ASTR 330: The Solar System Lecture 5: Planetary Astronomy Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Studying Matter From A Distance Dr Conor Nixon Fall 2006 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.

ASTR 330: The Solar System Spectroscopy of Planets Dr Conor Nixon Fall 2006 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.

ASTR 330: The Solar System Planetary Emission Dr Conor Nixon Fall 2006 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?

ASTR 330: The Solar System Spectral Windows Dr Conor Nixon Fall 2006 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.

ASTR 330: The Solar System Transmission Of the Earth’s Atmosphere Dr Conor Nixon Fall 2006 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?

ASTR 330: The Solar System Other Spectral Windows Dr Conor Nixon Fall 2006 Ultraviolet absorption is due to ozone, O 3 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?

ASTR 330: The Solar System Planets In Visible Light Dr Conor Nixon Fall 2006 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?

ASTR 330: The Solar System Different Albedos Dr Conor Nixon Fall 2006 The Moon Venus Enceladus Photos: The Nine Planets, LPL Arizona

ASTR 330: The Solar System Planetary Infrared Spectra Dr Conor Nixon Fall 2006 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.

ASTR 330: The Solar System Information From IR Spectra Dr Conor Nixon Fall 2006 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. clouds 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 clouds or not!

ASTR 330: The Solar System How Does a Telescope Work? Dr Conor Nixon Fall 2006 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: Most large astronomical telescopes are reflectors, which are lighter and suffer fewer problems at very large sizes.

ASTR 330: The Solar System Development of Telescopes Dr Conor Nixon Fall 2006 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 (photo: Mount Wilson) 200 in. (5 m) Hale, Mount Palomar. Largest (photo: Alain Maury) BTA-6 (6 m), Mount Pashtoukov. Largest (photo: SAO-RAS) Keck I & II (9.8 m), Mauna Kea. Largest (photo: WM Keck Observatory)

ASTR 330: The Solar System Does size matter? Dr Conor Nixon Fall 2006 Why build larger and larger telescopes: what advantages are there? 1. More light collection (proportional to D 2 ) 2. Spatial resolution (proportion to D). One other way to improve (2) is often to go up close, in a spacecraft!

ASTR 330: The Solar System Siting a Telescope Dr Conor Nixon Fall 2006 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?

ASTR 330: The Solar System The Future Of Telescopes Dr Conor Nixon Fall 2006 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.

ASTR 330: The Solar System Optical Interferometry Dr Conor Nixon Fall 2006 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

ASTR 330: The Solar System Space Telescopes Dr Conor Nixon Spring 2004 Hubble Space Telescope (HST, visible and near-IR): a 2.4 m diameter reflecting telescope launched into LEO in 1990, the Hubble suffered some initial technical problems. Since a spectacular repair mission in 1993, the Hubble has gone on to produce some of the finest astronomical pictures of all time. Compton (gamma ray): launched in … Chandra (X-ray): launched in … SIRTF (Infrared): NGST/JWST (next generation visible, near-IR):

ASTR 330: The Solar System Quiz - Summary Dr Conor Nixon Fall 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.