Biosignatures Part II: What is in the Pale Blue Dot—and Why?

Journey of NASA probes Source:

Journey of NASA probes

In Part I, I introduced terms, and an infrared spectrum that characterizes our planet from afar. The picture of the “Pale Blue Dot” is us—and it was taken in 1994 from Voyager 1. The Voyager 1 probe, at that point, was entering the Kuiper Belt; the probe had finished its primary tasks. The “little probe that could” currently is at the outer edges of the Solar System and (with its twin—Voyager II) is an inter-Solar System representative of who we were in 1977 (the date of launch). The probes continue to collect data and send the data back to Earth for analysis. You can follow the periodic updates (reference and link at end of post)**.

Firstly, the post will inform you what is important in the posted spectrum; and at the conclusion, tell a little of what is in store for the next post.

The spectrum from the previous post:

Biosignature spectra of Earth-like planetAuthors: Lisa Kaltenegger & Wesley A. Traub Astrophys. J. 698:519-527, 2009 Available on-line

Biosignature spectra of Earth-like planet
Authors: Lisa Kaltenegger & Wesley A. Traub
Astrophys. J. 698:519-527, 2009
Available on-line

A spectrum is a type of “fingerprint” that characterizes a molecule or atom. Infrared spectroscopy utilizes the manner of how the atoms of a molecule are interconnected—or how the “connections” between individual atoms can be characterized. Let me explain with a water molecule—

single water molecule

single water molecule

The water is correctly represented as having two hydrogen atoms and one oxygen atom—the hydrogen atoms are represented as the two white balls which are connected to the red oxygen. In the spectrum, the labeled “signatures” (or peaks) correspond to molecular “behavior” in the infrared. From left-to-right, the distinguishing peaks “tell” how the water molecule “moves” (or the manner in which connections vibrate). Thus, glancing at the top spectrum (in black)—there are three distinguishable labels for the water molecule. Each label corresponds to a type of vibration that the molecule undergoes—from highly energetic (far left) to weakly energetic (far right). Further investigation of the diagram reveals numerical labels—from the left-to-right (or high-energy to lower-energy) the numbers reveal the so-called wavelength at the excitation “signature.” The notion of low numbers to higher numbers is counter-intuitive when one tries to envision the energetics of each vibrational peak—but when speaking of a “frequency” (as scientists normally would), the notion is intuitive.

Further inspection of all the peaks in the spectrum reveals that there are a number of molecules—each of which can be discerned under “ideal” conditions. However, as might be inferred, this ideal condition occurs only under highly controlled circumstances. When the “earth-like” planet passed in front of a star (similar to our sun), we may/may not record similar data. One major factor is the opacity of the atmosphere through which the star-light passes. Accordingly, data recorded of the transiting planet (the earth-like planet passing in front of the star) may not “catch” every different molecule in the atmosphere. The spectrum cannot be recorded below a certain height because it is opaque.

In the next post I will speak of current efforts to identify signatures or “peaks” in current extra-solar planets.


For exhaustive information on the Voyager probes:


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