Jorge Pla-García, Antonio Molina, Javier Gómez-Elvira and REMS team

The eleventh month of the thirty-third Mars year [1] goes from sol [2] 1582 to 1631 since the Curiosity landing. According to the Sun position, this month goes from 300 to 330 solar longitude [4] (Ls). The rover drove uphill along half a kilometer (482 m) and climbed 17 meters in elevation on Aeolis Mons [3] –an average slope of 3.5%– during these 49 sols. Although the rate of ascent was similar to last month, Curiosity diverted its course to study Ireson Hill. Even a small relief as this one worth separate from the path, since it may provide valuable information about Gale’s past geologic and environmental conditions (see Figure 1). One of the main reasons to climb Aeolis Mons is precisely that: it is a time travel, looking into the results of millennia of activity. The rising in elevation is also interesting from an atmospheric point of view, and time still goes on, being the second summer month –of three– in the southern hemisphere.

Figure 1. Two drawings depict the same location on the northern half of Gale Crater at two points in time. North is to the left and central peak to the right. (Source: NASA/JPL-CALTECH) Click here to know more.

Atmospheric dust

This month the atmospheric dust is in the spotlight. The spring equinox (Ls 180) is the starting of the “dust season,” as the solar radiation reaches its maximum, and it will end with the autumn equinox (Ls 0). Dust is highly influential in the Martian atmosphere causing most of its variability. The particle size is so small that we could say it is smoke dust instead. The suspended dust particles have a double effect, retaining the infrared radiation coming from the ground but reflexing the incident visible radiation, providing an anti-greenhouse effect in this case. Because of that, nighttime temperature rises, while the daytime temperature decreases. The result is a reduction in the day temperature range, both ground and atmospheric, of about 20 °C. These changes in temperature have a substantial effect on pressure and winds that modify the transport and injection of dust on the atmosphere, feeding back the process.

Atmospheric pressure

The atmospheric pressure continues dropping during this month, similarly to the last one (Figure 2). As the autumn equinox approaches, the CO₂ in the atmosphere starts to freeze over the southern polar cap, decreasing the air pressure. As expected, the atmospheric pressure is lower this month compared to the same month of previous years, since the rover is climbing Aeolis Mons –the higher the elevation, the lower the air pressure.

Figure 2. Average pressure evolution measured by REMS instrument inside Gale crater (Source: CAB) Click image to enlarge.

The highest daily air pressure is registered during the sunrise and the lowest during the sunset due to the Mars thermal tides [6],as we talked about last month.

Air temperature

Temperatures were below zero during all the month, something common on Mars. The average temperature was around -42 °C, with an average daily maximum of almost -9 °C, while the minimum was around -69 °C (Figure 3). It is important to note, however, that the temperatures above 0 °C are rare, oscillating every day about 60 °C –more in a single sol than the annual daily average.

A higher dust concentration compared to last month caused a raising in the minimum daily averages –more greenhouse effect for the ground emitted infrared radiation during the night– and a decrease in the maximum daily averages –anti-greenhouse effect for visible radiation during the day.

Figure 3. Average air temperature evolution measured by REMS instrument inside Gale crater (Source: CAB) Click image to enlarge.

As it is noted in Figure 3, temperatures were very similar to the ones recorded the same month of the lasts years.

Atmospheric Circulation

This month ends the annual atmospheric behavior that we called “ventilation” or high mixing –that we explained in detail last month. The mountain waves [8] that inundated the crater with air from the outside start to weaken, so the mixing between inside and oustide air is becoming slower.

Due to the crater’s topography, the wind rises through the rims during the day and sink again during the night, as can be seen in Figure 4. This local circulation is called subsidence and occurs in Gale as well. The flows divert near the surface and converge again over the crater, suppressing the lower layer of the atmosphere, named as CLA [4].

CLA’s height is related to the topography, the surficial roughness, the wind speed, and the temperature difference with the ground, among others factors. During the daytime, the increase in temperatures and vertical air mixing widens the CLA. On the other hand, during nighttime, the cooling of the ground stop convection decreasing turbulence (thermodynamic, not mechanic one!) and CLA gets thinner.

Curiosity continues its journey upslope Aeolis Mons [3], getting further from the maximum subsidence zone (red star in Figure 4). The higher over Aeolis Mons, the more winds will flow (downslope during nighttime and upslope during daytime) and the higher the air temperatures –since the cold air masses trapped inside the crater stay behind.

Figure 4. Atmospheric subsidence of air masses inside Gale Crater. (Data source NASA/Goddar) Click image to enlarge.

Dust devils

Dust devils are common both on Earth and Mars surface. The intense solar radiation heating of a spot on the ground, that gets hotter that its surroundings, produces a turbulent convective process. The hotter air mass close to the ground floats creating a pressure depression below it. The low-pressure area pulls the air around it in, generating winds that rotate and rising dust within it (Figure 5). Dust devils have a major role controlling the suspension dust, as they are the primary dust injectors.

Figure 5. This sequence of images shows a dust-carrying whirlwind, called a dust devil, scooting across the ground inside Gale Crater at Sol 1597 (Source: NASA/JPL-Caltech/TAMU). Click here to know more.

During the first and second year since the arrival of Curiosity, the CLA was very weak (thin or suppresed), limiting therefore the turbulences reach and dust devils formation. The higher we are on the rims of Aeolis Mons, the stronger (wider) is the CLA, allowing the creation of more dust devils. As expected, Curiosity observed much more of them, like the one in Figure 5.

Ground temperature

This month the ground temperatures have been less constant that during the past one, recording a light decrease, but still being around -35 °C in daily average (Figure 6). As solar radiation is highly influenced by the seasons, this is an expected trend. The variability in the daily averages is something that is also repeating from last years during this month, highly influenced by suspension dust and the diversity in grounds along the Curiosity track. Rims, as Ireson Hill, favor the existence of mixed materials with different thermal behavior.

Figure 6. Evolution of the ground temperatures measured by REMS instrument inside Gale crater (Source: CAB) Click image to enlarge.

Thanks to REMS team for their effort and the valuable data.

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Jorge Pla-García, Antonio Molina, Javier Gómez-Elvira and REMS team

The tenth month of the thirty-third Mars year [1] goes from sol [2] 1534 to 1581 since the Curiosity landing. The rover drove uphill along 200 meters and climbed 15 meters in elevation on Aeolis Mons [3] –an average slope of 7.5%– during these 47 sols. The area is located on the Bagnold dunes that overlie the Murray formation, composed of a fluvial-lacustrine mixture of materials, mostly fractured mudstones, with dark sand banks covering them in patches. According to the Sun position, this month goes from 270 to 300 solar longitude [4] (Ls), being the first summer month –of three– in the southern hemisphere.

Atmospheric pressure

As can be noted in Fig. 1, the annual maximum of atmospheric pressure [5] was measured during the previous month, matching with the highest CO₂ sublimation (ice turning into gas) in the martian south pole. The atmospheric pressure begins to drop during this month. As the autumn equinox approaches, the CO₂ in the atmosphere starts to freeze over the southern polar cap, decreasing the air pressure. As expected, the atmospheric pressure is lower this month compared to the same month of previous years, since the rover is climbing Aeolis Mons –the higher the elevation, the lower the air pressure.

Figure 1. Average pressure evolution measured by REMS instrument inside Gale crater (Source: CAB) Click image to enlarge.

The highest daily air pressure is registered during the sunrise and the lowest during the sunset. Mars thermal tides [6] are the weather phenomenon responsible for these significant, daily variations. Together with thermal tides effect, the local air circulation causes air to flow away from the crater interior during the day and flow back during the night, as a consequence of the descending air along slopes.

The typical air pressure oscillation registered at ~08:00 pm during the previous seasons was not recorded this month. The local crater air circulation at that time is balanced with global and regional air currents.

Air temperature

Temperatures were below zero during all the month, something common on Mars. The average temperature was around -40 °C, with an average daily maximum of almost -12 °C, while the minimum was around -70 °C (Fig. 2). It is important to note, however, that the temperatures oscillate every day about 60 °C, that is, more in a single sol than the annual average.

Figure 2. Average air temperature evolution measured by REMS instrument inside Gale crater (Source: CAB) Click image to enlarge.

During this month, temperatures were stable. Gone is the spring, where solar radiation was maximum at Gale crater, and so the temperatures, which reached an absolute maximum value of +4 °C on hottest sol of the martian year. Also, in Figure 2, one can tell that the temperature evolution this year is very similar to the previous ones.

Atmospheric Circulation

On Mars, there is a dramatic difference between the lowlands of the northern hemisphere and the highlands of the south. This circumstance develops nocturnal winds from the south (at night, cold winds sink from the south due to being heavier) during most of the year. However, during this month, the winds from the northwest are especially intense at night, defeating the southern winds for the first time in the year.

On the other hand, during the evening, an inversion [7] develops at the crater rim and outside the crater, setting up the ideal conditions for a mountain wave [8], which descends to the surface during evening/night. Mountain waves are not thermally driven buoyancy circulations, unlike slope circulations. Instead, some mesoscale flows, like mountain waves, are dynamically generated by the interaction of the wind with the topography. These dynamic phenomena can oppose buoyancy forces and provide a mechanism for warm air to descend or for cold air to rise. Therefore, this is the ideal scenario for flush the crater out completely. This phenomenon occurs from mid-spring, peaking in summer solstice (Ls 270) and ending in mid-summer (Ls 225 – Ls 315).

These scenarios are fairly common near mountainous terrain on Earth and are responsible for downslope windstorms, like Foehn winds in the lee of the Alps and Chinook winds in the lee of the Rocky Mountains. The latter shows us an outstanding example in the midnight of February 3rd-4th. With cold dense air at the surface across most of Northeast Colorado and relatively warm air aloft over the Central Rockies, strong west winds aloft worked overnight to push the cold air out. A temperature battle happened around Boulder (CO) area with approximately ~12 °C of temperature change in about 4 hours. Temperatures went from -5 °C at 10:30 pm up to +7 °C by midnight. The temperature went back down to -6 °C as the westerly winds weakened temporarily before strengthening again allowing temps to rise back above +8 °C during all night long. This magnitude of temperature change along a cold/warm air boundary is fairly common in the winter months across the high plains, east of the Rocky Mountains. This process is fairly common in this area during the winter months, as it is in Gale crater on Mars during the summer, a unique time of the year when mountain waves develop (Fig. 4).

Figure 3. The temperature battle along Rocky Mountains base. When approaching February 4th midnight near Boulder (CO), temperatures oscillated around 12 °C due to warm downslope winds forced by mountain waves (Data source: The Weather Company) Click image to enlarge.

Figure 4. Strong winds develops mountain waves that sink warm air masses from the top of the Rockies to the cold plains triggering a temperature battle ensued overnight both near Boulder (right side, source NCAR edited) and at Gale crater (left side, source NASA edited) Click image to enlarge.

Ground temperature

The average ground temperature measured this month was very stable around -33 °C (Fig. 5), which is a bit higher than during the same month of previous years. These small daily variations could be caused by a low and constant thermal inertia of the materials that the rover found along her track. The multiple dune fields close to the rover is consistent with this behavior, since the small particle size of the sand reduces its capability to store heat, and follow fast the thermal variations on the air. Those temperatures change dramatically during the day, with a 80 to 100degree difference between day and night – much higher that the seasonal variations.

Figure 5. Evolution of the ground temperatures measured by REMS instrument inside Gale crater (Source: CAB) Click image to enlarge.

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