Earthshine scientific basis

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Features of Earthshine

The Earthshine spectrum will display a number of features, relating both to the atmosphere and surface of Earth, and interference features from the Moon. In order to find the light reflected from Earth, the interference from the Moon, as found by investigating the spectrum of light reflected from the sunlit side of the Moon, will need to be cancelled. In addition to this, the effect of Rayleigh scattering means that there will be a greater intensity of light with low wavelengths than of high wavelengths, and this effect will also need to be accounted for. Once this has been done, features of the Earth can usefully be observed.

Fluctuations in Earthshine results

Firstly, time will have a noticeable effect on the concentration of a number of gases in the atmosphere, and so on the Earthshine spectrum. The concentrations of oxygen and carbon dioxide fluctuate over the course of a year, with oxygen at a peak and carbon dioxide at a trough during October, and the reverse true around March. This fluctuation will be able to be observed using an Earthshine satellite, as the same point on the earth faces the Moon one fewer time than Earth rotates in a full lunar cycle, and so around 26.2 times. Since Earthshine can only be taken when the Moon is not sunlit, and half or more of the Moon is not sunlit for half of the lunar cycle, readings will be able to be taken of the same topography around 13.2 times every full lunar orbit of 27.3 days. This allows around 176 readings to be taken per year, which is enough to allow yearly fluctuations to be investigated thoroughly.

Secondly, terrain, dependent on topography, will change the Earthlight spectrum. At a basic level, some terrains, such as snow, have a far higher albedo than others, like sea, and generally land-based topographies will reflect more light than oceans. In addition to this, vegetation has a distinct absorption pattern, with a sudden change in the reflection of light beyond a wavelength of around 700nm. The rotation of the Earth relative to the Moon will cause the topography observed to fluctuate daily, and the angled axis of the Earth's rotation will cause fluctuations of a greater period length. These must be accounted for in developing a model of Earthshine, and then in later analysis.

Features of Life Observable in Earthshine

The presence of life on Earth is indicated by a number of different features in the light absorbed by Earth's surface and atmosphere, and so will also be shown by the reverse of those features in the light reflected from Earth, as can be observed in Earthshine. These features are based on the absorption spectra of different elements, different compounds, and different surface terrains, all of which reflect light in unique spectra. Identifying these spectra in Earthlight allows us to better understand the externally perceivable features of life on Earth, and so to understand general features of life which can be percieved from near Earth. This proposal outlines some general features, as listed below.

Firstly, oxygen is an important indicator of the presence of life. Without life, and especially photosynthesising plants and algae, oxygen would not be produced in anything but very low quantities. That which is in the atmosphere would be removed through reactions with various minerals to form stable oxide compounds, and as it would not be replaced through photosynthesis, the overall concentration of oxygen in the atmosphere would decrease from the current amount of approximately 21% of the atmosphere to less than 1% in around 10 million years. Therefore, the existence of a high concentration of oxygen, or another reactive gas, in the atmosphere of an exoplanet would be an strong indication of the existence of life on that planet. In the spectrum of light found in Earthshine, the presence of oxygen will be shown by dips in the intensity of light at the wavelengths absorbed by oxygen. The nature of these dips as observed in Earthshine can then be used to identify oxygen in the light reflected from exoplanets.

Secondly, water is a vital compound in the formation of life on Earth, and is likely to be important in the development of life on other planets. It's actions as a solvent allow complex solutions necessary for life, and it is necessary in the formation of enzymes. It is a liquid at a large range of temperatures, moderating climate to allow the the development of life, and at a high enough temperature so that biological reactions can occur quickly. Because of its importance, planets which lack liquid water are unlikely to sustain life, and so the Earthshine spectrum around the absorption spectra of water will be a useful reference for the spectra from exoplanets in order to determine whether they are capable of sustaining life.

Finally, life requires an external energy source to reduce entropy locally, creating self-regulating systems far from thermodynamic equilibrium. The most likely source for this energy is light from a nearby star, in the same way as, on Earth, most organisms rely directly or indirectly on the Sun's light to allow photosynthesis. Any organism which takes in light will absorb high quantities of light at certain frequencies, and in order to prevent overheating is likely to reflect light outside these frequencies, especially in the infra-red spectrum. On Earth, this is observable in terms of a rapid increase from 5% to 60% in the quantity of light reflected from vegetation at the “red edge”, at a wavelength of around 700nm. On other planets, the rapid change is unlikely to be at the same wavelength as on Earth, but will be observable by a sudden increase in the quantity of light reflected below a certain frequency of light, unexplained by other surface types, which generally have a similar albedo at all wavelengths. Closer analysis of Earthshine will allow the red edge to be studied in greater depth, and conclusions from this can be used in the search for extraterrestrial life.

The Electromagnetic Spectrum

As many people know, what we see as visible light is but one small part of an entire spectrum of radiation. These can be described as transverse waves of radiation, propagating in straight lines, while the wave itself is comprised of both an electrical and a magnetic component, hence the name, perpendicular to each other and to the direction the wave travels in. Light can also be described as being carried by subatomic particles, photons, as it possesses the characteristics of both waves and particles at the same time. Continuing to look at light as a transverse wave, the frequency, the number of peaks or troughs that pass by in a given time (usually measured per second, as Hertz, Hz), describes how much energy a beam of light carries. High frequencies carry more energy, as the magnetic and electrical components exchange more often, while low frequencies carry less energy. The speed of electromagnetic radiation in a vacuum is constant, the same for all frequencies of light in all situations, and is denoted by c (there is some debate as to whether c stands for constant, or celeritas, the latin for swiftness). It is important to note that the energy that a beam of light carries does not vary with speed or wavelength, such as when light travels through a denser medium, but only with frequency.

Spectral patterns

Different substances absorb light differently. Whereas carbon dioxide, for instance, absorbs mostly light at the lower frequency infra red, other elements have different patterns. These are called absorption spectra and can be displayed graphically, such as the pictures below.

Absorption spectra
imagine.gsfc.nasa.gov