Mars is a lot like Earth – but there are differences
Martian robots and spacecraft are sending gigabits of data to California (giga = billion) every day. Face to face, by e-mail and phone, the scientists and engineers who designed and flew the instruments collecting this data have been generous in sharing their time and expertise. But ask for a major conclusion (“What makes Mars tick?”) and they hold their cards close to the chest.
For good reason.
Life would be different if Earth’s gravity were weaker: Grasshoppers would hop higher; golf balls would fly farther; the blue of the oceans would be accented dry land colored the same ruddy red as Mars.
Measuring sediment carried by rivers, we calculate that erosion wears down mountains so fast that they – and the continents on which they rest – should have long ago washed to the sea. The entire surface of the Earth should be covered by ocean, a mile-and-a-half deep.
Mountains exist because, as fast as they are washed to the sea, they rise.
Push a balloon under water and release it. Water, denser than air, flows downward; flowing under the balloon, it pushes the balloon upward. The process is convection.
The rock mantling Earth’s core is rich in radioactivity, heating the mantle and making it plastic (solid, but easily deformable). Hot rock expands and floats upward, propelled by cooler, denser rock falling down around it.
The Earth is a “lava lamp.”
How fast the cold rock sinks and how fast the hot rock floats up depend largely on two factors – the differences in temperature and the strength of gravity. The bigger the temperature difference, and the stronger the gravitational field, the stronger the convection.
The moving mantle rubs against Earth’s outer crust, rearranging it. Convection drives plate tectonics.
Mars is half the diameter of Earth, and its gravity is less. To calculate your weight on Mars, multiply your Earth weight by two, then divide by five.
Rock is only so strong. If a mountain rises too high, rock at its base becomes plastic and flows out from under it. Planets are round because gravity pulls everything down, toward the center.
Under weak Martian gravity, mountains grow higher than they do on Earth … higher than on any other planet. The Martian volcano Olympus rises 15 miles above the Tharsis Plateau; the highest volcano on Earth, Mauna Loa, only five miles above the Pacific floor.
Weaker gravity drives weaker convection. The Martian lava lamp is different than Earth’s.
Ethyl alcohol boils at 78.5 degrees Celsius, water at 100. When alcohol and water are heated in a still, the alcohol boils off more quickly, creating high-proof brandy or whisky in the still’s “cold leg.” In the “hot leg,” the solution becomes depleted in alcohol, enriched with water.
Different minerals melt (or freeze) at different temperatures. As convection rearranges their innards, planets “distill”- “fractionate” – the minerals in their rocks. Whereas Earth’s lava lamp carried most of our iron down into the core, Mars’ weak gravity drove a less efficient convection, leaving significant amounts of iron near the surface.
Combined with oxygen, this surface iron stained the planet red.
Chemistry and physics – heating and cooling, melting and freezing – work the same on Mars as they do on Earth. Quartz is quartz; andesite in Ares Vallis (Mars) should resemble the rock on Andesite ridge (near Castle Peak, Earth). It’s tempting to look at Martian rocks and think we know what we’re seeing.
But the Martian lava lamp is different, and it would be foolhardy to jump to any conclusions before a lot more data are in.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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