The Planetary Boundary Layer (PBL) is an important region on Mars and other planets, including the Earth, because it is in contact with the surface. It is, therefore, the layer that controls the transfer of material, water, dust, pollutants, or other surface material into the upper atmosphere winds where it is then transferred around the planet.
On Mars, we study this region to further our understanding of the transport of volatiles, such as water, and sedimentary material, such as dust. On Earth, instruments are available to study the winds in this region; however, on Mars, these instruments have yet to be deployed. In our paper we analyze the displacement of spacecraft hardware, observed at various landing sites across Mars, to provide a unique set of direct in-situ wind measurements in the PBL.
To descend through the Martian atmosphere, the lander is housed inside an aeroshell (figure 1) fitted with a heat shield. The aeroshell uses the atmosphere to decelerate from hypersonic speeds down to supersonic speeds absorbing and dispersing the energy with its heat shield. Once the spacecraft is traveling at supersonic speeds, a parachute is deployed typically between 5 and 12 km above the surface. Coincidentally, the parachute deployment altitude occurs approximately at the top of the PBL. The parachute then slows the spacecraft down to subsonic speeds. Shortly after parachute deployment, the heat shield is jettisoned. This allows the final part of the landing system to operate, i.e. radar to measure altitude and deployment of landing legs or airbags.
After this point, landing systems may differ in their operation. Broadly speaking, the terminal landing systems can be divided into two types. Descent systems used by Viking, Phoenix, Curiosity, and Schiaparelli are similar in that the parachute is jettisoned at an altitude of around 1 km shortly followed by the ignition of liquid powered retrorockets that can perform a controlled touchdown.
For the Spirit and Opportunity rovers, high-powered solid rocket boosters attached to its backshell quickly decelerated the spacecraft momentarily to a standstill a few 10s of meters above the surface whereupon the lander, enveloped in airbags, were released and the retrorockets shot into the sky carrying away the parachute. Retrorockets and landing bags were also used by the Soviet landers of the 1970s. A slightly different approach was used by Beagle 2. Due to its low mass, a large subsonic parachute was able to slow it down sufficiently, without the use of retro rockets, so it could land with airbags.
All the successful landers, including the Viking landers, Pathfinder, Spirit, Opportunity, Phoenix, and Curiosity, have been imaged from orbit with parachutes and heat shields landed within a few hundred meters of the lander. Two partially-successful landing systems have also been imaged. One of these was Beagle 2, which landed intact on the surface but failed to deploy its antenna and communicate with Earth. Another partially-successful system was used by Schiaparelli. The lander successfully transmitted measurements made during its descent. However, after deploying its parachute and releasing its heat shield, the onboard software incorrectly interpreted its motion sensors and gave the command to jettison the parachute too early, ultimately resulting in a crash landing.
To extract the wind speed and direction from the images, we used a two-stage approach. The first stage involved measuring the distance and direction between the various jettisoned components on the surface. This was straightforward, as an application was available to view and measure the distances on the images. After this, we use an in-house developed numerical trajectory model to simulate the landing spacecraft. The trajectory model utilizes the equations of motion and is based on the physical modeling approach commonly used by flight and spaceflight simulators. A Monte Carlo technique was used to automatically find the wind speed and direction. A Hill climbing algorithm varied the wind speed and direction values in a random manner until the impact points from the model matched the impact points observed in the images.
We found that most of our wind measurements were broadly in line with those from a Mars climate model, suggesting the winds measured are representative of the ambient wind field and the atmosphere was relatively calm during these spacecraft landings, i.e. with little or no turbulence, during their descent. One outlier measurement was obtained for the Phoenix lander, which landed at a high latitude close to the northern cap of Mars. Here, the wind speed was relatively high during the descent of the lander on the parachute. It appears to have decreased shortly before the lander was released from the parachute. Closer to the surface, the wind was found to rapidly increase and change direction, suggesting a gust may have been blowing. This may not be surprising as Mars tends to experience more variable weather at northerly latitudes.
In terms of winds on Earth, the wind conditions on Mars during the spacecraft descents were generally light with most wind speeds below 8 m/s. Taking into account the thin Martian atmosphere these wind speeds correspond to a strength of 0 to 1 on the Beaufort scale. At this intensity, the wind may provide just enough energy on Earth to keep a large kite aloft. The high wind speed of 20 m/s, as possibly experienced by Phoenix descending on the parachute, would only correspond to 2 on the Beaufort scale or a light breeze.
We provide a unique set of direct in-situ wind measurements in the Martian PBL. It is intended that this information be useful for informing modelers and investigators in the conditions of this important region of the Martian atmosphere.
The wind measurements can be found in our paper.
These findings are described in the article entitled Measurement of Martian boundary layer winds by the displacement of jettisoned lander hardware, recently published in the journal Icarus. This work was conducted by M.D. Paton and A.-M. Harri from the Finnish Meteorological Institute, and H. Savijärvi from the University of Helsinki.
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