A year of convective vortex activity at Gale crater

Icarus 278 (2016) 180–193

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A year of convective vortex activity at Gale crater Kathryn Steakley∗, James Murphy Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003, United States

a r t i c l e

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Article history: Received 2 March 2016 Revised 10 June 2016 Accepted 13 June 2016 Available online 21 June 2016 Keywords: Mars, atmosphere Atmospheres, dynamics Meteorology

a b s t r a c t Atmospheric convective vortices, which become dust devils when they entrain dust from the surface, are prominent features within Mars’ atmosphere which are thought to be a primary contributor to the planet’s background dust opacity. Buoyantly produced in convectively unstable layers at a planet’s surface, these vertically aligned vortices possess rapidly rotating and ascending near-surface warm air and are readily identified by temporal signatures of reduced atmospheric surface pressure measured within the vortex as it passes by. We investigate such signatures in surface pressure measurements acquired by the Rover Environmental Monitoring Station aboard the Mars Science Laboratory rover located within Gale crater. During the first 707 sols of the mission, 245 convective vortices are identified with pressure drops in the range of 0.30–2.86 Pa with a median value of 0.67 Pa. The cumulative distribution of their pressure drops follows a power law of slope −2.77 and we observe seasonal and diurnal trends in their activity. The vast majority of these pressure signatures lack corresponding reductions in REMS-measured UV flux, suggesting that these vortices rarely cast shadows upon the rover and therefore are most often dustfree. The relatively weak-magnitude, dustless vortices at Gale crater are consistent with predictions from mesoscale modeling indicating that the planetary boundary layer is suppressed within the crater and are also consistent with the almost complete absence of both dust devils within Mars Science Laboratory camera images and Gale crater surface dust devil streaks within orbiter images. © 2016 Elsevier Inc. All rights reserved.

1. Introduction Convective vortices are small-diameter, vertically-oriented, warm-core, atmospheric circulations that develop as a result of superadiabatic, buoyantly unstable vertical atmospheric temperature gradients near the surface/atmosphere interface (Ryan and Carroll, 1970). When convective vortices lift dust from a planetary surface, they become dust devils, which have been observed on both Earth and Mars (Balme and Greeley, 2006). General circulation model studies of the martian atmosphere imply that dust devils might be the main contributor to the radiatively important background atmospheric dust load (Newman et al., 2002a, b; Basu et al., 2004; Kahre et al., 2006). This modeling work raises the important questions of: how often do convective vortices occur on Mars?; how much dust do they lift into the atmosphere?; and what local conditions enhance or inhibit vortex dust lifting? Observations of convective vortices by orbiters and landers on Mars have been and continue to be assessed in the pursuit of answering these questions. The meteorological measurements being provided by NASA’s Mars Science Laboratory (MSL) Curiosity rover offer

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Corresponding author. Tel.: +1 575 646 4834; fax: +1 575 646 1602. E-mail address: [email protected] (K. Steakley).

http://dx.doi.org/10.1016/j.icarus.2016.06.010 0019-1035/© 2016 Elsevier Inc. All rights reserved.

additional, unique insights into martian dust devil characteristics and are the focus of this paper. A convective vortex consists of a cylindrical column of rotating air possessing substantial vertical and tangential wind speeds (Sinclair, 1973) in the midst of a local reduced surface pressure environment. The hydrostatic surface pressure within a vortex decreases towards the vortex center (Sinclair, 1973; Ringrose et al., 2007). It is this pressure gradient that drives the vortex’s horizontal quasi-cyclostrophic circulation; surface friction induces radial mass inflow toward the vortex center. Increasing near-surface atmospheric temperature with decreasing radius within the vortex is sometimes also noted (Sinclair, 1973). In situ meteorological observations can indicate the occurrence of passing convective vortices via distinct signals present in measured time variations of surface pressure, as well as near-surface atmospheric temperature variations, wind speed and direction variations, and changes in received solar flux magnitude if the vortex contains suspended dust (Sinclair, 1973; Tratt et al., 2003; Ringrose et al., 2007). The detection of convective vortices based on their characteristic pressure ‘drops’ (decrease of measured pressure as the vortex passes over or near the pressure measuring instrument) has been utilized as a vortex identification method in Earth-based surface meteorology surveys (Lorenz and Lanagan, 2014; Jackson and Lorenz, 2015; Lorenz and Jackson, 2015) and in prior Mars

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surface missions including the Pathfinder Lander (Schofield et al., 1997; Murphy and Nelli, 2002) and the Phoenix Lander (Ellehoj et al., 2010). Pressure measurements from these Mars missions were obtained at 0.25–2.0 Hz sampling rates (Pathfinder: 0.25– 2 Hz, Phoenix: 0.5 Hz) and the characteristic vortex pressure drops had typical durations between a few to 60 s (Murphy and Nelli, 2002; Ferri et al., 2003; Ellehoj et al., 2010). Similar convective vortex signatures have now been identified in data obtained by the MSL Curiosity Rover (Kahanpää et al., 2013; Haberle et al., 2014; Harri et al., 2014; Moores et al., 2015; Kahanpää et al., 2016). Wind signatures of convective vortices, represented by wind vector rotation and speed increase as the core of the vortex is approached, have also been investigated extensively through field studies on Earth (Ryan and Carroll, 1970; Sinclair, 1973; Tratt et al., 2003; Ringrose et al., 2007; Balme et al., 2012) as well as in the lab with vortex generators (Greeley et al., 2003; Neakrase and Greeley, 2010). Dust devils were first detected in situ on Mars at the Viking Lander 2 site based on wind velocity signatures (Ryan and Lucich, 1983; Ringrose et al., 2003). The Curiosity rover landed within Mars’ near-equatorial 154-km diameter Gale crater (5.4°S and 137.8°E) on August 6th , 2012 (UTC), at a martian seasonal date of late southern winter (LS 151.15°). The rover’s scientific instrumentation includes the Rover Environmental Monitoring Station (REMS) meteorological instrument suite, designed to provide measurements of surface pressure, near-surface air temperature and ground temperature, downward solar UV flux, relative humidity, and wind speed and direction (Gómez-Elvira et al., 2012). Although there have been detections of convective vortices within the REMS pressure data (Haberle et al., 2014; Harri et al., 2014; Moores et al., 2015; Kahanpää et al., 2016), these events have not yet been thoroughly characterized. Haberle et al. (2014) and Harri et al. (2014) briefly discuss the detection of vortices in the first 100 sols of the MSL mission, show an example of a detected vortex, and discuss that it was unexpected to find pressure signatures of vortices given the suppression of the planetary boundary layer (PBL). Moores et al. (2015) identify 149 dustless vortices based on their pressure drop signatures, but do not observe visible dust devils in the MSL navigation camera images, which is also consistent with a shallow PBL. The PBL comprises the layers of the atmosphere within which energy and momentum exchange between the atmosphere and the surface are the dominant factors controlling the thermodynamic structure. The PBL can encompass the lower few 10’s of meters of the atmosphere under very stable conditions, extending to ∼10 km under conditions of intense convection (Petrosyan et al., 2011). Rennó et al. (1998) concluded that the maximum intensity (pressure drop magnitude within the vortex core) of a convective vortex can be related to the thickness (height) of the PBL within which the vortex develops. The PBL height is related to the intensity of near-surface convection, with the PBL being characterized by an adiabatic, vertical temperature profile (Deardorff, 1970; Petrosyan et al., 2011). The PBL height grows after sunrise as the Sun warms the surface and that thermal energy is conveyed to the near-surface atmosphere and thereafter convectively transferred upward. The unfettered PBL’s maximum vertical extent occurs in the early afternoon, after which its depth declines to a shallow dynamically-induced vertical extent after sunset (Haberle et al., 1993; Savijärvi, 1999; Petrosyan et al., 2011). Mesoscale numerical modeling of Gale crater indicates that the PBL there is likely suppressed (∼2 km at landing site) compared to its vertical extent at neighboring locations outside the crater (3–9 km) due to intracrater circulations driven by the crater’s rim wall and its kilometers-tall central Mount Sharp (Vasavada et al., 2012; Tyler and Barnes, 2013, 2015). Tyler and Barnes (2013) estimate PBL depth in the northern part of the crater at the landing site to be ∼2 km, while it is 3–5 km the southern part of the

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crater and 3–9 km in the surrounding region outside the crater. Such Gale crater PBL height suppression would be expected to influence the achieved magnitude of convective vortices occurring there. In this paper we present the identification and characterization of convective vortex signatures within measured surface pressures provided by Curiosity’s REMS during the first 707 sols of the MSL mission. We find that the pressure drop magnitudes of identified vortices are generally less intense than those of vortices detected at other Mars locations (Murphy and Nelli, 2002; Ellehoj et al., 2010), which is consistent with the suggestion that Gale crater circulations suppress the local PBL depth (Tyler and Barnes, 2013; Moores et al., 2015). In Section 2 we describe the MSL REMS data analyzed for this investigation. In Section 3 we describe our vortex detection method. Vortex identification results are presented in Section 4, while within Section 5 we discuss time of sol, seasonal, and intensity variations of identified vortices, and why they appear to lift little to no dust (Moores et al., 2015). Our conclusions are presented in Section 6. 2. MSL REMS data MSL’s REMS instrument suite (Gómez-Elvira et al., 2012, 2014; Harri et al., 2014) provides measurements that characterize the meteorological conditions at the rover’s surface location within Gale crater. The suite includes pressure sensors (Harri et al., 2014), thermistors measuring near-surface gas temperature (Gómez-Elvira et al., 2014), an infrared sensor measuring emitted flux from the surface from which ground temperature is derived (Hamilton et al., 2014), a 6-channel ultraviolet wavelength radiant energy sensor quantifying downward solar flux at the surface (Gómez-Elvira et al., 2014), a relative humidity sensor (Gómez-Elvira et al., 2014), and a wind velocity sensor. The REMS wind sensor was damaged during the rover landing (Gómez-Elvira et al., 2014) so we do not focus on the derived wind speed data; nor do we focus on humidity measurements herein. The REMS pressure measuring system consists of four separate sensors each comprised of a Vaisala Barocap single-crystal silicon micromachined sensor head and a Thermocap (Gómez-Elvira et al., 2012, 2014; Harri et al., 2014). There are two sets of paired sensors. One set consists of two sensors designed for rapid warmup pressure measurements, while the other set consists of a single rapid warm-up sensor coupled with a longer warm-up sensor designed for high-stability over extended time. The pressure measurements investigated in this work are those from the rapid warm-up sensor in the rapid warm-up/high-stability pair (transducer #2; Gómez-Elvira et al., 2014). Pre-flight instrument calibration indicated an instrument noise level of ∼0.2 Pa, defined as the peak-to-peak value of the noise (Harri et al., 2014); operation has shown sensor noise at the 0.1 Pa level (Gómez-Elvira et al., 2014). This noise magnitude is verified during instrument operation on Mars and is demonstrated in Sections 3 and 4. REMS’ transducer #2 s rapid warm-up pressure sensor output is sampled at 1 Hz when the REMS system is turned on, as are the temperature, wind, humidity, and UV flux instrument measurements. The high-stability pressure sensor is sampled at a 16-s interval and is not further considered here. Standard REMS operation consists of 5-min duration sessions initiated at the start of each hour of the sol (in Local Mean Solar Time, LMST), augmented by 60-min duration sessions spread through the sol with a pattern generally not repeating from one sol to the next (Fig. 1; compare with Fig. 1 in Gómez-Elvira et al., 2014, in which LMST is employed). There are also some 15-min and 45-min duration sessions (Gómez-Elvira et al., 2014). Prior to sol 71, the session lengths were defined using Earth hour (3600 s) and minute (60 s) durations. Subsequent to sol 71, the session durations were redefined

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Fig. 1. MSL REMS temporal measurement coverage (mission sols 1–707; LS 151° MY 31 through LS 171° MY 32). Local true solar time (x axis) vs. mission sol number (left y axis) or solar longitude (LS ) (right y axis). Horizontal lines indicate times that the REMS sensor was operational. REMS operated for five minutes at the beginning of each hour in LMST, and occasionally for 15 and 60 min segments. Thicker sections denote nearly complete time coverage; for example sols 600 through 630 had more than 80% time coverage around the 12 LTST hour. The apparent vertical curvature is due to the operational times being determined with respect to LMST rather than LTST.

in martian LMST time, where one LMST hour spans roughly 3699 s and 5 LMST minutes include roughly 308 s. REMS pressure measurements, as well as the measurements from REMS’ other sensors, are available from the data archive curated at NASA’s Planetary Data System (PDS) Atmospheres Node (Chanover et al., 2015). REMS atmospheric temperature measurements are provided by two thermistor systems, one mounted immediately beneath each of the two wind sensor booms which have their long axes in the horizontal plane and are attached to the vertical camera mast positioned near the front right of the rover. One wind sensor boom points in the rover forward direction (Boom #2) while the other (Boom #1) is positioned 120-degrees clockwise (as viewed from above) from the front-pointing boom (Gómez-Elvira et al, 2012). The archived ambient temperatures are the product of a model that accounts for three individual thermistor measurements from each of the two thermistor systems. Each system contains a thermistor at the far end of its 35 mm long fin, another positioned two-thirds of the distance along the downward-angled fin from its boom-mast juncture, and a third at the fin’s boom-mast juncture (see Gómez-Elvira et al, 2012, Fig. 9). The model additionally accounts for radiative, conductive, and advective/convective effects arising from the thermistor itself as well as the surrounding ground environment and a thermal characterization of the rover deck and the rover’s aft-mounted radioisotope thermoelectric generator (Gómez-Elvira et al, 2012). The ambient air temperature sensor response, 20–80 s depending upon environmental conditions (Gómez-Elvira et al., 2014), is long compared to the time extent of most vortex pressure signatures. This slow response time results in ambient air temperature measurements being ambiguously diagnostic of warm-core vortex passage. The REMS instrument suite includes a downward UV flux sensor mounted atop the rover deck. The UV sensor consists of six diodes collectively spanning the 200–380 nm wavelength range, with one of the diodes providing full wavelength coverage

(Gómez-Elvira et al., 2012, 2014). Each diode has its own transparent cover and is surrounded by a magnet intended to divert settling dust from accumulating on the diode cover. The six diodes are arranged in a circular pattern located near the rover’s frontright mast. The diodes are individually susceptible to shadowing from rover structures (the mast and the cameras it hosts, the mechanical arms) and any atmospheric obscuration (dust devil, dust or condensate cloud, etc.). Diode measurements are provided at the same sampling cadence as the pressure and temperature measurements. As previously mentioned, one or more components of the REMS wind sensor on Boom #1 became damaged at some point in time after launch (Gómez-Elvira et al., 2014). This has resulted in minimal reporting of wind speed or direction within the REMS archived data (Gómez-Elvira et al., 2014). However, there are some signals from the wind sensors that can provide qualitative value to this investigation. Each of the two REMS booms mounted on the Remote Sensing Mast (RSM) includes three longitudinally-aligned electronic boards spaced around the boom at 120° intervals. One board (designated #2) is positioned along the bottom of the boom. Boom #1’s board #1, located leftward around the boom from board #2 when viewed looking outward from the RSM (see GómezElvira et al., 2012, Fig. 19), provides no signals within the archived REMS data. The REMS wind sensor design is based upon thermal anemometry (Gómez-Elvira et al., 2012). Each of the six boards includes five ‘die’: four actively heated die arranged in square configuration and positioned at the mast-end of the board, and the fifth die that is not actively heated and which is positioned at the far end of the board. This fifth die provides a reference temperature against which the four other die are heated to maintain a constant specified overheat. Longitudinal (along boom/board long axis) and transverse (across board long axis) differential thermal conductance among the four heated die on each operating board are measured and in a fully operating wind sensor system would

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polynomial value. The “1σ ” duration of each event corresponds to the total time, in seconds, between consecutive pressure measurements that were more than 1 standard deviation (1σ ) below the interval’s polynomial-subtracted mean pressure value. Fig. 3 illustrates the detection and characterization of a candidate vortex event. 4. Identified convective vortices

Fig. 2. Convective vortex pressure drop signatures from three Mars surface robotic missions. Graphed pressure values are the result of centering the x axis around the pressure drop minimum and centering the y axis around 0 by subtracting the median pressure value of each data set over the time shown. Examples are from: MSL (LS 199.6°; LMST 10:37 – solid line), Pathfinder during a period with a 0.25 Hz sampling rate (LS 179.6°; LMST 13:29 – dashed line), and Phoenix (LS 140.6°; LMST 12:08 – dotted line).

be employed to derive the 3-D wind vector. Despite the board’s failure, the sensor does provide signals that during short, vortexscale time intervals can be interpreted as evidence of change in wind direction. 3. Vortex identification Convective vortices can be observationally characterized by a distinct decline and subsequent increase in measured surface pressure lasting a few to tens of seconds (Balme and Greeley, 2006). Such signatures have previously been detected in measured martian surface pressures including REMS pressures (Fig. 2). To identify vortex candidate events present within the first 707 sols of the REMS pressure data, a 3rd order polynomial was fit to the measured pressures within a specified time interval to characterize the long term (5–15 min) pressure trend. This trend was subsequently subtracted to isolate shorter period variations (Murphy and Nelli, 2002). Hour-long sessions were usually immediately followed by 5-min sessions, but were sometimes followed by 15-, 45-, or additional 60-min sessions, creating long continuous segments of varying length. Segments longer than 15 min were divided into consecutive 10 0 0-s intervals plus a final interval of varying length depending on the total continuous segment time. The 3rd order polynomial curve fitting and subtraction was then applied to these newly defined intervals, as well as to the individual 5- and 15min sessions. 45-min sessions were divided into three ∼10 0 0 s sessions. The polynomial-subtracted time series were then analyzed to identify pressure signature occurrences that satisfied each of three criteria: (1) the measured pressure drop magnitude exceeded three standard deviations from the polynomial-subtracted session average (with drop magnitude and standard deviation determined from the polynomial-subtracted pressure values), (2) the “1σ ” pressure drop duration, described below, was at minimum two seconds (to avoid the false identification of individual pressure value outliers as vortices), and (3) the pressure drop magnitude exceeded 0.3 Pa in order to distinguish events from the REMS pressure sensor peakto-peak noise of ∼0.2 Pa. Pressure drop magnitudes were calculated by subtracting the minimum pressure during an event from the contemporaneous

One thousand and eight daytime (080 0–180 0 LMST) pressure drop occurrences met the three vortex identifying criteria described in the previous section. Seven hundred and thirty two of these events exhibited pressure drops greater than 4σ , while 371 events exhibited pressure drops greater than 5σ in magnitude. Each of these 1008 occurrences was visually inspected to assess its validity as a candidate vortex. This visual assessment indicated that some identified events were obviously not vortexlike (556 events), with some containing 10–20 s data gaps within a session while others exhibit event variability almost indistinguishable from the overall variability spanning 1–2 min encompassing the event (Fig. 4a and b). Some identified events were potentially valid but too ambiguous to confirm and were excluded (193 events, Fig. 4c and d). An additional 14 events were removed due to a UV sensor shadowing effect described in Harri et al. (2014) and in Section 5.3 below. The remaining 245 events present what appear to be unambiguous, verified convective vortex characteristics, examples of which are shown in Fig. 5. Pressure drop magnitudes of all 245 verified vortex occurrences are presented in Supplementary Table 1. The pressure drop magnitudes, ࢞P, range from 0.3 Pa (the lower limit used in the identification criteria) to 2.86 Pa. Mean and median ࢞P values are 0.67 and 0.56 Pa, respectively. In addition to pressure drop magnitudes, other characterizing parameters of these vortex events include their duration and the mean pressure and ambient temperature at the time of the event (Table 1 in Supplementary Materials). The time duration of each vortex was characterized by two methods, the “1σ ” duration and also the full-width-at-half-maximum (FWHM) duration. The “1σ ” vortex durations, described in Section 3, ranged from 2 s (the minimum requirement for vortex identification) to 54 s, and had a median value of 11 s. Although long duration events have been observed in meteorological data obtained on Mars (Ferri et al., 2003; Ellehoj et al., 2010), the 54 s duration of the longest MSL event is not due to a large magnitude but to a pressure drop signature with long, asymmetrical, varying wings. This may be the result of a vortex with additional structure or one that has a non-liner, perhaps circular, translation path. The FWHM was calculated as the time span between the half-amplitude ingress and egress pressure values (Fig. 3). Ingress and egress slopes were calculated via linear interpolation. FWHM values ranged from 1.4 to 20.3 s with a median FWHM of 5.3 s. These vortex duration values are similar to those determined for vortices encountered by Mars Pathfinder (full durations between 15 and 51 s, Ferri et al., 2003) and Phoenix (FWHM values mostly between 5 and 15 s, Ellehoj et al., 2010). There is no evident correlation between vortex pressure drop magnitude and duration. The ambient pressure and ambient temperature values corresponding to each vortex were determined by averaging the ten measurements both preceding and following the period when the pressure event drops 3σ below the continuum trend. Since the REMS instrument suite does not provide continuous temporal coverage (Fig. 1), the 245 verified vortex occurrences are a lower estimate of the actual number of vortices encountered by MSL. By accounting for the percentage of integrated time REMS was in operation during each LTST hour over the 707 sols investigated here, the number of encountered vortices can be estimated for each LTST hour by normalizing the number of verified

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Fig. 3. Vortex characterization process, Sol 302, LTST 11:06:49. Top panel: REMS pressure measurements (solid line) for the sol 302 vortex are shown with the 3rd order polynomial continuum fit (dotted line). Middle panel: Continuum subtracted pressure (solid line) is shown for the same sol 302 event and was calculated by subtracting the continuum in panel 1 from the REMS pressure in panel 1. A dotted line for reference is shown at y = 0. Lower panel: Same as middle panel but with smaller axis range. Two horizontal bars depict the 1σ duration and FWHM for this pressure drop. The vertical bar shows the pressure drop magnitude.

occurrences per hour by the fraction of time that hour was sampled. Such a normalization, based upon the coverage indicated in Fig. 1, results in an ‘inferred’ vortex occurrence number of 724, approximately one per sol during the first 707 mission sols. 5. Discussion 5.1. Diurnal variation of vortex occurrence The time-of-sol distribution of the 245 verified vortex events within 1-h LTST time intervals accumulated over all 707 sols (Fig. 6a) indicates that the 11:00 to 13:00 LTST time period experienced the most frequent vortex encounters, a characteristic generally consistent with previous Mars measurements (Murphy and Nelli, 2002; Ellehoj et al., 2010; Greeley et al., 2010). The largest measured pressure drop magnitudes also occurred during this same time of sol (Fig. 6a). Inferred vortex activity peaks between the hours of 11:00 and 12:00 LTST (Fig. 6b). This LTST time-of-sol peak in inferred vortex activity at MSL occurs one hour bin earlier than the inferred activity at the Pathfinder landing site (Murphy and Nelli, 2002). The MSL time of day peak is also earlier than at the Phoenix landing site, although during LS 111–148° Phoenix exhibited a secondary

peak in activity around 10:00 LMST (Ellehoj et al., 2010). One note about these time-of-sol comparisons is that MSL data analyzed here span a full martian year (discussed further in Section 5.2) while neither Pathfinder nor Phoenix extended beyond northern spring and summer. The time-of-sol dependence of vortex occurrence is consistent with the daily growth and collapse of the PBL (Petrosyan et al., 2011, and references therein) and Rennó et al.’s (1998) correlation of vortex intensity with PBL depth. As the PBL expands vertically, vortices develop and become more intense. Unfettered PBL depth maximizes in early afternoon (130 0–140 0 LTST) so vortex intensity (and presumably occurrence) maximum would be expected to occur then. PBL depth models generally indicate that PBL depth is greatest in early afternoon for the Viking (Haberle et al., 1993) and Pathfinder (Savijärvi, 1999) landing sites and observations of martian convective vortices/dust devils from other landing missions typically observe the most vortex occurrences in the early afternoon (Murphy and Nelli, 2002; Ellehoj et al., 2010; Greeley et al., 2010). The indication that MSL’s inferred vortex occurrence maximizes prior to noon suggests that the PBL depth within Gale crater does not necessarily continue to increase after noon. This topic is further addressed in Section 5.3 when vortex intensity is discussed.

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Fig. 4. Examples of eliminated candidate convective vortex events. Four examples of candidates eliminated during the by-eye vortex identification: (a) a candidate eliminated due to the 16 s gap in the middle of the signature, (b) a candidate whose signature is indistinct from the surrounding times, (c) and (d) candidates referred to as “ambiguous eliminations,” or candidates which could potentially be vortices but whose signatures were not convincing enough to pass the by-eye identification.

5.2. Seasonal variation of vortex occurrence MSL’s long, ongoing mission duration provides a unique in situ opportunity to assess seasonal variations in vortex occurrence, augmenting the multi-year seasonal variability in vortex occurrence derived from imaging provided by the MER Spirit rover within Gusev crater (Greeley et al., 2010). At Gale crater’s nearequatorial location, seasonal variations primarily result from the variation in received solar flux arising from Mars’ obliquity and orbit eccentricity. Winter traveling baroclinic weather systems that affect middle-latitude (Viking Landers 1 and 2; Barnes, 1981; Murphy et al., 1990) and high-latitude (Phoenix; Ellehoj et al., 2010; Nelli et al., 2010) locations are absent at Gale crater’s equatorial location. The near phasing of orbit aphelion (LS 71°) with southern winter solstice (LS 90°) results in an annual minimum local noontime downward solar flux (at the atmosphere top) of 428 W m−2 (Fig. 7) at LS 82°. The maximum flux of 683 W m−2 occurs at LS 225°, which precedes both orbit perihelion (LS 251°) and southern summer solstice (LS 270°). The seasonal distribution of inferred vortex occurrence (Fig. 7) exhibits a variation that is in phase with the local noontime downward solar flux magnitude and with REMS-measured near-surface ambient temperature. Since the number of sols during the LS 180– 270° time interval (143 sols) is less than the number (193 sols) during LS 0–90°, the number of vortices per sol during local spring, LS 180–270° (∼1.5 per sol) is twice that during local autumn, LS 0–90° (∼0.75 per sol). The panel of four plots in Fig. 8 shows the LTST distribution for each season separately and uses the same representation for the logarithmic pressure drop magnitude bins as in Fig. 6a and b. High-magnitude pressure drops are detected throughout the year. Southern spring and summer appear to

either have secondary peaks in activity during the late afternoon hours between 14:00 and 16:00 LTST or a lull in activity in the early afternoon (14:0 0–15:0 0 LTST in spring and 13:0 0–14:0 0 LTST in summer). There do appear to be seasonal variations in vortex activity at Gale, with vortices occurring more frequently during southern spring and summer than during southern fall and winter (Fig. 7). This trend is similar to that observed visually by the Spirit rover at Gusev crater, where spring was the season of greatest dust devil activity (Greeley et al., 2010). At the MSL landing site, vortex activity peaks before 12:00 LTST during the spring and summer seasons but appears to peak an hour later during the fall and winter seasons (Fig. 8). The winter season is dominated by low-magnitude pressure drop occurrences (Fig. 8). The spring season exhibits a greater frequency of large-magnitude pressure drop events, but there is also a greater frequency of large-magnitude vortices during autumn (Fig. 8). It is difficult to make definitive conclusions about these seasonal variations in pressure drop magnitude given the low number of high-amplitude events observed. We note that because time coverage is more extensive between LS 0° and 180° than it is between LS 180° and 0° (Fig. 1) the inferred number of vortices during fall and winter are affected more by the normalization of time coverage than other seasons. Unfortunately there are no sols during the MSL mission thus far with continuous coverage over an entire sol. This would be beneficial for comparisons with the Phoenix mission (Ellehoj et al., 2010), which had continuous coverage, and the Pathfinder mission (Murphy and Nelli, 2002), which included 5 sols with continuous coverage. Murphy and Nelli (2002) noted that the frequency of detected events during these full coverage sols was greater than the frequency inferred from incompletely sampled sols, and

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Fig. 5. Examples of MSL pressure drops. Each drop is labeled with the LS and LTST (hr:min) at which it occurred. The x axis shows the time before and after each pressure drop minimum in seconds. Vertical dotted lines span the median FWHM for all detected MSL vortices, ∼5.3 s. The gray shading around the event on LS 211.4° shows the 0.2 Pa peak-to-peak noise of the pressure sensor. Note the double event on LS 318.3°.

Ellehoj et al. (2010) identified a single sol during which ∼40 vortices were detected. The seasonal variation in vortex frequency observed at Gale crater (Fig. 7) is consistent with the change in solar energy input into the local environment throughout the year, represented by the modeled flux at the top of Mars’ atmosphere (solid line, Fig. 7). More analysis is needed to assess the contribution of other factors that could affect these changes in activity, such as local topography or “weather”. It is conceivable that the change in incident solar flux with season could affect the depth of the planetary boundary layer. Mesoscale modeling has indicated that the PBL depth within Gale crater is likely suppressed by intracrater circulations (Tyler and Barnes, 2013, 2015) but those works did not assess seasonal variations of the possible suppression. 5.3. Pressure drop magnitude power law fitting Previous convective vortex/dust devil studies have found that for both Earth and Mars convective vortices, the cumulative

frequency distribution of their measured pressure drop magnitudes roughly follow power laws with slopes of approximately −2 (Lorenz, 2012; Jackson and Lorenz, 2015; Lorenz and Jackson, 2015). Since it is generally unknown if pressure measurements for any given vortex sampled the vortex central pressure, these power law fits are not necessarily representative of the full pressure drop magnitudes exhibited by the vortices encountered. Thus, power law coefficient comparisons between vortices at different locations are made under the assumption that pressure drops are sampled at random vortex radii for all data sets. The cumulative distribution of MSL pressure drop magnitudes (>0.5 Pa) exhibits a steeper slope, indicating relatively fewer intense vortices compared to results from the Pathfinder and Phoenix missions (Fig. 9). The power law coefficient for MSL pressure drop magnitudes greater than 0.5 Pa is −2.77, compared to the values of −1.72 and −2.48 calculated for Pathfinder and Phoenix, respectively. Inclusion of MSL pressure-drop magnitudes between 0.3 and 0.5 Pa results in a power law coefficient of −2.35. We will not speculate on potential causes for the power law variations between missions, though Jackson and Lorenz (2015) suggest that sample size can strongly affect power law slope magnitude. The two different MSL power law coefficients could possibly represent some fundamental difference in frequency statistics for small- and large-magnitude vortices, but the difference could also be illustrative of the difficulty in distinguishing true vortex events at the smallest magnitude pressure drops. MSL’s median pressure-drop magnitude (0.71 Pa) for its encountered vortices of magnitude ≥ 0.5 Pa does not noticeably differ from the median values for encountered vortices at Pathfinder (0.72 Pa) and Phoenix (0.74 Pa). The pressure-drop magnitude distributions indicate that vortices, especially large-magnitude vortices, occur less frequently within Gale Crater than at the Pathfinder or Phoenix landing sites. The extent of time (number of sols) spanned by the MSL measurements (707 sols) exceeds Pathfinder’s 83 sol duration by a factor of 8+ and Phoenix’s 157 sols by a factor of 4+. When these different durations are accounted for, as well as temporal sampling completeness, ignoring any seasonal variations for the moment, the annual number of inferred vortices at Pathfinder and Phoenix exceed the number at MSL by a factor of several. We also ignore potential instrument biases here, but note that the REMS pressure sensor on MSL does have the lowest pressure resolution of the three landing sites (0.2 Pa, Gómez-Elvira et al., 2012) while that of Pathfinder has the highest pressure resolution (0.25 microbars, Seiff et al., 1997). The distribution of inferred MSL vortices predicts ∼1 vortex greater than 0.3 Pa in magnitude per sol during the ∼1 martian year focused upon here. At Pathfinder, the 210 inferred vortices greater than 0.5 Pa in magnitude during the 83 sol mission extrapolate to ∼1600 vortices annually (>2.5 per sol; Murphy and Nelli, 2002) while the near continuous temporal sampling at Phoenix provides an estimate of ∼1.3 vortices per sol for pressure drop magnitude >0.5 Pa (Ellehoj et al., 2010). If the numbers from the cumulative distributions in Fig. 9 are normalized by total mission duration and the temporal coverage is roughly ∼30% at the MSL site and ∼25% at the Pathfinder site during daylight hours, vortices of 0.5 Pa in magnitude are predicted to have an occurrence rate of ∼0.7 per day at the MSL landing site compared with ∼1.2 per day at the Phoenix site and ∼3.8 per day at the Pathfinder site. For vortices 2 Pa in magnitude, the discrepancy increases, although this may be the result of small number statistics. At the MSL site, 2 Pa magnitude vortices are roughly predicted once every 38 days, while the Phoenix site would see one every ∼16 days and the Pathfinder site, one every ∼3 days. These are rough predictions that do not take into account seasonal effects, but they do imply that vortices, especially those of larger magnitude, occur less frequently at Gale crater than at other locations on Mars. While pressure drop distribution results in Fig. 9 are somewhat problematic to interpret

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Fig. 6. (a) Detected vortex diurnal activity, and (b) Inferred vortex diurnal activity. Histograms depict the number of vortices detected (a) and inferred (b) over 707 sols as a function of LTST in hours, with one hour bins. Shaded bars denote the ranges of pressure drop magnitudes of detected vortices and are binned logarithmically by factors of 21/2 .

Fig. 7. Seasonal vortex activity. Gray bars depict the number of inferred vortices as a function of solar longitude where LS 0–90° is southern fall (143 vortices inferred; 0.74 inferred/sol), LS 90–180° southern winter (166 vortices inferred; 0.93 inferred/sol), LS 180–270° southern spring (213 vortices inferred; 1.5 inferred/sol), and LS 270–360° southern summer (173 vortices inferred; 1.12 inferred/sol). Fall includes sols 350 through 543, winter includes sols 9 through 53 and sols 544 through 676, spring includes sols 54 through 195, and summer includes sols 196 through 349. The solid line represents top-of-atmosphere downward solar flux magnitude in W m− 2 (right y axis) provided by the NMSU 1-D model described in Vasavada et al. (2012). Triangles represent the maximum REMS temperature in K (left y axis) measured between 11:30 and 12:30 LTST.

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Fig. 8. Inferred vortex diurnal activity over four seasons (sols 9–676). Four panels show the number of inferred vortices as a function of LTST in one hour bins for winter (top left), spring (top right), summer (bottom left) and fall (bottom right). The sol spans for the seasons are the same as those in Fig. 7 and are labeled in the top left corner of each panel. The shaded bars represent the same pressure magnitude bins as those in Fig. 6a and b.

based on the unknown radial position sampled for each vortex, the suggestion that vortices are weaker and less frequent at Gale crater is consistent with the general lack of visually observed dust laden vortices and surface dust devil trails there (Haberle et al., 2014; Moores et al., 2015). A potential explanation for the relatively weak and infrequent vortices at Gale crater is the suppression of its PBL (see Section 5.3), which is supported by results from mesoscale modeling of the crater (Tyler and Barnes, 2013, 2015).

5.4. Dust-free vortices at Gale crater Quantifying the amount of dust lifted by observed vortices on Mars is central to determining the role of dust devils in replenishing background dust in Mars’ atmosphere (Metzger et al., 1999; Ferri et al., 2003; Greeley et al., 2010). There is no requirement that convective vortices be dust laden, so measurements in addition to pressure are necessary to determine if an encountered convective vortex is a dust devil. Inferred vortex wind speeds can provide some insight regarding whether or not a vortex is potentially capable of lifting dust from the surface. Measured downward solar flux at the surface can be diminished if a convective vortex contains sufficient dust (Mason et al., 2014). If a convective vortex at Gale crater lifts enough material from the surface, that column of debris could cast a shadow upon MSL’s six-channel UV flux sensor

(Gómez-Elvira et al., 2012) causing a temporary decrease in measured UV flux. Correlation of UV flux decreases with dust laden vortices proves to be problematic due to an instrumental issue described in Harri et al. (2014) as the “shadow effect.” When the REMS UV sensor is temporarily shadowed (by a rover structure is how Harri et al., 2014 describe the situation), a spurious decrease in measured REMS pressure with magnitude less than 1 Pa can be introduced into the pressure values within 2–3 min of the start or cessation of the shadow occurrence. MSL pressure measurements obtained within 2–3 min of a shadow receive a shadow effect flag in the REMS data files available from PDS Atmospheres Node. Seventeen of the verified-by-eye vortex pressure events coincided with shadow effect flags. Of these 17 shadow effect flagged events, 3 were larger than 1 Pa in pressure drop magnitude and were retained in the final sample of 245 vortices, while 14 events were less than 1 Pa in pressure drop magnitude and were eliminated from further consideration. Eliminating most vortex occurrences with coincident UV sensor shadow effects unfortunately removes potential dust lifting events that we are interested in studying. However, the fact that so few vortex detections coincided with shadow effect flags implies that the majority of the vortices are not lifting significant amounts of material. For the 14 eliminated events with shadow effect flags, we cannot confidently distinguish between pressure drop-UV shadow

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Fig. 9. Cumulative distribution of pressure drop magnitudes. Three distributions of pressure drop magnitudes from three separate Mars missions are shown above as the total number of vortices detected above a given pressure drop magnitude in pascals. MSL vortices are open circles, Pathfinder vortices are plus signs, and Phoenix vortices are triangles. The small filled squares show the MSL detected vortices plus the candidates which were eliminated because they were classified as too ambiguous to be confident detections (see Fig. 4). Power law fits were applied to the cumulative distributions of each mission (solid lines) and the magnitude of each power law slope, α , is shown in the legend (MSL α = −2.77, Pathfinder α = −1.73, and Phoenix α = −2.48).

pairs caused by dust devils and pressure drop-UV shadow pairs which are pressure instrument errors resulting from random shadows. However, two of the fourteen eliminated instances stand out for having a brief UV flux drop occur within a few seconds of a pressure drop (Fig. 10). Their pressure drop magnitudes were less than 1 Pa, thus the most likely dust lifting events in this data set are excluded from our detection list in Supplementary Table 1 because they cannot be ruled out as instrumental artifacts. These two events occurred on sols 105 (Fig. 10a) and 355 (Fig. 10b) at 12:00 and 12:01 LTST and have pressure drop magnitudes of 0.51 and 0.68 Pa respectively. On sol 105, the pressure drop minimum occurs several seconds before the UV flux minimum, while on sol 355, the flux minimum precedes the pressure drop by several seconds. Whether the flux drop or pressure drop occurs first would depend upon the position of the Sun, vortex direction of travel, and vortex tilt. The UV flux drop from sol 355 (Fig. 10b) appears to have two minima, which could theoretically be caused be the cylindrical structure of a vortex with more material in the wall than in the core (Mason et al., 2013). The short time duration of these two events and the position and motion of the rover at those times suggests that these shadows were probably not caused by the rover mast or arm. On both sols 105 and 355 the rover and its arm were stationary. REMS data at similar times on the preceding and subsequent sols do not have shadow effect flags, though the rover did change positions between sols 354, 355, and 356. The largest magnitude vortex in our sample received a shadow effect flag but was one of the three shadow flagged events retained because its pressure drop magnitude exceeds 1 Pa. The temporal character of the shadow coinciding with this vortex (Fig. 10c) differs substantially from the sol 105 and 355 events and is possibly representative of rover structure-induced shadowing. Interestingly, the eliminated sol 105 shadow effect event does coincide with REMS Wind Sensor signals that suggest an abrupt wind direction change coincident with the pressure and UV dips. Wind Sensor Boom 2, Board 2 longitudinal differential thermal conductance values (Fig. 11) exhibit an abrupt change in sign

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within several seconds of the pressure and UV dips. The abrupt wind sensor signal sign change can be interpreted as a substantial change in wind direction, which would be anticipated during vortex encounters. Thus, there is an indication that vortex events are accompanied by wind direction changes, as anticipated, but quantification of wind direction change magnitude as well as actual wind speed is not available. The sol 355 pressure and UV events (Fig. 10b) are also accompanied by Wind Sensor Boom 2 Board 2 signals suggesting an abrupt change in wind direction. Intriguingly, the Sol 403 UV shadow event (Fig. 10c) also coincides with an abrupt change in Boom 2 Board 2 longitudinal wind sensor signal coincident with the large-magnitude pressure signal, yet the character of that shadow is not suggestive of a dust laden vortex. Since only two of the UV shadow events provide temporal signals suggestive of dust laden vortices (Fig. 10), we infer that the majority of the detected vortices are dustless. The lack of corresponding UV flux drops coincident with pressure drops is consistent with other reported observations that Gale crater convective vortices are not lifting dust. Haberle et al. (2014) note that dust devil tracks have not been observed in orbiter images of the crater, and a search for dust devils in MSL Navigation Camera (Navcam) images resulted in only one potential detection during the first 360 sols of the mission (Moores et al., 2015). These findings are consistent with our observations of mainly dustless vortices, so the remaining question is what prevents vortices at Gale crater from lifting material? One possibility for the lack of dust within vortices is that the local availability of surface dust is limited. Reports on the availability of dust within Gale crater are varied, and suggest that there may be a significant difference in dust cover between the crater floor and central mount. Prior to the MSL mission, analysis of the dust cover index derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data implied that the floor of Gale crater was relatively dust free while the central mount had a moderate to thick layer of dust (Stockstill et al., 2007). For other sites at which dust devils have been imaged, the dust cover index implied that the Spirit landing site at Gusev crater was relatively dust covered while Pathfinder landing site was relatively dust free (Ruff and Christensen, 2002). Analysis of Mars Odyssey Thermal Emission Imaging System (THEMIS) data suggested that dust was present on the crater floor but it may vary spatially (Rogers and Bandfield, 2009). Contrastingly, the moderate thermal inertia measured by Fergason et al. (2012) suggests that there are not substantial amounts of dust available. After the MSL landing, CRISM spectra of the Bradbury Landing Site and the Hummocky Plains region, where the rover traversed during the first 360 sols of the mission, both show spectral signatures of dust, but that dust availability decreases towards the dune region to the south (Arvidson et al., 2014). Images of the dark, rover-wheel tracks in this region are consistent with the displacement of dust (Arvidson et al., 2014). Arvidson et al. (2014) estimate that the dust layer thickness in this area is greater than hundreds of microns but less than a centimeter. Dune morphology studies of Gale crater have revealed that aeolian processes are actively shaping the crater floor and grain transport by prevailing winds is common (Hobbs et al., 2010; Silvestro et al., 2013), suggesting that at least sand-sized grains are available to be saltated, but this does not necessarily guarantee the presence of dust. Taken together, these observations imply that there is probably dust available on the crater floor, but given the uncertainty in the amount of dust available and how that varies along the traverse, a lack of dust cannot be ruled out as a possible explanation for the empty vortices detected here. Another possibility is that the vortices at Gale crater are simply too weak to lift dust. The literature proposes two main mechanisms by which dust lifting occurs in vortices. First, the surface stress caused by the tangential winds within a vortex can be

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Fig. 10. REMS ultraviolet (UV) flux data. UV flux data corresponding to three different pressure events of interest: (a) a candidate from sol 105 that was eliminated due to the shadow effect, (b) a candidate from sol 355 which was eliminated due to the shadow effect, and (c) the largest pressure drop amplitude event in our sample from sol 403. Each panel shows measured REMS UV flux for 5 wavelength bandpasses in W m− 2 (left y-axes) and pressure in pascals (right y-axes) as a function of LTST with the format (hr:min:sec). Unlike the data in panels (a) and (b), the UV flux data in panel (c) are not available in the most reduced data set (REMS MODRDR) and are instead taken from the REMS ENVRDR data set in the PDS atmospheres node.

Fig. 11. REMS Boom 2 Wind Sensor signal. The measured signal (volts) from REMS’ Boom 2, Board 2 longitudinal wind sensor component (the high frequency curve above) during the time interval at ∼noon on mission sol 105 when a ‘UV shadow’ event occurred and is evident in all six UV sensor channels included above. Note the abrupt change in Wind Sensor signal coincident with the UV flux decrease, and with the pressure drop shown in Fig. 10a.

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capable of saltating sand grains which can then transfer dust to the atmosphere when those grains impact the surface (Greeley et al., 2003; Neakrase and Greeley, 2010). Second, the surface to subsurface pressure differential at the center of a vortex may produce the vertical transport of dust (Greeley et al., 2003; Balme and Hagermann, 2006). This second mechanism is known as the “P effect,” and the extent of its contribution to dust lifting by vortices is not well understood. Other processes have been suggested as contributors to dust lifting and are being explored (e.g. Wurm et al., 2008). Here, we present an estimation of the vortex tangential wind speeds to explore the lifting ability of the Gale crater vortices according to the saltation lifting mechanism. Laboratory experiments suggest that the vortex tangential wind speed threshold for saltating dust particles at Mars’ surface is approximately 20 m s−1 (Greeley et al., 2003; Neakrase and Greeley, 2010). Dust devil tangential wind speeds have been measured from images taken by the High Resolution Imaging Science Experiment (HiRISE) to be around 20–30 m s−1 with the exception of one very low speed dust devil at 9 m s−1 (Choi and Dundas, 2011). According to the Rankine vortex model, the tangential velocity within the vortex increases linearly with radius in the core and decreases inversely with radius beyond (Sinclair, 1973). The magnitude of tangential velocity is related to the magnitude of the pressure difference between the vortex core and the ambient surroundings. This assumes the vortex is in the cyclostrophic regime, where the centrifugal force is balanced with the pressure gradient (Sinclair, 1973; Rennó et al., 1998; Rennó et al., 20 0 0; Ellehoj et al., 2010). Thus, the central pressure drop within a vortex can be treated as a measure of vortex intensity. Tangential wind speeds of each detected MSL vortex were estimated using the formulation from Ellehoj et al. (2010) and references

P =

v2 Pav RT

(1)

therein, where P is the pressure drop magnitude in the vortex core, v is the tangential wind speed at the core boundary, Pav is the ambient REMS pressure, R is the gas constant 187 J kg−1 K−1 for CO2 , and T is the REMS ambient temperature. The quantified P for each detected REMS vortex provides a lower limit estimate of the core pressure drop; we emphasize lower limit because we cannot know whether the central most point of the vortex core, and thus the deepest pressure well, passed over the rover. Pav and T for each vortex were calculated by averaging the 10 pressure and ambient temperature data points before and after each event. The estimated lower limit tangential wind speeds for the 245 detected vortices range from 3.9 to 12.2 m s−1 and have a median value of 5.6 m s−1 . None of these values exceed the vortex wind speed lifting threshold for dust grains that Greeley et al. (2003) and Neakrase and Greeley (2010) experimentally determined to be around 20 m s−1 , though 8 events have tangential velocities greater than 9 m s−1 , the speed of the slow rotating dust devil observed by Choi and Dundas (2011). The estimated tangential wind speeds for two potential (but eliminated due to UV shadowing) dust laden vortex events from sols 105 and 355 (Fig. 10) are below this 20 m s−1 threshold, both at approximately 6 m s−1 . It could be that the suppression of the PBL within Gale crater (Tyler and Barnes, 2013) is limiting the frequency of formation of larger vortices capable of dust lifting. The pressure drop magnitude of a convective vortex has been shown to be mathematically related to the height of the PBL in Rennó et al. (1998). Thus, if the tangential wind speed of a vortex is approximately proportional to its maximum pressure drop and that pressure drop is related to PBL height, it is possible that the suppressed PBL within Gale crater is preventing the vortices from achieving tangential wind speeds capable of exceeding the dust lifting threshold. The boundary layer maximum depth along Curiosity’s traverse in Gale

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crater (∼2 km, Tyler and Barnes, 2013) is relatively low compared to the PBL depths at the Pathfinder (∼4–5 km, Savijärvi, 1999) and Phoenix (∼7–8 km, Tamppari et al., 2008) landing sites. While pressure drop measurements of vortices at the Pathfinder and Phoenix landing sites did not have significantly different median values from that of the MSL sample, the largest pressure drops occur less frequently at Gale crater (Fig. 9). If larger pressure drops are more likely to lift dust, it could be that the majority of vortices at all three sites are dust free, but that larger, dust lifting vortices at Gale crater are infrequent enough to not be seen. If it is the case that a low PBL depth can suppress dust lifting vortices, then perhaps other locations on Mars with low PBL depths also result in the majority of vortices being dust free. To investigate this further, we looked for other regions on Mars with low PBL depths measured from radio occultation data (Hinson et al., 2008) and compared these locations with distributions from various studies of orbital images of dust devils and their tracks (Cantor et al., 2006; Stanzel et al., 2008; Whelley and Greeley, 2008; Reiss et al., 2014). The only PBL depth close to that of Gale crater was determined to be around 2.6 km at 53.6°N, 206.3°E (Hinson et al., 2008). Unfortunately, this low-PBL-depth location is not covered in the Fisher et al. (2005) or Cantor et al. (2006) studies of dust devils in orbital images. None of the images of dust devils found in Stanzel et al. (2008) or Reiss et al. (2014) were taken at the low-PBL-depth location and a global map of dust devil tracks developed by Whelley and Greeley (2008) showed that this was a region of low dust devil activity. Although it is possible that other low-PBL-depth locations on Mars are not producing visible dust devils, we hesitate making any conclusions based on results from only this location and Gale crater. Further comparisons of measured PBL depth and dust devil activity on Mars are warranted to explore the extent of a potential connection between PBL depth and dust lifting ability. 6. Conclusions Surface pressure drop signatures of 245 convective vortices have been identified in the Curiosity Rover REMS pressure measurements acquired during the first 707 sols of the MSL mission. Despite the possibility that no vortices would be observed within Gale crater due to the suppressed planetary boundary layer (Tyler and Barnes, 2013, 2015) and the lack of observed dust devil tracks (Haberle et al., 2014), the frequency of our detections suggest an occurrence rate of about 1 vortex per sol when the incomplete temporal sampling provided by REMS is accounted for. Quantified pressure drop magnitudes range from 0.3 (an imposed lower limit) to 2.86 Pa, with their cumulative distribution suggesting that largemagnitude vortices within Gale crater generally occur less frequently than at the Pathfinder and Phoenix landing sites (Murphy and Nelli, 2002; Ellehoj et al., 2010). Vortex activity is most frequent during the 11:0 0–13:0 0 (LTST) hours, with a preference for a pre-noon LTST maximum which is earlier than previously reported on Earth (Balme and Greeley, 2006; Jackson and Lorenz, 2015) and Mars (Murphy and Nelli, 2002; Ellehoj et al., 2010; Greeley et al., 2010). The suggestion of a pre-noon occurrence maximum might imply that the PBL depth is greatest before noon. Seasonal changes in convective vortex activity at Gale crater are suggested, with vortices being more frequent during local spring and summer than during fall and winter. This is expected given the seasonal change in incident flux received at that location on Mars (Fig. 7) and the similar findings from the Spirit rover of high dust devil activity in the spring and summer months (Greeley et al., 2010). As the MSL mission continues, a second year of REMS data will result in additional vortex detections and will provide an opportunity for comparison between seasonal activity over subsequent Mars years.

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For Gale crater vortices greater than 0.5 Pa in magnitude, the cumulative distribution of pressure drops follows a power law of slope −2.77. This is consistent with the finding from Lorenz (2012) that the pressure drop magnitude distributions of Earth and Mars vortices tend to have power law slopes around −2 (logarithmically binned distributions followed power laws of slope −1.56 ± 0.49 for Pathfinder, −2.20 ± 0.47 for Phoenix, and −0.76 ± 0.74 for White Sands, NM). The distribution for Gale crater vortices shows fewer high-magnitude drops than those from the Pathfinder and Phoenix missions (Fig. 9). Compared to these other Mars locations, Gale crater appears to produce vortices that occur less frequently and are weaker in magnitude. Estimated vortex wind speeds dependent upon the measured pressure drops suggest that vortex winds infrequently achieve magnitudes of ∼20 m s−1 deemed necessary for saltation and subsequent dust lifting to be initiated (Greeley et al., 2003; Neakrase and Greeley, 2010). Examination of REMS UV flux data corresponding to pressure drops suggests that the Gale crater vortices are lifting little to no dust. Only two potential dust laden vortices were identified, occurring on sols 105 and 355, based on their UV flux signatures (Fig. 10). Unfortunately, the pressure signals of these events cannot be validated due to the shadow effect pressure sensor instrumental error (Harri et al., 2014). Intriguingly, these sol 105 and 355 UV shadow events are accompanied by Wind Sensor signals suggestive of wind direction variation coincident with the pressure and UV dips. A better understanding of the circumstances under which this shadow effect occurs for the pressure sensor, and if it also affects the Wind Sensor signals is warranted. The relatively weak, apparently dust free vortices at Gale crater support the Tyler and Barnes (2013, 2015) prediction that the local PBL depth is suppressed due to crater circulation. Such PBL depth suppression would be expected to result in less intense vortices (Rennó et al., 1998) within Gale crater compared to Pathfinder and Phoenix where modeling suggests PBL depths are greater (Savijärvi, 1999; Tamppari et al., 2008). Our results suggest that the lower PBL depth has inhibited convection within the crater, which in turn has produced less intense vortices to the point where their wind speeds no longer exceed the threshold for particle entrainment. In agreement with the findings of Moores et al. (2015), our observations suggest that convective vortices are not substantial contributors to the local dust opacity at Gale crater. The diurnal and seasonal variations in vortex activity at Gale crater can motivate future investigations of how PBL depth varies on those timescales. Over the first 707 sols of the MSL mission, vortex activity peaked before 12:00 LTST on average (Fig. 6b), which implies that the PBL depth might also be greatest before noon. The time of day variation of PBL depth at Gale crater could be examined with mesoscale modeling. It could be that the circulation within the crater, which was found to suppress PBL depth in general (Tyler and Barnes, 2013, 2015), also plays a role in the diurnal variation of PBL depth. Additionally, the change in PBL depth as a function of season could be explored with a mesoscale model and then compared to the seasonal changes observed in vortex activity at Gale (Figs. 7 and 8). Characterizing the behavior of the PBL depth at Gale crater can enhance our understanding of the uniquely weak and dust free vortices observed there. Acknowledgments The MSL REMS data set was obtained from the Planetary Data System (PDS). The authors thank Ralph Lorenz and Brian Jackson for helpful discussions, Alexander Thelen for assistance with data analysis, and reviewers Lori Fenton and Colin Dundas for their constructive comments. Support for this research

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