Planetary Surfaces: Why study them? Can they help us find life elsewhere?

Universe Today recently explored the importance of studying impact craters and what they can teach us about finding life beyond Earth. Impact craters are considered one of the many surface processes—others include volcanism, weathering, erosion, and plate tectonics—that shape surfaces on numerous planetary bodies, with all of them simultaneously occurring on Earth. Here, we will explore how and why planetary scientists study planetary surfaces, the challenges faced when studying other planetary surfaces, what planetary surfaces can teach us about finding life, and how upcoming students can pursue studying planetary surfaces, as well. So, why is it so important to study planetary surfaces throughout the solar system?

“Planetary surfaces record the history of the Solar System, a history that’s almost entirely lost to us here on Earth,” Dr. Paul Byrne, who is an Associate Professor of Earth, Environmental, and Planetary Sciences at Washington University in St. Louis, tells Universe Today. “Our planet is active and has processes that erode, bury, or destroy its ancient surfaces, so we have a limited understanding of the early days of our own planet. But that ancient record is (largely) preserved on the Moon, Mars, Mercury, and even smaller objects such as asteroids, so by studying them we’re getting a better understanding of our own planet. And it works both ways: by applying what we know of Earth, we’re able to get a better handle on why the surfaces of other worlds look the way they do.”

While the Earth is approximately 4.6 billion years old, the reason why it lacks ancient surface features is due to the surface processes mentioned above, as all of them are very active on the Earth and causing the surface to drastically change over the planet’s lifetime. However, it is plate tectonics that is arguably the biggest contributor for altering the Earth’s surface. This involves the recycling of the Earth’s surface and subsurface materials due to our planet’s seven major and eight minor tectonic plates interacting with each other over vast periods of geologic time as they spread, smash, and even slide past each other through the three types of plate boundaries known as divergent, convergent, and transform plate, respectively. While studying all these processes on the Earth are conducted through direct examination, laboratory analyses, and satellite imagery, what are some of the challenges that scientists encounter when studying planetary surfaces on other worlds?

Dr. Byrne tells Universe Today, “Studying the surfaces of other worlds is challenging for several reasons, the first (and biggest) being that we have to get there! We’re limited in what we can learn with telescopes from Earth (either on the surface or in space), because those telescopes are generally designed to study truly enormous and vastly distant features like nebulae. So, to properly ‘see’ the surfaces of bodies in the Solar System, we need to send spacecraft there—either to fly by or, preferably, orbit. In many cases, once we’re there we can image the surface and take other measurements relatively easily.”

Dr. Byrne continues by telling Universe Today, “But for worlds such as Venus and Titan, which have thick atmospheres, we need radar to see through to the surface. Then we have to make sense of what we’re actually seeing! That’s where we use ‘comparative planetology’, applying what we know of Earth (and other places we’ve visited) to piece together the story of what we’re seeing. It’s challenging, especially for a place we’ve never visited before, but also extremely exciting!”

Also called remote sensing, satellites and orbiters perform a variety of tasks during a flyby or once in orbit around a planetary body that range from direct images to scientific measurements, including spectroscopy, temperature, and surface composition, just to name a few. As its name implies, a flyby is when a spacecraft is designed to fly past a planetary body—or bodies—and conduct as much science as possible before the spacecraft passes it. Two of the most famous flyby missions in the history of space exploration are the Voyager 1 and Voyager 2 spacecraft. These brave, robotic pioneers conducted flybys of the outer planets that greatly expanded our knowledge and understanding of not only the planets, but their many moons, as well.

They discovered volcanic activity on Jupiter’s moon, Io, and a lack of craters on Europa, indicating the potential existence of a liquid water ocean beneath its icy surface. They obtained the first images of Saturn’s ravioli-like moon, Pan, to complement a few other Saturnian moons they also discovered. Additionally, they imaged several other previously discovered moons, including Titan, Rhea, Dione, Tethys, Enceladus, and Mimas. While Voyager 1’s trajectory took it out of the solar system, Voyager 2 continued to Uranus and Neptune, imaging their moons of Miranda and Triton, respectively, with the latter having geysers that shoot several kilometers into space. Most recently, NASA’s New Horizons spacecraft flew past Pluto, imaging its surface and revealing a landscape of mountains and valleys that scientists previously hypothesized didn’t exist.

For orbiters, NASA and a number of space agencies around the world have sent a plethora of spacecraft to orbit and study the Moon, Mercury, Venus, Mars, Jupiter, and Saturn, with Voyager 2 being the only spacecraft to have visited Uranus and Neptune. An example of one of countless past orbiter missions includes NASA’s Cassini spacecraft that explored the Saturnian system between 2004 and 2017. This historic mission provided scientists valuable new data about the ringed planet and its many moons, including confirming the existence of lakes of liquid methane on Titan and geysers shooting from the south pole of Enceladus, which Cassini flew through and sampled for organics. Additionally, the European Space Agency’s Huygens probe detached from Cassini and landed on Titan, becoming the first spacecraft to land on an outer solar system planetary body, where it imaged rounded pebbles on the surface that might have been shaped from fluid activity.

An example of an active orbiter mission is NASA’s Mars Reconnaissance Orbiter, which uses its High Resolution Imaging Science Experiment (HiRISE) camera to obtain some of the most stunning images of any planetary body ever explored. It not only provides valuable scientific data about the Red Planet but has also imaged active avalanches on the Martian surface on numerous occasions. But with all these missions and their scientific images and data, what can planetary surfaces teach us about finding life outside of Earth?

“One of the biggest questions we have is why and when life emerged on Earth, and as one part in answering those questions we need to understand when and why Earth became habitable,” Dr. Byrne tells Universe Today. “So, studying other planetary surfaces can tell us about the conditions during the early part of Solar System history, and also the processes that are common to planetary bodies generally or are seemingly unique to Earth. The full answer will come from a combination of chemistry, biology, fieldwork on Earth, and planetary science observations, but certainly a key part of the puzzle involves the study of other worlds.”

Dr. Byrne proudly tells Universe Today that his favorite planetary surface he has studied during his career is Venus, which is known for its extreme surface temperatures of approximately 464 degrees Celsius (867 degrees Fahrenheit) and crushing surface pressures that are more than 90 times greater than on Earth. Radar images obtained from NASA’s Magellan spacecraft in the 1990s revealed striking surface features that were indicative of volcanic activity, either in the past or present. This includes massive shield volcanoes, some of which are more than 100 times larger than any lava domes on the Earth and are likely due to the extreme surface temperatures and pressures. Despite this, scientists currently hypothesize that microbial life could still exist within its clouds, which possess temperatures and pressures that are more Earth-like. Also, despite its extremely harsh conditions today, scientists also hypothesize that Venus’ surface was once more Earth-like deep in its ancient past.

“It’s an incredible world, one almost the same size as Earth but with vastly different surface conditions—for reasons we still don’t understand,” Dr. Byrne tells Universe Today. “Venus has a horde of features we can recognize on Earth, including volcanoes, lava flows, tectonic structures, impact craters, and even a couple of dune fields. But it also has landforms we don’t see obvious counterparts to on Earth, or any other world for that matter. And perhaps most intriguing are Venus’ “tessera terrains”, a type of landscape that really doesn’t look like anything else in the Solar System except, perhaps, Earth’s continents. Figuring out why Venus is so fantastically different to Earth is a really important task for planetary scientists not just for understanding Venus, but for ultimately establishing how two large rocky worlds can end up so different, with one being home to us and the other being utterly inhospitable.”

Given the plethora of worlds that have been explored throughout the Space Age, studying planetary surfaces involves a multitude of scientific backgrounds and disciplines to help piece together the very intricate puzzle of how our solar system and its many planets, moons, and asteroids came to be what they are today. Along with studying satellite images, additional research involves laboratory experiments, real-world and computer simulations, data analysis, fieldwork, countless measurements and calculations, and much more. Therefore, what advice does Dr. Byrne offer to upcoming students who wish to pursue studying planetary surfaces?

“Certainly the standard training in mathematics, physics, and chemistry will help, but a solid grounding in geology is just as (if not more) important,” Dr. Byrne tells Universe Today. “But even that training can take a myriad of forms, say as part of an environmental science degree or from an Earth science, geology, or even geophysics degree. Experience with remote-sensing software and processing techniques is a big help. But arguably the most important thing is a familiarity with the different ways landscapes can look across Earth and the Solar System, and that familiarity can start in childhood by just being interested in what your locality looks like!”

What new discoveries will scientists make about planetary surfaces throughout the solar system, and possibly elsewhere, in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!