Day or night, the temperature on Venus varies little from its mean of about 460 °C. That’s hot enough to melt lead and an extreme demonstration of the runaway greenhouse effect of carbon dioxide, which makes up most of the planet’s thick atmosphere. With the planet’s crushing surface pressure of 9.4 MPa—93 times that on Earth—the gas exists in a supercritical state and behaves as a solvent. (On Earth, supercritical CO2 is used to decaffeinate coffee beans.) Overhead, sulfuric acid clouds tens of kilometers thick are blown around the planet by hurricane-force winds.
Of the 26 spacecraft sent to Venus since 1961, only a handful have even attempted to land on its harsh surface. In 1982 the Soviet Venera 13 lander sent the image shown in figure 1 before its instruments stopped working after a mere two hours on the surface, despite being housed in a protective pressurized vessel filled with heat-absorbing lithium salt. The quick demise is unsurprising: Silicon electronics start to fail at temperatures above 250 °C because too many electrons are thermally excited across the bandgap.
Figure 1. The surface of Venus, as captured in true color by the Venera 13 lander during the 127 minutes its instruments remained functional on the planet’s surface. The base of the spacecraft appears surrounded by flat rock slabs and soil, and the entire scene appears orange because the planet’s thick atmosphere efficiently filters blue light from incident solar radiation. (Adapted from ref. 7.)
Figure 1. The surface of Venus, as captured in true color by the Venera 13 lander during the 127 minutes its instruments remained functional on the planet’s surface. The base of the spacecraft appears surrounded by flat rock slabs and soil, and the entire scene appears orange because the planet’s thick atmosphere efficiently filters blue light from incident solar radiation. (Adapted from ref. 7.)
In the ensuing decades, silicon carbide technology has matured enough to make the semiconductor a more suitable choice for high-temperature applications.1 Its strong, short Si–C bonds help protect the material from radiation damage by protons and gamma rays, and its wide bandgap allows for transistor action even at temperatures as high as 1000 °C. For the past several years, a team of engineers and materials scientists led by Philip Neudeck of NASA’s Glenn Research Center in Cleveland, Ohio, has pursued the goal of developing durable SiC electronics sophisticated enough to reliably monitor the combustion conditions inside an aircraft engine, say, or the stability of an industrial drill bit deep underground.
To that end, the researchers have steadily toughened the various pieces of a SiC-based integrated circuit (IC). Those pieces include the metallic interconnects that run between field-effect transistors; the insulating dielectric layers that protect everything from oxidation, which can be particularly insidious in hot environments; the gold-capped contacts that help link the IC chip to an external circuit board; and the board’s gold- and platinum-embedded glass traces that connect the IC to other circuits. The effort has involved, among other things, choosing pure and relatively inert electronic materials that are stable at high temperature and whose thermal expansion coefficients are well matched to each other.2
Now Neudeck and colleagues have demonstrated that the robust ICs can operate successfully for several hundred hours in a simulated Venus atmosphere.3 The team placed two ICs in an 800-liter pressure chamber—NASA’s Glenn Extreme Environments Rig (GEER)—and exposed them for nearly 22 days to the same temperature, pressure, and oxidizing chemical environment that a Venus lander would endure. The ICs were ring oscillators—a common logic technology demonstration circuit—each composed of dozens of transistors and resistors. Figure 2 shows one IC test chip before and after exposure. Instruments outside monitored each oscillator’s constant-frequency signal. Although a feed-through cable short-circuited at the seal connecting one of the packages’ circuit board to the outside instruments, neither chip failed.
Figure 2. Before and after photographs of a silicon carbide integrated circuit (IC) that was exposed to a simulated Venus surface environment for 21.7 days in NASA’s Glenn Extreme Environments Rig. The IC, designed to output a constant-frequency signal, was connected to four leads on a ceramic substrate and to a fiberglass-insulated bundle of nickel-alloy wires that led to instruments outside the chamber. Although the feed-through wires short-circuited during the experiment and soot and other chemical debris discolored the steel housing, diagnostics performed after the experiment showed that the chip itself never failed. (Adapted from ref. 3.)
Figure 2. Before and after photographs of a silicon carbide integrated circuit (IC) that was exposed to a simulated Venus surface environment for 21.7 days in NASA’s Glenn Extreme Environments Rig. The IC, designed to output a constant-frequency signal, was connected to four leads on a ceramic substrate and to a fiberglass-insulated bundle of nickel-alloy wires that led to instruments outside the chamber. Although the feed-through wires short-circuited during the experiment and soot and other chemical debris discolored the steel housing, diagnostics performed after the experiment showed that the chip itself never failed. (Adapted from ref. 3.)
The degree of success was a “big surprise to me,” says Neudeck, who admitted that the test was the first Venus simulation his team had made on a working circuit. Besides the threat of material degradation and the diffusion of soot, he had worried that the dense gases would be electrically conductive enough to short-circuit the chips’ power-supply leads, which were held at a constant 50 V. “A bunch of things could have gone wrong but didn’t,” he added. “The only reason we ended the experiment was that our scheduled time with GEER ran out.”
According to Colin Wilson, a planetary physicist at Oxford University, the achievement is encouraging because it proves the durability of SiC electronics in harsh conditions beyond just high heat. “The demonstration paves the way for light, compact, more affordable missions to Venus,” he says.
Indeed, in November 2016 Wilson, Carl-Mikael Zetterling (KTH Royal Institute of Technology in Sweden), and Thomas Pike (Imperial College London) pitched to the European Space Agency the idea of a Venus lander mission that would measure, for a period of months, the planet’s seismicity.4 Although they have not reported 460–500 °C operating times comparable to Neudeck’s team, Zetterling’s research group and others are also developing an all-SiC collection of electronic circuits for a lander: CPUs, memory, analog–digital converters, environmental sensors, and a telecommunications system. The only additional piece they propose is a radioisotope thermoelectric generator to power the electronics. (Because of Venus’s thick cloud deck, little light reaches the surface.) For a similar mission planned by NASA, see reference 5.
Known unknowns
Despite its wildly different climate, Venus is sometimes known as Earth’s twin. The two are similar in size, mass, bulk composition, and proximity to the Sun. And although radar maps have revealed Venus to be covered in volcanic and tectonic features, no one knows whether volcanism and tectonic activity continue today. Establishing the current geological state would help scientists address a fundamental question about Venus that they are still struggling to answer about Mars—whether the planet ever had a habitable environment with liquid-water oceans. (See the article by Ashwin Vasavada on page 34 of this issue.)
Unlike Mars’s surface, whose study has benefited enormously from a succession of rovers crawling around the planet for the past decade, the surface of Venus remains almost completely unexplored. If roving on Venus’s rugged terrain turns out to be too challenging or otherwise impractical, the high atmospheric density near its surface—50 times that on Earth—enables another possible way to get around: using metal Chinese-lantern-like balloons whose bellows can expand or contract to control buoyancy.6
The concept of planetary ballooning is less outlandish than one might think. Back in 1985 the Soviet Union successfully deployed two balloons in the clouds of Venus, 55 km above the surface. Too high up to image the ground, the balloons and their payloads were carried more than 10 000 km by fast winds and enjoyed balmy temperatures around 20 °C. Today’s research into robust SiC circuitry could lead to high-temperature balloon missions that travel far closer to the ground—even touching down in spots—so that instruments can photograph passing terrain or sample its broiling soil and rock.
