Hubble did not look at Earth directly. Instead, the astronomers used the Moon as a mirror to reflect sunlight, which had passed through Earth’s atmosphere, and then reflected back towards Hubble. Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of transiting exoplanets. These atmospheres may contain chemicals of interest to astrobiology, the study of and search for life.

Though numerous ground-based observations of this kind have been done previously, this is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope. Hubble detected the strong spectral fingerprint of ozone, which absorbs some of the sunlight. Ozone is important to life because it is the source of the protective shield in Earth’s atmosphere.

On Earth, photosynthesis over billions of years is responsible for our planet’s high oxygen levels and thick ozone layer. That’s one reason why scientists think ozone or oxygen could be a sign of life on another planet, and refer to them as biosignatures.

“Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life,” explained Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, lead researcher of Hubble’s observations.

Although ozone in Earth’s atmosphere had been detected in previous ground-based observations during lunar eclipses, Hubble’s study represents the strongest detection of the molecule to date because ozone — as measured from space with no interference from other chemicals in the Earth’s atmosphere — absorbs ultraviolet light so strongly.

Hubble recorded ozone absorbing some of the Sun’s ultraviolet radiation that passed through the edge of Earth’s atmosphere during a lunar eclipse that occurred on January 20 to 21, 2019. Several other ground-based telescopes also made spectroscopic observations at other wavelengths during the eclipse, searching for more of Earth’s atmospheric ingredients, such as oxygen and methane.

“One of NASA’s major goals is to identify planets that could support life,” Youngblood said. “But how would we know a habitable or an uninhabited planet if we saw one? What would they look like with the techniques that astronomers have at their disposal for characterizing the atmospheres of exoplanets? That’s why it’s important to develop models of Earth’s spectrum as a template for categorizing atmospheres on extrasolar planets.”

Her paper is available online in The Astronomical Journal.

Sniffing Out Planetary Atmospheres

The atmospheres of some extrasolar planets can be probed if the alien world passes across the face of its parent star, an event called a transit. During a transit, starlight filters through the backlit exoplanet’s atmosphere. (If viewed close up, the planet’s silhouette would look like it had a thin, glowing “halo” around it caused by the illuminated atmosphere, just as Earth does when seen from space.)

Chemicals in the atmosphere leave their telltale signature by filtering out certain colors of starlight. Astronomers using Hubble pioneered this technique for probing exoplanets. This is particularly remarkable because extrasolar planets had not yet been discovered when Hubble was launched in 1990 and the space observatory was not initially designed for such experiments.

So far, astronomers have used Hubble to observe the atmospheres of gas giant planets and super-Earths (planets several times Earth’s mass) that transit their stars. But terrestrial planets about the size of Earth are much smaller objects and their atmospheres are thinner, like the skin on an apple. Therefore, teasing out these signatures from Earth-sized exoplanets will be much harder.

That’s why researchers will need space telescopes much larger than Hubble to collect the feeble starlight passing through these small planets’ atmospheres during a transit. These telescopes will need to observe planets for a longer period, many dozens of hours, to build up a strong signal.

To prepare for these bigger telescopes, astronomers decided to conduct experiments on a much closer and only known inhabited terrestrial planet: Earth. Our planet’s perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry of a terrestrial planet transiting its star.

But the observations were also challenging because the Moon is very bright, and its surface is not a perfect reflector because it is mottled with bright and dark areas. The Moon is also so close to Earth that Hubble had to try and keep a steady eye on one select region, despite the Moon’s motion relative to the space observatory. So, Youngblood’s team had to account for the Moon’s drift in their analysis.

Where There’s Ozone, There’s Life?

Finding ozone in the skies of a terrestrial extrasolar planet does not guarantee that life exists on the surface. “You would need other spectral signatures in addition to ozone to conclude that there was life on the planet, and these signatures cannot necessarily be seen in ultraviolet light,” Youngblood said.

On Earth, ozone is formed naturally when oxygen in the Earth’s atmosphere is exposed to strong concentrations of ultraviolet light. Ozone forms a blanket around Earth, protecting it from harsh ultraviolet rays.

“Photosynthesis might be the most productive metabolism that can evolve on any planet, because it is fueled by energy from starlight and uses cosmically abundant elements like water and carbon dioxide,” said Giada Arney of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, a co-author of the science paper. “These necessary ingredients should be common on habitable planets.”

Seasonal variability in the ozone signature also could indicate seasonal biological production of oxygen, just as it does with the growth seasons of plants on Earth.

But ozone can also be produced without the presence of life when nitrogen and oxygen are exposed to sunlight. To increase confidence that a given biosignature is truly produced by life, astronomers must search for combinations of biosignatures. A multiwavelength campaign is needed because each of the many biosignatures are more easily detected at wavelengths specific to those signatures.

“Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren’t strong enough,” Arney explained. “We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets.”

NASA has a forthcoming observatory called the James Webb Space Telescope that could make similar kinds of measurements in infrared light, with the potential to detect methane and oxygen in exoplanet atmospheres. Webb is currently scheduled to launch in 2021.

Video: https://www.youtube.com/watch?v=OHbiPO8bAts&feature=emb_logo

“The fitness of bacteria is more complex than expected,” explains Ulrich Gerland, professor for the theory of complex biosystems at the Technical University of Munich. The physicist has been studying the survival strategies of E. coli bacteria for several years.

The unicellular organisms, which go by the Latin name Escherichia coli and support digestion in the large intestine of mammals, are a popular model organism. They facilitate investigations into the way living beings can adapt to changing environmental conditions.

Surviving hard times

“We have known for some time that biological fitness depends on two things: the growth rate when food is available and the ability to survive periods of nutrient deficiency,” explains the scientist. “What was not clear is how these two factors are related.”

For the first time, Gerland and his team have now systematically investigated the extent to which fast or slow growth influences the survivability of E. coli bacteria: “It turns out that changes in growth conditions have a direct impact on death rates. These follow a simple law: The best-nourished and fastest-growing bacteria are the first to die when deprived of food.”

Fat bacteria — poor fitness

So, a good diet is bad for the fitness of bacteria. But why? To find an answer to this question, the TUM researchers carried out a number of experiments: First, cultures of E. coli bacteria were provided nutrient solutions with different compositions.

In the second step, the unicellular organisms were put on a zero diet. Throughout the entire period, the scientists examined whether and how quickly the cells multiplied, and how long they survived.

The fight for survival

The research showed that regardless of how well or poorly they were previously fed, bacteria stopped reproducing when they were deprived of food. In this “maintenance phase,” organisms struggle for bare survival. All available energy sources — for example, the cellular remains of dead bacteria — are used to sustain the metabolism.

In this extreme situation, many cells die of starvation within a few days. However, the death rate is particularly high among rapidly growing E. coli bacteria. “They are primed for rapid growth and waste energy resources. This leads to their demise during the hunger phase,” explains Gerland.

As it turns out, the abundantly fed bacteria have an increased need for energy, as further experiments prove. Surviving times of scarcity is more difficult for organisms with a high energy consumption. “We now understand why evolution doesn’t favor the fastest possible reproduction,” says Gerland. “The biological fitness that is crucial for the survival of a species builds on a balance between growth and survivability.”

Antibiotic therapy with a carrot and a stick

The research results may find application in the future, for example, to improve the effect of antibiotics: “Applying a carrot and stick principle, intestinal bacteria growth could be stimulated by consuming a sweet dish. This would weaken the bacteria if an antibiotic against an intestinal infection is then administered,” explains Gerland. However, it is still too early for concrete recommendations. More research will be necessary.

“These materials have potential applications in any technology that needs bright fluorescence or calls for designing optical properties, including solar energy harvesting, bioimaging, and lasers,” says Amar Flood, a chemist at Indiana University and co-senior author on the study along with Bo Laursen of the University of Copenhagen.

“Beyond these, there are interesting applications that include upconverting light to capture more of the solar spectrum in solar cells, light-switchable materials used for information storage and photochromic glass, and circularly polarized luminescence that may be used in 3D display technology,” Flood says.

While there are currently more than 100,000 different fluorescent dyes available, almost none of these can be mixed and matched in predictable ways to create solid optical materials. Dyes tend to undergo “quenching” when they enter a solid state due to how they behave when packed close together, decreasing the intensity of their fluorescence to produce a more subdued glow.

“The problem of quenching and inter-dye coupling emerges when the dyes stand shoulder-to-shoulder inside solids,” says Flood. “They cannot help but ‘touch’ each other. Like young children sitting at story time, they interfere with each other and stop behaving as individuals.”

To overcome this problem, Flood and colleagues mixed a colored dye with a colorless solution of cyanostar, a star-shaped macrocycle molecule that prevents the fluorescent molecules from interacting as the mixture solidified, keeping their optical properties intact. As the mixture became a solid, SMILES formed, which the researchers then grew into crystals, precipitated into dry powders, and finally spun into a thin film or incorporated directly into polymers. Since the cyanostar macrocycles form building blocks that generate a lattice-like checkerboard, the researchers could simply plug a dye into the lattice and, without any further adjustments, the structure would take on its color and appearance.

While previous research had already developed an approach to spacing the dyes apart using macrocycle molecules, it relied on colored macrocycles to do the job. Flood and colleagues found that colorless macrocycles were key.

“Some people think that colorless macrocycles are unattractive, but they allowed the isolation lattice to fully express the bright fluorescence of the dyes unencumbered by the colors of the macrocycles,” says Flood.

Next, the researchers plan to explore the properties of fluorescent materials formed using this novel technique, enabling them to work with dye makers in the future to realize the materials’ full potential in a variety of different applications.

“These materials are totally new, so we do not know which of their innate properties are actually going to offer superior functionality,” says Flood. “We also do not know the materials’ limits. So, we will develop a fundamental understanding of how they work, providing a robust set of design rules for making new properties. This is critical for putting these materials into the hands of others — we want to pursue crowd sourcing and to work with others in this effort.”

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