NASA has officially called an end to the historic Opportunity rover mission, ending a spectacular, 14-year adventure on the Red Planet. One of the rover’s most intriguing discoveries, however, came just two months after it landed, when it stumbled upon tiny objects bearing a startling resemblance to blueberries — the nature of which still divides scientists today.
On February 11, 2004, while exploring a rocky outcrop at Meridiani Planum, Opportunity ventured into an area where the Martian surface was covered with hard, round, grey pebbles. Because the objects were so eerily spherical in shape, the Mars Exploration Rover (MER) team dubbed them “blueberries”.
“[We] saw some very strange things,” noted Steve Squyres, a MER team member and Cornell University scientist, in his mission journal at the time. “[We] see these strange round objects we’re calling ‘spherules’ embedded in the outcrop, like blueberries in a muffin. The outcrop erodes away as it gets sandblasted, and the spherules (which seem to resist erosion better than the rest of the outcrop does) fall out and roll down the hill. Weird.”
Using Opportunity’s microscopic imaging camera, the MER team observed several batches of the tiny blueberries, which measured anywhere from between 100 micrometres to 6.2 millimetres in diameter. In a NASA press release issued soon after the discovery, Squyres said the features were “unlike anything ever seen on Mars before.”
Proposed explanations for the geological anomaly included accretion due to watery conditions, meteorite impacts, and volcanic eruptions. Two weeks after the discovery, the MER team settled on the accretion theory, claiming that the ball-shaped pebbles were a geological phenomenon known as “concretions”.
“The blueberries were a complete surprise when we landed and found that they carpeted the surface,” Harry McSween, a planetary scientist at the University of Tennessee and a MER team member at the time, recounted to Gizmodo. “We knew from orbital [observations] that the region contained a lot of hematite (iron oxide), and that was one of the reasons that landing site was chosen.”
The blueberries, he said, were the hematite seen from orbit — little balls of material that grew slowly from the watery solutions percolating through the rocks.
“We were able to identify the hematite in them, and saw them in the rocks themselves,” said McSween. “When the rocks weather, the concretions are more resistant, so everything else is winnowed away, probably by the wind, and the concretions remain behind and build up to high concentrations on the surface. Geologists call this a ‘lag deposit.’”
Similar hematite concretions are known to occur on Earth, he said, but they are usually larger. As for the apparent bluish colour of the blueberries, McSween said they’re actually grey. The MER team “chose certain wavelengths of light to use for the images, and that had the effect of making them appear blue,” he explained.
This conclusion, that the balls formed from accretion, meant that water once existed on the Martian surface. Today, this is a well-accepted fact, thanks mostly to observations made by the Curiosity rover, but this wasn’t known at the time the blueberries were discovered.
But the Martian blueberries story didn’t end there.
In 2005, a Lunar and Planetary Science paper, co-authored by Donald Burt from ASU School of Earth and Space Exploration, claimed the MER analysis made no sense, and that something else had to be responsible for these odd surface features.
“There are many problems with [the MER] hypothesis, the most important of which include their nearly perfectly spherical shape (unlike highly irregular normal concretions), their uniform size range (never larger than about 7 mm, whereas normal concretions can reach huge size), their uniform distribution across a huge volume and area of rock (many concretions tend to be concentrated at chemical reaction fronts), their failure to clump together, other than a couple of loose doublets and single triplet, and their unusual mineralogy,” Burt wrote to Gizmodo.
By “unusual mineralogy,” Burt is referring to their grayish-blue exterior, which suggests the objects formed under high temperatures. On Earth, such objects, known as specular or shiny hematite, typically form in hot springs or volcanic steams. Normal sedimentary concretions, he said, form at low temperatures, but they’re comprised of dull and not shiny iron.
To explain the Martian blueberries, Burt, along with his co-authors Paul Knauth and Ken Wohletz, revisited an earlier consideration: Meteorite impacts. In another 2005 paper authored by the same team, this one appearing in Nature, the scientists suggested that the pebbles were an unusual type of impact spherule, formed by the complex tumbling interactions between iron flakes and the sticky, acidic, and salty steam and dust clouds produced by major meteorite impacts on the iron-rich Red Planet.
“These clouds are analogous to the huge pyroclastic clouds that form from a major volcanic explosion, or from the ground explosion of a nuclear bomb,” said Burt. “The spherules are called ‘accretionary lapilli’ and are well known to form by tumbling accretion during such explosive events on Earth. They formed, for example, during the meteorite impact event that is believed to have killed off the dinosaurs about 65 million years ago. The process is somewhat like forming a hailstone by ice condensation in a tall thunderhead.”
Burt said the MER team considered this hypothesis after he and his colleagues suggested it to them, but ultimately rejected the idea of “hematite hailstones.” Nevertheless, Burt said, his team’s impact spherule hypothesis “explains their spherical shape, their strict size maximum (caused by their suspension in a turbulent gas), their very wide and uniform distribution, and their unusual bluish, shiny appearance and mineralogy.”
To which he added: “Our basic disagreement with the MER team, all along, has been that we think that the sedimentary environments on early Mars need not have been the same as those on present-day Earth, despite some apparent similarities between the two planets. Early Mars was still a relatively tiny planet, constantly bombarded by impacts, and was still twice as far from the Sun as Earth.”
Interestingly, a 2014 study led by geophysicist Anupam Misra from the University of Hawaii, published in Planetary and Space Science, reached the same conclusions — that the Martian blueberries are not accretions and that they formed from meteorites. Misra’s primary argument was that the concretion theory couldn’t explain the narrow sizes of the Martian blueberries, whereas the meteorite impact theory could.
Soon after the Misra paper was published, however, geologist Brenda Bowen from the University of Utah and Timothy Glotch from Stony Brook University told National Geographic that the concretion theory is far from dead. Bowen said the alien Martian environment could still produce the conditions required for accretion, while Glotch said tools onboard Opportunity pointed to a low-temperature geological process, and not a “high-temperature event.”
Not be outdone, a research team from Nagoya University and Kochi University in Japan posited yet another explanation in December 2018, arguing that the spheroids formed around cores of a mineral called calcite, with the iron-rich components existing only on the exterior. A post from Space.com explains:
Based on [field observations] and chemical modelling, the scientists suggested that floods of iron-rich, gently acidic water washed over the original calcite structures. Unlike the terrestrial versions, Martian blueberries seem to be made of hematite all the way through, no longer sporting any calcite heart. But that could point to a long period of overwash that ate through all the calcite, the researchers said.
Clearly, the debate about the Martian blueberries is still a lively one, but as Burt explained to Gizmodo, this ongoing scientific argument wouldn’t have happened without Curiosity and researchers committed to sharing information with the public.
“Opportunity was a wonderful mission that demonstrated NASA’s fabulous engineering capabilities, in that it lasted so long and revealed so much. The best part of both MER missions was the early decision by Steve Squyres, Jim Bell [the lead scientist in charge of Opportunity’s panoramic camera], and NASA, to share photographic images and scientific results with the general public as soon as they were obtained. In addition to adding to public enthusiasm, this major decision allowed for alternative scientific interpretations, such as ours, to be hypothesized and tested in real time for year after year, without us being members of the official team.”
The true nature of the Martian blueberries remains an enigma. It’s a testament to the wild success of the Opportunity rover that what was supposed to be a 90-day period of exploration lasted over 14 years, uncovering Martian mysteries that continue to perplex scientists. Opportunity and Spirit’s contributions will continue to reverberate through the scientific community for years to come.