Ptolemy is the first example of a new concept in space instrumentation, which has been devised to tackle the analytical challenge of making in situ isotopic measurements of solar system bodies. The instrument concept is termed 'MODULUS' which is taken to mean Methods Of Determining and Understanding Light elements from Unequivocal Stable isotope compositions.
MODULUS was named in honour of Thomas Young, the initial translator of the Rosetta stone, whose name is immortalised by the measure of elasticity known as Young's Modulus.
The scientific goal of the MODULUS concept is to understand the geochemistry of light elements, such as hydrogen, carbon, nitrogen and oxygen, by determining their nature, distribution and stable isotopic compositions.
The size of a small shoe box and weighing less than 5 kg, Ptolemy will use gas chromatography / mass spectrometry (GCMS) techniques to investigate the comet surface & subsurface.
The scientific objectives of the Ptolemy instrument are:
- Evaluate the link between water ice on a comet and major bodies of water on Earth
- Comprehend the internal balance of volatiles on a comet and describe the cosmochemical fundamentals of cometary formation
- Elucidate the nature of organic components present on a comet and assess the relationship with equivalent materials known from other Solar System reservoirs (the Earth, asteroids, planets and their satellites, interplanetary dust etc.)
- Determine the nature of low temperature mineral components present on a comet and decipher the formation history of such materials
- Document certain features of any high-temperature, refractory, minerals
- Assess the relevance of comets to the operation of widespread and important Solar System processes such as planet formation and the origin of life
Ptolemy is supplied with samples of cometary material by the SD2 sampling and drilling system. Once the lander is on the surface of the comet, SD2 will be used to obtain small cores of ice/dust from both the near-surface environment and at sub-surface depths of up to 200 mm. Solid samples collected in this way will be delivered to one of four ovens dedicated to Ptolemy, which are mounted on a circular, rotatable carousel. The carousel has a total of 32 ovens, with the remainder being used by COSAC and ÇIVA. Of the four Ptolemy ovens, three are for solid samples collected and delivered by SD2 whilst the fourth contains a gas-trapping substrate, which will be used to collect volatiles from the near-surface cometary atmosphere.
With an appropriate sample loaded into one of the ovens, the carousel rotates to a position whereby a device referred to as a "tapping station" is used to connect the oven to the inlet of the gas management system. At this point, sample volatiles can be released into the analytical system by heating the oven. Once in the analytical system, they are quantified, purified and chemically reacted (where necessary) to produce a relatively simple gas mixture. The gases are then passed to the ion trap mass spectrometer, either directly or through one of three analytical channels comprising gas chromatography columns and additional chemical processing reactors. For experiments requiring gas chromatography, a constant supply of helium carrier gas is delivered by a regulator, which ensures maintenance of the necessary pressure and flow rate. The helium is used to force the cometary gas mixture through the selected column and associated reactors in order to effect further separation, reaction and purification.
In either mode of operation, direct or via the analytic channels, the ion trap mass spectrometer is set to perform continuous sweeps over the mass range of interest , which can be anywhere between m/z = 12 and m/z = 150). The device works by ionising gases as they flow into the chamber which contains the mass spectrometer. Un-ionised gases and any excess helium flow to an external vent tube, while ionised species become trapped within the electrode structure of the mass spectrometer. Through a combination of rapidly changing radio frequency potentials applied to the spectrometer electrodes, the ions are sequentially ejected according to their mass and detected by an electron multiplier. The cycle of ionisation, trapping and ejection is repeated many times, with a typical duty cycle of 10 ms.
The ion trap has two main functions: to obtain qualitative analytical data by assessing what gases are present in any particular sample and to measure stable isotope ratios. With the ion trap in a qualitative analytical mode the instrument operates over a large mass range (for example, m/z = 12 to 100 or 40 to 150). In contrast, during isotope ratio determination, the mass range will be quite restricted (for instance, m/z = 43 to 47 for measurements of 12C/13C on CO2 gas). In either case, the ion beam measurements made by the electron multiplier are integrated over a number of consecutive mass sweeps and the appropriate information recorded digitally for subsequent analysis. Since Ptolemy aims to obtain isotope ratio measurements of the highest possible precisions, the ion trap instrument will be calibrated in situ during the same period of time over which the cometary analyses are made. For this calibration, equivalent analyses will be made of a reference gas taken from Earth to the comet and delivered to the instrument through the gas management system. In this way, analogous isotope ratio data will be acquired from the reference gas. With knowledge of the actual isotope ratio of the reference it will then be possible to correct the measured cometary data in order to obtain an absolute value for the ratio of interest.
Last Update: 09 Apr 2010