Earth Without Life?

On the night of 3 July 2003, the Mars Express spacecraft was pointed backwards to obtain a view of the Earth-Moon system from a distance of 8 million kilometers [4.8 million miles] while on its way to Mars.

The Earth-Moon banner image at top is the first picture of planetary objects obtained by the Mars Express’s High Resolution Stereo Camera (HRSC). Although the spatial resolution is low at this great distance, the picture gives a good indication of what to expect from Mars Express in its orbit around Mars. At only 250-300 kilometrers above Mars, the camera will obtain very high-resolution images, in brilliant colour and impressive 3D of most of the Martian surface, at resolutions of up to 2 meters [about the height of a person]. The image was built by combining a super resolution black-and-white snap-shot image of the Earth and the Moon taken by the HRSC with color information obtained by the blue, green, and red sensors of the instrument.

Earth in chemical elements that are biologically significant. Charles Darwin thought that life could have originated "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, electricity, etc. present." Researchers have reflected upon Darwin’s sunlit shallow pool ever since.
Credit: ESA/Mars Express


How the Earth May Look Lifeless

During a series of instrument tests, the OMEGA spectrometer on board Mars Express acquired ‘spectra’ of the Earth and the Moon, in visible and near-infrared light. This particular spectrum corresponds to the entire Earth’s illuminated crescent, dominated by the Pacific Ocean, and indicates the molecular composition of the atmosphere, the ocean, and some continents.

As the peaks in the image right indicate, water (H2O) and carbon dioxide (CO2) dominate. Molecular oxygen (O2) is also identified, as well as ozone (O3), methane (CH4) and several other minor constituents. During the observations, the Earth rotated so as to offer a varying observed surface and atmospheric composition.

These Earth observations by OMEGA have several unique features. In fact, OMEGA provided a global view of the Earth’s disc from a high-phase angle, contrary to low-orbit observations by previous space missions. Such global disc spectra are useful not only for observations at Mars, but also to prepare future observations of Earth-like planets, such as for the Darwin mission.

The evocative phrase describing the Earth as a ‘pale blue dot’ was coined by Carl Sagan after seeing our planet as a single pixel. The view was taken from the departing Voyager spacecraft. The entire earth could be encompassed as a flicker of light. The first image of Earth ever taken from another planet that actually shows our home as a planetary disk was captured by the Mars Orbital Camera on May 8th.

Comparison of Mars, Venus and Earth in water bands, showing the clear presence of water on Earth uniquely
Credit: NASA Workshop, Pale Blue Dot

As Anne Druyan , Sagan’s widow, later described this perspective image to Astrobiology Magazine, that Voyager shot made "us look at this tiny planet, at the pale blue dot, and to see it in its real context, in its actual circumstances, in its true tininess. I don’t know anyone who’s able to really see that one-pixel Earth and not feel like they want to protect the Earth; that we have much more in common with each other than we’re likely to have with anyone anywhere else."

Black Dots, Lifeless Oceans?

The Earth, as seen from a distant vantage point, has long captivated the imagination of planet finders. And in 1993, a team of researchers inspired by Carl Sagan, used an Earth fly-by of the Galileo spacecraft on its way to Jupiter to catch a glimpse of how the Earth might appear from afar. For astrobiologists, Sagan’s results were surprising.

Rather than seeing the Earth as an obvious candidate for life, the Galileo pictures gave surprisingly few clues of the biological potential of our own planet.

From afar, how Galileo missed the obvious signs of terrestrial life as we would have expected to see them, was at first disconcerting to the scientific community, because future missions aim to observe more distant extrasolar planets and detect what would be visible in the spectra–the ‘pale blue dot’ scenario.

One answer may lie in the fact that the spacecraft made its observations while still quite close to the Earth.

Earth and Moon
Image of the Earth and Moon taken by Galileo spacecraft.
Credit: NASA

"The spectrograph was designed to look at small areas of Jupiter, so the field of view of the spectrograph was quite small," said Nick Woolf of Arizona, in earlier discussions with the Astrobiology Magazine.

"Also, since the surface brightness of Jupiter [the Gaileo’s intended visual target] is far less than the Earth, the spectrograph detectors saturated except when the spectrograph was pointed at the darkest area of Earth – a cloud-free section of sea," Woolf noted. The cloud-free sea is considered very dark relative to the dominance of bright clouds in a global picture of Earth. Thus it should come as no surprise that Galileo was successful in only imaging a relatively dark and lifeless planet, mainly because its design was not intended to look at Earth, but to probe Jupiter instead.

Recipe from Afar

A spectroscope that will detect infrared or visible light mainly is focused on four gases that are found in Earth’s atmosphere and linked to life:

  • Water vapor A baseline sign, indicating the presence of liquid water, a requirement of known life.
  • Carbon dioxide Can be created by biological and non-biological processes. Because it is necessary for photosynthesis, it would indicate the possible presence of green plants.
  • Methane Considered suggestive of life, it also can be made both by biological and non-biological processes.
  • Molecular oxygen (O2) – or its proxy, ozone (O3). The most reliable indicator of the presence of life, but still not conclusive.

Unless molecular oxygen in the atmosphere is constantly replenished by photosynthesis, it is quickly consumed in chemical reactions, in the atmosphere, on land and in seawater. So the presence of a large amount of oxygen in an extrasolar planet’s atmosphere would be a sign that it might host an ecosystem like present-day Earth’s.

An additional oxygen-related biosignature is the possibility of detecting green plants that make oxygen. Chlorophyll reflects near-infrared light very strongly, a phenomenon known as the "red edge" because the light is just beyond the range of colors human eyes can see. (If humans could see the red edge, plants would look red instead of green.) Near-infrared cameras would have no trouble picking up this distinctive signal.

The doppler effect
Earth as seen by the departing Voyager spacecraft: a tiny, pale blue dot.
Credit: NASA

Although methane is often biogenic, detecting it on a distant world would not automatically indicate the presence of life. Jupiter and Saturn, for example, have traces of it. Most space detectors probably could not see methane at Earth’s present concentration, 1.6 parts per million (ppm), since its spectroscopic lines overlap those of water. However, methane levels around 1,000 ppm may have occurred on Earth between 2.3 and 3 billion years ago, produced as a waste byproduct by primitive microorganisms called methanogens. This strong a methane signal would probably be visible to a biosignature detector, although methane is largely considered a suggestive but not convincing biosignature. Finding oxygen along with methane might constitute the most convincing biosignature.

What’s Next

The discovery of about 100 extrasolar planets over the past decade has placed a momentous task on the scientific agenda: finding planets that could harbor life. Most of the newly discovered planets are gas giants that orbit close to their stars. They’re broiling hot, and probably dead. The job of Terrestrial Planet Finder (TPF) is to find "terrestrial" (Earthlike) planets, and then to scan them for biosignatures – chemical signs of life.

For most planet finders, the real challenge is to identify faint planets in the glare of their much brighter parent stars. To overcome the distortion of how our own atmosphere may further obscure this detection, both large land-based telescopes and space missions will likely combine in the future to complete the picture.

Both NASA and the European Space Agency (ESA) propose space missions to look for Earth-like planets in the infrared. NASA is developing the Terrestrial Planet Finder (TPF) project, part of the Jet Propulsion Laboratory Navigator Program, and ESA is developing its DARWIN project. The European Southern Observatory is exploring the possibility of ground-based searches using the future Overwhelmingly Large Telescope (OWL) project.

Related Web Pages

Terrafirma Now
The Earth as Seen by Galileo
Crescent Moon and EarthShine–courtesy Chris Cook, astrophotography
Lakeside Landing
Mars Exploration Rovers
Jet Propulsion Laboratory
Mars Exploration Program