astr 330: the solar system lecture 5: planetary astronomy dr conor nixon fall 2006

ASTR 330: The Solar System

Lecture 5:

Planetary Astronomy

Dr Conor Nixon Fall 2006


Studying Matter From A Distance


• Astronomers use almost exclusively the technique of remote sensing in their investigations.

• Remote sensing means studying the radiation of distant objects at a distance, as opposed to in situ investigations, where the object is directly sampled.

• Examples of remote sensing: Earth-based telescopes Spacecraft orbiting the Earth or other planets

• Examples of in situ science: Mars Viking lander or Sojourner rover Galileo atmospheric probe.


Spectroscopy of Planets


• We have discussed, in Lecture 3, the technique of splitting light into its component colors, called spectroscopy.

• Planets are cooler than the Sun, and cooler objects are redder. A metal poker when heated goes from red to yellow to white as it is heated.

• Planets (like human beings) are cool enough not to emit visible light at all, instead they radiate at longer wavelengths, in the infrared part of the EM spectrum.


Planetary Emission


• This figure shows the energy emission peaks from objects at 5780 K (the Sun), and 255 K, representative of a typical planetary temperature. Whereas the Sun emits most energy in the visible, the planets emit more energy in the infrared.

(figure credit: DC Griersmith, CNES)

• Therefore, when we look at the spectrum of planets, the most interesting information is often in the infrared.

• But, how easy is it to see planets at different wavelengths from the Earth? What problems might there be?


Spectral Windows


• The Earth’s atmosphere allows visible radiation through.

• As we venture into the infrared however, we find that the atmosphere does not transmit (allow through) all wavelengths from outside.

• Water vapor and carbon dioxide gas are responsible for absorbing in certain infrared spectral ranges. Therefore, we cannot see planetary radiation in these parts of the spectrum.

• Between the absorption bands however, we can see outside.

• These absorption-free parts of the spectrum are known as atmospheric or spectral windows.


Transmission Of the Earth’s Atmosphere


• Note that atmosphere is also opaque in the ultraviolet, and shorter wavelengths. If we want to see X-rays from the solar corona, we have to go into space!

• Question: would the Hubble Space Telescope be concerned by spectral windows?


Other Spectral Windows


• Ultraviolet absorption is due to ozone, O3 molecules in the stratosphere.

• Why do we worry about a hole in the ozone layer?

• The atmosphere is also transparent at wavelengths from 1 mm to about 30 cm: the spectral range of microwaves, radar, television, and FM radio.

• What are the implications for:

• Radio Astronomy (do we need to go into orbit?)

• Radar sensing of the Moon?

• What information aliens may have about our civilization?


Planets In Visible Light


• If planets radiate in the infrared, can we see planets at all in the visible light part of the spectrum?

• Sure! But why?

• What problems does this cause for spectroscopy of planets in visible light? (hint: what sources of lines are there?)

• We define the albedo of a planet as the amount of sunlight reflected back to space. For example, some albedos are:

• Moon = 0.11• Venus = 0.75• Enceladus = almost 1.00

• What happens to sunlight which is not reflected?


Different Albedos


The MoonVenus

Enceladus

Photos: The Nine Planets, LPL Arizona


Planetary Infrared Spectra


• A lot of information about the planets is contained in the infrared spectrum. Seen here are example spectra of Mars, Earth and Titan.

Figures: MGS TES Team; Greg de Boer, UCSU.


Information From IR Spectra


• The emission and absorption lines we see in a planetary IR spectrum correspond to energy transitions in gaseous molecules.

• By the height of the lines, we can gather information about:

• how much of the gas there is, and

• how hot it is.

• The width of the lines contains information about what pressure the gas species is at.

• By comparing a number of different lines from the various gases, we can gain information about the whole physical and chemical state of the atmosphere, even if there are cloudsclouds or not!


How Does a Telescope Work?


• A telescope is essentially just a means for collecting and focusing light, similar to the human eye, but many times more powerful.

• There are 2 main types: reflecting, which uses mirrors to collect and focus, and refracting, which uses lenses, like the eye. There are many sub-types of reflector, like the popular Schmidt-Cassegrain (below left).

Pictures: [email protected]

• Most large astronomical telescopes are reflectors, which are lighter and suffer fewer problems at very large sizes.


Development of Telescopes


• Remote sensing for 400 years was carried out on the Earth using successively larger diameter telescopes:

• 100 in. (2.5 m) Hooker, Mount Wilson. Largest 1917-1948. (photo: Mount Wilson)

• 200 in. (5 m) Hale, Mount Palomar. Largest 1948-1974. (photo: Alain Maury)

• BTA-6 (6 m), Mount Pashtoukov. Largest 1974-1993. (photo: SAO-RAS)

• Keck I & II (9.8 m), Mauna Kea. Largest 1993-(photo: WM Keck Observatory)


Does size matter?


• Why build larger and larger telescopes: what advantages are there?

1. More light collection (proportional to D2)

2. Spatial resolution (proportion to D).

• One other way to improve (2) is often to go up close, in a spacecraft!


Siting a Telescope


• altitude• humidity• proximity to light pollution• latitude

• Good sites:

• Mauna Kea• Andes• Antarctic• Space! Picture: Richard Wainscot/IfA

• What considerations might be important when planning where to site a telescope?


The Future Of Telescopes


• Recent developments which are likely to become increasingly important in future:

• segmented mirror design: using lots of small, light mirrors moving together instead of a single huge mirror: easier to build and maintain.

• adaptive optics/image stablization: this means using special compensation techniques to remove the ‘twinkle’ due to the Earth’s atmosphere.

• multiple mirror telescopes, visible interferometry: this means using multiple telescopes linked together to provide the spatial resolution, although not the light-collecting power, of a single huge telescope.

• space telescopes: above the atmosphere has many advantages.


Optical Interferometry


• At the W.M Keck Observatory, efforts are underway to link the two great 10 m telescopes into a single effective aperture 85 m across!

Picture: W.M. Keck Observatory


Quiz - Summary


1. Distinguish between remote sensing and in situ sensing, and give examples.

2. What is meant by an atmospheric spectral window?

3. What information can we tell about a planet from infrared spectral lines?

4. What are the two main types of telescopes, and name some recent advances in telescope technology.

astr 330: the solar system lecture 5: planetary astronomy dr conor nixon fall 2006

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