Curiosity has been on the surface of Mars for more than four years and has covered about 15 km of the red planet. In that time, the rover has drilled and analyzed numerous rocks to learn more about the planet’s mineralogy. According to Curiosity, Mars was once a wet world with much of the chemistry necessary for life.
The rover has also climbed about 200 vertical meters from the bottom of Gale Crater, near Yellowknife Bay, onto the lower slopes of Mount Sharp and into the Murray Buttes feature. During these travels, Curiosity has revealed the changing composition of the clays and rocks. For example, scientists working with data collected by Curiosity announced Tuesday that they recently found boron on Mars for the first time, an indicator of past habitability.
In addition, Curiosity’s rich chemical analysis of Martian rocks helps scientists understand the nature of the large lake that partially filled Gale crater billions of years ago. They have found evidence of ancient streams and deltas that fed the lake. Based on sediments left behind, scientists have confirmed that the lake’s water was not too acidic to ever support life.
Scientists discussed their latest findings Tuesday at a press conference at the fall meeting of the American Geophysical Union in San Francisco. As Curiosity climbed the slopes of Mount Sharp, it found an increased amount of certain clay minerals compared to older sedimentary layers previously explored. From this, scientists have gathered details about the climate of Mars billions of years ago and how it may have changed during the critical few hundred million years when water was abundant on the planet’s surface.
One of the ways scientists have come to this conclusion is through X-ray diffraction, which can identify minerals by their crystalline structure. At the lower elevations, Curiosity found more magnesium and less iron, while higher up the minerals contained more aluminum and oxidized iron. These differences point to the complex water chemistry of Mars in the past, said Thomas Bristow, a staff scientist at NASA’s Ames Research Center.
A further complication of these studies of ancient Martian chemistry is that minerals could be formed on site or transported by water. Even with four years of data from present-day Mars, it’s still challenging to build a model of how water moved on Mars three billion years ago.
“This puzzle makes my job fun,” said Bristow. “Fortunately, the picture becomes more coherent as we climb the mountain.”
A lovely lake
The scientists now believe that rivers brought sediment into Gale crater about 3.5 billion years ago. Those rivers deposited pebbles and sand higher up in the middle of the basin, where there was a stagnant water. As these sediments built up at the base of the crater, the lake level rose. Samples taken by Curiosity when it was in Yellowknife Bay allowed scientists to study the sandstone and mudstone that were buried by dust after the lake dried up.
While studying these Martian clays, scientists and geologists must also consider the movement of a larger groundwater system, which would have remained moist long after the surface lake dissolved as Mars’ atmosphere dissipated and the planet cooled. “Any microbes could have stayed happy in those cracks for a long time,” said Curiosity scientist John Grotzinger of the California Institute of Technology.
The intriguing new data has boosted Grotzinger and others’ confidence in a temperate past for Mars. About 3.8 billion years ago, he said, when life emerged on Earth, conditions on Mars would have been much the same. Snow on the tops of Mars would have melted during the warm season and flowed down as rivers into basins such as Gale Crater. That water was not too acidic. “We see all the properties that we like to associate with habitability,” he said. “There’s really nothing too extreme here.”
Finding drill is not boring
The discovery of boron, for the first time on Mars, also excited scientists. Curiosity’s ChemCam instrument recently found the element in mineral veins. Boron makes up about one-tenth of one percent of the rock’s composition.
Scientists have several ideas as to why they find the rare, highly water-soluble element on Mars. Boron indicates the presence of complex chemical processes that took place long ago in the rivers and lakes of Mars. These aren’t smoking weapons for life — Curiosity hasn’t found any microbes or fossils — but more complex chemistry raises the odds that life may have originated on Mars. In addition, some scientists believe that some form of boron is needed as part of a chemical reaction to make RNA. Unfortunately, Curiosity cannot discern differences in drill types.
With their latest observations on Mars, scientists have pushed the boundaries of Curiosity’s chemistry kit. Another rover similar in size and design to Curiosity, Mars 2020, should launch in July 2020. It will have an ultraviolet Raman spectrometer and the ability to determine if the boron on Mars is the mineral form believed to be part of the RNA reaction. Mars 2020 will also be able to detect organic compounds. So while Curiosity didn’t find any life, it certainly managed to pique astrobiologists’ curiosity and make them want more data from the red planet.
List image by NASA