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SCHOME Core Team.
|Baso Schomer||Decimus Schomer||Explo Schomer||Kali Schomer|
|KitKatKid Schomer||Marko Schomer||Mars Schomer||Topper Schomer|
|Faji Bing||Gaea SParker||Technomancer SParker|
1. Executive Summary
We propose an instrument, SCHOME (Spectroscopy, Climate and Habitability from Orbital Measurement of Earthshine), to observe Earthshine and identify key life markers in the reflected light of the Earth. The SCHOME instrument consists of an adapted visible-near infrared spectrometer and a pin-hole camera, with the ability to take 176 readings in one year. By using Earthshine to analyse the composition of the Earth’s atmosphere, we will be able to monitor O2, O3 and H2O. Results from this study will characterise habitability and life signatures of the Earth and serve as a reference when classifying the sustainability and potential life of an analogously observed exoplanet. Our study would tie in with current and future missions of international space agencies to observe exoplanets. In the long term, we could observe the effects of climate change and human influence on the Earth albedo in terms of global cloud, ice and water cover.
Our team comprises eight core team members aged 14 to 17 years old from all over the UK who are taking part in the Schome Park Project based at the Open University, Milton Keynes. The Schome Park Project is an education innovation using two private islands on the Teen Grid of the virtual environment Second Life, in conjunction with an online forum and wiki. As most of the students and staff have not met in real life, communication through these media has been a key aspect of the project. Unlike other entrants, our team cannot physically meet to discuss our entry. Instead, we have had to find ways to discuss and develop our ideas within Second Life and other outputs, thus making our entry unique.
3. Scientific Background
3.1 Visible Light and the Electromagnetic Spectrum
What we see as visible light is one small part of the entire electromagnetic spectrum of radiation (see Figure 1). 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 and is related to wavelength. Visible light is between 400 and 700 nm, with near infrared between 700 and 1400 nm.
Earthshine is the sunlight reflected back from the Earth onto the dark side of the moon, as illustrated in Figure 2. By measuring Earthshine, we can observe the reflected light (albedo) spectrum of the Earth, and use it to identify the composition of the surface and atmosphere. We can also monitor changes in atmospheric features and terrain such as cloud, ice and water cover of the planet. At a basic level, some terrains (such as snow) have a higher albedo than others (such as the sea). Generally, land-based topographies will reflect more light than liquid water. In addition, vegetation has a distinct reflectance pattern, with a sudden change in the reflection of light beyond a wavelength of around 700 nm. Traditionally, Earthshine is measured using ground-based telescopes; we propose to take measurements from Earth orbit.
3.2.1 Previous Earthshine Measurements
Earthshine will display a number of features relating to both the atmosphere and the surface of Earth, in addition to interference features from the Moon. In order to find the albedo spectrum of 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 out. Therefore, the Earth albedo EA(λ) is a ratio of Earthshine to moonlight (Equation 1) with geometrical factors added to account for positions of the Earth, Sun and Moon. MS(λ) is defined as moonlight spectra, ES(λ) is defined as Earthshine and g1 and g2 are geometrical factors.
EA(λ) = (ES(λ)xg1)/(MS(λ)xg2)
There have been several projects to observe Earthshine, notably at Big Bear Solar Observatory, California, and the GOME project on the satellite ERS-2. Although some Earthshine models have been created, not enough observations have been made to create a definitive model of Earthshine which would be useful in indicating the existence of life on other planets or identifying a habitable environment.
3.2.2 Frequency of Observations
The same point on the Earth faces the Moon approximately 26.2 times during one lunar cycle. Since Earthshine measurements 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, Earthshine readings of the same topography can be taken approximately 13.2 times every full lunar orbit of 27.3 days. This allows around 176 readings to be taken per year, unaffected by weather conditions during the observation period (unlike ground-based observations).
3.2.3 Features of Life Observable in the Earthshine Spectrum
The presence of life on Earth is indicated by features in the light absorbed or reflected by Earth's surface and atmosphere, which are observable in Earth’s albedo spectrum. These features are based on the absorption spectra of different elements, compounds and surface terrains, all of which have unique absorption bands. Identifying these spectral features in Earth albedo spectra allows us to understand externally perceivable features of life on Earth better, and this is transferable to observations of exoplanets (see section 3.3). Examples of Earth albedo spectra (observed and modelled) are given in Figure 3. Primary spectral features include:
- • O2 (630 nm, 690 nm and 760 nm) - a potential indicator of the presence of life. Without photosynthesising plants and algae, oxygen would typically be produced in low quantities. Oxygen in the atmosphere would be removed through reactions with minerals (forming oxides). Without replacement via photosynthesis, the overall concentration of oxygen in the atmosphere would decrease over time. The existence of oxygen in the atmosphere of an exoplanet may be one indicator of habitability and the presence of life.
- • H2O (700 – 720 nm) – a vital compound for life on Earth, it is likely to be important in the development of life on other planets. It is essential for biochemical reactions necessary for life. It remains a liquid at a large range of temperatures, moderating climate to allow the development of life at a temperature enabling biological reactions to occur quickly. Because of its importance, planets lacking liquid water are unlikely to sustain life.
- • Vegetation Red Edge (680-730 nm) – most organisms rely directly (or indirectly) on an external energy source, and this s most likely to be the local star. Any organism which takes in light will absorb high quantities of light at certain frequencies and, as an evolutionary adaptation to prevent overheating, is likely to reflect light outside these frequencies, especially in the infrared. On Earth, this is observable in terms of a rapid increase from 5% to 50% in the quantity of light reflected from vegetation beyond the “red edge”, a wavelength of around 700nm. This can be extracted from analysis of Earthshine allowing the red edge to be studied in greater depth. However, this effect would be small and therefore it is not a priority of the sensor to find it, though discovery would be a massive bonus.
- •O3 could also be monitored in relation to annual variations and human environmental impact, as its absorption band lies within the visible range O3 (broad feature over 450-750 nm). Its effect would, however, be small and so may not be observable.
An exoplanet (or extra-solar planet) is a planet beyond the Solar System. To date, 276 exoplanets have been detected and most of these are giant planets that are thought to be similar to Jupiter. Several ground-based and space-based methods are used to search for exoplanets. Current space missions used to identify exoplanets include COROT, Spitzer and the Hubble Space Telescope, as well as the secondary extended mission of Deep Impact (EPOCh) which will observe the light reflected from exoplanets. Presently, Spitzer is able to provide spectral information from which exoplanet compositions can be identified. For example, water vapour and methane have been discovered in the atmosphere of planet HD 189733b, but the exoplanet itself is unable to sustain life as it is too hot (around 1000 K). Future missions include NASA’s Kepler mission (launch 2009) and ESA’s Gaia mission (launch 2011), which will both observe planetary transits (crosses) in front of the parent star to identify new exoplanets. The James Webb Space Telescope (set for launch in 2013) will observe exoplanets using infrared imaging. It will gather the infrared images and used them to characterise the age and mass of the exoplanets in addition to measuring their spectra. ESA’s Darwin mission (estimated launch 2015) is a space telescope which aims to find and study the properties and composition of Earth-like exoplanets in the infrared. Our instrument will tie in with current and future missions to observe and search for life on exoplanets. By looking at the spectra of Earth, we can characterise what makes it suitable for sustaining life, information that can be related to present and future exoplanet observations.
3.4 Signal to Noise
See here for preliminary write-up of signal to noise calculations
4. Scientific Objectives
The primary research objectives of SCHOME are to:
- 1. determine life signatures, habitability features and albedo changes related to topography and weather of the Earth from Earthshine spectra
- 2. relate our findings to current Earth climate models using the satellite’s ability to observe the Earth’s spectrum around 176 times a year
- 3. relate the findings from present and future exoplanet spectral studies to those of this study.
5. Scientific Rationale
SCHOME allows us to observe the Earthshine spectrum and to identify what makes Earth hospitable for life by identifying the signatures of life and assessing habitability. We will be able to characterise different terrain and vegetation as each has a different albedo, and we will be able to identify signatures of life such as oxygen. During ground-based measurements of Earthshine, the weather e.g. clouds, can make the moon difficult to observe and measurements can also be affected by local atmospheric effects. Earthshine measurements taken from space, will not be affected in the same way. They will also benefit from the Earthshine having to travel through the atmosphere one less time (compared to ground to ground based observations) hence improving the signal. We aim to collect data for as long as possible (i.e. for the entire time that the satellite will be in orbit) to obtain an accurate idea of the factors affecting key features of the Earthshine spectrum. We will be able to measure the Earthshine spectrum an estimated 170 times per year. A long-term study using this satellite could potentially observe Earthshine spectral changes resulting from the impact of climate change on our planet. This is an ambitious project and we would seek to collaborate with those from the international astronomy community to aid the data reduction process. However, we intend the identification of key Earthshine features to be conducted by our team, with additional supervision and support provided by PhD students at the Open University. We can use the results of these Earthshine observations as a reference when assessing exoplanets in terms of habitability, climate, terrain and the presence of life.
Extracting the Earth albedo from the Earthshine spectrum requires the measurement of moonlight spectra (lit side of the moon) and the Earthshine spectra (unlit portion of the moon) in the visible wavelengths and calculating it using Equation 1. To measure these parameters we will require a) a spectrometer (visible waveband) to acquire spectral information b) optics to focus reflected light from the moon to the spectrometer.
6.1 Visible Spectrometer
A spectrometer will split the incoming light of Earthshine by wavelength, having previously been focussed by the optical section of the instrument. The intensity of light at each wavelength will be measured and recorded by the detector. It can then be plotted on to a graph of wavelength against light intensity, which can be analysed to find a number of features. In order to pick up the key features of oxygen, water and vegetation red edge, we will need to investigate the visible and near infrared sections of the electromagnetic spectrum, with light of wavelengths between around 500 to 1000 nm. The proposed instrumentation is a miniaturised visible and near infrared spectrometer, which is a stripped down and more rugged version of the AvaSpec-2048 Fibre Optic Spectrometer. The SCHOME spectrometer is based on a UV-Vis spectrometer currently being developed for the ESA ExoMars mission. Table 1 summarises key features of the core instrument, comparing the UV-Vis instrument developed for ExoMars and the proposed SCHOME instrument.
|Properties||AvaSpec-2048 Fibre Optic Spectrometer||Uv-Vis Spectrometer for ExoMars 9||SCHOME Instrument|
|Optical Bench||75mm focal length||75mm focal length||75mm focal length|
|Wavelength range||200-1100 nm||200 – 650 nm||400 – 800 nm|
|Resolution||0.04 – 20 nm||1 – 1.5 nm||1 – 1.5 nm|
|Sensitivity||5000 counts/µW per 2ms integration time||5000 counts/µW per 2ms integration time||5000 counts/µW per 2ms integration time|
|Detector||CCD linear array, 2048 pixels||CCD linear array, 2048 pixels||CCD linear array, 2048 pixels|
|Power||1.92 W||200 mW||200 mW|
|Dimensions||175 x 110 x 44 mm||100 x 130 x 30 mm||Reconfiguration of the ExoMars instrument allows us to comply with competition constraints|
|Mass||719 g||~100 g||~100 g|
Due to mass and space constraints, a pin-hole camera with an additional lens is proposed to focus Earthshine and Moonlight to the spectrometer. The camera consists of a tiny hole (pin-hole) that focuses incoming light from a wide field of view to the lens, which then focuses the light to the spectrometer.
Figure 4. Pin-hole camera with a lens focusing light into the spectrometer
Our team, consisting of eight core team members from different parts of the UK, has developed a proposal for this competition within the virtual world of Second Life, using the Schome Park Project forum and wiki to organise our ideas. We propose an instrument, SCHOME (Spectroscopy, Climate and Habitability from Orbital Measurement of Earthshine), to measure the reflected light of the Earth (Earthshine). The SCHOME instrument consists of an adapted visible-near infrared spectrometer and a pin-hole camera and has the ability to take 176 readings per year over the lifetime of the satellite. The main objectives of the instrument are to 1) determine life signatures, habitability features and albedo changes related to topography and weather on the Earth and 2) relate these results to current Earth climate models as well as present and future exoplanet spectral data.
Kaiser, P. York University. http://www.yorku.ca/eye/
Meadows, V., Crisp, D. and Tinetti, G. Characterization of Terrestrial Planets From Disk-Averaged Spectra: Spatially and Spectrally Resolved Planetary Models. http://nai.arc.nasa.gov/team/customtags/projectreports.cfm?teamID=57&year=7&projectID=1385