Record-setting Mars dust storm was overdue
As a global dust storm imperils the Opportunity rover and encircles Mars, scientists are getting their best look yet into the rare phenomenon. Data gleaned from this rare atmospheric event could also help provide important clues as NASA develops plans for crewed landings on the surface of the Red Planet.
This is not the first dust storm to impact mission operations at Mars. When Mariner 9 arrived in 1971 as the first spacecraft to orbit the planet, it was treated to a global dust storm already in progress that delayed imaging and science operations by nearly two months. Dust storms were monitored by the Viking landers and orbiters, however the nuclear-powered landers were not affected by reduced sunlight as Opportunity is experiencing. A global dust storm in 2001 was closely monitored by the Mars Global Surveyor from orbit and provided scientists with their best look to date of dust storm formation and evolution.
The last global storm in 2007 put the solar-powered Spirit and Opportunity rovers to the test. However sky conditions were never poor enough to force either rover into a protective low-power fault mode as is suspected with Opportunity.. Nearly 6 Mars years have elapsed since the last event and over a year has passed since scientists issued a 2016 forecast that a global dust storm could occur by early 2017. While that may not be a terrible forecast for a planet that lacks the extensive network of weather monitoring stations and satellites as Earth, it begs the question of what causes these events and makes them difficult to predict?
WHAT CAUSES GLOBAL DUST STORMS?
Global dust storms were probably observed by astronomers as early as the 1870’s, but very few could be verified prior to the start of the space age. Since the Mariner 9 dust storm in 1971, large dust storms have frequently been observed during the southern hemisphere’s summer, however they don’t happen every Mars year. On average, Mars experiences a global storm every 5 1/2 Earth years (or roughly every 3 Mars years). The most recent confirmed global dust storms occurred in 1977, 1982, 1994, 2001, and 2007 although orbital coverage was limited from the early 1980’s through mid-1990’s and the total number of storms could be higher.
Regional dust storms are far more commonplace and global storms usually start off as a single regional storm or a collection of several regional storms. While the precise trigger that causes storms to expand into a global event is unclear, scientists have a fairly good understanding of the processes that form smaller storms.
Dust is thought to be lofted into the Martian atmosphere when sunlight warms air near the surface causing it to rise into cooler air above it. These updrafts operate in a similar manner to their cousins on Earth, but if an updraft on Mars is strong enough it can also loft fine-grained dust into the air with it. This sometimes manifests in the form of dust devils that have been observed by rovers.
As on Earth, winds on the surface of Mars can be driven by sharp differences in temperature gradients. Across much of the surface, day to night temperatures can vary by as much as 100 degrees Celsius or more. Although in the lower pressure atmosphere of Mars (only about 1 percent of Earth’s), these result in relatively weak winds that occur in the evening and morning hours. For stronger winds, a larger temperature gradient during peak daytime heating is needed, and this effect is increased in areas where lower and higher elevations are in close proximity.
Weather generated by mountain ranges on Earth is a common phenomenon, often resulting in summertime thunderstorms and generating high amounts of snowfall during the winter. A similar effect has been noted on Mars as certain regions are more susceptible to recurring dust storm activity than others due either to terrain or proximity to the polar caps. These storms typically occur at regular intervals under favorable conditions, especially during local summer.
For example, the greatest variety of topographic relief on Mars is in the southern hemisphere and includes the lowest point on the planet in the Hellas Basin. At over 26,000 feet (7,924 meters) below the reference datum (or “sea level”) for Mars, the deepest portions of Hellas experience some of the highest atmospheric pressures on Mars. This allows for increased temperatures during daytime heating and higher temperature gradients with nearby mountains than are possible elsewhere. As a result, dust storms frequently originate from the Hellas Basin.
Higher temperature gradients between cooler highlands and warmer lowlands result in stronger winds that are capable of lofting dust into the atmosphere. If multiple storms occur simultaneously, the collective volume of dust in the atmosphere can be sufficient enough to cause a widespread regional storm. What causes the evolution of regional storms into global events is an ongoing source of investigation for scientists.
IMPACT OF DUST STORMS ON ASTRONAUTS
The opening scene of the movie adaptation of Andy Weir’s novel The Martian involves a dust storm forcing the evacuation of a crew from the surface and stranding one of their crewmates, Mark Watney, on Mars in the process. The scene depicts a dust storm with what appear to be hurricane-force winds that threaten to topple the lander and impales Mark Watney with an antenna that breaks free from their habitat. While the storm serves as a critical plot element for the rest of the story, the severity of the storm depicted has not been observed on Mars to date.
Due to lower atmospheric pressure on Mars, a wind of 60 miles-per-hour (96 kilometers-per-hour) exerts much less force than it would on Earth, perhaps just enough to wave a flag. Even during the strongest winds Mars can generate, the overall risk to the physical integrity of spacecraft is fairly low. So the impacts of a dust storm as depicted in The Martian are unlikely. However dust storms will still likely have an impact on mission operations for astronauts exploring the surface of Mars.
Operational conditions on crewed missions include reduced visibility making EVA’s dangerous and increasing the amount of dust carried into the spacecraft by returning astronauts. Concerns also exist regarding possible static build-up and electrical discharges that could pose a threat to critical spacecraft systems. If any of the spacecraft systems rely on solar power (like the Opportunity rover) astronauts may need to shut off non-critical systems and scale back power-intensive mission activities until sky conditions improve.
FORECASTING FUTURE EVENTS
As with weather forecasting on Earth, predicting dust storms on Mars will get easier as more data is collected about these events. In this respect, the current dust storm is likely to be the most studied global event on Mars to date. A total of six spacecraft are monitoring the planet from orbit and the nuclear-powered Curiosity rover is monitoring changing sky conditions at the surface on a daily basis. Even Opportunity provided valuable data before falling silent as it sent back observations about the amount of dust in the atmosphere before skies became too dark to recharge its batteries.
NASA’s MAVEN (Mars Atmospheric and Volatile EvolutioN) orbiter and ESA’s ExoMars Trace Gas orbiter are watching the storm closely to help address outstanding questions on how dust storms impact trace elements and compounds in the atmosphere. Instruments on other spacecraft, including the Mars Climate Sounder (MCS) and Mars Color Imager (MARCI) on the Mars Reconnaissance Orbiter (MRO) are also collecting daily surface and atmospheric measurements of the storm’s progress.
Weekly Mars weather reports that are relayed to mission teams have been issued by Malin Space Science Systems since MRO’s arrival in late 2007. These weather reports help mission planners to schedule spacecraft operations accordingly and enable forecasting of future events to a limited extent. Going forward, these weather reports will also help to monitor the atmosphere as dust settles out over the next several months.
As scientists glean new information from observations of the current storm, improvements can be made to existing climate models and forecasting methodologies that can be used to predict the next big event. Until then, 2018 will enter the history books as the most recent (if not overdue) global dust storm to impact Mars.
Paul is currently a graduate student in Space and Planetary Sciences at the University of Akransas in Fayetteville. He grew up in the Kansas City area and developed an interest in space at a young age at the start of the twin Mars Exploration Rover missions in 2003. He began his studies in aerospace engineering before switching over to geology at Wichita State University where he earned a Bachelor of Science in 2013. After working as an environmental geologist for a civil engineering firm, he began his graduate studies in 2016 and is actively working towards a PhD that will focus on the surficial processes of Mars. He also participated in a 2-week simluation at The Mars Society's Mars Desert Research Station in 2014 and remains involved in analogue mission studies today. Paul has been interested in science outreach and communication over the years which in the past included maintaining a personal blog on space exploration from high school through his undergraduate career and in recent years he has given talks at schools and other organizations over the topics of geology and space. He is excited to bring his experience as a geologist and scientist to the Spaceflight Insider team writing primarily on space science topics.