APOD: Mercury's Sodium Tail (2020 Jul 08)

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APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by APOD Robot » Wed Jul 08, 2020 4:05 am

Image Mercury's Sodium Tail

Explanation: What is that fuzzy streak extending from Mercury? Long exposures of our Solar System's innermost planet may reveal something unexpected: a tail. Mercury's thin atmosphere contains small amounts of sodium that glow when excited by light from the Sun. Sunlight also liberates these molecules from Mercury's surface and pushes them away. The yellow glow from sodium, in particular, is relatively bright. Pictured, Mercury and its sodium tail are visible in a deep image taken in late May from Italy through a filter that primarily transmits yellow light emitted by sodium. First predicted in the 1980s, Mercury's tail was first discovered in 2001. Many tail details were revealed in multiple observations by NASA's robotic MESSENGER spacecraft that orbited Mercury between 2011 and 2015. Tails are usually associated with comets. The tails of Comet NEOWISE are currently visible with the unaided eye in the morning sky.

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by RocketRon » Wed Jul 08, 2020 4:40 am

Presumably planet Earth would be emitting quite a tail/trail itself ?
Oxygen/Hydrogen/CFCs/assorted Carbon/organics for starters, and possibly Nitrogen ??

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by XgeoX » Wed Jul 08, 2020 10:24 am

Cool shot but I get annoyed when they talk about Mercury’s atmosphere. It’s actually more accurately termed an exosphere. It’s just a bunch of stray atoms flying around that aren’t even a gas because they never collide with one another.
Other than that Mercury is a fascinating planet. It’s a bitter disappointment that the Europeans cancelled the lander portion of their mission, would have been great to actually get surface data in situ.

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by orin stepanek » Wed Jul 08, 2020 10:26 am

MercuryTail_Alessandrini_960.jpg


Mercury; our planet comet! :mrgreen:
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by heehaw » Wed Jul 08, 2020 11:28 am

Another absolutely FABULOUS APOD!

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by Ann » Wed Jul 08, 2020 12:05 pm

Very interesting. I'm reminded of the fact that the atmosphere of Mars is also carried away by the solar wind, so obviously Mars must have a tail. REad about the solar wind's destructive influence on the atmosphere of Mars here.

Surely Earth should have a tail, too?

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by neufer » Wed Jul 08, 2020 12:55 pm

Ann wrote:
Wed Jul 08, 2020 12:05 pm

Surely Earth should have a tail, too?
  • Unlike Mars & Mercury, Earth has both a high escape velocity and a protective magnetic field.
https://en.wikipedia.org/wiki/Atmosphere_of_Mercury\ wrote:

<<Because of Mercury's proximity to the Sun, the pressure of solar light is much stronger than near Earth. Solar radiation pushes neutral atoms away from Mercury, creating a comet-like tail behind it. The main component in the tail is sodium, which has been detected beyond 24 million km (1000 RM) from the planet. This sodium tail expands rapidly to a diameter of about 20,000 km at a distance of 17,500 km.In 2009, MESSENGER also detected calcium and magnesium in the tail, although these elements were only observed at distances less than 8 RM.

Sodium in Mercury's exosphere was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm. The average column density of this element is about 1 × 1011 cm−2. Sodium is observed to concentrate near the poles, forming bright spots. Its abundance is also enhanced near the dawn terminator as compared to the dusk terminator. A year after the sodium discovery, Potter and Morgan reported that potassium (K) is also present in the exosphere of Mercury, though with a column density two orders of magnitude lower than that of sodium. The properties and spatial distribution of these two elements are otherwise very similar. In 1998 another element, calcium (Ca), was detected with column density three orders of magnitude below that of sodium. Observations by the MESSENGER probe in 2009 showed that calcium is concentrated mainly near the equator—opposite to what is observed for sodium and potassium. Further observations by Messenger reported in 2014 note the atmosphere is supplemented by materials vaporized off the surface by meteors both sporadic and in a meteor shower associated with Comet Encke.>>
https://en.wikipedia.org/wiki/Exosphere wrote:
<<The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the thermosphere. Very little is known about it due to lack of research. Mercury, the Moon and three Galilean satellites of Jupiter have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The gases that can be found in the Earth's exosphere are mostly hydrogen and carbon dioxide.

In principle, the exosphere covers distances where particles are still gravitationally bound to Earth, i.e. particles still have ballistic orbits that will take them back towards Earth. The upper boundary of the exosphere can be defined as the distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth's gravitational pull. This happens at half the distance to the Moon. The exosphere, observable from space as the geocorona, is seen to extend to at least 10,000 kilometres from Earth's surface.>>
https://en.wikipedia.org/wiki/Atmospheric_escape wrote:
<<Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and impact erosion. The relative importance of each loss process depends on the planet's escape velocity, its atmosphere composition, and its distance from its sun. Escape occurs when molecular kinetic energy overcomes gravitational energy; in other words, a molecule can escape when it is moving faster than the escape velocity of its planet. Categorizing the rate of atmospheric escape in exoplanets is necessary to determining whether an atmosphere persists, and so the exoplanet's habitability and likelihood of life.
..............................................................
Atmospheric escape of hydrogen on Earth is due to Jeans escape (~10 - 40%), charge exchange escape (~ 60 - 90%), and polar wind escape (~ 10 - 15%), currently losing about 3 kg/s of hydrogen. The Earth additionally loses approximately 50 g/s of helium primarily through polar wind escape. Escape of other atmospheric constituents is much smaller. A Japanese research team in 2017 found evidence of a small number of oxygen ions on the moon that came from the Earth. In 1 billion years, the Sun will be 10% brighter than it is now, making it hot enough for Earth to lose enough hydrogen to space to cause it to lose all of its water (See Future of Earth#Loss of oceans).
..............................................................
Recent models indicate that hydrogen escape on Venus is almost entirely due to suprathermal mechanisms, primarily photochemical reactions and charge exchange with the solar wind. Oxygen escape is dominated by charge exchange and sputtering escape. Venus Express measured the effect of coronal mass ejections on the rate of atmospheric escape of Venus, and researchers found a factor of 1.9 increase in escape rate during periods of increased coronal mass ejections compared with calmer space weather.
..............................................................
Primordial Mars also suffered from the cumulative effects of multiple small impact erosion events, and recent observations with MAVEN suggest that 66% of the 36Ar in the Martian atmosphere has been lost over the last 4 billion years due to suprathermal escape, and the amount of CO2 lost over the same time period is around 0.5 bar or more. The MAVEN mission has also explored the current rate of atmospheric escape of Mars. Jeans escape plays an important role in the continued escape of hydrogen on Mars, contributing to a loss rate that varies between 160 - 1800 g/s. Oxygen loss is dominated by suprathermal methods: photochemical (~ 1300 g/s), charge exchange (~ 130 g/s), and sputtering (~ 80 g/s) escape combine for a total loss rate of ~ 1500 g/s. Other heavy atoms, such as carbon and nitrogen, are primarily lost due to photochemical reactions and interactions with the solar wind.
..............................................................
Saturn's moon Titan and Jupiter's moon Io have atmospheres and are subject to atmospheric loss processes. They have no magnetic fields of their own, but orbit planets with powerful magnetic fields, which protects these moons from the solar wind when its orbit is within the bow shock. However Titan spends roughly half of its transit time outside of the bow-shock, subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the transit of Titan, causing neutral hydrogen to escape. The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud. Interaction with the plasma cloud induces sputtering, kicking off sodium particles. The interaction produces a stationary banana-shaped charged sodium cloud along a part of the orbit of Io.>>
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by johnnydeep » Wed Jul 08, 2020 6:00 pm

neufer wrote:
Wed Jul 08, 2020 12:55 pm
Ann wrote:
Wed Jul 08, 2020 12:05 pm

Surely Earth should have a tail, too?
  • Unlike Mars & Mercury, Earth has both a high escape velocity and a protective magnetic field.
<...much snipped...>
Despite all that good info, I'm still not clear on whether earth has a tail or not. It is "losing" both hydrogen and helium at fairly low rates, and that these come to make up earth's exosphere, and atoms in the exosphere can still be gravitationally bound to the earth. But does that mean that ALL such atoms are so bound, leaving none left to make a tail?
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by neufer » Wed Jul 08, 2020 7:15 pm

johnnydeep wrote:
Wed Jul 08, 2020 6:00 pm

Despite all that good info, I'm still not clear on whether earth has a tail or not. It is "losing" both hydrogen and helium at fairly low rates, and that these come to make up earth's exosphere, and atoms in the exosphere can still be gravitationally bound to the earth. But does that mean that ALL such atoms are so bound, leaving none left to make a tail?
  • 3 kg of hydrogen per second makes for a pretty febble tail.
https://en.wikipedia.org/wiki/Atmospheric_escape wrote:
<<Atmospheric escape of hydrogen on Earth is due to Jeans escape (~10 - 40%), charge exchange escape (~ 60 - 90%), and polar wind escape (~ 10 - 15%), currently losing about 3 kg/s of hydrogen. The Earth additionally loses approximately 50 g/s of helium primarily through polar wind escape. Escape of other atmospheric constituents is much smaller. A Japanese research team in 2017 found evidence of a small number of oxygen ions on the moon that came from the Earth. In 1 billion years, the Sun will be 10% brighter than it is now, making it hot enough for Earth to lose enough hydrogen to space to cause it to lose all of its water (See Future of Earth#Loss of oceans).
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by Chris Peterson » Wed Jul 08, 2020 7:50 pm

johnnydeep wrote:
Wed Jul 08, 2020 6:00 pm
neufer wrote:
Wed Jul 08, 2020 12:55 pm
Ann wrote:
Wed Jul 08, 2020 12:05 pm

Surely Earth should have a tail, too?
  • Unlike Mars & Mercury, Earth has both a high escape velocity and a protective magnetic field.
<...much snipped...>
Despite all that good info, I'm still not clear on whether earth has a tail or not. It is "losing" both hydrogen and helium at fairly low rates, and that these come to make up earth's exosphere, and atoms in the exosphere can still be gravitationally bound to the earth. But does that mean that ALL such atoms are so bound, leaving none left to make a tail?
Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
Chris

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by johnnydeep » Wed Jul 08, 2020 9:50 pm

Chris Peterson wrote:
Wed Jul 08, 2020 7:50 pm
johnnydeep wrote:
Wed Jul 08, 2020 6:00 pm
neufer wrote:
Wed Jul 08, 2020 12:55 pm
  • Unlike Mars & Mercury, Earth has both a high escape velocity and a protective magnetic field.
<...much snipped...>
Despite all that good info, I'm still not clear on whether earth has a tail or not. It is "losing" both hydrogen and helium at fairly low rates, and that these come to make up earth's exosphere, and atoms in the exosphere can still be gravitationally bound to the earth. But does that mean that ALL such atoms are so bound, leaving none left to make a tail?
Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
Every body? Even small rocky satellites of the outer planets, or asteroids in the asteroid belt? Why would they preferentially lose rather than gain matter from dust, say?

EDIT: ok, never mind about the gaining matter part of that, which can still happen whether the lose matter in other ways or not. So, disregarding any gain, why would small satellites or asteroids lose matter?
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by Chris Peterson » Thu Jul 09, 2020 12:10 am

johnnydeep wrote:
Wed Jul 08, 2020 9:50 pm
Chris Peterson wrote:
Wed Jul 08, 2020 7:50 pm
johnnydeep wrote:
Wed Jul 08, 2020 6:00 pm


Despite all that good info, I'm still not clear on whether earth has a tail or not. It is "losing" both hydrogen and helium at fairly low rates, and that these come to make up earth's exosphere, and atoms in the exosphere can still be gravitationally bound to the earth. But does that mean that ALL such atoms are so bound, leaving none left to make a tail?
Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
Every body? Even small rocky satellites of the outer planets, or asteroids in the asteroid belt? Why would they preferentially lose rather than gain matter from dust, say?

EDIT: ok, never mind about the gaining matter part of that, which can still happen whether the lose matter in other ways or not. So, disregarding any gain, why would small satellites or asteroids lose matter?
Because they get bombarded with high energy particles, which means they have atoms kicked off by spalling processes. For most it's not much, but it's ongoing, and as they're all in the solar wind or magnetic fields or other non-isotropic effects, those atoms will move preferentially in some particular direction. Which we might reasonably describe as a tail.
Chris

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RocketRon

Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by RocketRon » Thu Jul 09, 2020 4:46 am

neufer wrote:
Wed Jul 08, 2020 7:15 pm
3 kg of hydrogen per second makes for a pretty feeble tail.
3 kg of hydrogen per second is about a tonne every 5 or 6 minutes, 10 tonnes per hour,
200 + tonnes per day.
Doesn't sound too insignificant.
(Although on the scale of the Universe, just a few drops in the bucket)

How much does a good comet emit. ?
Obviously it will be more diverse in its composition,
but Earth is unlikely to be just emitting pure hydrogen either ?

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by Chris Peterson » Thu Jul 09, 2020 5:28 am

RocketRon wrote:
Thu Jul 09, 2020 4:46 am
neufer wrote:
Wed Jul 08, 2020 7:15 pm
3 kg of hydrogen per second makes for a pretty feeble tail.
3 kg of hydrogen per second is about a tonne every 5 or 6 minutes, 10 tonnes per hour,
200 + tonnes per day.
Doesn't sound too insignificant.
(Although on the scale of the Universe, just a few drops in the bucket)

How much does a good comet emit. ?
A typical value would be tens of tons per second.
Chris

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by neufer » Thu Jul 09, 2020 1:15 pm

http://www.esa.int/Science_Exploration/Space_Science/When_a_planet_behaves_like_a_comet wrote:
When a planet behaves like a comet
29/01/2013 : ESA / Science & Exploration / Space Science


<<ESA’s Venus Express has made unique observations of Venus during a period of reduced solar wind pressure, discovering that the planet’s ionosphere balloons out like a comet’s tail on its nightside.

The ionosphere is a region of weakly electrically charged gas high above the main body of a planet’s atmosphere. Its shape and density are partly controlled by the internal magnetic field of the planet.

For Earth, which has a strong magnetic field, the ionosphere is relatively stable under a range of solar wind conditions. By comparison, Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere.

The extent to which this shaping depends on the strength of the solar wind has been controversial, but new results from Venus Express reveal for the first time the effect of a very low solar wind pressure on the ionosphere of an unmagnetised planet.

The observations were made in August 2010 when NASA’s Stereo-B spacecraft measured a drop in solar wind density to 0.1 particles per cubic centimetre, around 50 times lower than normally observed; this persisted for about 18 hours.

As this significantly reduced solar wind hit Venus, Venus Express saw the planet’s ionosphere balloon outwards on the planet’s ‘downwind’ nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions.

“The teardrop-shaped ionosphere began forming within 30–60 minutes after the normal high pressure solar wind diminished. Over two Earth days, it had stretched to at least two Venus radii into space,” says Yong Wei of the Max Planck Institute for Solar System Research in Germany, lead author of the new findings.

The new observations settle a debate about how the strength of the solar wind affects the way in which ionospheric plasma is transported from the dayside to the nightside of Venus.

Usually, this material flows along a thin channel in the ionosphere, but scientists were unsure what happens under low solar wind conditions. Does the flow of plasma particles increase as the channel widens due to the reduced confining pressure, or does it decrease because less force is available to push plasma through the channel?

“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions,” says Markus Fraenz, also of the Max Planck Institute and co-author on the paper.

A similar effect is also expected to occur around Mars, the other non-magnetised planet in our inner Solar System.

“We often talk about the effects of solar wind interaction with planetary atmospheres during periods of intense solar activity, but Venus Express has shown us that even when there is a reduced solar wind, the Sun can still significantly influence the environment of our planetary neighbours,” adds Håkan Svedhem, ESA’s Venus Express project scientist.>>
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by TheZuke! » Thu Jul 09, 2020 1:50 pm

Chris Peterson wrote:
Wed Jul 08, 2020 7:50 pm

Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
I'm pretty sure my "body" doesn't have a tail (visible, anyway), and I am certainly not losing mass, but unfortunately gaining some with each solar orbit.
:D

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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by Chris Peterson » Thu Jul 09, 2020 2:07 pm

TheZuke! wrote:
Thu Jul 09, 2020 1:50 pm
Chris Peterson wrote:
Wed Jul 08, 2020 7:50 pm

Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
I'm pretty sure my "body" doesn't have a tail (visible, anyway), and I am certainly not losing mass, but unfortunately gaining some with each solar orbit.
:D
Well... you may be experiencing a net mass gain (which is true of the the Earth, as well). But you carry a cloud of personal microbes around you, as well as billions of shedding skin cells, and exist in a world of air currents. So I reckon you actually do have a tail, which in fact is analogous in most respects to those of planetary bodies.
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by johnnydeep » Thu Jul 09, 2020 2:42 pm

Chris Peterson wrote:
Thu Jul 09, 2020 2:07 pm
TheZuke! wrote:
Thu Jul 09, 2020 1:50 pm
Chris Peterson wrote:
Wed Jul 08, 2020 7:50 pm

Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
I'm pretty sure my "body" doesn't have a tail (visible, anyway), and I am certainly not losing mass, but unfortunately gaining some with each solar orbit.
:D
Well... you may be experiencing a net mass gain (which is true of the the Earth, as well). But you carry a cloud of personal microbes around you, as well as billions of shedding skin cells, and exist in a world of air currents. So I reckon you actually do have a tail, which in fact is analogous in most respects to those of planetary bodies.
Yup, and of course, lately, some of us unfortunately have tails partially comprised of SARS-CoV-2 virids :(
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by johnnydeep » Thu Jul 09, 2020 2:44 pm

Chris Peterson wrote:
Thu Jul 09, 2020 12:10 am
johnnydeep wrote:
Wed Jul 08, 2020 9:50 pm
Chris Peterson wrote:
Wed Jul 08, 2020 7:50 pm


Every body in the Solar System has a tail, because every body is losing atoms to the surrounding space, and there are directional fields and forces present everywhere. But in most cases, these tails are extremely rarefied.
Every body? Even small rocky satellites of the outer planets, or asteroids in the asteroid belt? Why would they preferentially lose rather than gain matter from dust, say?

EDIT: ok, never mind about the gaining matter part of that, which can still happen whether the lose matter in other ways or not. So, disregarding any gain, why would small satellites or asteroids lose matter?
Because they get bombarded with high energy particles, which means they have atoms kicked off by spalling processes. For most it's not much, but it's ongoing, and as they're all in the solar wind or magnetic fields or other non-isotropic effects, those atoms will move preferentially in some particular direction. Which we might reasonably describe as a tail.
Ok, I'll buy that :)
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Re: APOD: Mercury's Sodium Tail (2020 Jul 08)

Post by XgeoX » Thu Jul 09, 2020 6:37 pm

Yes, I am surrounded by a gaseous discharge!

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The spalling process.

Post by neufer » Thu Jul 09, 2020 7:50 pm

Click to play embedded YouTube video.
johnnydeep wrote:
Thu Jul 09, 2020 2:44 pm
Chris Peterson wrote:
Thu Jul 09, 2020 12:10 am
Because they get bombarded with high energy particles, which means they have atoms kicked off by spalling processes. For most it's not much, but it's ongoing, and as they're all in the solar wind or magnetic fields or other non-isotropic effects, those atoms will move preferentially in some particular direction. Which we might reasonably describe as a tail.
Ok, I'll buy that :)
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