Rovers probe extraterrestrial processes

Mars: ‘Weird Mix of Alien and Familiar ...’

As a planetary field geologist, John Grant gets excited when he completes a 150-meter-long traverse in just one day.

From his office at the Smithsonian Institution’s National Air and Space Museum – where Grant analyzes images beamed to Earth from NASA’s two Mars Exploration Rovers – he interprets micro- and macro-scale planetary geology on-the-fly. His job is to maximize the science conducted, and to help keep the Mars Exploration Rovers safe while they explore the surface of the Red Planet.

Grant’s mission is, indeed, to boldly go where no geologist has gone before.

At the recent AAPG Annual Convention and Exhibition in New Orleans, Grant, chair of the Center for Earth and Planetary Studies at the Smithsonian Institution’s National Air and Space Museum, told the EMD and DEG luncheon attendees that sedimentary geologists continue to play a key role in documenting evidence that water existed on Mars in the geological past.

“The geological processes are basically the same (on Earth and Mars), but the manifestations and details are different on Mars,” Grant said. “There’s some stunning landscapes on Mars, including a weird mix of alien and familiar land forms.”

Ever cognizant of the geomorphic term “equifinality” – that different processes can create similar looking features – Grant said:

“You have to keep an open mind in Mars exploration, and think about new ways to test your hypotheses.”

The presence of at least ephemeral water on Mars, three to four billion years ago, he said, has been confirmed in sedimentary rock outcrops of fluvial and lacustrine depositional origins. Grant illustrated this point with stunning photos – taken by the Rovers and also from space – of alluvial fans, mass wasting features (caught in motion), cross-bedded sandstones, dunes with tell-tale water ripples and the presence of chalky white evaporites.

“We’re seeing things from orbit, on the surface of Mars, that have the pixel space of one square foot, and that’s incredible spatial resolution,” Grant said. “It looks like there were places in the early history of Mars where there were hundreds of meters of standing water,” he added, citing sedimentary outcrops, complete with cross-bedding, which measure up to 150 meters thick.

The Holden Crater, one of Grant’s ongoing areas of investigation, contains clay-bearing minerals – by his calculations, the associated water in the Holden Crater may have been some 300 meters deep at one time.

Still Going Strong

In 2004, Spirit and Opportunity, the two Mars Exploration Rovers, were landed on opposite sides of the planet, commencing what was to be a 90-sol (“sol” is a Mars day, which is about 40 minutes longer than an Earth day) scientific mission. Nearly seven years later, in a science project that’s exceeded all expectations, the solar powered rovers are still transmitting data back to Earth.

“You design the rovers for the worst case scenario: maximum impact on landing, maximum life and maximum shaking,” Grant explained.

Good luck, combined with good design and longevity, have enabled the rovers to traverse unique features on Mars, unlocking the planet’s geological history in the process.

Grant affectionately calls Spirit and Opportunity the “identical twins” that have six-wheels each and are about the size of golf carts. However, the golf cart analogy ends there: the Athena Science Payload of sophisticated remote sensing instrumentation, high definition cameras and rock abrasion tools transform the rovers into travelling field laboratories.

Using HiRISE camera images to create digital elevation models of the surface traversed by the rovers, Grant identifies intriguing geological features that merit further scientific investigation. Working with engineers at the Jet Propulsion Lab (JPL), he helps chart navigational routes – free of hazardous boulders and cliffs – to interesting scientific targets that don’t compromise the rovers’ safety.

Early on in Opportunity’s mission in the Eagle Crater, near an outcrop dubbed “Stone Mountain,” the rover photographed layered sedimentary rocks containing “blueberries,” spherical concretions that contain hematite. In fact, Grant said, the ground was littered with spherical concretions, weathered from the nearby bedded rocks and somewhat reminiscent of concretions found in the Navajo Sandstone in Utah (May 2005 EXPLORER).

“We traversed to Stone Mountain,” he said, “based upon geochemical evidence from the rover’s investigations that revealed the existence of hematite, a mineral often associated with water.”

Step By Step

Grant described how the rovers have been buffeted by dust devils racing across the Martian surface, often operating on reduced power because their solar panels are covered with dust. Luckily, he said, the passage of subsequent dust devils helps remove the layer of dust occluding the solar panels.

According to Grant, there are two ways to drive the rovers at JPL:

  • Advance “blindly,” on a non-threatening track, at a speed of two meters per minute.
  • When faced with potentially hazardous obstacles, proceed more slowly and cautiously, at about 10 meters per hour.

One of Spirit’s wheels has failed, leaving five functional wheels with one that drags along the ground. However, out of adversity came a serendipitous scientific discovery: As the wayward wheel dragged, it plowed through the ubiquitous oxidized surface, excavating white, silica-rich sediments below.

"We may well have missed that otherwise," Grant mused.

Spirit currently is stuck in soft, fine sediments along the margin of a small, filled-in crater, in an area called the "Rock Garden."

"Statements about the future viability of Spirit may be exaggerated," Grant said, sounding like a proud father defending his offspring. Tilted southward – the sun is currently in the north – Spirit’s solar panels are less effective during the southern hemispheric winter.

"Spirit has basically gone into hibernation. Come September, when ‘spring time’ happens and the sun returns in the southern sky, we hope to hear from Spirit – and resume exploration of the Gusev Crater – when it calls back to the JPL."

In 2011, with the launch of the Mars Science Lab, the focus of exploration will shift to investigate whether the ancient aqueous environments were habitable. According to Grant, the Mars Science Lab will be geared to search for – and study – the existence of organic carbon, the building block of potential life forms on the Red Planet.

Already well in the planning stages, the next generation of Mars rovers will launch in 2018. The current plan calls for dual rovers that will carry complementary tools – including ground-penetrating radar and a drill on a European rover capable of penetrating one meter into the subsurface – increasing the depth of subsurface investigation.

According to Grant, the future exploration of Mars could involve bringing back samples to Earth. The next generation rovers will be searching for the necessities of human space travel and exploration – potential fuel sources on Mars, perhaps gas hydrates or near-surface water that could be converted to hydrogen energy.

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