[1] The exhaust plume of Phoenix's hydrazine monopropellant pulsed descent thrusters will impact the surface of Mars during its descent and landing phase in the northern polar region. Experimental and computational studies have been performed to characterize the chemical compounds in the thruster exhausts. No undecomposed hydrazine is observed above the instrument detection limit of 0.2%. Forty‐five percent ammonia is measured in the exhaust at steady state. Water vapor is observed at a level of 0.25%, consistent with fuel purity analysis results. Moreover, the dynamic interactions of the thruster plumes with the ground have been studied. Large pressure overshoots are produced at the ground during the ramp‐up and ramp‐down phases of the duty cycle of Phoenix's pulsed engines. These pressure overshoots are superimposed on the 10 Hz quasi‐steady ground pressure perturbations with amplitude of about 5 kPa (at touchdown altitude) and have a maximum amplitude of about 20–40 kPa. A theoretical explanation for the physics that causes these pressure perturbations is briefly described in this article. The potential for soil erosion and uplifting at the landing site is also discussed. The objectives of the research described in this article are to provide empirical and theoretical data for the Phoenix Science Team to mitigate any potential problem. The data will also be used to ensure proper interpretation of the results from on‐board scientific instrumentation when Martian soil samples are analyzed.

1. Introduction [2] The final descent and touchdown phase of the Phoenix Lander is controlled by twelve hydrazine (N 2 H 4 ) monopropellant retro‐rocket engines in pulsed mode, which could result in the impingement of some of the exhaust products onto the Martian regolith at the landing site during the landing phase. The Phoenix engines use pulse frequency and duty cycle control to dynamically throttle power during descent [Wong and Masciarelli, 2002]. [3] There is concern among the Phoenix Science Team that the exhaust products could interact with and alter the natural compounds in the Martian soil. There is also concern that the undecomposed hydrazine and potential impurities in the fuel could contaminate the landing site. Finally, site alteration and dust lifting by the interaction of the pulsed jet with the Martian regolith is also a potential concern. [4] To address these issues, the Phoenix Science Team has performed extensive analysis of the hydrazine fuel, the thruster exhaust products, and the dynamical interaction of the exhaust plume with the ground surface. These efforts included (1) selecting the cleanest, driest propellant available for the mission; (2) assaying and documentation of the mission propellant; (3) conducting experimental plume signature identification during the hot fire engine qualification testing of the flight motors using a batch of the mission propellant; (4) capturing sample exhaust gases from these tests for use in future laboratory analysis; (5) extensive laboratory testing, computational modeling and analysis; and (6) preserving a batch of the mission propellant and a back‐up rocket motor for potential future testing, should this be warranted. Computational and experimental studies have been performed on the products of the catalytic hydrazine decomposition in order to understand the potential physical interactions of the rocket plume with the Martian surface. [5] A very brief discussion of two of the Phoenix Lander primary scientific instruments that will benefit from the plume diagnostics data and analysis efforts is included in this section to provide some context for this investigation. A more detailed description of these instruments is included in other manuscripts in this special issue [Kounaves et al., 2008; W. V. Boynton et al., The Thermal and Evolved‐Gas Analyzer on the Phoenix Mars lander, manuscript in preparation, 2008]. [6] The Microscopy, Electrochemistry and Conductivity Analyzer (MECA) is a combination of scientific instruments including a wet chemistry laboratory (WCL), optical and atomic force microscopes, and a thermal and electrical conductivity probe. MECA will determine acidity, saltiness, and composition by mixing soil samples with small amounts of water. MECA will also examine the soil grains to provide information on mineralogy and origin. [7] The Thermal and Evolved Gas Analyzer (TEGA) is a combination of high‐temperature ovens and a mass spectrometer that will be used to perform chemical analysis of Martian soil and ice samples. TEGA will be used to detect volatiles, soil mineralogy, and potential organics that may be resident on the Martian surface. [8] Understanding the physics of rocket plume impingement on planetary surfaces is important for the survivability of the spacecraft during terminal descent and touchdown phases of Entry, Descent and Landing (EDL) [Albee et al., 2000]. [9] Limited investigations of steady state rocket plume interactions with the Martian soil surface were conducted for the Viking mission by NASA researchers in the 1970s [Grover et al., 2005]. The dynamics of the interaction of an underexpanded jet plume flow field, where the nozzle exit pressure is greater than the ambient (back) pressure, with the surface is quite complex. Indeed, instabilities in the flow occur because of the coexistence of subsonic, transonic, and supersonic regions within the flow [Krothapalli et al., 1999]. For this reason, computational simulations must be tested with data from experiments [Janos and Hoffman, 1968]. [10] This paper discusses the objectives of plume diagnostics research efforts and explains why they are important to the Phoenix science mission. We present the methodology and results from three research efforts: (1) spectral diagnostics of the rocket engine exhaust gases, (2) analysis of plume gases using gas chromatography and mass spectrometry, and (3) physical interactions of the rocket plume with the regolith. We conclude with a brief discussion on planned future work and how these data will help with the scientific measurements on Mars.

2. Objectives [11] The understanding of the physical interaction of thruster plumes with the Martian surface is also crucial for assessments of dust lifting and spacecraft contamination. However, detailed experimental investigations of pulsed rocket plume interaction with the ground and the Martian regolith are currently not available [Mehta et al., 2007]. We conducted computational simulations and laboratory experiments to study these interactions and assess their effects on dust lifting. We report such measurements here and show that they are consistent with numerical simulations. The hydrazine fuel used in the Phoenix Lander is high purity grade (99% by weight) but still contains impurities such as water (<1%), ammonia (<0.3%), aniline (<0.003%), and trace organics (<0.005%). There was a significant level of uncertainty among the science team as to the exact composition of the exhaust products and it was decided to perform additional experimental and numerical studies on the Phoenix landing system rocket engines. [12] The objectives of this research are to determine the chemical compounds present in the rocket engine exhaust and to characterize the physical interaction of the thrusters exhaust plumes with the Martian surface. The affects of the exhaust products on the inorganic chemical analysis will be addressed in a separate paper.

3. Plume Gas Sampling and Analysis Using Fourier Transform Infrared Diagnostics 3.1. Chemical Reaction Modeling [13] The primary product of hydrazine decomposition is ammonia 2 and H 2 2 H 4 with an iridium catalyst can be expressed by [ Lyon, 1971 2 and H 2 . A rhodium catalyst produces nitrogen and hydrogen in equal quantities [ Sayer, 1970 The primary product of hydrazine decomposition is ammoniaIn the presence of sufficient heat, the ammonia will further decompose into Nand HThe catalyst beds in the Phoenix landing system engines are made of iridium and rhodium metals affixed to a porous ceramic alumina. The decomposition of Nwith an iridium catalyst can be expressed by [where x is the fraction of originally formed ammonia that has dissociated into Nand H. A rhodium catalyst produces nitrogen and hydrogen in equal quantities [ [14] Chemical equilibrium modeling was performed to provide some insight on interpretation of equations (1)-equations (1)–(4). The predicted plume properties are shown in Table 1. The equilibrium model column in Table 1 is the output from the NASA Gordon‐McBride [McBride and Gordon, 1996] chemical equilibrium code. The quenched model column in Table 1 is the results from similar calculation performed by the engine manufacturer. The manufacturer's code is a modified version of the Gordon‐McBride code that allows for quenching the NH 3 decomposition inside the combustor. Equations (1)-Equations (1)–(4) show that the products in hydrazine monopropellant rocket exhaust plume can significantly vary, with possible major species mole fractions of NH 3 10%–80%, H 2 0%–67%, N 2 20%–33%, and N 2 H 4 unknown. Table 1. Chemical Equilibrium Calculated Mole Percent at the Rocket Engine Nozzle Exit Species Equilibrium Model Quenched Model NH 3 9.1% 36% N 2 32% 27% H 2 59% 36% [15] As mentioned in the section 2, the hydrazine fuel used in the Phoenix Lander is high purity grade. A purity analysis was performed by the engine manufacturer to ensure that the fuel was in compliance with military specification MIL‐P‐26536. The results from the purity analysis are shown in Table 2. Table 2. Hydrazine Purity Analysis Results Compound Acceptable Values for High Purity Grade (by Weight) Measured Values N 2 H 4 99.00% min 99.69% H 2 O 1.00% max 0.25% NH 3 0.30% max 0.06% Trace Organics (Excluding Aniline) 0.005% max 0.001% Aniline 0.003% max <0.0006% Total Nonvolatiles 0.0010% max 0.0004% Particulates 1 mg L−1 max 0.8 mg L−1 Corrosivity 0.00125% Fe max 0.00025% Chlorine 0.0005% max 0.00017% CO 2 0.0030% max 0.0003% 3.2. Test Procedure [16] The thrusters exhaust products were characterized during the operational test phase of the engine manufacturing process. Three plume analysis systems were used (in addition to modeling and simulation) to ensure thorough characterization of the exhaust products. Exhaust gas samples were extracted through a heated sample line and analyzed using a Fourier transform infrared (FTIR) multigas analyzer (MGA) [Markham et al., 2004]. A separate plume sample was extracted through the heated sample lines and stored in a passivated sample canister for off‐site analysis using gas chromatography/mass spectrometry (GC/MS) methods. An additional gas sample was collected and stored for future analysis, if required. An instrumentation schematic is shown in Figure 1. Figure 1 Open in figure viewer PowerPoint Schematic of the FTIR instrumentation installed on the rocket engine test cell. [17] As mentioned in the Introduction, the same fuel lot as used on board the Phoenix Lander was used during the engine test firings. The results from the MGA analysis were previously reported by Plemmons et al. [2007] and are summarized below. 3.3. Plume Analysis Results [18] On the basis of the hydrazine decomposition analysis, the concern over unreacted hydrazine, and the results from the fuel purity analysis, the FTIR MGA was calibrated to measure high levels of NH 3 , and low levels N 2 H 4 , and water vapor. Exhaust gas samples were acquired and analyzed for two separate engine firings. The MGA measured NH 3 concentrations in excess of 50% during engine start and settled to 45% after the engine reached operating temperature, which is 19% higher than the engine manufacturer's performance model and 5 times larger than the equilibrium model prediction. The measured water vapor levels are just under 0.25%, which is in agreement with the pretest fuel purity analysis shown in Table 2 and indicates that water does not participate significantly in the reaction. [19] No hydrazine was observed in the thruster exhaust plume. However, the hydrazine FTIR absorption features on this instrument are coincident with the ammonia absorption spectra. Hence, the high ammonia concentration will mask low concentration hydrazine. On the basis of posttest analysis of the FTIR spectra where the spectra were manually searched for hydrazine absorption features, it is estimated that the unreacted hydrazine levels in the thruster plume is less than 0.2%. [20] The ammonia and water vapor concentration measured for one of the engine firings is shown in Figure 2. The engine was fired for approximately 40 s. At the end of the firing, the FTIR absorption cell was isolated and the MGA continued to analyze the final sample to obtain a baseline estimate on the instrument precision under test conditions. After approximately 2 min 40 s, the absorption cell was purged with high‐purity nitrogen and the ammonia and water vapor concentration readings returned to zero. Figure 2 Open in figure viewer PowerPoint Multigas analyzer results for ammonia and water vapor. [21] Gas samples were extracted from the exhaust plume of two Phoenix retrorocket thrusters during hot fire acceptance testing. Recirculation gas samples were taken from inside the test cell for two additional engine firings. The samples were analyzed for ammonia, water vapor, and hydrazine concentrations using a FTIR absorption spectrometer. The measured ammonia and water vapor were in reasonable agreement with expected values. Using a value of x = 0.3 in equation (3), yields 45% NH 3 , 26% N 2 , and 29% H 2 , on a molar basis.

4. Analysis of the Plume Gases Using Gas Chromatography–Mass Spectrometer [22] The purpose of this analysis was to corroborate the spectroscopic infrared gas analysis via the gas sample probe that was performed during the test firings. The samples were obtained during the test firings by diverting the plume gases into two passivated gas sample canisters. One was charged with the plume gases at 300 torr and the other at 1,277 torr, respectively. The analysis reported here was performed using the 1,277 torr sample. The analytical procedures for the instrumental analysis were developed for simple sample extraction, and used standard methods on a standard configuration GC‐MS. The 300 torr sample and the remaining 1,170 torr sample have been stored for future testing should specific questions arise during surface measurements on Mars or should they arise regarding interpretation of the chemistry results. 4.1. Analytical Methodology [23] During two of the test firings, the stainless steel heated sample lines carrying hot rocket exhausted gases outside the test chamber, through a heated boost pump, delivered the gases to a sample canister rather than to the FTIR‐MGA instrument. One of those sample containers collected only 300 torr of exhaust gases. Since this is below atmospheric pressure, the sample can only be accessed through complicated line work and was thus not analyzed in these tests. The other sample canister collected at a pressure of 1,277 torr, well above 760 torr (1 atm) was easily extracted and required no complicated processing. [24] A septum was attached to the canister outlet. The attachment included a rubber stopper plate through which a syringe could be inserted to extract samples. A small space of less than 0.25 mL volume was formed between the stopper and the closed valve of the canister. The canister had to be opened and bled significantly (3 or 4 times for several seconds) to flush air out of the chamber. Samples were tested each time. The oxygen peak reduced significantly with each flush. When the oxygen peak disappeared, it was assumed the chamber was emptied of air. [25] With the septum closed tight and the canister fully open, the small chamber was allowed to come to equilibrium with the gases in the canister. The canister was then fully closed off from the chamber. A 100 μL sample was extracted from the chamber, through the septum, with a 500 μL (Superco SGE) gas tight syringe. The sample was carried 15 feet and manually injected into the GC‐MS (Shimaduz QP‐5050A). [26] The injection temperature was 200°C and interface temperature 280°C. The instrument control mode was split because the samples were large and plentiful. The total flow rate was 21.7 mL min−1 under an inlet pressure of 100 kPa. The final reported analyses were run for 5 min at a steady oven temperature of 50°C. Higher temperatures and ramped temperatures were unnecessary since the entire sample was gaseous and did not need to be volatilized. With significantly longer runs (>1 h) only the peak at 1.25 min was observed in the gas chromatogram and thus a 5 min run was more than sufficient. [27] Prior to the sample analysis, air was run by identical sample delivery and GC‐MS method. The GC‐MS method was also run without injection of any sample, and provided a true blank. After all plume analyses were complete, the GC‐MS was baked out using a very slow temperature ramp of 50°C to 470°C. There were no indications of any less volatile substances remaining on the column. 4.2. GC‐MS Results [28] Table 3 shows the results for the mass spectrum of the air blank and plume sample. The blank, containing approximately 77% N 2 and 21% O 2 , displays a mass peaks at 28 m/z (14 + 14 atomic mass units (amu)) with a relative intensity (I rel ) of 100 and at 32 m/z (16 + 16 amu) with an I rel = 40.4. Table 3. Results of GC‐MS Analysis m/z Air, I rel Sample, I rel Fragment 15 0.0 5.2 NH 16 3.1 88.8 NH 2 17 0.4 100.0 NH 3 18 0.8 3.8 ? H 2 O, NH 4 28 100.0 75.6 N 2 31 0.0 0.0 32 40.4 1.2 O 2 [29] The chromatogram for the 100 μL plume sample shows a single sharp peak with a retention time of 1.2 min. This peak in turn resulted in a mass spectrum with three major peaks at 16m/z (I rel = 88.8), 17m/z (I rel = 100), and 28 m/z (I rel = 75.6). These peaks can be assigned to NH 2 (16 amu), NH 3 (17 amu), and N 2 (28 m/z), respectively. Other very minor ammonia‐related and water peaks are also present, probably for NH (15 amu) and H 2 O (18 amu). It should be noted that detection and analysis of H 2 is extremely complex and requires special containers, handling, and procedures; thus no attempt was made to measure the H 2 content of the plume gases. It is also important to note that no standards were used to construct calibration curves and thus no quantitative results are provided for any of the other constituents. 4.3. Interpretation of GC‐MS Results [30] The most significant results of the GC‐MS analysis are (1) that a major fraction of the thruster plume is composed of NH 3 and (2) that no hydrazine is present. As discussed above, the quenched equilibrium code for the Phoenix hydrazine thrusters predicts 36% NH 3 , 36% H 2 , and 27% N 2 . The in situ plume gas analysis yielded 45% NH 3 and 0.23% H 2 O. Using absolute intensities and setting the level of NH 3 + NH 2 + NH at 45%, we can calculate the levels of N 2 and H 2 O, as 18% and 0.8%, respectively. Even though our results are not rigorously quantitative, these levels are reasonably close to those predicted by the model and performance criteria. Taking into account all these results, it is reasonable to estimate that the plume contains about 45% (±5%) NH 3 . [31] The more significant result is that we found no indication in the mass spectrum, within our detection limits, of N 2 H 4 (32 amu). If any is present it is <1%, however, given that there is no confirmation peak at 31amu for N 2 H 3 , the peak at 32 amu is most likely due to O 2 contamination from air not fully evacuated from the container. [32] The other major reaction product found in the plume sample is N 2 . This can typically range from 20% to 35% and the model predicts 27%. Our estimated result of 18% is low, but can be accounted for by the errors introduced if we allow for some recombination of N 2 and H 2 fragments to give NH.

6. Planned Future Work [50] We are planning to use the data presented here to test the effects of NH 3 adsorption on a cross section of Mars stimulant soils. The test are designed to provide information as to what effects such adsorption may have on the chemical analyses that will be performed by the Wet Chemistry Laboratory (WCL) on Phoenix. [51] We are planning to conduct experiments with our CFTB at the NASA Ames Planetary Aeolian Laboratory (PAL) Mars chamber (Aeolian wind facility) to quantify soil erosion and uplifting during spacecraft landing on Mars (M. Mehta and N. O. Renno, manuscript in preparation, 2008). Our main goal is to understand the flow physics and the effects of the pulsed underexpanded rocket exhaust impinging on the Martian soil. We will use scaling laws (see section 5.1.2) to relate experimental results to the potential site alteration of Phoenix's pulsed rocket plumes at the landing site. [52] These experimental results will provide the Phoenix Science Team with a first‐order approximation to the crater contour as well as the crater depth and dust deposition at the landing site. Results from the tests will be primarily used for scientific interest, providing information about possible dust contamination in the sampling areas for MECA and TEGA, on calibration targets, and general albedo‐induced changes to the thermal properties of the science deck [Marshall et al., 2007]. Subsequently, these tests can also be valuable for determining if significant erosion or deposition occurs at the digging location. Since MECA only has four sample analysis opportunities for the wet chemistry cells, it seems important that we understand soil disturbance in the digging area, both its lateral and vertical extent. These top few centimeters are also the zone of chemical contamination from the plumes that could potentially influence TEGA results. Results from these tests can therefore lead to mitigation strategies still within the flexibility of our operational (digging) schemes [Marshall et al., 2007].

7. Conclusion [53] The FTIR and GC‐MS analysis of the thruster plume gases show no detectable hydrazine. This is the primary result of the FTIR and GC/MS analysis, since the presence of hydrazine has the potential to significantly complicate the interpretation of the in situ analysis of the Martian soil. The analysis however did show significant amounts of NH 3 and N 2 . The N 2 posses no problem for the Phoenix wet chemistry, but the presence of NH 3 in the plume must be further investigated and a good understanding of the possible reactions obtained. [54] Our experiments and numerical simulations suggest that the ground impingement pressure of the Phoenix thruster plumes at the surface of Mars generates large transient pressure overshoots which correlates to a ground shock frequency of approximately 20 Hz and a 10 Hz quasi‐steady state ground pressure perturbation. These large ground pressure overshoots potentially occur due to the formation and collapse of the plate shock and stagnation bubble near the surface due to high instability in the plate shock dynamics during P C start up and shutdown cycles. This has the potential to significantly increase soil erosion and uplifting. The amount of cratering and soil uplifting depends on the soil properties, ambient pressure and engine thrust. Experiments with University of Michigan's cold flow thruster at the NASA Ames Aeolian wind facility will be used to quantify soil erosion and uplifting before Phoenix lands on Mars. The results of this experiment will be used to refine the landing and digging strategy so that contamination processes can be properly understood and mitigated if necessary. This will also describe the jet‐induced soil dynamic processes taking place at the landing site during the landing phase of the spacecraft.

Acknowledgments [55] This work was supported by NASA Jet Propulsion Laboratory and the Phoenix Mars Mission. In particular, we would like to thank Peter Smith (Phoenix Mission PI) of University of Arizona; Robert Shotwell (Project Systems Engineer), Rob Grover (EDL Systems Lead) and Mike Hecht (MECA Lead) of NASA JPL; Bill Boynton (TEGA Lead) of University of Arizona; Greg McAllister, Tim Fisher, Pete Huseman, Doug Gulick, and Tim Priser of Lockheed Martin Space Systems; Matt Dawson of Aerojet, Inc.; Chuck Davis of KSC; John Marshall of SETI; Ron Greeley of Arizona State University; Ray Arvidson of Washington University; and Jasper Kok and Robb Gillespie of the University of Michigan for all their support and guidance.

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