When trying to understand ourselves, sometimes it helps to look to our neighbors. As NASA sets course for Mars once again, we examine how important the Red Planet has become in understanding planets, including our own.
—A streak of rocket fire pierced the foggy predawn skies of southern California Saturday, as NASA sent off its latest Mars mission.
The InSight mission is set to rack up a series of “firsts.” It’s already NASA’s first interplanetary launch from the West Coast. It will also be the first time CubeSats will deploy in deep space. And, if the mission is successful, it will be the first time that scientists gather direct data on the interior of another planet and detect quakes on another planet.
Despite all these firsts, the mission marks the 45th time humans have sent robotic envoys to uncover Mars’s secrets (although only about half of those missions are considered a success).
“I think it’s fair to say that our level of understanding of Mars is now only second to Earth in the solar system by the sheer number of missions that we’ve sent to Mars,” says Ashwin Vasavada, project scientist for NASA’s Mars Science Laboratory at the Jet Propulsion Laboratory in Pasadena, Calif.
Through all those missions, scientists have learned a lot about Mars. But Mars missions are about much more than just Mars. The information gleaned during these missions has opened the door to planetary science at a cosmic level. And in that process, the Red Planet has also given us a better understanding of our blue world.
In grade school science classes, students learn that science requires plentiful data to analyze patterns and come up with testable models to explain a phenomenon. If scientists have just one example of, say, a planet, they cannot confidently extrapolate that therefore every planet must look just like that one.
And there is just one planet that we know intimately: our own. So planetary scientists have launched mission after mission to gather data on the other worlds in our solar system and spot new planets beyond.
“Every time we go to another body and acquire a new type of data, we get a chance to test out all of the theories that we’ve developed for Earth and see if they really stand up when we bring in another case,” says Suzanne Smrekar, deputy principal investigator for the InSight mission.
These missions have revealed tantalizing glimpses of worlds surprisingly unlike our own. But with flybys and orbiters scoping out most of the other planets in our solar system, scientists are just beginning to scratch the surface in understanding these worlds.
But with Mars, the data has gone deeper. Decades of orbiters and rovers have yielded data that are bringing the Red Planet into sharper focus – and building a second model of a planet for scientists to test their theories against.
“It helps us put things into context,” says Tanya Harrison, the director of research for the Space Technology and Science (“NewSpace”) Initiative at Arizona State University and a science team collaborator on the Mars Exploration Rover (MER) Opportunity. “If you were a geologist, you wouldn’t just go pick up a rock in Scotland and then try to extrapolate the entire history of the Earth from that one rock. It’s kind of the same thing if you just look at one planet and try to take that and apply it to the entire solar system.”
And that’s exactly what scientist want to do: reconstruct the history of our nearest neighbors.
“There are some deep existential questions that looking at the comparative histories of the planets can help answer,” says Selby Cull-Hearth, a planetary scientist at Bryn Mawr College in Pennsylvania. For example, she says, “One of the great driving questions of our existence is, why are we here? How did this happen?”
Earth and three of its neighbors – Mars, Venus, and Mercury – are all thought to have originated from the same rocky material in the early days of the solar system. But today, they all look dramatically different. Mercury is a super dense, hot and fiery yet icy enigma. Venus is a super hot, molten, sulphur-clouded orb. Earth is a dynamic water world. And Mars is a frozen red desert. “It’s baffling why we should get such different outcomes,” Dr. Cull-Hearth says.
Two paths diverged in a solar system
Mars is a great place to start. It’s relatively easy to get to (a trip to Mars takes less than a year), it bears some striking similarities to Earth, but has some key differences too.
“Scientists like to have controlled experiments, and then you turn a knob and you see what happens,” Dr. Vasavada explains. “When you look at it that way, Mars has turned knobs that are not able to be turned on Earth.”
For example, scientists have uncovered extensive evidence that rain once fell on Mars, filling up lakes and rivers – much like the lush Earth life thrives on today. But some 2 to 3 billion years ago, that Martian hydrological cycle ground to a near halt. Understanding how that happened is one of planetary scientists’ big questions.
There are some ideas. Today, Mars has an extremely thin atmosphere. On Earth, the atmosphere serves as a blanket of sorts to keep the surface of the planet warm enough for liquid water. So what happened to Mars’s atmosphere? Some scientists have suggested that it was stripped away by the solar wind – a barrage of charged particles flowing from the sun.
On Earth, the atmosphere is protected from that process by a global magnetic field. So the idea is that Mars must have lost its magnetic field at some point. The magnetic field on Earth is thought to be created by the convection of fluid in the core. So if scientists discover that Mars has a solid, inactive core, that data would support this model.
One hitch in that idea is that Mars still does seem to have a bit of a magnetosphere. NASA’s MAVEN orbiter mission spotted some aurora activity, which is thought to require a magnetic field of some sort.
The InSight mission (aka Mars Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) may help resolve some of this puzzle.
When the lander touches down on the Red Planet in November, it will deploy a small suite of instruments programmed to study the planet’s mysterious magnetosphere, the internal heat flow, and the structure of the interior.
Mars’s surface has been thoroughly mapped by orbiters, but scientists have only been able to model its interior based on external gravity data until now. So they know Mars has a core, but they don’t know what it’s made of, how big it is, or if it’s geologically active.
InSight mission scientists plan to use “marsquakes” to figure out what’s inside the Red Planet. Although Mars is not thought to have tectonic plates like those on Earth that shift and grind against each other, triggering earthquakes, a planet can shake and quake for other reasons. Mars is cooling and shrinking, so the crust likely cracks from time to time under that pressure, triggering a marsquake. Impactors can also jostle the planet.
The InSight seismometer is so sensitive it can pick up quivers the size of a single atom all over the planet. Because seismic waves move through different materials at different speeds and angles, scientists can use marsquake data to figure out the structure and composition of the interior of the planet.
With all the data InSight gathers, scientists hope to piece together the structure of the interior of Mars, which could have implications beyond Mars, too.
Without plate tectonics turning over its crust, Mars’s interior has probably remained largely the same since its formation, Dr. Smrekar says. As a result, the inside of the Red Planet offers a snapshot of the materials that are the building blocks for the terrestrial planets. It could offer many clues into the early evolution of these planets.
“It’s going to give us our first real look at the interior structure of another planet beyond Earth,” Dr. Harrison says. “And if it’s what we expect, if it’s what we think we should see based on what we understand from Earth, then that’s great. It helps us validate our models. But maybe we’ll get there and find something completely unexpected. Then it becomes one of those instances when we have to rethink what is happening.”