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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, E00E14, doi:10.1029/2009JE003420, 2010
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Full Article
Initial results from the thermal and electrical conductivity probe (TECP) on Phoenix Aaron P. Zent,1 Michael H. Hecht,2 Doug R. Cobos,3 Stephen E. Wood,4 Troy L. Hudson,2 Sarah M. Milkovich,2 Lauren P. DeFlores,2 and Michael T. Mellon5 Received 5 May 2009; revised 18 August 2009; accepted 30 September 2009; published 27 March 2010.
[1] The thermal and electrical conductivity probe (TECP), a component of the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA), was included on the Phoenix Lander to conduct in situ measurements of the exchange of heat and water in the Martian polar terrain. TECP measured regolith thermal conductivity, heat capacity, temperature, electrical conductivity, and dielectric permittivity throughout the mission. A relative humidity sensor returned the first in situ humidity measurements from the Martian surface. The dry overburden above the ground ice is a good thermal insulator (average = 0.085 W m−1 K−1 and average Cr = 1.05 × 106 J m−3 K−1). Surface thermal inertia (I) calculated from these values agrees well with daytime orbital determinations, but differences in the spatial and temporal scale of heat transport lead to very different measurements at night. Electrical conductivity was consistent with open circuit throughout the mission; an upper limit conductivity of 2 nS cm−1 is derived. Bulk dielectric permittivity ("b) shows several puzzling signals but also a systematic increase overnight in the latter half of the mission, contemporaneous with H2O adsorption. The magnitude of the increase is difficult to reconcile with expected changes in unfrozen water. Atmospheric H2O averages around 1.8 Pa during the day, corresponding to a RH < 5%. At night, much of the H2O disappears from the atmosphere, and RH increases to ∼100%. Temperature and H2O partial pressure data suggest that adsorption on mineral surfaces plays a major role in scrubbing H2O, with a possible contribution from perchlorate salts. Citation: Zent, A. P., M. H. Hecht, D. R. Cobos, S. E. Wood, T. L. Hudson, S. M. Milkovich, L. P. DeFlores, and M. T. Mellon (2010), Initial results from the thermal and electrical conductivity probe (TECP) on Phoenix, J. Geophys. Res., 115, E00E14, doi:10.1029/2009JE003420.
1. Introduction [2] The Phoenix Lander touched down on 25 May 2008 at 68.22°N, 234.25°E, an area that is fairly typical of the Vastitas Borealis plains. Operations lasted 152 sols, from Ls 78° through Ls 148°, early to mid northern summer. At that site and season, mass and energy transfer between the surface and atmosphere; the details of that exchange are key to the function of the current Martian climate. Characterization and understanding of exchange processes, their role in the present climate, and ultimately, the past and future Martian climates was a major Phoenix goal.
1
NASA Ames Research Center, Moffett Field, California, USA. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. 3 Decagon Devices, Pullman, Washington, USA. 4 Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA. 5 Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA. 2
Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JE003420
[3] The thermal and electrical conductivity probe (TECP), a component of the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) was mounted on the Robotic Arm (RA) of the Phoenix Lander, to conduct in situ measurements of the exchange of heat and water in the Martian polar terrain. Based on the small, dual‐probe sensors that are routinely used to monitor soil thermal properties and water content, the TECP performs 6 distinct measurements: dielectric permittivity, electrical conductivity, temperature, thermal conductivity, volumetric heat capacity, and relative humidity. 1.1. Current Climate [4] Polar summer on Mars is dynamic, and the interactions between the Martian volatile inventory, the cycles of solar insolation, and the geologic materials that constitute key volatile reservoirs were a key focus for TECP. Fundamental scientific questions addressed by TECP bear on many processes and time scales that collectively define the Martian climate. 1.1.1. Diurnal Cycle [5] The diurnal variability of the H2O column reflects the fundamental physics that control its transport through the
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Figure 1. The TECP, (top) mounted on the end effector, at the distal end of the 2.3 m long Robotic Arm. (bottom) Close‐up of TECP, with the function of the various needles identified, and the location of the humidity sensor identified. Planetary Boundary Layer (PBL). Observational evidence has accumulated that suggests day‐night variations on the order of factors of 2–3, [Sprague et al., 1996; Titov et al., 1999; Formisano et al., 2001], but the sources, sinks, and exchange mechanisms remain uncertain [Zent et al., 1993; Määttänen and Savijärvi, 2004]. [6] The vertical distribution of water vapor is another important factor in the consideration of regolith exchange [Jakosky et al., 1997]. The vertical distribution is governed by processes of vertical mixing within the atmosphere, saturation at some altitudes and times of day, depending on the temperature and the vapor partial pressure, as well as the microphysics of any resulting condensates. Results from the Planetary Fourier Spectrometer (PFS) suggest the possibility that atmospheric H2O is preferentially concentrated in the lowest few kilometers of the atmosphere at high northern latitudes during summer [Formisano et al., 2001; Tschimmel et al., 2008]. [7] The diffusion of H2O through the regolith is a rate‐ limiting process in regolith atmosphere exchange, and considerable laboratory work has been done to elucidate the key physics and rates [Hudson et al., 2007; Bryson et al., 2008; Sizemore and Mellon, 2008]. Regolith diffusion is a barrier to equilibration, and can act to preserve ice. Thus, for example, present subsurface midlatitude to low‐latitude water deposits might be remnants from periods of high obliquity [Head et al., 2003]. 1.1.2. Seasonal Cycle [8] The Phoenix landing site is part of the circumpolar terrain covered annually by the seasonal CO2 cap. Within that perimeter, the surface energy balance and atmospheric transport processes are thought to control the condensation and sublimation of H2O and CO2, the dominant climatic processes
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in the current epoch. TECP data, alone, and in concert with data from other Phoenix instruments, are used to determine the thermophysical properties of the regolith at the Landing site, and to monitor, in situ, the thermal response of the regolith. Energy balance calculations, tuned for the Phoenix site, can be compared with observations to indicate shortcomings in physical models [Haberle et al., 2008]. [9] During ice‐free summers, the high‐latitude regolith serves as a H2O reservoir, exchanging continually with the atmosphere [Jakosky, 1983a; Fouchet et al., 2007]. Within the regolith, exchangeable H2O occurs either as ground ice, hydrated salts, or as adsorbate. Depending on the thermal and diffusive properties of the regolith, H2O vapor exchange with the ground ice could have a major impact on the annual atmospheric water budget [Schorghofer and Aharonson, 2005]. [10] Böttger et al. [2005] argued, based on 3‐D GCM simulations that the regolith adsorbs water preferentially in high latitudes. This is especially true in the northern hemisphere, where perennial subsurface water ice builds up poleward of 60°N. Without the actions of the adsorbing regolith the equilibrated water cycle is found to be a factor of 2–4 too wet. The process by which this occurs is by adsorption of water during northern hemisphere summer in northern midlatitudes and high latitudes where it remains locked until northern spring when the seasonal CO2 ice cap retreats. 1.2. Synergistic Observations: Ground Truth [11] Additional power is gained from combining the TECP results with those from other Phoenix payload elements. 1.2.1. H2O Exchange [12] While the TECP measures the atmospheric H2O abundance, these data would be difficult to interpret absent additional constraints. Combining H2O, wind, and atmospheric temperature measurements from the Meteorology package, as well as coordinated observations from the Mars Reconnaissance Orbiter [Whiteway et al., 2009; Tamppari et al., 2010], H2O exchange mechanisms can be elucidated. 1.2.2. Thermophysical Ground Truth [13] The thermal response of the Martian regolith depends primarily on the physical structure of the surface layer. The thermal conductivity and heat capacity, and hence the surface thermal inertia, which can be determined globally from orbit, provide information about the nature of the surface of Mars and the types of materials from which it is composed [Mellon et al., 2008]. [14] Interpreting thermal inertia can be complicated by the variety of structures and material properties that result in the same thermal inertia value. By determining the surface thermal inertia, and comparing it to characterization of the regolith particulate components, provided primarily from the MECA Optical Microscope experiment [Pike et al., 2009], valuable thermophysical ground truth determinations can be made, which can then be applied to interpretations of orbital data far removed from the Phoenix site.
2. Instrument Description [15] The TECP is described by Zent et al. [2009], and only a high‐level description of the instrument will be given here. TECP is implemented as a single electronics box, fitted with four needles, which are inserted into the medium of interest (Figure 1).
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2.1. Science and Measurement Objectives [16] The TECP was included on Phoenix specifically to determine the regolith thermophysical properties that control energy fluxes between the atmosphere and subsurface, to characterize the processes that control the distribution and exchange of H2O between the atmosphere and subsurface in the current climate, and to document the occurrence of any unfrozen water, both as thin films in the regolith, and as atmospheric vapor. 2.1.1. Science Objectives 2.1.1.1. Energy Fluxes [17] Regolith thermal properties, particularly the thermal conductivity () and the volumetric heat capacity (Cr) are key parameters in any model that purports to address the distribution of Martian ground ice, the climatic cycles associated with variations in orbital parameters, or the annual cycles of H2O and CO2. These properties determine how effectively heat conducts from the illuminated surface to depth, and fix the thermal profile from the surface down at least as far as the ice table [Mellon et al., 2000]. [18] Based upon these measurements, estimates of the thermal inertia of the surface can be made, and compared with thermal inertias that are derived from measurement of the surface temperature throughout a thermal cycle, whether by TECP itself, or with thermal inertias derived from orbital data [e.g., Mellon et al., 2008]. 2.1.1.2. Regolith H2O [19] Another set of objectives involves understanding the abundance, state, and mobility of H2O in the regolith. H2O redistributes itself continuously through the regolith, continually accumulating in the lowest‐energy reservoir that is thermodynamically available. Because soil and ice coexist in the regolith, it is necessarily true that there is a finite and variable population of unfrozen H2O molecules; if sufficiently thick films of unfrozen water occur, surface diffusion in the film can outstrip diffusion in the vapor phase. These mobile H2O molecules are most easily detected in measurements of soil electrical properties, which are otherwise dominated by highly insulating silicates and ice. 2.1.1.3. Atmosphere [20] The atmosphere serves as the conduit through which H2O is distributed around the planet, and as the upper boundary condition controlling the state and distribution of H2O throughout the regolith. Exchange between the atmosphere and regolith is controlled substantially by the absolute humidity of the atmosphere, and somewhat to a lesser extent by the wind fields. Therefore, TECP also measured atmospheric relative humidity. 2.1.2. Measurement Objectives [21] The measurement objectives for TECP are as follows. [22] 1. Measure the temperature of the Martian regolith as a function of Local Mean Solar Time (LMST), and the subsolar angle (Ls). [23] 2. Measure the regolith thermal conductivity () with 10% accuracy over the range 0.03–2.5 W m−1 K−1, and the volumetric heat capacity of accessible Martian regolith materials with 10% accuracy over range 0.4 to 4 MJ m−3 K. [24] 3. Measure the electrical conductivity (inverse of the electrical resistivity) of the regolith materials, from 107 nS cm−1 to 1 nS cm−1 with 10% accuracy, particularly as a function of measured regolith temperature.
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[25] 4. Measure the bulk dielectric permittivity of the regolith materials over the range of 1 to 20, with resolution of 0.005, as a function of regolith temperature. We chose not to measure a wider range because high sensitivity (hence high resolution) is desired at the low end of the range to maximize sensitivity to small changes in the abundance of unfrozen water. [26] 5. Measure the H2O vapor density of the Martian atmosphere as a function of LMST, Ls, and height above the surface, including measurements made just above the ice layer. The requirement is to measure the H2O vapor pressure from 0–0.5 kPa over the temperature range of 195–270 K, with accuracy of ±10% of reading. 2.2. Technical Approach [27] The TECP needles are fitted into a plastic housing, which in turn is attached to the instrument electronics box. The plastic interface is machined from polyethylethylketone (PEEK), which has low thermal and surface electrical conductivity ( = 0.249 W m−1 K−1; Cr = 2.87 × 106 J m−3 K−1; rs = 2 × 1014 ohms per square (W sq−1), and provides insulation between the thermally and electrically activated needles and the electronics box. The aluminum electronics housing was anodized to increase emissivity, which enhanced thermal radiation, and minimized solar heating. A port in the instrument housing allowed the relative humidity sensor to access the Martian atmosphere. The sensor was shielded behind a porous Teflon membrane that prevented dust from fouling the humidity sensor. 2.2.1. Temperature [28] The temperature of three of the needles (1, 2, and 4) was measured as the difference between the sense and reference junctions of a Type E (chromel‐constantan) thermocouple. The reference junction of the thermocouple is on the TECP analog electronics board, adjacent to a temperature‐sensing current source. Typically reported soil temperature are from needle 4, which is unaffected by the heat pulse applied to the regolith by needle 1 (the furthest needle) in the course of thermal properties experiments. 2.2.2. Thermal Properties [29] The in situ thermal properties of the Martian regolith are determined via the transient heated needle technique [de Vries, 1952]. In this approach, a heat pulse is applied via a heater that approximates a line source, and the thermal response of the medium is measured at some radial distance from the heat source [Cobos et al., 2006]. [30] In addition to thermocouples, needles 1, 2 and 4 are also equipped with resistance heaters. In typical operations, needle 1 is heated for 30 s, and the temperature change of both needle 1 and needle 2 is monitored. The temperature change of needle i, between time 0 and time t is determined from Ti;t ¼ Ti;t T4;t Ti;0 T4;0
ð1Þ
Needle four (farthest from the heated needle) is used as a temperature reference, as it is unaffected by the heat pulse from needle 1 (for heat pulses of normal duration). The experimental protocol conducted a thermal property measurement every 15 min, in order to allow the regolith temperature to reequilibrate. This method also has the advantage of removing the effects of thermal drift of the bulk material
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or the TECP electronics board during the measurement. Because the TECP has only three thermocouples, the data from them are typically referred to as TC1, TC2, and TC3; it is important to bear in mind that TC3 is recorded in needle 4, rather than needle 3. [31] When a constant amount of heat, q W m−1, is emitted from unit length of a zero‐mass heater over a period of time, the temperature response at radial distance r, is [Carslaw and Jaeger, 1959] 2 q r C Ti;t ¼ Ei 0 < t < t1 4 4t
ð2Þ
where t1 is the heating time, and Ei is the Exponential Integral. The temperature change after the heat is turned off at time t1, is given by: Ti;t ¼
2 q r C r2 C Ei þ Ei 4 4t 4ðt t1 Þ
t > t1 ð3Þ
Material thermal properties, thermal conductivity, (), and volumetric heat capacity (Cr), are determined by fitting the time series temperature data during heating to equation (2), and during cooling to equation (3); note that the absolute temperature T does not enter into the calculation, only the change in temperature, DT. [32] Thermodynamically, heat capacity is defined for both constant pressure (CP) and constant volume (CV) conditions. Formally, the Phoenix measurement conditions do not correspond to either; however, in practical terms, the distinction is of little consequence, since typically (CP‐CV) is no more than a few percent of CV when T > 1000 K, and less at lower temperatures [Navrotsky, 1994]. [33] The values for and Cr found via straightforward fitting of the time series data to equations (2) and (3) are inherently inaccurate due to inevitable discrepancies between the TECP and the mathematically idealized model. A functional calibration of the TECP thermal properties measurement was performed against materials of known thermal properties [see Zent et al., 2009]. [34] Although in principle the heat pulse technique can also be used to determine the volumetric H2O content, [Ren et al., 2003], the specific heat of the dried particulate material must be known independently. In addition, over the diurnal range of Mars surface temperatures, the typical assumption made in terrestrial analyses, that C depends only on the H2O abundance, is no longer reliably valid. 2.2.3. Dielectric Permittivity [35] To measure the dielectric permittivity of the regolith, two of the needles (3 and 4) are used as the plates of a capacitor; with the regolith serving as the dielectric. The measurement involves putting a microwave frequency (∼6.25 MHz) voltage square wave on the capacitor, and measuring the time history of the charge on the plates. TECP measurement limits were between 1 and 20, with a resolution of 5 × 10−3. [36] Because TECP measured the effective permittivity of a mixture of phases, rather than a pure phase, we refer to the measured quantity as the bulk dielectric permittivity ("b) of the regolith.
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2.2.4. Electrical Conductivity [37] The electrical conductivity (s) of the soil is measured between needles 1 and 2. The regolith conductance (GR), the inverse of the regolith resistance (RR), is measured via a simple voltage divider. Three fixed resistors (RF = 1 kW; 100 kW; 3 MW) are used to allow the 12 bit data word to cover the entire dynamic range specified in the requirements with sufficient precision. This results in three distinct measurement channels, each with its own effective range, which are measured sequentially. 2.2.5. H2O Vapor [38] The TECP H2O measurement is based on a GE Panametrics MiniCap 2 polymer relative humidity sensor. It is a capacitance‐based instrument that measures the permittivity of a polymer film which adsorbs H2O. The circuit in the TECP is read out in a manner similar to the capacitance measurement already described for the soil measurement. [39] Solid‐state humidity sensors of this type are sensitive to relative humidity, almost independent of temperature [Anderson, 1995]. Specifically, the RH is measured relative to the equilibrium pressure of liquid H2O, rather than ice, regardless of temperature. At T < 273 K, the saturation vapor pressure of supercooled water is used [Murphy and Koop, 2005]. [40] The physical basis for this behavior is likely that atmospheric H2O forms an adsorbed film on the sensing polymer in equilibrium with the atmosphere, and it is the thickness of this adsorbate that is actually sensed as a change in permittivity. It can be shown that, for many adsorbents, the mass (or thickness) of this adsorbed layer is nearly the same for a given RH, regardless of temperature [Anderson, 1995]. [41] In TECP, the humidity sensor is located adjacent to the board temperature sensor on the analog electronics board, and Tb is used as a measure of the RH sensor. Due to power dissipation on the electronics board, Tb is typically slightly higher than the ambient air temperature during operation. This works to our advantage, in that it maintains the sensor above the frost point when the atmospheric RH is high. However, it also means the RH of the sensor is lower than that of the atmosphere. In practice however, we can use the measured RH along with Tb to calculate the absolute humidity of the atmosphere. Conversion to atmospheric RH is accomplished by comparing the atmospheric frost point to the atmosphere temperatures measured by the MET package temperature sensors.
3. Operations [42] A comprehensive list of observations made by TECP is presented in Table 1. Although TECP did not perform in‐soil measurements until sol 46, due to competing demands on the RA, atmospheric measurements were made throughout the mission. During some in‐air measurements, TECP data was acquired while the RA was in motion, and those instances are noted in Table 1. [43] When TECP was placed in‐soil by the RA, the goal was to fully insert the needles, but not the collars of the PEEK mounting. This placement strategy causes minimal disturbance to the preexisting soil structure. It was discovered, during characterization testing that the thermal and electrical property measurements are sensitive to small departures from the experimental design configuration.
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Table 1. TECP Observations Sol
LMST Start
Duration (min)
Type
Intent
1 4 5 8 8 8 9 9 11 11 12 13 13 14 15 15 15 15 15 16 16 16 17 17 18 18 18 19 19 19 20 20 21 21 21 21 21 21 21 21 21 22 22 24 24 25 26 28 28 29 29 30 31 31 31 31 31 32 32 35 35 36 36 36 36 36 37 37 37 37 38
14:45 17:04 10:30 11:15 12:45 14:47 14:55 16:13 9:58 12:05 15:35 10:02 15:23 9:58 9:29 12:04 14:31 21:53 23:08 10:47 12:37 15:59 9:57 16:27 10:51 13:46 15:25 9:41 12:41 17:11 10:06 12:29 10:53 11:08 11:21 11:35 11:49 12:03 13:46 14:56 16:39 9:12 13:44 9:14 14:00 10:02 16:06 11:34 15:46 12:34 17:05 18:30 16:56 17:11 17:25 17:39 17:53 18:38 21:54 17:04 22:22 10:06 11:56 15:03 16:29 18:45 9:58 12:15 14:54 17:26 16:43
21.91 26.71 26.83 26.71 28.01 27.69 27.68 27.69 27.82 27.83 27.68 27.82 27.68 27.68 27.82 27.83 27.69 27.81 27.81 27.82 206.93 27.81 27.83 27.68 27.69 27.77 27.75 27.81 27.69 27.68 28.00 27.69 13.34 13.34 13.33 13.33 13.33 13.34 27.69 27.69 27.69 27.68 62.71 62.68 62.72 27.69 27.68 62.69 62.71 62.68 62.70 27.68 13.34 13.34 13.33 13.35 13.33 27.69 62.69 27.69 77.78 27.69 27.69 27.69 27.71 27.68 27.70 27.70 27.70 27.69 27.69
air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air
Checkout
Target
.
Holy Cow
TT
Elevation (m)
Raduis (m)
Theta (deg)
1.08 1.43
0.37 0.25
175.63 156.6
1.41 1.40 1.30 0.51 0.51
0.95 1.08 0.81 1.63 1.63
149.47 158.25 40.25 36.38 36.38
0.35
1.97
31.14
0.66
1.35
31.14
1.68
1.19
198.36
1.35 0.77 0.95 0.95
0.6 1.55 1.55 1.55
223.69 52.91 34.38 34.38
0.64 0.78
1.55 1.03
31.2 31.2
0.38 2.02 1.52 1.27 1.02 0.52 0.27 0.51 0.14 0.56 0.6 0.51
1.36 1.81 1.81 1.81 1.81 1.8 1.8 1.63 0.4862 1.67 1.24 1.47
31.14 93.81 69.13 58.65 49.46 36.03 32.44 36.32 31.4 31.2 34.57 28.25
0.67 1.45 1.43
2.02 0.85 1.19
210.32 140.05 184.76
1.39
1.06
152.18
1.39 0.51 2.02 1.27 1.02 0.52 0.27 0.07 0.07 1.46 1.46
1.04 1.47 1.81 1.81 1.81 1.8 1.8 1.81 1.81 0.92 0.92
151.54 28.31 93.81 58.65 49.46 36.09 32.49 31.14 31.14 162.55 162.55
1.43
1.19
184.76
MRO MRO
lidar lidar t, TT lidar, t, SSI structural column lidar t, TT lidar, TT TT profile profile profile profile profile profile TT Snow Queen doc. TT lidar lidar, SSI sky survey, MRO lidar lidar lidar, TT lidar, TT lidar, t, MRO lidar, t, MRO lidar, TT lidar, TT lidar, TT profile profile profile profile profile lidar, t, TT lidar, MRO lidar, TT, MRO lidar, TT, MRO lidar, MRO TT lidar, TT, MRO photometry photometry photometry photometry lidar, TT
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Table 1. (continued) Sol
LMST Start
Duration (min)
Type
Intent
39 39 40 40 42 42 43 43 44 44 44 44 46 46 47 48 48 48 48 50 51 52 52 52 52 52 52 52 52 53 54 54 55 55 56 57 58 58 59 59 59 61 61 62 63 64 64 65 66 67 67 67 67 69 69 69 69 69 70 70 70 70 70 70 70 70 70 71 71 71 71
0:23 17:23 10:00 15:18 12:07 16:33 6:30 8:00 7:02 9:30 14:55 18:48 0:23 11:42 5:46 11:07 14:35 18:49 22:02 22:23 15:53 18:45 20:14 20:28 20:42 20:55 21:10 21:23 22:00 14:32 10:53 11:27 9:11 22:09 5:45 23:53 11:35 15:38 9:46 11:57 16:37 20:36 22:21 23:11 16:57 13:57 19:40 6:39 9:20 0:37 6:46 8:18 16:51 0:21 7:30 14:44 16:45 17:19 15:37 16:37 17:38 18:39 19:40 20:41 21:42 22:43 23:44 0:45 1:46 2:47 3:48
27.69 27.69 27.69 27.69 27.69 27.68 92.15 27.69 152.03 27.74 27.69 27.68 27.69 224.56 96.88 48.78 27.71 27.69 289.19 27.68 73.31 27.70 13.34 13.34 13.34 13.34 13.34 13.34 72.87 27.69 32.68 412.86 515.19 400.87 127.87 27.74 27.69 27.69 27.70 27.68 27.69 27.74 27.69 170.68 27.70 26.77 677.08 27.69 27.70 92.69 27.70 27.69 27.74 27.69 115.27 27.69 32.69 32.87 60.96 62.69 62.70 62.70 62.69 62.69 62.69 62.69 62.70 62.69 62.69 62.69 62.69
air air air air air air air air air air air air air soil soil air air air air air air air air air air air air air air air soil soil soil soil air air air air air air air air air air air air air air air air air air air air air air soil soil soil soil soil soil soil soil soil soil soil soil soil soil soil
lidar TT lidar, t lidar, TT RA scraping TT
Target
lidar lidar, Holy Cow TT lidar soil thermal soil thermal MRO MRO MRO MRO lidar, SSI water column permittivity in air lidar, SSI, MRO
permittivity in air lidar, SSI, MRO insertion soil thermal soil thermal soil thermal
Vestri Vestri
Vestri Vestri Vestri Vestri
lidar,TT lidar, TT TT lidar, TT lidar lidar, TT TT lidar lidar TT lidar TT lidar, t lidar, TT, t lidar, t, MRO lidar, t, MRO lidar, Snow Queen lidar, TT TT lidar, photometry, MRO soil insertion soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal soil thermal
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Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri Vestri
Elevation (m)
Raduis (m)
Theta (deg)
0.84 1.21
1.88 2.23
91.09 240.48
0.52 0.52 0.51 0.52
1.46 1.46 1.42 1.46
33.38 33.38 35.32 33.32
0.04 0.04
1.82 1.82
30.9 30.9
1.08 0.48
2.19 1.39
230.82 145.94
1.11 0.04 0.03 0.03 0.03 0.8 0.61 0.84 0.84
1.08 1.82 1.82 1.82 1.82 1.6 1.58 1.4 1.4
224.34 30.91 30.9 30.9 30.9 161.78 203.37 31.14 31.14
0.87 1.68 1.68
1.4 1.19 1.19
31.14 198.42 198.42
1.42 1.43
1.06 1.19
154.6 184.76
0
1.4
69.01
0.72 1.01 0.04 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03
1.38 1.42 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89 1.89
45.25 196.54 34.92 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09 35.09
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Table 1. (continued) Sol
LMST Start
Duration (min)
Type
Intent
Target
71 71 71 71 72 72 73 73 73 74 74 75 75 75 75 76 76 76 77 78 78 79 79 79 79 79 79 80 80 81 81 81 81 81 82 82 83 83 84 84 84 84 85 86 86 86 86 88 88 90 90 90 91 91 91 91 91 91 92 92 93 93 94 94 95 95 96 97 98 98 99
4:49 5:50 6:52 17:02 5:56 16:29 9:14 12:07 17:18 6:05 22:28 7:10 11:30 17:20 21:40 9:33 14:12 16:28 9:44 9:55 20:38 6:02 9:25 12:22 15:32 17:49 22:30 6:08 9:23 14:05 15:15 19:02 20:57 23:23 14:47 16:55 5:45 18:03 0:00 16:22 20:08 22:03 17:16 0:31 10:59 14:57 16:24 8:12 18:33 0:12 9:11 11:56 0:10 7:42 8:40 10:07 15:36 17:39 6:48 12:48 9:12 17:48 11:03 16:40 8:11 13:38 21:59 2:44 12:11 12:45 15:52
62.69 29.38 378.55 27.69 27.79 27.68 26.78 99.86 27.68 27.68 377.68 27.69 27.69 27.68 27.68 27.70 27.68 27.69 27.70 27.82 27.69 27.83 27.69 32.49 27.68 27.69 325.27 67.68 67.69 27.69 67.76 67.76 67.76 67.75 27.83 27.83 27.84 27.69 243.03 67.68 44.16 67.69 47.76 238.00 52.87 27.68 67.69 27.82 67.68 242.68 169.36 89.77 112.69 61.69 63.31 91.72 99.34 67.69 241.76 96.08 67.69 67.69 64.70 103.30 112.78 94.28 337.87 101.01 32.70 337.88 82.68
soil soil soil air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air air soil air air air air air air air air air air air air air air air air air air air air air air air soil soil air
soil thermal soil thermal soil thermal
Vestri Vestri Vestri
lidar lidar, TT, ice zenith movie lidar, TT snow queen lidar lidar lidan, SSI Atm, UHF lidar, SSI Atm SSI Atm SSI Atm SSI Atm
trenching trenching lidar, SSI half‐az survey lidar, SSI atmos, TT SSI atmos, TT SSI water sky mini SSI water column, ice zenith movie lidar, SSI atmos, TT
lidar TT TT TT, RA temp lidar, SSI ice TT, MRO TT, lidar, MRO soil thermal SSI SSI
Upper Cupboard
horizon movie lidar TT optical depth lidar TT humidity UHF pass TT TT TT TT TT TT zenith movie permittivity in air soil insertion soil thermal
Gandalf Gandalf
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Elevation (m)
Raduis (m)
Theta (deg)
0.03 0.03 0.03 0.41 0.88 1.05 1 0.36 0.74 0.74 1 1 1 0.21 0.21 1 1.43 1.43 0.86 1 1 1 0.18 0 0 0 1 1 1 0 0 0 0 0 0 0 0.97 1.43 1.43 1.43 1.43 1.43 0.58 1.4 −0.09 0.74 0.74 0.989 0.74 0.4 0.82 1.43 1.43 1.43 1.43 1.43 1.43 1.43 1.43 0.91 0.87 1.1 1.68 1.68 1.28 1.43 1 0.61 −0.01 −0.04 1.26
1.89 1.89 1.89 2.22 0.45 1.23 0 0.77 1.41 1.41 0 0 0 1.4 1.4 0 1.19 1.19 1.48 0 0 0 1.38 1.38 1.38 1.38 0 0 0 1.38 1.38 1.38 1.38 1.38 1.38 1.38 0.04 0.9 0.9 0.9 0.9 0.9 1.3 1.646 1.59 1.41 1.41 2.0400846 1.41 0.23 0.46 1.19 1.19 1.19 1.19 1.19 1.19 1.19 1.19 0.49 0.27 1.03 1.19 1.19 1.29 1.19 0 0.85 1.87 1.86 1.61
35.09 35.09 35.09 37.62 36.62 81.74 36.9 31.14 31.14 30.73 36.9 36.9 36.9 79.79 79.79 36.9 184.76 184.76 155.7 36.9 36.9 36.9 78.61 67.89 67.89 67.89 36.9 36.9 36.9 66.24 66.24 66.24 66.24 66.24 66.24 66.24 37.69 157.96 157.96 157.96 157.96 157.96 41.62 70.99 31.67 31.14 31.14 63.8 31.14 31.2 65.49 184.76 184.76 184.76 184.76 184.76 184.76 184.76 184.76 76.79 51.99 49.78 198.42 198.42 170.98 184.76 36.9 34.36 34.17 33.5 216.86
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Table 1. (continued) Sol 100 100 103 103 103 103 104 106 109 109 111 112 112 112 112 112 114 117 117 118 118 118 118 119 119 122 122 122 123 123 124 125 127 128 133 133 133 137 140 141 143 144 145 145 148 148 149 150
LMST Start
Duration (min)
16:57 21:12 5:45 16:25 16:59 23:47 9:13 7:24 9:17 12:44 18:28 17:31 17:45 17:58 18:12 18:25 9:12 9:59 16:45 6:01 8:46 12:34 15:49 9:06
82.68 338.28 237.69 32.70 32.87 208.34 407.23 363.91 211.76 68.20 302.67 13.33 13.33 13.33 13.33 13.34 112.71 210.00 67.68 67.68 67.71 96.02 67.69 236.18
0:59 13:55 14:20 9:12 15:04 10:00 12:38 18:57 14:34 7:16 10:12 15:37 19:25 14:18 11:11 13:50 11:39 10:37 14:20 12:57 13:55 15:19 11:43
67.68 26.69 187.08 153.73 96.30 178.49 67.69 112.87 27.04 18.57 24.72 67.70 67.70 67.70 107.81 202.70 92.78 67.71 12.97 84.65 18.93 26.70 67.71
Type air air air soil soil soil soil air air air soil air air air air air air air air air air air air soil air air soil soil soil soil soil air air air air air air air air air air air air air air air soil soil
Intent
Target
soil insertion soil thermal soil thermal soil thermal extra long extra long extra long soil thermal profile profile profile profile profile
Sindr Sindr Sindr Sindr
Rosy Red
TT TT/MRO TT/MRO TT/MRO soil thermal extraction
UpCu
soil insertion soil thermal soil thermal soil thermal soil thermal
Vestri Vestri Vestri Vestri Vestri
Elevation (m)
Raduis (m)
Theta (deg)
0.14
1.59
31.14
0.74 −0.01 −0.02 −0.02 −0.02 0.55 1.13 1.43 0.11 2.02 1.27 1.02 0.52 0.27 0.44 0.86 0.27 0.27 0.51 1.68 1.68 −0.05 0.27
1.4 1.96 1.96 1.96 1.96 1.04 0.48 1.19 1.83 1.81 1.81 1.81 1.8 1.8 1.74 0.41 1.81 1.81 1.26 1.19 1.19 1.83 1.78
31.14 40.91 41.39 41.39 41.39 188.66 93.11 184.76 32.01 93.81 58.65 49.46 35.97 32.38 153.28 51.49 152.6 152.6 139.68 198.36 198.36 34.93 31.3
0.1 0.03 0.03 0.03 0.03 2.27
1.85 1.89 1.89 1.89 1.89 1.23
37.06 35.03 35.03 35.03 35.03 31.14
0.75
1.76
200.54
0.42 2.27
1.78 1.23
100.75 31.14
−0.06
1.31
80.09
0.47
1.94
59.54
0.44 0.45 0.11 0.49 0.13
1.76 1.83 1.7 1.48 1.72
94.01 69.07 104.04 92.34 37.87
TT
TT extra long extra long TT TT periodic periodic soil temperature soil thermal
Inserting the TECP further than the needle collar results in compression of the underlying regolith, and systematically higher thermal conductivity, heat capacity, and dielectric permittivity measurements [Zent et al., 2009]. Unfortunately, due to small uncertainties in the Digital Elevation Map (DEM) of the workspace, and the difficulty of controlling the 2.3 m RA with submillimeter precision, ideal TECP placement was impossible to guarantee. Operationally, the preference was to slightly over insert the needles, and then to operate the RASP, a small drill bit used to collect icy samples, for 15 s to collapse any voids caused by RA start and stop transients. During prelaunch testing in the Spacecraft Interoperability Testbed, this protocol yielded reproducible measurements of thermal and electrical properties, in spite of the inherent limitations in fine motion control [Zent et al., 2009].
Alviss Alviss
3.1. Protocols [44] There were essentially two software blocks that were used to run TECP, depending upon whether data was collected while TECP was held aloft (the Wind Block), or whether it was inserted into the regolith (the Trench Block). The primary difference is that the Wind Block did not make electrical properties measurements. On sols 51, 52 and 96, the Trench Block was run while TECP was suspended above the surface, in order to perform in situ characterizations of the temperature dependence of the dielectric permittivity circuit. [45] The flight software specified the number of samples to acquire in each measurement mode; the measurement, frequency and hence measurement duration on any given sol was slightly variable, depending on the processor workload.
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Figure 2. TECP measurement sequences.
In practice, this worked out most often to about 1.2 s per sample. The measurement sequence of both blocks is specified in Figure 2. Both the Wind Block and the Trench Block conducted a fixed sequence of measurements, and then acquired relative humidity data until block execution reached 16 min total duration. This allowed time for the heat generated during the thermal properties experiment to dissipate before the next measurement sequence. 3.2. Atmospheric Measurements [46] The TECP Wind Block was run repeatedly throughout the mission, and was generally coordinated with either Solid State Imager (SSI) observations of the telltale (TT) [Holstein‐Rathlou et al., 2010], lidar measurements, optical depth measurements (t), SSI water column measurements, or MRO overflights, during which periods Phoenix and MRO conducted coordinated science observations [Tamppari et al., 2010]. On sols 21, 31, and 112 vertical profiles were conducted, where TECP gathered data for 15 min periods, at heights between 35 cm and 2 m. From time to time, TECP was also parked above two exposures of ground ice, Holy Cow, and Snow Queen, in order to monitor the atmospheric humidity immediately above these ice exposures. 3.3. Regolith Measurements [47] The regolith soil insertion sites are shown in Figure 3. They are, for the most part, between 1.8 and 2.0 m, radially, from the RA shoulder joint. Because the scoop and TECP share the single end effector, the RA wrist joint rotation was limited. As a consequence, the TECP needles could only be inserted vertically into a horizontal surface, at distances between 1.85 and 2.01 m from the shoulder joint. In one instance, an insertion was made in the bottom of a preexisting trench, which allowed TECP to be used just inside the 1.8 m contour. [48] The strategy for TECP was to preserve the 1.8–2.0 m annulus as much as possible. Early in the mission, a prime site, named Vestri, was selected for repeated measurement throughout the mission. Figure 4 shows the TECP, during initial in‐soil measurements at Vestri. The site was selected because it was accessible, but behind (from the lander’s perspective) a large rock (Headless) that would otherwise impede trenching and sampling operations. Vestri is located
near the middle of the workspace; on the shoulder of the polygon known as Wonderland. Approximated as a plane on the scale of the TECP, the surface strike is N75°E, dipping 4° to the NW. A backup site (Sindr) was also selected. [49] Vestri was monitored throughout full diurnal cycles on sols 46–47 (Ls = 97.4°), 54–56 (Ls = 101.1°), and 69–71 (Ls = 108.0°). Additional measurements were made at Vestri on sol 122 (Ls = 133.1°). Late in the mission, Headless was moved, and the entire site was excavated to ground ice, which established the small‐scale stratigraphy of the site for thermal modeling, and allowed the thermal effects of Headless on the ice table to be examined [Sizemore et al., 2010]. [50] Shorter duration measurements were acquired at several other sites to assess the heterogeneity of the surface soils, as well as search for systematic variations related to polygonal and stratigraphic structure. Two attempts were made, on sols 86 and 119, to insert the TECP needles in the bright ice‐rich deposit exposed in the Dodo‐Goldilocks– Upper Cupboard trench complex. Neither placement was
Figure 3. Predigging map of the Phoenix workspace. The TECP regolith insertion sites are indicated by name in red (Rosy Red is white for readability).
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4.1. Thermal Properties 4.1.1. Thermal Conductivity [55] The thermal conductivity of the regolith, as measured by TECP, is shown in Figure 5a. The regolith is a remarkably good thermal insulator, at least over the uppermost 15 mm. No significant differences were detected among the eight measurement series, at six unique locations in the Phoenix workspace (thermal properties were not measured at Alviss). The overall average thermal conductivity is 0.085 W m−1 K−1. 4.1.2. Volumetric Heat Capacity [56] The volumetric heat capacity data are also shown in Figure 5a. The average value is ∼1.05 × 106 J m−3 K−1, and, more so than thermal conductivity, there is a clear correlation of Cr with regolith temperature. This result is not unexpected; the Debye temperature (D) of all minerals lies well above ambient Martian surface temperatures. At T < 0.1D, heat capacity in solids increases as T3, between
Figure 4. TECP making in‐soil measurements at Vestri, sol 46 (Ls = 97.4°). The large rock just inboard of TECP is Headless, which was later moved to examine the thermal effects of rocks on the ice table [Sizemore et al., 2010].
successful because the RA reached its programmed force limit, and arrested its motion. The high forces associated with the attempt to insert into the bright material are consistent with the hard nature of icy soil [Arvidson et al., 2009]. [51] TECP was inserted at Gandalf on sol 98 (Ls = 121.8°), chosen because it was in the trough separating the Wonderland and Humpty Dumpty polygons. On sol 104 (Ls = 124.3°), the backup site, Sindr, was characterized. [52] On sol 111 (Ls = 128.1°), measurements were made within a preexisting trench (Rosy Red 3). At the time, Rosy Red 3 had a flat bottom, and it was estimated that only about 2 cm of residual regolith separated the bottom of the trench from ground ice. In what came to be known as the Overdrive Experiment, the RA pressed the TECP into the trench bottom with the goal of bringing the needles as close as possible to the surface of the ice. The intent of this experiment was to survey vertical variations in soil properties, and to detect and characterize the expected diurnal variations in unfrozen H2O at the base of the dry permafrost. [53] Finally, on sols 149 and 150, (Ls = 147.1°), the final sol of operations, TECP investigated a site known as Alviss, which is radially outboard from the Upper Cupboard site, again, with the focus being an surveying of the regolith thermal properties.
4. Results [54] The results of the TECP measurements are presented rather briefly, followed by a more quantitative discussion of their significance.
Figure 5. (a) The regolith thermal conductivity (circles) and volumetric heat capacity (squares) measured by TECP as a function of the needle 3 temperature (TC3) for the 7 TECP insertions which produced thermal properties data. Note the change in scale. (b) The calculated thermal inertia of the regolith, based upon the and Cr values plotted in Figure 4a. Insertion site and sol number are indicated in the legend. The Ls of each measurement is indicated by the color code, and specified by the white bars on the right hand scale.
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binned 10 s averages from all TECP atmospheric temperature data are plotted against local mean solar time. The substantial increase in standard deviation is the result of small scale eddies in the turbulent planetary boundary layer [Moores et al., 2010; Holstein‐Rathlou et al., 2010]. The variable momentum imparted to the 2.3 m Robotic Arm results in continual jostling, and a much enhanced range of all measurements that depend upon contact between the TECP needles and the regolith soil particles.
ð4Þ
4.2. Soil Temperature [59] Measured temperatures (TC3) acquired during in‐soil measurements ranged from 253 K to 181 K (Figure 7). Since the TECP needles are 15 mm in length, and were inserted as near to vertical as possible, the recorded temperatures are actually an integration of the regolith thermal gradient over that scale, not the temperature of the surface. This is particularly important to note because of the low thermal conductivity of the regolith, which results in very steep thermal gradients near the surface; the diurnal thermal skin depth is ∼5 cm (section 5.1.2). Several effects are evident in the data, which distort the idealized sinusoidal pattern that would be expected from a uniform half‐space. [60] First, because the RA shoulder joint, and hence the Phoenix workspace, were on the north side of the lander (by design), daytime temperature maxima occurred near the time that the shadows of the Robotic Arm, and the attached scoop and RAC, as well as the SSI, crossed the TECP insertion sites. Shadowing by the Robotic Arm (RA), the Robotic Arm Camera, and scoop are most evident between 1300 and 1400 LMST, although self‐shadowing of the insertion site by the TECP itself also alters the radiative balance. In general, temperatures depart from even an approximate sinusoidal curve between 0800 and 2000 LT. Overnight temperatures however, which are less affected by shadowing,
The thermal inertias calculated from the thermal properties are shown in Figure 5b. The thermal inertia is temperature dependent, as would be expected from the temperature dependence in Cr, and consequently has different values from day to night. 4.1.4. Effects of Spacecraft Motion [58] Figure 5 shows the first occurrence of a phenomenon that will be noted in other data sets, and which requires some explanation. TECP data in general exhibit a greatly increased range at the warmest temperatures. This same variability occurs in the dielectric permittivity measurements (section 4.4). As part of the prelaunch characterization work, the effects of gaps around the needles, due to partial insertion, partial retraction, or lateral movement were characterized [Zent et al., 2009], and in some cases, relatively small gaps were observed to introduce significant errors. Because the enhanced variability is restricted to the warmest part of the day, and appears in multiple measurements, the working hypothesis is that the increased variability during the warmest part of the day results from small spacecraft movements due to atmospheric turbulence. Support for this hypothesis comes from both the Meteorology mast air temperature data, which exhibit high‐frequency, large‐amplitude variability between approximately 0800 and 1800 LT, and several TECP data sets. In Figure 6, the standard deviation of
Figure 7. Temperatures from needle 3 (TC3) during in‐soil measurements, as a function of Local Mean Solar Time and color coded for Ls. Insertion site and sol number are indicated in the legend. The Ls of each measurement is indicated by the white bars on the right hand scale.
Figure 6. The standard deviation of 10 s binned in‐air temperature, recorded by needle 3 (TC3), throughout the mission. The substantial increase in s(TC3) during the day is the result of enhanced atmospheric convection. 0.1 D and D, the dependence is less extreme, and only above D does heat capacity reach its maximum value (3nR), where R is the gas constant and n is the number of atoms per unit formula.. The cumulative effect of multiple geologic materials, each with its own such Cr(T), produces the heat capacity signature in Figure 5a. 4.1.3. Thermal Inertia [57] Given and Cr, it is then possible to calculate (rather than derive from surface temperature variations) the thermal inertia of the uppermost ∼15 mm of regolith. I¼
pffiffiffiffiffiffiffiffiffi C
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and more by radiative and conductive processes, do follow an approximate sine curve. [61] More problematically, because of the extremely low thermal conductivity of the regolith, which therefore couples poorly to the TECP needles, TC3 is some function of the temperature of the soil into which they are inserted, and the temperature of the PEEK head into which they are mounted. The relative influence of TPEEK is a function of the thermal conductivity of the soil; the lower the regolith thermal conductivity, the greater the proportional heat transfer from the TECP body. For this reason, the temperature is reported as TC3, and not regolith temperature. We will limit ourselves to noting a few qualitative trends in TC3 here. [62] Daytime thermal maxima vary little from Ls ∼ 98° through Ls 120°. By Ls 130° however, daytime temperatures are lower by several K, and by Ls 148°, noon temperatures were over 20 K cooler than they had been at the beginning of the mission. Overnight lows decrease steadily from Ls 108° onwards, even though sunset was not until Ls 119°. However, the cosine dependence of insolation on solar zenith angle means radiative cooling becomes increasingly important as the mission proceeds. A more quantitative treatment of the regolith temperature field and its derivation from TECP and other data, is described in section 5.2. 4.3. Electrical Conductivity [63] The TECP was calibrated over a range of 107 to 1 nS cm−1, although the practical detection limit was nominally considered to be 2 nS cm−1. However, all measurements of regolith electrical conductivity throughout the mission were consistent with an open circuit. The upper limit of 2 nS cm−1 means that at no time was there adequate unfrozen H2O to facilitate conduction on the scale of the TECP needles. 4.4. Dielectric Permittivity [64] Regolith dielectric permittivity measurements were made to identify occurrences of H2O molecules with rotational degrees of freedom (i.e., neither ice nor vapor, referred to here as “unfrozen” H2O). A total of 12,889 dielectric measurements were made over the course of 9 in‐soil and 5 in‐air observations. [65] Unfortunately, relating the measured dielectric permittivity directly to an absolute unfrozen H2O content is not straightforward [Topp et al., 1980; Jones and Or, 2003; Lebron et al., 2004; Sweeney et al., 2007]; measured permittivity depends on details of the contact between the regolith particles and the TECP needles, in addition to other environmental factors such as temperature and bulk density [Ulaby et al., 1982; Campbell, 2002]. Indeed, sensors used in terrestrial investigations are not moved during the experimental period, and require unique experimental calibration for each soil to obtain unfrozen water content from probe output [Yoshikawa and Overduin, 2005]. [66] Although the measured baseline "b changes discontinuously each time contact with the regolith is made and broken, variations within each in‐soil observation sequence are significant. The simplest approach to evaluating "b variations related to the diurnal cycle is to normalize the permittivity at a fixed temperature, which allows us to discriminate effectively between nighttime and daytime behavior. Figure 8 shows one such cut through the data, where
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Figure 8. Measured dielectric permittivity as a function of regolith temperature Data gathered up to sol 55 (Ls 101°) are in black. Data acquired beginning sol 69 (Ls 108°) are in gray (no regolith data was taken in the interim). The data are normalized to " = 2.4 at 230 K, due to the dependency of permittivity on the contact geometry between the needles and the regolith grains.
the dielectric permittivity is normalized to 2.5 at 230 K, for all soil measurements. Prior to sol 70, there is no clear dependence of permittivity on regolith temperature. However, after sol 70, a distinct pattern of increasing overnight permittivity is seen. This is not a result of small variations in the permittivity of solid mineral particulates, which would have an opposite dependency on temperature from that which is observed. In section 5, we evaluate the hypothesis that the systematic increase in dielectric permittivity overnight, subsequent to sol 70, is due to the accumulation of unfrozen H2O by the cooling regolith during the overnight hours. 4.5. Humidity [67] Polymer humidity sensors are nonlinear in their response to relative humidity close to 0 and 100% [Anderson, 1995]. TECP sensor data with a relative humidity (with respect to water) less than 1.6% are not well fit by the calibration function and are rejected. [68] To reduce instrumental noise, individual data points at 1 s intervals are averaged into 10 s bins, comparable to or shorter than the response time of the sensor. These methods result in 1,465 data points taken with the TECP in the soil, and 5,972 points with the TECP in air. Figure 9 shows all data converted into vapor pressure plotted against local mean solar time. [69] The daytime averaged H2O pressure is steady throughout the mission (LS 78° to 147°) at ∼1.8 Pa. Integrated throughout a scale height, this would require a col-
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Figure 9. H2O vapor pressure (Pa) as a function of Local Mean Solar Time (LMST). Data acquired while TECP was held aloft by the RA are color coded for subsolar longitude. Data acquired with the TECP in‐soil are black.
umn of 130 precipitable micrometers (pr mm). however, it is likely that H2O is mixed only through the depth of the PBL [Whiteway et al., 2009], suggesting that the H2O column abundance decreases with PBL depth as the season progresses, decreasing from ∼60 pr mm to ∼30 pr mm. [70] Daytime RH was initially
