Curiosity Page 29
For about 400 million years, things seemed to be finding a groove, but then came what is known as the Late Heavy Bombardment. There are a number of theories how it happened, but, in short, it did. One broadly acknowledged idea is that the gravitational forces of the large outer planets caused gravitational “resonances,” which seem to have propelled wandering rocky material in toward our part of the solar system. The mass of these larger planets—Jupiter, Saturn, and the rest—caused this drifting rocky material to change orbit from roughly circular to an ellipse, and as they dipped into the inner solar system, some stayed there. It's worth mentioning that though the gas giants are big, they are not very heavy (in terms of mass) and that the sun still accounts for almost 99 percent of the total mass in the solar system, so our star is like a big gravitational magnet. This lasted several hundred million years more and resulted in the scarred planetary surfaces we see today, as these clumps of rock slammed into the inner planets. Interestingly, the end of this era seems to coincide generally with the beginnings of life on Earth.
On Mars, this era included and was followed by the geological epochs below:
PRE-NOACHIAN: About 4.5 to 4.1 billion years ago. This era encompasses the worst of the bombardments, and some of Mars's worst global scars came about during this time, notably an area called the Hellas basin and the differential elevations between the northern (basins) and southern (highlands) hemispheres.
NOACHIAN: About 4.5 to 3.6 billion years ago. The major craters and basins we see today were probably formed about this time. The Tharsis Bulge—a huge, three-thousand-mile-wide, four-mile-high lump on the Martian northern hemisphere—came into being. Mars was also deluged with water-bearing meteors and asteroids during this period, resulting in sufficiently large bodies of water to shape some of the larger fluvial (water-worn) features we still see today.
HESPERIAN: About 3.6 to 2.6 billion years ago. The extensive lava plains formed during this era, and possibly Olympus Mons, Mars's largest volcano. More water arrived as well, creating more fluvial features and possibly oceans. The later part of this era is also perceived as the general boundary between the warm, wet Mars and the cold, dry Mars we know today.
AMAZONIAN: About 2.6 billion years ago to the present. Fewer large meteors were slamming into the surface, but other activities continued. Lava flows and glaciers were active during this time. Mars was drying out and getting colder.
Note that these dates are estimates and especially the last boundary—Hesperian to Amazonian—could range from over 3 billion to 1.5 billion years ago.
The big news in terms of gross geological theory is that of sedimentation. Mars was long thought to be primarily volcanically formed and sculpted, with some wind and possibly water at work. But since the 1990s, it has become increasingly clear that a huge amount of interaction between water and the surface of Mars occurred, with the resulting sedimentary records included in its wake. Weather (with water) and not volcanism formed the face of the planet. Some of these formations are small and local, but they can also range up to hundreds of miles. The oldest of them go back over 4 billion years, to the later Noachian period. Nothing similar exists on Earth, with its more active (and comparatively poorly preserved) geological history. And Mars does not appear to have measureable tectonic activity, so what happens on Mars stays on Mars, so to speak. Also, without tectonics, there is not the uplift and rifting that occurs on Earth, so sedimentation seems to occur mostly within craters and water-worked channels. This is good for finding flat sedimentation, but it's not so good for seeing extreme or deep strata exposures, except for water-worn valleys and craters.
Catastrophic flooding, groundwater bursts, and even glaciers seem to be some of the bigger engines of water-caused change on Mars. The old Mars—warmer, wetter, and more active—caused weathering and change through these watery activities. As Mars lost its atmosphere and much of its moisture, the environment became dry, cold, and more chemically nasty. Wind and chemistry do more to erode what's left than anything else. The planet also oxidized as it cooled, resulting in the rust-red color we know today. The majority of the large water-formed canyons, and the resulting generation of sediment, seems to have occurred between the Noachian and the early Hesperian periods.
The largest strata belts—huge layers of sedimentation—appear to be concentrated in a roughly fifty-degree-north to fifty-degree-south equatorial band. This makes some sense, as this would be the more temperate part of Mars, and it would allow for vast amounts of rapidly flowing water. In places like Arabia Terra, such stratification can be seen in amazing relief—it looks like a topographic map brought to life, complete with bands of strata such as we are used to seeing on Earth. Whether wind or water were more responsible for this region's appearance is still under debate.
Most of the strata are more subtle than that seen in Arabia Terra and are only evident upon closer examination—like the heap of sedimentary rock at the center of Gale Crater. This eighteen-thousand-foot-high mound is one of the larger single exposures of continuous sedimentary strata on the planet, which made landing there a fine choice.
Mount Sharp, then, is like a time machine. At its base, Curiosity will be staring back almost 4 billion years into the past and, as it climbs the serpentine foothills, will slowly advance toward the modern era. But not all the fun must wait for the central peak to be reached.
Exploring Mars, or indeed any world, via robots requires a lot of planning and patience. But perhaps the most essential skill is inventiveness. So often, the ideas and theories held upon the inception of a Mars mission have changed radically by the time it is ready to fly. Then, concepts held dear at launch may have evolved by the time your machine lands. Finally, once on the surface, new discoveries force old ideas to adapt and mature. It's a constantly changing kaleidoscope of theory, observation, and problem solving.
One magnificent example of this is the search for habitable environments on Mars and how this primary element of Curiosity's mission has already shifted after less than two years. As John Grotzinger said it, “We have reached a turning point in this mission—it has changed from a mission to search for habitable environments to the search for organic carbon.” Why would he say that so soon?
The answer lies in learning and adapting. When Curiosity drilled at John Klein and Cumberland, the geologists completed their primary analyses over the course of a couple of weeks. It became clear that these two drill holes, only nine feet apart and more flat and safe (for the rover) than remarkable in nature, held secrets we could only have dreamed of a decade ago. Remember red Mars/gray Mars? Those soil samples showed that the environment extant when those deposits were laid down in that wet, flowing stream were nearly perfect for life, at least of a microbial nature. The water was not salty. It was not too acidic or too alkaline. It was cold but not freezing. And it was filled with minerals in varying states of oxidation. These conditions are not only benign for life but provide enough energy and the proper nourishment for some types of microbes.
So we know that life could have existed…but how do we find out if it actually did? That's the 2.5-billion-dollar question.
Some form of organic molecules have been detected on the surface of Mars, as elaborated upon in the AGU press release. How much might have been a sampling-system contaminant from Earth, how much might have been meteoritic in origin, and how much might be biological versus abiotic (nonlife) in origin are all open questions. The key now is to test, retest, and test again. Then, validate results and look for overlaps and relationships.
Hold that thought for a moment—about the process of finding organic molecules, and possibly a biosignature of life. There is another huge complication to finding life on Mars—one that goes beyond the understanding of ancient environments. This complication is radiation.
Curiosity is the first mission to survey the radiation environment on the surface of Mars. This simple instrument, the RAD instrument, has been wonderfully successful at its task. But what it has told us is not
a happy story. Mars is like your brain on drugs—one big, hot skillet. For although it is a cold place, the surface is hot with radiation.
A primary purpose for the RAD instrument was to measure how much radiation exposure astronauts would receive during a typical Mars mission—and it's a lot. About 1,000 millisieverts, or ten times what they would be bombarded with during a long mission on the International Space Station (ISS). But this exposure would be manageable if there were some shielding and protection during their voyage to Mars and their stay there.
But this same radiation, the result of high-energy particles streaming in from the sun and from deep space, has another downside. It may be actively frying any organic molecules in the surface soil and rocks of Mars. It is estimated that any area exposed for some tens of millions of years, as most are on this relatively unchanging world, would be sterile down to about ten feet below the surface. The drill can penetrate only a few inches. You can see the problem. A longer drill would be better, but such a device will have to wait for a later mission.
Add to this the chemistry in the soil—particularly the existence of toxic perchlorate—and you have a recipe for sterility. Moreover, perchlorate also causes organic molecules to burn up in the very tests designed to find them, as was discovered in relation to the Viking life-detection experiments. Drat.
But remember that I praised JPL's ingenuity? Read on.
The same team that compiled the RAD readings worked with the geologists to invent a way to skirt the issue of surface radiation bombardment and the havoc it wreaks on samples. In essence, let's say that you are on Earth and you want to find worms in your garden, what would you do? You could dig deep, where the soil is wetter and cooler, or you might simply turn over a rock to expose the damp soil, right? That soil would not have been baked out by the sun.
As it turns out, if you can calculate the rate at which the sedimentary outcrops in Gale Crater were being eroded, you can also calculate the amount of time the area previously “shaded” by the sedimentary overhang has been blasted by the sun and other radiation. For example, in a million years, wind alone would be enough to wear away about a yard of rock. So, if you drive the rover to the base of one of these eroded areas and snug right up to the edge of the part that is wearing away, what you are drilling into might have been exposed to radiation for only 500,000 or 900,000 years, instead of tens of millions. Which means—you guessed it—you wouldn't have to go as deep to possibly find organics because this surface has not been sitting under the blasting sun and deep-space radiation for as long as the “unshaded” areas. Of course, a half-million years is still a long time, but it's better than ten or twenty times that.
This sort of experiment is exactly what JPL sees ahead at its next major stop at Kimberley, which Curiosity reached in early April 2014. It is located about halfway to Mount Sharp. The rover will get closer to the site, snoop around, take some initial measurements, and then, if the scientists like what they see, pick a place to drill. If they can find the type of eroded overhand discussed above, they will drill there, into rock that has not been exposed to radiation for as long as the area surrounding it.
And now, let's look at some recent progress of the mission. In the latest round of announcements, which coincided with a slew of papers coming out in the journal Science, the results of recent data from the rover were published. I'll go over some of the highlights:
In a planetary exploration first, the geologists are figuring out how to date some of the rocks on Mars. Prior to this development, dating was done almost entirely by counting craters and guessing at their rate of creation, which is an inexact method at best. But, using new investigative methods that allow the science team to understand the rate at which potassium in rocks slowly converts to the gas argon, they are now able to accurately date rocks way, way back…we're talking 4.3 billion years old. There is a fudge factor of +/– 300–400 million years, but it's still a remarkable thing to achieve on a planet other than our own. And as it turns out, the results of this dating fits the old crater-counting methodology almost perfectly, validating that previous technique as a generally reliable tactic for determining general ages of broad areas of Mars. This means that there is far less guesswork involved in the dating of other regions that have so far been seen only by the Mars orbiters—that is, the vast majority of the planet. So it's a big deal. Expect some new theories about the evolution and nature of the planet's surface in the next few years.
It is clear that there was both a source of carbon—CO2—and nitrogen back when the habitat represented by the drill samples was extant. Between that and the benign nature of the ancient environment, life had a good chance of taking root. And the age of the sample was about 3.5 billion years, or about the same time that life first appeared on Earth. Coincidence? Nobody on MSL is speculating publicly—yet. But recall that there are plenty of smart people who posit that life may have formed on Mars first, and then migrated to Earth on meteorites. If evidence of past life on Mars, or even organic material with a biosignature, is found, then the timing of its formation will be critical. It may lead us toward an understanding of where life on Earth may have originated—and it may not be local to our own planet. It ain't over till the fat microbe (if any) sings.
The lake bed in Gale Crater, the site of the drill samples, may have been as big as three by thirty miles in size, and it was probably much larger—similar to an upstate New York Finger Lake. Curiosity has given us the first conclusive evidence of a body of standing water that size on any planet besides Earth (this is excepting the possible oceans on Jupiter's moon Europa and Saturn's moon Enceledus—but these bodies of water, if extant, exist far below a layer of ice and are of a very different nature).
There is more, but those are the main points. Curiosity has already achieved one of its primary goals: detecting ancient habitable environments on Mars. Finding some organic molecules would be the biggest Christmas present anyone on the mission could ask for. Of course, nobody can predict that outcome. But, like the ten-year-old boy looking through the telescope at Griffith Observatory, I remain buoyantly optimistic.
Fig. 31.1. DUNKIN’ DUNES: This image of the Shaler outcrop shows the cross-bedding the geologists get so excited about. An ancient flowing stream caused small sand dunes on the streambed, which eventually become these inclined layers. This area is thought to be younger than much of the rest of Yellowknife Bay. Image from NASA/JPL-Caltech/MSSS.
When this book went to press, Curiosity had left the treacherous region that was smashing holes in its wheels and had taken a risky detour through Dingo Gap, crossing a thirty-two-foot sand dune, then driving across smoother terrain to reach the area named Kimberley. Currently, the rover has covered almost four miles. And, of all things, it is driving backward. Apparently this takes some of the strain off the wheels and would help to even out any more damage that might occur.
Fig. 32.1. THE ROAD TO MOUNT SHARP: From Dingo Gap, Curiosity sought a smoother route, one that would incur less damage to the wheels while still allowing for relatively swift progress. This image shows the pathway it took to continue the journey to its prime destination—Mount Sharp. Image from NASA/JPL-Caltech/MSSS.
On a recent autonomous drive, the rover crawled over three hundred feet in a single sol. That's a far stretch from earlier drives and a big vote of confidence in the onboard navigation. In the current terrain, there are far fewer sharp rocks, and those that do exist appear to be loose on the ground, meaning that they would shift and possibly sink a bit, causing less damage to the rover.
On the way to the mountain, Curiosity will of course be stopping at various waypoints or points of interest. That brings us back to its current location, and one where it will likely spend a fair amount of time, Kimberley. And what was Kimberley named after? Tick-tock…wrong, it was nothing in Canada. The earthly Kimberley is a spot of geological interest in Australia that contains very old rocks. And, as it turns out, both gold and about a third of the world's diamonds. On a side note, t
he only landing in Australia by the Japanese in World War II was at Kimberley. Four Japanese military officers spent about twenty-four hours there doing a bit of spying before wisely returning to their base. Isn't that interesting?
Kimberley (the Mars version) affords another chance to grab a drill sample, and at this time a couple of prime locations to do so have been identified. These provide the best chance yet for an overhang-protected (shaded) and younger sample. It is also one of the stops roughly bounding an area that is special for another reason.
“These are points of interest that help us piece together the geology between Yellowknife Bay and the lower slopes of Mount Sharp,” says Ashwin Vasavada. “At some point the materials stop coming from the crater rims and start coming from Mount Sharp, and there's some very complex inner bedding that we can't figure out from orbit. It's probably also very difficult to figure out from the ground, but at least we can try. And so we've chosen four spots on the way to Mount Sharp that are almost like natural road cuts into the planes of these layers, places where wind probably eroded the hollows where we can see layers of materials exposed.”
Melissa Rice added that this was a region the scientists had been coveting for over a year: “This is the spot on the map we've been headed for, on a little rise that gives us a great view for context imaging of the outcrops at Kimberley.” Context imaging is the process of looking at areas near the one you are interested in to get a better idea of how the overall region was formed.