Mission Planning Briefing: Checking the Satellite on the Ground
1A space agency is getting a new Earth-observing satellite ready for launch. From orbit, it will measure heat coming out of the ground and the colors of minerals in bare rock. To check its readings, the mission team needs test sites on the ground where scientists already know what lies below. Mission scientist Dr. Ayumi Ferraro briefed the team, along with Noor Petrakis and Tadeo Marchetti, seventh graders in the agency's summer intern program. She described two resources the satellite should help find: heat for geothermal power plants and diamonds.
2Earth's interior is hot. In most places, rock gets about 25 °C warmer for each kilometer you go down, and the ground surface averages about 15 °C. Where magma rises close to the surface, the heat is much closer, and groundwater heated by it can be over 200 °C just a few kilometers down. Most magma rises where plates pull apart, where one plate sinks beneath another, and over hot spots. A geothermal power plant that spins turbines with steam usually needs water of at least 150 °C at a depth of 2 km.
3Diamonds are made of carbon, and they form only under enormous pressure, about 150 km or more below the surface. The oldest parts of continents, which formed more than 2.5 billion years ago, sit on cold, thick roots of solid rock that reach down 200 km or more. Diamonds can form and stay unchanged in these roots for billions of years. Under younger parts of the plates, the cold, solid rock does not reach that deep, and few diamonds are kept. Rarely, a very fast, explosive eruption from deep in the mantle has punched up through these roots and left a carrot-shaped pipe of rock called kimberlite. Such eruptions are extremely rare, and most known pipes are tens of millions of years old or older.
4Table 1 lists six possible heat test sites. Table 2 lists five kimberlite pipes that have been tested for diamonds. Gems are weighed in carats, and one carat is 0.2 g.