Getting RADical about REMs while Odyssey flips

The recent focus on the NASA / JPL Mars Science Laboratory mission has been on Curiosity’s soil sampling activities in the region of Gale Crater scientists have called “Rocknest”.  However, this is not all that the rover has been up to. Through the sample gathering operations, two other instruments have been hard at work, measuring and monitoring the environmental conditions around the rover. These are the The Radiation Assessment Detector (RAD) and the Rover Environmental Monitoring Systems (REMS) – each of which has been working away day and night since Curiosity first arrived on Mars.

RAD is particularly focused on the amount of radiation, both from the sun (solar radiation) and in the form of cosmic rays, reaching the surface of Mars. While Mars has an atmosphere, it is extremely thin and tenuous in comparison to that of Earth – at ground level it is about as dense as Earth’s atmosphere at an altitude of several miles. The Martian atmosphere is primarily carbon dioxide and it does not have any ozone layer. This, coupled with the lack of a strong magnetic field means that the surface of Mars is subjected to far higher levels of both solar and cosmic radiation than are experienced on Earth.

However, even though the atmosphere around Mars is tenuous, it is still enough to have an effect on incoming radiation, and RAD is designed to measure the levels of radiation common to the surface of Mars and, working with REMS, help give further insight into the processes which go into altering that radiation as it passes through the atmosphere. Both of these studies will in turn help scientists understand the impact incoming radiation is having on the local environment and increase out understanding of surface conditions on Mars in preparation for human missions there in the future.

Curiosity: increasing our understanding of the Martian surface environment for future human missions

REMS is the rover’s meteorological station, comprising instruments mounted both on the body of the rover and on the forward mast. It is responsible for monitoring wind, pressure, humidity and temperature, and is being used to establish a track record of atmospheric conditions and changes experienced by Curiosity. Despite the wind measurement instrument on the rover’s mast being damaged during the landing phase of the mission – mostly likely due to a stone being thrown up by the Descent Stage engines and striking the instrument – REMS has been returning huge amounts of data about the Martian atmosphere, helping scientists develop a clearer understanding of the complex mechanics at work in the Martian atmosphere.

As mentioned about, the Martian atmosphere is largely carbon dioxide and very tenuous. Both of these points factor into large seasonal variances in the Martian atmosphere. Due to the tenuous nature of the atmosphere, temperatures are extremely low. During the colder winter months, these low temperatures cause a significant amount of the atmosphere to “freeze out” into the polar ice caps (most notably the southern polar cap, which is predominantly carbon dioxide ice).  In the southern hemisphere, the warmer temperatures, while still low by Earth terms, are enough for much of this carbon dioxide to sublimate into the atmosphere with the result that season changes can cause the Martian atmosphere to shrink / grow by some 30% through the course of a year.

These thermal processes also operate on a day/night cycle, and also affect the radiation signature being recorded by Curiosity’s RAD instrument. Essentially, what is happening is this: during daylight hours, the atmosphere heats up rapidly and expands, causing the atmosphere to “bulge out”. Convection currents cause the atmosphere to flow outwards from this bulge to equalise the pressure either side of it. This leaves the atmosphere below the bulge at a lower pressure than the air on the night side of the planet. As the day passes and the sun sets, the atmosphere cools and the bulge contracts increasing the surface air pressure beneath it.

The daily thermal cycle on Mars: by day, as the planet rotates, the atmosphere warms and expands. Air flows out from the heated “bulge” in order to equalise the pressure with the atmosphere around it, with the result that while the bulge causes a “thicker” atmosphere, it is one that is less dense and at a lower pressure than the air on the “night side” of the planet

Overall, this daily fluctuation can amount to a 10% variation in air pressure over the day / night cycle as measured by REMS. Taken alongside the RAD measurements, this has revealed an interesting correlation with the amount of radiation being measured around the rover. As the air temperature increases through the day and the atmosphere expands to lower the local air pressure, so to does the amount of radiation being measured by RAD increase. Then, as the temperature drops during the evening into night, so to does the atmospheric density and pressure increase – and surface level radiation doses fall, with between a 3% and 5% variation in radiation levels being recorded by REMS during a single day / night period.

A 5-Sol chart showing the relationship between radiation and air pressure during the day / night cycle. As the air is warmed during each day, so the air pressure drops (blue) and the amount of radiation being recorded (red) increases. As night draws is, so the blue line increases, indicating an increase in atmospheric pressure – and radiation levels drop

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