Mass Spectrometry in Space: Insights From a NASA Expert

When we design an instrument to go to a planetary environment, we have to tailor it to be successful in that environment. There are two aspects to that: one is ensuring it can function in a place that's not a cozy laboratory environment, and the other is adapting it to do the best science possible in that environment.

With the Dragonfly mass spectrometer, our goal is to analyze surface samples – solid samples that are drilled and delivered from the surface of Titan. We are looking to understand the complex organic chemistry and the range of molecules that are present in these surface environments. We know there will be a lot of organics – we learned that from the Cassini-Huygens mission – but we don’t know exactly what they will be. We're also very interested in understanding how these organics in the atmosphere have been processed on the surface, where they may have been exposed to water, etc. We are very interested in seeing if this processing has led to the formation of prebiotic molecules – such as amino acids and nucleobases.

We have to design the instrument to be able to get these surface samples, deliver them to the mass spectrometer inlet and then have the capability to look for those molecules. Titan's surface is extremely cold – 94 Kelvin, which is -180° C or -290° F. That means that the samples we take from the surface are extremely cold, so we had to design a sample system that functions in that environment and minimally impacts the sample.

However, a mass spectrometer doesn’t want to be cold; it prefers to be at room temperature or maybe a little cooler. So, we have to keep the samples cold while also interfacing with an instrument where the electronics have to be kept warm. That’s an example of where the science and the function parts of the mission can be in conflict.

At the Goddard Space Flight Center where we're designing this instrument, we work with experts who send telescopes into space and regularly deal with sub-Kelvin temperatures. They’ve helped us design a thermal system that keeps the samples cold on one side and the instrument warm on the other, whether the samples are sitting in the carousel or during the moment of analysis.

The idea is to keep the sample as cold as possible, especially if it contains water or ammonia, so it stays frozen.

It’s all relative. For spacecraft science payloads, yes, this type of instrument is on the higher end of power requirements. The sample cooling aspect is passive, but the heating is not. We have to heat the sample oven and heat all the plumbing lines to keep the helium flow moving, as well as the gas chromatograph columns and electronics. Most of our power needs go toward heating everything – keeping it hot, trying not to have cold spots and keeping it as isothermal as possible.

This mission uses a nuclear power source, similar to the Curiosity and Perseverance rovers. It’s powered by plutonium, which decays and generates heat. This heat charges the battery, which is our power source, but it also gives off waste heat that we can use. We distribute that heat around the insulated lander like a heating, ventilation and air conditioning system in your house – moving heat to keep things at the right temperature.

However, it turns out that we are so well insulated against Titan’s atmosphere during hibernation that, when we start operating, we can get too hot! To manage this, we have special ducts that pass the internal air close to the outside of the spacecraft, and this helps to cool it down.

The thermal management on this mission is the most challenging I’ve ever seen or worked on for a spacecraft.

A few centimeters – it's very surface level. But that’s because what we're really interested in are these different surface materials.

For example, Titan has equatorial dunes, but these are not made from silicate sand, like on Earth; they're made of organics. How they are formed is not entirely known. They are certainly composed of a good component of organics, possibly with an ice core. We want to sample these dune sands.

There are also inter-dune flats where we see exposed water ice, and we're interested in those materials as well.

Finally, a big target of interest is what I’ve already mentioned – surface processing that might lead to the formation of things like amino acids. If you have moments of transient liquid water on the surface – for example, if there is a meteor impact which melts the ice crust and might take hundreds of years to freeze back over – you can have organic material that rains down from the atmosphere and all over the surface. If this happens during that period of melt, we could see the formation of interesting prebiotic molecules.

One of the big interests in Titan is that we see a lot of organic chemistry happening in the atmosphere. It forms a haze, which covers the moon and makes Titan look like a fuzzy orange ball. This is because the organic haze in the atmosphere is formed by photochemistry between nitrogen and methane, which builds up bigger and bigger molecules.

The Cassini mission dipped through the very upper edges of the atmosphere and was able to measure organic ions that were thousands of Daltons in size – chemically, huge. We see the signatures of complex molecules in spectroscopy, both from ground observations and from the spacecraft in the Titan system, that resemble aromatic features – possibly polycyclic aromatic hydrocarbons, maybe with some heteroatoms in them.

They are very different. To start with, they are different types of mass spectrometers. The Dragonfly mass spectrometer is an ion trap mass spectrometer. On Titan, we want the ability to look at higher molecular weights. We’re interfacing with a laser desorption source, so we know we’ll be able to look at molecules that are 1000 Daltons in size, maybe more. And we have time to sit on the surface and do this – measure something, then possibly go back, measure another sample and look at it differently.

Venus is the complete opposite. On the DAVINCI mission, we have a descent probe that will go through the atmosphere, and it has about an hour to transect from the upper atmosphere to the surface. The science happens during that descent. This has been done before – like the Galileo probe at Jupiter or the Huygens mass spectrometer on Titan – so it’s something we know how to do, but the focus is different. We can’t go back and retake a sample.

Our goals are to look for atmospheric gases on Venus, not large organic molecules. What we are most interested in on Venus is what the chemical profile will be as we go through the atmosphere, and to be able to pin certain chemicals to different altitudes and see how their abundance changes and how they interact. This will help us learn more about the chemistry of the deep atmosphere.

Because of all this, our goals for the Venus instrument are very different. We need something that is really robust that makes very accurate measurements of isotope abundances. The measurements must be stable, well-calibrated and the instrument must also be mechanically strong enough to do all of this while falling through the atmosphere.

We have a strong history with instruments built at Goddard, particularly with the quadrupole mass spectrometer. This is the type of instrument we’ve used on the Galileo probe, Huygens and even previous Venus missions. The Sample Analysis at Mars (SAM) instrument on the Curiosity rover also used this type of spectrometer. Essentially, we are taking the major components of the SAM instrument, which is the mass spectrometer and the tunable laser spectrometer, and we’re putting them on DAVINCI to make complementary measurements at Venus, which we can then compare directly to the measurements from Mars.

I’m very biased – as we all are here at this conference – but I think that mass spectrometers will continue to be one of the key instruments we send on any planetary mission. They are so powerful in terms of flexibility and adaptability to the environment. There are so many ways that the technology and state-of-the-art equipment are being pushed on Earth for applications here, which we can utilize and adapt for spaceflight.

I see a lot of progress in pushing the boundaries, especially in aspects such as enhancing resolution, mass range and in different techniques for introducing samples. How we adapt the sample, interface with the sample, how we get those samples in to the apparatus and how we look at them will all influence what we’re seeing. With Dragonfly, for example, we have two modes of analysis: one where we look at volatile compounds or compounds that can be made volatile which are sent to a gas chromatograph, and another mode – laser desorption – where we can look at larger molecules.

Another key thing I’m excited about is the advancement in power systems. Power is a major challenge for us, as we touched on earlier. Another significant challenge is communication and data bandwidth – that is, how we are getting the data back. The more we can advance onboard machine learning and AI techniques that will help to process huge amounts of data and distill it into what we know is meaningful, the better. This will allow us to send only the important data back. That’s going to be a critical area for us to push forward, as ultimately, data is power – it takes power to transmit it back.

We are working on that in our group with MS data, with a focus on enhancing the rapid analysis of data when it comes back, helping to pick out what is the “good” data versus the “bad” data. When data comes back, we need to make decisions quickly about what to do next. Our goal is to enhance onboard processing along those lines. Right now, we’re still gathering all the data back and analyzing it ourselves, but we're on track to implement onboard intelligence to really help with this data bandwidth problem.