This article first appeared in the St. Louis Beacon, July 31, 2012 - Shortly after midnight on Aug. 6, the Mars Science Laboratory rover named Curiosity is scheduled to complete its eight-month journey with an intricately choreographed daytime descent into Gale crater on Mars.
Raymond Arvidson, James S. McDonnell Distinguished Washington University professor, and graduate student Abigail Fraeman will be among the 270 or so planetary scientists gathered at the Jet Propulsion Laboratory in Pasadena to hold their collective breath during the complicated landing maneuver and then cooperate in giving Curiosity the best possible scientific start to its two-year mission. Arvidson has participated in every Mars landing since Viking 1 in 1976, except the 1997 Mars Pathfinder mission, and is a “participating scientist” for this mission.
Curiosity is the biggest Mars rover yet, weighing in at about 1 metric ton. About the size of a small SUV, it will be eight feet high when its camera arm is fully extended and will move on six huge wheels 50 cm in diameter. It will be powered by a radioactive thermal generator that converts the heat given off by decay of plutonium 238 into electricity.
Because of Curiosity’s size and its array of precision scientific instruments, NASA has described the landing sequence as “Seven Minutes of Terror.” In those seven minutes from when Curiosity enters the thin Mars atmosphere at 13,200 miles an hour, its computer must direct a tight sequence: deploy a parachute, discard the heat shield, disconnect the back section and parachute while firing off retrorockets, lower the rover to the crater floor on cords from its "sky crane” and finally cut the cords and send the sky crane off to crash some distance away.
Mars missions follow the water
From the first Viking Lander through the still-working Opportunity rover, Mars exploration has concentrated on finding water and characterizing its cycles -- all with the aim of learning whether life has ever existed on Mars. The types of minerals found and their patterns of deposition have all confirmed the presence of water. The Phoenix lander (2007-2008) near the planet’s north pole dug down to glacial ice just about an inch below the surface.
The twin Spirit and Opportunity rovers that landed in 2003 have added so much detailed data that Arvidson can say today “Mars has a modern hydrologic cycle probably associated with snow that occasionally falls near the equator, in addition to evidence for warm, wet conditions early in geologic time."
And thanks to Spirit and Opportunity, scientists are sure that Mars has gone through three general climate periods as characterized by its geology:
- Early Mars had a liquid core, a magnetic field, greenhouse gases and active volcanic eruptions. Its clay rocks are characterized by minerals formed at neutral pH.
- Middle Mars geology is characterized by shallow lakes and swampy areas filled with sulfate salts like gypsum and epsom salts. These minerals are formed in acidic conditions. Arvidson describes the environment in this intermediate "playa" as “swamps of acidic sulfate goo.”
- Today’s Mars is dry, cold and dusty. The liquid core has gone, as has the magnetic field, and the very thin atmosphere is mostly carbon dioxide.
Curiosity’s scientific payload will include the tools to analyze for carbon-containing molecules typical of life, such as amino acids. In addition, its use of the thermal radioactive generator instead of solar panels will enable it to work day and night -- traveling by day and doing lab work on collected samples at night. Its mission is planned to last a full Mars year, or about two earth years.
Arvidson says, “The beauty of Mars is that the ancient rocks are so well preserved because the planet has petered out. Mars is frozen in the time of the solar system’s youth.” The materials preserved from the ancient times offer a glimpse into the era when life on Earth was beginning. Because Earth’s geologic environment is so active with volcanoes and tectonic plate movement, the records from that era here have been mostly erased.
So why is Earth still geologically active after its first 4.6 billion years while Mars seems to have ground to a halt? Early on, Mars had a liquid core, active volcanos and a magnetic field that could maintain an atmosphere. But Mars is farther from the sun than Earth, and more important, is half its size. Its size allowed the heat to dissipate. As the planet cooled, the core solidified, and the magnetic field disappeared. Without the magnetic field, greenhouse gases disappeared, and cooling accelerated.
Curiosity climbs a mountain
The landing site in the 96-mile wide Gale crater will be near Mount Sharp, a 3.4 mile high sedimentary mountain. As Curiosity ascends the lower slopes of Mount Sharp, it will acquire images, collect samples and chemically analyze these ancient clay and gypsum rocks. The geology is varied within the crater, so Curiosity’s science lab will sample a spectrum of rocks.
The chosen landing site was selected to bring the rover as close as possible to Mount Sharp. Original plans had put the landing farther away because of fear that Curiosity could get stuck in the sand dunes close to the mountain.
The Surface Materials and Mobility Working Group that Arvidson co-chairs spent time in the Mojave desert with a Rover simulator one-third the weight of the real one and that should therefore duplicate Martian gravity. They found that Curiosity’s big wheels enabled it to navigate those dunes at slopes up to 12 degrees -- and so it should travel without getting stuck.
In the meantime, the Opportunity rover continues to roll along, exploring a new site and collecting more data. Last August, Opportunity reached the rim of the 14-mile wide Endeavor crater, where the instruments on the Mars Reconnaissance orbiter had detected minerals whose chemistry had been altered by water. They appeared to be clay type rocks from the earliest Martian time period, similar to those at the bottom of the crater that Curiosity will explore. Opportunity is gathering data about these rocks whose age measures years in the billions.
The Washington University team is heavily involved with Opportunity, for which Arvidson is deputy principal investigator.
Ed Guinness, senior research scientist, described the daily routine of operating Opportunity. The science team of up to 50 from all over the country meets every day or every other day by teleconference. It decides what it wants the rover to do in the next day or two. (Since the Mars day or "sol" is 40 minutes longer than Earth’s day, the time between planning sessions depends upon whether the two planets’ daily cycles are relatively in-sync or out-of-sync.)
Women with WU connections active in Curiosity mission
Women are generally in the minority in the physical sciences. But in Mars rover missions, five women who are or have been students of Ray Arvidson are active participants in the Curiosity Mars Science Laboratory. Among them:
- Graduate student Abigail Fraeman was sent to the Jet Propulsion Laboratory (JPL) to witness the landing of the Spirit and Opportunity rovers as a high school student, and she met Arvidson there. She met Bethany Ehlmann through the Arvidson connection.
- Ehlmann, an undergraduate at WU,is now an assistant professor at Caltech. She credits Arvidson with “exposing students to the most exciting parts of the process of doing science."
- Kirsten Siebach, WU graduate and now a Ph.D. student at Caltech, does outreach with school children. “I love the way that space exploration inspires people to think about the world around them from a different perspective and wonder about the way the Earth works.”
Mars story continues
Not so long ago, fiction writers wrote of advanced “life” on Mars because telescopes showed what seemed to be a system of canals. Then, our space program put an end to those fantasies. Mars was cold and dry with a thin atmosphere of carbon dioxide.
But since the 1970s science has known that “the red planet” had a watery past and now know it still has water today. Some believe that microbes that can live in extreme conditions may actually exist on Mars today. Much more likely is that such microbes lived when the planet was warm and wet.
With luck, the Curiosity mission will be able to find if Mars was ever “habitable.” But even if evidence of life past or present remains elusive, Curiosity’s findings will add vastly to our understanding of the solar system and its history.