Everyone has seen awe-inspiring pictures of Saturn’s rings. Less well known is that Jupiter has rings too, so faint they are normally all but invisible. The rings of Jupiter are just one of the little-known subjects that Heidi Becker has photographed over the past decade or so. She is a co-lead investigator on NASA’s Juno mission, which arrived at our solar system’s largest planet in 2016 and has been studying it in detail ever since.
Many of the most eye-catching images of Jupiter have been taken by the craft’s main camera, JunoCam. But that isn’t what Becker works with. She uses a special low-light detector that was never intended to take proper images for people to view.
Nonetheless, she has discovered she can use it to image dark and shadowy areas of the planet and its moons and to reveal things no other instrument can. That goes far beyond those feathery rings – to the dark sides of the planet’s moons, mysterious craters and new kinds of lightning. 91av caught up with Becker to hear about the best pictures she has taken, and what we have learned from them.
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Joshua Howgego: When you were younger, you worked on the stage. How did you end up studying Jupiter? That’s quite a transition.
Heidi Becker: Growing up, I was a dancer, and I also did a lot of experimental theatre in New York. But when I was in my early 20s, I had an injury to my knees, which made dancing not possible for me any more. But I was always fascinated by light and the stars. I was looking through a telescope on a visitor night at an amateur astronomy club in Connecticut, and the astronomer guiding us showed us the Orion Nebula. He described the nebula, the magnetic field lines, the gas that was collapsing, how it was a nursery for stars – it was so beautiful. It was the moment when I knew I wanted to go into that field.
You eventually wound up at NASA.
I decided to get a degree in physics and go on to optical sciences – I was interested in cameras and light. Then, towards the end of my degree, I needed some extra money and got a part-time job at NASA’s Jet Propulsion Laboratory, which ended up becoming a full-time job working on the effects of space radiation on electronics. One of my colleagues was an expert on star tracking and was involved in the proposal for Juno.
What is a star tracker and why are they so important?
The idea is that you keep track of the positions of the stars so that you know where you are in space. If you don’t know that, you don’t know where your data were collected. It’s crucial to the science and for navigation.
The biggest challenge for Juno’s mission success at the time was: how do you have a star tracker on a spacecraft that is going to be constantly spinning – and that works in the harsh radiation environment around Jupiter?
A tiger team formed: a small group of experts aggressively trying to solve this problem. I joined this team, and we had only a few months to do a lot of unprecedented radiation testing on sensors that would go into our star tracker before Juno was approved. Most missions experience solar proton events and galactic cosmic rays, but not electrons travelling at relativistic speeds, which is what you get near Jupiter. So, there was no body of information about how a sensor from a camera would behave or degrade. We found a lot of odd places to simulate the environment. One was a cancer hospital in Paris – they let us in at midnight to use their medical equipment to irradiate our sensor.
How did this go from being an instrument used for navigation to a science camera?
Juno’s star tracker is called a Stellar Reference Unit, or SRU. Its function is to take pictures of the dark sky and find known stars. Then the spacecraft figures out where it is pointing.
However, Juno flies where no one has flown before, and so [Juno principal investigator] dzٳDZٴDz asked me if there was any data we’d be getting that we could use to understand the radiation environment. I was complaining one day to a colleague about how limited the information was that we could get from the SRU. They said, “Well, why don’t you just take a picture?” And so, we reached an agreement that we would take about 20 pictures per orbit of Jupiter and transmit them to Earth – something we don’t do with the SRU’s usual star images. These would just be images filled with the signatures of the surrounding radiation; it just looks like the snowy static you can get on a TV. What that shows us is the energy and concentration of high-energy penetrating particles around the spacecraft.
So, at that stage the pictures weren’t exactly going to be much to look at…
Before Juno arrived at Jupiter, there was a lot of talk on the team about Jupiter’s ring system. It wasn’t clear how well the other cameras would be able to see it. The main camera, JunoCam, is designed for bright light, to take beautiful pictures of the cloud tops in sunlight. That made me wonder if we could do it with the SRU. And that started this cascade of searching for different types of opportunities within the system that were low light, because that first ring image was so successful.
What did we know about the rings before you photographed them?
It was known that there is this ring system. There are different parts to it, and it was thought that the rings are fed by dust coming from meteoroid impacts on its four inner moons that orbit at the same radius as the rings: Amalthea, Thebe, Metis and Adrastea. However, there were very limited observations of the rings, because many previous missions have approached it equatorially, or from the side, making it hard to see.
Those previous observations also raised this laundry list of mysteries about the rings. The New Horizons spacecraft observed bright clumps of material following Adrastea, and it’s not known if those are other small moons, or whether something hit Adrastea, and for a brief moment in time, there were little pieces trailing it. Other observations show that the rings were brighter at certain longitudes, and it’s not clear why.
I can see how they would be awkward to photograph. How did you manage it and what did you see?
Because we’re flying so close to Jupiter, we had the opportunity to look straight down on the rings and see the distribution of the dust. When you let Jupiter’s shadow get cast across part of the image, you see that sharp boundary between dust and darkness. So, if you’re trying to find a very dim signal from very diffuse, faint dust, it becomes a little easier, because you get that contrast. That’s a trick we’ve been using a lot. We took the first ring image in 2016, and we’ve done as many as we can since then.
Why is it important to study rings?
ճ’r mini laboratories for planetary formation or even solar system formation. You start with a great big ball of gas that accretes and turns into a star or a planet. Knowing how the dynamics of that work in the rings around planets like Jupiter gives us a framework for understanding how that works on larger scales. We’re finding that Jupiter’s rings are a part of a highly interconnected system. It involves those inner moons, the inner radiation belts of Jupiter, which are very strange and very non-uniform, the magnetic fields. All of this is very mysterious, and there may be other influences from Jupiter itself.
You have taken so many other great pictures. Tell me about Ganymede.
Our closest approaches to three of Jupiter’s moons – Ganymede, Europa and then Io – were on the night side. Jupiter was our light bulb, and the geometry was such that it shone on just a particular region of the surface of Ganymede. We were flying past extremely quickly, tens of kilometres per second, which meant that for each of those moons, the SRU could take only one picture. We had to get it right. But it worked. The SRU took Juno’s highest-resolution image of Ganymede, and the special lighting conditions revealed an incredible amount of previously unseen features. This has allowed us to massively improve the geologic map of that region of Ganymede.
One of the things we identified is an intriguing spray of ejected material. Ganymede has a very large crater called Tros. But many kilometres away from the crater is this streak of ejecta that we saw in the shape of a ray. It sort of looks as though it could have come from Tros crater, but the weird thing is that there is this very old, dark terrain in between. So, did it come from there? Or is it a comet? Or what?
That sounds like a real mystery. If it was an impact crater, wouldn’t you expect it to spread out in a sort of a circle?
In a star pattern, yeah. It’s very strange. We’ve brought it to the attention of the JUICE team [another mission due to study Jupiter] because I think once they’re there and doing this amazing survey of Ganymede, it’d be interesting if they can figure out what it is.
You also made an amazing discovery about a new type of lightning…
Well, at a certain point in our mission, we started approaching Jupiter from the night side. The very first picture that we took was a really exciting moment. At that point in the mission, the closest approach to Jupiter was always at 2 or 3 in the morning, California time. So, I would wake up instinctively and look at my computer – sort of like a mother with a baby that needs to be fed in the middle of the night. And the picture came down, and we could see the cloud tops at high resolution on the night side. The reason for that was because they were illuminated by moonlight from Io. And there were little flashes of light in the image that were lightning in the cloud tops.
Were you the first to see that image in the middle of the night?
Absolutely. When you’re the first person seeing some part of nature for the first time – in this case before others wake up – it’s an incredibly intimate and awe-inspiring moment. It’s one of the things about what I do that is rarefied and precious. I think I put my hands on the screen and said “Oh, my God”, because it was such high-resolution imagery.
What was new about this lightning?
Lightning has been seen by every mission that orbited or flew past Jupiter on its night side. We know that the larger the flashes we saw on the cloud tops, the deeper it originated in the atmosphere. Ever since the Voyager mission [in 1979], we’ve seen lightning coming from a particular depth in the atmosphere, where the conditions allow water to exist in a liquid, solid and gaseous state, which is the condition that on Earth is necessary for most lightning and thunderclouds. So, scientists were content with the fact that lightning on Jupiter was very Earth-like.
When I analysed the lightning I saw, I found it was coming from above the water cloud, where it is too cold for liquid water to exist. I remember one scientist, who is a real expert on Jupiter’s lightning, saying to me: “This is a problem.” But really, it was a discovery. It was a different kind of lightning that doesn’t occur on Earth.
Right at the time that this discovery was made, we were finding that Jupiter’s atmosphere wasn’t as well-mixed as we had always thought – there were these pockets of missing ammonia – and Juno’s atmospheric scientists were developing ideas to explain why. The theory was that, at a certain high region in the atmosphere, you have high-energy storms that are throwing up water ice crystals and interacting with the ammonia gas at a specific altitude, which can act like an antifreeze and create liquid ammonia-water droplets. These droplets get tumbled around in thunderstorms, forming an icy crust, and would eventually form into hailstones. These hailstones could grow and fall deep into Jupiter’s atmosphere, taking the ammonia with them and creating the missing pockets of ammonia scientists previously measured. So, when I saw lightning coming from exactly the region where this was predicted to happen, it was evidence that these high-altitude storms may actually exist.
What are you excited about snapping in your next photograph?
We’re doing a lot of imaging of Jupiter’s aurora on the night side of the planet. We’re interested in the vertical structure of the aurora, and what is happening at different depths in the atmosphere. And there has been a lot of imaging of the aurora at infrared and ultraviolet wavelengths, but some of the chemistry cannot be seen at those wavelengths. Visible cameras can see them, though, so we have this unique, close vantage point, and we’re in the middle of a campaign of investigating this.
If you could take a picture of anything in the Jupiter system, what would you go for?
When we did the Jupiter shine trick with Europa, we found a region that we nicknamed the platypus [above], which looks like the ice shell has been disrupted and where there could be liquid water very close to the surface. There’s a theory that proposes that if you have liquid water close to the surface, it can cause the ice shell above it to collapse and create a sinkhole. As it’s in the process of refreezing, you might have big ice blocks that shoot up from the water. Our photograph makes it look a lot like that is going on there. So, if I could do anything, I would probably fly back there, even closer, and see if we could prove that – I think that would be very cool.
