Curiosity's science team announced this week that Gale crater, on Mars,was habitable to potential life, billions of years ago. This is the first place off of Earth ever to be confirmed where life could have possibly had a place to survive, or even thrive. The science team was all smiles, looking joyful and relieved. Curiosity has been on Mars for 7 months, and has already accomplished one of her major goals - to determine if Gale crater could possibly have been habitable for microbes in Mars' ancient past. In February, she drilled into a rock and analyzed the minerals there, and from the results we now know, for the first time ever, if microbes were on Mars at this site, they would have had a place to live.
What makes a place habitable for life? On Earth, we have so much life, living in all kinds of conditions, that it seems like life could live anywhere, given enough time to adapt. We take for granted that in general, life will find a way. But there are a few very specific conditions that it's thought all life needs to exist, not only life as we know it on Earth, but most kinds of possible life, anywhere. Looking for these conditions to exist in the same spot, during the same period of time, is one of the main goals of astrobiology.
Twilight in Yellowknife Bay.
Image credit: NASA/JPL-Caltech/MSSS/Damien Bouic.
Curiosity took this image in February, 2013. Today, Gale crater is cold and dry, with freezing temperatures and no liquid water. But 3 billion years ago, twilight in ancient Gale crater may have been balmy. This area on Mars was much warmer, with so much liquid water that a lake may have filled the crater, with sediments slowly building to reach between 1 and 5 kilometers deep. Leading scientists to now think that this lake may have been habitable for over 2 billion years.
There are four things astrobiologists look for when searching for habitable places: water, energy, elemental building blocks, and friendly physical conditions.
First, the site has to have liquid water, to let any microbes use it in their cells. Water dissolves chemicals and lets the microbes move the chemicals around in their cells to stay alive.
Secondly, the site needs a source of primary energy, which until recently was thought to necessarily be sunlight, but in the past couple of decades microbes on Earth have been found and studied that get their energy from the dissolved chemicals in rocks. When looking for signs of life or habitability in our solar system, this chemical energy is now considered the most likely to be used.
Thirdly, the site needs to have a combination of elements that most life would probably need, because of how these elements interact with other elements: carbon, hydrogen, nitrogen, oxygen, phosphorous, and also probably sulfur.
And the fourth thing that is looked for in a potentially habitable site is if the physical environment would allow life to be there. The temperatures need to let water exist in a liquid state, and that water needs to be friendly enough (particularly a pH balance and mineral content that are hospitable) to not destroy little cells and their organic chemistry.
Other things help, too, like some shielding from radiation, and an environment stable enough that gives life time to develop and adapt to changes. Curiosity's scientists feel confident that Gale crater at Yellowknife Bay had all of these conditions.
Source: Anderson and Bell 2010, Mars: The International Journal of Mars Science and Exploration.
In the image above, the white circle is where Curiosity landed and has been exploring for the past seven months. She has been slowly heading towards the hill to the lower right of the circle, Mount Sharp, and is now in an area called Yellowknife Bay, which is the specific area considered to have been habitable long ago. This map shows elevation, with the blues and greens being lower than the reds and oranges, and you can see how deep the elevations are in the entire crater, especially where Curiosity is spending her time. Mount Sharp is made of layers of sediments.
Curiosity's scientists aren't yet sure how the sediments of Mount Sharp were laid down - if they were wind-blown or lake sediments - but the evidence is showing more and more that this crater was a water environment, with water running over the crater rim and filling the deep parts. Mount Sharp may be a deposit of lake sediments kilometers-high, built up in a lake that filled the crater. In this possible scenario, as conditions changed and the lake level dropped, the base of Mount Sharp could have been eroded as water was able to flow over the rim and run downhill in fast-moving streams. If that's the case, Yellowknife Bay may be much younger than the sediments of Mount Sharp, because it is in an area that looks like it was once where flowing water pooled up, well below the crater rim.
Image credit: NASA/JPL-Caltech/ASU
The image above combines a picture of Curiosity's area of the crater, with false-colors that mark how orbiters around Mars mapped the area to show different kinds of rocks. This mapping was based on how much heat the different rocks soak up and release from sunlight. The red areas have a lot of rock that hold more heat than the surrounding rock, during the daytime and into the evening. The black oval is the area that was targeted for Curiosity to land in last August, and the tiny black dot is where she actually landed. She has been traveling along since then, inspecting rocks and testing her geology and chemistry instruments, and finding evidence of ancient watery conditions along the way. The blue dot shows where she is now and where she drilled into the rock. This area, Yellowknife Bay, is only a few hundred meters from where she landed, but has different kinds of rocks from the landing site. Look closely and you can see that Yellowknife Bay is just barely into a red area. The tested rock is part of some mudstone clay bedrock called Sheepbed. This clay is a big reason that Gale crater was picked as Curiosity's landing site, because clay rocks are generally formed from watery minerals.
The area marked as “alluvial fan” is where, millions or billions of years ago, fresh water ran down from the rim of Gale crater, and washed out into a less-steep area. The water would have brought sediment with it, and then water and sediment pooled up into what could have been a lake, altering the minerals into clay. It's not known how long ago this lake area existed, or for how long, but the kinds of sediments it left behind are the kinds of rocks that form from an environment of fresh water - like a lot of lakes we know on Earth.
Image credit: NASA/JPL-Caltech/MSSS
You may recognize this image from last September. It was very exciting! This is a ledge of Hottah, the rocky remnants of an ancient stream bed. Water is considered so vital for life to exist, and this looks so much like dried up stream beds found on Earth, that when we were shown the banks of this old stream, many people thought that this must have been a habitable spot. But water alone doesn't mean a place is habitable for microbes - the area has to have all those other conditions of energy, nutrients, and good physical conditions.
Image credit: NASA/JPL-Caltech/Cornell/MSSS
This picture shows two different places on Mars: on the left is some rock from the area of Mars where the Opportunity rover landed in 2004 and is still exploring, and on the right are rocks in the area where Curiosity did her drilling. Both areas had liquid water in their past, but only Curiosity's site is considered to have been habitable. They both have that red-orange surface of oxidized iron (rust), and they both have rocks formed from interaction with water, and crevices filled with other minerals washed in. But the rocks at Opportunity's site are of a kind known on Earth to form from very hot, acidic, salty water, and even though we do have microbes on Earth who love all those things, they have adapted to live in that kind of water that would kill most other life. Not enough is known about Opportunity's area to be able to say for sure that it wasn't habitable, but it definitely seems to have been unfriendly.
Curiosity's area in Yellowknife Bay is very different from this: the minerals found here were formed from interaction with fresh water with just a tiny bit of salinity, and not acidic at all, but neutral or slightly alkaline. That's why project scientist John Grotzinger said that the water here was so fresh, we could have drunk it. If any life was here, it would have had a friendly environment, not having to adapt to challenging, extreme conditions.
Image credit: NASA/JPL-Caltech/Cornell/MSSS
This image shows how much more advanced Curiosity's instruments are compared to Opportunity's. On the left, you can see where Opportunity used her Rat Abrasion Tool to scrape away the surface dust to reveal the kind of rock that lay beneath. She could then do a bit of scraping to the top layer of the rock, but that was as far as she could go. On the right is the hole where Curiosity used her drill to go well into the rock and see what it is made of. That is why it was such a big deal when she used her drill for the first time, because that is the first time a drill had ever been used off Earth, to let us see inside a rock on another planet. Curiosity's chemistry lab then analyzed the powder, but just visually we can see a lot here: the dust that Opportunity made while scraping is red, while Curiosity's dust is gray. The red means that the minerals are oxidized iron, hematite, and that alone shows that the water environment there wasn't so friendly, and it also shows that there was a lot of oxygen reactions going on. That isn't good for microbes. Oxygen is very reactive, and it can use up all the chemical energy sources by spontaneously reacting with the minerals, and not leave much behind for the microbes. The gray dust of Curiosity's rock shows it being in a “reduced” form, meaning that it has lots of hydrogen that microbes could use for energy, the way plants use the photons in sunlight. It hadn't reacted with oxygen yet, which would have knocked off the hydrogens. It shows that the water environment in Curiosity's Yellowknife Bay was good for microbes, energy-wise.
Image credit: NASA/JPL-Caltech/MSSS
Here's the nice green-gray powder that Curiosity made by grinding up what she drilled out of the rock (above). This energy-filled powder looks beautiful to astrobiologists. Curiosity is beautiful, too - look how shiny and new she still looks, even after seven months on Mars!
Image credit: NASA/JPL-Caltech/Ames
The gray powder in Curiosity's scoop is from ground-up mudstone clay, the kind formed from a lake environment. To give you a really clear idea visually, the image above shows lake sediments on Earth, from Australia. On the left you can see a thick layer of gray lake sediment. On the right you can see the core of sediment taken out of the lake bed. The lower parts of the core are orange, because those sediments were laid down in some kind of different climate and water conditions, before the lake sediments really started. The gray part at the top is what was laid down on the lake floor, and is what is thought to have been similar to what may have happened in the Yellowknife Bay area on Mars. The similar minerals found at Curiosity's site show there was long-standing liquid water, it was fresh, neutral water that had a chemical energy source for microbes, and the physical conditions in the area allowed all this to happen.
Image credit: NASA/JPL-Caltech/Ames
Here you can see how Curiosity's chemistry lab detected clays. You don't need to know what the different arches mean at all, just see the difference near the bottom on the one at the right. It has the extra light blue area that shows the existence of smectite, a kind of fresh water clay.
Image credit: NASA/JPL-Caltech/GSFC
Now this is the real exciting picture!
After Curiosity ground up the drilled pieces of rock and turned them into that nice gray powder, her chemistry lab heated the powder to release gases which would show what the rock is made of. The water, shown in the graph was part of the clay, chemically bound to the minerals. The oxygen and carbon dioxide may have been there in the rock, or may have been the result of reacting with something when the powder was heated.
The really impressive part is in the two forms of sulfur. “m64” is sulfur dioxide, and “m34” is hydrogen sulfide. Hydrogen sulfide is used as an energy source by some Earthling microbes. The way it works is like a battery - the hydrogens are pulled along a chemical chain in the microbe's metabolism, pulled by something with oxygen at the other end, something like sulfur dioxide. This kind of energy gradient in minerals is literally how our chemical batteries work, and it's how microbes run their own internal batteries, to live and get the work done in their cells. This area in Yellowknife Bay not only had energy sources, but it had both ends of the chemical gradient, with different niches for microbes to take advantage of both ends of the energy chain. Not only that, but her chemistry lab also found all the elements considered necessary for life - carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. The oxygen, carbon, and nitrogen need to be examined more to see exactly what forms they were in, but everything is there.
The drilled rock showed that this area had all the conditions needed for habitability - liquid water, energy, elemental nutrients, and friendly physical conditions over a very long time, perhaps two billion years. And it's thought that this was going on during the same time that life was starting and then flourishing on Earth.
Curiosity's science team feels that these results show not only the very first place considered habitable off Earth for life to have had a possible home, but the results also will push other investigations forward. They feel like they have now ground-verified the minerals that the orbiters found, and so they can have a lot of confidence not only in what the orbiters have shown for Gale crater, but for what orbiters around other planets and moons in our solar system are showing. John Grotzinger hopes that this opens up a dynamic field of “comparative planetary habitability”, where scientists can study worlds remotely from orbiter data and feel confident in that data, even if we don't have robots on the surface.
There are plans to send orbiters to the Jupiter and Saturn systems to study icy moons where landing a rover will be a huge challenge. And, the team members feel that Curiosity's mission has been such a huge success so far, that NASA's next Mars rover, planned to be launched in 2020, can build on some of the same platforms, like the landing technique, and so allow the engineers to develop even more advanced instruments.
Now that they have found a past habitable environment on Mars, the team wants to concentrate on looking for a bright organic carbon signal. Finding that signal wouldn't necessarily mean that life was there, but it would be more evidence of a habitable place and allow the next generation of instruments to know how organic carbon on Mars is preserved. The 2020 rover will know a lot more about how to find organic carbon on Mars, and will also be looking directly for evidence of past life itself.