Using a technique developed by researchers at Brown, scientists have discovered a strange warm spot deep in the interior of the Red Planet.
PROVIDENCE, R.I. [Brown University] - The satellites that orbit the Moon and Mars have provided scientists with a wealth of information about the surfaces of those other worlds. They have found traces of water on the Moon and spied potentially habitable surface environments on Mars.
For all that success, using those tools to understand what's happening beneath the surface, however, has been a taller order. But a technique called tidal tomography, developed by Brown University researchers, is enabling scientists to use existing satellites to glean key information from deep within planetary interiors.
In a study published in the journal Nature, a team led by Alexander Byrne from the University of Arizona used the technique to show that Mars has a massive heat anomaly in the mantle beneath its southern hemisphere. The study suggests that the hot spot is 200 to 400 degrees Celsius warmer than the rest of the planet, a finding that sheds new light on how the Red Planet has evolved over time.
Two Brown researchers who co-authored the study have played key roles in the development and application of tidal tomography. Harriet Lau, an associate professor in the Department of Earth, Environmental and Planetary Sciences at Brown, previously used tidal tomography to reveal new insights about Earth's interior. Working with Nick Wagner, a postdoctoral researcher at Brown, they've adapted it for Mars, and they hope it will soon reveal new details about the Moon as well.
Wagner discussed the work in an interview.
Q: Could you explain how this technique for investigating planetary interiors works?
As Mars travels around the Sun, the Sun's gravity gently stretches and squeezes the planet. Because Mars' orbit is not a perfect circle, the strength of this pull changes over the course of a Martian year, so the amount that Mars deforms changes as well. This deformation is called a tidal bulge. This tidal bulge causes mass inside the planet to shift slightly, producing tiny changes in its gravitational pull. An orbiter passing over a region with slightly stronger gravity speeds up a little, while one passing over slightly weaker gravity slows down. So, we can use the measured speed changes of orbiters to estimate the amount Mars is squishing.
For this study, we used about 16 years' worth of satellite data from three separate spacecraft to accomplish this. Once we know how the planet is responding to these gravity changes during its orbit, we can do a bit of math and figure out what interior state causes it. A hotter or less rigid interior deforms more easily, while a colder or more rigid interior resists deformation. In this case, we found that the southern hemisphere of Mars would need to be about 20% less rigid than the northern part, which we can infer is due to a difference in temperature.
Q: Why is it important to learn about the interior of Mars, or any planet?
Earth is one of the most unique planets in our solar system - I promise that's an unbiased statement! - and learning what made it different helps us to understand a lot of things, including how life happened. Therefore, learning what the structure is of other planets and how they formed can help us understand Earth. We call this way of thinking "comparative planetology." In addition to focusing on how unique Earth is, we can flip this around. Deciphering the interior of Mars can help us understand why the Red Planet looks so much different than Earth. Mars used to have water on its surface, so what conditions allowed that water to flow freely on its surface, but not today? One large consideration is the total "heat engine" of Mars, which this difference in internal heat today may shed light on.
Q: What does this new finding tell us about how Mars has evolved over time?
There's a bit of a chicken-or-egg problem now as to how primordial this temperature difference came to be. Did Mars somehow form with this asymmetry in place, and that then governed the geodynamic evolution of Mars - or did something happen to Mars early in its history that then caused this temperature difference? There's one hypothesis that the northern hemisphere of Mars was hit by a large impactor early in its history. Could that impact have left its mark until the present? On the other hand, it might also be likely that the southern hemisphere has thicker crust than in the north, and that has insulated the mantle there comparatively and kept it warmer. Both theories are possible. I think disentangling these theories is the next big step after this study.
Q: Can you relate this finding to Earth? Does Earth's interior have these kinds of warm spots?
Definitely. Using a very similar methodology, except the data came from GPS stations instead of from satellites, my postdoctoral supervisor at Brown, Harriet Lau, helped constrain the buoyancy of two large "blobs" underneath the Pacific Ocean and Africa called the Large Low Shear Velocity Provinces. What's interesting is that there is also a compositional component, meaning it's not just a temperature difference. I think this Martian anomaly may also have a compositional component, but a follow-up study will need to be done.
Q: What happens next with this line of research?
I think for Mars we need to disentangle the two theories I mentioned earlier of the origin of this difference and what it means for the formation and evolution of Mars. Also, this methodology is relatively new and is proving to be quite powerful in measuring these internal variations in planets, so we're working on applying this to the Moon.
