The Curiosity Mars rover is in the prime of its robotic life, approaching dramatic layered deposits on the slopes of Mt. Sharp. But even as the four and a half year-old mission reaches the features it was initially sent to investigate, scientists and engineers are feverishly planning for the next rover mission, Mars 2020.
2020 is shaping up to be a busy year on the Mars exploration calendar: in addition to the NASA rover, the European Space Agency and China have missions slotted for the favorable launch window. But where to go? Making the decision is a complex process, as teams of scientists and engineers develop navigational software, optimize the payload, and establish the geologic context of potential landing sites.
The importance of site selection is magnified by the role of Mars 2020 as the first step in a sample return mission – a longtime grail of Mars scientists. In its current configuration, the rover can collect about 30 canisters of soil, air, or rock particles. A future, planned-but-not-yet-officially-on-the-books mission will return the cache to Earth, where a robust analytical arsenal will be waiting to conduct a battery of detailed tests.
For many scientists, one shift from previous missions is a tighter focus on investigating rocks that preserve signals from Mars’ active past – a period between about three and four billion years ago during which liquid water flowed across the surface, altered rocks, and may have created habitats for opportunistic organisms. And because the rover can only collect samples at the planet’s surface, researchers are avoiding places where more recent forces like craters or windswept sand have covered the ancient rocks.
At a meeting last week in Monrovia, California, hundreds of Mars scientists and mission engineers gathered to narrow the list of landing sites from eight to three. The selection process has been in full swing since 2014, and the final decision will be made in the next year or two, as the instrument teams finalize their flight-ready hardware and prepare for launch.
The workshop was a civil yet high-stakes environment, as proponents of the eight sites had a few hours each to make their case. Ken Farley, a professor of geochemistry at the California Institute of Technology, serves as the mission’s Project Scientist. “It’s hard to imagine any decisions on this mission that will be as impactful to the overall mission as selecting the landing site,” he told the attendees. “For decades to come, the decision we make will be important – it will drive a lot of science that gets done.”
In other words, no pressure.
After two days of presentations and debate, participants were polled on the sites, asked to consider the three main criteria: 1) Could the samples, if returned to Earth, lead to fundamental discoveries about Mars and the potential for extraterrestrial life? 2) Can the site be investigated over the course of the primary mission (roughly three Earth years)? 3) Are there significant water resources that may be advantageous for future, human-based exploration?
(The water issue is a double-edged sword, as the mission is also being steered away from water resources that are too active. A mandate of the planetary protection office is to avoid cross-contamination of Earth and Mars, so any habitats that could support hitchhiking terrestrial microbes need to be avoided. Sites with water ice within five meters of the surface are not being considered.)
And the winners – drumroll please – were, in some sort of preferential order, Jezero Crater, Northeast Syrtis, and the Columbia Hills. Jezero is an ancient lake – life may have taken hold in its waters, and the inflow of sediments from a wider watershed could concentrate compelling material. NE Syrtis maintains mineralogical signatures from water-rock interactions that could be similar to those powering rock-hosted life on Earth. The Columbia Hills site – which has been investigated by the Spirit rover – possesses an ancient hydrothermal spring. The idea of re-visiting a site with Mars 2020’s more powerful set of tools – at the expense of a geologically distinct portion of the planet – is a subject of heated debate.
In the coming weeks, the Extremo Files will profile each of the three sites still in the running. Astrobiology’s next great hope for transformative discovery may well hang in the balance.
Climate change on Earth is a well-established phenomenon, but scientists have long struggled to explain an even more dramatic change of conditions, long ago in a far-off land.
Mars is a dry, frigid planet today, with an average ground temperature of about -60 °C. Liquid water seems to be possible only under a narrow range of circumstances, but for the most part, water sublimates directly from solid ice to gaseous water vapor. And yet, features on the surface of Mars tell a very different story. Dramatic river channels and canyons indicate that vast quantities of water once flowed from the southern highlands to northern plains.
Clearly, Mars must have been warmer to allow all of this liquid water to persist on its surface. But how were such distinct conditions created? What was different about the planet billions of years ago?
Launching rockets into space has traditionally been the domain of nation states: only a handful of countries over the last several decades have mounted the technical expertise and financial resources to put payloads in orbit. With so few players, outer space was governed by the “Cold War principles” outlined in the Outer Space Treaty of 1967, which holds nation states accountable and reserves the use of space for scientific study and other peaceful purposes.
Today, space law needs an overhaul, according to James Gilley, an Instructor of International Studies at Louisiana State University. After all, some of the key assumptions that underlie the Outer Space Treaty no longer apply. A wide range of entities, from treaty non-signatories to private companies like SpaceX and Blue Origin, are charging into low Earth orbit and beyond, and their intentions are unlikely to be restricted to scientific research.
“We’ve kind of forgotten as an international community that we’ve written laws about this stuff,” says Gilley, “because nothing has truly upset the apple cart.” But that could change soon, and Gilley points to a few plausible cases where rogue actors could set off an international legal crisis.
Natural selection is a rigorous master, demanding tough choices and efficiency of all living organisms. Even when things are good and the living is easy, trade-offs are required.
For example, as a microbe is enjoying a nutritionally replete buffet, it begins to grow, laying the groundwork for proteins that will build biomolecular scaffolds and ultimately generate a new cell through replication. But as quickly as this process begins – as DNA is transcribed into the RNA that will serve as the template for proteins – it all comes grinding to a halt. The cell has run out of phosphorous.
Take a look outside your window. How many species do you see?
This question of how geography influences biodiversity has bedeviled biologists for centuries. But according to a new study led by Marcell Peters from the University of Wurzburg, the number of distinct species you’re seeing – or, more accurately, the number you would see in the nearest natural environment – depends most strongly on temperature.
Several hypotheses have percolated through the scientific literature over the years:
1) The temperature hypothesis, in which the greater rate of biological processes, interactions, and evolution associated with higher temperatures is the main determinant of diversity.
2) The water availability hypothesis, proposing that water supply underlies primary productivity and enables more diversity at higher trophic levels.
3) The productivity hypothesis, linking maximal diversity with a greater abundance of life’s required nutrients.
4) The area and geometric constraints hypotheses, which link diversity and the availability of distinct niches to more space or geographic gradients.
5) The plant diversity hypothesis, connecting animal diversity to the number of consumable plant species.
All of these different possibilites have remained in the discussion for so long because diversity studies typically examine a single type of organism. So while vascular plants are subject to the water availability hypothesis, arthropod distributions are best described by the plant diversity hypothesis.
After waking up this morning, your body began a well-established set of procedures to prepare for the day: heart rate ramped up, body temperature increased, and hormone levels shifted. And tonight, you’ll probably start to feel sleepy around the same time you did last night, as other physiological changes start to take place. This predictable cycle, of course, is your circadian rhythm, the hard-wired biological changes that help your body achieve its full set of diurnal activities, from sleep at night to optimal coordination around 2:30 PM and greatest strength at roughly 5:00 PM.
To qualify as a “circadian,” a biological cycle must be endogenous (self-programmed, rather than merely a response to environmental conditions), temperature resistant (despite the changing kinetics of different thermal regimes), and re-settable (think about your latest transition to a new timezone, jetlagged as it may have been). This doesn’t just happen in animals that sleep at night: plants, bacteria, and fungi have all been shown to prepare biochemically for daily cycles.
When NASA’s New Horizons mission flew by Pluto last year, it sent back images that recalled a past era – a time when a few photos of initial reconnaissance could drastically change our sense of an entire celestial body. For The Moon and Mars, these initial glimpses happened decades ago; satellites of the gas giants, like Europa, Enceladus, and Titan were ready for their close-ups next. And after that? Well, it seemed as if the geologically active, charistmatic targets in our Solar System were spoken for. As illuminating as subsequent investigations have been – and to be sure, there are fundamental aspects of all of these bodies that still await discovery – at least we knew what kind of world we were dealing with.
That all changed with Pluto. Expectations were low, given the outer Solar System’s reputation as an icy wasteland, but when initial photos showed potential cryovolcanoes and recent resurfacing, the questions came fast and furious. Many of them centered around the heart-shaped feature a thousand kilometers across located at 25 degrees north latitude.
How hot is too hot for life to survive? Ever since microbes were discovered squirming around in hydrothermal springs several decades ago, the limit of heat-loving (thermophilic) organisms has been a moving target. The current record-holder is “strain 121,” an archaeon isolated from the Mothra hydrothermal vent deep beneath the surface of the northeast Pacific Ocean; suitably enough, it can grow at a blistering temperature of 121 °C.
There are some physical limitations of biological activity at high temperature. Enzymes unfold, as rapid molecular bond vibrations tear intricate molecular structures apart, obliterating their functional capabilities. Individual amino acids lose their “handedness” as geometrical arrangements of molecular structures equilibrate with heat; since all known biological amino acids are “right-handed,” organisms must spend a lot more energy re-shaping amino acids to fit the template. DNA bases “A” and “G” falter at exponentially higher rates as the temperature goes up, leading to murderous rates of mutations.
Despite these assessments of biomolecule stability and the search for the thermal outliers, there haven’t been a lot of systematic studies of how habitability at seafloor sites changes with temperature. Sure, an impressive organism or two can eke out a living at 121 °C in the lab, but is that also true in the real world, where conditions could be much more variable? And are viable communities around continuously up to that thermal limit?
In 2007, the Mars Exploration Rover (MER) Spirit came across a slightly raised platform, roughly pentagonal in shape and 90 meters across, which scientists named Home Plate. The rocky outcrop had a base of solidified ash, with nearby deposits of gas-filled basalts. Next to the plateau, nubby, nodular chunks of rock showed up, and light-colored soil just beneath the surface was exposed by the rover’s wheels. Mineralogical spectra of the bright soil were beamed back to Earth, revealing, to the scientists’ surprise, that it was composed almost entirely of silica.
When the geological puzzle pieces were assembled, two main options emerged: Home Plate may have been a volcanic fumarole, spewing sulfuric acid at basaltic rocks and leaving silica behind, or it could signify the remnants of a mineral rich hot spring, whose silica-saturated water generated the knobby rocks. Either way, water and heat were likely involved, and the discovery led to an onslaught of new questions and exciting plans for further studies.
But then, the Spirit rover went silent, forcing MER scientists to get creative. To pursue the Home Plate mystery, they’ve scoured the Earth for mineralogical signals most similar to those found on Mars. By determining the conditions that best recapitulate the martian data, the thinking goes, we might be able to piece together the events of Mars’ ancient past.
Astronauts flying long-duration missions in space have been known to suffer substantial health effects: they grow nauseous, lose bone density, and watch their muscles atrophy. These large, human-scale changes are pretty easy to observe, as trillions of cells’ responses to microgravity are compiled into the physiological response of one organism. But what’s happening at the cellular scale? How do single-celled life forms respond when launched into space?
Gravity is an aspect of life that we – and all other organisms on our planet – have taken for granted for all but the last 50 years of our 3+-billion year evolutionary journey. This makes it hard to think about ways in which gravity could be hard-wired into biology, as it never represented an actionable variable for evolutionary pressures.
By taking life into space, biologists are seeing how gravity affects the cellular environment in fundamental ways. It’s been suspected for years: past experiments have shown that microbial cultures tend to form biofilms more easily, and pathogens become stronger. But a genetic, mechanistic understanding of these processes hadn’t been explored.