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Venus’ clouds appear to contain a smelly, toxic gas that could be produced by bacteria, a new study suggests.

Chemical signs of the gas phosphine have been spotted in observations of the Venusian atmosphere, researchers report September 14 in Nature Astronomy. Examining the atmosphere in millimeter wavelengths of light showed that the planet’s clouds appear to contain up to 20 parts per billion of phosphine — enough that something must be actively producing it, the researchers say. 

If the discovery holds up, and if no other explanations for the gas are found, then the hellish planet next door could be the first to yield signs of extraterrestrial life — though those are very big ifs.

“We’re not saying it’s life,” says astronomer Jane Greaves of Cardiff University in Wales. “We’re saying it’s a possible sign of life.”

Venus has roughly the same mass and size as Earth, so, from far away, the neighboring planet might look like a habitable world (SN: 10/4/19). But up close, Venus is a scorching hellscape with sulfuric acid rain and crushing atmospheric pressures.

Still, Venus might have been more hospitable in the recent past (SN: 8/26/16). And the current harsh conditions haven’t stopped astrobiologists from speculating about niches on Venus where present-day life could hang on, such as the temperate cloud decks.

“Fifty kilometers above the surface of Venus, the conditions are what you would find if you walk out of your door right now,” at least in terms of atmospheric pressure and temperature, says planetary scientist Sanjay Limaye of the University of Wisconsin–Madison, who was not involved in the new study. The chemistry is alien, but “that’s a hospitable environment for life.”

Previous work led by astrochemist Clara Sousa-Silva at MIT suggested that phosphine could be a promising biosignature, a chemical signature of life that can be detected in the atmospheres of other planets using Earth-based or space telescopes.

On Earth, phosphine is associated with microbes or industrial activity — although that doesn’t mean it’s pleasant. “It’s a horrific molecule. It’s terrifying,” Sousa-Silva says. For most Earthly life, phosphine is poisonous because “it interferes with oxygen metabolism in a variety of macabre ways.” For anaerobic life, which does not use oxygen, “phosphine is not so evil,” Sousa-Silva says. Anaerobic microbes living in such places as sewage, swamps and the intestinal tracts of animals from penguins to people are the only known life-forms on Earth that produce the molecule.  

Still, when Greaves and colleagues searched Venus’ skies for signs of phosphine, the researchers didn’t expect to actually find any. Greaves looked at Venus with the James Clerk Maxwell Telescope in Hawaii over five mornings in June 2017, aiming to set a detectability benchmark for future studies seeking the gas in the atmospheres of exoplanets (SN: 5/4/20), but was startled to find the hints of phosphine. “That’s a complete surprise,” Greaves says. When she was analyzing the observations, “I thought ‘Oh, I must have done it wrong.’”

So the team checked again with a more powerful telescope, the Atacama Large Millimeter/submillimeter Array in Chile, in March 2019. But the signature of phosphine — seen as a dip in the spectrum of light at about 1.12 millimeters — was still there. The gas absorbs light in that wavelength. Some other molecules also absorb light near that wavelength, but those either couldn’t explain the whole signal or seemed improbable, Greaves says. “One of those is a plastic,” she says. “I think a floating plastic factory is a less plausible explanation than just saying there’s phosphine.”

Phosphine takes a fair amount of energy to create and is easily destroyed by sunlight or sulfuric acid, which is found in Venus’ atmosphere. So if the gas was produced a long time ago, it shouldn’t still be detectable. “There has to be a source,” Greaves says.

Greaves, Sousa-Silva and colleagues considered every explanation they could think of apart from life: atmospheric chemistry; ground and subsurface chemistry; volcanoes outgassing phosphine from the Venusian interior; meteorites peppering the atmosphere with phosphine from the outside; lightning; solar wind; tectonic plates sliding against each other. Some of those processes could produce trace amounts of phosphine, the team found, but orders of magnitude less than the team detected.

“We’re at the end of our rope,” Sousa-Silva says. She hopes other scientists will come up with other explanations. “I’m curious what kind of exotic geochemistry people will come up with to explain this abiotically.”

The idea of searching for life on Venus “has been regarded as a pretty out-there concept,” says Planetary Science Institute astrobiologist David Grinspoon, who is based in Washington, D.C. Grinspoon has been publishing about the prospects for life on Venus since 1997, but was not involved in the new discovery.

“So now I hear about this, and I’m delighted,” he says. “Not because I want to declare victory and say this is definite evidence of life on Venus. It’s not. But it’s an intriguing signature that could be a sign of life on Venus. And it obligates us to go investigate further.”

Because of the planet’s acidic atmosphere, extreme pressures and lead-melting temperatures, sending spacecraft to Venus is a challenge (SN: 2/13/18). But several space agencies are considering missions that could fly in the next few decades.

In the meantime, Greaves and colleagues want to confirm the new phosphine detection in other wavelengths of light. Observations they had planned for the spring were put on hold by the coronavirus pandemic. And now, Venus is in a part of its orbit where it’s on the other side of the sun.

“Maybe when Venus comes around on the other side of the sun again,” Greaves says, “things will be better for us here on Earth.”

About seven years ago, Kristin and Josh Mohagen were honeymooning in Napa Valley in California, when they smelled something surprising in their glasses of Cabernet Sauvignon: green pepper. A vintner explained that the grapes in that bottle had ripened on a hillside alongside a field of green peppers. “That was my first experience with terroir,” Josh Mohagen says.

It made an impression. Inspired by their time in Napa, the Mohagens returned home to Fergus Falls, Minn., and launched a chocolate business based on the principle of terroir, often defined as “sense of place.”

Different countries produce cocoa with distinct flavors and aromas, Kristin Mohagen says. Cocoa from Madagascar “has a really bright berry flavor, maybe raspberry, maybe citrus,” she says, while cocoa from the Dominican Republic “has a little more nutty, chocolaty taste.”

The couple estimates that back in 2013, when they founded Terroir Chocolate, about 50 other small batch chocolate companies in the United States were also touting terroir as integral to their products’ flavors.

Since then, terroir has continued to take hold as a marketing strategy — and not just for wine and chocolate. Terroir labels are also becoming more common for products like coffee, tea and craft beer, says Miguel Gómez, an economist at Cornell University who studies food marketing and distribution. Consumers “are increasingly interested in knowing where the products they are eating are produced — not only where but who is making them and how,” he says. People “value differences in the aromas, the flavors.”

The definition of terroir is somewhat fluid. Wine enthusiasts use the French term to describe the environmental conditions in which a grape is grown that give a wine its unique flavor. The soil, climate and even the orientation of a hillside or the company of neighboring plants, insects and microbes play a role. Some experts expand terroir to include specific cultural practices for growing and processing grapes that could also influence flavor.

The notion of terroir is quite old. In the Middle Ages, Cistercian and Benedictine monks in Burgundy, France, divided the countryside into climats, according to subtle differences in the landscape that seemed to translate into unique wine characteristics. Wines produced around the village of Gevrey-Chambertin, for example, “are famous for being fuller-bodied, powerful and more tannic than most,” says sommelier Joe Quinn, wine director of The Red Hen, a restaurant in Washington, D.C. “In contrast, the wines from the village of Chambolle-Musigny, just a few miles south, are widely considered to be more fine, delicate and light-bodied.”

Some scientists and wine experts are skeptical that place actually leaves a lasting imprint on taste. But a recent wave of scientific research suggests that the environment and production practices can, in fact, impart a chemical or microbial signature so distinctive that scientists can use the signature to trace food back to its origin. And in some cases, these techniques are beginning to offer clues on how terroir can shape the aroma and flavor of food and drink.

Coffee’s chemical fingerprint

Ecologist Jim Ehleringer of the University of Utah in Salt Lake City studies trace elements that plants passively take up. Those elements are a direct reflection of the soil. “Trace elements do not decay and so they become characteristic of a soil type and persist over time,” Ehleringer says.

To see if they could trace a coffee to its origin using the coffee’s blend of trace elements, Ehleringer and his team recently measured the concentrations of about 40 trace elements in more than four dozen samples of roasted arabica coffee beans from 21 countries. Roasting beans to different temperatures can affect the concentrations of individual elements. To correct for this roasting effect, Ehleringer calculated the ratio of each element to every other element in a sample, which remains fairly constant, even with roasting.  

In the Aug. 1 issue of Food Chemistry, his team reports that coffee beans from different regions can have distinct chemical fingerprints. A coffee’s chemical quality “comes down to geology,” Ehleringer says. Three samples of coffee beans from Yemen, for example, had a ratio of boron to manganese that was shared by less than 0.5 percent of coffee samples grown elsewhere.

Other researchers have used similar elemental analyses to find chemical signatures of place in products ranging from wines produced in distinct growing regions in Portugal to peanuts grown in different provinces in China.

The technique is valuable for validating origin when terroir is part of a product’s allure. Coffee farmers in Kona on Hawaii’s Big Island, for example, are using the results of an elemental analysis to support a class action lawsuit, scheduled for trial in November, against 21 major retailers. The suit claims those companies falsely market their coffees as “Kona” when the beans were actually grown elsewhere.

While an elemental analysis can authenticate a product’s terroir, it does not suggest that geology shapes flavor. Trace elements alone, says Ehleringer, “impart no flavor or taste.”

Tracking cocoa to its source

To try to link flavor to place, some scientists go after different chemical signatures altogether. At Towson University in Maryland, chemist Shannon Stitzel is tracing cocoa to its roots using organic compounds, which are mostly produced by the cocoa plant itself. The concentration of specific organic compounds in a plant can result from a complex mix of interacting factors — from the genes of a particular variety to components of terroir like climate and agricultural practices.

Stitzel works with samples of cocoa liquor — cocoa beans that have been fermented, dried, roasted and ground into a paste — from across the globe. At room temperature, cocoa liquor is a solid. But with a bit of heat, the paste melts into a glossy liquid that Stitzel describes as “a little thicker than honey.”

Using organic compounds to assign the cocoa liquor samples to their countries of origin is “not nearly as clean as when you do it with elemental analysis,” she says. In unpublished work, she was able to use an elemental analysis to accurately link cocoa liquor to its country of origin about 97 percent of the time.

But Stitzel turned to organic compounds because their presence may ultimately help explain the flavor differences that she, like the Mohagens, thinks very clearly exist between cocoa liquors from different countries. “You can open up each of the containers and the aroma is entirely different,” she says.

Stitzel recently identified concentrations of organic compounds in cocoa liquor from Vietnam, Indonesia, Honduras, Ecuador and Mexico. She then used a statistical technique known as a discriminant analysis to group samples based on similar concentrations of nine organic compounds, including caffeine, a similar compound called theobromine and an antioxidant called epicatechin.  

On the American Chemical Society’s SciMeetings online platform in April, Stitzel reported that this chemical fingerprint was enough to accurately identify the correct country of origin for about 90 percent of the samples. In some cases, however, the samples didn’t form neat groups by country. Cocoa liquor samples from Honduras formed two different groups, depending on roasting temperature. Samples in the Honduras group that were roasted at the highest temperature were hard to tell apart from samples from Ecuador and Vietnam.

Stitzel now wants to add more compounds to the analysis to boost her sourcing accuracy and to connect regions to specific flavor compounds. “We’re still … trying to understand which compounds might be related to flavor,” she says. Her recent analysis already shows that caffeine, theobromine and epicatechin, which all produce a bitter flavor, can help set apart one country’s chocolates from another’s.

The aroma of place

Other researchers are finding that terroir leaves an imprint on the molecules that shape food’s aroma. Plants produce compounds known as aroma glycosides, which contain a sugar component linked to a volatile aromatic compound. When intact, aroma glycosides have no scent. But breaking the sugar-volatile bond — via high temperatures, low pH or enzymes from yeast — sets the volatile and its aroma free. The bouquet of a nicely aged bottle of wine is made up, in part, of aroma volatiles in the grapes that yeast enzymes let loose over time.

Many beer brewers, however, would rather your IPA have the same reliable flavor whether you pop open the bottle this Friday or in October. When volatile aromatics let loose in a bottled beer, that’s no good for large-volume brewers who need to ship consistent-tasting products. Brewers call that volatile release “beer creep,” says Paul Matthews, a senior research scientist in the Washington state branch of Hopsteiner, an international commercial hop grower and processor headquartered in New York City.

If brewers add hops (the flower of the hop plant) to beer early in the brewing cycle, heat breaks the sugar-volatile bond and the aroma from aroma glycosides is largely lost before bottling. The remaining flavor is more consistent over time. But when craft brewers make “dry hopped” beers like IPAs, adding the hops after the boiling stage, this late addition allows many aroma glycosides to go into fermentation and then into the bottle intact. The compounds release volatile aromatics as yeast enzymes break bonds even after the bottle is capped. So the aromas of these beers are more likely to “creep” over time.

Because genetics influences aroma and flavor, Matthews is exploring whether it’s possible to better control aroma glycoside concentrations through breeding. Breeding hop varieties to have lower concentrations could diminish the “beer creep” problem faced by large-volume craft brewers who distribute their beer over long distances.

At the same time, Matthews and colleagues are investigating the potential of breeding hop varieties to have higher aroma glycoside concentrations for use by smaller craft brewers, who are less concerned about shelf life but want to enhance the aroma of their beers.

Matthews recently tested whether aroma glycoside concentrations in individual hop cultivars are determined more by genetics or by terroir. “Of course, they are determined by both,” he says. “But if they are more genetic, we can breed for them.”

In collaboration with colleagues, including phytochemist Taylan Morcol of Lehman College in the Bronx, part of the City University of New York, Matthews grew the same 23 genetically distinct hop cultivars at two commercial fields with distinct terroirs. Matthews calls the Yakima Valley site in Washington state “desert in the shadow of Mount Rainier.” The other site, in the Kootenay River valley in Idaho, is “much more boreal — pine forest and humid,” he says.

At each location, the team measured the concentrations of four aroma glycosides in each hop cultivar. Genetics indeed played the biggest role in determining how much aroma glycosides a hop plant produces, the researchers report in the Aug. 15 Food Chemistry. The concentrations of three of the aroma glycosides differed across cultivar types but remained fairly similar within the same cultivar grown in the two locations.

But for one aroma glycoside, terroir trumped genes in a big way. At the Kootenay site, all of the cultivars produced low concentrations of hexyl glucoside, a molecule that gives off a grassy aroma when its sugar bond is broken. But at the Yakima site, every one of these same cultivars, with genetics matching the plants in Kootenay, produced about two to eight times as much hexyl glucoside.

“There is a terroir difference,” Matthews says. The team can’t yet pinpoint which component of terroir causes the spike in hexyl glucoside at the Yakima site. The best guess: mites and aphids.

At Yakima, those critters, which munch on the hop plants, hang around for a longer portion of the growing season than at the Kootenay site. Matthews and his colleagues hypothesize that the plants might produce hexyl glucoside chemicals as a defense against the pests. When a mite or aphid munches on the plant, the volatile may be released to attract insects that will eat the mites or aphids.

The researchers are planning a follow-up experiment to test whether hop plants exposed to these pests in environmentally controlled chambers produce more of this grassy hexyl glucoside than hop grown under the same environmentally controlled conditions without the pests.

Microbes leave their mark

People have understood the importance of yeast in wine fermentation for at least two centuries. About six years ago, food microbiologist David Mills of the University of California, Davis and graduate student Nicholas Bokulich, now a food microbiologist at ETH Zurich, discovered that groups of microbes may help shape the flavor of wine. Unique microbial communities in different California growing regions can predict which metabolites will be present in the finished wine, Mills, Bokulich and colleagues reported in 2016 in mBio. “Metabolites are any product of metabolism in any organism,” Bokulich says, adding that yeast, other fungi and bacteria each make varying contributions of metabolites in different wines.

“Those metabolites … have an aroma and a flavor,” says Kate Howell, a biochemist at the University of Melbourne in Australia. One of Howell’s own studies, she and her team reported online in August in mSphere, suggests that fungal species in particular shape the metabolites — and thus aroma and flavor — in wine from different growing regions in Australia.

Howell and colleagues studied microbes at 15 vineyards growing Pinot Noir grapes across six wine regions in southern Australia. At each vineyard, the team extracted fungal and bacterial DNA from the soil, as well as from what’s known as the “must” — destemmed, crushed grapes that haven’t yet been fermented. Then, the team identified 88 metabolites in the finished wine.

Different wine growing regions had distinct microbial communities in both the soil and the must, which appeared to influence the unique compositions of metabolites in the finished wine. The researchers found that over 80 percent of the metabolites found in the various wines were linked to the diversity of fungi found in the grape must. High levels of Penicillium fungi, for example, resulted in wine with low levels of octanoic acid, a volatile compound that can give wine a mushroom flavor.

Howell hopes vintners may someday be able to manage microbes in the soil and throughout the fermentation process to bring out the best of the local microbial terroir. Today, nearly all of the yeasts that vintners purchase to add to their grape must are isolated from French vineyards and other famous wine regions, she says. “That doesn’t present the same value of place as encouraging diversity in the fermentation in the place that the grapes were grown.”

For his part, Quinn, of The Red Hen, eagerly awaits more scientific explorations of terroir. He would especially like to know why wines produced from the limestone-dominated Kimmeridgian soils in Chablis, Sancerre and Champagne, France, all have a chalky, saltlike mineral taste. Scientific research helps explain how wine reflects its place, Quinn says, “from the climatic elements to the microbial elements, what the earth is saying, and why [a particular] wine is so delicious.”

New seafloor maps reveal the first clear view of a system of channels that may be helping to hasten the demise of West Antarctica’s vulnerable Thwaites Glacier. The channels are deeper and more complex than previously thought, and may be funneling warm ocean water all the way to the underside of the glacier, melting it from below, the researchers found.

Scientists estimate that meltwater from Florida-sized Thwaites Glacier is currently responsible for about 4 percent of global sea level rise (SN: 1/7/20). A complete collapse of the glacier, which some researchers estimate could happen within the next few decades, could increase sea levels by about 65 centimeters. How and when that collapse might occur is the subject of a five-year international collaborative research effort.

Glaciers like Thwaites are held back from sliding seaward both by buttressing ice shelves — tongues of floating ice that jut out into the sea — and by the shape of the seafloor itself, which can help pin the glacier’s ice in place (SN: 4/3/18). But in two new studies, published online September 9 in The Cryosphere, the researchers show how the relatively warm ocean waters may have a pathway straight to the glacier’s underbelly.

From January to March 2019 researchers used a variety of airborne and ship-based methods — including radar, sonar and gravity measurements — to examine the seafloor around the glacier and two neighboring ice shelves. From those data, the team was able to estimate how the seafloor is shaped beneath the ice itself.

These efforts revealed a rugged series of high ridges and deep troughs on the seafloor, varying between about 250 meters and 1,000 meters deep. In particular, one major channel, more than 800 meters deep, could be funneling warm water all the way from Pine Island Bay to the submerged edge of the glacier, the team found.

While searching for primate fossils in northern India, paleontologist Christopher Gilbert noticed something small and shiny poking out of the dirt. It turned out to be a roughly 13-million-year-old molar from a small-bodied ape related to modern gibbons.

The tooth is the oldest known fossil from a gibbon ancestor, says Gilbert, of Hunter College at the City University of New York. He and colleagues assigned the fossil, which was eroding out of previously dated sediment at a site called Ramnagar, to a new genus and species, Kapi ramnagarensis.

Until now, the oldest remains of an ancient gibbon species consisted of a small number of teeth found in China, which date from around 7 million to 9 million years ago. Possibly older fossils of a gibbonlike creature are controversial (SN: 10/29/15). Genetic studies of living primates have suggested that gibbon ancestors emerged by at least 20 million years ago in Africa.

After finding the Ramnagar molar in 2015, Gilbert’s team compared it with corresponding teeth of living and extinct apes and monkeys. Features including low, rounded cusps on the edges of the chewing surface link the ancient tooth to modern gibbons and the gibbon predecessor in China, the scientists report September 9 in Proceedings of the Royal Society B.

K. ramnagarensis comes from deposits that previously yielded fossils of an orangutan ancestor, suggesting to Gilbert that both apes reached South Asia from Africa around the same time. “We’re catching a window into that event” as small-bodied gibbons and large-bodied orangutans headed to their recent and current home ranges in East and Southeast Asia, he says.

In October 2019, a state court determined that North Carolina’s congressional districts had been severely gerrymandered and struck down the state’s map. The court’s ruling was informed, in part, by tens of thousands of alternative maps demonstrating that the district boundaries had very likely been manipulated for political gain, the very definition of gerrymandering.

Researchers had generated a slew of alternative, computer-generated maps designed to help identify potential patterns of bias. The approach is increasingly used, alongside other tests, to ferret out alleged gerrymandering. District manipulations can be so subtle that they’re undetectable just by looking at them. “The eyeball test is no good,” says Jonathan Katz, a political scientist and statistician at Caltech.

U.S. states redraw their district lines every 10 years to adjust for changing demographics picked up by the national census. The last round a decade ago raised eyebrows, most notably for districts drawn in Michigan, North Carolina and Pennsylvania.

“The post-2010 round of redistricting is widely viewed as a time of extreme, even egregious, partisan gerrymandering,” retired political scientist Richard Engstrom wrote in the January 2020 Social Science Quarterly.

A 2017 report by the nonpartisan Brennan Center for Justice at New York University School of Law noted that existing congressional maps were largely biased in favor of the Republican Party. In 26 states, which account for 85 percent of U.S. congressional districts, as many as 17 Republicans in the House of Representatives owe their seats to gerrymandering. In states where Democrats controlled the redistricting process, partisan bias is also a problem, but the effect is smaller because those states are often made up of fewer districts.

Since 2011, legal scholars, political scientists and mathematicians conducting gerrymandering research have served as expert witnesses in more than 250 state and federal court cases regarding redistricting.

With the 2020 U.S. census now under way, legislators will soon be revising electoral districts again. This time around, researchers hope that instead of serving as expert witnesses in court, they can help identify problematic districting before the new maps even go into effect.

“In 2010, the politicians were thinking very hard on how to draw maps,” while the public and academics weren’t paying a lot of attention, says Jon Eguia, an applied game theorist at Michigan State University in East Lansing. “So in 2011, [those politicians] drew a lot of very bad maps in very many states. Now we’re all paying attention.”

Measuring bias

According to the U.S. Constitution and the Voting Rights Act of 1965, a state’s voting districts must each contain about the same number of people and be drawn in a way that doesn’t disenfranchise racial or ethnic minorities. States can have additional rules, such as contiguity of districts or that cities or counties be kept intact.

That leaves two main tactics for gerrymandering: “packing” the opposing political party’s supporters into a few districts in hopes your party dominates the rest, or “cracking” these supporters by spreading them across many districts to dilute their collective voting power. Those skilled in gerrymandering draw district boundaries that ensure decade-long protection for the ruling party.  

Early methods to test a state map for gerrymandering, which are still in use today, rely on a concept known as partisan symmetry. If a map provides symmetric, or equal, opportunity for all parties in a contest to convert votes into seats, that map is deemed fair. States with packed and cracked districts won’t pass tests based on symmetry.

Symmetry-based tests gained prominence after LULAC v. Perry, a 2006 Supreme Court case that reviewed a mid-decade redistricting plan in Texas. In that case, scientists entered into evidence a partisan bias test — a simulation of what would have happened in an election if the parties’ vote shares were reversed. For instance, if Party A wins 10 out of 15 seats with 70 percent of the vote in an actual election, then Party B, in a hypothetical election, should win 10 seats if it had 70 percent of the vote. Deviation from that “symmetry” equals the level of partisan bias.

In LULAC, the justices largely upheld Texas’ map. But the court indicated willingness to use a symmetry-based method in a future case, though perhaps not solely the partisan bias test. Justice Anthony Kennedy expressed concerns about that particular test, saying it was unclear how much bias was too much. Kennedy also questioned the method’s reliance on statistical simulations rather than real-world results. “We are wary of adopting a constitutional standard that invalidates a map based on unfair results that would occur in a hypothetical state of affairs,” he wrote in the plurality opinion for LULAC.

Real-world symmetry

Kennedy’s feedback on LULAC prompted researchers to develop other symmetry-based tests that used actual election results.

One such approach is the median-mean difference test. That test arises from the basic statistical principle that the difference between the median and the mean indicates the level of skew in the data, with values closer to zero less skewed and vice versa. In 2015 in the Election Law Journal, political scientists Michael McDonald and Robin Best of Binghamton University in New York explained how a median-mean analysis could help identify partisan skew in a state.

A party’s mean vote share comes from averaging vote shares across all districts in a state. A party’s median vote share comes from the district in the middle of a distribution, with the party’s worst-performing district in terms of vote share at one end and the best-performing district at the other. If the difference between a party’s median and mean vote shares is high, that indicates a possibly biased skew, though McDonald and Best noted that some natural skew is inevitable and must be factored into any analysis.

Another approach is called the efficiency gap, which measures the difference in packing and cracking between parties by tallying “wasted” votes. So, if Party A draws an electoral map that spreads Party B’s voters across districts, votes cast for Candidate B in districts won by A are wasted. Conversely, if Party A packs Party B’s voters into a few districts, votes cast for Candidate B beyond the majority needed to win are also wasted. Each party should have about the same number of wasted votes in an election. So the efficiency gap, first described in 2015 in the University of Chicago Law Review, is calculated by taking the difference in wasted votes between parties and dividing that by the total number of votes cast.

PlanScore, a nonpartisan group of legal, political and mapping experts, has shown how the various symmetry tests can be used in tandem to flag possible gerrymandering. In its analysis of the congressional map used in North Carolina’s 2018 congressional elections, where Republicans won 10 seats and Democrats three, PlanScore found bias using all three tests: The partisan bias test showed that in a hypothetical, tied election, Republicans would win 26.9 percent extra seats. In the median-mean test, the median Republican vote share was 5.8 percent higher than its mean vote share. And the efficiency gap was 27.7 percent in favor of Republicans.

Natural advantage

Critics of symmetry-based tests, though, say that the presence of packing and cracking does not prove that the governing party has intentionally gerrymandered the state’s districts. Instead, asymmetries can also arise from natural variations in where voters live. Crucially, Republicans tend to be more dispersed across states while Democrats are concentrated in cities.

“The party that’s more spread out has a geographic advantage,” says applied mathematician Jonathan Mattingly of Duke University. “That’s our system.”

Tests that have emerged over the last few years, known broadly as ensemble methods, accept that natural advantage. These tests rely on computers to generate thousands, or even millions, of possible maps. The programs draw myriad boundary lines around a given district and estimate which party would win that district under each scenario.

Consider a state in which one party wins four to seven seats in almost all maps generated, while the state is using a map that regularly gives that party nine seats, Eguia says. That discrepancy suggests partisan gerrymandering.

Lawyers working on gerrymandering cases tend to use ensemble and symmetry methods in tandem, says Ruth Greenwood, an attorney in Cambridge, Mass., with PlanScore and the nonpartisan nonprofit Campaign Legal Center. That’s because each alternative map can also be evaluated for symmetry using the various tests. Averaging symmetry scores in the ensemble maps and comparing those values with the symmetry scores of the real-life map can illuminate when the difference is stark and therefore can’t be chalked up to any “natural advantage.”

Using all methods together also means that when a map’s fairness is questioned, the lawyers can show that “on any metric, the result is terrible,” Greenwood says.

States at the helm

The issue of gerrymandering was weaving through the federal courts until June 2019, when the Supreme Court ruled 5–4 in Rucho v. Common Cause that partisan gerrymandering fell outside federal jurisdiction. The decision was in response to an appeal challenging a federal court’s January 2018 ruling that North Carolina’s congressional district map had been gerrymandered along partisan lines, giving Republicans 10 out of 13 seats in 2016 and 2018, despite the fact that in both elections Republicans won only about half the popular vote.

By removing the federal court system from gerrymandering cases, the Supreme Court left the issue to the states, a charge a North Carolina state court quickly took up in Harper v. Lewis. The state court relied on much of the same evidence presented in Rucho. That evidence included testimony by Mattingly showing that in a universe with 24,518 alternative congressional district maps, the map North Carolina was using was an egregious outlier. In the maps Mattingly generated, Republicans carried 10 or more seats less than 1 percent of the time.

In October 2019, the three-judge panel in the Harper case ruled that state lawmakers needed to redraw the congressional district map for 2020. Plaintiffs, however, then challenged the state’s remedial map, which would likely result in Republicans winning eight out of 13 seats. Mattingly again provided expert testimony, this time adding 57,202 more maps made using less stringent constraints than the first batch. His research showed a much more purple state, with a 7–6 or 6–7 party split arising in the majority of those alternative maps.

But with the state’s 2020 primary fast approaching, a state court approved the remedial map in December 2019. That new map will stand for only a short time, though. Along with every other state, North Carolina will go through redistricting again in 2021 after the census tallies are in. New Jersey and Virginia — which hold statewide elections in odd years — may struggle to finish their maps before the 2021 elections, since the census was delayed due to the pandemic.

Meanwhile, census watchers worry that the Trump administration’s recent decision to move the completion deadline for the population count from October 31 to September 30 could lead to undercounts among hard-to-reach groups, thereby affecting the placement of district boundaries.

Greenwood expects a spike in lawsuits, though now at the state rather than federal level, challenging the newly drawn maps. And a small cadre of researchers is now ready to serve as expert witnesses in those cases, Eguia says.

The upcoming mapmaking process doesn’t have to repeat the mistakes of a decade ago, he adds. In 2011, lawmakers drew maps largely behind closed doors without researchers present — resulting in a decade of lawsuits, he says. Why not, instead, bring researchers into the room, with their tests and myriad maps, from the get-go? “That’s what the community of experts would prefer,” he says. “That’s what I would advocate for.”