Research Stories

by Skip Derra

The Red Planet has long sparked the interest of those of us on the Blue Planet. Considered a planetary cousin to Earth, Mars garners continuing attention by planetary geologists and NASA scientists of many flavors. Everyone is intently searching for signs of Martian life.

For years, orbiters have been photographing Mars from every possible angle. Robot landers have been roving and digging into its surface with the purpose of finding water on the Red Planet. For the vast majority of planetary geologists, those images and data suggest that Mars was once home to vast lakes, rivers, and streams. They suggest that an Earth-like climate existed at some point in the past. It may have even harbored some basic ancient life forms.

The evidence has been so convincing to most scientists that they now are meeting to determine exactly where to land the next rover and continue the hunt for ancient life.

But for other scientists, that same convincing data means something totally different. The data in question comes from the highly successful Mars Exploration Rovers (MER) named Opportunity and Spirit. The machines have gathered a mighty mass of information. Still, the numbers and images do not scream “Earth-like climate” to L. Paul Knauth and Don Burt, both planetary geologists at Arizona State University, or to Ken Wohletz, an explosion researcher at Los Alamos National Laboratory in New Mexico. These three have read the same data and have looked at the same images. But for them, the data have consistently provided an alternative view of Mars surface features.

To these three, Mars is not a place where vast bodies of water once existed as much as its surface is a product of vast bombardment by meteors. Knauth, Burt, and Wohletz wrote a 2005 paper published in the journal Nature that gave an alternative view of the evidence. They read the surface features on Mars as being the result of massive cratering from relentless rounds of meteorite blasts that took place eons ago. And, they say, new data only bolsters their case.

The image shows a fairly recent surge deposit on Mars. The deposit flowed in from the left in the direction of the arrows. The lower half of the image shows terrain blanketed by the surge. The source crater that produced the surge is out of the frame about 25 kilometers to the left. The arrows are drawn to be 100 meters on the scale of the photo. (Image courtesy of NASA/JPL/University of Arizona)

“There is a near hysteria about water on Mars right now,” Knauth says. “They keep discovering water on Mars. It’s the big buzzword. Because if you have water, then you have the first big condition you need for life. That is what is driving the current interest. The big question is whether we had Earth-like rivers, lakes, and precipitation and all of that.”

Knauth says that the most incredible thing is that scientists know nothing about the first 400 to 500 million years of Earth history or Mars history. “That history was completely obliterated at 3.8 billion years by the late heavy bombardment,” he explains. “Mars could’ve been an Eden. But we have no record of any of that. On Earth, we have a few scraps of crystals that we can age date, which were blasted around and wound up in sediments that were younger.

“Mars is cratered from one end to the other. Because it has an atmosphere, all of these should have made base surges,” Knauth continues. Impact surges “present a simple alternative explanation involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulfides, brines and ice produced by a meteorite impact,” the three wrote in their Nature article “Impact Origin of Sediments at the Opportunity Landing Site on Mars.”

The three researchers don’t dispute that there is water on Mars. Not at all. What they do dispute is what the Red Planet’s geologic features are saying about water and where it would be logical to look for it.

“There’s water on Mars,” Burt adds. “But it’s in the form of ice. There are salts as well. But if you freeze water you are going to gets salts out, because they won’t dissolve in ice. There was flowing water, but it was ephemeral. The water occurred right after the impacts. Those impacts by meteorites and pieces of asteroids melted a lot of rock, generated a lot of heat, and threw off a lot of steam. Just like right after a giant nuclear explosion, it would be hot, it would heat everything up and eventually cool off. But eventually, it would return to what it always has been.”

The Opportunity lander touched down on Meridiani Planum in January 2004. That landing began a very important period in planetary exploration. The rover has returned many breathtaking images of the Martian surface, as well as measurements of the surrounding geologic features and chemistry.

Researchers on the MER team feel the observations of this site point to an area once drenched in water. They think that environment might have supported life. The body of water gradually evaporated away leaving sulfates behind. Because the sulfates point to an area that once was a lake, or large body of water, scientists think it would be a good choice to further explore the Meridiani Planum for fossils or other relics of previous life forms.

“The current emerging view, the NASA view, is that this was originally a giant lake or spring deposit and winds blew and reworked the sedimentary layers into wind deposits,” Knauth says. “There is no place on Earth where this has happened. We have lots of playa lakes. There is nowhere where the whole lake was reworked into a wind deposit, in situ. It is completely untenable in our view.”

Knauth, Burt, and Wohletz believe the Opportunity data can be explained in terms of a meteorite strike or, more likely, many strikes.

“When a meteorite hits there is a tremendous blast. It is much like a nuclear explosion,” Knauth says. “On a planet with an atmosphere, around the base of the impact site you get a turbulent ground-hugging, roiling cloud of gas and debris that goes out and makes a sedimentary deposit. You get deposits that can go up to almost 100 kilometers from big volcanoes. A big [meteorite] impact can provide deposits over tens of thousands of square kilometers.

This image shows volcanic surge deposits on Earth near the Koko Crater area on Oahu, Hawaii. They were caused by volcanic explosions and are the result of seawater explosively mixing with molten rock. These flat-bedded layered rocks are very similar to rocks imaged by the Mars Rovers Spirit and Opportunity. ASU geologists Don Burt and Paul Knauth think that similar rocks on Mars could have been deposited by the cratering-type explosions caused by meteorite impacts. (Don Burt photo)

Some of the questions concerning the observed sediments at the Opportunity site include a mixing of evaporative salts, textures of the sediments, and the existence of small spherules of iron oxide.

“The mixed chemistry of the salts is all wrong for an evaporated lake at the Opportunity site,” Burt says. “If it were a large lake that slowly evaporated, then the salt deposits would be more uniform going from least soluble (calcium sulfate, jarosite) to most soluble (halides and magnesium-sulfate).

“With evaporated deposits you wouldn’t get what you are seeing chemically or mineralogically on Mars,” Burt adds. “At the Mars site, they have their most soluble salts mixed with the least soluble salts. On Earth, the least soluble evaporates first (like a bathtub ring) and the most soluble last. However, in this deposit it is a complete mix.”

“They say the wind mixed everything up. But wind is an efficient segregator of phases, not a mixer,” Knauth explains. “It doesn’t work.”

“The interesting thing is that since the publication of our Nature article, scientists have found the identical type of deposit on the other side of the planet. It’s called ‘Home Plate,’” Knauth says. “They had to interpret it as a base surge because it actually had a ‘bomb sag.’ That is a common feature found in base surge deposits.”

“But they see the same features without a bomb sag at Meridiani. Yet they still say that is a wind-reworked lake bed,” Knauth adds.

If the NASA view is correct at Meridiani, the chemistry requires a long sequence of post-depositional events. Both Knauth and Burt think this is unlikely.

“For every change in chemistry they find in the deposit they have to invoke a new surge of ground water,” Knauth adds. “I think they are up to nine intrusions of ground water coming in, each having a different chemistry, in order to explain what they see. They acknowledge it. They use the terms ‘a remarkably complex history.’”

All of which to Burt points to the key feature of their hypothesis—simplicity.

“Our hypothesis doesn’t leave anything out that is important,” Burt says. “We think, hands down, our story is the simplest. We explain all of the features, all of the data that come back from the Mars rovers. But our story explains it in far fewer steps, with no internal contradictions.”

For more information about their work, contact L. Paul Knauth, Ph.D., or Donald Burt, Ph.D., School of Earth and Space Exploration, College of Liberal Arts and Sciences, at: Knauth@asu.edu or Donald.Burt@asu.edu