Cushing, N. M. Bardabelias, and N. G. Barlow. Department of Astronomy and Planetary Sciences, Northern Arizona University, Flagstaff, AZ; chellykearney@gmail.com; Department of Biological Sciences, Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ; jut.wynne@nau.edu; USGS Astrogeology Science Center, Flagstaff, AZ; gcushing@usgs.gov; Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ; nmb23@lpl.arizona.edu; deceased.

The transport of H 2 O ice along the retreating north polar seasonal CO 2 ice cap has previously been modeled and observed. Spectral observations show that H 2 O ice forms on the interior of the seasonal cap, while thermal observations show these regions to be consistent with CO 2 ice. Prior to the sublimation of the seasonal CO 2 , the observed H 2 O ice deposits are diminished—and because H 2 O ice sublimation rates are extremely slow while in direct thermal contact with CO 2 ice, an alternate removal process must be operating. We propose a model where the process of removing these H 2 O deposits starts with insolation‐induced basal sublimation of the underlying CO 2 ice. This sublimed gas would “seep” upward and into the interface between the two ices, increasing pressure until the gas pressure fractures the cold‐trapped H 2 O ice. Small fragments would be suspended while larger fragments would be pushed aside, exposing the underlying CO 2 ice.

Introduction: Mars polar processes are dominated by the exchange of the predominately CO2 atmosphere and the seasonal CO2 ice caps (e.g. [1-4]). This process is generally referred to as the CO2 cycle. H2O, even though it only comprises ~100 ppmv, also cycles in and out of the seasonal polar caps. The H2O is cycled between the surface as ice and the atmosphere as both vapor and ice. Ice clouds are observed over the springtime seasonal cap. This presentation will focus on these observations over the northern cap. Background: The CO2 and H2O cycles are intimately coupled as H2O can be cold trapped on top of colder seasonal CO2 and the H2O ice can modify the CO2 ice sublimation rates, primarily by changing the effective surface emissivity (e.g. [5-6] and references within). Observations of the receding seasonal northern CO2 cap have shown an annulus of H2O ice that surrounds the CO2 cap throughout much of the northern spring [7-10]. As the annulus of H2O ice sublimes, the H2O vapor can be transported over the CO2 cap edge where it is cold trapped as a layer of H2O ice [11-13]. This layer can be optically thick, thus hiding the spectral signature of the underlying CO2 ice. This layer of H2O ice has been observed to extend across much of the interior CO2 cap [9-10], thus raising the question – is the H2O delivered to the interior of the cap by the same process as near the edges? Data: The Mars Odyssey spacecraft is currently in an orbit that allows for early morning observations. Onboard Odyssey is the THermal EMission Imaging System (THEMIS) [14] which contains both a multiband thermal infrared imager and a multiband visible imager. These instruments enable us to look for morning clouds over the seasonal cap. Lee clouds are clouds that form near the surface and have a wave-like structure that is caused by winds blowing over a topographical obstacle. Thus, lee waves can be used to determine wind direction. The lee clouds are also composed of H2O ice (based on THEMIS IR observations). Therefore, the lee clouds are a tracer of H2O circulation above the seasonal polar caps. Examples of lee clouds observed by the Odyssey THEMIS VIS camera are shown in Fig. 1. Figure 1: Topographically induced atmospheric lee waves. Each panel shown is ~ 18 km wide. Left panel shows waves in H2O clouds south of Escorial crater. Right panel shows waves in possible CO2 ice clouds northeast of Koralev crater. Credit: [15] Fig 3.

Introduction: Mars is a dynamic planet where 25%-30% of the predominately CO2 atmosphere [1, 2] is annually exchanged between the atmosphere and surface CO2 ice [3-6]. The mass of seasonal CO2 ice has been estimated to be 3.5 x 10 kg in the north and 6.5 x 10 kg in the south [7]. While bulk estimates of the CO2 mass are available in the literature from a variety of measurements, including pressure curves, high energy particles (gamma rays and neutrons) and gravity, spatial resolution of the CO2 ice distribution has usually been limited to zonal means (and then usually making several assumptions). Insolation-mass balance calculations have also been used to estimate local column densities [e.g. 8-10]. Here we use the insolation-mass balance (IMB) approach over both seasonal caps and validate the results with comparison to the bulk mass as estimated from gravity data [7]. Data: The Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) [11] provides near continuous monitoring of the solar albedo, planetary temperature and surface temperature over several annual cycles for both the northern and southern polar regions. As such, TES is well-suited to monitor the advancing and retreating seasonal cap edge, as well as estimate the energy balance of condensing or sublimating CO2 ice. If the date of complete CO2 ice sublimation is known (often referred to as the CROCUS date [8]), then the energy balance can be integrated backwards in time from that date. A more complete description can be found in [10]. This method provides a spatially resolved estimate of the CO2 ice column density as a function of season. The determination of the CROCUS date can be achieved either using the thermal infrared (TIR) spectral channels at 25 microns or from the solar albedo bolometer (SAB). A sudden increase in temperature of more than 5 K above the CO2 frost temperature (~145 K) usually indicates that the CO2 ice is no longer buffering the surface temperature. The SAB can also be used to determine the disappearance of seasonal CO2 ice by observing a slight increase in brightness followed by an immediate darkening. For the northern seasonal cap, the TIR method is best as the seasonal CO2 ice cap is surrounded by an annular layer of H2O ice during much of the retreat phase. [9, 12] In the south, the SAB method is best as the warm dusty atmosphere can affect observed brightness temperatures even at 25 microns. Results: A comparison between the northern and southern seasonal caps can provide context for assumptions about seasonal CO2 ice deposition and sublimation. Figure 1: North polar seasonal cap column density (kg/m) at Ls 0°.

Introduction: Recent adjustments to the Mars Odyssey spacecraft’s orbit have enabled the THEMIS instrument [1] to acquire observations in the first hour after sunrise at both visible (VIS) and thermal-infrared (TIR) wavelengths. The first hour of daylight is an interesting (and not well-observed) time on Mars—when solar energy initiates katabatic winds and interacts with volatiles freshly deposited overnight. The thin martian atmosphere does little to stabilize surface temperatures—which, during polar springtime, often plunge to CO2 condensation temperatures overnight before rapidly increasing from nearly the first instant of sunshine. Accordingly, CO2 and H2O frost or snow can accumulate overnight [2,3] and sublime rapidly at sunrise – possibly influencing fog and/or cloud formation through atmospheric density variations. Observations: Searching for examples of early morning ground fog, we surveyed all concurrently acquired THEMIS VIS/TIR early-morning imagery (incidence angle between 75°-90°) covering the north polar region (>60° latitude) in mid-to-late spring (Ls=30°90°) of Mars Year (MY) 33. While the great majority of early-morning observations (with the sun < 15° above the horizon) show no visible increase in atmospheric opacity due to clouds or fog, we have identified about a dozen observations with at least some form of cloud or fog coverage. Here we discuss four of these observations where different circumstances have induced the formation of ground fogs or low-level clouds with topographically induced lee waves. The temperatures discussed henceforth are THEMIS Band-9 brightness temperatures. For each observation, these temperatures were re-calibrated to CO2 temperatures using clearly identifiable CO2 ice deposits, if present. V60793009 / I60793008: The VIS image (Figure 1) shows a small scene in the first hour of daylight with the sun only 11.1° above the horizon (Ls=33.7°, Latitude = 68.2°). An optically thick ‘fluffy-looking’ layer of either ground fog or blowing CO2 snow closely blankets the local terrain. The concurrent TIR image is nearly featureless (which is expected of regions covered with CO2 ice) except for a darker region corresponding to the region in Figure 1—consistent either with reduced IR emissivities from opaque CO2 fog or with small grain sizes of freshly deposited CO2 snow. According to [3, 4], regions of enhanced precipitation will likely show decreased surface emissivity because the accumulating deposits would have relatively small grain sizes compared with the surrounding frosts. [3] modeled CO2 snowfall rates as high as 0.75 g/cm2/hr. However, if Figure 1 is showing ground fog, this would imply that atmospheric conditions were already near the CO2 condensation point and either a decrease in atmospheric pressure (possibly from Bernoulli’s principal and earlymorning katabatic winds passing over Heimdal crater to the north) or a decrease in near-surface temperatures caused ice-crystal formation resulting in ground fog. V61625003 / I61625002: Figure 2 shows an interesting scene with a 40-km fog-filled crater with snowcovered rim walls and a frosted dune field on its floor (Ls=64.1°, Latitude = 69.9°). Brightness temperatures for the scene were calibrated assuming CO2 snow on the rim walls (~145 K). The dune-field temperatures are more consistent with frozen H2O (~165 K), and the fog regions warmer still (~180 K), indicating a composition of H2O ice vapor.

1 , J. J. Wynne 2,3 , M. D. Jhabvala 4 , G. E. Cushing 1 , P. Shu 4 , N. A. Cabrol 3 ; 1 U.S. Geological Survey, Astrogeology Science Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001 (ttitus@usgs.gov); 2 Colorado Plateau Research Station, Northern Arizona University, Flagstaff, Arizona 86011; 3 The SETI Institute, Carl Sagan Center, Mountain View, CA 94043; 4 NASA Goddard Space Flight Center, Greenbelt, Maryland 20771.