Mars mission will offer clues to solar system
HKen Hicks
ave you ever tried to spin an egg in its shell? A hard-boiled egg spins quite easily. An uncooked egg will wobble and slow down quickly because of friction between layers of liquid as it spins.
The same principle can be applied to planets. If a planet has liquid (molten lava) below the surface, it will wobble differently than a solid planet. Determining the composition below a planet’s surface helps us to understand how a planet formed.
We have good information about the subsurface composition of Earth and its moon. But our moon is not a planet — and is much smaller than Earth — so it’s not really a good indicator of other planets. It would be better to get the subsurface makeup of Mars.
In fact, Mars could be an even better indicator than Earth of how our solar system formed because Earth’s surface and interior are always changing, at least on geological time scales. In contrast, Mars seems almost pristine.
For example, we know that Earth’s magnetic field has reversed direction many times over millions of years, based on the magnetism locked into iron crystals at different levels of sedimentary rocks. Earth’s magnetic field is known to be produced by movements of its fluid outer core.
Mars doesn’t have an overall planetary magnetic field, even though weak fields are found at various locations. From measurements of the Mars Global Surveyor, which orbits Mars and uses radar to map out the Martian surface, the tidal deformation of Mars means that at least part of its core must be liquid. That leaves many questions.
To know more about how our solar system formed, we need to learn about the interior of at least one other planet. And now we have NASA’s new InSight mission.
InSight landed on Mars this past week after a sixmonth journey. Its purpose is to take measurements that will allow scientists to study the interior of the Red Planet. It has three primary instruments: a seismograph to measure “Marsquakes,” a heat probe that will burrow 16 feet below the surface, and a radar reflector to provide InSight’s precise location.
Much of what we know about Earth’s interior is from measurements of earthquakes. As the waves from an earthquake pass through Earth, both on the surface and its interior, the wave speed depends on Earth’s composition. Similarly, measurements of Marsquakes will tell us whether it has a solid or liquid core (and how big it is).
The heat probe will tell us about variations in temperature below the surface, which can be compared with models of the Martian crust. In essence, the heat probe will tell whether there is a heat source at the center of Mars.
The source of heat at Earth’s core is radioactive decay of elements such as uranium, which are plentiful below the surface. We expect there is a similar heat source for Mars, but we don’t know how big it is.
The radar reflector will allow us to measure the position of a point on Mars (where InSight is sitting) relative to Earth to an accuracy of a few centimeters. Considering that Mars is, on average, about 140 million miles away, that’s remarkable accuracy.
The point is that we can now measure the wobble of Mars as it spins. Earth wobbles slightly, because of the moon, about every 18 years. Previous measurements show that Mars also wobbles about once a year. Just like the spinning egg, measurements of the wobble will tell us about the interior of Mars.
With InSight, we’ll learn new things about the origin of the planets.