DAKAR, Senegal — It’s the largest piece of Mars ever found on Earth — a 54-pound (25-kilogram) meteorite that fetched more than $5 million at a New York auction last month, setting a world record. But in the West African nation of Niger , where the rusty-red rock was unearthed in the Sahara Desert, officials have launched an investigation into what they call possible “illicit i

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