The goal of the practice, that has been performed during three months within an aerospace engineering internship course, is to train students in the investigation of the present day environmental conditions on the surface of Mars through data gathered by REMS discerning, by using some of the tools and techniques that are used to face this kind of analytic and scientific work, the perturbation that the platform (the rover itself) introduces in several measured parameters.
This practice had three major objectives:
•To estimate the radiative heat-interchange between the MSL platform and the Martian surface.
•To investigate its variation along the year comparing its magnitude with the solar radiation
•To quantify the influence of this heat-interchange produced by the presence of the spacecraft
on the Martian ground temperature.
In order to develop the basic skills for the use of the computer tools to perform the analysis, the practice started with a preliminary study of the radiative thermal contamination of the ground by the RTG using the program gnu plot to plot initially raw data from the PDS. This first exercise permitted to appreciate the usual artifacts produced by the rover instrument noise, and air thermal fluctuations around the hot platform, or the effect of self-heating of the HS electronics on his relative humidity measurements (Figure 3).
In Figure 3 the data are shown as a function of LMST. Among other things, it showed the noisy signal of the ground temperature sensor measurements, which is more pronounced at night, when the real values are hidden in noise measurements that range up to 40 K. The data processing was performed by developing a fortran95 program that averages consecutive data-sets within a period of 5 minutes, in the case of nominal acquisitions, and over intervals of 15 minutes in the extended (1 hour) acquisitions. Furthermore, if within the sequential analysis of REMS observations a gap of 2 minutes or more is detected, a new averaging sequence is started. This program evaluates then several physical magnitudes derived from these observables, such as the radiative heat-flux, averaged over one sol. The results of the 5 and 15 minutes averages are shown in Figure 4.
Next we evaluated the radiative heat flux between the rover platform and the ground. In order to have a temperature that represents that of the metallic structure of the rover, we used the temperature of the UVS, which is embedded within the rover deck. The radiative flux between two elements can be calculated using the Stefan-Boltzmann equation. Then:
q=σ·ε1·ε2·F (1→2) · (Tr4-Tg4)
Where q is the radiative flux in W/m2
, σ is the Stefan-Boltzmann constant (5.6704·10-8
), Tr is the rover temperature in K, and Tg is the brightness temperature of the ground in K. ε1 is the rover surface emissivity, which is determined by the properties of the white paint that covers it. We shall take ε1=0.88, which is the typical infra-red (IR) emissivity of some space qualified paints; ε2 is the ground surface emissivity, which for the IR range and the site explored by Curiosity can be assumed to be ε2=0.9. In radiative heat transfer, a view factor F (1→2)
, is the proportion of the radiation from surface 1 that strikes surface 2. The view factor of the rover side to the ground can be approximated by that of a vertical wall to the ground, which is F (1→2)
=0.5. The remaining energy is radiated to the sky and air.
The results are shown in Figures 5a and 5b using the 5 minutes averages of the magnitudes Tr and Tg. For comparison the measured incident UV irradiance is taken as a proxy for the full incident solar irradiance. Notice that in Figure 5a, the rover temperature is higher than that of the ground all along the day, with the exception of a few hours around local noon, when they are very similar. This is because during the central hours of the day the air temperature is much lower than the temperature of the ground. Convection at the boundary layer is active and this results in a natural cooling of the rover surface, whose temperature is reduced almost to that of the ground. Thus, the radiative
heat-flux from the rover to the ground is always positive and can reach values as high as 22 W/m2
. In the example of Figure 5b, there is an anomaly in this thermal profile: there were shadows cast by the mast over the location of the UV sensors, and thus the local temperature of the platform was lower than that of other illuminated areas, including the ground. This results in an apparent negative radiative heat-flux, which is in fact only an artifact of this transient shadow. For this sol also, maximal heat-fluxes reach values of about between 17 W/m2
and 22 W/m2
. This energy flow is large and thus we foresee that this shall have a detectable effect on the local temperature of the ground at the site where the rover is located .
Next, this analysis is performed for all the sols during one Martian year. The daily average of the heat-flux is calculated and shown as a function of sol. The incident daily insolation at the Top of the Atmosphere (TOA) is calculated for comparison using
is the solar insolation at a distance of rav
=1.52 AU, which is the mean Mars-Sun distance in Astronomic Units, in W·m-2
(with SO=590 W·m-2
) for a given latitude θ(for the landing site of MSL θ=-4.5°), δ is the solar declination and depends on the inclination angle of Mars rotation axis and of Ls·ravr
, which is the relative orbital radius depending in turn on the orbit eccentricity e=0.0934 and on the difference of the solar angle Ls and the angle of reference at perihelium Lsp=250°; ε=25.2 is the obliquity of Mars, and finally, H is the hour angle at sunset.
These results are compared with the atmospheric opacity which is a measurement of the amount of dust in the atmosphere and varies along the course of the mission. These magnitudes are shown in Figure 6.
In the second step, the thermal contamination of the ground around the rover was used as a reference to quantify the effect of this radiative heat-flux on the ground. When the rover changes positions, the soil is not contaminated initially so, observing the temperature evolution when it remains still on a new position after a shift, it is possible to infer, with relation to the different kind of ground the rover has gone over, the magnitude of the contamination induced by rover alone . This analysis required the implementation of the routines splint
to compare measurements at different days and times. An example is shown in Figure 7, where the period of time when the rover moved is shadowed. Similar difference curves have been calculated for other sols, (Figure 8). The temperature difference between the fresh uncontaminated soil and the contaminated site is indicated with an arrow bar. Once the rover stays still, there is a slow ground temperature increment due to the heating of the ground by the standing rover. However, since this rover-induced heating takes place after noon, the heating rate is strongly compensated by the natural cooling rate of the ground during these hours, resulting in a small net heating trend and a much smaller temperature recovery. These results are summarized in Table 1.