Daily Archives: October 14, 2008

astronomy, comet, plasma tail, solar physics, solar wind

Gasbag Dies Down

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All Quiet on the Solar Wind Front

No, this isn’t about politics, although it’s interesting to note that when I DID write about politics, I had a HUGE spike in readership… but alas, now that I’m back to talking about astronomy, the excitement has died down…

… heavy sigh …

A visible light image of the Sun on October 14, 2008
A "visible light" image of the Sun on October 14, 2008

…where was I…

oh yes… gasbags.

Specifically, the Sun.

Yes, it’s a “bag” of gas and it blows a stream of gas and charged particles out in a constatn stream called the solar wind.  Traditionally, the Sun goes through some cycles where it’s windier and gasbaggier than usual. These normally tend to correspond to periods of higher sunspot activity and the like.

Well, we had a meeting of our little vodcasting production group today — that’s the small collection of us who are creating vodcasts for MIT Haystack Observatory on the subject of space weather — the stuff that happens in the near-planet environment when our local gasbag star decides to belch some material our way.

Among other things, we got to talking about the current, relatively quiescent solar wind. Everybody who monitors the solar wind (basically solar physicists and atmospheric scientists and space weather experts from around the world) has been remarking on how low the wind levels are, almost like the Sun is holding out on us for some unknown reason. It so happens that they’re the lowest in 50 years. What this means for the long term in solar behavior is probably not a big deal right now, but suffice to say, right now, everybody who was expecting some ramping up of activity as the current solar cycle wears on is ready for some outburst action.  Or even just a sunspot, please, anything… do we have to get a famous blonde actress out here to beg the Sun to just “think of the children” now?

Well, perhaps things are not that desperate. The Sun has its little behavioral crotchets, and the more data we take, the more we learn about this star (and presumably others like it). The folks who monitor the Sun continue to do so, adding in this quiescent data to the ever-growing store of solar behavior knowledge.  And, they’re hoping for some solar action sooner rather than later.

The Sun on October 14, 2008.
The Sun on October 14, 2008. The sunspot is the bright spot in the upper center of the Sun.

Now, we did have the promise of a sunspot a couple of days ago, and there was a coronal hole spewing a stream of material that Earth went through. Everybody got ready just in case this portended some new break-out activity. But, as you can see by the picture on the left, there’s not a whole lot of sunspottin’ going on here. The coronal hole (which you can’t see in this particular view) is moving out of our line of sight and there’s another small one moving into our view. So, everybody waits, and we’re pretty sure there’ll be some action soon… we just don’t know when.

The solar wind is of more than just passing interest to me, and not just because I’m working on vodcasts about space weather. Back in the dark ages I worked on a whole passel (technical term meaning “hundreds if not thousands, so many that I had dreams about them at night”) images of Comet Halley during its last apparition in 1985 and 1986. Our research interest was really with Halley’s tail, specifically its plasma tail. A comet’s plasma tail interacts with the solar wind. Specifically, a plasma tail forms when gases streaming off the comet get ionized by interactions with the solar wind. In the process, the plasma tail gains an electrical charge and has the same electrical current propreties (polarity, etc.) as the “regime” (area) of the solar wind in which it forms. (And those properties are “encoded” into the solar wind as it leaves the Sun.)

As the comet goes along in its orbit (and it’s important to know that the plasma tail only exists within about 1.5 to 2.0 astronomical units from the Sun for reasons I’m not going to go into here because this post is already longer than it should be), it experiences the solar wind. If a comet stays in the regime of the solar wind that has the same polarity as its plasma tail, everything is fine and dandy. But, eventually the comet will encounter a regime that has the opposite polarity. The old plasma tail can’t exist there, and so it “breaks off” and a new one starts to grow, capturing the polarity of the new regime. The loss of the old tail is called a “disconnection event.”

The technical term for the underlying process that occurs when the magnetic field line entrained in the plasma tail collides with the magnetic field lines in the solar wind is “magnetic reconnection” and it’s quite a complex process that we see in magnetic fields in places like the solar atmosphere and near-Earth space (and if you want to read more about it, be my guest.)  Anyway, I spent a lot of time studying those Halley pictures to pinpoint when its plasma tail disconnection events took place and our team of stalwart grad students and principal investigator (i.e. the PhD astronomer) tried to relate what WE were seeing to what part of the solar wind the comet was encountering.

Chasing comet plasma tail got a lot easier when the Ulysses spacecraft was launched to measure the solar wind as it was streaming away from the Sun. UIysses’s instruments would tell us (essentially) “Okay, I saw the solar wind at this location and it had this polarity and temperature and speed and this many particles loaded into it.”

And we’d say, “Okay, we’re going to look at a comet that we know about that will be experiencing that very piece of solar wind you just measured today, and it will pass through that piece of solar wind in two days. We’ll be able to see what happens to the plasma tail. And, based on what you’re telling us Ulysses, we will predict what will happen to the plasma tail as a result.”

We did that for a bunch of comets (where “bunch” is secret scientist jargon that means “more than two or three”) and by golly, we were able to use solar wind readings to predict what would happen to the plasma tails of those comets.  Which was a lot of fun and got me through graduate school (along with some work I did on HST’s GHRS instrument team).

Comet Encke encounters a coronal mass ejection collide. (Courtesy NASA/STEREO, as shown on Boston.com)

I thought of all that fun today when I saw this really cool set of images from the STEREO mission. I actually saw them last year but I must have been busy or something because I didn’t blog about them then.

Anyway, they show Comet Encke experiencing a coronal mass ejection (a huge blast of solar wind) in April 2007.  Essentially this little spermy-looking comet is moving along in the solar wind, doing its little comety thing and it collides with a fast-moving jet of material (plasma) spewed out by the Sun. The interaction with the comet and both of its tails (the dusty, inert tail and the electrically charged plasma tail) basically tears them off and we watch new ones grow over the space of a few hours.  It was a great thing to see a disconnection event (with the associated magnetic reconnection) happen in “real” time after years of looking at static comet images in sequences and just imagining what the action must have looked like when it was occurring.

exoplanets, life

Time and Tides Have An Effect on Life

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The Recipe for Life-bearing Planets Gets More Complex

The creation of life on our planet was a long, drawn-out affair, taking more than a billion years of chemical and biochemical processing to accomplish after the planet formed some 4.5 billion years ago. Technically, life began some 3.8 billion years ago in some life-friendly oasis on the planet, meaning that the correct conditions were there for some chemically-rich soup to react to something like an influx of heat or a zap of lightning, producing the first living things.  There’s enough ambiguity in there that just about anybody can come up with a theory about what happened (including some pretty jaw-dropping ones about LGMs, travelling deities, and so on), but the scientific consensus (based on verifiable research) is that the primal ooze finally combined in ways that led to the first life forms, and from there it was evolution all the way, baby.

Before all this happened, though, the planet had to form, and it had to do it in the right place. There’s the rub.  If a planet forms too close to its star, its surface gets broiled. Mercury’s a good example here — its surface is alternately flame-roasted and then chilled as it rotates on its axis only 69 million kilometers from the Sun. Get too far away from the Sun, say out in the realm of the gas giants, and it’s too cold for a hard-body planet (i.e. rocky) to form life.

Distance isn’t the only characteristic you have to consider, however.  There’s also a little thing called “tides” — and I’m not talking simply about the ocean tides we experience here on Earth, although they’re part and parcel of the same phenomenon.

Jupiters moon Io
Jupiter's moon Io is heated by tidal friction.

When two bodies interact with each other, gravitational interactions can push and pull on their surfaces, creating tides — and that also heats them.

Jupiter’s moon Io shows an extreme case of tidal heating — gravitational interactions between Jupiter and this tiny moon and its sibling moons Europa and Ganymede cause the surface to bulge up and down. This also heats Io’s interior, and the end result is a volcanic moon.

Tidal heating between a star and its planet (or even a planet and its moons) can drive plate tectonics. Earth has plates, is heated from within, and also has a “tidal” relationship with the Moon. Our planet’s “basement” is basically made up of seven major plates (and several smaller ones) and the continents and oceans ride along on top of them. (For more about plate tectonics on Earth, go here or here.).  Among other things, tectonics  keeps excessive carbon dioxide from accumulating in a planetary atmosphere. If it hadn’t performed this service on Earth, we might have a deadly greenhouse atmosphere like the one at Venus.

A group of scientists at University of Arizona is looking into the role that such tides play on planets and what influence they may have on whether life could evolve on rocky planets around other stars. Brian Jackson, Rory Barnes and Richard Greenberg of UA’s Lunar and Planetary Laboratory gave a paper at the Division of Planetary Sciences meeting in Ithaca,  New York, and in it they say that tides can play a major role in heating terrestrial planets. Such tides could create scenes of unbelievable hellishnesson rocky alien worlds that would be livable if conditions were better. And tidal heat can work in reverse, creatiing conditions favorable to life on planets that would otherwise be unlivable.

A map of Earths tectonic plates -- did they help life get started?
A map of Earth's tectonic plates -- did they help life get started?

What this means is that as astronomers search out worlds on other planets, they might need to examine exoplanets in great detail to see if tidal heating (from their stars or interactions with possible moons) is playing a role in their livability factors. Recently there have been so-called “super Earths” discovered around other stars. These planets are somewhere between two and ten times as massive as Earth. If they really ARE Earthlike (meaning that they’re rocky bodies around the size of the Earth or bigger) then it’s possible that tidal heating from interactions with their star or nearby moons may be great enough to melt them, or at least produce volcanism at a level that we see at Io. This would make them pretty poor prospects for being life-bearing planets, and they’d be more like  “super-Ios.”

The more massive a planet is, the greater the effects of tidal heating will be on its surface and interior.  This means that the most easily detectable super-Earths could be dominated by volcanic activity, which is one of the big conclusions that the University of Arizona team came to in their research.  So, the first Earth-like planets found are going to be the most easily spotted, and thus they’ll be big. This means they’ll probably going to be strongly heated and have big volcanoes.

A super-Earth with possible plates?
A super-Earth with plate tectonics and experiencing tidal forces needs the right amount of both to support life.

And as astronomers find Earth-like planets in what they cal,l the “habitable zone” around other stars, those planets may well NOT be habitable if they’re gobsmacked by tidal heating.

On the other hand, if a planet is smaller than it should be, or maybe lies outside the habitable zone, it could still support life if it is heated by tidal interactions that could cause outgassing of volatiles (gases, ices) that enrich a planet’s atmosphere with the right stuff needed for life. Tidal heating also can generate sub-surface liquid oceans on water-rich rocky planets that would otherwise be frozen, just as tidal heating is believed to warm a sub-surface liquid water ocean on Jupiter’s moon Europa.

Also, tidal heating could produce enough heat to drive plate tectonics for billions of years, long enough for life to appear and flourish.

So, for those of you keeping score at home, the ingredient list for life is getting more and more refined. And, when we look at other planets in our search for life, we need at where the planet exists in relation to its star, how long it’s been around, whether it can supply the water, warmth, and “food” for life, and now, whether or not it is subject to the correct application of tidal force.