Estimating the Altitude of Martian Clouds at the Mars Science Laboratory Landing Site

Our second LPSC installment continues the focus on martian clouds, but this time, instead of looking at them from orbit, we examine them from the surface. The animated gif above shows a view from Curiosity. I, for one, find these animations quite relaxing. Over now to Charissa:

By Charissa Campbell

Clouds have been observed on Mars from the Curiosity rover's current location. This is interesting because it makes Mars seem more like another Earth and makes one wonder if this could possibly be our future home. However, as everyone knows, not all clouds are the same and have a variety of morphologies (shapes) and altitudes. Since Mars does not have a very thick atmosphere, in contrast to the Earth, the clouds are typically cirrus clouds which are the thin, high altitude clouds that are very wispy looking. One reason we want to study these clouds is to better understand the Martian climate.

Onboard the rover is a camera, called the NavCam, that takes snapshots of the atmosphere. There is a total of 8 images in a single observation which are combined to make what we call a Zenith Movie (ZM). It has this name because each image includes the zenith point as viewed from the rover, a term for the point in the sky right above your head. Since we are creating a movie out of the images it will allow us to view the movement of any cloud features. However, like any data, the images must be processed to be able to see these clouds. 

An intrinsic property of clouds is its opacity, which is defined by how much light can get through the object you’re measuring. For instance, in high opacity (thick) Cumulonimbus clouds, not much light gets through and it may seem darker than usual when those are overhead. In contrast, cirrus clouds are low opacity (thin) and do not significantly obstruct the light from reaching the ground. This is especially true on Mars where many of the clouds have such a low opacity, they cannot be seen through the raw images. The mean-frame subtraction technique is then used that find the slightly-brighter pixels in each frame that have “shifted” compared to the background (largely atmospheric dust) where these shifted pixels are the clouds.

One conflict we must overcome first is to distinguish if these features are indeed clouds or in fact just Martian dust flying about. It has already been seen that Martian dust does move around, and even create dust devils (see: http://www.lpl.arizona.edu/~lemmon/mer_dd.html), so therefore we have to be able to distinguish if what we are seeing is truly clouds, or simply dust. This can be estimated by simply seeing how the features move across the frame. If you were to look up into the sky and observe the clouds, you’d notice that over time, they stay relatively the same shape while moving across the sky. If you were to view dust being flown about by wind, you’d see that it does not keep the same shape. This same technique can be used when analyzing the processed movies. If the features that are being observed are moving at the same speed relative to each other, they are more likely to be clouds while ones that do not stay the same shape are more likely to be Martian dust.

The best way to analyze a ZM is to find one that has noticeable features in it. The example that I first worked with was Sol (the word for a Martian day) 1211, which is the gif shown at the top of this article, which has really distinct features that made it easier to analyze. Here we want to look at the wind speed of the cloud and the direction it is moving relative to the rover. These parameters are used because they will relate to the computer modelling done on atmospheric processes on Mars. Computers are very useful in today’s scientific research as they can help us estimate certain parameters on places that we cannot visit (or at least any time soon!). Luckily, our collaborators use one of the supercomputers at NASA to run the Mars Regional Atmospheric Modeling System (MRAMS) at the current location Curiosity is at and at different times of the Martian year. MRAMS plots the wind speed and direction during an entire Martian day at a specific season. This way, we can use the time of day that the ZM took place and compare what the model found for that same time versus what we found through our hand analysis and further estimate the altitude of the clouds that are being observed through Curiosity’s camera.

There has to be two types of analysis done for this project. First, we needed to see how far the features moved across the screen and in which direction. This was done by overlaying a grid over the images and comparing the difference in grid location between the first and last image. Since we also know the field of view, in degrees, of the rover, we can estimate how much the cloud moved. To get the direction, we then overlaid a wind-direction compass and following the path that the cloud moves.  
We can compare both these values to the MRAMS models to find likely altitudes for our features. This was done using basic trigonometry, which is shown by the first triangle below. The angle theta (θ) would correspond to the angular velocity we found from the ZM analysis. But without knowing the altitude it is impossible to know the absolute windspeed of the features. Furthermore,  in MRAMS, the velocity vector has two components (u & v) and we must find the angle these two components make in order to retrieve the wind direction and speed from the model output.  It’s funny how something that might seem so complicated takes simple trigonometry to figure out. No matter how many times I have worked with trigonometry, I will never forget SOH CAH TOA.

 

Since we now can directly compare the model’s data to the ones found in the ZM, we can see if a certain altitude stands out between both the wind speed analysis and the wind direction one. When comparing these values we can see if there is any range of plausible altitudes. With enough observations and comparisons, we can narrow down the list of potential altitudes for clouds above the landing site. For now, we are looking only at a single season (Ls 90°). However, eventually we will also analyze the clouds at different seasons on Mars. That way, we can analyze an multiple Martian years to begin to look for trends.