planetary science

Mars’ water is being skimmed off the top. NASA’S MAVEN spacecraft found water lofted into Mars’ upper atmosphere, where its hydrogen and oxygen atoms are ripped apart, scientists report in the Nov. 13 Science.
“This completely changes how we thought hydrogen, in particular, was being lost to space,” says planetary chemist Shane Stone of the University of Arizona in Tucson.
Mars’ surface was shaped by flowing water, but today the planet is an arid desert (SN: 12/8/14). Previously, scientists thought that Mars’ water was lost in a “slow and steady trickle,” as sunlight split water in the lower atmosphere and hydrogen gradually diffused upward, Stone says.
But MAVEN, which has been orbiting Mars since 2014, scooped up water molecules in Mars’ ionosphere, at altitudes of about 150 kilometers. That was surprising — previously the highest water had been seen was about 80 kilometers (SN: 1/22/18).
That high-up water varied in concentration as the seasons changed on Mars, with the peak in the southern summer, when seasonal dust storms are most frequent (SN: 7/14/20). During a global dust storm in 2018, water levels jumped even higher, suggesting dust storms lift water in a “sudden splash,” Stone says.
The top of Mars’ atmosphere is full of charged molecules that are primed for rapid chemical reactions, especially with water. So water up there is split apart quickly, on average lasting only four hours, leaving hydrogen atoms to float away (SN: 11/27/15). That process is 10 times faster than previously known ways for Mars to lose water, Stone and his colleagues calculated.
This process could account for Mars losing the equivalent of a 44-centimeter-deep global ocean in the past billion years, plus another 17-centimeter-deep ocean during each global dust storm, the team found. That can’t explain all of Mars’ water loss, but it’s a start. More

Detecting phosphine in Venus’ atmosphere made headlines, but reanalyses and new searches call into question the original discovery of the molecule. More

Comet 67P/Churyumov-Gerasimenko has its own version of the northern lights.
Observations taken by the Rosetta spacecraft reveal the comet’s aurora, which — unlike Earth’s eye-catching light shows — shimmers in invisible ultraviolet light, researchers report online September 21 in Nature Astronomy. Comet 67P joins comet C/Hyakutake 1996 B2, Mars (SN: 3/19/15), Saturn (SN: 4/6/20) and moons of Jupiter as known hosts of extraterrestrial auroras.
Electrons in the solar wind — a stream of charged particles continually flowing from the sun — interact with the gas surrounding 67P to create the auroral glow, planetary scientist Marina Galand of Imperial College London and colleagues report. Solar wind electrons are drawn toward the comet by an electric field surrounding 67P, similar to the way electrons cascade into Earth’s atmosphere to produce the northern and southern lights (SN: 7/25/14).
Electrons strike oxygen in Earth’s atmosphere to paint the sky red and green. But solar wind electrons strike water molecules in 67P’s coma, or shroud of gas. That shatters the water molecules and makes some of the resulting oxygen and hydrogen atoms glow ultraviolet. A similar water-smashing interaction creates auroras on Jupiter’s moons Europa and Ganymede (SN: 3/12/15).
Also unlike Earth, 67P has no magnetic field to steer incoming electrons toward the poles and form auroras with distinct patterns in the sky (SN: 2/7/20). If 67P’s ultraviolet aurora were visible, it would look like a diffuse halo around the comet.
Such cometary auroras could someday be used to probe variations in the solar wind, Galand says. That may lead to better forecasts for space weather, which can mess with satellites and power grids (SN: 7/5/18). More

Earth’s deep stores of water may have been locally sourced rather than trucked in from far-flung regions of the solar system.
A new analysis of meteorites from the inner solar system — home to the four rocky planets — suggests that Earth’s building blocks delivered enough water to account for all the H2O buried within the planet. What’s more, the water produced by the local primordial building material likely shares a close chemical kinship with Earth’s deep-water reserves, thus strengthening the connection, researchers report in the Aug. 28 Science.
Earth is thought to have been born in an interplanetary desert, too close to the sun for water ice to survive. Many researchers suspect that ocean water got delivered toward the end of Earth’s formation by ice-laden asteroids that wandered in from cooler, more distant regions of the solar system (SN: 5/6/15). But the ocean isn’t the planet’s largest water reservoir. Researchers estimate that Earth’s interior holds several times as much water as is found at the surface.
To test whether or not the material that formed Earth could have delivered this deep water, cosmochemist Laurette Piani of the University of Lorraine in Vandœuvre-lès-Nancy, France, and colleagues analyzed meteorites known as enstatite chondrites. Thanks to many chemical similarities with Earth rocks, these relatively rare meteorites are widely thought to be good analogs of the dust and space rocks from the inner solar system that formed Earth’s building blocks, Piani says.

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She and her team measured the abundance of hydrogen in these meteorites — a proxy for how much H2O they could produce — and calculated that local interplanetary debris had the potential to deliver at least three times as much water as is found in all the oceans. The meteorites don’t contain water, Piani says. Rather, they house enough of the raw ingredients to create water when heated.
In the meteorites, the team also found a close match to the type of water found in Earth’s mantle. A smattering of all water molecules on Earth contain a heavy variant of hydrogen known as deuterium. The ratio of deuterium to hydrogen in the enstatite chondrites lies within the range measured in Earth’s deep water. That similarity, the team argues, makes a strong case for local building blocks being the source of much of the planet’s water.
“This work is something I wanted to do myself or had been waiting for someone to do,” says Lydia Hallis, a planetary scientist at the University of Glasgow in Scotland. In 2015, she led a team that measured the deuterium abundance in lava plumes that tap deep into Earth’s mantle (SN: 11/12/15). “I’m really happy that [the new data] sits within the region where our previous data from deep mantle samples is sitting.”
Hallis and others stress that these new measurements are difficult. Once the meteorites hit the ground, they quickly absorb hydrogen from Earth’s environment. “They did a really good job of picking the right meteorites and making the right measurements,” she says. “This is pretty convincing that this hydrogen that’s measured is from the enstatite chondrites rather than from terrestrial contamination.”
The enstatite chondrites could have also contributed a lot of water to the oceans as well — but they are not the full story. The deuterium-hydrogen ratio in ocean water, which is a bit higher than that of mantle water, is better matched to the ratio found in icy asteroids from the outer solar system. “We still need a bit of water coming from the outer solar system,” Piani says. So, while local materials may have delivered the bulk of Earth’s water, the oceans were likely topped off a bit later by collisions with remote space rocks. More

It takes a certain amount of heat to keep an ocean wet. For Jupiter’s largest moons, a new analysis suggests a surprising source for some of that heat: each other.
Three of the gas giant’s four largest moons, Ganymede, Callisto and Europa, are thought to harbor oceans of liquid water beneath their icy shells (SN: 5/14/18). The fourth, the volcanic moon Io, may contain an inner magma ocean (SN: 8/6/14).
One of the primary explanations for how these small worlds stay warm enough to harbor liquid water or magma is gravitational kneading, or tidal forces, from their giant planetary host. Jupiter’s huge mass stretches and squishes the moons as they orbit, which creates friction and generates heat.
But no studies had seriously considered how much heat the moons could get from gravitationally squishing each other.

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“Because [the moons are] so much smaller than Jupiter, you’d think basically the tides raised by Io on Europa are just so small that they’re not even worth thinking about,” says planetary scientist Hamish Hay of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
Together with planetary scientists Antony Trinh and Isamu Matsuyama, both of the University of Arizona in Tucson, Hay calculated the size of the tides that Jupiter’s moons would raise on each other’s oceans. The team reported the results July 19 in Geophysical Research Letters.
The researchers found that the significance of the tides depends on how thick the ocean is. But with the right-sized ocean, neighboring moons could push and pull tidal waves on each other at the right frequency to build resonance. It’s a similar effect to pumping your legs on a swing, or synchronized footfalls making a bridge wobble, Hay says.
“When you get into one of these resonances, those tidal waves start to get bigger,” he says. Those waves would then rush around the moon’s interior and generate heat through friction, the researchers calculated. If the conditions are right, heat from the gushing tidal waves could exceed heat from Jupiter.
The effect was biggest between Io and Europa, the team found.
“Basically everyone neglected these moon-moon effects,” says planetary scientist Cynthia Phillips of NASA’s Jet Propulsion Laboratory, who was not involved in the new work. “I was just astonished … at the amount of heating” that the moons may give each other, she says.
The extra infusion of energy into Europa’s ocean could be good news for the possibility of alien life. Europa’s subsurface ocean is thought to be one of the best places in the solar system to look for extraterrestrial life (SN: 4/8/20). But anything living needs fuel, and the sun is too far away to be useful, Phillips says.
“You have to find other sources of energy,” she says. “Any kind of frictional or heating energy is really exciting for life.” More

NASA’s Perseverance rover took off at 7:50 a.m. EDT on July 30 from Cape Canaveral, Fla., and is now on its way to Mars with a suite of instruments designed to search for ancient life. The launch is the third this month of spacecraft en route to the Red Planet.
This is the 22nd spacecraft NASA has aimed at Mars (16 of those missions were successful). But Perseverance will be the first mission to cache rock samples from the Red Planet for a future mission to bring back to Earth.
It will also be the first NASA mission in more than 40 years to directly search for life on Mars. The rover will land in a region called Jezero crater (SN: 7/28/20). That crater was once an ancient lake bed, and scientists think its rocks and sediments could preserve signs of life, if life was ever there (SN: 7/29/20). The spacecraft will take video and audio recordings of its own landing as it touches down — another first for a NASA Mars mission.
“This mission has more cameras on it than any we’ve ever sent before,” said Lori Glaze, director of NASA’s Planetary Science Division, on July 30 during a news conference. “It’s going to feel like we’re actually there, riding along with Perseverance on the way down.”
Perseverance, shown here in an artist’s illustration, will seek signs that Mars once hosted alien life.JPL-Caltech/NASA
Mars launches tend to come in clumps thanks to Mars’ and Earth’s orbits. The planets line up on the same side of the sun every two years, so scientists have narrow windows to launch for the most efficient trip. All three of this year’s missions will arrive in February 2021.
The other missions launched in July represent firsts for their respective countries. The United Arab Emirates’ first interplanetary mission, which carries an orbiter called the Hope Probe, launched from Japan on July 19. Hope will measure Mars’ weather, from daily temperature changes to the significance of dust in the planet’s atmosphere (SN: 7/14/20).

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Next up was China’s first Mars mission, Tianwen-1, which means “questions to heaven” and launched on July 23. China has previously sent spacecraft to orbit and land on the moon (SN: 1/3/19). And it is the first nation to send an orbiter, lander and rover all at once on its first attempt to reach Mars. “No planetary missions have ever been implemented in this way,” mission scientists wrote July 13 in Nature Astronomy. “If successful, it would signify a major technical breakthrough.”
Tianwen-1’s lander and rover will touch down in Utopia Planitia in April 2021. Instruments on the rover and lander will test Mars’ soil composition and magnetic and gravitational fields and will probe Mars’ interior.
Utopia Planitia is the same region where the first long-lived Mars lander, NASA’s Viking 1, touched down in 1976 (SN: 7/20/16). Viking was the first spacecraft to search for life on Mars, but its results were inconclusive. Perhaps with the rush of spacecraft this year, and the plans to bring red rocks home, scientists will finally learn whether Mars ever did — or does — host alien life. More

NASA’s next rover is a connoisseur of Martian rocks. The main job of the Perseverance rover, set to launch between July 20 and August 11, is to pick out rocks that might preserve signs of past life and store the samples for a future mission back to Earth.
“We’re giving a gift to the future,” says planetary scientist Adrian Brown, who works at NASA Headquarters in Washington, D.C.
Most of the rover’s seven sets of scientific instruments work in service of that goal, including zoomable cameras to pick out the best rocks from afar and lasers and spectrometers to identify a rock’s makeup. After the rover lands in February 2021, it’s capable of collecting and storing 20 samples within the first Martian year (about two Earth years). The NASA team plans to collect at least 30 samples over the whole mission, says planetary scientist Katie Stack Morgan of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
Fortunately, Perseverance is headed to a spot that should be full of collection-worthy rocks. The landing site in Jezero crater, just north of the Martian equator, contains an ancient river delta that looks like it once carried water and silt into a long-lived lake.

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“We can already predict which parts of that delta might give us the highest return for possible biosignatures,” Stack Morgan says. The crater has a “bathtub ring” of carbonates, minerals that settle in shallow, warm waters that are especially good at preserving signs of life. “That makes Jezero special,” she says.
But Perseverance is more than a rock collector. The rover will probe the ground beneath its wheels, fly a helicopter, track the weather and test tech for turning Martian air into rocket fuel. Every part of the rover has a job to do.

RIMFAX
RIMFAX, or Radar Imager for Mars’ Subsurface Experiment, will use radio waves to probe the ground under the rover’s wheels. The instrument will take a measurement every 10 centimeters along the rover’s track and should be able to sense 10 meters deep, depending on what’s down there. The InSight lander, currently on Mars, has a seismometer that listens for Marsquakes, but a ground-penetrating radar to understand the Martian interior is a first.

MOXIE
Human explorers will need oxygen on Mars, but not just for breathing, says former astronaut Jeffrey Hoffman. “It’s for the rocket,” says Hoffman, now an engineer at MIT. To take off from the Martian surface and return home, astronauts will need liquid oxygen rocket fuel. Bringing all that fuel from Earth is not an option.
To demonstrate how to make fuel from scratch, MOXIE, or Mars Oxygen In-Situ Resource Utilization Experiment, will pull carbon dioxide out of the Martian atmosphere and convert it to oxygen. MOXIE will produce about 10 grams of oxygen per hour, which is only about 0.5 percent of what’s needed to make enough fuel for a human mission over the 26 months between launch windows. But the effort will teach engineers on Earth how to scale up the technology.

Mastcam-Z
Set atop Perseverance’s neck, Mastcam-Z, the rover’s main set of eyes, can swivel 360 degrees laterally and 180 degrees up and down to view the surrounding landscape. Like its predecessor on the Curiosity rover, the camera will take color, 3-D and panoramic images to help scientists understand the terrain and the mineralogy of the surrounding rocks. Mastcam-Z can also zoom in on distant features — a first for a Mars rover.

SuperCam
How can Perseverance look for signs of ancient microbes in rocks too far away to touch? Enter SuperCam, a laser spectrometer mounted on the rover’s head. SuperCam will shoot rocks with a laser from more than seven meters away, vaporizing a tiny bit of the minerals. Researchers will then analyze the vapor to help figure out what the rocks are made of, without having to drive the rover down steep slopes or up rugged crags. The laser will also measure properties of the Martian atmosphere and dust to refine weather models.

MEDA
MEDA, or Mars Environmental Dynamics Analyzer, is the rover’s weather station. Six instruments distributed across the neck, body and interior will measure air temperature, air pressure, humidity, radiation and wind speed and direction. The tools will also analyze the physical characteristics of the all-important Mars dust. Scientists hope to use the information from these sensors to better predict Mars weather.

PIXL, SHERLOC and WATSON
Geologists never go into the field without a hand lens. Likewise, Perseverance will be prepared with three arm-mounted magnifying instruments. PIXL, the Planetary Instrument for X-ray Lithochemistry, will have a camera that can resolve grains of Martian rock and dirt to scales smaller than a millimeter. It will also detect the chemical makeup of those rocks by zapping them with X-rays and measuring the wavelength of light the rocks emit in response. SHERLOC, or Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals, will take similar measurements using an ultraviolet laser. WATSON, the Wide Angle Topographic Sensor for Operations and Engineering, will take pictures with a resolution of 30 micro­meters to put the chemistry in context. The instruments will seek signs of ancient microbes preserved in Martian rocks and soil, and help scientists decide which rocks to store for a future mission to return to Earth.
Ingenuity
This helicopter will be a test case for future reconnaissance missions to help the rover see further on Mars.JPL-Caltech/NASA
Perseverance will also carry a stowaway folded up origami-style in a protective shield the size of a pizza box: a helicopter called Ingenuity. At a smooth, flat spot, Ingenuity will drop to the ground and unfold, then take about five flights in 30 Martian days. These flights are mainly to show that the copter can get enough lift in the thin Martian atmosphere. If Ingenuity is successful, future helicopters might help run reconnaissance for rovers. “There’s always a question with the rover, what’s over that cliff? What’s over that rise?” says planetary scientist Briony Horgan of Purdue University in West Lafayette, Ind. “If you have a helicopter, you can see those things ahead of time.”

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It started with a spring breeze. The Opportunity rover watched with its robotic eyes as the wind blowing through Perseverance Valley kicked puffs of rusty Mars dust into the air. In more than 14 Earth years of exploring the Red Planet, the rover had seen plenty of this kind of weather.
But the dust grew thicker. Small flecks swirled like wildfire smoke through the atmosphere, turning sun-filled midday into dusk, then night. Within a week, the dust storm spanned more than twice the area of the contiguous United States and eventually encircled the whole planet, allowing just 5 percent of the normal amount of light to reach Opportunity’s solar panels. The rover went quiet.
“It got so bad so quickly, we didn’t even have time to react,” says Keri Bean of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Bean had joined Opportunity’s rover-operating team just before that May 2018 storm.
Dust storms like that one, which snuffed out Opportunity for good, are the most dramatic and least predictable events on the Red Planet (SN: 3/16/19, p. 7). Such storms can make the nail-biting process of landing on Mars even more dangerous and could certainly make life difficult for future human explorers.

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Despite almost 50 years of study, scientists are missing some key data that would help explain how dust gets kicked into the air to form planet-wide storms and what keeps it circulating for weeks or months at a time.
“We just do not understand how dust storms form on Mars,” says planetary meteorologist Scott Guzewich of NASA’s Goddard Space Flight Center in Greenbelt, Md. History has shown that certain regions and seasons are more prone to dust than others. “Other than that, we’re … blind.”
Mars missions set to launch this summer, from the United States, China and the United Arab Emirates, will help solve that pressing mystery. NASA’s new rover, Perseverance, will carry a suite of weather sensors called MEDA, for Mars Environmental Dynamics Analyzer. Those sensors will build on decades of Mars exploration and fill in missing puzzle pieces.
“Predicting dust is the ultimate goal” for MEDA, says planetary scientist Germán Martínez of the Lunar and Planetary Institute in Houston. The data MEDA will collect will be “the most substantial contribution to this topic so far.”
Dust, dust everywhere
Dust is as important to weather on Mars as water is on Earth. With no oceans, scant water vapor and a thin atmosphere, Martian weather can be monotonously calm for about half the Martian year, which lasts close to 687 Earth days. But when the Red Planet’s orbit brings it closer to the sun, dust storm season begins.
In the 10-month dusty season, which corresponds to spring and summer in the southern hemisphere, extra sunlight warms the atmosphere. That warmth generates strong winds as air moves from warm to cool regions. Those winds lift more dust, which absorbs sunlight and warms the atmosphere, generating still stronger winds, which lift even more dust.

The storms come in a range of sizes: Local storms can cover an area about the size of Alaska and last up to three Martian days (each of which lasts about 24.5 hours); global storms can engulf the planet for months. The storm that defeated Opportunity raged from the end of May through late July. Such global storms probably result when several smaller storms merge.
Global dust storms have affected Mars exploration since the arrival of the first long-term robotic visitor in 1971, when NASA’s Mariner 9 orbiter found the planet’s surface entirely obscured. Opportunity and its twin rover, Spirit, both survived a global dust storm in 2007, yet a large regional dust storm ended the Phoenix lander’s mission in 2008.
There has never been a Mars mission that didn’t worry about dust.
A farmer’s almanac
Luckily, Mariner 9 was an orbiter, with no plans to land. It just had to wait for the skies to clear to start snapping pictures of the Martian surface. But the same 1971 storm is probably to blame for vanquishing two Soviet landers that arrived at almost the same time.
Spacecraft that must land to do their work can’t just wait for better timing. Launch windows for missions between Earth and Mars open only every 26 months or so. Engineers who design landing systems need to know what conditions a spacecraft will face when it gets there, says Allen Chen of the Jet Propulsion Lab, who leads the entry, descent and landing for Perseverance.
The most important factor is the density of the atmosphere. Even though Mars’ atmosphere exerts just 1 percent of the pressure of Earth’s on the planet’s surface, both the thin Martian air and the wind blowing through it slow down the spacecraft and affect where it lands, Chen says.
Perseverance will take pictures of the ground while parachuting through the atmosphere and match the images to an onboard map made with images from NASA’s Mars Reconnaissance Orbiter. Based on those details, an in-flight navigation system will steer the rover to a safe landing spot, helping the rover touch down within an area 25 kilometers wide — the most precise Mars landing ever.
“But that’s dependent on being able to see the ground,” Chen says, without dust obscuring the view.
To land a rover, engineers like Chen rely on forecasts that use the past to tell the future — ​similar to weather forecasts on Earth, but with less data. Atmospheric scientist Bruce Cantor of Malin Space Science Systems in San Diego, a self-described Mars weatherman, put out a Mars weather report every week until September 2019. His forecasts are based on statistics and historical data, mostly taken from orbit. “It’s almost like a farmer’s almanac in my head,” he says.
Cantor’s forecasts for Mars landings since 1999 have been “pretty accurate,” he says, and he boasts that he predicted the storm that ended the Phoenix mission to within three days. More accuracy wouldn’t have saved Phoenix, he says. The lander’s batteries were already low from low winter sunlight levels and the buildup of dust on the solar panels. “It was just a matter of what storm was going to be the mission-ending one,” he says.
He foresees clear skies for Perseverance’s touchdown in February 2021. Based on the season and weather patterns in the past, the probability of a dust storm hitting within 1,000 kilometers of the center of Perseverance’s landing area is less than 2 percent, Cantor and colleagues reported in the journal Icarus in March 2019.
But just in case, Chen’s team trained the navigation system to “deal with it being pretty darn dusty,” Chen says.
A constellation of weather stations
As Mars missions get more complex, and especially as NASA and other groups contemplate sending human explorers, being able to prepare for dust storms takes on extra urgency.
“Someday, somebody is going to go to Mars, and they’re going to want to know when and where storms occur,” Cantor says. “That’s when this stuff becomes really important.”
Cantor would know. Well over a decade ago, while testing a different rover system in Southern California, he jumped into a 2-meter-tall dust devil just to see what it would feel like. “Not one of my smartest moves,” he says. He wasn’t injured, but “it did not feel good. It felt like getting sandblasted.”
Martian astronauts would be protected by more than shorts and a T-shirt, but dust could easily invade human habitats and clog air filters — or damage astronauts’ lungs if they breathe it in. The dust may even carry poisonous and carcinogenic materials that could make astronauts ill over the course of a mission.
Astronauts will need to know when to stay inside. Part of the problem in predicting storms is a sheer lack of data. For Earth’s weather, meteorologists use thousands of ground-based weather stations, plus data from satellites, balloons and airplanes. Mars has only six active satellites, run by NASA and the European and Indian space agencies. And just two sets of weather instruments report from Mars’ surface: one on the Curiosity rover, which has been collecting data since 2012 (SN: 5/2/15, p. 24), and a nearly identical set that arrived with the InSight lander in 2018.

But those two spacecraft are practically neighbors, a big weakness for understanding the whole planet. “It’d be like having one of your weather stations in D.C. and the other in Buffalo,” Guzewich says.
Perseverance will help fill in the gaps. So might China’s first Mars rover, Tianwen-1, set to launch in July with an instrument to measure air temperature, pressure and wind. The Russian and European ExoMars mission, scheduled to launch in 2022, includes a lander called Kazachok equipped with meteorology and dust sensors (SN Online: 3/12/20).
From the air, the UAE’s Emirates Mars Mission, known as Hope, will observe weather, including storms, and how the atmosphere interacts with the ground. Over one Martian year in orbit, Hope will help build a global picture of how the atmosphere changes day to day and between the seasons.
Just having a few more weather stations will be a big boost, says José A. Rodríguez Manfredi of the Center for Astrobiology in Madrid, principal investigator for MEDA, the weather sensors on Perseverance. “We will have a mini network working on Mars in a few years.”
But four or five weather stations on the ground probably won’t be enough. To reliably predict dust storms, what Mars scientists need is a global network collecting data all the time.
To cut down on the cost of such a network, Guzewich suggests figuring out which measurements “would give us the most bang for our buck.” For Earth, NASA and other agencies use a type of study called an Observing System Simulation Experiment to figure out which variables are most important for predicting the weather. Satellites are then designed to focus on those most valuable observations. Such a study has never been done for Mars, but the only obstacle is funding, Guzewich says.
“Mars atmospheric scientists have been clamoring” for such experiments, he says. “We’re not going to reproduce Earth’s observing network before humans go to Mars. It’s not going to happen…. But maybe we could do something that is financially and technologically reasonable that really does make a difference and gets us to the point where we can predict the future a couple days in advance.”
China’s space agency plans to launch its first Mars mission, called Tianwen-1, in July. Its rover (illustrated atop the lander) will measure air temperature, pressure and wind, among other things.Xinhua
Blowing in the wind
Mars forecasts also suffer from a lack of fundamental information, Martínez says. How hard does the wind have to blow to lift the dust? And what does the dust do once it’s airborne?
This is where Perseverance will shine. The rover will make the best direct measurements yet of wind speed and direction on Mars, especially the vertical wind that lifts dust upward.
For a long time, scientists struggled to understand how dust was lifted into the air at all. “It seemed like it couldn’t be possible,” Guzewich says. “The atmosphere is so thin, a single particle of dust or sand is so heavy, it just shouldn’t work.” Observations and experiments over the last 20 years suggest that once sand grains start bouncing along the surface, they can knock into other grains and knock smaller particles upward. But it’s still not possible to tell which of those bouncing grains will lead to a storm — or which of those storms will go global.
Mars climatologists have tried to make detailed wind measurements for decades, Martínez says, but have hit several stretches of bad luck. Only five surface missions — the Viking 1 and 2 landers in 1976, the Pathfinder lander in 1997 and the ongoing Curiosity and InSight missions — have provided useful data on wind speed and direction near the surface. And even those have had mixed results.
NASA’s InSight lander, shown here in a mosaic of selfies the spacecraft took, carries a set of weather sensors called TWINS, or Temperature and Wind for InSight. The lander is one of just two weather stations on the Martian surface. Mars atmospheric scientists say they need more to predict dangerous dust storms.JPL-Caltech/NASA
“Arguably, the best wind record on Mars is still the one from the Vikings, 40 years ago,” Martínez says. Curiosity was supposed to take direct wind measurements in all directions with a pair of electrically heated booms that jutted away from the rover’s neck. “We had great expectations,” Martínez says.
But photos the rover took of itself showed that one boom was damaged as the rover landed, and out of commission. For the first 1,490 Martian days of Curiosity’s mission, the rover could take measurements only when the wind was blowing head on. Then, in October 2016, the second boom broke. In April, researchers suggested a way to hack Curiosity’s temperature sensors to get wind data, but there’s no plan to use that hack at the moment, Guzewich says.
That leaves InSight, but its wind readings are muddled by other parts of the lander getting in the way of airflow. The readings are still useful, but the MEDA team hopes to do better.
Taking lessons from InSight and Curiosity, Perseverance’s MEDA will have more wind sensors that reach farther from the rover’s body. The sensors will be protected by a shield until after the rover has landed safely.
“We are very excited,” Martínez says. “The vertical wind has never been measured before on Mars. We’re going to do that.”
Measuring wind speeds will help scientists determine how hard the wind must blow to kick up dust, the first step in triggering a dust storm.
That figure has personal resonance for Bean, the former Opportunity rover operator. Her first shift was exactly two weeks before the mission-ending global dust storm. She told the rover to use its arm to brush the surface of a rock.
“My coworkers blamed me for starting a whole butterfly effect,” she says. “You brushed the surface,” they joked, “the dust went up, you started the whole dust storm.”
In its end-of-mission report, the Opportunity team admits it will never really know what ended Opportunity’s nearly 15-year run. One possibility is that the dust grew too thick on the solar panels for gentle wind in the calm season to blow the dust off.
One potential fix would be to design future rovers to vibrate their solar panels fast enough to make dust skitter off, Bean says. Once humans are on the planet, they could just clear dust with their arms.
A week or so before Opportunity was officially declared lost, Bean decided to memorialize the rover. “I’d always liked tattoos, but nothing ever spoke to me,” she says. In college, she had studied Mars’ atmospheric opacity — the amount of light that can penetrate an atmosphere’s dust, represented by the Greek letter τ. So Bean got a tattoo on her arm of the last measurement Opportunity sent to Earth: “τ=10.8.” That stands for a night-dark sky in the middle of the day.

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