Review the text.
Rovers on the Red Planet
A robotic rover sits on the surface of Mars.
Although no human has landed on Mars yet, scientists have been gathering information from the red planet's surface since the 1990s. Researchers at the National Aeronautics and Space Administration (NASA) are able to study the planet's history by examining rocks and images obtained from Mars. Wheeled robotic explorers, also called Mars rovers, gather these samples. Similar to remote control cars, the rovers are controlled by operators back on Earth.
The first Mars rover, Sojourner, landed in 1997 and spent 83 days taking photographs and exploring the Martian surface. Proving that a robot could relay meaningful data to the waiting scientists, Sojourner sent more than 550 photos back to NASA. Roughly the size of a microwave, the rover weighed about 25 pounds and carried specialized instruments to examine the planet's soil and rocks. It relied on batteries and solar power to operate.
Next came the twin rovers, Spirit and Opportunity, which touched down on Mars in 2004. These rovers were sent to search for any indications of water on the planet. Since all known life-forms require water, these rovers helped scientists investigate whether Mars might have ever been capable of sustaining life. Operating independently, the rovers succeeded in transmitting photos from different areas of the planet, discovering evidence that water likely did once exist on Mars. The rovers also provided evidence that Mars was once warmer than it is today. It was thought that these rovers would be functional for 90 days, but both greatly exceeded those expectations. While Spirit roved around Mars for more than six years, Opportunity continued to function for almost fifteen years and traveled twenty-eight miles over the surface of Mars.
Another rover, Curiosity, landed on Mars in 2012. This rover's mission was to seek evidence not only of water but also of carbon in Mars's soil. Carbon is important because it is the basis for organic compounds, which are necessary ingredients for living things. Curiosity was designed to collect soil samples from the planet's surface and heat them in an onboard oven. The heated soil emits gases that scientists back on Earth can analyze to reveal the composition of the soil. In 2018, scientists studying some of Curiosity's soil samples detected carbon dioxide and methane, two organic compounds that indicate there was once organic carbon on Mars. Although the presence of these gases suggests that life could have once existed on the planet, there are other ways carbon dioxide and methane might have formed on Mars. These possibilities raised by Curiosity's discovery motivated scientists to keep investigating.
The fifth and most advanced rover yet, Perseverance, arrived on Mars in 2021. It carries an advanced array of instruments to search for evidence of ancient life on Mars. The rover is the size of a car, weighing 2,260 pounds and standing seven feet tall. Perseverance also brought a helper along for the ride—a helicopter named Ingenuity. NASA sent this solar-powered helicopter on a mission to test the ability of aircraft to fly in the thin Martian atmosphere. Once on Mars, Ingenuity made a successful flight, so NASA started sending it on exploratory missions, collecting even more information about the planet.
Sometime in the 2030s, NASA hopes to conduct a complex operation to fly rock samples collected by Perseverance back to Earth. Meanwhile, to prepare for the next steps in Mars exploration, scientists continue to study the evidence of water and organic materials on the planet. They are also studying how oxygen might be produced there. Someday in the future, NASA hopes that human explorers will follow the rovers and touch down on Mars.
What text structure does the author use throughout the passage to explain the different Mars rovers?