Friday, January 30, 2009

Methane on Mars. Does this mean Life?

So, the science and popular press was all in a huff in the last week or two about the possibility of life on Mars because of the confirmation (first reported in 2004 in many popular press reports) that there is methane (natural gas to you earthlings; CH4 to us nerds) being generated on Mars.

Now, part of the problem is that NASA's PR people think nobody cares if it's not instantly life-changing for everybody. And in part, they're right.

But, come on! Is this really appropriate?

NASA: Martian Methane Reveals the Red Planet is not a Dead Planet

Who besides a geologist is going to interpret that as anything other than meaning there's life on Mars?

So, here's the deal.

In 2004, Krasnopolsky et al. reported that they had discovered methane in the martian atmosphere. In 2009, Mumma et al. reported that they had seen methane appear, disappear, and reappear over several martian seasons. The methane appeared in the summer time. The amount of methane detected is in the parts per billion (ppb).

Methane has, at most a ~350 year half-life in the martian atmosphere. The amount detected should have disappeared long ago unless it is currently (as-in right this minute, not "currently" as-in geologically) being generated.

So, there are two ways to make methane: geologically and biologically.

  1. Hydrothermal systems generate methane. We see no evidence for elevated temperatures or other features signifying a hydrothermal system.
  2. Methane Clathrates can generate methane when heated.
  3. Various volcanoes release methane.
  4. Methane hydrates (clathrates are a form of hydrate).
  5. I'm sure there are others that I don't know about
  1. Most (90%) of methane production on Earth is biogenic.
  2. That's really all I know. I'm not a biologist... ;)

So, we know that on Earth there's life, and most of the methane we find comes from that life. So, obviously this means there's life on Mars, right? Of course not.

Each of the geologic origins of methane has problems on Mars, but extraordinary claims require extraordinary evidence, and I think a claim that methane on Mars proves the existence of life on Mars is extraordinary.

Would I be excited? Of course! But, there's no reason yet for President Obama to open a new cabinet position dedicated to intrasystem relations.

Here's another thing. That 350 year life of methane I quoted above is for photodisassociation due to photons (from the sun) striking methane and imparting enough energy to break the molecular bonds that hold the carbon atoms to the hydrogen atom...


So, why is the methane disappearing so much more quickly than 350 years? That's the geologic question of interest to me. It's certainly not life destroying the methane. I don't know what it is. Perhaps there's a regular/periodic absorption and release of methane from clathrates or other hydrates? I'm not sure. I would love to know. Someone with more geochemistry background than I will answer this in the next few years. Whatever it is, Mars is becoming more interesting the more we study it... :)

In the meantime, rest assured that as soon as I hear anything about better evidence for life on Mars, I'll post it here.

Monday, January 26, 2009

Why do bubbles form in water

How and why do bubbles form in water?

Son and dad played with some water the other day. First, we blew into a glass of tap water with a straw. A few bubbles formed on the surface and quickly popped.


Surface tension.

Surface tension arises from the fact that a liquid is held together by cohesion. The molecules that make up the liquid are attracted to all adjacent molecules. Well, at the surface of a liquid, there are fewer adjacent molecules. This means the cohesive forces between the remaining adjacent molecules are enhanced. This is surface tension.

When you blow air through the water, the air displaces the water and we see buoyancy in effect. We also see surface tension take action (yes, even at the bottom of the glass of water). This is because there is a surface at the air-liquid interface, and the water molecules on that surface have fewer adjacent molecules; the forces are enhanced. The tension on the bubble would like to be minimized and the best way to do that is to maximize the surface area over which the force is distributed, giving you a roughly spherical shape to the air bubble.

So, due to surface tension a roughly spherical bubble forms. Due to buoyancy, that bubble moves upward through the water.

Why does a bubble form at the surface? Surface tension again. When the air moves through the top "layer" of water, the water tries to stick together and a bubble forms.

It pops so quickly because the surface tension of water is so great that the water is pulled back to itself quickly. Here's a poor-quality, high-speed-camera video of a soap bubble popping. If you squint and watch closely, you'll see that the bubble does not just collapse. It pulls back upon itself, like a retracting dome. This is due to the surface tension keeping the water together.


So, after that, we put a little dish soap into the water and again blew air through. This caused a lot more bubbles to form on the surface, and most of those bubbles lasted a long time.


Surface tension.

Dish soap is a detergent, which decreases surface tension. Because the surface tension is decreased, the bubbles on the surface of the water are able to stay together longer; the cohesive forces are not as strong now that the detergent has been mixed in and the water+soap doesn't pull itself back together as quickly. If the surface tension was 0, the liquid would be a gas...

Detergents are used specifically because they lower the surface tension of water, which stops its beading behavior and allows it to soak into clothing or better dissolve junk on dirty dishes. Also, higher temperatures decrease water's surface tension, allowing better cleaning behavior.

Monday, January 19, 2009

Why do the glass blocks at the Desert Botanical Gardens float on the water?

So, we went to the Desert Botanical Gardens in the Phoenix. They had a show exhibiting the art of Dale Chihuly. He's an artist who works with glass (blowing, shaping, etc.).

We saw this there and Sonny boy asked how does the glass float on the water.

Buoyancy, baby!

Basically, buoyancy works because of two properties of fluids:

1) In a fluid (water in this instance), pressure increases with depth.
2) Pressure, at any given depth, is exerted in all directions.

So, what this means is that for any real object that is submersed, the bottom of the object will experience a greater, upward-directed force than the downward-directed force on the top of the object. This results, after all the forces exerted by the fluid are summed, in an upward-directed force being applied to the object by the fluid.

So, why don't all objects float in water?

Well, there are other forces acting besides the buoyancy forces. Specifically, gravity.

Let's imagine two balls of equal size, one made of cork and one made of lead, say 1 m^3.

The density of cork is about 240 kg/m^3, so a 1 m^3 ball of cork has a mass of 240 kg, and will weigh 2352 newtons.
The density of lead is 11340 kg/m^3, so a 1m^3 ball of lead has a mass of 11340 kg and will weigh 111132 newtons.
The density of water is 1000 kg/m^3, so a 1m^3 ball of water has a mass of 1000 kg and will weigh 9800 newtons.

So, if you displace 1 m^3 of water with 1 m^3 of cork, you've replaced 9800 newtons of water with 2352 newtons of cork. Since the 1m^3 of water was sitting where it was, happily not moving (let's assume the water has a constant temperature throughout its depth), we can successfully argue that the water was feeling a buoyant force of 9800 newtons. That is, the weight of the water (mass * gravity) was perfectly balanced by the buoyant force of the water.

Now, let's put that 1m^3 of cork (2352 newtons) in place of the water. Suddenly, the buoyant, upward force of 9800 newtons is being opposed by only 2352 newtons of downward force. The water pressure pushes the cork up.

When we put the 1m^3 of lead in place of the water, the 9800 newtons of buoyant, upward force is met with 111132 newtons of downward force. The lead sinks.

Specific gravity is the term used to tell you if something will sink or float in water, but all it really is is a ratio of densities. Something less dense than water will float on the water; the volume of the water displaced times the density of the water times the gravitational acceleration tells you the weight of the water displaced and that tells you the buoyant force that must be met for an object to sink in water (or any fluid, once you know the fluid's density).

Now, hang on! The density of glass is higher than the density of water!

Ah, but the glass sculptures are filled with air, decreasing their bulk density to less than water. This is the same way boats float. The volume of the water they displace is very high, allowing a high weight of water displaced, which means a large buoyant force opposing a (relatively) small downward force due to gravitational acceleration; the boat/water bulb floats.

[aside] When I say "weigh" above, I mean it will produce a force of xxx newtons at the Earth's surface, at the equator. People often mix-and-match the concept of weight and mass. An object's mass is a fundamental quantity that defines how much material there is. An object with mass has weight when a gravitational acceleration is applied to it. Weight (w) is defined by:
w = m*g
where w is the weight, m is the mass, and g is the gravitational acceleration. You'll notice that weight has the units of force. That's because it is a force.
You can't (correctly) interchange your weight in pounds with mass in kilograms. It's often done, especially in popular literature, and even by many conversion tables. It's incorrect. A kilogram of lead on the Earth is the same as a kilogram on the moon, but it'll weigh about 1/6 on the moon.

Friday, January 2, 2009

Why the leap second?

So, at the new year celebration with friends, one friend, Trish, asked, "why do we have a leap second?" I gave a vague answer, mostly because I'd just downed the last of my DW's rum and coke; she wasn't there due to an unfortunate sledding accident recently discussed, and she wanted me to drink some "yummy" drinks for her. I know she likes rum and coke, so I had one. Ugh (the rum was pretty good, but the coke was horrid, as are all colas). I had to be goaded into finishing it; I did as I do with most alcoholic drinks...I downed it so I didn't have to taste it. So, I was a little flighty.

Let me more properly answer this question here...

First, we need to discuss how we keep time. One of the many ancient ways of keeping time was based on the position of the sun, which is dependent on the rotation of the earth. Everyone "knows" that a day is one full rotation of the earth and that the the length of a day is 24 hours. This is, in fact, how a day is defined by the National Institute of Standards: A day is exactly 86,400 seconds long (60 seconds/minute * 60 minutes/hour *24 hours/day).

A second is now thusly defined: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Interestingly, even this seemingly super-precise definition isn't quite good enough. According to Einstein's theory of general relativity, the length of time a second takes (huh?) depends on how far into a gravity well the clock is. So, a clock in Earth orbit will keep time at a slightly different rate from one at sea level (the orbital clock will, in about 10,000 years, gain a second on the sea level clock; relativity has been tested by GPS satellites, which need very precise time). So, whenever we talk of counting seconds, we are speaking of an ideal atomic clock at the geoid (just follow the link; there isn't enough space to explain everything).

So, we now have a day that is exactly 24 hours long. However, the earth itself doesn't care how we define a "day". Therefore, for science purposes, we define a "sidereal day" based on the position of the "fixed" stars. That is, we assume the earth's position does not change relative to very distant stars (obviously this is incorrect, but is pretty good). That is, there is rotation of the Earth, but not translation. We define the sidereal day as one full rotation of the earth such that the position of a star, say Alpha Centuari, is in the exact same position as it was the day before as viewed from the same point on the earth. This takes 23 hours, 56 minutes, and 4.090530833 seconds. For now.

Clearly our two precise definitions of time do not agree. Nor is the word "day" in every-day usage self-consistent. So?

Well, it gets even worse. Everyone "knows" that the length of a year is 365 days, except leap years, which are 366 days long. Also everyone knows that a year isn't exactly 365 days, which is why we need leap years (because otherwise our calendars would be wrong relative to our expectations). The amount of time it takes for the earth to complete one orbit around the sun is actually about 365.242 days. You'll notice that this is NOT 365.25 days, which is what you would expect given that a leap year happens every four years. Which is why a leap year does NOT happen every four years. If that were the case, three extra days would be added every 400 years. Therefore, any century year (1800, 1900, 2000, 2100, etc.) that is not divisible by 400 is not a leap year, even though it is divisible by 4. 2000 was a leap year, and 2400 will be a leap year. 1600 was the last leap year before 2000.

So, what does any of this have to do with leap seconds?

Well, it all points to the complication of using either observed, earthly phenomena (the position of the sun or stars in the sky) or precisely defined times (electron movement in a caesium atom) as a way to keep track of scientifically interesting stuff as well as day-to-day stuff with the same clocks. An atomic clock that uses the definition of the second as shown above will eventually tell you that it's noon when it has been dark for hours. That's obviously not useful.

And I haven't even spoken of the fact that the Earth's rotation is slowing due to tidal interactions with the moon, of the fact that Earth's axial tilt precesses with respect to the fixed stars. or...

Because the Earth's rotation rate is changing ever-so-slightly, we need a way to keep our clocks up-to-date. We wouldn't want to have the situation of having to suddenly add a 1/2 day to our calendars because we let our precise clocks get so out-of-sync with our perceived time; this would be much more distressing to the world than adding a second every few years. So, since 1972, we've been adding a leap second (so far, they've all been positive, but it's possible that at some time, we'll end up losing a second from our year. We'll probably get many protests when that happens). Since 1972, we've added 24 leap seconds to our clocks, sometimes in December, sometimes in June.

Note that leap seconds and leap years only share the name "leap" and the fact that our planet does not behave precisely according to our time keeping devices. A leap second can be added in any year whether it's a leap year or not, leap seconds are added irregularly while leap years are very predictable, and they account for different phenomena.