tide.  At the horizon, it is basically pulling the water along the surface of the Earth, and we have low tide.

In reality, since the Earth is rotating, the tides actually follow a couple hours after the location of the Moon.  But for our purposes, we can safely assume the Moon is directly overhead or at the horizon.

During the lunar cycle, there are times when the Sun, Earth, and Moon line up in a condition called syzygy.  These are when the Moon is full or at new phase.  Tides are higher than normal because of the combined gravitational attraction of the Sun and the Moon.  During the Full Moon and New Moon, the high tides are called spring tides. During the first quarter or third quarter, the Moon and the Sun are 90° apart in the sky and high tides are at the lowest heights.  We refer to these high tides as neap tides.

If there hadn't been a Moon, we would still have tides, but they would only be affected by the Sun.  High tides and low tides would be much different than we have today.  In fact, if there hadn't been a Moon, ground-based life might not exist.  Biologists believe that life began in the oceans and as tides rose and fell, some of that life might have been left behind on the shores, especially during spring tides.  This would force that life to adapt to life on land and evolve into air-breathing creatures.  If there hadn't been a Moon, intelligent life might still have evolved, but would have developed in cetaceans, rather than primates.

The Moon also affects atmospheric tides, but since air is less dense than water, the tides are not as pronounced.  Atmospheric tides do add to weather and climate on Earth.

Lastly, as mentioned in a previous post (The Origin of the Moon), the Moon is slowly receding from the Earth.  As it gets farther and farther away, the size of the tides will decrease.  Since the recession is only 2 cm/century, it will take millenia for the size of the tides to be noticable.

Supermoon

I probably should have posted this a couple of days ago, but oh well.

The Supermoon of 2014 just occured this past weekend (August 10). 

The Supermoon is not the newest superhero of DC or Marvel.

The Supermoon is the full moon happened to occur during perigee.  The Moon appeared the largest because it happened to be at its closest in its orbit to Earth.  Tides (which will be discussed further in a future post) happened to be a tad higher, but nothing else really is affected by a Supermoon.  It's just cool to see the Moon so large.

Return to the Moon

We have not been to the Moon since Apollo 17 in 1972.  The reason the United States went to the Moon, originally, was not for science or exploration, but rather for political reasons.  The US wanted to beat the Soviet Union to the Moon.  On July 20, 1969, when Neil Armstrong made one small step for man and a giant leap for all mankind, a human being stood on another celestial body other than Earth for the first time in history.

Why haven't we been back?  One, it is extremely expensive to travel to the Moon.  Not only do we have to have enough fuel to get there, but we need enough to get back.  We'd also have to worry about keeping the astronauts safe while on the Moon.  Two, there really is no economic or political gain from going to the Moon.  At the moment, the only gains we would receive would be purely scientific.  There is no profit to travelling to the Moon, though sometime in the future, it may be profitable to mine the Moon.  Politically, it wouldn't make one country better than any other.  The only advantage would be if there was a multi-nation coalition to go the Moon and make it worthwhile for all humanity.  Lastly, we don't have the technology to go back to the Moon.  A whole class of new spacecraft would have to be designed, tested, and constructed for man to go to the Moon once again.

Why is this important?  There has been talk of a crewed mission to Mars, which is all well and good.  But to skip going back to the Moon first would be a huge mistake.  The Moon is much easier to get to from the Earth than Mars; it would only take a few days travel to get to the Moon, with a round trip only taking about a week.  To get to Mars, it would require at least 6 months of travel from Earth to Mars and almost two years for a round trip.  If humanity built a lunar base, it would be easier to use as a launching point for exploration of the rest of the Solar System.  It would require less energy to launch a ship from the Moon than the Earth because the Moon is much smaller and its gravity would not work as hard against launching a rocket or spacecraft.  Once we set up a permanent presence on the lunar surface, exploration of the Solar System should follow, with Mars being the most logical first step.

Another nice thing about using the Moon as a launching pad is that the materials needed to build rockets and habitats and create fuel for spacecraft are already on the Moon.  The challenge would be to harvest the material and convert it into useful products. That is obviously many years in the future, but we still need to return to the Moon before thinking about going to Mars.

The Origin of the Moon

When we first explored the Moon, we weren't sure what we would find.  We expected to find similar material that we find on Earth.  What we found is even more unusual.

First, why did we expect similar material on the Moon?  Based on the location of the Earth in the Solar System (and by default, the location of the Moon), we expected to find refractory elements because they have a higher boiling point than volatile elements, i.e. they vaporize at high temperatures which in the early Solar System included the location of the Earth.  All planets close to the Sun are generally made up of this type of material because they are close to the Sun.  This is why we believed that terrestrial planets are close to their central star and Jovian planets are far from their star.  Because of this, we expected to find the Moon was made up of refractory elements.

However, when we actually went to the Moon, we discovered something really strange; the composition of the Moon is nearly identical to that of the interior of the Earth.  Not that we found the same material, but that the concentrations were the same.  So what does this tell us?

This tells us that when the Earth was very young, something collided with the infant Earth to create the Moon.  A Mars-sized body collided with the Earth to create the Moon and to leave some material on the Earth.  This helps explain three things:

1. The Moon's composition and why it is nearly identical to the Earth's composition
2. The Moon is receding from Earth. Recall that the Moon is moving about 2 cm away from the Earth every century (See blog post on the eclipses)
3. The 1:1 Moon-Earth resonance since the Moon formed from the Earth

Daily Galaxy

Synodic Period vs. Sidereal Period

As mentioned before, the sidereal period of an object depends on that object's alignment with the body it orbits and with a distant, background star.  When we talk about the Moon, we need to clarify the difference between the sidereal month and the synodic month.

Last time, we talked about the lunar resonance, explaining why the same face of the Moon is always pointed towards Earth.  When we measure the time from Full Moon to Full Moon (or New Moon to New Moon), it takes about 29.5 days to complete one cycle of phases.  We call this the synodic month and it is also the reason why months are approximately 30 days.  But the sidereal month is only 27.3 days.  The reason why it is shorter than the synodic month is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.

Lunar Resonance

Despite what my wife thinks (and the Pink Floyd albums says), there is no such thing as a "Dark Side of the Moon". Granted, the face of the Moon facing away from the Sun is dark, but that face changes as the Moon orbits the Earth.  A better description of the faces of the Moon would be to call them the Near Side of the Moon and the Far Side of the Moon.  The near side is the face that always pointed to the Earth, and the far side is always pointed away.  Why is this?

The reason why we should use near side and far side is because the Moon is in a 1:1 resonance with the Earth.  Remember that the Sun-Mercury system has a 3:2 resonance, so for every two orbits around the Sun, Mercury rotates three times on its axis.  For the Moon-Earth system, this means that for every one complete orbit around the Earth, the Moon rotates on its axis just one time.  Because of this, the same side of the Moon is always facing Earth.  It wasn't until 1959, when the Soviet Union's Luna 3 space probe photographed the far side.  In 1968, it wasn't observed by human eyes for the first time during the Apollo 8 mission.

Eclipses

Previously, we learned about the phases of the Moon. We saw how it goes from new Moon to full Moon back to new Moon again. We know that during the new phase, the Moon is between us and the Sun. During the full phase, the Earth is between the Sun and the Moon. We also know the apparent diameter of the Sun and the Moon are nearly identical, at about 0.5°.  So how come we don't see the Moon blocked out when its full and why isn't the Sun always eclipsed during the new Moon phase?

The main reason why is the Earth and the Moon do not orbit in the same plane.  The Moon's inclination is about 5° compared to the ecliptic.  Therefore, during most new Moon phases or most full Moon phases, the Moon is not in the same alignment with the Earth and the Sun.  This 5° inclination leads to the eclipses to happen every six months.

 



There are two types of eclipses: solar eclipses and lunar eclipses.  Solar eclipses occur when the Moon crosses in front of the Sun, blocking us from seeing the Sun on Earth.  Lunar eclipses occur when the Moon passes through the shadow of the Earth.  A lunar eclipse (or a solar eclipse) follows two weeks after the associated solar (or lunar) eclipse.  Solar eclipses and lunar eclipses never occur more than 14 days apart.

Lunar eclipses occur during the full Moon phase as the Earth is between the Moon and the Sun.  If the Moon is completely in the shadow, or umbra, of the Earth.  Since the Earth is so much larger than the Moon, the shadow of the Earth completely envelopes the Moon during a lunar eclipse.  Also, this allows a lunar eclipse to occur even if the Earth, Sun and Moon are not completely in line.  If the Moon is slightly out of the umbra and partially in the penumbra, partial lunar eclipses will occur.  From viewing partial lunar eclipses, ancient astronomers could tell that the Earth was round, not flat, because of the shape of the Earth's shadow on the Moon.

Solar eclipses occur during the new Moon phase when the Moon crosses in front of the Sun.  These are only total if the Moon is near perigee and the Sun, Moon, and Earth are completely aligned.  If the Moon is nearer apogee than perigee, a solar eclipse will be annular, or ring-like, because there will be a ring of sunlight around the outline of the Moon.  As with lunar eclipses, if the Moon is slightly off center of the ecliptic, then a partial solar eclipse will occur.

Total Solar Eclipse (NASA)

Annular Solar Eclipse (Wikipedia)

In the future, total solar eclipses will not longer occur because the Moon is moving away from the Earth at a rate of 2 cm/century.  We will learn more about the recession of the Moon later.

Our Nearest Neighbor

The nearest celestial body to the Earth is obviously our Moon.  We've already discussed the phases of the Moon and how they relate to the Moon's position with respect to the Sun and the Earth.  In the next few posts, we will learn about the Moon.

For example, how come eclipses don't happen every month? Why is there a supermoon? Where did the Moon come from?  What would life be like if the Moon had never existed or what would life be like if we decided to blow up the Moon (it was actually thought of at one point)?  Why should did we go to the Moon originally, why haven't we gone back, and should we go in the future?

Before we get into these topics, let's learn a little about our Moon. 

• It is the only natural satellite of the Earth and is the fifth largest satellite in the solar system, behind Ganymede, Titan, Callisto, and Io. It is the only celestial object, besides the Earth, that humans have stood on.
• It is approximately 238,900 miles from Earth or about 384,400 kilometers, and it took Neil Armstrong, Buzz Aldrin, and Michael Collins about 4 days to reach it from Earth. 
• It has a sidereal period of 27.3 days and a synodic period of 29.5 days. We will talk about this in a future post. 
• It has no atmosphere to speak of and therefore, no life. 
• The Moon has a radius of 1,737 km (27.3% of Earth's) and a mass of 7.35x10²² kg (1.23% of Earth’s). On the Moon, you would weight approximately 1/6 of what you weigh on Earth. That’s why in videos, the astronauts on the moon always looked like they were bouncing.  It would actually be more difficult to walk normally.
• At one point in its history, the Moon was active. This was likely to the cooling it went under after it formed and its constant bombardment by asteroids early in its life.
• It has an inclination of about 5° to the ecliptic, which I realize now I have never talked about.  To learn about the ecliptic and what it is, click here.

 

The Phases of the Moon

We are going to take a break from talking about Venus to explain about the phases of the Moon.  This background is necessary when we get into the next post about Venus.

The Moon goes through basically eight phases as it orbits the Earth.  These phases are a result of the Moon's alignment with both the Earth and the Sun.  Before we go into the descriptions of the phases, we should define two terms:

The sidereal lunar cycle, or "sidereal month" is how long it takes the Earth, Moon, and a background star to line up.  This period last about 27.3 days.  After another 2.2 days, the Earth, Moon, and Sun are lined up in what is called the lunar month, or synodic month.  This period is approximately 29.5 days.  The difference between these periods is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.

The first phase during the lunar month is called the "New Moon".  Basically, it is when the Moon is between the Earth and the Sun.  The unilluminated face of the Moon is pointed towards the Earth.  However, the new Moon is not completely dark.  Earthshine, reflection of sunlight off the Earth's surface can illuminate the surface of the Moon.

Farmer's Almanac Online

 

The next phase is the Waxing Crescent phase.  Waxing in astronomy means that something is increasing in size.  As the Moon moves around the Sun, a sliver of the Moon on the right limb is illuminated by the Sun as seen from the Earth.  What we see is the crescent shape of the Moon.





 

Farmer's Almanac Online

The Moon is said to be aging as it goes from New Moon to Waxing Crescent.  About a week after the New Moon, the Moon reaches the First Quarter phase because it has reached the end of one quarter of the lunar cycle.  The entire right half of the Moon is illuminated as seen from the Earth.

10 Minute Astronomy



 

The next phase is called Waxing Gibbous.  The illuminated surface of the moon crosses the center of the moon and spills over into the left half of the Moon.  The illuminated portion of the Moon resembles an oval with pointed ends.

Farmer's Almanac Online



Halfway through the lunar cycle, we hit the Full Moon. No, werewolves do not appear during the Full Moon.  People do not get crazy during the Full Moon, though it may appear to be so.  The Full Moon is the phase where the Earth is between the Sun and the Moon and from Earth we see the entire face of the Moon illuminated.

Farmer's Almanac Online

After the full Moon passes, the illuminated portion of the Moon begins to decrease, or wane.  The next phase is the Waning Gibbous phase.  The right limb has a darkened crescent shape, and the left half and a portion of the right half of the Moon are illuminated, opposite the Waxing Crescent phase.



Farmer's Almanac Online

After three weeks in the lunar cycle, the Moon reaches the Last Quarter or sometimes called Third Quarter.  It is called this because either the Moon is at the beginning of the last week of the lunar cycle or the end of the third week of the lunar cycle.

Astropixels



In the final week before the Moon is "reborn" into the New Moon, the Moon goes through the Waning Crescent phase.  Only a sliver on the left limb of the Moon is illuminated. 

Farmer's Almanac Online

 

Here is an image showing the relative position of the phases with respect to the Earth and the Sun.





From Swinburne Astronomy Online