A 2006 photo from the Cassini spacecraft shows the mighty planet Saturn, and if you look very closely between its rings you can see a faint pinprick of light. That tiny dot is Earth bustling with life as we know it, and we have marked it on the image.  The image is the second ever taken of our world from deep space.  The first, captured by the Voyager spacecraft in 1990, stunned many people, including the famous astronomer Carl Sagan who called our seemingly miniscule planet a "pale blue dot" and "the only home we've ever known.”  Last September, a portion of Cassini’s picture showing Earth was unveiled to an auditorium full of scientists attending the third Pale Blue Dot workshop at the Adler Planetarium in Chicago. The workshop brought together scientists from across the country and beyond to talk about how to find life elsewhere in our universe, a central theme in the interdisciplinary field of astrobiology.  When the workshop participants were presented with the picture, they spontaneously began to clap.  One of their fundamental goals is to capture a portrait like Cassini's showing another "pale blue dot" like Earth in a planetary system beyond our own.

If scientists do ever acquire such a photo, how will they figure out whether anybody is home?  Well, a photo of a light-blue extra-solar planet – might indicate the planet has an atmosphere, a tantalizing sign that it could support life.  In our solar system, we are the only small light-blue planet, and Earth appears blue because its atmosphere scatters blue light around, literally filling the sky with the colour blue.  But colours are only clues. A planet with an atmosphere might be blue, or not.  Mars has an atmosphere, albeit a thin one, and it's red.  The blue light in the Martian skies is soaked up by iron-containing molecules on the surface that radiate red.  Ultimately, astrobiologists will need additional data to determine whether an extra-solar planet is habitable, and if so, whether it harbours life.  Future planet-finding missions like NASA's Terrestrial Planet Finder are expected to return both valuable "spectral" data and imagery of planets looking small enough to fit on a pin. Spectra are surveys of an object's rainbow-like array of different wavelengths of light. They reveal the presence of specific molecules.

To determine if a planet is liveable, scientists will look for carbon dioxide and water vapour, signposts that a planet has an atmosphere and oceans, respectively.  Atmospheres not only provide air to breathe, but also act like blankets to keep a planet warm and help buffer potential residents from damaging ultraviolet and cosmic rays.  Oceans help regulate a planet's temperature and provide liquid water, an essential ingredient for life on Earth.  Other molecules enveloping a planet, such as oxygen, ozone and methane, can suggest that life itself has taken root.  On Earth, oxygen is "breathed out" by plants, and methane by micro-creatures living in swamps and animals.  These chemicals don't survive for long on their own, so if they are hanging around an extra-solar planet, then something must be pumping the stuff out.  That something could be life, but this isn't always the case.  Saturn's moon's Titan is shrouded in an atmosphere containing lots of methane not produced by life.

Scientists say that oxygen is a more reliable sign of life than methane, but if they found large quantities of both, they'd be more convinced.  Finding two of these molecules together would be much better than one - the more, the better.  For example, if we found carbon dioxide, oxygen and water vapour, in addition to methane, then we'd be pretty convinced that we were looking at an environment like our own.  But not all molecules are a good sign.  For example, abundant amounts of sulphur dioxide suggest a dead, dry planet.  This chemical would dissolve into a planet's ocean if it had one, so its presence means it is unlikely there's much water around.  Venus is one such parched planet, with a thick atmosphere containing sulphur and carbon dioxide.  When it comes to exotic planets, scientists say that we should be prepared for the unexpected.  Planets are potentially as diverse as people, and in the last few years, they've found evidence that extra-solar planets may not be anything like those in our solar system.

Taking in the rings in their entirety was the focus of this particular imaging sequence. Therefore, the camera exposure times were just right to capture the dark-side of its rings, but longer than that required to properly expose the globe of sunlit Saturn. Consequently, the sunlit half of the planet is overexposed.  Between the blinding light of day and the dark of night, there is a strip of twilight on the globe where colourful details in the atmosphere can be seen. Bright clouds dot the bluish-grey northern polar region here. In the south, the planet's night side glows golden in reflected light from the rings' sunlit face.  Saturn's shadow stretches completely across the rings in this view, taken on 19th Jan. 2007, in contrast to what Cassini saw when it arrived in 2004.   This view is a mosaic of 36 images -- that is, 12 separate sets of red, green and blue images -- taken over the course of about 2.5 hours, as Cassini scanned across the entire main ring system.  It looks toward the unlit side of the rings from about 40 degrees above the ring plane.  The images in this natural-colour view were obtained with the Cassini spacecraft wide-angle camera at a distance of approximately 1.23 million kilometers (764,000 miles) from Saturn. Image scale is 70 kilometers (44 miles) per pixel.  There is a full sized image (1.6Mb) here.

This view looks toward the unlit side of the rings from about 9 degrees above the ring plane.  The rings glow feebly in the scattered light that filters through them and the darkened rings seem to nearly touch their shadowy reverse images on the planet below.  The image was taken in visible light with the Cassini spacecraft wide-angle camera on Feb. 4, 2007, at a distance of approximately 1.2 million kilometers (800,000 miles) from Saturn. Image scale is 75 kilometers (47 miles) per pixel.

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