Monday, July 11, 2011

Enceladus Was A Child Of Gaia

Today’s post is going to be a little strange.

I’m working on a bunch of stuff right now. Unfortunately I have no idea exactly how or when everything or even anything will come together. Maybe late this week I will have some new music or a new video (or some new drawings!). Or maybe next week. Or maybe later. I’m just not sure right now.

(An aside: Today’s post talks about Enceladus. Gaiaamong other things—is the name of a cool synthesizer. I’ve more or less decided not to buy a synthesizer—On Not Playing A Synth Workstation #1 and On Not Playing A Synth Workstation #2—but if I were going to buy a synthesizer, I’d probably buy a Gaia.)

Anyway, I want to do a post to start setting up something of what I will be posting about later. That’s what today is about. So here goes.


First, here is a story from NASA about one of Saturn’s moons, Enceladus, interacting electrically with Saturn’s magnetosphere:

Electrical Circuit Between Saturn and Enceladus

This artist's concept shows a glowing patch of ultraviolet light near Saturn's north pole that occurs at the "footprint" of the magnetic connection between Saturn and its moon Enceladus. The footprint and magnetic field lines are not visible to the naked eye, but were detected by the ultraviolet imaging spectrograph and the fields and particles instruments on NASA's Cassini spacecraft. The footprint, newly discovered by Cassini, marks the presence of an electrical circuit that connects Saturn with Enceladus and accelerates electrons and ions along the magnetic field lines. In this image, the footprint is in the white box marked on Saturn, with the magnetic field lines in white and purple.

A larger white square above Enceladus shows a cross-section of the magnetic field line between the moon and the planet. This pattern of energetic protons was detected by Cassini's magnetospheric imaging instrument (MIMI) on Aug. 11, 2008.

The patch near Saturn's north pole glows because of the same phenomenon that makes Saturn's well-known north and south polar auroras glow: energetic electrons diving into the planet's atmosphere. However, the "footprint" is not connected to the rings of auroras around Saturn's poles (shown as an orange ring around the north pole in this image).

The Cassini plasma spectrometer complemented the MIMI data, with detection of field-aligned electron beams in the area. A team of scientists analyzed the charged particle data and concluded that the electron beams had sufficient energy flux to generate a detectable level of auroral emission at Saturn. Target locations were provided to Cassini's ultraviolet imaging spectrograph team. On Aug. 26, 2008, the spectrograph obtained images of an auroral footprint in Saturn's northern hemisphere.

The newly discovered auroral footprint measured about 1,200 kilometers (750 miles) in the longitude direction and less than 400 kilometers (250 miles) in latitude, covering an area comparable to that of California or Sweden. It was located at about 65 degrees north latitude.

In the brightest image the footprint shone with an ultraviolet light intensity of about 1.6 kilorayleighs, far less than the Saturnian polar auroral rings. This is comparable to the faintest aurora visible at Earth without a telescope in the visible light spectrum. Scientists have not yet found a matching footprint at the southern end of the magnetic field line.

The background star field and false color images of Saturn and Enceladus were obtained by Cassini's imaging science subsystem.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif. manages the mission for NASA's Science Mission Directorate, Washington, D.C. The ultraviolet imaging spectrograph team is based at the University of Colorado, Boulder. The magnetospheric imaging team is based at the Johns Hopkins University Applied Physics Laboratory, Laurel, Md. The Cassini plasma spectrometer team is based at the Southwest Research Institute, San Antonio, Texas.

Now the situation in local space, here, around the Earth, is quite a bit different.

The Earth is much smaller than Saturn, and the Earth’s magnetosphere is also smaller. It is still, however, interesting. Here are some tid-bits about Earth’s magnetosphere from Wikipedia. Most of these excerpts will use the radius of the Earth as a measure. For scale, the Moon generally orbits at around 60 times the Earth’s radius. This is an interesting distance, in this context, and I’ll discuss that a little below.

Here is a general diagram of the Earth’s magnetosphere:

Notice that the magnetic field lines are “compressed” on the side of the Earth facing the Sun, and extended on the side of the Earth away from the Sun.

On the Sun-ward side, the magnetosphere typically extends for about twelve times the radius of the Earth.

The magnetosphere of Earth is a region in space whose shape is determined by the Earth's internal magnetic field, the solar wind plasma, and the interplanetary magnetic field (IMF). In the magnetosphere, a mix of free ions and electrons from both the solar wind and the Earth's ionosphere is confined by electromagnetic forces that are much stronger than gravity and collisions.

Despite its name, the magnetosphere is distinctly non-spherical. All known planetary magnetospheres in the solar system possess more of an oval tear-drop shape because of the solar wind.

On the side facing the Sun, the distance to its boundary (which varies with solar wind intensity) is about 70,000 km (10-12 Earth radii or RE, where 1 RE = 6371 km; unless otherwise noted, all distances here are from the Earth's center). The boundary of the magnetosphere ("magnetopause") is roughly bullet shaped, about 15 RE abreast of Earth and on the night side (in the "magnetotail" or "geotail") approaching a cylinder with a radius 20-25 RE. The tail region stretches well past 200 RE, and the way it ends is not well-known.

The outer neutral gas envelope of Earth, or geocorona, consists mostly of the lightest atoms, hydrogen and helium, and continues beyond 4-5 RE, with diminishing density. The hot plasma ions of the magnetosphere acquire electrons during collisions with these atoms and create an escaping "glow" of energetic neutral atoms (ENAs) that have been used to image the hot plasma clouds by the IMAGE and TWINS missions.

The upward extension of the ionosphere, known as the plasmasphere, also extends beyond 4-5 RE with diminishing density, beyond which it becomes a flow of light ions called the polar wind that escapes out of the magnetosphere into the solar wind. Energy deposited in the ionosphere by auroras strongly heats the heavier atmospheric components such as oxygen and molecules of oxygen and nitrogen, which would not otherwise escape from Earth's gravity. Owing to this highly variable heating, however, a heavy atmospheric or ionospheric outflow of plasma flows during disturbed periods from the auroral zones into the magnetosphere, extending the region dominated by terrestrial material, known as the fourth or plasma geosphere, at times out to the magnetopause.

Earth’s magnetosphere provides protection, without which life as we know it could not survive. Mars, with little or no magnetic field is thought to have lost much of its former oceans and atmosphere to space in part due to the direct impact of the solar wind. Venus with its thick atmosphere is thought to have lost most of its water to space in large part owing to solar wind ablation.

Due to the size of Jupiter's magnetosphere there is a possibility of very weak and very brief seasonal head-tail interaction between Earth's and Jupiter's magnetospheres. The magnetospheres of the outer gas planets may weakly interact, although their magnetospheres are much smaller than Jupiter's.


A magnetic tail or magnetotail is formed by pressure from the solar wind on a planet's magnetosphere. The magnetotail can extend great distances away from its originating planet. Earth's magnetic tail extends at least 200 Earth radii in the anti-sunward direction well beyond the orbit of the Moon at about 60 Earth radii, while Jupiter's magnetic tail extends beyond the orbit of Saturn. On occasion Saturn is immersed inside the Jovian magnetosphere.

The extended magnetotail results from the energy stored in the planet's magnetic field. At times this energy is released and the magnetic field becomes temporarily more dipole-like. As it does so that stored energy goes to energize plasma trapped on the involved magnetic field lines. Some of that plasma is driven tailward and into the distant solar wind. The rest is injected into the inner magnetosphere where it results in the aurora and the ring current plasma population. The resulting energetic plasma and electric currents can disrupt spacecraft operations, communication and navigation.

So when the Moon is waning, or new, the Moon is quite outside the Earth’s magnetosphere. When the Moon is waxing, and full, and just starting to wane, the Moon is often within the Earth’s magnetosphere.

Now the Moon is believed to have a very weak magnetic field. But there are local magnetic fields on the Moon, and Sun light/shade on the surface dust can generate electrostatic effects that we’ve seen in an earlier post, Flying Saucers And Beethoven.

This is one of the very few interesting astronomical topics that almost never gets covered on the internet or in the hobby press. There are sometimes articles written about what are termed “transient lunar phenomenon,” but those articles are usually just quick summaries of suspected strange light effects on surface features or, more rarely, speculation about out-gassing from active geology on the Moon.

However—and I’m not a physicist!—this seems like a very interesting area to speculate about.

During the three or four days per month when the Moon is within the Earth’s magnetosphere, the Moon will be cutting through magnetic lines of force tied to the Earth’s north and south magnetic poles. If there is out-gassing on the Moon and the gas interacts electrostatically and becomes plasma, I believe one would expect atoms (or molecules?) to transit along the magnetic lines of force all the way to the Earth and disperse into the Earth’s upper atmosphere. Similarly, if lightening from storms or other high-energy events generates plasma on Earth, atoms or molecules could be carried out into space along the magnetic lines of force and—if the Moon is within the Earth’s magnetosphere at the time—interact in some way with lunar dust or lunar surface features.

These would be, of course, tiny, tiny interactions. But over large stretches of time even tiny interactions could add up.

The distances in astronomy are vast, very hard to imagine sometimes. But, nonetheless, one of the lessons of modern science seems to be that everything is more inter-connected than one would suppose.


To me this is one of the three most interesting topics in astrophysics right now. I’m most interested in what is going on in the outer system; I’m also interested in what is happening at Jupiter and Saturn and just how small a dwarf star can be; and then this stuff, I’m interested in wondering about just how inter-connected the Earth and the Moon may be or may not be.

I’ll be talking more about this stuff, but I wanted to get this topic started. And so I did.

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