Planetary+Temperatures


 * Quest 1: What makes Earth habitable?**

Based on our “What is Life?” discussion we now need to investigate what characteristics of Earth make it a habitat for life. Once again, go to the NASA Space wiki and in the section “Quest 1: What makes Earth habitable?” list qualities that Earth the planet provides that permits life to exist here. When you finish come back here to begin investigating some of those important characteristics.

Energy for warmth Need a source that is long lasting and stable – life has been here continuously at least 3.5 billion years. Evidence for liquid water across all that time, so what does that say about temperature?

Answer: It must not have significantly fluctuated beyond the range that water is liquid – that is between 0 and 100 degrees C – where it freezes and evaporates.

The energy source for Earth and presumably all the planets in our solar system and others is a central star, the Sun.


 * What determines our T on Earth?**

Distance from the Sun.

Energy from any heat source falls off with distance – lets do a virtual experiment to discover how.

__Simulation:__ Point source Sun has a cone of rays that fall on a flat surface that can be positioned at different distances from the Sun. Make the cone be square shaped rather than round so it is easier to measure change in area. Target surface can have same size squares so students just count the squares illuminated.

At a distance of 1 unit measure area of illuminated cone and temperature. (Make area equal to 1 square km at a distance of 1 unit and intensity of 100% temperature of 120°C)

What is the area covered by the light beam and how intense is it?

Now move the target surface to a distance of 2 units, and then 4 and then 8 and repeat the measurements of illuminated area and intensity. Plot those on a graph.

Make online graph that student can enter distance, area and intensity. How get students to “discover” that light intensity falls off as inverse square? How does the number of squares of light change with distance? (at distance of 1 unit - 1 sq of light, at 2 units, 4 squares, at 3, 9 squares) What can say about the intensity of the light? (same amount of illumination, but it covers 1, 2, 4, 9 times as much area so falls off by same amount, 1/1, 1/2, 1/4, 1/9 etc)

Notes: Mercury’s noon T is 407°C (765°F) and just before sunrise it is -180°C (-300°F) Pluto: hi T -240°C (-400°F)

__Physical Experiment.__ Students use flashlight to conduct inverse sq experiment – can measure changing areas but not intensity.

Conclusion: Sun’s heating of an object decreases quickly – as the sq of its distance from the Sun. That is why outer planets very cold – they only get 10% or even 1/1000th (0.1%) of energy from Sun as Earth does. For example, Earth receives 1400 watt/m^2 vs 1.5 watt/m^2 for Neptune at 30 AU.

How far is each from the Sun? Same distance, but are their temps equal? No. Earth during the day the temp is about 20°C and Moon is about 150! Why the huge difference? Students put ideas on wiki.
 * So consider Earth and Moon.**

__What is different about Earth and Moon?__ Earth is bigger – should that make it less hot? No, since we talking about sunlight/unit area of surface. Earth has atmosphere, Moon doesn’t. Could that make a difference? Does atmosphere affect T on Earth? Yes – clouds make it cooler by providing shade. And another way: reflecting light that otherwise would heat surface.

This called reflectivity or albedo. You already know that different surfaces interact differently with sunlight. Walk barefoot in summer. Black asphalt on road hotter than white sidewalk because dark colors absorb heat and bright reflect more, absorb less.



Moon is made up of dark rocks – average albedo 12% - reflects 12% of sunlight, 88% gets absorbed to warm it up! Earth has clouds, snow and ice caps that all strongly reflect sunlight. But these vary over the year, so Earth albedo changes, but average is 30% - so 30% of sunlight get reflected back to space - Earth gets cooled by material that makes it up.

__Any other way?__ Yes – air moves. Satellite images show movement of air masses carrying heat around parts of globe – both longitudinally within a lat zone and n-S across zones. Atmosphere mixes heat, tending to equalize. On Moon, night time side is -180 check (and day side 150C!), but not on Earth – typically just tens of degrees cooler, because of atmosphere effectively redistributes heat.

__Simulation: Affect of Atmosphere__

Two spheres – Earth and Moon – use space images. Earth smaller and has clouds. Moon bare rocks – exposed to heat. Have gauge that shows T on day side and night side. Student can write down Ts – why – what do with data?? Add atmosphere to Moon and see how it affects T [where get accurate model data?] T extremes will modulate, but still not as small as Earth. Why?

Students guess / make suggestions.

How long does a piece of Earth face the sun – about 12 hrs, followed by 12 hrs of night – no warming. And for Moon? 14 days in sunlight, 14 days of night. Slow rotation means that even with atmosphere there would be restricted transport of heat – in fact, atmo get very hot on sun side and maybe boil off, and on night side maybe condense and freeze.

__Simulation: Importance of Rotation__ Need sim Moon able to rotate at different speeds with day and night Ts being measureable. How find relation of rotation rate to T??

Yes, greenhouse warming. Earth received sunlight that warms surface, but surface doesn’t get hotter and hotter every day so it reradiations into atmosphere heat in form of radiation called infrared – long wavelengths. This passes through clouds and most of atmosphere and escapes to space. But some gases reflect IR, bouncing it back down to surface, causing extra heating. Greenhouse warming increases Earth’s T by 33°C, without it Earth would be permanently covered with ice – snowball earth. Greenhouse gases are CO2, methane, even water vapor.
 * Finally what of composition of atmosphere? Does it affect T?**

Moon has no greenhouse warming so at night its heat just radiated away getting colder and colder.

On Mars, much thinner atmos but 95% CO2 so still some greenhouse warming – abut 7° - still mostly below freezing. Brr.

On Venus, 100 times as much atmo as Earth annd x% CO2 so 450 degrees of greenhouse warming – runaway greenhouse!

__Sim or just state?__

Interesting fact: For 3 days after 9-11 attack all aviation canceled in US. For those days temperature average 1.8°C warmer than 3 days before and 3 days after when flights resumed. Why? Contrails – ice droplets forming on jet exhausts added to greenhouse effect trapping some outgoing radiation, warming Earth.

End up with students filling in a table of effects of different parameters influencing T (for example: how can he parameter vary to make a planet colder?)



And another variable. But we know on Earth that sometimes it is very hot – Aug has Ts in 90sF, but 6 month later might be 30s! Why? Seasons. What causes seasons? Titlt of Earth axis of rotation 23.5° for Earth. Parameters above predict average T, tilt causes seasonal extremes. We won't worry about that!

Let's evaluate each of our solar system's planets based on these factors.

Table of planets and factors - students enter + or - to choose how factor influences planet T (compared to Earth).

Notes: Atmosphere relates to how thick it is, from 0 for none, 1 for thin, 2 for Earth-like, 3 for thicker. Atmospheric Composition relates to whether there are greenhouse gases, water, methane, CO2.

How would you apply what you learned in our solar system to other stars? If you travel to another star and your science officer makes a few measurements for each planet your job will be to make smart guesses of a planet's average T and thus suitability for life.

Cautions: Other stars may be brighter or fainter than our sun so same distances would have different Ts – habitability zone would move in and out depending on star brightness.