The winds in the Martian nocturnal boundary layer at the time of the Pathfinder landing

Abstract

In 1997 Pathfinder descended on its parachute through the nocturnal boundary layer of Mars. Pathfinder descended early in the morning a few hours before the possible formation of a low level jet [1]. We use an EDL trajectory model to simulate the landing of Pathfinder and its jettisoned hardware and together with HiRISE images of the landing site we are able to constrain the wind speeds and directions during its descent.

Background

Pathfinder parachute deployment occured at an altitude of around 10 km. The heat shield was jettisoned 15 s later. Shortly afterwards Pathfinder enveloped in its deflated airbags was lowered down on a bridle. The airbags were then inflated at an altitude of a few hundred metres. High powered rockets, attached to the back shell, were fired at an altitude of about 80 m reducing the vertical velocity to nearly zero at an altitude of 20 m above the surface. The lander was then released and dropped to the surface while the rockets used the remaining propellant to shoot the backshell and parachute upwards and away from the lander. The lander proceeded to bounce about 1 km towards the northeast coming to rest after about two minutes [2].

Method

We numerically integrate the equations of motion to model the trajectories of the Pathfinder spacecraft and its heat shield. The relative distance between the heat shield and the lander in HiRISE images is compared to that in the model using a similar method used in [3]. The first impact point of the airbags, which we require to help constrain the trajectory of the lander, is not visible in the HiRISE image so we reconstruct the likely impact point based on information in [2]. To infer wind speeds and directions we vary the wind speeds in the EDL model using a Hill climbing algorithm until the predicted impact points match those of the heat shield in the HiRISE image and the reconstructed impact point of the airbags. There are two levels of winds between 0 and 9.6 km in the EDL model an upper and a lower one. The division line between the two levels is a free parameter. In figure 1 the division line is set to 1 km altitude.

Results

Figure 1 shows the ground track of Pathfinder starting from parachute deployment at 9.6 km. The dots on the tracks are intervals of 1 km. The predicted impact points from the EDL model have been fitted to the locations of the jettisoned Pathfinder heat shield in the HiRISE image and the inferred first bounce location.    

Figure 1 : Ground track of Pathfinder and the heat shield.

Summary

We varied the line dividing the upper and lower wind levels in the EDL model from 500 m to 2000 m. We also varied the distance between the first bounce and the lander at rest from 400 and 1000 m. We found the wind directions can be fairly well constrained even if these two parameters are varied. Above 500-2000 m the upper level wind was inferred to be coming from the west (240-310°) and below 500-2000 m the lower level wind was inferred to be coming from the southeast (120-150°).  To obtain fits between the model and observations the upper level wind speed was required to be between 15 and 30 m s^-1 while the lower level wind speed in the model was found to be in a similar range.

References

[1] Savijärvi, H., Määttänen, A., Kauhanen, J. and Harri, A.-M.: Mars pathfinder: New data and new model simulations, Quarterly Journal of the Royal Meteorological Society, 130, 669-683, 2004.

[2] Golombek, M.P., Anderson, Robert, Barnes, Jeffrey, Bell, J.F., Bridges, Nathan, Britt, Daniel, Brückner, J., Cook, R.A., Crisp, David, Crisp, J.A., Economou, Thanasis, Folkner, W., Greeley, R., Haberle, R., Hargraves, R.B., Harris, J.A., Haldemann, A., Herkenhoff, K., Hviid, S.F., Wilson, G.R.: Overview of the Mars Pathfinder Mission: Launch through Landing, Surface Operations, Data Sets, and Science Results, Journal of Geophysical Research, 104, 8523-8554, 1999.

[3] Paton, M. D., Harri, A.-M. and Savijärvi, H.: Measurement of Martian boundary layer winds by the displacement of jettisoned lander hardware, Icarus, 309, 345-362, 2018.