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Phoenix LIDAR measurements of Mars atmospheric dust
Icarus 223 (2013) 649–653
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Phoenix LIDAR measurements of Mars atmospheric dust L. Komguem a, J.A. Whiteway a,⇑, C. Dickinson b, M. Daly a, M.T. Lemmon c a
Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada MacDonald Dettwiler and Associates (MDA), Space Missions, Brampton, Ontario, Canada c Atmospheric Sciences Department, Texas A&M University, College Station, TX, United States b
a r t i c l e
i n f o
Article history: Received 31 October 2012 Revised 15 January 2013 Accepted 23 January 2013 Available online 4 February 2013 Keyword: Mars, Atmosphere
a b s t r a c t The LIDAR instrument on the Phoenix mission obtained measurements of atmospheric dust and clouds from the surface in the Arctic region of Mars (68.22°N, 234.25°E) during late spring through the middle of summer. The observed vertical distribution of dust indicated that the planetary boundary layer (PBL) was evenly mixed up to heights of 4 km by daytime convection and turbulence. The values of the dust optical extinction coefficient derived from the LIDAR measurements within the PBL reached a maximum of 0.15 km1 during the period around summer solstice and then decreased to values approaching 0.03 km1 over the next 60 martian days (sols). The ratio of the LIDAR backscatter coefficient at wavelength 1064 nm to that at 532 nm for dust was obtained in three cases and found to have values in the range 1.18–1.35, relative to an assumed value of unity for the near surface water ice clouds. Mie scattering calculations were performed to determine that the measured dust 1064/532 backscatter color ratios were consistent with particle size distributions having effective radii in the range of 1.2–1.4 lm. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The optical properties of the atmosphere on Mars are determined primarily by airborne dust. This is most obvious during planetary scale dust storms (Strausberg et al., 2005; Smith, 2009), regional dust activity associated with weather systems (Cantor et al., 2001), and small-scale convective vortices (Cantor et al., 2006; Ellehoj et al., 2010). Even during quiescent conditions, the atmospheric optical depth in the absence of clouds is attributed to suspended dust. In comparison to Earth, absorption of solar radiation by dust has a much greater effect on temperature since the atmospheric molecular density on Mars is smaller by a factor of 100. It has been demonstrated with theoretical and modeling studies that solar heating of dust in the atmosphere of Mars can induce a dynamical feedback that would enhance the intensity of deep convection, analogous to latent heating in moist convection on Earth (Fuerstenau, 2006; Heavens et al., 2011). Observing the temporal and spatial variation of the optical properties of the suspended dust is thus key to advancing our understanding of atmospheric dynamics on Mars. The Phoenix spacecraft landed in the Arctic region of Mars (68.22°N, 234.25°E) on 25 May 2008, 30 martian days (or sols) prior to summer solstice, and operated over the next 5 months
⇑ Corresponding author. Address: Centre for Research in Earth and Space Science, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3. Fax: +1 416 736 5626. E-mail address: [email protected] (J.A. Whiteway). 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2013.01.020
(Smith et al., 2008, 2009). The mission included a LIDAR (light detection and ranging, or laser radar) instrument that was capable of measuring the vertical distribution of atmospheric dust and clouds through the planetary boundary layer, and up to heights of 20 km (Whiteway et al., 2008, 2009; Dickinson et al., 2010, 2011). This paper provides a description of the observations of atmospheric dust with the LIDAR and also an estimate of the effective radius associated with the dust particle size distribution. 2. Measurements and analysis The Phoenix LIDAR measurement technique involved emitting pulses of light vertically into the atmosphere and detecting the backscatter from dust and cloud particles (Whiteway et al., 2008). The transmitter was based on a Nd:YAG laser that emitted pulses of light with wavelengths of 1064 nm and 532 nm at a repetition rate of 100 Hz. The light scattered back from the atmosphere was collected by a 10 cm diameter telescope, and detected by an avalanche photodiode at wavelength 1064 nm and a photomultiplier at wavelength 532 nm. The signal acquisition involved 14-bit amplitude to digital conversion (ADC) from both detectors and also photon counting from the photomultiplier. The sampling interval was equivalent to 5-m height bins, but the standard operating mode during the mission used on-board averaging to 20 m vertical resolution for the analog detection and 50 m for photon counting. Temporal averaging over 20 s (2000 laser shots) was applied to the signals before on-board storage and transmission back to Earth. In the analysis for the results presented here, the recorded
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L. Komguem et al. / Icarus 223 (2013) 649–653
Fig. 1. (a) LIDAR backscatter signals from atmospheric dust on mission sol 48 (Ls = 98.57°), acquired with amplitude to digital conversion (ADC) and photon counting at wavelength 532 nm. (b) Dust optical extinction coefficient retrieved from the separate ADC and photon counting measurements, joined at height 3 km. (c) Ratio of dust extinction coefficient (m2/m3) to molecular density (kg/m3), which is equivalent to the integrated dust cross sectional area per unit mass (m2/kg) of molecular atmosphere.
backscatter signals were further smoothed with a running mean over 6 points (120 m) for the analog, and 12 points (600 m) for the photon counting. Fig. 1a shows the analog and photon counting backscatter signals recorded for wavelength 532 nm on mission sol 48 (solar longitude, Ls = 98.57°). In this case the signal is due only to dust. The combination of the ADC and photon counting was used to accurately record the entire dynamic range in the signal from ground to height 20 km. The analog detection was used at heights below 3 km since in the near range the optical signal strength was great enough that photon counting detection was nonlinear and saturating due to pulse pile up (Donovan et al., 1993). At heights above 5 km, only the photon counting (532 nm) was used to detect the weak optical signals. The measured signal is described in terms of the optical properties of the atmosphere by the lidar equation as:
SðzÞ ¼
Z z C 0 0 bðzÞ exp 2 r ðz Þdz z2 0
where S(z) is the measured backscatter signal as a function of height z. The backscatter coefficient, b(z) is the fraction of optical energy scattered back per unit length and per unit solid angle. The extinction coefficient, r(z) is the fractional decrease in the optical energy per unit length through the atmosphere due to scattering. The backscatter and extinction coefficients are related to the physical properties of the dust and cloud particulates in the atmosphere. The calibration constant, C, takes into account factors such as the laser pulse energy, area of the receiver aperture, throughput of the system optics, and detector efficiency. The retrieval of the extinction and backscatter coefficients was carried out using the method of Fernald (1984), with a constraint applied such that the extinction coefficient integrated from ground to height 20 km matches the total optical depth measured coincidentally by the Phoenix Surface Stereo Imager (SSI) (Lemmon et al., 2008), minus a value of 0.04 to account for dust optical depth above a height of 20 km. The value of 0.04 is an approximation based on the assumption of constant dust mixing ratio above 20 km. The analog and photon counting signals were analyzed independently assuming a constant ratio of extinction to backscatter coefficients (the lidar ratio) of 40 steradians for dust particulates. A sensitivity analysis was performed to study the effects of varying the lidar ratio on the derived extinction coefficient. It
was found that the retrieved extinction coefficient changed by no more than 3% at any height when the dust lidar ratio was varied from 20 Sr to 100 Sr, which is the range of values published for aerosols in the atmosphere of Earth (e.g. Ansmann et al., 2003; Pappalardo et al., 2004). The independently measured atmospheric optical depth provided a direct constraint on the extinction coefficient integrated over the vertical profile. Thus the value of the extinction coefficient is constrained, but the derived backscatter coefficient would depend on the assumed lidar ratio. For this reason, the extinction coefficient was used as an indicator of atmospheric dust content, rather than the backscatter coefficient. The ratio of backscatter at the separate wavelengths of 1064 nm and 532 nm was used to estimate dust particle size (Figs. 3 and 4), but that was obtained directly from the lidar signals and did not involve an assumed lidar ratio. The error bars in the derived extinction in Fig. 1b contain both the uncertainty carried over from the SSI measurement and also the contribution of random uncertainty in the measured LIDAR signals (e.g. standard deviation in photon counting), which were propagated through the calculations. The use of a constant lidar ratio is equivalent to assuming that the dust particle size distribution does not change significantly with height. This was necessary since there have been no previous measurements in the atmosphere of Mars that provide the aerosol size distribution and changes with height. Greater fall speeds for larger particles would produce a tendency to smaller particles remaining in the atmosphere with increasing height, but this would depend on the rate of vertical mixing and convection. The measurements reported in this paper show that the planetary boundary layer (PBL) dust loading is approximately evenly mixed up to height 4 km (e.g. Figs. 1 and 2). This gives support to the assumption that the dust particle size distribution was not strongly height dependent in this region, and that the assumption of constant lidar ratio is sufficient up to height 4 km. Above the PBL, and in the absence of significant convection, one could argue that the effective radius for the size distribution would gradually shift toward smaller values with height and this could add to the uncertainty in the measured extinction coefficient. However, without any knowledge of the dominant mechanism for transporting dust through the atmosphere above the PBL, the best approximation was that the lidar ratio was constant with height. The LIDAR backscatter signal from dust could be detected to heights of 20 km during late evening and early morning, when
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Fig. 2. (a) Atmospheric dust optical extinction coefficient profiles at wavelength 532 nm for five sols that are representative of the dust loading at different times during the mission. (b) Ratio of dust extinction coefficient (m2/m3) to molecular mass density (kg/m3), which is equivalent to the integrated dust cross sectional area per unit mass (m2/kg) of molecular atmosphere.
Fig. 3. Mie scattering calculation of the ratio of optical backscatter coefficient at wavelengths 1064 nm and 532 nm as function of the effective radius for the particle size distribution. Values of refractive index and effective width are given. The three measured values of color ratio and corresponding effective radii are indicated with dashed lines.
the background of scattered sunlight was a minimum. During midday the detectable signal level was limited more by background skylight such that the dust backscatter signal could be detected up to maximum heights of approximately 7 km. Thus the midday LIDAR measurements are not directly comparable to the total optical depth measurement by the SSI and the contribution above 7 km was assumed to be the same as the nearest nighttime LIDAR measurement. It is estimated that this assumption adds no more than 2% uncertainty to the extinction coefficient derived from midday LIDAR measurements, based on the variability of the optical depth at heights above 6 km in the nighttime LIDAR measurements. Fig. 1b shows the extinction coefficient derived from the signal shown in Fig. 1a. The extinction coefficient can also be considered as the product of the particle effective cross sectional area (m2) and the number density per unit volume (m3). The extinction coefficient is thus also proportional to the mass per unit volume of the particulate material. The vertical distribution of enhanced dust loading from ground to height 4 km in Fig. 1b is similar to what is observed above deserts on Earth (e.g. Dickinson et al., 2011), where dust is lifted from the surface and mixed through the planetary boundary layer (PBL) by wind stress, convection, and turbulence. In Fig. 1c the extinction coefficient (in units of m2/m3) was divided by an estimate of the background molecular density (in units of kg/m3) obtained from a numerical model of the Mars
atmosphere (Davy et al., 2009). This ratio is referred to as the density scaled extinction. It can be considered as the dust crosssectional area per unit mass of molecular atmosphere (in units m2/kg) and it is a conserved tracer. In this case the dust is mixed sufficiently through the planetary boundary layer (0–4 km) that the density scaled dust extinction is approximately constant. This is as expected for any conserved tracer in a well-mixed turbulent layer (e.g. Dickinson et al., 2011). The depth of the convective boundary layer on Mars was previously determined from radio occultation measurements of the vertical profile of temperature by the Mars Express mission (Hinson et al., 2008). While the season and location of the measurements do not overlap, the boundary layer depths obtained from the Phoenix LIDAR measurements (3–5 km) are within the range of the Mars Express results in the northern high latitudes in spring (2.6– 5.4 km). During the latter half of the mission (after sol 80) the LIDAR measurements were conducted mostly in the late evening or early morning in order to avoid a temperature dependent instrument issue that occurred only during the warmest part of the day. During this period there were nearly always clouds at the top of the boundary layer in the early morning hours (Whiteway et al., 2009; Dickinson et al., 2010) that obscured the observation of dust at heights below 5 km and attenuated the signal such that measurements of dust at heights above 5 km were not possible. Thus there were fewer LIDAR observations of dust in the second half of the mission that could be directly compared with observations during the first half. Fig. 2 shows the vertical profiles of the 532 nm extinction coefficient measured on mission sols 14 (Ls = 83.05°), 38 (Ls = 93.64°), 48 (Ls = 98.57°), 67 (Ls = 106.95°), and 96 (Ls = 121.11°). The total atmospheric optical depth measured by the SSI during those sols was, in the same order, 0.55, 0.48, 0.40, 0.35, and 0.30. Most of the change in atmospheric optical depth over this period was due to a decrease in the dust loading within the PBL. In each case the density scaled extinction (Fig. 2b) was approximately constant, or well mixed, through the PBL (ground to height 4 km). The atmospheric dust loading was a maximum during the first 40 sols of the mission, in the period around summer solstice. Ellehoj et al. (2010) have used images from the Mars Color Imager (MARCI) instrument on the Mars Reconnaissance Orbiter to show that a dust cloud passing over the Phoenix site on mission sol 25 followed a trajectory that traced back to the polar ice cap. It was noted that this was a period when CO2 ice was sublimating from the polar cap such that the dust that previously settled there would have become available to be lifted into the atmosphere. It is also noteworthy that increased dust activity has been observed near
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Fig. 4. Measurements from mission sol 109 (Ls = 127.05°). (a) Time–height contour and line plot of the backscatter signal at wavelength 532 nm and (b) time–height contour and line plot for the backscatter signal at wavelength 1064 nm. (c) 1064/532 backscatter color ratio contour and line plot. The line plots represent the average over the time interval.
the edge of the retreating north polar cap during spring with the Hubble Space Telescope (James et al., 1999). Numerical simulations have been used to demonstrate that there would be enhanced dust lifting due to strong wind speeds and weather activity driven by the thermal contrast at the polar cap edge (James et al., 1999; Toigo et al., 2002). The Phoenix LIDAR observations of maximum atmospheric dust loading during the period near summer solstice are not inconsistent with the hypothesis that the polar cap is a source region for atmospheric dust, but more detailed analyses of imaging from orbit and numerical simulations would be required to advance this interpretation. All of the LIDAR measurements presented above were obtained with the emitted wavelength of 532 nm. The signals from the emitted wavelength of 1064 nm were also recorded, but were not of sufficient signal-to-noise-ratio to provide measurements at heights above 2 km. The 1064 nm signals were useful for comparison with those at 532 nm in order to obtain an estimate of the effective radius associated with the dust particle size distribution. For example, Fig. 3 shows a calculation of the ratio of backscatter coefficients at 1064 nm and 532 nm as a function of the effective radius of a particle size distribution. The effective radius is the crosssectional area weighted mean radius for the particle size distribution. Formally it is defined as the ratio of the third moment to the second moment of the particle size distribution. The Mie scattering calculations (Grainger et al., 2004) assumed spherical particles with a Gamma distribution for particle size that was equivalent to that applied by Wolff et al. (2009) and Dickinson et al. (2011). Thus a measurement of the 1064/532 backscatter color ratio can provide an estimate of the effective radius of for an assumed particle size distribution. The relative calibration of the 532 nm and 1064 nm analog channels in the LIDAR instrument is required to obtain an accurate measure of the 1064/532 backscatter color ratio in the optical signals. This was not part of the instrument characterization since the relative laser pulse energy at the two wavelengths was dependent on the instrument internal temperature, and there was no accurate measurement of the laser output energy at either wavelength while operating on Mars. However, the atmospheric ice water clouds
provided a means to obtain a relative calibration. The clouds formed at temperatures and water vapor densities that are similar to the conditions in which cirrus clouds are known to form in the atmosphere of Earth (e.g. 65 °C, and 6 mg/m3). It has been argued that the ice particle size distribution would have been similar to that found in cirrus clouds on Earth, where ice crystal radii range in size from less than 10 lm to greater than 200 lm (Whiteway et al., 2009; Dickinson et al., 2011), and this has been demonstrated with numerical modeling (Daerden et al., 2010). The observation of fall streaks in the Mars clouds also indicates ice crystal radii of a few tens of micrometers (Whiteway et al., 2009). Ice particles of this size would have a 1064/532 backscatter color ratio that is approximately unity. The measured vertical profile of 1064/532 backscatter color ratio was normalized so that it was unity within the cloud near the surface (ground fog), and thus the value just above the cloud is accurate for estimating the color ratio for backscatter from the atmospheric dust. The dust contribution to the signal within the cloud was removed before obtaining the relative calibration. It was assumed that there was no difference in the optical extinction coefficient at 1064 nm and 532 nm within the cloud. There were three cases during the second half of the mission when the signal at wavelength 1064 nm was sufficient to measure dust above a surface cloud. The measurements on mission sol 109 (Ls = 127.05) are shown in Fig. 4. There was an enhancement in the signal at both wavelengths due to the surface cloud and a distinct cloud top in the height range between 500 and 600 m. With the 1064/532 backscatter color ratio normalized to unity for cloud scattering, the value in the dust above height 600 m was 1.18. Referring to Fig. 3, a 1064/532 backscatter color ratio of 1.18 corresponds to an estimated effective radius of 1.2 lm. In the two other cases in which this method could be applied the 1064/532 backscatter color ratio for dust was found to be 1.2 on mission sol 128 and 1.35 on mission sol 99. The color ratio of 1.35 corresponds to an estimated effective radius of 1.4 lm. This limited sample thus provides a dust effective radius in the range 1.2–1.4 lm. The dust particle number density can be estimated using the approximate expression for the extinction coefficient: a ¼ 2pR2eff N. With an extinction coefficient of a = 0.04 km1, and an
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effective particle radius in the range of Reff = 1.2–1.4 lm, the particle number density can be estimated as being in the range N = 3.2– 4.4 cm3. It should be stressed that the estimate of dust effective radius is not a direct measurement. A substantial contribution to the uncertainty in this estimate is due to the assumed value of unity for the 1064/532 backscatter color ratio for scattering from cloud ice crystals. Previous measurements and simulations for ice crystals in cirrus clouds have indicated deviations of the 1064/532 color ratio from unity within 20% (Bi et al., 2009; Vaughan et al., 2010), due to variations in size distributions and cloud ice crystal habits. That uncertainty would also be associated with the estimate of the 1064/532 backscatter color ratio obtained from the measurements in this study. Another uncertainty is associated with the assumed parameters for the distribution of dust particle size and optical properties. There has not yet been a direct measurement of the airborne dust particle optical and physical properties on Mars. The form of the size distribution and the refractive index used in previous studies (Wolff et al., 2009; Dickinson et al., 2011) was applied in the analysis here for consistency. The retrieved dust effective radius of 1.2–1.4 lm is within the range of previous estimates based on passive remote sensing measurements from the surface (e.g. Lemmon et al., 2004) and orbit (e.g. Clancy et al., 2003) during quiescent atmospheric conditions. 3. Conclusion The LIDAR instrument on the Phoenix Mars mission detected the backscatter of emitted laser pulses from ground to height 20 km. The optical extinction coefficient was derived from the recorded signals to provide a measure of the vertical distribution of atmospheric dust loading. It was observed that most of the dust resided within the planetary boundary layer (PBL) from ground to height 4 km. The ratio of the dust extinction coefficient to the background molecular density indicated that the dust was approximately evenly mixed throughout the PBL. The ratio of the signals at wavelengths 1064 nm and 532 nm was used to estimate that the effective radius associated with the dust particle size distribution was in the range of 1.2–1.4 lm. Acknowledgments The research reported here was carried out with support from the Space Science Enhancement Program of the Canadian Space Agency (CSA). Support for the LIDAR instrument Science Team during the Phoenix mission was provided by the CSA under Contract 9F007-070437/001/SR. The Phoenix mission was led by Prof. Peter H. Smith at the University of Arizona, on behalf of NASA, and managed by the Jet Propulsion Laboratory. References Ansmann, A. et al., 2003. Long-range transport of Saharan dust to northern Europe: The 11–16 October 2001 outbreak observed with EARLINET. J. Geophys. Res. 108 (D24), 4783. http://dx.doi.org/10.1029/2003JD003757. Bi, L. et al., 2009. Simulation of the color ratio associated with the backscattering of radiation by ice particles at the wavelengths of 0.532 and 1.064 lm. J. Geophys. Res. 114, D00H08. http://dx.doi.org/10.1029/2009JD011759.
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Contents lists available at SciVerse ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Phoenix LIDAR measurements of Mars atmospheric dust L. Komguem a, J.A. Whiteway a,⇑, C. Dickinson b, M. Daly a, M.T. Lemmon c a
Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada MacDonald Dettwiler and Associates (MDA), Space Missions, Brampton, Ontario, Canada c Atmospheric Sciences Department, Texas A&M University, College Station, TX, United States b
a r t i c l e
i n f o
Article history: Received 31 October 2012 Revised 15 January 2013 Accepted 23 January 2013 Available online 4 February 2013 Keyword: Mars, Atmosphere
a b s t r a c t The LIDAR instrument on the Phoenix mission obtained measurements of atmospheric dust and clouds from the surface in the Arctic region of Mars (68.22°N, 234.25°E) during late spring through the middle of summer. The observed vertical distribution of dust indicated that the planetary boundary layer (PBL) was evenly mixed up to heights of 4 km by daytime convection and turbulence. The values of the dust optical extinction coefficient derived from the LIDAR measurements within the PBL reached a maximum of 0.15 km1 during the period around summer solstice and then decreased to values approaching 0.03 km1 over the next 60 martian days (sols). The ratio of the LIDAR backscatter coefficient at wavelength 1064 nm to that at 532 nm for dust was obtained in three cases and found to have values in the range 1.18–1.35, relative to an assumed value of unity for the near surface water ice clouds. Mie scattering calculations were performed to determine that the measured dust 1064/532 backscatter color ratios were consistent with particle size distributions having effective radii in the range of 1.2–1.4 lm. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The optical properties of the atmosphere on Mars are determined primarily by airborne dust. This is most obvious during planetary scale dust storms (Strausberg et al., 2005; Smith, 2009), regional dust activity associated with weather systems (Cantor et al., 2001), and small-scale convective vortices (Cantor et al., 2006; Ellehoj et al., 2010). Even during quiescent conditions, the atmospheric optical depth in the absence of clouds is attributed to suspended dust. In comparison to Earth, absorption of solar radiation by dust has a much greater effect on temperature since the atmospheric molecular density on Mars is smaller by a factor of 100. It has been demonstrated with theoretical and modeling studies that solar heating of dust in the atmosphere of Mars can induce a dynamical feedback that would enhance the intensity of deep convection, analogous to latent heating in moist convection on Earth (Fuerstenau, 2006; Heavens et al., 2011). Observing the temporal and spatial variation of the optical properties of the suspended dust is thus key to advancing our understanding of atmospheric dynamics on Mars. The Phoenix spacecraft landed in the Arctic region of Mars (68.22°N, 234.25°E) on 25 May 2008, 30 martian days (or sols) prior to summer solstice, and operated over the next 5 months
⇑ Corresponding author. Address: Centre for Research in Earth and Space Science, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3. Fax: +1 416 736 5626. E-mail address: [email protected] (J.A. Whiteway). 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2013.01.020
(Smith et al., 2008, 2009). The mission included a LIDAR (light detection and ranging, or laser radar) instrument that was capable of measuring the vertical distribution of atmospheric dust and clouds through the planetary boundary layer, and up to heights of 20 km (Whiteway et al., 2008, 2009; Dickinson et al., 2010, 2011). This paper provides a description of the observations of atmospheric dust with the LIDAR and also an estimate of the effective radius associated with the dust particle size distribution. 2. Measurements and analysis The Phoenix LIDAR measurement technique involved emitting pulses of light vertically into the atmosphere and detecting the backscatter from dust and cloud particles (Whiteway et al., 2008). The transmitter was based on a Nd:YAG laser that emitted pulses of light with wavelengths of 1064 nm and 532 nm at a repetition rate of 100 Hz. The light scattered back from the atmosphere was collected by a 10 cm diameter telescope, and detected by an avalanche photodiode at wavelength 1064 nm and a photomultiplier at wavelength 532 nm. The signal acquisition involved 14-bit amplitude to digital conversion (ADC) from both detectors and also photon counting from the photomultiplier. The sampling interval was equivalent to 5-m height bins, but the standard operating mode during the mission used on-board averaging to 20 m vertical resolution for the analog detection and 50 m for photon counting. Temporal averaging over 20 s (2000 laser shots) was applied to the signals before on-board storage and transmission back to Earth. In the analysis for the results presented here, the recorded
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Fig. 1. (a) LIDAR backscatter signals from atmospheric dust on mission sol 48 (Ls = 98.57°), acquired with amplitude to digital conversion (ADC) and photon counting at wavelength 532 nm. (b) Dust optical extinction coefficient retrieved from the separate ADC and photon counting measurements, joined at height 3 km. (c) Ratio of dust extinction coefficient (m2/m3) to molecular density (kg/m3), which is equivalent to the integrated dust cross sectional area per unit mass (m2/kg) of molecular atmosphere.
backscatter signals were further smoothed with a running mean over 6 points (120 m) for the analog, and 12 points (600 m) for the photon counting. Fig. 1a shows the analog and photon counting backscatter signals recorded for wavelength 532 nm on mission sol 48 (solar longitude, Ls = 98.57°). In this case the signal is due only to dust. The combination of the ADC and photon counting was used to accurately record the entire dynamic range in the signal from ground to height 20 km. The analog detection was used at heights below 3 km since in the near range the optical signal strength was great enough that photon counting detection was nonlinear and saturating due to pulse pile up (Donovan et al., 1993). At heights above 5 km, only the photon counting (532 nm) was used to detect the weak optical signals. The measured signal is described in terms of the optical properties of the atmosphere by the lidar equation as:
SðzÞ ¼
Z z C 0 0 bðzÞ exp 2 r ðz Þdz z2 0
where S(z) is the measured backscatter signal as a function of height z. The backscatter coefficient, b(z) is the fraction of optical energy scattered back per unit length and per unit solid angle. The extinction coefficient, r(z) is the fractional decrease in the optical energy per unit length through the atmosphere due to scattering. The backscatter and extinction coefficients are related to the physical properties of the dust and cloud particulates in the atmosphere. The calibration constant, C, takes into account factors such as the laser pulse energy, area of the receiver aperture, throughput of the system optics, and detector efficiency. The retrieval of the extinction and backscatter coefficients was carried out using the method of Fernald (1984), with a constraint applied such that the extinction coefficient integrated from ground to height 20 km matches the total optical depth measured coincidentally by the Phoenix Surface Stereo Imager (SSI) (Lemmon et al., 2008), minus a value of 0.04 to account for dust optical depth above a height of 20 km. The value of 0.04 is an approximation based on the assumption of constant dust mixing ratio above 20 km. The analog and photon counting signals were analyzed independently assuming a constant ratio of extinction to backscatter coefficients (the lidar ratio) of 40 steradians for dust particulates. A sensitivity analysis was performed to study the effects of varying the lidar ratio on the derived extinction coefficient. It
was found that the retrieved extinction coefficient changed by no more than 3% at any height when the dust lidar ratio was varied from 20 Sr to 100 Sr, which is the range of values published for aerosols in the atmosphere of Earth (e.g. Ansmann et al., 2003; Pappalardo et al., 2004). The independently measured atmospheric optical depth provided a direct constraint on the extinction coefficient integrated over the vertical profile. Thus the value of the extinction coefficient is constrained, but the derived backscatter coefficient would depend on the assumed lidar ratio. For this reason, the extinction coefficient was used as an indicator of atmospheric dust content, rather than the backscatter coefficient. The ratio of backscatter at the separate wavelengths of 1064 nm and 532 nm was used to estimate dust particle size (Figs. 3 and 4), but that was obtained directly from the lidar signals and did not involve an assumed lidar ratio. The error bars in the derived extinction in Fig. 1b contain both the uncertainty carried over from the SSI measurement and also the contribution of random uncertainty in the measured LIDAR signals (e.g. standard deviation in photon counting), which were propagated through the calculations. The use of a constant lidar ratio is equivalent to assuming that the dust particle size distribution does not change significantly with height. This was necessary since there have been no previous measurements in the atmosphere of Mars that provide the aerosol size distribution and changes with height. Greater fall speeds for larger particles would produce a tendency to smaller particles remaining in the atmosphere with increasing height, but this would depend on the rate of vertical mixing and convection. The measurements reported in this paper show that the planetary boundary layer (PBL) dust loading is approximately evenly mixed up to height 4 km (e.g. Figs. 1 and 2). This gives support to the assumption that the dust particle size distribution was not strongly height dependent in this region, and that the assumption of constant lidar ratio is sufficient up to height 4 km. Above the PBL, and in the absence of significant convection, one could argue that the effective radius for the size distribution would gradually shift toward smaller values with height and this could add to the uncertainty in the measured extinction coefficient. However, without any knowledge of the dominant mechanism for transporting dust through the atmosphere above the PBL, the best approximation was that the lidar ratio was constant with height. The LIDAR backscatter signal from dust could be detected to heights of 20 km during late evening and early morning, when
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Fig. 2. (a) Atmospheric dust optical extinction coefficient profiles at wavelength 532 nm for five sols that are representative of the dust loading at different times during the mission. (b) Ratio of dust extinction coefficient (m2/m3) to molecular mass density (kg/m3), which is equivalent to the integrated dust cross sectional area per unit mass (m2/kg) of molecular atmosphere.
Fig. 3. Mie scattering calculation of the ratio of optical backscatter coefficient at wavelengths 1064 nm and 532 nm as function of the effective radius for the particle size distribution. Values of refractive index and effective width are given. The three measured values of color ratio and corresponding effective radii are indicated with dashed lines.
the background of scattered sunlight was a minimum. During midday the detectable signal level was limited more by background skylight such that the dust backscatter signal could be detected up to maximum heights of approximately 7 km. Thus the midday LIDAR measurements are not directly comparable to the total optical depth measurement by the SSI and the contribution above 7 km was assumed to be the same as the nearest nighttime LIDAR measurement. It is estimated that this assumption adds no more than 2% uncertainty to the extinction coefficient derived from midday LIDAR measurements, based on the variability of the optical depth at heights above 6 km in the nighttime LIDAR measurements. Fig. 1b shows the extinction coefficient derived from the signal shown in Fig. 1a. The extinction coefficient can also be considered as the product of the particle effective cross sectional area (m2) and the number density per unit volume (m3). The extinction coefficient is thus also proportional to the mass per unit volume of the particulate material. The vertical distribution of enhanced dust loading from ground to height 4 km in Fig. 1b is similar to what is observed above deserts on Earth (e.g. Dickinson et al., 2011), where dust is lifted from the surface and mixed through the planetary boundary layer (PBL) by wind stress, convection, and turbulence. In Fig. 1c the extinction coefficient (in units of m2/m3) was divided by an estimate of the background molecular density (in units of kg/m3) obtained from a numerical model of the Mars
atmosphere (Davy et al., 2009). This ratio is referred to as the density scaled extinction. It can be considered as the dust crosssectional area per unit mass of molecular atmosphere (in units m2/kg) and it is a conserved tracer. In this case the dust is mixed sufficiently through the planetary boundary layer (0–4 km) that the density scaled dust extinction is approximately constant. This is as expected for any conserved tracer in a well-mixed turbulent layer (e.g. Dickinson et al., 2011). The depth of the convective boundary layer on Mars was previously determined from radio occultation measurements of the vertical profile of temperature by the Mars Express mission (Hinson et al., 2008). While the season and location of the measurements do not overlap, the boundary layer depths obtained from the Phoenix LIDAR measurements (3–5 km) are within the range of the Mars Express results in the northern high latitudes in spring (2.6– 5.4 km). During the latter half of the mission (after sol 80) the LIDAR measurements were conducted mostly in the late evening or early morning in order to avoid a temperature dependent instrument issue that occurred only during the warmest part of the day. During this period there were nearly always clouds at the top of the boundary layer in the early morning hours (Whiteway et al., 2009; Dickinson et al., 2010) that obscured the observation of dust at heights below 5 km and attenuated the signal such that measurements of dust at heights above 5 km were not possible. Thus there were fewer LIDAR observations of dust in the second half of the mission that could be directly compared with observations during the first half. Fig. 2 shows the vertical profiles of the 532 nm extinction coefficient measured on mission sols 14 (Ls = 83.05°), 38 (Ls = 93.64°), 48 (Ls = 98.57°), 67 (Ls = 106.95°), and 96 (Ls = 121.11°). The total atmospheric optical depth measured by the SSI during those sols was, in the same order, 0.55, 0.48, 0.40, 0.35, and 0.30. Most of the change in atmospheric optical depth over this period was due to a decrease in the dust loading within the PBL. In each case the density scaled extinction (Fig. 2b) was approximately constant, or well mixed, through the PBL (ground to height 4 km). The atmospheric dust loading was a maximum during the first 40 sols of the mission, in the period around summer solstice. Ellehoj et al. (2010) have used images from the Mars Color Imager (MARCI) instrument on the Mars Reconnaissance Orbiter to show that a dust cloud passing over the Phoenix site on mission sol 25 followed a trajectory that traced back to the polar ice cap. It was noted that this was a period when CO2 ice was sublimating from the polar cap such that the dust that previously settled there would have become available to be lifted into the atmosphere. It is also noteworthy that increased dust activity has been observed near
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Fig. 4. Measurements from mission sol 109 (Ls = 127.05°). (a) Time–height contour and line plot of the backscatter signal at wavelength 532 nm and (b) time–height contour and line plot for the backscatter signal at wavelength 1064 nm. (c) 1064/532 backscatter color ratio contour and line plot. The line plots represent the average over the time interval.
the edge of the retreating north polar cap during spring with the Hubble Space Telescope (James et al., 1999). Numerical simulations have been used to demonstrate that there would be enhanced dust lifting due to strong wind speeds and weather activity driven by the thermal contrast at the polar cap edge (James et al., 1999; Toigo et al., 2002). The Phoenix LIDAR observations of maximum atmospheric dust loading during the period near summer solstice are not inconsistent with the hypothesis that the polar cap is a source region for atmospheric dust, but more detailed analyses of imaging from orbit and numerical simulations would be required to advance this interpretation. All of the LIDAR measurements presented above were obtained with the emitted wavelength of 532 nm. The signals from the emitted wavelength of 1064 nm were also recorded, but were not of sufficient signal-to-noise-ratio to provide measurements at heights above 2 km. The 1064 nm signals were useful for comparison with those at 532 nm in order to obtain an estimate of the effective radius associated with the dust particle size distribution. For example, Fig. 3 shows a calculation of the ratio of backscatter coefficients at 1064 nm and 532 nm as a function of the effective radius of a particle size distribution. The effective radius is the crosssectional area weighted mean radius for the particle size distribution. Formally it is defined as the ratio of the third moment to the second moment of the particle size distribution. The Mie scattering calculations (Grainger et al., 2004) assumed spherical particles with a Gamma distribution for particle size that was equivalent to that applied by Wolff et al. (2009) and Dickinson et al. (2011). Thus a measurement of the 1064/532 backscatter color ratio can provide an estimate of the effective radius of for an assumed particle size distribution. The relative calibration of the 532 nm and 1064 nm analog channels in the LIDAR instrument is required to obtain an accurate measure of the 1064/532 backscatter color ratio in the optical signals. This was not part of the instrument characterization since the relative laser pulse energy at the two wavelengths was dependent on the instrument internal temperature, and there was no accurate measurement of the laser output energy at either wavelength while operating on Mars. However, the atmospheric ice water clouds
provided a means to obtain a relative calibration. The clouds formed at temperatures and water vapor densities that are similar to the conditions in which cirrus clouds are known to form in the atmosphere of Earth (e.g. 65 °C, and 6 mg/m3). It has been argued that the ice particle size distribution would have been similar to that found in cirrus clouds on Earth, where ice crystal radii range in size from less than 10 lm to greater than 200 lm (Whiteway et al., 2009; Dickinson et al., 2011), and this has been demonstrated with numerical modeling (Daerden et al., 2010). The observation of fall streaks in the Mars clouds also indicates ice crystal radii of a few tens of micrometers (Whiteway et al., 2009). Ice particles of this size would have a 1064/532 backscatter color ratio that is approximately unity. The measured vertical profile of 1064/532 backscatter color ratio was normalized so that it was unity within the cloud near the surface (ground fog), and thus the value just above the cloud is accurate for estimating the color ratio for backscatter from the atmospheric dust. The dust contribution to the signal within the cloud was removed before obtaining the relative calibration. It was assumed that there was no difference in the optical extinction coefficient at 1064 nm and 532 nm within the cloud. There were three cases during the second half of the mission when the signal at wavelength 1064 nm was sufficient to measure dust above a surface cloud. The measurements on mission sol 109 (Ls = 127.05) are shown in Fig. 4. There was an enhancement in the signal at both wavelengths due to the surface cloud and a distinct cloud top in the height range between 500 and 600 m. With the 1064/532 backscatter color ratio normalized to unity for cloud scattering, the value in the dust above height 600 m was 1.18. Referring to Fig. 3, a 1064/532 backscatter color ratio of 1.18 corresponds to an estimated effective radius of 1.2 lm. In the two other cases in which this method could be applied the 1064/532 backscatter color ratio for dust was found to be 1.2 on mission sol 128 and 1.35 on mission sol 99. The color ratio of 1.35 corresponds to an estimated effective radius of 1.4 lm. This limited sample thus provides a dust effective radius in the range 1.2–1.4 lm. The dust particle number density can be estimated using the approximate expression for the extinction coefficient: a ¼ 2pR2eff N. With an extinction coefficient of a = 0.04 km1, and an
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effective particle radius in the range of Reff = 1.2–1.4 lm, the particle number density can be estimated as being in the range N = 3.2– 4.4 cm3. It should be stressed that the estimate of dust effective radius is not a direct measurement. A substantial contribution to the uncertainty in this estimate is due to the assumed value of unity for the 1064/532 backscatter color ratio for scattering from cloud ice crystals. Previous measurements and simulations for ice crystals in cirrus clouds have indicated deviations of the 1064/532 color ratio from unity within 20% (Bi et al., 2009; Vaughan et al., 2010), due to variations in size distributions and cloud ice crystal habits. That uncertainty would also be associated with the estimate of the 1064/532 backscatter color ratio obtained from the measurements in this study. Another uncertainty is associated with the assumed parameters for the distribution of dust particle size and optical properties. There has not yet been a direct measurement of the airborne dust particle optical and physical properties on Mars. The form of the size distribution and the refractive index used in previous studies (Wolff et al., 2009; Dickinson et al., 2011) was applied in the analysis here for consistency. The retrieved dust effective radius of 1.2–1.4 lm is within the range of previous estimates based on passive remote sensing measurements from the surface (e.g. Lemmon et al., 2004) and orbit (e.g. Clancy et al., 2003) during quiescent atmospheric conditions. 3. Conclusion The LIDAR instrument on the Phoenix Mars mission detected the backscatter of emitted laser pulses from ground to height 20 km. The optical extinction coefficient was derived from the recorded signals to provide a measure of the vertical distribution of atmospheric dust loading. It was observed that most of the dust resided within the planetary boundary layer (PBL) from ground to height 4 km. The ratio of the dust extinction coefficient to the background molecular density indicated that the dust was approximately evenly mixed throughout the PBL. The ratio of the signals at wavelengths 1064 nm and 532 nm was used to estimate that the effective radius associated with the dust particle size distribution was in the range of 1.2–1.4 lm. Acknowledgments The research reported here was carried out with support from the Space Science Enhancement Program of the Canadian Space Agency (CSA). Support for the LIDAR instrument Science Team during the Phoenix mission was provided by the CSA under Contract 9F007-070437/001/SR. The Phoenix mission was led by Prof. Peter H. Smith at the University of Arizona, on behalf of NASA, and managed by the Jet Propulsion Laboratory. References Ansmann, A. et al., 2003. Long-range transport of Saharan dust to northern Europe: The 11–16 October 2001 outbreak observed with EARLINET. J. Geophys. Res. 108 (D24), 4783. http://dx.doi.org/10.1029/2003JD003757. Bi, L. et al., 2009. Simulation of the color ratio associated with the backscattering of radiation by ice particles at the wavelengths of 0.532 and 1.064 lm. J. Geophys. Res. 114, D00H08. http://dx.doi.org/10.1029/2009JD011759.
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