ASTR 330: The Solar System
Lecture 5:
Planetary
Astronomy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Transmission Of the Earth’s Atmosphere
• Question: would the Hubble Space Telescope be concerned by spectral
windows?
• 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!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Different Albedos
The Moon
Venus
Enceladus
Photos: The Nine Planets, LPL Arizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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 clouds or not!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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).
• Most large astronomical telescopes are reflectors, which are lighter
and suffer fewer problems at very large sizes.
Pictures: angolano@geocities.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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 19741993. (photo: SAO-RAS)
• Keck I & II (9.8 m), Mauna
Kea. Largest 1993(photo: WM Keck Observatory)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Siting a Telescope
• What considerations might be important when planning
where to site a telescope?
• altitude
• humidity
• proximity to light pollution
• latitude
• Good sites:
• Mauna Kea
• Andes
• Antarctic
• Space!
Picture: Richard Wainscot/IfA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Dr Conor Nixon Fall 2006