What's up in the sky: Comet Lulin and Venus

The currently visible planets from Earth are always changing. In fact, the term "planet" comes from the Greek term, "wandering star", since they don't move like the rest of the sky.

That being said, I thought it would be informative to have a "What's up in the sky" section every couple of months to let people know what's visible if they just step outside and look up in the evening. Even from the middle of the most light-polluted cities, bright planets are still visible. I've already received a few questions pertinent to this.

Fitz asks:

I'm in Chicago, and I keep seeing what looks like a really bright star in the west. Is that Venus? What's the best way for a non-astronomer to find out these sorts of things (other than asking you!).

That's definitely Venus you're seeing. Other than the occasional airplane or bright satellite pass, Venus is the third brightest object in our sky after the Sun and the Moon. In fact, it's the object most frequently reported as a UFO.

Venus will be visible in the early evening sky in the West for the next month or so. As a planet on an orbit interior to Earth's orbit, it's currently "rounding the track" to pass in between us and the Sun, a phenomenon known as inferior conjunction. Because of this geometry, it's currently exhibiting a nice crescent phase and getting larger each night as it approaches us. At this point, with a good pair of binoculars and some keen vision, you should be able to make out that it's not merely a point of light, but a tiny crescent.

To answer the last part of your question, Sky & Telescope's This Week's Sky at a Glance provides a good overview of what's visible at any given time. Additionally, picking up some Planetarium software is also a good idea since it can be customized to your location and any date you want...there are some really good options out there. If you're a fan of open source software, I'd highly recommend Stellarium which can be downloaded from their subversion repository (side note: props to my subversion peeps).

Ben asks:

Comet Lulin is coming closest to earth tonight. Do I have a snowball's chance ... in space ... of seeing it?

Similarly, Tami asks:

I overheard a conversation yesterday that sounded like there was a comet in view the last few days, however it has been hazy here so there wasn't much view. Was there a comet visible recently? Which one? When? Where? Do you have any pics? How close is it to the earth? Is Armageddon inevitable?

Every couple of years we get a comet which is able to break the visible brightness barrier and can be seen with just the naked-eye. Comet Lulin is an example of this. Current estimates of its magnitude (brightness) place it at just above the naked-eye limit for a dark-sky site.

This comet is a bit of an interesting one. Its closest approach to the Sun was over a month ago, at a distance a good 20% greater than the Earth-Sun distance, but the geometry works out so that its closest approach to Earth was just four days ago at a distance just 40% of the Earth-Sun distance. (No worries, though, this is not even close to hitting us.)

Its orbit carries it *very* far from the Sun - over 3 light-years, in fact. At this distance, the Sun is not the only gravitational force acting on its orbit, but the gravity of other stars may start to be significant, so it's a little unclear if this comet will actually return to our solar system. Even if it does return, it won't be for another 50 million years. Whoa.

Now, that all said, it's not terribly spectacular with just the naked-eye...don't expect something similar to Comet Hale-Bopp back in 1997. You'll need a very dark sky to see it unaided, and it won't look like much more than a smudge. On good nights I can see the Milky Way from my yard, but I was unable to spot this comet with just the naked eye. With a decent pair of binoculars, though, it should stand out.

However, you'll have to hurry if you want to see this one. It's now moving away from Earth on its way out of the solar system, and won't be visible for long. It's currently rising over the Eastern horizon around sunset in the constellation Leo, though a finder chart is almost certainly necessary. You can find one here.

As for images, I'd recommend Spaceweather's Comet Lulin gallery...over 16 pages of images submitted by amateur astronomers.

Light: Particle & Wave

Karl asks:

So, photons. Radio waves. Electromagnetic radiation. Why do we only hear "photons" referred to for electromagnetic radiation that happens to fall within the visible spectrum for the human eye, plus some distance on either side of it, and then sometimes for anything above it (like gamma rays). But way down there in the radio wave regions, for some reason we're never talking about photons.

What's up with that? Is it just a matter of convention, or is there some kind of qualitative change as you go up the spectrum?

Good question. A pretty common confusion in physics is the dual nature of light: sometimes it acts like a wave, other times it acts like a particle. Specific to your question, though, you can absolutely talk about a "radio photon" or a "gamma ray wave", we simply don't encounter those terms in large part due to our methods of detection.

The only real qualitative change through the spectrum is energy per photon. Your average gamma ray photon will have an energy on the order of a trillion times more than your average radio photon, with a visible light photon somewhere in the middle - a million times less energy than gamma rays, and a million times more than radio waves.

Now, it's the deposition of this energy which allows us to detect it. In the case of modern visible light detectors (such as the CCD chip in your digital camera or at any observatory), a visible photon comes and hits the CCD, causing an electron to jump from being bound to a silicon atom to floating around in the sea of conduction electrons, which gets spilled out for reading at the end of an exposure. The point here is that the energy from a single photon gets filtered through to produce a noticeable, macroscopic effect in the electronics. Something similar applies to even shorter wavelength electromagnetic radiation like UV and X-rays - albeit in slightly different ways - that permits us to observe them on the individual photon level.

Radio, on the other hand, is a bit of a different beast. (I should also state that the radio astronomy world itself is somewhat disconnected from visible/infra-red astronomy.) A single radio photon isn't really enough to cause a noticeable difference in anything macroscopic. The wavelength of radio waves, however, *are* macroscopic. So, it's more reasonable to conceptualize radio energy as a sea of electromagnetic waves altering the electric field of your macroscopic antenna and producing a noticeable signal.

That's not to say radio photons don't exist. For example, atomic hydrogen which flips the spin state on its electron will cause a single radio photon (with a wavelength of 21 centimeters) to be emitted, and is a very important probe of gas in the galaxy.

So, I think the answer to your question is ultimately an observational predicament: which method of light conceptualization (particle or wave) causes the macroscopic change to your instrument.

Mars' Pole Star, and Blog of Note!

Well, it would seem that we've been listed as blogspot's blog of note! Yay, us!

Now, back to your regularly scheduled planetary astronomy questions. It's been a week since I've answered questions due to a massive computational fluid dynamics project I've been working on, so let's take these in order...Michael writes:

If you're standing on Mars, what star is closest to Mars' north pole?

I looked up the coordinates of Mars' north pole with JPL's Horizon ephemeris generator. This is the same web form that we planetary astronomers use when we get time on a big telescope and need to know where an object is, how high up in the sky it is, what phase it's in, etc. This is a huge resource for us, so a big thanks to the folks at JPL for providing for us.

It turns out the Earth-centric coordinates currently are:
RA: 317.67°
Dec: 52.88°

While there's no bright star right at that location, the closest one at about 7° away would be Deneb, an incredibly luminous white supergiant. Depending on which constellation scheme you go with, that star is either the head of the Northern Cross, or the tail of Cygnus the Swan.

Michael also asks:

Where's the Sun on the first day of Spring on Mars?

This is a good question. For the first day of Northern Spring on Earth, the Sun is located at a position known as the "First Point of Aries", a position that by definition is RA: 0°, Dec: 0°. The math gets a little tricky if we want to do this for Mars while staying in Earth-centric coordinates of RA and Dec (some icky spherical trigonometry is involved), so let's take advantage of JPL Horizons again.

It turns out to be on the edge of Sagittarius, almost perfectly lined up with the center of our galaxy.

Followup on escape velocity

Mike said:

Seven miles per second. That's the Earth's escape velocity, and the energy required to escape its gravity well...any slower and you'll fall back to Earth.

Karl wrote:

That's always confused me.

Sure, if you get moving to 7 m.p.s, then you'll escape without any further power. But couldn't you be going slower than that and still escape, as long as you continue to provide thrust?

Sure, that's possible, too. In general, though, orbital mechanics equations are generally done with instantaneous velocity changes (i.e. you suddenly go from zero to seven miles per second). This is for two reasons:

1) The orbital equations are just a whole lot simpler to work using instantaneous changes in velocity - it's only algebra. With acceleration over a prolonged period, though, it suddenly becomes a calculus equation.

2) Until very recently, velocity changes to get into and subsequently alter orbits of spacecraft were only done with chemical propellants. Since these are short-lived accelerations, they can be treated as instantaneous to first-order. This is particularly the case for large booster rockets, where once you start the rocket, the whole thing keeps going until it's finished.

Now, reason #2 has recently changed with the creation of spacecraft with ion engines. These engines work on a very different principle...they continuously accelerate individual atoms past an electrified grid, which means very small thrust but over very long periods of time. For more info on this, check out NASA's FAQ on Ion propulsion currently featured on such spacecraft as the Dawn mission to asteroid Vesta.

What you're ultimately trying to do with escape velocity is overcome the Earth's gravity well by raising your potential energy to that of an object at an infinitely far distance. Even though it sounds counter-intuitive, it turns out that this is actually a finite quantity of energy because as distance increases to very large values, the force of gravity becomes infinitesimally small.

As long as you can get your kinetic energy moving away from the Earth equal to this potential energy at infinity, you can escape. Seven miles per second is the oft-quoted figure because that velocity provides a body with enough kinetic energy to be equal to the difference in potential energy between Earth's surface and an object at infinity.

So, to be clear, you ultimately don't need to be going seven miles a second to begin with. You can alter your velocity on the way up however you please...but in the end, you'll have to end up spending at least as much energy as you would've going 7 miles per second initially.

Happy Anniversary, Pluto!

Jennifer wrote:

I tried the 5 year old explanation on my son today, with a few extra details thrown in to set the scene. I swear I saw stars sparkling in his eyes when I finished. Worked like a charm :).

Nice, glad it worked!

One final postscript I forgot to mention on all of this to make it particularly fitting: today is the 79th anniversary of Pluto's discovery.

In honor of the anniversary, I just came from a department event announcing the newly created position of the Clyde Tombaugh endowed chair. Patsy Tombaugh, Clyde's widow, spoke at the event to a packed room. She quoted Clyde in what I thought was a particularly nice sentiment: "How can people not be interested in astronomy? Don't they want to know where they are?"

Mike.

Mars, Escaping Earth, and Why Pluto isn't a Planet

Jennifer wrote:

Dear Planetary Astronomer Mike,

I have a 5 year old son who's heart is set on traveling to mars (and a three year old daughter who would like to rocket ship to mars). He often has questions about the planets, the sun, and earth.

Well, we'll see what happens to Bush's lofty goal of getting humans to Mars. It's not such a terrible idea, just completely underfunded by the last administration.

Most planetary astronomers I know are waiting with baited breath to see who Obama appoints as NASA administrator, as it will deeply affect future science policy. From reading the tea leaves, the guess is that there will a lot of new funding for Earth climate satellites and research...which also isn't such a terrible idea.

Jennifer continued:

My daughter (who is determined to be in this email because she must be just like her brother) wants to know how fast a rocket ship has to move to get her to the moon.

Seven miles per second. That's the Earth's escape velocity, and the energy required to escape its gravity well...any slower and you'll fall back to Earth.

Jennifer concluded with:

For the last week my son's been asking me why Pluto is not a planet and what it is/how it formed.

Okay, this is actually a pretty popular question, and I've given a couple talks about it. It's also a question that that hits home - our astronomy department was founded by Clyde Tombaugh, discoverer of Pluto.

So, imagine the following scenario:

1. Astronomers have mapped out our solar system, and after some analysis are expecting to find a planet where one has not yet been found.
   
   
2. After careful searching and some serendipity, an observer finds a tiny light moving among the heavens. Plotting its course, it turns out to be an object exactly where a planet was expected...though it's somewhat smaller than expected.
   
   
3. Astronomers rejoice! A new planet has been found! We're so smart!
   
   
4. Time passes...when suddenly a new object is discovered at almost the exact same distance from the Sun...curious. Another planet? In the same orbit?
   
   
5. Another object at the same distance is discovered.
   
   
6. Another object at the same distance is discovered.
   
   
7. Another object at the same distance is discovered.
   
   
8. etc.
   
   
9. After compiling a vast array of objects all orbiting in roughly the same orbit, maybe these aren't planets after all.
   

The above scenario perfectly outlines what actually happened...with Ceres, the first asteroid ever discovered. (Ha! Tricked you!)

By the late 1700's, astronomers were disturbed by the conspicuous gap between Mars at 1.6 AU and Jupiter at 5.2 AU (1 astronomical unit = 1 AU = the distance between Earth and the Sun). Titius and Bode drew up an entirely empirical equation to calculate the distance of planets from the Sun which worked out quite nicely. There was a missing term in their equations, though...precisely in this gap, around 2.5 AU, suggesting there should be a planet there. Quoting Titius from 1768:

"After Mars there follows a space of 4+24=28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not."

The stage was further set by Herschel, who had discovered the planet Uranus in 1781. After his discovery, astronomers had warmed to the idea of finding new planets telescopically...it was new, it was happening, it was the in thing to do. Moreover, Uranus fits perfectly as the next term of the Titius-Bode equation. A coordinated observing campaign was begun to search for the missing planet.

Along comes Giuseppe Piazzi, an Italian monk and hobby astronomer. After dutifully scanning the heavens, on the first night of the new millennium, January 1st, 1801, he observes a tiny "star" where none had been before. He follows it night-to-night and observes it moving...at first it was thought to be a comet, but after careful calculation astronomers realize this is exactly what they've been looking for. It is dubbed Planet Ceres. All is well, and astronomers pat themselves on the back for being so very clever.

Then in 1802, Heinrich Olbers finds another object at roughly the same distance as Ceres...the, uh, Planet Pallas! In 1804, Planet Juno is discovered...in 1807, Planet Vesta is is discovered...something is amiss. By 1850, there were 13 of these new planets...Here's a page from the 1850 Annual of Scientific Discovery which documents all 18 planets at the time.

It wasn't until 1852 that these objects were reclassified as "minor planets"...which is a good thing, since we know now of roughly 300,000 minor planets that are hardly on the same footing as the classic 8. Science-wise, it makes a lot more sense to group these objects into a family of astronomical bodies, the asteroid belt. Still, for 51 years Ceres enjoyed full planet status.

Now, also in this time period (1846) Neptune was discovered at 30 AU. Small gravitational perturbations in the motion of Uranus caused scientists to theorize the existence of a large planet further out. Le Verrier and Adams both independently calculated where such a planet should be...but Le Verrier was given telescope time first, and Neptune was found very close to its predicted position. This was a huge triumph of predictive science (and one of the few cases in history where the scientific method was more important to discovery than serendipity...but that's another topic).

Hoping to follow on this success, Percival Lowell hoped to do something similar some 80 years later. Having then detected subtle perturbations in the orbit of Neptune, he funded the search for "planet X", a large planet even further out. Initial observations were less than successful...planet X's position had to be recalculated 3 times after initial searches turned up nothing. But, in 1930, Clyde Tombaugh observing at Lowell Observatory discovered a tiny light moving night-to-night. 40 AU out...this was it! Planet Pluto! Once again, astronomers congratulated themselves on their sheer brilliance, and their fancy scientific method.

At first, Pluto was calculated to have a mass 7 times larger than the Earth. Over time, though, this was amended, and it turned out the new planet was a bit smaller. A lot smaller, in fact. In 1978, Pluto was found to have a moon, which allowed a precise determination of its mass...Pluto was found to be about 5 times less massive than our Moon. But, hey, it's a planet, who are we to argue?

By 1989, the scientific method took another blow...the Voyager spacecraft passed by Neptune allowing for a precise orbit determination, and discovered that those initial gravitational perturbations which Lowell measured simply didn't exist at all. They were observational errors. Planet X was found, again, by sheer serendipity...the third predicted position based on flawed Neptune data just happened to line up with the position of an actual object.

Still a planet, though, right? Things get trickier in 1992...David Jewitt discovers 1992 QB1 orbiting at almost the same distance as Pluto from the Sun. It's only one-fifth the radius of Pluto, though, so that's hardly planet-y at all.

Time passes. More objects are discovered around this distance from the Sun...about 1,000 more, though they're all smaller than Pluto. This situation is starting to sound really familiar, when suddenly in 2006 Mike Brown discovers 2003 UB 313 (now known as Eris). Radius determinations find that Eris is actually larger than Pluto...uh oh.

Crisis ensues. If Pluto is a planet, and Eris is bigger, then Eris is a planet, too, right? Or was Pluto never really a planet, after all? The International Astronomical Union, the body responsible for all solar system object naming, steps in. It's put to a vote, and the criteria for a planet are established:

• It has to orbit a star. (Pluto: check.)
• It has to have enough mass to be round. (Pluto: check.)

Initially this was it...it would have included Pluto as a planet, but 52 other solar system objects as well. That's a few too many to be useful, so, one more criterion was added:

• It must have cleared its orbit of all other bodies. (Pluto: crap.)

That final criterion is the problem for Pluto. It's a simple calculation: just compare the mass of a planet to the mass of all the stuff in the planet's orbit. Earth is responsible for 99.999% of all the mass of in its orbit. Pluto, on the other hand, is responsible for just 7% of the mass in its orbit.

Objects meeting criteria 1 & 2 but not criterion 3 are now "dwarf planets"...officially this include Pluto, Eris, Haumea, Makemake, and Ceres so far. Pluto enjoyed full planetary status for 76 years...not terribly different than Ceres, really.

It also makes sense...just as all the objects between Mars and Jupiter are collectively known as the asteroid belt and have a common origin, so all the object around 40 AU are collectively known as the "Kuiper Belt" and have a common origin. This bring us to the final part of your question: what is it, and how did it form.

Until recently, there were two competing theories for solar system formation: gas instability and core accretion. Both start with the extremely early solar system, where a swirling disc of gas and dust surrounded the proto-star that would become our Sun. Gas instability says that little over-dense nuggets of gas and dust in the disc gravitationally collapse to make planets. Core accretion states that first dust particles stick together to start forming larger and larger planetesimals, whose gravity then grows to the point that they can start pulling in the surrounding gas. For several reasons I won't get into (unless someone asks), gas instability has fallen out of favor with the astronomical community, though the debate isn't quite over yet.

Based on core accretion, particularly in the outer solar system, you start with these planetary "seeds" that suck in gas and make gas giant planets...at some point, though, you run out of gas to accrete. Add to that the slow orbital speed and huge distances between objects out at 40 AU, and you'll find that seeds at Pluto's distance never merged together enough to get in on the big gas feast.

Anyway, we're pretty sure this is what happened to Pluto...put in terms for a 5-year-old, its big brothers Jupiter, Saturn, Uranus, and Neptune ate up all the gas before Pluto got any, so it's been left as just a lonely little planet seed. I think there's probably a moral in there somewhere for him about sharing with his sister, too.

Mike.

Another Followup: Does Glass Flow as Fast as the Mantle?

Mike said:

...if water has a viscosity of 1 and honey has a viscosity of a few thousand, rheids in the mantle are on the order of a few billion.

Roland asked:

How does glass compare?

Oh boy, the whole "glass is a liquid" thing. Yes, I was taught this in a grammar school science class, and it is wrong, wrong, wrong! Sorry, don't mean to get excited here, but oft-repeated scientific fallacies are a big pet peeve of mine..."glass is a liquid", "there's no such thing as centrifugal force", "the moon is bigger on the horizon", "toilets flush backwards in the southern hemisphere", etc. They all drive me nuts. For a while I even flirted with the idea of registering scientificfallacies.com as a domain name for a site specifically designed to debunk these.

Anyway, yes, glass is an amorphous solid...I think the Corning site (a manufacturer of glass) has an excellent write-up about this.

I particularly like the bit about lead flowing 1 billion times faster than glass. For perspective, the whole justifying argument of "glass is thicker at the bottom than the top of old church windows" would mean that astronomical telescopes with large glass mirrors would go out of focus in a matter of weeks.

Now, how does glass flow compare to rheids in the mantle? Well, it's a little hard to do so, particularly since the mantle covers such a wide range of temperatures. At depth, the magma viscosity is lower (i.e. it flows more easily) than near the crust, simply because the temperature increases as you go down.

That said, though, the upper mantle is usually pegged with a viscosity in the neighborhood of 10^20 poise, shockingly almost the same as that given for glass. The reason why the rheids in the mantle flow better than glass in the church window is simply a matter of pressure. In the church window, the only force compelling the glass to flow is gravity. This is extremely weak in comparison to the mantle being forced by the pressure of gigatons of material above it.

Put in another way, remember that viscosity is just a measure of resistance to flow, not flow itself. A pool of honey will move much more slowly if you poke it with your finger than if you hit it with a hammer, even though the viscosity doesn't change. Similarly, mantle rheids and glass both have about the same resistance to flow...just that in the mantle case the applied force is much, much greater such that it flows on the order of centimeters per year.

Mike.

More Followups on the History of our Planet

Mike said:

...years after the impact to do so), but would certainly have been obliterated once there was nothing but hot lava to stand on.

Roland wrote:

How certainly and how obliterated? Do the temperatures involved clearly preclude the survival of any molecules more complex than those that existed during the earlier "hot" period? I find it slightly more romantic to imagine that some little step of material complexity on the way towards what we now call life might have carried over from an older world.

Estimates of surface temperature post-impact that I've seen are around 2500K, or roughly the surface temperature of your average red star. The only molecules that survive those temps are pretty simple...metal hydrides and metal oxides, mostly. This is definitely not a regime where amino acids can exist, and depending on the the exact temperature, even H2O can dissociate. One of the few surviving minerals from before the impact is zirconium dioxide (from which we radiometrically date Earth's formation)...it has a melting point around 3000K, so that does place an upper limit, at least.

---

Mike said:

Well, the whole Precambrian era was not the most exciting time-period, kinda like how in a Western Civ class they'll skip straight from "End of the Roman Empire" to "The Renaissance".

Roland then asked:

Was there something amazingly cool going on simultaneously next door (whatever that means) at the time? (The geological analogue to how the heights of the West's sister societies under Islam was taking place during this "boring period".)

Well, Mars certainly had *something* going on around 3 billion years ago. It's pretty much settled at this point that Mars had an abundance of liquid water on its surface, and probably even had a massive northern ocean. Just look at a surface topography map and you'll notice that not only is the entire northern hemisphere much lower in elevation, but also relatively devoid of craters. Moreover, there are very obvious drainage patterns, e.g. Warrego Valles. Moving from the southern highlands into the northern lowlands, which abruptly fade away right where you'd expect there to be a "beach".

Now, what happened to all the water is another matter, and still somewhat debated. The likely scenario involves a few key observations...Mars definitely had active volcanoes in the past, but they're all extinct now. Remember that Mars is significantly smaller than the Earth, about half the radius. This means its surface area-to-volume ratio is about twice as large as Earth's, so it should cool approximately twice as fast. As far as we can tell from a lack of magnetic field, there is no longer any molten material in Mars' interior...so it looks like the volcanoes simply shut off once the mantle cooled.

Once your volcanoes stop working, it's a very bad day for your planet. Atmospheres continually "sputter" off into space - high-energy photons for the Sun will hit gas molecules, which then have enough energy to make escape velocity and leave the planet forever. On Earth and Venus, our active volcanoes are the only things which continually resupply the atmosphere to keep it in a quasi-steady state. Add to that Mars' escape velocity, which is less than half of Earth's, and atmospheric sputtering becomes a big deal.

Another significant observation in this scenario is Mars' lack of a big moon. Earth's habitability depends strongly on having a relatively large moon...from regular torsion forces exerted by our Moon on Earth's slightly oblate shape, our North Pole precesses every 26,000 years, just like a top which both spins (rotation) and wobbles (precession). The downside to this is that we don't always have a pole star to navigate by (we happen to be lucky to be born at at a time when our North pole points to Polaris). The upside is way more important, though - we have a relatively constant axial tilt of 23.5 degrees. Since axial tilt determines the strength of a planet's seasons, ours have been relatively constant since the moon formed.

Mars is a different story, though. It gets tidally pulled in a very non-regular manner...sometimes by the Sun, sometimes by Jupiter. Although its current axial tilt is only 24 degrees, numerical simulations have shown its axial tilt in the past to be anywhere from 0 to 60 degrees over the course of millions of years. This would mean crazy seasons...with an axial tilt of 60 degrees, your arctic circle will be down at 30 degrees latitude. Constant sunshine during the summer over most of a hemisphere would only help the sputtering phenomenon, further decreasing the atmospheric pressure.

Recall that the temperature range at which liquid water can exist decreases as atmospheric pressure drops...this is why there are things like high-altitude baking directions. Eventually, at a pressure right around Mars' current atmosphere, it can't exist at any temperature. Goodbye, ocean.

Mike

Does Water Fall More Slowly on the Moon?

Karl wrote:

I was asked by a seven-year-old recently if things fall more slowly on the Moon. But as it happens, the specific example he used was: if you pour a glass of water, will it take longer to reach the ground than if you were to pour it here on Earth?

(This was asked indirectly via his mother, but the example was his. Seven-year-olds are more excited by spilling liquids all over the floor than by carefully dropping stones from the Leaning Tower of Pisa, I guess! He already knew that all objects fall at the same rate here on Earth, and he understands about atmospheric drag.)

So I divided my answer into two parts. For the first part, I said:

"Yes, things fall more slowly on the moon, or in other places with less gravity than Earth, like Mars. So if you drop your shoes on the Moon, they take longer to hit the ground. And it's not just because there's no air to slow them down -- it's really because there's just less gravity."

(This is all paraphrased. Unlike Barack Obama, I don't perfectly remember all the conversations I've ever had. However, when I write my memoirs, I will.)

I went on: "But the specific example you chose is interesting. If you pour out water on the Moon, something special will happen. It won't stay liquid water -- it will break up into a fine ice dust. Because there's no air pressure, it boils at room temperature, in other words, it evaporates right away, because there's no pressure keeping the water inside itself. The water molecules are all bouncing around, and now they are free to bounce away in all directions. But the water also freezes as it boils, because its surface area increases so much (it's just little tiny specks of water now) that it radiates all its heat and turns to ice. So boiling and freezing can happen at the same time, when there's no air."

The rough outlines of my answer are confirmed by

http://answers.google.com/answers/threadview/id/20227.html

But there's something I'm not sure of:

In the rare-to-nonexistent atmosphere of the Moon (let's assume it's a perfect vacuum, for the sake of discussion), why would water lose any of its heat? In other words, I get that it would boil right away. But would it also freeze? And would it behave differently in a shadowy part of the Moon versus a sun-drenched part?

I told him I'd ask Planetary Astronomer Mike. He's counting on you.

So, In the middle of the day on the moon, we'd expect the water to completely boil away. However, it will cool down in this process, though, because of latent heat.

This has to do with the idea that even though 100° C liquid water and 100° C steam have the same temperature, the steam has way more heat. If we want to raise a gram of water from room temperature (20° C) to its boiling point at 100° C, we need to add 80 calories of heat. If we actually want to boil that gram of water once its at that temperature, though, we need to add significantly more heat: 540 calories. This second injection of heat is what's known as latent heat, since it doesn't go into raising the temperature, but rearranging the molecules.

In its liquid state, water is actually pretty tightly bound. A hydrogen atom from one water molecule has a strong magnetic affinity for the oxygen side of another water molecule, an interaction known as hydrogen bonding. That extra heat for boiling is required to break the hydrogen bonds. This hydrogen bonding gives water most of its unique properties: strong surface tension, ice less dense than water, huge latent heat, etc. A great, albeit technical, website is the "anomalies of water" page.

So in the case of the moon during daytime, we drop the water, it begins to boil it away, but that boiling process will remove enough heat that the water's temperature will drop to the point that it just freezes. Only after our subsequent lump of ice either radiatively absorbs enough sunlight or conductively absorbs enough surface heat can it boil away completely.

On the moon at night, some of the water will again boil (just because of the initial heat in it), freeze, and our subsequently block of ice will lay relatively dormant. Over long time periods, some of that ice will sublimate...just due to random molecular vibrations, occasionally a surface molecule will get enough energy to break its bonds and roam free as a steam particle. That process is highly temperature dependent, though...if I'm recalling correctly, ice needs to get down to around 150K or below to be stable against sublimation.

There have actually been a couple space missions now looking for water ice buried in the North and South Pole craters on the moon, where permanent darkness should be capable of maintaining ice in this stable state. The jury is still out on this (there's another similar mission due to launch later this year), but what's even more weird is that there's been very good radio evidence of extremely thick ice sheets in the craters at the poles of Mercury.

I have no idea how you'll explain this to a seven-year-old.

Mike.

Atmosphere and Centrifugal Force

Roland wrote:

Thanks for the Mars answer. That's exactly what I was looking for. On that one: from my years of experience half-watching the Science Channel, I learned that we still have an atmosphere because of the magnetosphere deflecting solar winds that would otherwise strip it away. Is that entirely erroneous, or how do the lack of geological replenishment and the lack of magnetic protection compare in the care and feeding of an atmosphere?

Well, that too is important, but only to an extent. The magnetosphere shields us from charged particles - such as energetic protons and electrons emitted in solar flares - but can't do anything about the uncharged ones like energetic photons. Sputtering will certainly happen faster without a magnetosphere, but extreme UV can still get in and do its worst.

Roland then wrote:

And, continual resupply: this means the molten hooey down there contains lots of N and about a fifth that much O, or what?

Well, sort of. The nitrogen that's emitted is largely in the form of various nitrous oxide compounds...but eventually gets to N2 in one way or another, as that's a really low energy state for N to occupy. Oxygen, on the other hand, also gets emitted via a much larger supply of CO2, (as well as SO2) but there's a lot of extra sinks for the oxygen to go to, such as carbonates, silicate rocks, iron oxide, and such. O2 only gets into that form through plants, and is actually quite a high-energy state...left to its own devices with no photosynthesis, O2 would pretty quickly disappear from the atmosphere.

Roland continued:

There are really centrifugal force deniers? Do they think that Wonder Woman's lasso has magical levitation properties in addition to its magical truth-extraction properties? What do they think is happening, you know, in a centrifuge? Have they ever been on a Tilt-O-Whirl? The mind boggles, and it's making me queasy just recalling how much I concretely, viscerally accept that that there is such a thing as centrifugal force. (You really shouldn't have mentioned the taffee right before I was going to have to mention the Tilt-O-Whirl.)

Hmm, I recall in high school physics that we learned "there's really no such thing as centrifugal force, it's just inertia and centripetal force." In college physics, I then learned that depending on your coordinate system (e.g. a turning car), there really is centrifugal force. It's all summed up rather nicely in this xkcd.

Roland concluded with:

Are there really people who think the moon *is bigger* on the horizon? Or do you just mean those who think the moon appears bigger because of atmospheric lensing? Because one is just insane and the other is just the kind of clever idea your high school science teacher would be wrong about. (I think the moon appears bigger because of my brain and seeing it near the horizon and other large-looking things without other scale reference. It seems likely I got both this and the atmosphere idea from people like high school teachers.)

I just mean the atmospheric lensing hypothesis, which is absolutely not true. The moon appearing larger is entirely psychological...as you stated, you don't realize how large an angular size the moon actually subtends until you see something like a distant building next to it. The moon is actually slightly smaller on the horizon (but only in the vertical direction) due to a steady gradation in atmospheric refraction.

But your statement, "just the kind of clever idea your high school science teacher would be wrong about," is, in general, precisely the kind of thing I'm railing against.

Mike.

Followups on the History of our Planet

Mike said:

Again, the Earth was already pretty geologically active prior to this impact, but definitely way moreso afterwards.

Ben wrote:

I'd like to know more here. How exactly was Earth "more geologically active" after Theia collided and merged?

Oh, I just mean that the crust probably cooled and solidified sometime between when it formed 4.6 billion years ago, and the impact 3.9 billion years ago. Just from a pure energy calculation, you can guess it almost certainly remelted after the impact.

It's even quite possible that primitive life could've arisen in that 700 million year window (since it only took a couple hundred million years after the impact to do so), but would certainly have been obliterated once there was nothing but hot lava to stand on.

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Mike said:

Whoa, super fast-forward! We just skipped *most* of the history of Earth. The Giant impact happened about 3.9 billion years ago...Pangaea formed only 250 million years ago.

Ben then asked:

Anything interesting happen during those 3.9 billion years there?

Well, the whole Precambrian era was not the most exciting time-period, kinda like how in a Western Civ class they'll skip straight from "End of the Roman Empire" to "The Renaissance".

Mostly it was just a bunch of blue-green algae floating around the primitive oceans (think: primordial goo), spending a couple billion years turning the CO2-rich atmosphere into the oxygen-rich atmosphere we enjoy today. In fact, that was a prerequisite for multi-cellular life to arise.

One event that is notable, though, is the Cryogenian period, some 800 million years ago. This is still a subject of heavy debate, but it looks probable that for some time the entire Earth was completely covered in glaciers, leading to the term "Snowball Earth". There's definitely evidence based on banded iron deposits that the ocean was completely sealed off from the atmosphere...the most probable explanation is that a global ice sheet separated the two.

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Mike said:

Imagine you put small plates of styrofoam on top of some boiling water...at certain times they'll all lump together as they're pushed by the convective flow. As they drift around as a single mass, they might pass over a convective plume...when this happens, they'll get pushed apart again, and the cycle repeats.

Ben asked:

Wow, that is an *awesome* explanation.

But why is it so slow? I mean, is the molten magma convection really
so slow as to take *millions* of years to travel? And if it's
happening at such an insanely slow speed, is it really the same
convection-phenomenon we see in boiling water? The same differential
equations?

Well, first let's understand how convection in normal fluids works.

Imagine we have a "parcel" of air at sea level and at the ambient temperature there. Due to some random perturbation, it rises slightly. Now, the atmospheric pressure at that greater height is less, so our parcel will expand and cool in the process.

Since it's colder, we'd expect it to fall back to the surface...but remember that temperature decreases with height, too. So, we ask the question, "Is our cooled parcel of air still warmer than the surrounding air?"

The answer ends up depending sensitively on the temperature gradient with height. If the gradient is steep - i.e. temperature drops quickly with height - then our parcel will be warmer than the surrounding medium. This means it's buoyant, and continues rising...so convection sets in. In the gradient is shallow - i.e. temperature drops slowly with height - then our parcel is colder than the surrounding medium and just falls back to its original position and our atmosphere is stably stratified.

It turns out that the critical threshold gradient separating the two answers, known as the "adiabatic lapse rate", goes as:

dT/dz = -g/Cp

where T is the temperature, z is height, g is gravitational acceleration, and Cp is the specific heat. In plain English, that means to maintain convection you have to have a big enough temperature difference between hot-at-the-bottom and cold-at-the-top, modulated by the local gravity and how much heat the convective medium can hold.

So to get back to your question, yes, this differential equation governs convection *everywhere*...in your lava lamp, in the water on the stove, in the atmosphere, and even in the magma in the mantle.

Now, I've been a little disingenuous in describing the mantle as a molten liquid. Technically, it's what's known as a "rheid". For short term-phenomena, a rheid can pretty much be thought of as a solid...it will even permit transverse seismic S-waves to pass through it (a wave mode that will not pass through liquid). Over the long-term, though, it will flow as an extremely viscous, plastic material when put under strain. We're talking *really, really* viscous here...if water has a viscosity of 1 and honey has a viscosity of a few thousand, rheids in the mantle are on the order of a few billion.

It's this viscosity that slows the process down. Think of it like this: if you drop a rock in air, it'll fall much faster than a rock dropping through water, which will fall much faster than a rock dropping through yogurt. Gravity is still acting the same way in all three cases, but the damping term increases in each case. Similarly, the buoyant acceleration of convection is still just as strong for magma as it is for any other convective process...but the extreme viscosity acts as such a strong damping term that the terminal velocity is very, very slow. A single convective overturn in the mantle is on the order of a million years.

Mike.

Questions about the History of our Planet

In the email which started it all, Ben wrote:

Dear Planetary Astronomer Mike:

I was watching a show on the Science Channel yesterday, and it was this super-dramatic documentary about the history of our planet. The whole story was told from the tabloid angle of "our world was created via a series of terrifying catalysmic events!!" Cheesy hype aside, I'm wondering which of the events below is considered well-accepted/mostly-agreed-upon by planetary scientists, which are controversial theories, and which are laughable. (The show presented every event as an amazing "fact" in glorious CGI.)

Ok, there's a lot here, so let's take it step-by-step.

First, let me make a caveat, though: cataclysmic change is all the rage in the sciences these days, from biology to astronomy. One-hundred years ago this was not the case...Barringer had a very difficult time trying to prove to people that Meteor Crater in Arizona was caused by an impact. Back then, people believed geological events only occurred on geological timescales.

Nowadays, possibly due to a massive philosophical shift resulting from the World Wars, the science world has embraced the concept of sudden change. This is not to say that it doesn't happen - there's very good evidence to suggest that it does - just that the pendulum may have swung a little too far in the other direction.

So, point by point, then:

Ben wrote:

The timeline went something like this:

After cooling down from the nebula, Earth was a big chunk of dead, lifeless rock.

I think this first one is actually the least scientific claim of all. Soon after the early Earth formed, there'd be a lot of self-gravity holding the thing together...easily enough to melt rock and metals due to pressure heating. Moreover, a lot of the relatively short half-life radioactive materials (such as aluminum-26) would still be hot from the last supernova from which our solar system formed...these radioactive materials act as another energy source to melt material in larger planetesimals.

All this molten rock swimming around with molten metal would cause differentiation - heavier materials like iron would sink to the core, lighter materials like silicates would rise to the surface. You don't have to be *that* big for differentiation to occur. Some of the larger planetesimals in the asteroid belt differentiated before they were rended apart due to Jupiter's tidal forces, leaving us with almost entirely metallic asteroids such as Vesta.

This ends up leaving a lot of hot magma on the surface exposed to the vacuum of space...the vacuum pressure alone would cause serious outgassing from the lava. This early Earth would probably have had a pretty significant CO2 and water vapor atmosphere, so I guess I'm not seeing the "dead, lifeless rock" here.

Ben continued:

An anonymous mars-sized planet collided with Earth, and the planets *fused together*. This caused Earth to get much bigger, and the sheer pressure and force of the collision caused our planet to become super geologically-active -- molten iron core, lava-ey mantle, drifting tectonic plates on top.

Okay, so technically it's not anonymous - they've actually provisionally named it "Theia".

Current n-body simulations (remember "orbit7"?) have shown it'd be pretty likely for an early Earth to accrue Trojans. These are essentially mini-planetesimals which orbit at the same distance from the Sun as a parent body (in this case, the early Earth), but at Lagrangian points 60 degrees ahead and 60 degrees behind the parent body's position in its orbit. We've actually seen tons of asteroids 60 degrees ahead and 60 degrees behind Jupiter, but in the same orbit.

Here's the catch, though: those objects at the Lagrangian points are only in stable orbits if they're much less massive than their parent body. These same simulations have shown that it's quite possible for these early Trojans to start accreting to a mass over the stability limit...at which point they come spiraling to the Earth.

Getting to your later question, then, we are pretty sure this event is what formed our moon. A mars-sized object (roughly 1/10th the mass of the current Earth) hit us in a sidelong collision, mostly fusing, but leaving a whole bunch of debris in Earth orbit. This debris later coalesced into our Moon.

This scenario explains two seemingly contrary properties of the moon:

1. The moon composition is surprisingly close to Earth's rocks.
2. The moon's orbit is really close to the plane of the solar system.

Statement 1 implies that the Earth and Moon formed side-by-side (the "sister theory")...but if this were true, we'd expect the Moon's orbit to be aligned with Earth's rotation axis, 23 degrees tilted to Earth's orbital axis. Statement 2 implies that the moon was captured from elsewhere in the solar system (the "capture theory")...but if this were true, we'd expect the Moon to be significantly different in composition.

The "Giant Impact Hypothesis" neatly ties both observations together. Statement 1 can be true because the orbital debris which formed the Moon came from the Earth and Theia. Statement 2 can be true because the initial angular momentum to establish the Moon's orbit was in line with Theia coming in from elsewhere in the solar system.

Now, why they didn't cover this in the documentary, I don't know...it'd make for some pretty sweet CGI. Again, the Earth was already pretty geologically active prior to this impact, but definitely way moreso afterwards.

Ben said:

Somehow (?) this caused water to form and cover the whole planet.

Umm, again, the water should already have been there, though possibly in vapor form depending on Earth's pressure and temperature prior to impact. Simulations I've seen at planetary conferences have actually shown that after the impact the Earth's surface was hot enough that, at least for a few thousands years, a significant component of the atmosphere was *rock vapor*. Whoa.

Eventually it'll cool down enough for rock, and then even water to rain out of the atmosphere. Note that there's still some modest debate about why Earth has *so much* water...some people point to a continual barrage of comets. Other people point to those first people and laugh.

Ben continued:

Plates started drifting around. Their collisions caused them to buckle up, revealing islands, and eventually whole continents.

Well, again, I don't know why there wouldn't have already been plates prior to impact, but whatev'...

Ben then wrote:

Continents aggregated into ever-bigger continents, and eventually the last 2 super-continents collided to form Pangaea

Whoa, super fast-forward! We just skipped *most* of the history of Earth. The Giant impact happened about 3.9 billion years ago...Pangaea formed only 250 million years ago.

It's believed there were actually several supercontinents formed prior to Pangaea...Vaalabara, Columbia, Rodinia, etc. Each of them came together, drifted apart, came together, drifted apart. The best analogy I've heard for this is:

Imagine you put small plates of styrofoam on top of some boiling water...at certain times they'll all lump together as they're pushed by the convective flow. As they drift around as a single mass, they might pass over a convective plume...when this happens, they'll get pushed apart again, and the cycle repeats.

Ben wrote:

The collision which form Pangaea created super-mountains across the center, bigger than the Himalayas. They were so high that they blocked all clouds and weather. This meant that the coastlines were lush and rainy. but the center of Pangaea (inner 60% land mass) was one giant "super desert", more than 10x the size of the Sahara. The rainy coasts set the stage for the development of multicellular marine life.

Ok, it probably wasn't Pangaea this happened on, but the prior supercontinent Rodinia (750 million years ago, roughly). It *is* a general property of supercontinents that they form deserts in their interior...you don't need to have big mountains, but it helps. The zonal flow picks up moisture passing over the super-ocean, and it rains out as it passes over the landmass. In the case of supercontinents, though, there's just more landmass than available rain.

I'm not sure what "bigger than the Himalayas" means...you really can't form a mountain much taller than Everest, since rock liquefies under the pressure of anything taller. Only on planets with lower gravity can you do this - for example, Mars has 1/3 the gravity of Earth, and Olympus Mons (tallest mountain on Mars) is 3 times taller than Everest.

By bigger do you just mean the range was more extensive? For ther record, the Himalayas already push up into the prevailing jet stream, causing vortices further downwind. and alternating highs and lows that we experience here in N. America.

Ben continued:

Later on (250 million years ago?), some Big Nasty Event caused 90% of all life to go extinct -- most likely planet-wide volcanic activity which poisoned the atmosphere with methane and warmed the planet excessively. This set the stage for tiny lizards (which survived) to evolve into dinosaurs and cover the pangaeic "hot earth".

Ah, the Permian-Triassic extinction event - the largest extinction event *ever*. Over 95% of marine life died.

There's still heavy debate about the cause. There were definitely huge crazy volcanoes unlike any seen before...the "Siberian Traps". It wasn't so much a single volcanic explosion from a mountain as it was a giant fissure in the crust of the Earth...lava just oozed from the gash for upwards of a million years, paving over most of what is now current-day Siberia with basalt. That kind of event would release massive, massive amounts of CO2 in the air and easily kill off lots o' life.

What precipitated the eruption is the debated part. There's extensive evidence of iridium at this geological layer, which suggests an impact event. Iridium is one of the two densest materials, so most of the iridium the Earth formed with quickly sunk to the center of the core during the early differentiation period. In general, if you find iridium on the surface, it came from an extra-terrestrial source.

However, there doesn't seem to be any really good candidate crater from this time period. It's interesting to note, though, that there are similar formations to the Siberian Traps on the Moon, known as maria - the large, dark flat regions you see when looking at it. These seem to have been caused by impact events so massive that the impactor pierced through the crust, creating the fissure from which lava pours out...it's possible there's no obvious impact crater because most of the impactor went all the way through the Earth's crust and melted in the mantle.

Ben added:

65 million years ago, an asterioid hit earth, wiped out the dinosaurs, left the mammals.

There's very little debate about this now...Chicxulub crater is the smoking gun, and the geological evidence shows iridium planet-wide at exactly 65 million years ago.

Ben then wrote:

Pangaea starts to break apart over the next 60 million years. The great plain of North America is a giant inland ocean.

An ice age happens about 12,000 years ago. Ice caps cover 1/3 of the planet, causing ocean levels to fall and the N. American inland ocean to completely drain. Glaciers carve the hell out our landscape.

Well, ice ages are super common...the one 12,000 years ago was just the most recent. The theory is they should happen every 41,000 years, as the period of Earth's precession comes into phase the period of change of Earth's perihelion (known as the "Milankovitch" cycle).

Ben concluded with:

There's an ongoing cyclical process of continents drifting apart and coming back together into a supercontinent (why??). Folks hypothesize a 'new' pangaea happening in the future, whereby Asia rotates clockwise, pushing the UK into the polar cap and Siberia into the equator. Africa smashes into Europe, replacing the Meditarrean sea with a huge mountain range: see here and here.

Yup, "Pangaea Ultima". It happens for the same reason that this cycle has always occured...unstable plates floating along magma convection flows.

Mike.