EPISODE 77: VERITAS

Transcript

John Brophy: Venus is a big place. It has three times the solid surface area of the entire Earth and three times the total surface area of Mars.

VERITAS‘ global mapping and high-resolution topography that we’ll get will define Venus exploration for a generation.

Deana Nunley (Host): Welcome back to Small Steps, Giant Leaps, a NASA APPEL Knowledge Services podcast where we tap into project experiences to share best practices, lessons learned and novel ideas.

I’m Deana Nunley.

Glad you’re along for the second segment in our two-part series on NASA’s Venus missions. We’re shifting our attention today from the DAVINCI mission to VERITAS. And our conversation is with VERITAS Project Systems Engineer John Brophy at NASA’s Jet Propulsion Laboratory. John, thank you for being our guest.

Brophy: Oh, thank you. It’s my pleasure to be here.

Host: What intrigues you most about exploring Venus?

Brophy: Well, I would really like to know what happened to Venus. Venus is basically the same size as the Earth. It has the surface gravity of about 0.9 Gs and instead of one for the Earth, but it’s made of the same stuff. But Venus is literally hell while Earth is like heaven, and it looked like Venus should have all the building blocks of a habitable planet. So what went wrong? How did Venus turn out this way? And what does it tell us about the evolution of rocky planets elsewhere in the galaxy? That’s what really intrigues me about going there.

Host: And so, what’s the aim of the VERITAS mission?

Brophy: The VERITAS seeks to answer really four essential questions about Venus. What processes shape rocky planet evolution? Is there evidence of past oceans on Venus? What geological processes are currently active on Venus? And is there evidence for significant amounts of water still present in the interior of Venus? So it’s really those four questions that we’re looking for answers for.

Host: What are the main science objectives of the mission?

Brophy: So, the primary objective is really that first one I talked about, which is to understand the processes that shape rocky planet evolution. So, the way to understand this is let’s start by looking at our favorite rocky planet, which is Earth and what things on Earth are important for long-term habitability. And there’s really four things.

So, on Earth, plate tectonics. It links together the atmosphere, the surface of the planet and its interior in a way that strongly affects the Earth’s long-term habitability. It’s kind of the ultimate in resource recycling.

The second key feature is that plate tectonics is also the dominant heat loss mechanism for the Earth. Heat loss from the interior maintains the magnetic dynamo in the Earth’s core, which generates the magnetic field that protects us from the charged particles that make up the solar wind that are blasted out by the Sun.

The third feature is that the vast majority of Earth’s present-day oceans and atmosphere are now believed to have come from the interior, and they’re degassed through volcanoes. Previously, it was thought that water is brought to the Earth by comets and asteroids but now it’s thought that it most likely comes from the interior.

And then the fourth key feature is of course, liquid water oceans that enabled the formation of continents. So what’s important for habitability on Earth? Plate tectonics, an active magnetic dynamo in the core, volcanoes, and of course water, lots of water. But we don’t actually know how plate tectonics got started. It’s likely that subduction was the first step in getting this started, but there’s really a lack of data that doesn’t allow us to distinguish between all the different theories on how plate tectonics gets going. And Earth’s active surfaces destroyed all the records. Most of the rocks from billions of years ago have been destroyed.

So, this is where Venus comes in. So remember, Venus is about the same size as Earth. It’s made of the same stuff as Earth and importantly, Venus today has what’s called a hot lithosphere. So the lithosphere is the crust of the planet and the upper part of the mantle, and it’s believed that this hot lithosphere is similar to what the early Earth’s lithosphere was like. And so, on Venus, we’re going to look for evidence of subduction, which is we think how plate tectonics gets started. We’re going to look for evidence of continents, of past volcanic activity, of current volcanic activity and out-gassing of water, and these are all the processes that affect the evolution and long-term habitability of rocky worlds. And so that’s what we’re going after.

Host: What impact do you think this mission’s global mapping and high-resolution images of Venus will have on future missions and exploration?

Brophy: So, it’s important to recognize that VERITAS is an orbiter mission. And so we’ll go into a nearly polar orbit at an altitude of about 215 kilometers above the surface. And over the course of nearly three Earth years, Venus will slowly rotate beneath us and give us global coverage of the surface, and in some cases allow us to image some parts of it up to seven different times. And VERITAS‘ global mapping and high-resolution topography that we’ll get will define Venus exploration for a generation. It will answer the questions that we just talked about, but almost certainly, will raise a great many more questions, ones that we don’t even know to ask yet, ones like what Isaac Asimov would say, ‘That’s funny,’ as we look at the data or ‘I wonder what that means,’ or ‘how do you explain that? Where did that come from?’ The VERITAS‘ global data set will tell future generations where to go, what things to measure, what instruments to develop, and most importantly, what questions to ask.

Host: Could you tell us about the instruments planned for the mission?

Brophy: Oh, I’d be happy to. So, VERITAS carries just two instruments, a high-power X-band radar called VISAR. X-band has a wavelength of about four centimeters. We chose that so we can get really high-resolution images and VISAR stands for Venus Interferometric Synthetic Aperture Radar. I know that’s quite a mouthful. And the second instrument is called VEM, which stands for Venus Emissivity Mapper. So apparently, we felt compelled to include the name of the planet in each of the instrument names. So, VISAR, the Venus Interferometric Synthetic Aperture Radar, we use this to look through the Venus cloud cover and to measure the topography and to image the surface at high resolutions. This radar is so good that we get 30-meter resolution all over the planet and 15-meter resolution for about a quarter of the planet.

So how does this work? So the interferometer part of the instrument uses two transmit antennas. So the spacecraft carries these two large antennas and they create an interference pattern. Now, if this interference pattern hits a smooth flat surface, you’ll see a nice regular pattern, but the surface of Venus isn’t smooth or flat. It has lots of elevation differences, and those differences distort that interference pattern. By measuring those distortions, we can back out what the elevation differences are on the surface, which is exactly the topography and so VISAR, our VISAR instrument will provide topography that is 100 times finer resolution, and up to 10 times more accurate than that provided by the Magellan mission back in the 90s.

The synthetic aperture radar part of the instrument makes use of the spacecraft motion to create a much larger receiving antenna than its physical size, and this has been a common radar technique for decades. It provides us the ability to take high-resolution images from orbit and our images will be up to 10 times higher resolution than those provided by Magellan. And the combination of this high-resolution topography and these high-resolution synthetic aperture radar images enable us to search for actually buried topographic features and allows us to construct, reconstruct Venus’ past geological record. We can determine how craters have been modified over time, whether the volcanic resurfacing occurred in a steady manner or all at once. It’ll provide us a lot of information about what happened to Venus.

And then finally, the instrument has a feature that we call repeat-pass interferometry, and this is the first time it’ll be done on another planet. We do this by passing over exactly the same spot on Venus separated by several hundred days. Remarkably, this technique is so accurate that we can detect changes on the order of a few centimeters on the surface. So, this allows us to look for all sorts of interesting changes, including active volcanoes, active lava flows, subduction, as we talked about before for plate tectonics, and it’s a very powerful tool for understanding how planets change. So that’s the radar.

The other instrument is VEM, the Venus Emissivity Mapper. This is a camera, basically a camera that’s sensitive to wavelengths around one micron. And it turns out that even though Venus is covered in clouds, it has these windows that are roughly transparent at this wavelength. It still causes a lot of scattering. So, you can’t really get high-resolution images with VEM. That’s what we do with the radar. But what VEM can do is tell us what types of rocks are on the surface. Importantly, you can distinguish between volcanic rocks and granite. So why do we care? Well on Earth, granite forms when the mantle material interacts with water. So, if we see a lot of area on Venus with granite rocks, it would indicate that these are likely continents similar to Earth’s granite continents, and that these formed in the presence of large quantities of water, that is oceans. Geologists have identified features on Venus that are called tesserae, and it’s believed that these are the oldest landforms on the planet and are good candidates that are potentially continents on Venus.

So, interestingly enough, this allows the VEM instrument to look essentially billions of years into the past to see if there were once oceans on the surface of Venus. And it seems like magic, but it’s actually fairly straightforward. VEM works by just measuring how bright the light is coming from the rocks. The rocks are glowing, they glow in the infrared and the VEM instrument can see that glow. And if you know what the temperature of the rocks are, you can relate that brightness, that VEM measures to effectively the color of the rock. Technically, it’s the emissivity but the volcanic rocks are very, very black. They have an emissivity around 0.9 On a scale with a maximum of one and the granite rocks are grayer. Their emissivity’s around 0.7.

So, we’ll go to great lengths to calibrate the instrument in Germany, where the VEM instrument’s being developed and where they have the world’s best laboratory to recreate the temperatures and pressures on Venus and before flight, we’ll use them to measure all sorts of different rock types in this environment and create a catalog of responses to different rock types as a function of temperature and pressure and so forth. But all this is complicated. On Earth, the temperature of the surface varies with altitude, and this is also true on Venus. So to know the temperature of the rocks, which is important, you have to know the elevation and so that’s where the VISAR topography comes in. The topography maps we get from VISAR provide that elevation, and that’s used by VEM to then determine the emissivity. So there’s an intimate coupling between these two instruments.

And there are other corrections for the thick atmosphere and the clouds and all sorts of things. But in the end, we can tell what kinds of rocks there are on the surface. So as cool as that is, it’s not the only thing that VEM can do. It can also look for active volcanoes as we fly over and it does that by looking for the glow that the volcano shines onto the cloud cover. So if we’re lucky, VEM might literally see glowing volcanoes on Venus.

VEM has one other trick up its sleeve and that is, it can also actually observe near-surface water vapor using some of its measurement bands. And if there are large Earth-like concentrations of water remaining in the interior of Venus, then VEM will be able to detect volcanically out-gassed water. And based on some estimates, there could be as many as something like 10 volcanic eruptions per year that could produce enough water that is detectable by them. If we see these, it would indicate that the planet still retains substantial interior water and establishing that Venus has this volatile, rich interior despite the fact that the surface and the atmosphere are desiccated, it would be a major discovery. It’s also important because subsurface volatiles are believed to be key to enabling plate tectonics, and so their presence would also provide fundamental insights into how terrestrial planets evolve.

So those are our two instruments, but we also have a Gravity Science Investigation. And I always thought gravity science, it’s almost like magic. It provides an X-ray like interior view of the planet and allows us to see what’s going on inside Venus. With this, we’ll be able to tell the size of the core, the innermost part of the planet, and even determine how viscous that core is. As we mentioned earlier, the behavior of the core is responsible on Earth for the Earth’s magnetic field, but Venus doesn’t have a magnetic field, and so the question is what’s going on with its core? VERITAS‘ gravity science will tell us.

As we move out from the core, the next layer up in rocky planets is the mantle, and from VERITAS‘ gravity science, we’ll also be able to greatly improve the knowledge of the mantle’s viscosity on Venus. And those two things together, the core size and how viscous the mantle is, will tell us a lot about how the interior of Venus evolves and how this behavior shows up on the surface and how it affects these subduction zones if they’re there in possibly plate tectonics. So it’s a pretty amazing thing, and it’s actually fairly straightforward how you get these measurements. Understanding them is a little bit more complicated, but basically gravity science just uses the telecom subsystem. We use that to radio back to Earth, the science data that we collect with the VISAR and VEM instruments, and as VERITAS flies around Venus, slight variations in the Venus gravity field will cause the spacecraft to accelerate or decelerate slightly, and these variations in the gravity field are caused by concentrations of mass in different locations of the planet.

And back on Earth, the receiving antennas, they see these slight accelerations of the spacecraft as small shifts in the radio frequency of the data being radioed back to Earth. And we use, what’s called a K-band system, which is a higher frequency than is normally used on a lot of planetary missions and it gives us really good gravity science data. If you make these measurements for a long enough time, and VERITAS will do this for three years at Venus, then you can reconstruct what the interior looks like. Again, like I said, it’s feels almost like magic. So those are our science investigations.

We do have one other thing that we’re carrying as a technology kind of demo thing, and this is called the Deep Space Atomic Clock, which we abbreviate to DSAC because everything at NASA has an acronym. And as its name implies, it provides very accurate time on board the spacecraft. You might wonder, well, who cares how accurately time on board the spacecraft, but it turns out it’s all about navigation and just like ancient seafaring explorers, they needed the accurate time on board to know where they were, to know what longitude they were at. It turns out on spacecraft, you still need accurate time to know where you are. If you don’t have a really accurate time on board, the way this is normally done is the Earth sends out a signal to the spacecraft, spacecraft receives it, sends it back to Earth and on Earth they measure the round trip time that that signal took, and since speed of light is a constant, you can calculate how far the spacecraft is from Earth.

The better you can measure time, the more accurately you can tell where the spacecraft is. Light travels at roughly a foot a nanosecond. So one foot, every billionth of a second. And so you need to measure the time very accurately if you want to know where the spacecraft is very accurately. And this works on Earth because we have really good atomic clocks on the Earth, but now we want to put that atomic clock on board the spacecraft. So now the spacecraft can just receive the signal, a timestamped signal from Earth when it was sent. Now the spacecraft knows how long it took that signal to get to it, and it can calculate where it is relative to Earth. And so now the spacecraft can do its own navigation and be less reliant on help from Earth. And this will make future Deep Space missions more autonomous and less expensive to fly. So that’s our add-on. It’s not really an instrument. It’s a technology demonstration that we’re going to do as well.

Host: John, what significant engineering challenges does your team have to overcome to achieve success on this mission?

Brophy: Yeah, so there’s lots of things, but two things kind of stand out. One is the management of the radar data. The radar generates a huge amount of data because we’re talking about high-resolution images and topography all over the planet. And Venus, as we said, is big. It’s the same size as the Earth, but 70 percent of the Earth is covered in water and the land on Earth only makes up about 30 percent of the surface, but on Venus, it’s all surface, it’s all rocky surface. So Venus is a big place. It has three times the solid surface area of the entire Earth and three times the total surface area of Mars. And so handling all that data, processing that data and getting it back to Earth is a significant challenge. So we do a lot of the synthetic aperture radar data processing onboard the spacecraft and then transmit those products back to Earth. And in some cases we can do a data compression up to a factor of 1,000 in order to get that data, but that’s a significant challenge to make sure that all works properly.

The second challenge, interestingly enough, is called aerobraking. So nice thing about Venus is it has an atmosphere. So, we take advantage of that and we use aerobraking. Aerobraking uses the upper fringes of the atmosphere to gradually slow down the spacecraft so that we can reach the operational orbit that we need to collect the data, and this saves a huge amount of propellant. If we didn’t do this, we’d have to carry much more than twice our current propellant load and our current propeller load is already half the spacecraft mass at launch. So this would increase the mass of the spacecraft at launch so much that we would need a much, much bigger, much more expensive launch vehicle and it would make the whole mission unaffordable.

But in order to make this work, we’re doing much more aerobraking over a longer period of time than any previous mission. So, the engineering of the spacecraft and the operational procedures have to be done right, to make this process robust to uncertainties in the atmosphere and to minimize the opportunities for human errors over this year-long aerobraking. So those are our kind of our two big challenges.

Host: You’ve mentioned Magellan a couple of times. How have experiences and lessons learned from previous missions such as Magellan been instrumental in development of the VERITAS mission?

Brophy: Magellan, it was really a terrific mission. It demonstrated the use of synthetic aperture radar, which we’re going to take advantage of. It demonstrated that to obtain high-resolution images of the surface of Venus, and it also was the first mission to demonstrate aerobraking. So we’re definitely building on the heritage from Magellan. It showed that the surface of Venus is very interesting, but left us only with tantalizing clues as to what happened to Venus. So VERITAS, we benefit from now roughly 30 years of technology advances since Magellan and it allows us to improve on Magellan’s synthetic aperture radar resolution by an order of magnitude, and we improve on its topography by two orders of magnitude. And we do this with a mission that’s only a small fraction of the cost and that’s because of the technology advances.

Magellan was a flagship mission. That’s kind of the most expensive class of NASA Deep Space science missions, and VERITAS is a relatively low-class, low-cost — sorry, a high-class mission, relatively low-cost discovery-class mission. But VERITAS, the instrumentation’s so good that it’s basically like putting on a pair of high-resolution glasses compared to the images we got from Magellan, and it’s all because the technology has advanced over the past 30 years.

One of the neat things about VERITAS and Magellan is that VERITAS will compare data from Magellan by looking at the same terrain essentially 40 years later and look for any evidence of geological changes that have occurred since Magellan made those measurements. So that’s a really neat opportunity.

Host: What drew you to the VERITAS mission?

Brophy: Well, for me, it’s not really so much as what drew me to VERITAS. I actually got drafted to work on this mission, but it’s rather what has kept me interested in it. And it’s because Venus has largely been neglected for a long time, decades, in fact. But it’s a very interesting place because of its similarities and of course, obvious differences from Earth. And VERITAS will help lock the secrets that Venus has kept for so long about how rocky planets evolve and how they go bad. It’s clear for my time that I’ve spent at JPL and I’ve been at JPL for a long time, that the number of amazing Deep Space missions is much, much larger than NASA can provide funding for, and all of these Deep Space science missions are both scientifically interesting and technically challenging. So it’s a rare privilege to get to work on one of these missions. VERITAS is basically the last mission that I’ll get to be intimately involved with in my career, and so I’m very interested in working on this mission.

Host: Is there a particular aspect of the mission that especially energizes you?

Brophy: Yeah. I would really, really like to know if Venus ever had a liquid water ocean on its surface. People just think of Venus as hell, but it may not always have been that way. Was it the first habitable planet in the solar system? That’s possible. If it did have a liquid water ocean on it, what went wrong? And if it never had a liquid water ocean, why didn’t it? These are questions of really, I think, significant importance for understanding not only about Venus, but about the possibility of life elsewhere in the universe, and it’s also a significant contemporary interest with our own planet. So that’s what really energizes me about this mission.

Host: Well, John, it’s been an absolute pleasure getting to chat with you today. Thank you so much for being on the podcast with us.

Brophy: Oh, it’s my pleasure. I appreciate the opportunity.

Host: Do you have any closing thoughts?

Brophy: I would like to just emphasize the fact that VERITAS is an international effort. We have significant contributions from Germany, Italy and France, and these help make the mission affordable. So, this is truly an international effort to improve humanity’s knowledge of the cosmos.

Host: Links to topics covered during our conversation are available at APPEL.NASA.gov/podcast along with John’s bio and a show transcript.

If you’d like to listen to more interviews and get information about what else is happening at NASA, we encourage you to check out other NASA podcasts at NASA.gov/podcasts.

As always, thanks for listening to Small Steps, Giant Leaps.