## Archive for the ‘Planets in General’ category

### Solar System Overview

May 27, 2010

Welcome to the solar system! It’s a really interesting place, and there’s a lot to cover. First lets get a basic idea of what our solar system looks like. There are eight planets in the solar system and five “dwarf planets” and they all orbit around the sun. The four planets closest to the sun are called the “inner” planets. They are all pretty small and made mostly of rocks. Earth is the third inner planet. Here’s a picture of their orbits (click for a bigger version):

Between the inner and outer planets is a region with a lot of rocks floating around, left over from when the solar system was forming. It’s not like in Star Wars, though. If you were on an asteroid in the belt, you probably wouldn’t be able to see any of the other ones without a telescope. The outer planets are much larger than the inner ones. They are huge balls of gas with dense cores. Because they are so large, they have very strong gravity, so most of them have many moons. Out beyond the giant planets is the dwarf planet Pluto. It doesn’t really fit in with the rest of the outer planets because it is tiny, smaller than earth’s moon. It also has a strange orbit, which makes scientists think it is part of a cloud of rocks and bits of ice called the Kuiper belt. Here is a picture of the outer planet orbits. You can’t see the orbits of the inner planets because they are too small. All the inner planets are right near the sun, inside Jupiter’s orbit.

Below you can see pictures of the sun and all the planets (and some of the dwarf planets) in our solar system. This shows them all at about the right sizes relative to each other, but much closer together than they actually are.

This post was originally part of a website made for the UMich Student Astronomical Society 2005 Astrophysics inreach by Ryan Anderson, Sarah Springsteen, and Ben Ruskin. Pictures were shamelessly stolen from various sites. It has been updated with more current information as of May, 2010.

### AGU 2009: Day 4 – Enceladus and Exoplanets

December 22, 2009

Click for the full comic from "Saturday Morning Breakfast Cereal"

Thursday at AGU started with a tough choice. At 8 am there was a talk about methane on Mars, and a special lecture about the water plumes on Enceladus, and plate tectonics on Venus! In the end I decided to go to the Enceladus lecture, given by Sue Kieffer. She explained that there are two primary models for how the Enceladus plumes form. The first is dubbed the “cold faithful” model, and involves pure water coming into contact with the vacuum of space, perhaps in an underground cavern, and boiling to create a plume of vapor and ice particles. (remember, if the pressure is low enough it doesn’t need to be hot for water to boil) The problem with this model is that gases detected in the plume would have to also be in the water, and when exposed to the vacuum they would come out of solution, much like a shaken can of pop, and so the amount of ice in the plume would be different from observations.

The other model, which Kieffer prefers, is the “frigid faithful” model, which relies on “clathrates” – gas molecules trapped in cages of water ice. In this model, dense fractures in the subsurface ice act as conduits for gases, including water vapor, released from the clathrates due to the geothermal heating in the crust. In this scenario the fractures are constantly sealing themselves as ice condenses in the mouth of the vent, but re-open as tidal stresses fracture the crust.

This clathrate has enough methane molecules trapped in the ice to burn!

The enceladus lecture finished with a long question and discussion session. Some of the concerns with the “frigid faithful” model were that it is hard to explain how the observed sodium got into the vapor plume. Also, it looks like argon is concentrated in the plume, which is best done with liquid water. Kieffer admitted that this was “troublesome” but another commenter pointed out that radioactive potassium in the ice could decay to provide the Argon. There is still the problem that the amount of argon coming out with the plume is way more than could be continually produced, suggesting that argon built up for a long time and is now being released. That would mean we are seeing a “special” event, and scientists are always hesitant to invoke special circumstances to explain an observation.

After Enceladus, I headed over to the session on planetary interiors. This session talked about all sorts of different objects, including everyone’s favorite two-faced moon of Saturn, Iapetus. Iapetus has a bizarre ring of mountains encircling its equator and a couple of talks tried to explain why that might be. Previous theories suggested that the ridge could be due to Iapetus changing shape as it began to spin slower, but Mikael Beuthe said that scenario wouldn’t produce the right kind of faults in the crust. He suggested that a better explanation would be if the crust was slightly thinner at the equator and Iapetus shrank as it cooled, causing the crust to buckle along the equator. A second talk by David Sandwell suggested the somewhat strange idea that the ridge was formed when the planet contracted due to melting of the interior ice, causing the outer spherical shell to collapse in two hemispherical halves. After these talks, there was some discussion and several scientists pointed out that it’s hard to explain massive faults right along the equator but nowhere else. Those people favored an origin related to material accreted from the rings.

Iapetus' equatorial ridge towers above the limb in this view from Cassini.

The other reason that I attended the planetary interiors session was that several talks speculated about “super-earth” exoplanets. C. Sotin (I didn’t catch his full first name) addressed the question of whether super-Earths would be more or less likely to have plate tectonics than the Earth. One paper, based on scaling up the equations that govern the process on Earth, suggested that super-earths would have plate tectonics, but a different paper based on simulations said exactly the opposite. Sotin took a look at both sides and concluded that it’s not just a simple function of the mass of the planet. Lots of unknown factors like water content, changes of phase deep in the mantle, and the degree to which the primordial crust is fractured all play a role.

Cartoon of the solar wind interacting with Earth's magnetosphere.

Another talk by Peter Driscol took a look at how to get “optimal” geodynamos in rocky exoplanets. He showed that, to get the maximum magnetic field from an Earth-sized planet, it should be slightly hotter than the Earth, but that Earth is still pretty darn good. He also said that it could be possible to detect the radiation from charged particles in the stellar wind spiraling through the planet’s magnetic field! The observation would have to be made from above earth’s ionosphere, and the most likely candidate would be a planet with a big convecting core (and therefore a powerful magnetic field) orbiting a nearby star which has a strong stellar wind.

One of the comments after Driscol’s talk, directed at everyone working on super-earth dynamics struck me as extremely significant. I didn’t catch the person’s name, but he pointed out that everyone assumes that the mantles of super-earths behave like the earth’s mantle, but in actuality, under the enormous heat and pressure silicates would actually break down and behave like a metal, much the same way that hydrogen becomes metallic inside Jupiter and Saturn! If Si really does become metallic in super-earths, then that would have lots of bizarre consequences, likely powering much stronger magnetic fields and possibly changing the heat-flow out of the planet.

Artist's impression of a possible super-earth discovered around the star Gliese 876. © NSF

Finally, Linda Elkins-Tanton talked about what the mantle convection would be like on a tidally-locked rocky planet. Such a planet would always have one side facing its star while the other would be in constant darkness. Elkins-Tanoton and her student Sarah Goldman found that the powerful heating on one side of the planet and cooling on the other side would drive a continuous upwelling plume on the hot side of the planet and a matching downwelling plume on the cold side. In extreme cases, this could lead to the presence of “magma ponds” on the sunlit side of the planet, which are constantly spilling away from the hottest point and eventually freezing. That sort of mass redistribution could make the planet want to re-orient itself. They have not yet considered what an atmosphere would do to the magma ponds, but likely it would transfer the heat away more effectively, making the ponds more prone to freezing.

I have always loved the idea of a tidally-locked terrestrial planet, particularly since there might be a sweet-spot near the terminator where the surface temperature is habitable. I’ve even started a short story set on such a world. It might be time to dig that back up…

This computer model shows that the sun-lit side of a tidally locked planet can get extremely hot, and the dark side can get extremely cold, but temperatures might be moderate right on the day-night line.

### How Habitable is the Earth?

November 4, 2009

Charlie Stross has an interesting post on his blog that asks the question “How habitable is the Earth?” He goes on to conclude, through a great discussion of the evolution of our planet, that the fraction of time that the earth has been habitable to humans is a tiny sliver of the time the Earth has been around, and that furthermore, much of the earth is not habitable for humans because it is water or ice or mountains. If much of our planet, even now, is not habitable, he argues, what hope is there of finding other habitable worlds out there in space?

It’s an interesting discussion, but I find it somewhat misleading. It makes the rather large assumption that for a world to be considered habitable, a human would have to be able to survive for 24 hours, naked, on the surface. Ok, that’s one definition of habitable. But if you are postulating that these humans are capable of interstellar travel, it seems like you might make allowances for the use of clothing and the ability to build shelter. After all, we’ve known about those ones for a while. You could go even further and suggest that these humans might be able to alter the air they breathe, either through individual gas masks, or on a planetary scale. We used CO2 scrubbers on the Apollo missions to make the air breathable, maybe that would work on a planet with otherwise unbreathable air?

I think he’s fundamentally right in terms of human habitability: the likelihood of a planet being perfectly attuned to humans is extremely low. We evolved to live on Earth and nowhere else. We are going to have to make some adjustments to ourselves or our environment to live anywhere else.

The problem is that he then extrapolates and suggests that this might explain the Fermi paradox (aka. if there are so many stars and planets out there, why haven’t we heard from any little green men?). But that is completely off-base! He is essentially saying that, because humans evolved to live on Earth and nowhere else, it is unlikely for anything else to be living out there because there are likely few earth-like places. That does not follow. There could be aliens out there that are completely happy on their planets that would be instantly lethal to us. And it’s entirely possible that if they set foot on Earth they would find it a very hostile and uninhabitable environment (and not just because of the terrified earthlings).

Anyway, it’s an interesting article. Go take a look.

And speaking of interesting articles, have you gone and voted for my MSL: Mars Action Hero article over at scientificblogging? I’m one of the finalists for their science writing competition, so take a look and vote for me if you like it. To see the other entries, click here. Feel free to vote for as many as you like, and remember you can vote daily until the 23rd!

### Solar System Creator

July 10, 2009

As I mentioned last month, on top of research and grad school duties, I’m in the process of planning out a sci-fi novel. It began with the month-long outlining challenge “Midsommer Madness” over at the Liberty Hall writing site, and I am continuing with it in my spare time.

I am trying to make my novel grounded in reality whenever possible. It is set in a known star system, 55 Cancri. The 55 Cancri system has 5 known planets, but I also took some artistic license and added moons and small planets that observations would likely have missed. Then, once I had planets and moons, I needed to figure out which ones would be habitable!

I happen to know a thing or two about planets, so I put together a handy spreadsheet to use to calculate things like surface temperature, surface gravity and orbital period given things like how bright the star is, how far away the planet is, etc. Once I had the spreadsheet made, I realized that there are likely other people out there who might find it useful.

So, whether you are a writer trying to come up with a plausible setting for your bestselling sci-fi epic, or a student learning about the solar system, or just plain curious about planets, please feel free to use and modify this spreadsheet. Right now it is set up for our solar system to give you an idea of what reasonable values are for the different variables, and to show that the results are generally pretty good despite the simplicity of the calculations. If you find this useful or have any questions, feel free to contact me by leaving a comment on this post.

I described how I calculated the surface temperature below. You don’t need to read the explanation to use the spreadsheet: it should just work if you enter numbers, but I encourage you to try to follow the derivation. Even if you don’t follow the algebra, I tried to explain everything in words to give some conceptual understanding of the ideas behind the math, and the ideas are what matter.

Note for Students: The spreadsheet is free for you to use, but be sure you cite this page as a source. Also show your work for all calculations! Copying values from the spreadsheet without showing your work probably won’t earn you any points, and may be considered plagiarism, which is grounds for failure and/or expulsion at most schools. And really, it’s not that hard to do the calculations, especially since the rest of this post is spent walking you through them! You might even learn something!

Ok, so how does it work? Well, the calculation of a planet’s surface temperature is based on the very simple idea that if its average temperature is not changing, then the amount of energy the planet absorbs must match the amount that it emits. Pretty much common sense! If the amount in and out were different, then the temperature would change until they balanced!

First, the absorption. The energy source is the star, which has a certain luminosity $L$ (given in watts). This says how much energy the star puts out in all directions per second. We want to know how much energy per square meter hits the planet, so we take the luminosity and spread it out evenly over the surface of a sphere with a radius $R$ equal to the distance from the planet to the star. The surface area of a sphere is $A=4\pi R^2$, so the amount of energy from the star hitting each square meter of the planet’s cross section is: $L/A=\dfrac{L}{4\pi R^2}$

The planet’s cross section is just the area of a circle with the planet’s radius: $\pi r^2$. Note that we’re using the area of a circle and not a sphere! That’s because the starlight doesn’t hit the whole planet, it just hits the part of the planet that is visible. Can you see all sides of a sphere at once? Neither can I, and neither can the star. What we see is the 2 dimensional cross section: a circle.

So now we have an equation for how much energy the planet absorbs per second: $Energy Absorbed Per Second= \dfrac{L \pi r^2}{4\pi R^2}$

But that is assuming that the planet absorbs every bit of light that hits it, which we know isn’t true: we see planets in the night sky by their reflected light! So we can add a correction called albedo. Albedo, $A$, is the fraction of starlight that the planet reflects back out into space, and $(1-A)$ is the fraction of starlight a planet absorbs. So with that correction, our equation becomes: $Energy Absorbed Per Second= \dfrac{(1-A) L \pi r^2}{4\pi R^2}$

Now we have to figure out an expression for the energy that the planet emits. Here we have to make an assumption to simplify things: we assume that the energy absorbed by the planet is immediately redistributed evenly over the whole planet. Obviously this isn’t right, it is much warmer on the day side than the night side, but this assumption makes our lifes much easier. We just have to remember that the value we get is going to be an average of day and night temperatures.

We also assume that the planet radiates away its energy like a blackbody. A blackbody is something that absorbs and emits all radiation perfectly. We’re not going to worry too much about this assumption. For our purposes, planets are close enough to being blackbodies that it doesn’t matter much. I know, I know, we just made an adjustment for albedo two paragraphs ago, implying that the planet is not a perfect blackbody! Just calm down. It works pretty well, and that’s all we need.

Anyway, if we assume the planet is a uniform temperature blackbody, then we can use the handy equation for blackbody emission: $Energy Emitted Per Square Meter Per Second = \sigma T^4$. Sigma is called the Stefan-Boltzmann constant, and is given by $\sigma = 5.67\times 10^{-8} W m^{-2} K^{-4}$ To get rid of that pesky “per square meter” part of the equation, we just multiply by the surface area of the thing doing the emitting: in this case, the planet. Here we do use the surface area of a sphere, remember our assumption that the energy absorbed gets spread out over the whole surface? This is why we did that. The result is:

$Energy Emitted Per Second = 4 \pi r^2 \sigma T^4$

Now that’s a fine equation if your planet emits every bit of energy that it receives straight back to space. But that’s not how it works for planets with atmospheres. There’s this effect where the atmosphere traps energy in the system for a longer time, resulting in a warmer planet…  you may have heard of it: the Greenhouse Effect! It would be nice if we could add that to our model! If we don’t, we’ll never get the surface temperature right for a planet like Venus, where the greenhouse effect dominates.

To actually do a proper simulation of the greenhouse effect is very difficult and complicated, so instead we are going to use a fudge factor. The bottom line is that the greenhouse effect $GE$ reduces the amount of energy radiated from the surface that escapes to space. So we can do something very similar to our albedo adjustment: $GE$ gives the amount of energy that the atmosphere absorbs, and $1-GE$ gives the amount of energy that actually escapes to space. For the Earth $GE \approx 0.4$ and for Venus $GE \approx 0.99$. Our modified equation is now:

$Energy Emitted Per Second = (1-GE) 4 \pi r^2 \sigma T^4$

Now, remember why we were doing all of this? We want to find the planet’s average surface temperature $T$. To get this, we have to set our two equations equal to each other and solve:

$Energy Emitted Per Second = Energy Absorbed Per Second$

$(1-GE) 4 \pi r^2 \sigma T^4 = \dfrac{(1-A) L \pi r^2}{4\pi R^2}$

Look! The planet’s radius appears on both sides of the equation! That means it cancels out, and that a planet’s radius has no effect on its surface temperature! Ok, don’t get too excited, we still need to solve for T.

$T^4 = \dfrac{L (1-A)}{16\sigma\pi R^{2}(1-GE)}$

$T = \left(\dfrac{L (1-A)}{16\sigma\pi R^{2}(1-GE)}\right)^{1/4}$

Voila! There is the expression for the equilibrium surface temperature of a planet, taking into account the planet’s reflectivity and the greenhouse effect. I hope this sheds a little light into how to think about the energy budget of a planet, and how my spreadsheet works. Again, if you have any questions, post them in the comments and I’ll answer them!

### Impact Crater

April 28, 2009

In my posts about our field trip to Arizona, I showed my best pictures of meteor crater, but really none of them come close to expressing the feeling of standing on the brink of such a feature and trying to imagine an explosion big enough to carve it out. I just came across a photo by Stan Gaz that does a much better job than my snapshots (click to follow a link to a bigger version):

### Discoveries in Planetary Science

April 26, 2009

The Division for Planetary Sciences of the American Astronomical Society just released several short sets of slides summarizing recent important discoveries in planetary science that aren’t yet in textbooks. They are very nice, easy to understand summaries so I encourage you to check them out. The topics so far are: Mars Methane, Extrasolar Planet Imaging, The Chaotic Early Solar System, Mars Sulfur Chemistry, and Mercury Volcanism. Follow those links to PDF files for each topic, or click here to go to the DPS “Discoveries in Planetary Science” page.

April 21, 2009

I got a bunch of questions about the BigPicture feature on the Cassini extended mission from an “enthusiastic” commenter, with whom I happen to be related (Hi mom!), and I thought I would dedicate a post to answering them.

1. How does a Jovian equinox work? Start by reviewing how one on earth works.

Well, the pictures are of Saturn, not Jupiter, but that doesn’t really matter since equinoxes work the same on all planets. An equinox occurs when, like the word implies, the night and day are of equal length. On Earth, this occurs on ~March 21 and ~September 22. During an equinox, the tilt of a planet on its axis is oriented parallel to the planet’s direction of motion. It’s a lot easier to show with a picture:

2. #5 Photo What are we seeing here? Rhea in front of Titan in front of the Sun?

Yeah, pretty much. Except that Titan is probably not exactly in front of the sun. I suspect the sun is off-screen, but the geometry is still essentially that Titan is between the sun and the spacecraft. Small particles like the ones in Titan’s hazy atmosphere tend to scatter light forward a lot more effectively than they scatter it back toward the light source, so that’s why Titan’s atmosphere looks so bright (plus the camera exposure, of course).

3. How many moons does this planet have, anyway?

Saturn has 61 moons. But that’s sort of an artificial number, because where do you draw the line between a moon and a large ring particle? Anyway, you can read all about Saturn’s satellites on Wikipedia.

4. How long does it take for info from Casini to come to Earth? Is it just a matter of a few minutes? A week?

Saturn’s orbit is 9.6 times as far from the sun as the earth, so that means that Saturn ranges from 8.6 to 10.6 astronomical units away from Earth (an astronomical unit is the distance from the earth to the sun). It takes light about 8 minutes to travel one AU. So, a radio signal from Cassini takes 1.2-1.5 hours to get back to earth.

5. What is a “shepard moon”? Why is Prometheus one?

A shepherd moon is a moon that orbits near a ring and whose gravity causes the ring to maintain a nice, sharp edge. Normally, rings want to spread out as the individual particles bump into each other and change their orbits slightly. If they were drifting toward the planet, that would mean that they were losing angular momentum. But then along comes the shepherd moon, on an orbit slightly closer to the planet. That means it is orbiting faster than the ring particles, so its gravity tends to try to drag them along with it. But by doing so, it gives them more angular momentum, so their orbit expands and ends up back where it began. The opposite happens for particles whose orbits are expanding when a more distant shepherd moon comes by. You can read more about this here, but sadly I’m not finding many good explanations of the phenomenon. Apparently there is an Enya album called “Shepherd Moons” though…

6. What point of view makes and eclipse? See Photo #10 and explain this to us.

An eclipse occurs when the shadow of one object falls on another object. So, a solar eclipse on Earth happens when the moon’s shadow falls on the Earth. A lunar eclipse occurs when the Earth’s shadow falls on the moon. In photo #10, the caption neglects to explain how Enceladus can be visible if it is in Saturn’s shadow. But remember what I said earlier about small particles scattering light forward? That applies to Saturn’s rings too. So, in the image, Enceladus is illuminated by Saturn’s glowing rings. Here’s an absolutely spectacular view of Saturn and its rings taken by Cassini a couple of years ago:

### LPSC: The Masursky Lecture

March 28, 2009

Every year at LPSC one of the big events is the Masursky lecture, given by that year’s winner of the Masursky prize recognizing “individuals who have rendered outstanding service to planetary science and exploration through engineering, managerial, programmatic, or public service activities”.

This year’s winner was Alan Stern, and he gave a thought-provoking talk about everyone’s favorite subject: What is a Planet? The official title was “Planet Categorization and Planetary Science: Coming of Age in the 21st Century.”

Artist's concept of an extrasolar planet and its moons.

Stern made it clear that he favors a broad definition of what constitutes a planet, essentially saying that if it’s round, it’s a planet. Not a surprising view for the leader of a mission to Pluto. But at the same time, he did not try to push his views too hard. Instead he took a look at the state of planetary science and suggested that the whole debate over nomenclature is really an indicator that we’re in the midst of a revolution driven by the startling diversity of planets being discovered.

All of a sudden, we are discovering planets and planet-like objects everywhere we look. Right now there are 344 known extrasolar planets, many of which have bizarre, unexpected attributes. Some orbit pulsars, others have scorching orbits that circle their star every few days or hours, others plunge deep into their solar system, and then retreat to icy distances are they follow extremely oblong orbits. Some extrasolar planets are puffed up until they have the density of balsa wood, others are extremely dense. Even in our solar system, we’re finding a whole new population of objects out in the Kuiper Belt, and we know of many moons that are interesting worlds themselves.

Stern’s premise is that this startling diversity is what is driving the debate over what is and isn’t a planet and that, essentially, we just have to keep debating as we gather more information. How do we organize planetary objects? What properties are the most important in classification? What subtypes make the most sense? And who decides whether a new object is a planet? Stern suggested that these are the types of questions that planetary scientists need to be mulling over and chatting about in the halls at conferences because that’s where it will be figured out, bit by bit. Eventually the scientific method will prevail and a logical system will emerge.

Exoplanet discoveries by year as of Early 2009.

I hope that’s how it works, but in the meantime, Stern made another point that I think was the most valuable one of the lecture, though he did not spend much time on it. He talked about the common complaint about the possibility of having more than nine planets: “But how will kids learn all those names?” His snarky response was: “I guess we’d better go back to nine states then.” But the serious response was that this is a fantastic teaching moment! This is an issue that the public is really interested in, and it’s a great example of how science works! We can use this to start conversations about planetary science and to help people start thinking critically and scientifically.

I’ll admit, I get tired of the “what is a planet” debate, but Stern is right. We are experiencing a revolution in what we consider a planet. It may be confusing and frustrating, but it’s an excellent teachable moment, and I plan to make the most of it the next time someone asks me about poor little pluto.

### A Tidally Locked Earth

February 25, 2009

A while ago, I posted about an interesting abstract and poster at the Lunar and Planetary Science Conference discussion the possibility that tidally locked exoplanets might still be habitable.

Well, apparently the new Discovery series entitled “The World Without…” is doing an episode about what would happen if the Earth stopped rotating. One of their associate producers contacted me after reading my blog post about tidally locked exoplanets and asked me some more about what the earth would be like if it were tidally locked! I wrote a long response that I thought you might find interesting, so here it is!

The first thing that I should make clear is that a tidally locked planet still rotates once per orbit. If it didn’t, then the same side would not always face the star! I don’t know if that rules out a tidally-locked earth for your episode, but I will assume that it doesn’t.

If Earth were tidally locked, there would be no seasons. The only change in the amount of sunlight would come from the slight variation in distance from the sun due to Earth’s orbit being slightly out of round. Instead of seasons I suspect there would be zones of different climates depending how far away you are from the center of the side that always faces the sun. Right on the equator of the sun-facing side, I would expect very high temperatures. In the center of land masses you would probably have scorching hot deserts, and near the coasts there would be huge thunderstorms due to the rapid evaporation of the water. As you go farther away from the sub-solar point, the sun would get lower in the sky and you would have gradually cooler climates in rings. I think that the intense heating on the sunlit side and the cooler climates surrounding that area would set up circulation in the atmosphere similar to the Hadley cells that transport heat away from our equator (http://en.wikipedia.org/wiki/Hadley_cell) but the winds caused by this would not be affected by the coriolis force since the planet is rotating so slowly. So in general you would expect surface winds to blow cooler air toward the sub-solar point, where it would be heated, rise and then circulate back toward cooler climates.

The far side of the planet would be frigid, since it would never see the sun. Its only source of warmth would be ocean circulation and wilds from the warm half of the planet. Even on the sunlit side, much of the planet would never see the sun rise very high, and would be quite cold.

The slow rotation of the Earth would have an effect on the moon too. Due to the moon’s gravity, the Earth bulges a little bit toward the moon. Right now, the earth rotated much faster than the moon orbits, so the tidal bulge is always a little bit ahead of where the moon’s gravity “wants” it to be. This means that the moon’s gravity is actually slowing the earth’s rotation down and the moon is gradually moving away from the earth. If the earth were rotating really slowly, then the exact opposite would happen (I’m assuming that the moon starts off in its current orbit). The moon would gradually try to make the Earth spin faster, and in doing so the moon would lose energy and come closer to the Earth.

I would imagine that the lack of days, and the sun being at the same place in the sky all the time would have some interesting effects on life too, but I know very little about biology so I’ll leave that for someone more qualified.

They asked a follow up question, requesting more detail about the evolution of the Moon’s orbit around a non-rotating planet, so I dug out my old “physics of the planets” notes and did my best to answer:

Assuming that you start with the earth not rotating but the moon in its current orbit, the moon will exert a torque on the earth until the earth is spinning as fast as the moon is orbiting. To spin-up the earth, the moon will give up some of its angular momentum, which will cause it to gradually drop to a lower orbit. If I’m doing things correctly, the moon will be about 1/4 as far away as it currently is by the time it has transferred half of its angular momentum to the earth.

When the moon stopped moving inward, the Earth and moon would be locked facing each other, much like Pluto and its moon Charon. There would be no changing tides on earth because the tidal bulge would always face the moon. All of this tidal evolution wouldn’t happen immediately, it would probably take hundreds of millions, if not billions of years. During that time, the tides would occur with a frequency equal to the moon’s orbital period minus whatever the earth’s slow rotational period is.

Through all of this I ignored the torque and tides from the sun. Its effect will be less than that of the moon, so it might slow things down but I think the moon’s orbit and earth’s spin would still evolve in the same way, generally.

Again, this sort of bends the rules of the episode because the earth is technically rotating. If you held the earth fixed, then I think the tidal evolution would just continue until the moon crashed into us, since the moon would continue to try to give the earth angular momentum, but that angular momentum would “disappear” due to whatever magic is holding the earth fixed. This is not physically realistic because angular momentum is supposed to be conserved.

### Blogging from the American Geophysical Union Fall Meeting

December 14, 2008

Tomorrow I’m heading to San Francisco along with 15,000 other scientists to participate in the 2008 fall meeting of the American Geophysical Union (AGU). Yes, you read that right: fifteen thousand scientists in one place.  This will be my first AGU and my first time in San Francisco so I’m excited. There’s tons of planetary science stuff scheduled, and I will be taking notes and blogging as much as possible during the meeting. I’ll also be presenting a poster about my work with the North Meridiani and Gale Crater potential landing sites for MSL, which is why this week has been a little light on blogging: I’ve been running around trying to get my poster finished and write papers and study for finals. Happily all of that is done now and next week will be a blogging bonanza. Stay tuned!