This and That: Using CASSINI INMS Data to Study Atmospheres and Plumes

Well, this is embarrassing. This is not what I had planned to discuss this week. This is the point where I go over the different test cases I’ll be using Tekton on. Unfortunately, my time has been almost entirely monopolized by my attempts to prepare a lab for the Astrobiology Class. The lab is done. The trouble has been with the results I keep getting which are slightly off from what all the published literature is saying. After much review, I think I’ve figured out the issue which relates to over-saturation of the main (higher sensitive) sensor. So it wasn’t all for not, but given the amount of time its taken me, I think its worth something discussing so we can all get a better idea of how to deal with other types of data sets, especially non image ones.

Cassini-Huygens Mission

 

Cassini-Huygens is an unmanned spacecraft sent to the Saturnian system. Since its arrival in 2004, it has provided many discoveries that have fundamentally changed the course of future planetary exploration.  With its many moons and rings, many consider Saturn to be like a mini solar system.

 

The Cassini-Huygens Spacecraft is made up of two main components: the Cassini Orbiter and the Huygens Probe. The orbiter hosts a range of science instruments in:

  • Optical Remote Sensing: used to study Saturn, the rings, and moons in visual and infrared wavelengths.
  • Fields, Particles, and Waves: used to study the dust, plasma, and magnetic fields around Saturn and its moons.
  • Microwave Remote Sensing: uses radio waves to map atmospheres, determine mass of moons, and collect data on the rings. It also unveils details about the surface of Titan.

More details about the spacecraft instruments are available online: https://saturn.jpl.nasa.gov/mission/spacecraft/cassini-orbiter/

 

The Huygens probe hosted its own set of instruments to more closely study Titan.

 

 Ion and Neutral Mass Spectrometer (INMS)

 INMS characterizes the neutrally charged particles and low energy ions in the gases in Titan’s atmosphere, Saturn’s magnetosphere, and the ring environment to determine their chemical, elemental, and isotopic compositions (Fig. 1).

62_PIA10356
Fig 1: Example of compositions from INMS at Enceladus (from JPL)

The instrument counts the number of particles present. These counts are converted to a specific density of particles in a region for a range of masses from 10 to 100 Daltons (the atomic mass per elemental charge). These correlate with specific compounds by their molecular weights, thereby revealing the abundances of each.

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Ion measurements are done in two modes: open source and closed source (Fig. 2). The closed source mode is used to measure non-reactive neutrals such as CH4 and Nitrogen. Open source mode measures positive ion species which tend to be less abundant. If you’re interested in identifying neutrals, the best route is to filter out the open sourced data, but this will impede your ability to study both ions in the spectrum. This allows for separating neutrally charged species from the charged ion species. Closed source increases the pressure of the incoming species to help facilitate readings. However, reactive species would react with instrument walls and thus require a different approach.

Calibrations and Sensitivities of INMS Data

 

Each species (mass value) must be calibrated to account for instrument sensitivities and other factors. These factors vary for each species, so it requires a close analysis of each potential species. A sensitivity factor,  of units counts/s/cm3, varies for each potential species measured. These are listed in a calibration file on the Planetary Data System (PDS) website. The results are also effected by the angle and velocity of the spacecraft and is accounted for using the Ram enhancement factor,  which is unitless. It’s dependent on several factors: the velocity of the spacecraft relative to the target (e.g. Titan), the angle of the INMS aperture ( theta), the temperature of the air (Ta ) and the instrument (Ti ), as well as the mass of the molecule (m) being considered. Then density, , can then be ascertained,

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2.JPG

where N is the number of counts that the instrument measures and tIP is the instrument integration period (0.031032 seconds). Needless to say, converting the raw data to physical number is a long and complex process. However, we will be circumventing the calibration process and calculating the results for the three known major species: hydrogen, methane, and nitrogen. The Sensitivity and Ram enhancement factor are shown in Table 1.

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There are two sensors on the INMS instrument. One is a high sensitivity sensor and the other is low. The high sensitivity works such that as it approaches saturation, the counts become throttled. This begins being significant after 40,000 counts. When it becomes completely saturated, the high sensitivity counter goes to 0. In order to get an accurate reading, you have to design a method that uses both counters, following these rules:

4.JPG

where C1 represents the first column of counts (high sensitivity) and C2 represents the second column of counts (low sensitivity). If the high sensitivity response is completely saturated, C2 will be much higher. Occasional readings may give 0 and 1 for C1 and C2, but these are still false and need to be filtered out. To be extra cautious, we choose values higher than 12 because the sensor becomes unreliable below this point.

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Another issue associated with saturation is dead time, where the receiver is overloaded with so many points that the detector misses some point as it lags in a way. However, this barely produces a 20% lag for even 20,000 counts, well above the highest counts we will deal with. For that reason, we will not worry about this correction.

Lastly, during closest approach, CASSINI begins to enter Titan’s atmosphere. Drag slows the spacecraft down, so thrusters must be used to maintain velocity. The exhaust includes high quantities of hydrogen, which can contaminate the hydrogen (2 Daltons) counts. You will be dealing with a flyby where this effect is minimal, but it is important to be aware that it may skew your results slightly.

Titan
Titan is the only moon in the solar system with a thick atmosphere. Nitrogen is the dominant species in the atmosphere, with a few percent methane present. Other components have been suggested based on a mix of ground studies and Voyager and Cassini observations.

The major constituents that were expected to be found in Titan’s atmosphere are shown in Table 1. Solar UV radiation and energetic particles interact with Titan’s atmosphere, ionizing and dissociating compounds. Over time, the most abundant constituents (methane and nitrogen) combine to form larger compounds. This is visualized in Figure 3.

Enceladus

Saturn has many icy moons other than Titan. Enceladus stands out because it is well known for its huge plumes of water emanating from the tiger stripes in the south pole. The ocean extends under the entire ice shell but is likely largest in the south pole. Its close proximity to Titan could prove hopeful for potential mixing of compounds between the two worlds.

8.JPG

PDS: The Planetary Data System

The Planetary Data System is an archive of all the public scientific data from NASA planetary missions, astronomical observations, and laboratory measurements. It offers several ways of browsing data whether by the body of interest, a particular mission, or by specific fields of science. We will navigate using the PDS Nodes which are located on the left side of the PDS home page. The nodes are broken down by area of research as follows:

  • Atmospheres: specializes in non-imaging atmospheric data from all non-earth missions.
  • Geosciences: specializes in data related to the study of surfaces and interiors of terrestrial planetary bodies.
  • Cartography and Imaging Sciences: specializes in all the digital image collections from past, present and future planetary missions.
  • Navigational & Ancillary Information (NAIF): specializes primarily in engineering related information such as system navigation and other mission functions.
  • Planetary Plasma Interactions: specializes in data related to solar wind, magnetospheres, ionospheres and their interactions with planetary atmospheres and surfaces.
  • Ring-Moon Systems: specializes in all data related to planetary ring systems.
  • Small Bodies: specializes in data related to asteroids, comets, and interplanetary dust.

There are a lot of other ways of navigating the site, and this is just one way of trying to find what you have.

Learn more at pds.nasa.gov

Several flybys have been studied for Titan. One example is the one done by Waite et al. 2005 to get an idea of the range of values you might see a signature from a particular molecule.

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