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Lander rocket exhaust effects on Europa regolith nitrogen assays
Planetary and Space Science 127 (2016) 91–94
Contents lists available at ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
Short communication
Lander rocket exhaust effects on Europa regolith nitrogen assays Ralph D. Lorenz n Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 9 March 2016 Received in revised form 24 April 2016 Accepted 30 April 2016 Available online 7 May 2016
Soft-landings on large worlds such as Europa or our Moon require near-surface retropropulsion, which leads to impingement of the rocket plume on the surface. Surface modification by such plumes was documented on Apollo and Surveyor, and on Mars by Viking, Curiosity and especially Phoenix. The low temperatures of the Europan regolith may lead to efficient trapping of ammonia, a principal component of the exhaust from monopropellant hydrazine thrusters. Deposited ammonia may react with any trace organics, and may overwhelm the chemical and isotopic signatures of any endogenous nitrogen compounds, which are likely rare on Europa. An empirical correlation of the photometrically-altered regions (‘blast zones’) around prior lunar and Mars landings is made, indicating A¼0.02T1.5, where A is the area in m2 and W is the lander weight (thus, thrust) at landing in N: this suggests surface alteration will occur out to a distance of 9 m from a 200 kg lander on Europa. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Europa Astrobiology Nitrogen Thruster exhaust
1. Introduction Europa has attracted much interest as a target of astrobiological exploration (e.g. Hand et al., 2009; Hand, 2015). While long of scientific interest, the claimed prospect of habitable conditions or even life in Europa’s ice-covered water ocean has attracted the attention of politicians and the public. The United States Congressional budget language (Consolidated Appropriations Act, 2016) for FY2016 mandates “ an orbiter with a lander.” Thus renewed efforts are underway to evaluate landing concepts to explore Europa's habitability and search for extant life. Living things contain substantial amounts of nitrogen (the Redfield ratio of Carbon:Nitrogen is 106:16).Nitrogen is an essential element for life: obvious examples include the amino acids that form proteins to execute living functions, and the purine and pyrimidine bases that encode information in DNA and RNA. However, Europa may be nitrogen-starved, as it formed in the warm protoJovian nebula in conditions which did not favor the accumulation of materials more volatile than water (Lunine and Stevenson, 1982). This formation scenario was unlike Titan whose abundant nitrogen (e.g. Lorenz and Mitton, 2008) accreted as ammonia and whose methane presumably accreted as clathrate. Like Europa’s meager carbon inventory, the only supply of nitrogen to Europa’s surface may be the small sporadic delivery of cometary material. Understanding the amount and form of any nitrogen in the n
Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.pss.2016.04.008 0032-0633/& 2016 Elsevier Ltd. All rights reserved.
Europan regolith (and by implication, its ocean) is a key goal of future exploration, and a lander in particular. However, since Europa is an airless body with a substantial gravitational field, soft landing will likely require rocket propulsion which may deposit nitrogen-bearing compounds from thruster exhaust around the landing site. In this paper I briefly explore quantification of this concern, drawing on observations of landing site disturbances on previous lander missions and with a simple thruster plume deposition and chemistry model. To set some historical context, this problem received considerable attention during preparation of the Viking lander missions, whose focus was similarly on astrobiology. Studies of thruster plume impingement (e.g. Clark, 1970) led to a clusterednozzle design to attempt to minimize mechanical disturbance, and some laboratory work was performed (Holzer and Oro, 1977) to assess how thruster exhaust compounds might perturb chemical analyses of the regolith. The Phoenix lander demonstrated quite strong excavation underneath its thrusters, with the fortuitous effect (Plemmons et al., 2008; Mehta et al., 2011) of exposing subsurface ice deposits. This effect seems to be a result of the pulsed thrust modulation (Mehta et al., 2013) which causes transient flow in the regolith (Scott and Ko, 1968; Metzger et al., 2009, 2011 and Morris et al. (2015) also discuss recirculation vortices in rocket exhaust and pit formation in regolith. However, astrobiology was not a focus of this mission, so deleterious effects of exhaust were not a concern. The Mars Science Laboratory (MSL), Curiosity, however would have confronted issues with site modification, even though its sky-crane landing and throttleable rather than pulsed thrusters (e.g. Sengupta et al., 2009; Dawson et al.,
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R.D. Lorenz / Planetary and Space Science 127 (2016) 91–94
2007; Vizcaino and Mehta, 2015) would be less damaging for a given lander weight. While this mission does have astrobiology as a focus, it is a rover and thus chemical concerns at the landing site could be addressed simply by driving out of the blast zone. It may be noted that in part because the lander weight and thrust was high, there was appreciable site modification, and indeed grit blasted up from the landing damaged one of the meteorology instruments.
2. Empirical observations of soft-lander disturbances Although the topographic excavation has been documented in small regions directly beneath various planetary landers by imaging from those platforms themselves (e.g. Hutton et al., 1980; Mehta et al., 2011; Arvidson et al., 2014) or from walking around nearby (Scott, 1969), the much wider areas that are altered more subtly, by chemical and/or microstructural changes, are most easily observed from orbit. Photometric disturbances of the lunar regolith were detected in Clementine data by Kreslavsky and Shkuratov (2003), and more recently in Lunar Reconnaissance Orbiter imaging data by Kaydash et al. (2011), Clegg et al. (2014) and Clegg-Watkins et al. (in press). The latter paper gives a compilation of the measured disturbance areas for five Apollo landers, four Lunas, four Surveyor landers and the recent Chang-E 3 landing. Here we have plotted those areas against the landed mass multiplied by lunar gravity (i.e. using weight as a proxy for the thrust at landing – since descent at constant velocity is a common control law for landing guidance, vertical force balance requires equivalence of thrust and weight and this estimate will in general be accurate to better than 10%) – see Fig. 1. To those lunar data, we have added the two most recent Mars landings (i.e. those that have occurred since the arrival of the Mars Reconnaissance Orbiter with its high-resolution imaging capability), namely Phoenix and Curiosity. The disturbed region for Phoenix (350 kg at landing) was outlined by eye on HiRISE public release image PSP 008591-2485 (Fig. 2) using the polygon tool in ImageJ: an area of 1020 m2 was derived. The disturbed region for Curiosity ( 2000 kg rover plus descent stage) was reported by
Fig. 1. Correlation of photometrically-disturbed areas for lunar and mars landers with weight at landing (i.e. thrust). It is seen that an empirical correlation is rather steeper ( weight1.5, solid line) than a simple linear dependence (dashed line). No systematic difference between Mars landers and lunar landers is apparent.
Fig. 2. HiRISE image PSP 008591-2485 showing the northern plains of Mars with a large discolored region around the Phoenix lander, disturbed by its retrorocket exhaust. For scale the heat shield is 150 m away: the much smaller disturbed patches associated with it and the parachute/backshell are due to the direct impact of those passive elements.
Arvidson et al. (2014) to extend 50–100 m from the landing site: adopting 75 m gives an area of 17,000 m2. Although the presence of the martian atmosphere influences the expansion of the rocket plume and the characteristics of the martian and lunar regoliths are different, on a log-log plot at least, it is seen that these two Mars data points are entirely consistent with the lunar landings of corresponding thrust. All else being equal, if a photometrically disturbed region corresponds to that where some threshold of pressure loading (for example) is exceeded, then one might expect the region to be proportional to the landing thrust (dashed line in Fig. 1). This does not quite seem to be what is observed: the aggregate dataset is rather better described by steeper dependence. Clegg-Watkins et al. (in press) offer a combined linear þquadraticþ constant fit to a smaller dataset, without theoretical justification (e.g. their constant term is unphysical). We suggest a simple power law dependence: although the scatter in the data might allow a reasonable linear fit, or an exponent as high as 2, the best fit appears to be a function of the form A¼0.02 T1.5, where A is the disturbed area in m2, and T the thrust ( ¼weight) at landing. A range of engineering factors (e.g. nozzles higher off the ground on larger vehicles, use of clustered nozzles as on Viking rather than single large motors) and physical processes (aerodynamic and pseudoballistic transport of sand and dust, gas flow through the regolith, heating, chemical deposition etc.) may be at work, and it is beyond the scope of the present paper to attempt to explain this dependence. A small ( 200 kg) lander requires a thrust for soft-landing of 260 N in Europa’s gravity, and thus if the correlation above that succeeds for Mars and lunar landings (the latter similar to Europa, both in terms of negligible atmospheric pressure and in surface gravity), it follows that a region about 80 m2 should be disturbed. In other words, regolith 9 m from the lander will have been perturbed to a degree comparable with photometrically-disturbed regions on the Moon. 9 m is larger than the reach of practical robotic arm designs, so sampling undisturbed surface material would demand some kind of deployable sampler, or mobility of
R.D. Lorenz / Planetary and Space Science 127 (2016) 91–94
the lander (the latter philosophy being embodied in the Curiosity rover).
3. Simple model for ammonia deposition For descent at a constant speed v (a typical control law applied for spacecraft landing, until engine cut-off a short distance above the ground h0), the engine thrust T equals the weight W. With Europa’s surface gravity of 1.314 m/s2, a 200-kg lander has a W¼262 N. Assuming monopropellant hydrazine thrusters with a vacuum specific impulse Isp of 220 seconds (as used on Curiosity and Phoenix), the mass flow ṁ = T /g0Isp required to generate thrust equal to the vehicle weight is 0.1 kg/s (from the definition of specific impulse: note this definition uses g0, the acceleration due to gravity on Earth). If thrusters are canted sideways, then thrust required to balance weight must be higher by a 1/cosine factor and the corresponding downward mass flux remains effectively the same. Plume expansion and interaction with the ground is complex (see e.g. Tosh et al., 2011) so for now we assume a simple uniform 45° half-cone as representative. It then follows for a height h above the ground that the mass flux d within the cone (i.e. over an area πh2) is d = T /πh2g0Isp . For vertical descent from infinity to height h0, the mass fluence D (i.e. the time integral of the above expression for d) for any point within h0 of directly beneath the lander can be shown by simple algebra to be D = T /πvh0g0Isp . Clearly, the higher above the ground the engines are cut off, the lower the deposition. Similarly, a faster descent speed leads to a reduced exposure time and thus lower D. Setting T¼ W, and using the MSL/Curiosity descent law (v ¼0.75 m/s, h0 10 m) we find D¼ 0.005 kg/m2. At some distance x (x 4h0), the deposition is simply obtained by substituting x for h0 in this expression (i.e. it falls off with the inverse of distance). To obtain an ammonia deposition rate we scale this result by factors ε and η which embody the fraction of exhaust gas that is ammonia, and the fraction of that which'sticks’ to the ground, respectively. This latter factor, an'accommodation coefficient’, depends on regolith texture and temperature: at very low temperatures it approaches unity (i.e. cryotrapping). It should be noted that in this respect landings on Mars and on the daytime moon may be quite different from landing on Europa, where equatorial surface temperatures (e.g. Spencer et al., 1999) are in the range 80– 130 K (and higher-latitude temperatures are lower still.) At these low Europa temperatures, ammonia is much more likely to ‘stick’ to the surfaces of regolith grains: laboratory experimentation and/ or detailed modeling to assess the retention of ammonia on cold ice regolith would be of some value. It may be noted that no indication of rocket exhaust contamination was noted in the (much warmer) regolith samples returned by Apollo. Measurements on Phoenix plume exhaust suggest an ammonia concentration of ε 45% (Plemmons et al., 2008), somewhat higher than some models and previous measurements (Lyons, 1971). For now we will assume εη 0.1, and thus some 500 mg/m2 of ammonia is deposited1. It is not clear how deep such material 1 Unreacted hydrazine was estimated at less than 0.2%. Note that if a more efficient bipropellant thruster system were used (typically monomethyl or dimethyl hydrazine, with nitrogen tetroxide), the specific impulse would be somewhat higher, reducing the mass fluxes required for a given thrust. However, the exhaust would now contain not just ammonia, hydrogen and nitrogen, but also water vapor, carbon dioxide and most likely some organics from partially-combusted fuel. These organics would likely include amines which – if endogenous – would be of great astrobiological interest. Christner et al. (2006) report, e.g. 10 8 mol/l of amino acids in the accretion ice above Lake Vostok; see also Priscu et al. (1999).
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might be deposited – if a regolith has significant porosity, exhaust gases may migrate substantially beneath the surface (e.g. Scott and Ko, 1968). Even if this ammonia deposition is spread through a 10 cm column of regolith (comparable with the depths excavated by previous landers such as Viking and Phoenix), the increment in nitrogen abundance would be 5 ppm. This may be compared with, for example, the abundance of available nitrogen compounds (i.e. excluding dissolved N2) in terrestrial ocean water of a few tens of micromol/kg (i.e. o1 ppm), while the abundance of ammonium ions specifically is almost three orders of magnitude smaller. In the absence of nitrogen fixation by lightning, nitrogen-fixing bacteria, and industrial fertilizer production (e.g. by the Haber process) which dominate fixation on Earth, available nitrogen including ammonium on Europa might be imagined to be very scarce, and thus the contamination of the surface by rocket exhaust may be problematic for assessing the habitability of the ocean.
4. Conclusions This study has pointed out that the chemistry of rocket propulsion, and the particulars of the Europa surface environment, make conventional soft-landing approaches problematic for any attempt to examine nitrogen compounds as biomarkers or habitability indicators at the immediate landing site: deposited nitrogenous material may substantially exceed the accessible ambient inventory. A more detailed examination (e.g. studying plume impingement and molecular accommodation on the regolith with more sophisticated flow and trapping models, e.g. Discrete Simulation Monte Carlo DSMC, Lane et al. (2008, 2010), and/or by laboratory experimentation) may be required to support any contention of accurate assaying of regolith nitrogen compounds insitu. Mitigations might include the use of alternative propellant chemistries (e.g. hydrogen peroxide) or landing approaches that terminate propulsion at a higher altitude to reduce exhaust deposition. These approaches (e.g. penetrators or semi-hard landers) will degrade lander mass performance and/or introduce higher impact loads, posing strong challenges to lander and instrument engineers. Mobility or novel remote sample acquisition techniques may be more practicable overall, but also pose cost and risk.
Acknowledgment This work was supported by NASA via the Jet Propulsion Laboratory.
References Arvidson, R.E., Bellutta, P., Calef, F., Fraeman, A.A., Garvin, J.B., Gasnault, O., Grant, J. A., Grotzinger, J.P., Hamilton, V.E., Heverly, M., Iagnemma, K.A., 2014. Terrain physical properties derived from orbital data and the first 360 sols of Mars Science Laboratory Curiosity rover observations in Gale Crater. J. Geophys. Res.: Planets 119 (6), 1322–1344. http://dx.doi.org/10.1002/2013JE004605. Christner, B.C., Royston-Bishop, G., Foreman, C.M., Arnold, B.R., Tranter, M., Welch, K.A., Priscu, J.C., 2006. Limnological conditions in subglacial Lake Vostok, Antarctica. Limnol. Oceanogr. 51 (6), 2485–2501. Clark, L.V., 1970. Effect of retrorocket cant angle on ground erosion – A scaled Viking study. NASA Technical Memorandum, NASA TM X-2075, NASA Langley Research Center. Hampton, VA. Clegg, R.N., Jolliff, B.L., Robinson, M.S., Hapke, B.W., Plescia, J.B., 2014. Effects of rocket exhaust on lunar soil reflectance properties. Icarus 227, 176–194. Clegg-Watkins, R.N., Jolliff, B.L., Boyd, A., Robinson, M.S., Wagner, R., Stopar, J.D., Plescia, J.B., Speyerer, E.J. in press. Photometric characterization of the Chang’e3 landing site using LROC NAC images, Icarus. Consolidated Appropriations Act, 2016. Text of House amendment #1 to the Senate amendment to H.R. 2029, Military Construction and Veterans Affairs and Related Agencies Appropriations Act 2016. (Rules Committee print 114-39, 〈http://
94
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docs.house.gov/billsthisweek/20151214/CPRT-114-HPRT-RU00-SAHR2029AMNT1final.pdf〉, downloaded April 23, 2016). Dawson, M., Brewster, G., Conrad, C., Kilwine, M., Chenevert, B. and Morgan, O., 2007. Monopropellant hydrazine 700 lbf throttling terminal descent engine for Mars Science Laboratory. AIAA-2007-5481 presented at the 43rd AIAA/ASME/ SAE/ASEE Joint Propulsion Conference and Exhibit. Cincinnati, OH, July, pp. 8– 11. Hand, E., 2015. Plumes on Europa tease NASA mission planners. Science 347 (6225), 932–933. Hand, K.P., Chyba, C.F., Priscu, J.C., Carlson, R.W., Nealson, K.H., 2009. Astrobiology and the potential for life on Europa. Europa. University of Arizona Press, Tucson, pp. 589–629. Holzer, G., Oro, J., 1977. Pyrolysis of organic compounds in the presence of ammonia. The Viking Mars lander site alteration experiment. Org. Geochem. 1 (1), 37–52. Hutton, R.E., Moore, H.J., Scott, R.F., Shorthill, R.W., Spitzer, C.R., 1980. Surface erosion caused on Mars from Viking descent engine plume. The moon and the planets 23 (3), 293–305. Kaydash, V., et al., 2011. Photometric anomalies in the Apollo landing sites as seen from the Lunar Reconnaissance Orbiter. Icarus 211, 89–96. Kreslavsky, M.A., Shkuratov, Y.G., 2003. Photometric anomalies of the lunar surface: results from Clementine data. J. Geophys. Res.: Planets 108 (E3). http://dx.doi. org/10.1029/2002JE001937. Lane, J., Metzger, P.T., Immer, C.D., 2008. Lagrangian trajectory modeling of lunar dust particles. In: Proceedings of Earth and Space 2008, Paper Presented at 11th Biennial ASCE Aerospace Division International Conference on Engineering, Construction and Operations in Challenging Environments. Long Beach, CA. Lane, J., Metzger, P.T., Carlson, J.W., 2010. Lunar dust particles blown by lander exhaust in rarefied and compressible flow. Paper Presented at 12th Biennial ASCE Aerospace Division International Conference on Engineering, Construction, and Operations in Challenging Environments. Honolulu, HI. Lorenz, R.D., Mitton, J., 2008. Titan Unveiled. Princeton University Press. Lunine, J.I., Stevenson, D.J., 1982. Formation of the Galilean satellites in a gaseous nebula. Icarus 52 (1), 14–39. Lyon, W.C., 1971. Monopropellant thruster exhaust effects upon spacecraft. J. Spacecr. Rocket. 8 (7), 689–701. Mehta, M., Renno, N.O., Marshall, J., Grover, M.R., Sengupta, A., Rusche, N.A., Kok, J. F., Arvidson, R.E., Markiewicz, W.J., Lemmon, M.T., Smith, P.H., 2011. Explosive
erosion during the Phoenix landing exposes subsurface water on Mars. Icarus 211 (1), 172–194. Mehta, M., Sengupta, A., Renno, N.O., Norman, J.W.V., Huseman, P.G., Gulick, D.S., Pokora, M., 2013. Thruster plume surface interactions: Applications for spacecraft landings on planetary bodies. AIAA J. 51 (12), 2800–2818. Metzger, P.T., et al., 2009. ISRU implications for lunar and martian plume effects. In: Proceedings of 47th AIAA Aerospace Sciences Conference. Orlando, FL. AIAA 2009-1204. Metzger, P.T., Smith, J., Lane, J.E., 2011. Phenomenology of soil erosion due to rocket exhaust on the Moon and the Mauna Kea test site. J. Geophys. Res. – Planets 116, 1–22. Morris, A.B., et al., 2015. Approach for modeling rocket plume impingement and dust dispersal on the Moon. J. Spacecr. Rockets 52, 362–374. Plemmons, D.H., Mehta, M., Clark, B.C., Kounaves, S.P., Peach, L.L., Renno, N.O., Tamppari, L., Young, S.M.M., 2008. Effects of the Phoenix Lander descent thruster plume on the Martian surface. J. Geophys. Res.: Planets 113 (E3), 113. http://dx.doi.org/10.1029/2007JE003059. Priscu, J.C., Adams, E.E., Lyons, W.B., Voytek, M.A., Mogk, D.W., Brown, R.L., Avci, R., 1999. Geomicrobiology of subglacial ice above Lake Vostok, Antarctica. Science 286 (5447), 2141–2144. Scott, R.F., 1969. Apollo Program Soil Mechanics Experiment: Final Report. NASA CR144435. Scott, R.F., Ko, H.Y., 1968. Transient rocket-engine gas flow in soil. AIAA J. 6 (2), 258–264. Sengupta, A., Kulleck, J., Sell, S., Van Norman, J., Mehta, M. and Pokora, M., 2009. Mars lander engine plume impingement environment of the Mars Science Laboratory. In: Proceedings of the IEEE Aerospace Conference. Big Sky, MT, March 2009. Spencer, J.R., Tamppari, L.K., Martin, T.Z., Travis, L.D., 1999. Temperatures on Europa from Galileo photopolarimeter-radiometer: nighttime thermal anomalies. Science 284 (5419), 1514–1516. Tosh, A., Liever, P.A., Arslanbekov, R.R., Habchi, S.D., 2011. Numerical analysis of spacecraft rocket plume impingement under lunar environment. J. Spacecr. Rockets 4 (1), 93–102. Vizcaino, J., Mehta, M., 2015. Quantification of plume-soil interaction and excavation due to the sky crane descent stage. AIAA SciTech, 8th Symposium on Space Resource Utilization, AIAA 2015-1649. Kissimmee, Florida. 5–9 January 2015, p. 10.
Contents lists available at ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
Short communication
Lander rocket exhaust effects on Europa regolith nitrogen assays Ralph D. Lorenz n Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 9 March 2016 Received in revised form 24 April 2016 Accepted 30 April 2016 Available online 7 May 2016
Soft-landings on large worlds such as Europa or our Moon require near-surface retropropulsion, which leads to impingement of the rocket plume on the surface. Surface modification by such plumes was documented on Apollo and Surveyor, and on Mars by Viking, Curiosity and especially Phoenix. The low temperatures of the Europan regolith may lead to efficient trapping of ammonia, a principal component of the exhaust from monopropellant hydrazine thrusters. Deposited ammonia may react with any trace organics, and may overwhelm the chemical and isotopic signatures of any endogenous nitrogen compounds, which are likely rare on Europa. An empirical correlation of the photometrically-altered regions (‘blast zones’) around prior lunar and Mars landings is made, indicating A¼0.02T1.5, where A is the area in m2 and W is the lander weight (thus, thrust) at landing in N: this suggests surface alteration will occur out to a distance of 9 m from a 200 kg lander on Europa. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Europa Astrobiology Nitrogen Thruster exhaust
1. Introduction Europa has attracted much interest as a target of astrobiological exploration (e.g. Hand et al., 2009; Hand, 2015). While long of scientific interest, the claimed prospect of habitable conditions or even life in Europa’s ice-covered water ocean has attracted the attention of politicians and the public. The United States Congressional budget language (Consolidated Appropriations Act, 2016) for FY2016 mandates “ an orbiter with a lander.” Thus renewed efforts are underway to evaluate landing concepts to explore Europa's habitability and search for extant life. Living things contain substantial amounts of nitrogen (the Redfield ratio of Carbon:Nitrogen is 106:16).Nitrogen is an essential element for life: obvious examples include the amino acids that form proteins to execute living functions, and the purine and pyrimidine bases that encode information in DNA and RNA. However, Europa may be nitrogen-starved, as it formed in the warm protoJovian nebula in conditions which did not favor the accumulation of materials more volatile than water (Lunine and Stevenson, 1982). This formation scenario was unlike Titan whose abundant nitrogen (e.g. Lorenz and Mitton, 2008) accreted as ammonia and whose methane presumably accreted as clathrate. Like Europa’s meager carbon inventory, the only supply of nitrogen to Europa’s surface may be the small sporadic delivery of cometary material. Understanding the amount and form of any nitrogen in the n
Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.pss.2016.04.008 0032-0633/& 2016 Elsevier Ltd. All rights reserved.
Europan regolith (and by implication, its ocean) is a key goal of future exploration, and a lander in particular. However, since Europa is an airless body with a substantial gravitational field, soft landing will likely require rocket propulsion which may deposit nitrogen-bearing compounds from thruster exhaust around the landing site. In this paper I briefly explore quantification of this concern, drawing on observations of landing site disturbances on previous lander missions and with a simple thruster plume deposition and chemistry model. To set some historical context, this problem received considerable attention during preparation of the Viking lander missions, whose focus was similarly on astrobiology. Studies of thruster plume impingement (e.g. Clark, 1970) led to a clusterednozzle design to attempt to minimize mechanical disturbance, and some laboratory work was performed (Holzer and Oro, 1977) to assess how thruster exhaust compounds might perturb chemical analyses of the regolith. The Phoenix lander demonstrated quite strong excavation underneath its thrusters, with the fortuitous effect (Plemmons et al., 2008; Mehta et al., 2011) of exposing subsurface ice deposits. This effect seems to be a result of the pulsed thrust modulation (Mehta et al., 2013) which causes transient flow in the regolith (Scott and Ko, 1968; Metzger et al., 2009, 2011 and Morris et al. (2015) also discuss recirculation vortices in rocket exhaust and pit formation in regolith. However, astrobiology was not a focus of this mission, so deleterious effects of exhaust were not a concern. The Mars Science Laboratory (MSL), Curiosity, however would have confronted issues with site modification, even though its sky-crane landing and throttleable rather than pulsed thrusters (e.g. Sengupta et al., 2009; Dawson et al.,
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2007; Vizcaino and Mehta, 2015) would be less damaging for a given lander weight. While this mission does have astrobiology as a focus, it is a rover and thus chemical concerns at the landing site could be addressed simply by driving out of the blast zone. It may be noted that in part because the lander weight and thrust was high, there was appreciable site modification, and indeed grit blasted up from the landing damaged one of the meteorology instruments.
2. Empirical observations of soft-lander disturbances Although the topographic excavation has been documented in small regions directly beneath various planetary landers by imaging from those platforms themselves (e.g. Hutton et al., 1980; Mehta et al., 2011; Arvidson et al., 2014) or from walking around nearby (Scott, 1969), the much wider areas that are altered more subtly, by chemical and/or microstructural changes, are most easily observed from orbit. Photometric disturbances of the lunar regolith were detected in Clementine data by Kreslavsky and Shkuratov (2003), and more recently in Lunar Reconnaissance Orbiter imaging data by Kaydash et al. (2011), Clegg et al. (2014) and Clegg-Watkins et al. (in press). The latter paper gives a compilation of the measured disturbance areas for five Apollo landers, four Lunas, four Surveyor landers and the recent Chang-E 3 landing. Here we have plotted those areas against the landed mass multiplied by lunar gravity (i.e. using weight as a proxy for the thrust at landing – since descent at constant velocity is a common control law for landing guidance, vertical force balance requires equivalence of thrust and weight and this estimate will in general be accurate to better than 10%) – see Fig. 1. To those lunar data, we have added the two most recent Mars landings (i.e. those that have occurred since the arrival of the Mars Reconnaissance Orbiter with its high-resolution imaging capability), namely Phoenix and Curiosity. The disturbed region for Phoenix (350 kg at landing) was outlined by eye on HiRISE public release image PSP 008591-2485 (Fig. 2) using the polygon tool in ImageJ: an area of 1020 m2 was derived. The disturbed region for Curiosity ( 2000 kg rover plus descent stage) was reported by
Fig. 1. Correlation of photometrically-disturbed areas for lunar and mars landers with weight at landing (i.e. thrust). It is seen that an empirical correlation is rather steeper ( weight1.5, solid line) than a simple linear dependence (dashed line). No systematic difference between Mars landers and lunar landers is apparent.
Fig. 2. HiRISE image PSP 008591-2485 showing the northern plains of Mars with a large discolored region around the Phoenix lander, disturbed by its retrorocket exhaust. For scale the heat shield is 150 m away: the much smaller disturbed patches associated with it and the parachute/backshell are due to the direct impact of those passive elements.
Arvidson et al. (2014) to extend 50–100 m from the landing site: adopting 75 m gives an area of 17,000 m2. Although the presence of the martian atmosphere influences the expansion of the rocket plume and the characteristics of the martian and lunar regoliths are different, on a log-log plot at least, it is seen that these two Mars data points are entirely consistent with the lunar landings of corresponding thrust. All else being equal, if a photometrically disturbed region corresponds to that where some threshold of pressure loading (for example) is exceeded, then one might expect the region to be proportional to the landing thrust (dashed line in Fig. 1). This does not quite seem to be what is observed: the aggregate dataset is rather better described by steeper dependence. Clegg-Watkins et al. (in press) offer a combined linear þquadraticþ constant fit to a smaller dataset, without theoretical justification (e.g. their constant term is unphysical). We suggest a simple power law dependence: although the scatter in the data might allow a reasonable linear fit, or an exponent as high as 2, the best fit appears to be a function of the form A¼0.02 T1.5, where A is the disturbed area in m2, and T the thrust ( ¼weight) at landing. A range of engineering factors (e.g. nozzles higher off the ground on larger vehicles, use of clustered nozzles as on Viking rather than single large motors) and physical processes (aerodynamic and pseudoballistic transport of sand and dust, gas flow through the regolith, heating, chemical deposition etc.) may be at work, and it is beyond the scope of the present paper to attempt to explain this dependence. A small ( 200 kg) lander requires a thrust for soft-landing of 260 N in Europa’s gravity, and thus if the correlation above that succeeds for Mars and lunar landings (the latter similar to Europa, both in terms of negligible atmospheric pressure and in surface gravity), it follows that a region about 80 m2 should be disturbed. In other words, regolith 9 m from the lander will have been perturbed to a degree comparable with photometrically-disturbed regions on the Moon. 9 m is larger than the reach of practical robotic arm designs, so sampling undisturbed surface material would demand some kind of deployable sampler, or mobility of
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the lander (the latter philosophy being embodied in the Curiosity rover).
3. Simple model for ammonia deposition For descent at a constant speed v (a typical control law applied for spacecraft landing, until engine cut-off a short distance above the ground h0), the engine thrust T equals the weight W. With Europa’s surface gravity of 1.314 m/s2, a 200-kg lander has a W¼262 N. Assuming monopropellant hydrazine thrusters with a vacuum specific impulse Isp of 220 seconds (as used on Curiosity and Phoenix), the mass flow ṁ = T /g0Isp required to generate thrust equal to the vehicle weight is 0.1 kg/s (from the definition of specific impulse: note this definition uses g0, the acceleration due to gravity on Earth). If thrusters are canted sideways, then thrust required to balance weight must be higher by a 1/cosine factor and the corresponding downward mass flux remains effectively the same. Plume expansion and interaction with the ground is complex (see e.g. Tosh et al., 2011) so for now we assume a simple uniform 45° half-cone as representative. It then follows for a height h above the ground that the mass flux d within the cone (i.e. over an area πh2) is d = T /πh2g0Isp . For vertical descent from infinity to height h0, the mass fluence D (i.e. the time integral of the above expression for d) for any point within h0 of directly beneath the lander can be shown by simple algebra to be D = T /πvh0g0Isp . Clearly, the higher above the ground the engines are cut off, the lower the deposition. Similarly, a faster descent speed leads to a reduced exposure time and thus lower D. Setting T¼ W, and using the MSL/Curiosity descent law (v ¼0.75 m/s, h0 10 m) we find D¼ 0.005 kg/m2. At some distance x (x 4h0), the deposition is simply obtained by substituting x for h0 in this expression (i.e. it falls off with the inverse of distance). To obtain an ammonia deposition rate we scale this result by factors ε and η which embody the fraction of exhaust gas that is ammonia, and the fraction of that which'sticks’ to the ground, respectively. This latter factor, an'accommodation coefficient’, depends on regolith texture and temperature: at very low temperatures it approaches unity (i.e. cryotrapping). It should be noted that in this respect landings on Mars and on the daytime moon may be quite different from landing on Europa, where equatorial surface temperatures (e.g. Spencer et al., 1999) are in the range 80– 130 K (and higher-latitude temperatures are lower still.) At these low Europa temperatures, ammonia is much more likely to ‘stick’ to the surfaces of regolith grains: laboratory experimentation and/ or detailed modeling to assess the retention of ammonia on cold ice regolith would be of some value. It may be noted that no indication of rocket exhaust contamination was noted in the (much warmer) regolith samples returned by Apollo. Measurements on Phoenix plume exhaust suggest an ammonia concentration of ε 45% (Plemmons et al., 2008), somewhat higher than some models and previous measurements (Lyons, 1971). For now we will assume εη 0.1, and thus some 500 mg/m2 of ammonia is deposited1. It is not clear how deep such material 1 Unreacted hydrazine was estimated at less than 0.2%. Note that if a more efficient bipropellant thruster system were used (typically monomethyl or dimethyl hydrazine, with nitrogen tetroxide), the specific impulse would be somewhat higher, reducing the mass fluxes required for a given thrust. However, the exhaust would now contain not just ammonia, hydrogen and nitrogen, but also water vapor, carbon dioxide and most likely some organics from partially-combusted fuel. These organics would likely include amines which – if endogenous – would be of great astrobiological interest. Christner et al. (2006) report, e.g. 10 8 mol/l of amino acids in the accretion ice above Lake Vostok; see also Priscu et al. (1999).
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might be deposited – if a regolith has significant porosity, exhaust gases may migrate substantially beneath the surface (e.g. Scott and Ko, 1968). Even if this ammonia deposition is spread through a 10 cm column of regolith (comparable with the depths excavated by previous landers such as Viking and Phoenix), the increment in nitrogen abundance would be 5 ppm. This may be compared with, for example, the abundance of available nitrogen compounds (i.e. excluding dissolved N2) in terrestrial ocean water of a few tens of micromol/kg (i.e. o1 ppm), while the abundance of ammonium ions specifically is almost three orders of magnitude smaller. In the absence of nitrogen fixation by lightning, nitrogen-fixing bacteria, and industrial fertilizer production (e.g. by the Haber process) which dominate fixation on Earth, available nitrogen including ammonium on Europa might be imagined to be very scarce, and thus the contamination of the surface by rocket exhaust may be problematic for assessing the habitability of the ocean.
4. Conclusions This study has pointed out that the chemistry of rocket propulsion, and the particulars of the Europa surface environment, make conventional soft-landing approaches problematic for any attempt to examine nitrogen compounds as biomarkers or habitability indicators at the immediate landing site: deposited nitrogenous material may substantially exceed the accessible ambient inventory. A more detailed examination (e.g. studying plume impingement and molecular accommodation on the regolith with more sophisticated flow and trapping models, e.g. Discrete Simulation Monte Carlo DSMC, Lane et al. (2008, 2010), and/or by laboratory experimentation) may be required to support any contention of accurate assaying of regolith nitrogen compounds insitu. Mitigations might include the use of alternative propellant chemistries (e.g. hydrogen peroxide) or landing approaches that terminate propulsion at a higher altitude to reduce exhaust deposition. These approaches (e.g. penetrators or semi-hard landers) will degrade lander mass performance and/or introduce higher impact loads, posing strong challenges to lander and instrument engineers. Mobility or novel remote sample acquisition techniques may be more practicable overall, but also pose cost and risk.
Acknowledgment This work was supported by NASA via the Jet Propulsion Laboratory.
References Arvidson, R.E., Bellutta, P., Calef, F., Fraeman, A.A., Garvin, J.B., Gasnault, O., Grant, J. A., Grotzinger, J.P., Hamilton, V.E., Heverly, M., Iagnemma, K.A., 2014. Terrain physical properties derived from orbital data and the first 360 sols of Mars Science Laboratory Curiosity rover observations in Gale Crater. J. Geophys. Res.: Planets 119 (6), 1322–1344. http://dx.doi.org/10.1002/2013JE004605. Christner, B.C., Royston-Bishop, G., Foreman, C.M., Arnold, B.R., Tranter, M., Welch, K.A., Priscu, J.C., 2006. Limnological conditions in subglacial Lake Vostok, Antarctica. Limnol. Oceanogr. 51 (6), 2485–2501. Clark, L.V., 1970. Effect of retrorocket cant angle on ground erosion – A scaled Viking study. NASA Technical Memorandum, NASA TM X-2075, NASA Langley Research Center. Hampton, VA. Clegg, R.N., Jolliff, B.L., Robinson, M.S., Hapke, B.W., Plescia, J.B., 2014. Effects of rocket exhaust on lunar soil reflectance properties. Icarus 227, 176–194. Clegg-Watkins, R.N., Jolliff, B.L., Boyd, A., Robinson, M.S., Wagner, R., Stopar, J.D., Plescia, J.B., Speyerer, E.J. in press. Photometric characterization of the Chang’e3 landing site using LROC NAC images, Icarus. Consolidated Appropriations Act, 2016. Text of House amendment #1 to the Senate amendment to H.R. 2029, Military Construction and Veterans Affairs and Related Agencies Appropriations Act 2016. (Rules Committee print 114-39, 〈http://
94
R.D. Lorenz / Planetary and Space Science 127 (2016) 91–94
docs.house.gov/billsthisweek/20151214/CPRT-114-HPRT-RU00-SAHR2029AMNT1final.pdf〉, downloaded April 23, 2016). Dawson, M., Brewster, G., Conrad, C., Kilwine, M., Chenevert, B. and Morgan, O., 2007. Monopropellant hydrazine 700 lbf throttling terminal descent engine for Mars Science Laboratory. AIAA-2007-5481 presented at the 43rd AIAA/ASME/ SAE/ASEE Joint Propulsion Conference and Exhibit. Cincinnati, OH, July, pp. 8– 11. Hand, E., 2015. Plumes on Europa tease NASA mission planners. Science 347 (6225), 932–933. Hand, K.P., Chyba, C.F., Priscu, J.C., Carlson, R.W., Nealson, K.H., 2009. Astrobiology and the potential for life on Europa. Europa. University of Arizona Press, Tucson, pp. 589–629. Holzer, G., Oro, J., 1977. Pyrolysis of organic compounds in the presence of ammonia. The Viking Mars lander site alteration experiment. Org. Geochem. 1 (1), 37–52. Hutton, R.E., Moore, H.J., Scott, R.F., Shorthill, R.W., Spitzer, C.R., 1980. Surface erosion caused on Mars from Viking descent engine plume. The moon and the planets 23 (3), 293–305. Kaydash, V., et al., 2011. Photometric anomalies in the Apollo landing sites as seen from the Lunar Reconnaissance Orbiter. Icarus 211, 89–96. Kreslavsky, M.A., Shkuratov, Y.G., 2003. Photometric anomalies of the lunar surface: results from Clementine data. J. Geophys. Res.: Planets 108 (E3). http://dx.doi. org/10.1029/2002JE001937. Lane, J., Metzger, P.T., Immer, C.D., 2008. Lagrangian trajectory modeling of lunar dust particles. In: Proceedings of Earth and Space 2008, Paper Presented at 11th Biennial ASCE Aerospace Division International Conference on Engineering, Construction and Operations in Challenging Environments. Long Beach, CA. Lane, J., Metzger, P.T., Carlson, J.W., 2010. Lunar dust particles blown by lander exhaust in rarefied and compressible flow. Paper Presented at 12th Biennial ASCE Aerospace Division International Conference on Engineering, Construction, and Operations in Challenging Environments. Honolulu, HI. Lorenz, R.D., Mitton, J., 2008. Titan Unveiled. Princeton University Press. Lunine, J.I., Stevenson, D.J., 1982. Formation of the Galilean satellites in a gaseous nebula. Icarus 52 (1), 14–39. Lyon, W.C., 1971. Monopropellant thruster exhaust effects upon spacecraft. J. Spacecr. Rocket. 8 (7), 689–701. Mehta, M., Renno, N.O., Marshall, J., Grover, M.R., Sengupta, A., Rusche, N.A., Kok, J. F., Arvidson, R.E., Markiewicz, W.J., Lemmon, M.T., Smith, P.H., 2011. Explosive
erosion during the Phoenix landing exposes subsurface water on Mars. Icarus 211 (1), 172–194. Mehta, M., Sengupta, A., Renno, N.O., Norman, J.W.V., Huseman, P.G., Gulick, D.S., Pokora, M., 2013. Thruster plume surface interactions: Applications for spacecraft landings on planetary bodies. AIAA J. 51 (12), 2800–2818. Metzger, P.T., et al., 2009. ISRU implications for lunar and martian plume effects. In: Proceedings of 47th AIAA Aerospace Sciences Conference. Orlando, FL. AIAA 2009-1204. Metzger, P.T., Smith, J., Lane, J.E., 2011. Phenomenology of soil erosion due to rocket exhaust on the Moon and the Mauna Kea test site. J. Geophys. Res. – Planets 116, 1–22. Morris, A.B., et al., 2015. Approach for modeling rocket plume impingement and dust dispersal on the Moon. J. Spacecr. Rockets 52, 362–374. Plemmons, D.H., Mehta, M., Clark, B.C., Kounaves, S.P., Peach, L.L., Renno, N.O., Tamppari, L., Young, S.M.M., 2008. Effects of the Phoenix Lander descent thruster plume on the Martian surface. J. Geophys. Res.: Planets 113 (E3), 113. http://dx.doi.org/10.1029/2007JE003059. Priscu, J.C., Adams, E.E., Lyons, W.B., Voytek, M.A., Mogk, D.W., Brown, R.L., Avci, R., 1999. Geomicrobiology of subglacial ice above Lake Vostok, Antarctica. Science 286 (5447), 2141–2144. Scott, R.F., 1969. Apollo Program Soil Mechanics Experiment: Final Report. NASA CR144435. Scott, R.F., Ko, H.Y., 1968. Transient rocket-engine gas flow in soil. AIAA J. 6 (2), 258–264. Sengupta, A., Kulleck, J., Sell, S., Van Norman, J., Mehta, M. and Pokora, M., 2009. Mars lander engine plume impingement environment of the Mars Science Laboratory. In: Proceedings of the IEEE Aerospace Conference. Big Sky, MT, March 2009. Spencer, J.R., Tamppari, L.K., Martin, T.Z., Travis, L.D., 1999. Temperatures on Europa from Galileo photopolarimeter-radiometer: nighttime thermal anomalies. Science 284 (5419), 1514–1516. Tosh, A., Liever, P.A., Arslanbekov, R.R., Habchi, S.D., 2011. Numerical analysis of spacecraft rocket plume impingement under lunar environment. J. Spacecr. Rockets 4 (1), 93–102. Vizcaino, J., Mehta, M., 2015. Quantification of plume-soil interaction and excavation due to the sky crane descent stage. AIAA SciTech, 8th Symposium on Space Resource Utilization, AIAA 2015-1649. Kissimmee, Florida. 5–9 January 2015, p. 10.