Twinkling Stars and Static Planets

Twinkle, Twinkle Little Star
How I wonder what you are.




Everyone remembers this nursery rhyme. The question is why do stars twinkle? And how come planets do not?




The short answer is that it has to do with the apparent diameter of the star and the planet. Stars are generally so far away that they have no apparent diameter, so that when light from the star enters our atmosphere, the light is easily diffracted due to different pockets of air. Since the width of the light is so small, these diffractions make it seem as if the star's light is constantly blinking in and out as the light is diffracted away from our eye as the light travels through the atmosphere.




For planets, however, they do have an apparent diameter, albeit much tinier than that of the Sun or the Moon. But this size is enough that when light from the planet travels through different pockets of air in the atmosphere, only a small portion of the light is diffracted away from our light of sight, so the brightness of the planet does not waver, and therefore does not twinkle.

The Hexagonal Storm on Saturn

Saturn's Hexago

Image Credit:

NASA/Cassini

North Polar Hexagon on Saturn

Image Credit:

NASA/JPL-Caltech/Space Science Institute

 

One of the strangest features on Saturn is the hexagon-shaped storm in its northern hemisphere. The storm was first discovered by Voyager in 1981-1982 and has been persistent since. It is located at 78° N latitude, but does not change longitude over time, unlike other storms on Saturn. The sides of the storm are about 13,800 km, longer than the Earth's diameter of abotu 13,500 km.

The Cassini mission also has taken images of the storm since its arrival in 2006. And if the conditions are right, i.e. the storm is in daylight, fuzzy images of the storm can be seen from Earth-based telescopes, even by amateur astronomers.

Oxford University astronomers proposed a hypothesis for the formation of the storm. In the lab, regular shpaes were created in a circular tank of liquid that had different rotation rates at the center and at the edges. Squares, hexagons, and octagons were all created, with hexagons being the most common shape. These latitudinal gradients in the rotation are one probable cause for the hexagon on Saturn.

Titan

Cassini Image of Titan showing the atmosphere of the moon

Image Credit:

NASA/Cassini

 

Titan is the largest Saturnian moon and the second largest in the Solar System behind Jupiter's moon Ganymede. Like Ganymede, it is also larger than Mercury. Christian Huygens, who discerned the rings of Saturn, also discovered Titan in 1655, making it the fifth satellite discovered with the telescope.

 

It is the only moon known to have a dense atmosphere, where atmospheric pressure is measurable on the surface. Though Europa, Ganymede, and Callisto may have liquid oceans below their outer crusts, Titan is the only body in the Solar System to have surface liquid besides the Earth. However, you wouldn't want to swim in those oceans as they are bodies of methane.

 

Because it has liquid methane oceans and lakes, much like Earth has a water cycle, Titan experiences a methane cycle. Surface methane evaporates and forms clouds in the Titan sky. The rain Titan experiences is methane.

 

Although Titan is smaller than Earth, its atmosphere is dense enough to create higher surface pressure than on Earth, at about 1.45 atmospheres (146.7 kPa). One atmosphere on Earth is the normal pressure at sea level which is 101.5 kPa (kilopascals). As shown in the above image, the atmosphere is extended with a composition of 98.4% Nitrogren (N₂), 1.4% methane (CH₄), and trace other molecules including water in the stratosphere (higher levels) and 95% N₂, 4.9% CH₄, and other molecules in the troposphere. Because of the methane clouds, the sky is very hazy on Titan so would have poor visibility on the surface when we make our first crewed mission to Titan sometime in the future.

 

The gravity on Titan is only 85% of that on the Moon even though it is larger because of its smaller density. If you were to stand on Titan, your weight would only be 15% of that on Earth.

 

Much like the Moon and Earth are tidally locked, Titan is tidally locked to Saturn, so for every orbit around Saturn, which is almost 16 Earth days, it only rotates once on its axis.



Saturn

Image Credit:

NASA/Voyager 2

 

Saturn is the second largest planet in the Solar System, about 95 times the mass of the Earth. However, compared to Jupiter, Saturn is tiny. It is only 30% the mass of Jupiter. Its radius at the equator is 9.44 times that of Earth and its polar radius is only 8.5 times Earth's. Despite this, if you could stand on Saturn, you would feel the same gravity as you do on Earth. Its day is just a little longer than Jupiter's at 10.57 Earth hours. Its average distance from the Sun is 9.5 AU which gives it an orbital period of 29.46 Earth years. It has an inclination of 26.5° with respect to its orbit and its orbit is only tilted at 2.5° to the ecliptic (the orbit of the Earth). With respect to the Sun's equator, it is tilted at 5.51°.

 

The thing Saturn is most known for is seen in the above image from Voyager 2. Its ring system is the most extensive of all the Jovian planets and Saturn has been known to have rings since the 1600s. The rings are a fascinating aspect of the most beautiful planet (in my opinion) in our Solar System, so that I can't talk about all rings in only one post. Stay tuned to learn a lot about Saturn's rings.

 

Saturn also has many moons, almost three times as many as Jupiter with 150 known, though only 51 have formal names. The largest satellite in the Solar System, Titan, belongs to Saturn and could be considered a mini-world in its own right. It also has moons that keep some of Saturn's rings in line, called shepherd satellites. It has a moon that doesn't look like a moon and moons that share an orbit.

 

Saturn's composition is similar to Jupiter, containing the same gases, but in different concentrations. This difference in concentrations, the thickness of its atmosphere, and the size of its heavy element core give rise to a strange phenomenon when looking at Saturn's density.

Composition of Jupiter

Jupiter's density is about 1.3 times that of water. So even though we know that Jupiter has a heavy element core, we know that the composition of Jupiter is mostly hydrogen and helium like the Sun. The outer layers of Jupiter are the lighter elements, like hydrogen and helium.

Jupiter itself is about 78% molecular hydrogen (H₂) and 19% helium with trace amounts of methane (CH₄), ammonia (NH₃) and water (H₂O). The pressure on Jupiter is so great that the gases which are gas at normal pressure and temperatures on Earth are liquid on Jupiter. Really deep in the interior, something even stranger happens to hydrogren. Not only is it liquid, but it becomes something called liquid metallic hydrogen. Metallic hydrogen just means that the molecules are organized in such a way that the atoms share electrons easily, allowing the liquid hydrogen to be electrically conductive. Try that with normal liquid hydrogen (though there really isn't anything normal about liquid hydrogen because it needs to be at 14 Kelvin (-253°C or -423°F) to be liquid. Obviously, liquid metallic hydrogen does not occur naturally on Earth, but can be created in labs under the correct pressure and temperature conditions.

The outer layers of Jupiter are its atmosphere which is "only" 1000 km thick, but that is only 1% of Jupiter's radius of 72,000 km. Compare that to Earth which has an atmosphere that is 2.5% of the Earth's radius. The atmosphere is composed of hydrogen gas with different cloud layers. The topmost cloud layer is ammonia crystals at a temperature of 150K, followed by ammonia hydrosulfide at 200 K, with the lowest cloud layer being water at 280K. The Galileo probe, whose images have been included in some of these posts, was also used to explore the atmosphere of Jupiter. After entering the atmosphere, the probe reached darkness about 80km down and was completely destroyed by the high pressure and heat at only 130 km down.

The Banded Atmosphere of Jupiter

Infrared and Visible Light Image of the Bands of Jupiter 

Image Credit:

NASA

One of the most striking features when you look at images of Jupiter is that the atmosphere is not homogeneous, but banded. The Jovian planets are the only planets that display this feature, though Jupiter's is the most pronounced.

The bands are regions of high pressure, rising gas and low pressure, sinking gas separated by high winds. The bands that have high pressure, rising gas are called zones. The zones are bright but cold. They are made up of ice particles that are either ammonia or water ice. The darker bands, called belts, are low pressure, thin clouds that are sinking towards the planet. The belts do not reflect as well as the zones, so appear darker in comparison. The high winds separating the bands are called jets, travel along lines of latitude (east-west), and blow in either the prograde direction or retrograde direction. The prograde jets travel along with the direction of rotation of the Jupiter (from west to east) and the retrograde jets travel against the direction of rotation (east to west). The prograde jets are the transitions from zones to belts (as you move away from Jupiter's equator) and the retrograde jets transition from belts to zones..

The zones and belts are distinct on the surface of Jupiter. Just like Earth, Jupiter has distinct temperature regions based on their location on the planet. They are named as follows:

• The Equatorial Zone: as the name implies, this is the zone at the equator. Its range is from 7°S to 7°N.
• The North and South Equatorial Belts: the belts just north and south of the Equatorial Zone. They are reddish in color and range from 7°N (S) to 18°N (S).
• The North and South Tropical Zones: the South Tropical Zone is where the Great Red Spot is located
• The North and South Temperate Belts: the South Temperate Belt contains Oval BA (Red Jr.)
• The North and South Temperate Zones
• The N-N and S-S Temperate Belts
• The N-N and S-S Temperate Zones
• The North and South Polar Regions

A description of the belts and zones

Image Credit:

Sky and Telescope



 

The jets between the zones and belts travel at over 100 m/s (360 kph or 224 mph) and keep the bands from mixing.

Venus' Atmosphere

The atmosphere of Venus is one of the thickest atmospheres in the Solar System, and is by far, the thickest of the terrestrial planets.  It is mainly carbon dioxide (CO2) with 96.5% of the atmosphere made up of CO2.  It has about 3.5% nitrogren (N2) and trace other elements with sulfur compounds beign a major portion.  Compare this to the Earth with 78% N2, 21% O2, and trace other gases (argon being the chief among those gases).

The atmosphere is so thick that the atmospheric pressure at the surface (what we would call sea level on Earth) is 92 times that of Earth.  A cubic meter of air on Earth has a mass of about 1.2 kilograms, or weighs about 10 pounds.  On Venus, the same volume of air has a mass of 67 kilograms, or weighs 600 pounds on Venus (on Earth, that volume of air would weigh 670 pounds).  This weigh is so heavy, that its atmosphere at the surface would squash you flat and kill you, if the oppressive heat didn't get you first.

Besides being oppressive, the heat is the most impressive thing about Venus' atmosphere.  Despite being farther from the Sun than Mercury, its surface temperature is hotter.  Because Mercury has such a thin, if lacking, atmosphere, it does not retain heat well.  With Venus thick atmosphere composed of mostly carbon dioxide, the atmosphere does a great job of retaining heat reflected and emitted by the surface of the planet.  Carbon dioxide a really good job of preventing infrared radiation from escaping into space which in turn heats up the atmosphere.  This lead to a runaway greenhouse effect which increases the heat on Venus' surface.  On Venus, surface temperatures can reach 462°C (864°F) where on Mercury, in sunlight, reaches "only" 420°C (788°F).  Mercury does drop below freezing on the side facing away from the Sun at -220°C (-364°F) because the lack of an atmosphere.

At the same time, it is nearly impossible to see the surface of Venus without some help.  The intense cloud cover does not allow visible light to escape.  On Earth, our clouds are made of water vapor and droplets.  Venus' clouds are hydrogen sulfide and sulfuric acid.  Not easy material for visible light to traverse.  These clouds allow 50% of the visible light to come through and heat the ground, leading to the reflection and emission of infrared light, while the other 50% is reflected into space.  What we see when we look at Venus is the cloud cover.

To see Venus itself, we use radio waves which have long enough wavelengths to travel through the clouds.  The reflected radio waves can then be detected and map the surface.  This was what the spacecraft Magellan did to show us the planetary features.  Any images of the surface of Venus are all false color.

Barren Mercury

As mentioned before, Mercury is a planet.  But it is unique in our Solar System in that it is the only planet without any type of atmosphere.

There may be a couple of reasons for it:

1. It may be too small to hole an atmosphere.  Mars is the second smallest planet but has a very thin atmosphere.  Because of their small sizes, the escape velocity for the planets are relatively small.  Most gas molecules have a relatively fast speed when moving about in the atmosphere, so they could escape easily.  However, this explanation may not be true. Callisto, as you recall, is similar in size to Mercury. But it does have a thin atmosphere.  The next bullet point is probably the correct explanation.
2. Mercury is extremely close to the Sun, approximately 0.4 AU out.  The Sun has a very active solar wind and that solar wind is extremely hot.  Possibly in the early formation of the Solar System, the primordial solar wind stripped Mercury of its atmosphere, which would have likely been very tenuous to begin with.  If Mercury had an atmosphere, it would have been similar to that of Callisto, with probably and composition similar to that of Mars, mostly carbon dioxide.

Next time, we will learn about Mercury's strange orbit.