Cameron S. Dickinson

Mars Water-Ice Clouds and Precipitation

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. Laser remote sensing from Mars’ surface revealed water-ice clouds that formed during the day and precipitated at night. The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface.

Lidar on the Phoenix mission to Mars

[1]xa0A lidar system for atmospheric measurements will operate from the surface of Mars as part of the Phoenix mission. This will measure the height profile of backscattered laser light from airborne dust and clouds. These observations will be coordinated with solar radiation measurements and in situ sampling to study the climate and the water cycle. The design and testing of the lidar system are described, and measurements are presented that demonstrate the analysis methods and the performance characteristics.

Phoenix and MRO coordinated atmospheric measurements

[1]xa0The Phoenix and Mars Reconnaissance Orbiter (MRO) missions collaborated in an unprecedented campaign to observe the northern polar region summer atmosphere throughout the Phoenix mission (25 May to 2 November 2008; Ls = 76°–150°) and slightly beyond (∼Ls = 158°). Five atmospherically related campaigns were defined a priori and were executed on 37 separate Martian days (sols). Phoenix and MRO observed the atmosphere nearly simultaneously. We describe the observation strategy and history, the participating experiments, and some initial results. We find that there is general agreement between measurements from different instruments and platforms and that complementary measurements provide a consistent picture of the atmosphere. Seasonal water abundance behavior matches with historical measurements. Winds aloft, as measured by cloud motions, showed the same seasonally consistent, diurnal rotation as the winds measured at the lander, during the first part of the mission (Ls = 76°–118°). A diurnal cycle recorded from Ls ∼ 108.3°–109.1°, in which a dust front was approaching the Phoenix Lander, is examined in detail. Cloud heights measured on subsequent orbits showed that in areas of active lifting, dust can be lofted quite high in the atmosphere, doubling in height over 2 h. The combination of experiments also revealed that there were discrete vertical layers of water ice and dust. Water vapor column abundances compared to near-surface water vapor pressure indicate that water is not well mixed from the surface to a cloud condensation height and that the depth of the layer that exchanges diurnally with the surface is 0.5–1 km.

Temperature, pressure, and wind instrumentation in the Phoenix meteorological package

[1]xa0The meteorological package (MET) on the Phoenix Lander is designed to provide information on the daily and seasonal variations in Mars near-polar weather during Martian late spring and summer. The present paper provides some background on the temperature, pressure, and wind instrumentation on the Phoenix MET station and their characterization. A separate paper addresses the MET lidar instrument. Laboratory studies in a Mars wind tunnel confirm estimates that the time constant of the thermocouples should be less than 0.5 s for wind speeds of 5 m s−1 or greater. Solar radiation falling on the thermocouples could raise the reported temperatures by up to 0.7 K for wind speeds of 5 m s−1. The increase will be wind speed dependent and will increase to 0.8 K at U = 3 m s−1 under peak solar radiation. Pressure sensors will give Mars surface pressures accurate to 10 Pa or better while Telltale deflections should provide reliable wind speed information up to at least 10 m s−1. The paper also discusses, to a limited extent, how the MET instruments will be used in conjunction with other instruments on the Phoenix Lander to provide an enhanced meteorological data set. We also describe instrumentation related to the Atmospheric Structure Experiment during entry, descent, and landing (EDL). These instruments will provide deceleration data. Together with drag coefficient information and a surface pressure measurement from MET, these data will allow us to infer the density, pressure, and temperature structure throughout the vertical column during EDL.

On pressure measurement and seasonal pressure variations during the Phoenix mission

[1]xa0In situ surface pressures measured at 2 s intervals during the 150 sol Phoenix mission are presented and seasonal variations discussed. The lightweight Barocap®/Thermocap® pressure sensor system performed moderately well. However, the original data processing routine had problems because the thermal environment of the sensor was subject to more rapid variations than had been expected. Hence, the data processing routine was updated after Phoenix landed. Further evaluation and the development of a correction are needed since the temperature dependences of the Barocap sensor heads have drifted after the calibration of the sensor. The inaccuracy caused by this appears when the temperature of the unit rises above 0°C. This frequently affects data in the afternoons and precludes a full study of diurnal pressure variations at this time. Short-term fluctuations, on time scales of order 20 s are unaffected and are reported in a separate paper in this issue. Seasonal variations are not significantly affected by this problem and show general agreement with previous measurements from Mars. During the 151 sol mission the surface pressure dropped from around 860 Pa to a minimum (daily average) of 724 Pa on sol 140 (Ls 143). This local minimum occurred several sols earlier than expected based on GCM studies and Viking data. Since battery power was lost on sol 151 we are not sure if the timing of the minimum that we saw could have been advanced by a low-pressure meteorological event. On sol 95 (Ls 122), we also saw a relatively low-pressure feature. This was accompanied by a large number of vertical vortex events, characterized by short, localized (in time), low-pressure perturbations.

Simulating observed boundary layer clouds on Mars

[1]xa0A microphysical model for Mars dust and ice clouds has been applied in combination with a model of the planetary boundary layer (PBL) for the interpretation of measurements by the LIDAR instrument on the Phoenix Mars mission. The model simulates nighttime clouds and fall streaks within the PBL that are similar in structure to the LIDAR observations. The observed regular daily pattern of water ice cloud formation and precipitation at the top of the PBL is interpreted as a diurnal process in the local water cycle in which precipitation of large ice crystals (30–50 μm effective radius) results in downward transport of water vapor within the PBL. This is followed by strong vertical mixing during daytime, and this cycle is repeated every sol to confine water vapor within the PBL.

Lidar measurements of clouds in the planetary boundary layer on Mars

[1]xa0The LIDAR instrument on the Phoenix mission provided observations of clouds within the Planetary Boundary Layer (PBL) on Mars. In mid to late summer there was a regular Sol-to-Sol pattern with cloud formation at around midnight and dissipation before midday. The ice water content (IWC) of the clouds was estimated from the measurements with peak values at 6 am of 1 mg/m3, associated with total column IWC of up to 5 g/m2. The time of cloud formation did not change throughout the second half of the mission. This is consistent with the expected atmospheric cooling, if the observed decreasing trend in the column amount of water occurred mainly within the PBL.

Observations of near-surface fog at the Phoenix Mars landing site

The Surface Stereo Imager (SSI) on the Phoenix Mars Lander was able to complement the operations of the LIDAR on four occasions during the mission by observing the laser beam while the LIDAR laser was transmitting. These SSI observations permitted measurement of the scatter from atmospheric aerosols below 200 m where the LIDAR emitter and receiver do not overlap fully. The observed laser scattering was used to estimate the ice-water content (IWC) of near surface fog. Values of IWC up to 1.7 ± 1.0 mg m−3 were observed. Compared to air aloft, fog formation was inhibited near the surface which had accumulated at least 30 ± 24 mg m−2 (0.030 pr-μm) on sol 113. Microphysical modeling shows that when precipitation is included, up to 0.48 pr-μm of water may be present on the surface at the time of measurement. Integrated over the entire night, this represents up to 2.5 pr-μm of water taken up diurnally by the surface, or 6% of the total water column.

The OSIRIS-REx laser altimeter

The OSIRIS-REx Laser Altimeter (OLA) is a scanning laser altimeter onboard the NASA mission to the near-Earth asteroid 101955 Bennu. We will describe the operation and unique capabilities of the instrument for an asteroid mission.

Compact high-speed scanning lidar system

The compact High Speed Scanning Lidar (HSSL) was designed to meet the requirements for a rover GN&C sensor. The eye-safe HSSLs fast scanning speed, low volume and low power, make it the ideal choice for a variety of real-time and non-real-time applications including: 3D Mapping; Vehicle guidance and Navigation; Obstacle Detection; Orbiter Rendezvous; Spacecraft Landing / Hazard Avoidance. The HSSL comprises two main hardware units: Sensor Head and Control Unit. In a rover application, the Sensor Head mounts on the top of the rover while the Control Unit can be mounted on the rover deck or within its avionics bay. An Operator Computer is used to command the lidar and immediately display the acquired scan data. The innovative lidar design concept was a result of an extensive trade study conducted during the initial phase of an exploration rover program. The lidar utilizes an innovative scanner coupled with a compact fiber laser and high-speed timing electronics. Compared to existing compact lidar systems, distinguishing features of the HSSL include its high accuracy, high resolution, high refresh rate and large field of view. Other benefits of this design include the capability to quickly configure scan settings to fit various operational modes.