Ptolemy: A report, and future plans

D. Andrews, S. Barber, G. Morgan, A. Morse, S. Sheridan, I. Wright, C. Pillinger

Ptolemy is the result of a collaboration between The Open University and the Rutherford Appleton Laboratory.

Philae Lander Science Working Team meeting, Venezia, March 30th – April 1st 2009.

Ptolemy status

• PC #8: Successful operation of the mass spectrometer, and of the carrier gas operations mode.– Injector valve VC non-operational, no further testing is required, and

plans are in place to recover the lost capability– Status of carrier gas tank 1 unknown, a test is scheduled for PC #10 to

investigate– Carrier gas flow control not demonstrated to a satisfactory standard,

due to non-optimised PID control coefficients being used. PID control coefficients now optimised through laboratory studies.

• PC #9: Successful completion of EAFT.

Ptolemy plans

• PC #10: Operation of the carrier gas system, the mass spectrometer, conditioning the instrument and testing science modes.

• ESB3: No operations.• PC #12: Upload of new sequence tables, and operation of

modes already conducted.• Lutetia: Science operations in a collaborative ‘exosphere

hunt’.

Ptolemy• Ptolemy is Gas Chromatograph, Isotope

Ratio, Mass Spectrometer.

• Ptolemy uses three gas chromatography columns to separate samples, and has an ion trap mass spectrometer as a detector.

• The instrument is located aboard the Philae lander.

• Ptolemy uses a peak power of 28 W, however typical operating powers are lower than this, with an average operating power of 6-7 W.

Ptolemy sampling methods

• Ptolemy requires sample gas to analyse.

• There are several ways in which samples are delivered to Ptolemy.

• The most direct method is ‘sniffing mode’, where ambient atmosphere is detected.

• Ptolemy typically collects sample gas via a tapping station, which interfaces with either a Medium or High Temperature Oven as part of the SD2 system.

• One of these ovens can be used to concentrate coma material for analysis.

Ptolemy schematic diagram.

Ptolemy helium carrier gas• Ptolemy uses helium which has

been doped to 100 ppm with argon, as a carrier gas for gas chromatography.

• This carrier gas is stored in two 150 ml pressure vessels, at a pressure of 50 Bar.

• The carrier gas tanks must be actuated (FM tank #2 has been), and then carrier gas flow is controlled by the action of a thermal expansion valve.

• The thermal expansion valve fills the plenum to a set pressure, with feedback from a pressure sensor.

• The pressure inside the plenum governs the flow velocity and elution times of the GC columns.

Ptolemy flight data, from PC #8• During PC#8, mass spectra were obtained of first the Ptolemy instrument background,

and then repeated with the addition of a flow of carrier gas.

• In the background spectra, peaks can be seen representing water, possibly nitrogen, carbon dioxide, as well as organic peaks.

• The pressure at which this data was obtained was not known, however Ptolemy had a method to admit a known partial pressure of an analyte.

• Ptolemy carries helium to act as a carrier gas for the gas chromatogaphy columns, and this helium has included a 100 ppm doping of argon gas, to give a known reference peak, and to allow D/H isotope studies.

• With a flow of 1 ml.min-1 of helium, the pressure inside the ion trap is ~1x10-4 mbar, giving a partial pressure of argon of 10-8 mbar. During PC#8, the helium flow rate was lower, being 0.5 ml.min-1. This gave an argon partial pressure of the order of 5x10-9 mbar. This was detected as a peak with an area of ~500 ion counts, during 80 seconds of experiment time. Given that a peak area of 10 ion counts is acceptable for chemical identification with a clean instrument, the demonstrated Ptolemy FM detection limit in ‘sniffing mode’ is 10-10 to 10-11 mbar.

PID control of the Ptolemy helium supply

• In order to provide consistent GC results, the flow velocity of carrier gas must be constant.

• Therefore, the pressure inside the plenum must be constant, to within a few percent.

• Using the standard PID constants (P=0.0015625, I=0.025, D=0) pressure control was poor, with control only being possible to +/- ~10%.

• Using the Ziegler-Nicholls method, the PID control coefficients for the Ptolemy thermal expansion valves were optimised at P=0.001, I=2.5, D=0, a result which gives plenum pressure control to better than +/- 0.5%.

Expansion of the initial transient region

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The CASE study• The SD2 ovens are essentially platinum

buckets, heated to 180˚C in the case of the Medium Temperature Ovens, or to 800 ˚ C for the High Temperature Ovens.

• One of the SD2 ovens dedicated to Ptolemy contains a fill of Carbosphere™ molecular sieve, which when exposed to gases at low temperature (as found on the Philae lander balcony), traps the gas down.

• This trapped gas, adsorbed onto the molecular sieve can be released for analysis by heating the oven.

Plans for PC #10• Ptolemy plans for PC #10 involve improvements to sequences already performed,

and practice for possible future exosphere studies.• Ptolemy operations during PC #10 fall into three separate tests:

– Test 1, Helium pressure control– Test 2, Gas Chromatography conditioning– Test 3, CASE operations

• Test 1 involves using the experimentally derived PID control coefficients to control the pressure inside the plenum chamber at 0.3 bar, 0.6 bar, and then 1 bar, holding the pressure at each level for 600 seconds.

• In test 2, immediately following test 1, a series of mass spectra are obtained of the carrier gas entering the mass spectrometer, followed by 3600 seconds where the gas chromatography columns are baked at 120°C. Following the bake out, further mass spectra will be obtained to monitor the contamination removed from the columns. The mass spectra will be of higher resolution than those gathered to date with the FM, with better than unit mass resolution.

• Test 3 involves a complete run through of a CASE experiment, with the Ptolemy tapping station interfacing with the default Ptolemy oven. ROSINA have shown an interest in being active during the Ptolemy CASE experiment.

PC #10 Ptolemy expectations• Following PC #10, the Ptolemy team will have greater confidence in operating the

FM.• The helium carrier gas system will have been validated, and a successful test will

mean operations with the GC system will be possible.• The GC system itself will have been baked out, to remove Earth contamination

from the columns.• As part of the validation of the helium carrier gas system, the upper detection limit

for the current Ptolemy parameters will be determined, validating Ptolemy for direct ‘sniffing mode’ study of exospheres.

• The CASE study will have been validated, for use later during the comet mapping phase, SDL, and post-landing operations.

• Ptolemy may be ready to start hunting for exospheres earlier than in the draft plan….

The case for 21 Lutetia CASE studies.

• It is known that the large asteroid 1 Ceres has an exosphere, and there were tentative results, presented at the PI working group meeting in late February 2009, which show possible evidence for an exosphere surrounding Steins.

• Lutetia has been modelled by B. Schläppi et al., and theoretically may show an exosphere several orders of magnitude more substantive that Steins.

• Under ideal conditions, Ptolemy may be able to detect traces of this exosphere, using the sniffing mode and CASE study. 0

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Proposed Ptolemy plan for the 21 Lutetia encounter: CASE

• It is proposed that Ptolemy operate the CASE study both several hours before and after close approach.

• The operation several hours before close approach would heat the CASE oven to drive off any trapped volatiles, in a repeat of a test to be carried out during PC#10. Mass spectra would be obtained via the direct analysis channel before, during and after the oven heating, with no carrier gas flow.

• The CASE oven would passively trap any exosphere encountered during the duration of the close approach, with Ptolemy switched off to reduce the risk of interfering with the already planned orbiter and lander operations.

• The pre-encounter CASE study would be repeated several hours after close approach, with mass spectra obtained again before, during and after heating of the CASE oven.

Proposed Ptolemy plan for the 21 Lutetia encounter: Sniffing

• As well as the CASE study, the Ptolemy team feel that operating Ptolemy in ‘sniffing mode’, once per hour from two hours before to two hours after close approach would allow direct detection of the asteroid exosphere, if the pressure is greater that ~10-11 mbar. The ‘sniffing modes’ would last 2 minutes, with a power of 6W, with Ptolemy off for the remainder.

• Timeline:

– CA – 5 hrs: Ptolemy CASE study, 30 mins, 10 W, start passive collection.– CA – 2 hrs: Ptolemy sniffing mode, 2 mins, 6 W, producing background spectra.– CA – 1 hr: Ptolemy sniffing mode, 2 mins, 6 W, producing background spectra.– CA: Ptolemy sniffing mode, 2 mins, 6 W, searching for the exosphere.– CA + 1 hr: Ptolemy sniffing mode, 2 mins, 6 W, producing background spectra.– CA + 2 hrs: Ptolemy sniffing mode, 2 mins, 6 W, producing background spectra.– CA + 5 hrs: Ptolemy CASE study, 30 mins, 10 W, generating an integrated spectrum of all

gases trapped down during the preceding 10 hours (outgassing and ?exosphere?).

• The timing is not essential, although the nearer to CA for a sniffing mode, the greater the chance of directly ‘sniffing’ the exosphere.

Possible pitfalls for the Lutetia CASE

• Because of the flow regime, only molecules directly ‘run into’ by the 3 mm2 open end of the oven would be available for trapping.

• It is likely that the orbiter body, lander body, or other structures will mask the CASE oven from the free stream of exosphere molecules.

• The trapping efficiency of the molecular sieve for gas molecules impinging at ~15 km.s-1 is not known.

• Additional volatiles seen in the post-close approach CASE study could either be from the exosphere, or from spacecraft outgassing, or from a mix of the two sources.

• If the flyby orientation precludes the CASE oven trapping down any exosphere material, then any volatiles seen in the post-close approach CASE would be from a known source – spacecraft outgassing, thus providing useful comparison data for other instruments.

Potential Ptolemy SDL activities

• At SDL, Ptolemy will have a set of tried and tested sequences, which are capable of determining the chemical composition of the cometary coma, and how it changes with time (height).

• The same sequences will already have been used in PC #10, PC #12, and potentially during the Lutetia flyby.

• The potential Ptolemy SDL sequences have the following consumptions:• For each sniffing mode: 2 minutes duration, 6 W peak power, 60 kBytes of data, 20 mass spectra.• For each CASE mode: 30 minutes duration, 10 W peak power, 65 kBytes of data, 20 mass spectra.

• The sequences can be modified to reduce their duration and data production, by reducing the number of mass spectra to be obtained.

Ptolemy during LTS:• CASE on a weekly basis, to determine the chemical and isotopic evolution of the

lower coma as the comet approaches perihelion.

• Sniffing mode in a similar fashion, to give confirmation of the chemical composition, and to give an indication of the local partial gas pressure.

• Further solid samples, from different locations and/or depths, to give increased knowledge of the surface and subsurface.

• Towards the end of the mission, undertake the Silicate Mode, using Fluorine to conduct oxygen isotopic analysis of cometary silicates.

Work to be done:• The Ptolemy instrument must be baked out to reduce the background

contamination, this is due to be accomplished during PC#10.

• The CASE study must be tested on the FM, to validate the experiment and to clean the oven of Earth contamination. This is to be carried out in PC#10.

• To investigate the possibility of Ptolemy operating during CA, interferences with other instruments must be determined.

• Collaboration needs to be arranged between Ptolemy and other instruments currently planning to attempt detection of the exosphere of 21 Lutetia.

Conclusions• Ptolemy has been shown to be able to produce isotopic data in the laboratory given flight-like

conditions.

• The Ptolemy FM has been shown to be able to detect partial pressures of as low as 10 -11 mbar in ‘sniffing mode’, and direct detection of lower partial pressures may be possible with further optimisation.

• Assuming an exosphere pressure of greater than 10-11 mbar around Lutetia close approach, Ptolemy would be able to identify component species from mass 10 to 150 amu using ‘sniffing mode’.

• The CASE study acts as a sample concentrator, in effect integrating many hours (or even weeks in the case of C-G) of exposure to an exosphere into an experiment lasting a few minutes.

• Whilst spacecraft pointing will in all likelihood preclude Ptolemy CASE observations of the exosphere of 21 Lutetia, valuable engineering and background environmental data can still be obtained by operating the Ptolemy CASE study several hours either side of close approach.

• Following PC #10, Ptolemy will be in essence prepared for asteroid/comet operations.

• Thanks to SONC, LCC and ESA.