Ceres appears especially bright because Dawn captured the imagery while the dwarf planet was at "opposition" — that is, when the spacecraft was directly between Ceres and the sun.

"Based on data from ground-based telescopes and spacecraft that previously viewed planetary bodies at opposition, scientists correctly predicted that Ceres would appear brighter from this opposition configuration," NASA officials wrote in a description of the new video, which was released Tuesday (May 16). [More NASA photos of the dwarf planet Ceres]

"This increase in brightness, or 'surge,' relates the size of the grains of material on the surface, as well as the porosity of those materials," they added.

Dawn took the photos that make up the video on April 29, from a distance of about 12,000 miles (19,300 kilometers). Mission team members shifted the spacecraft into a special orbit to view the 56-mile-wide (90 km) Occator Crater from this opposition perspective, NASA officials said. 

Occator stands out clearly in the video: It harbors the brightest of Ceres' mysterious bright spots, which scientists think reveal the presence of some type of salt on the body's surface.

The $467 million Dawn mission launched in September 2007 to study Vesta and Ceres, the two largest objects in the main asteroid belt between Mars and Jupiter. Dawn circled Vesta from July 2011 through September 2012 and reached Ceres in March 2015. In the process, it became the first spacecraft ever to orbit two different bodies beyond the Earth-moon system.

"Dawn's observations of Ceres during its more than two years there cover a broader range of illumination angles than almost any body in the solar system," NASA officials wrote in the same statement. "This provides scientists with an opportunity to gain new insights into the surface properties. They are currently analyzing the new data."

Last month, Dawn lost the third of its four orientation-maintaining reaction wheels. But the failure has not unduly affected the probe or its mission, NASA officials said; Dawn's handlers are used to orienting the spacecraft with its hydrazine thrusters and will continue to do so into the future.

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

Human civilizations dating back thousands of years left behind structures and records documenting their studies of the stars as they sought to chart the seasons, help travelers find their way and interpret the world around them. Stargazers among the ancient Greeks, Maya, Egyptians, Middle Easterners and Asians likely also pondered if there were other planets like ours among those distant points of light — and if so, what might live there.

Over the last century, science-fiction storytellers have used books, movies, comics and television to speculate at great length about contact with creatures from other worlds — to our benefit and our detriment. These creatures have been imagined as sometimes benevolent and sometimes bloodthirsty, and they have come in a wide range of shapes and sizes — from inquisitive "little green men" to human-parasitizing, chest-bursting Xenomorphs in the "Alien" movie franchise.

Present-day astronomers have likewise been probing this question, using sophisticated equipment to listen farther and peer deeper into the universe than ever before, to find evidence of our cosmic neighbors. From detecting unexplained radio signals to investigating the atmospheres and liquid water on distant worlds — how are scientists searching for signs of extraterrestrial life? [Greetings, Earthlings! 8 Ways Aliens Could Contact Us]

For an alien-seeking scientist, "life" means any living form — including microbes, astronomer Mercedes López-Morales, at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, told Live Science.

But even the smallest microbe living on a distant exoplanet — a planet orbiting a star other than our sun — could still broadcast a chemical signal that would be visible to sensitive telescopes, in the form of atmospheric gases that probably wouldn't be there in the absence of life, López-Morales explained.

"Life affects the atmosphere of a planet," she said. "You have gases that are only there because they are constantly being replenished by something — otherwise, they would react with other gases and disappear. For that gas or that molecule to be in the atmosphere of a planet, it must have some mechanism that is continuously producing it," López-Morales said.

One of the atmospheric gases astronomers are searching for in exoplanets is oxygen, which is plentiful in Earth's atmosphere because it is continuously being replaced by plants through photosynthesis.

However, the presence of unusual atmospheric gases doesn't necessarily mean that something living is generating them, López-Morales added.

"Sulfur molecules, for example, could come from active volcanoes," she explained. "For oxygen, there are at least two or three ways to produce it that involve irradiation in the ultraviolet light coming from stars. But we know that oxygen appeared on Earth because life appeared on Earth," she said.

Of course, even if these chemical signatures can be detected, there's no way to tell what forms of life are producing the signal, Sara Seager, an astrophysicist and planetary scientist at MIT, told Live Science in an email.

And what type of exoplanet is a good candidate for life? Our familiarity with our own world nudges efforts toward those that resemble Earth — "a rocky planet with a thin atmosphere with surface water," Seager said.

"Right now, we can tell — for some planets — if they are rocky, based on planet size and planet mass, which gives average density. But we can't yet tell if a planet has liquid water," she said. [A Field Guide to Alien Planets]

Location, location, location

What else makes an exoplanet a promising candidate? "Anything close by," López-Morales told Live Science. For an astronomer, that means less than 30 light-years away, which would enable humans to actually visit the world where life was detected, she said. (One light-year is about 5.9 trillion miles, or 9.5 trillion kilometers.)

"Eventually, I hope humans will have the technology to get that far, within a reasonable number of years. So for us, the holy grail is to find something within 30 light-years of Earth," she said.

Scientists are also investigating worlds within our own solar system — such as the Saturn moons Titan and Enceladus — which are close enough to be visited by probes that can collect samples and capture images. Several NASA missions are also looking closely at Mars, which once had abundant liquid water on its surface, and where brackish water still flows today, researchers announced in 2015.

"Humans are creatures that want to know — where we came from, where we're going, how we appeared on Earth," López-Morales said. "Our research might start providing answers to that." [FAQ: Significance of Liquid Water on Mars]

Radio signals

But scientists aren't just looking for signs of extraterrestrial life — they're also listening for them.

For more than two decades, SETI, the Search for Extraterrestrial Intelligence Institute, has conducted research to understand the origins of life in the universe, and to detect and analyze evidence of life emanating from places other than Earth. This effort includes investigations of microbial life within our solar system, such as on the surface of Mars or under the icy crust of Jupiter's moon Europa. SETI scientists are also monitoring the universe for signals in light or radio wavelengths that originate far away and could be signs of technologically advanced alien life, SETI explains on its website.

At SETI, astronomers use the Allen Telescope Array (ATA) of 42 radio antennas to "listen" for signals over a range of radio frequencies, tuned to "hear" the regions around 20,000 red dwarf stars (a broad term describing stars smaller than our sun and in a certain spectral range) that are closest to Earth, Seth Shostak, a senior astronomer at the SETI Institute, told Live Science.

Investigating red dwarf stars for life-supporting worlds is a relatively recent development at SETI. In the past, stars that were more like our own sun — a yellow dwarf — were thought to be the most likely candidates to host planets harboring life. But over the last few decades, astronomers have determined that many red dwarf stars host planets that could be at the right distance from the star to be habitable, according to Shostak.

"That's something we didn't know when we started," he said.

And SETI radio-signal monitoring is accelerating, as telescopes become more sensitive and technological developments increase the number of radio channels and locations in the sky that can be studied at once, Shostak explained.

"Until now, the total number of star systems that have been looked at carefully over a wide range of the radio dial is measured in the thousands. In the next 20 years, with new technology, you could increase that number to maybe a million," he said. [4 Places Where Alien Life May Lurk in the Solar System]

An alien megastructure?

Shostak also reviews images of alleged alien spacecraft sent to him by hopeful photographers, he told Live Science. A photographer himself, Shostak said that he invariably identifies all the purported "UFO" sightings as tricks of the light or internal reflections in the camera lens — much to the dismay of the observers.

"That never makes them happy," he said.

But even among astronomers, unusual observations can sometimes turn the conversation toward the likelihood of alien technology.

In 2015, when scientists discovered the star KIC 8462852 — also known as Tabby's Star, located more than 1,400 light-years from Earth — they were puzzled by repeated and significant dips in its brightness that took place over several years. During the dips, the star dimmed by as much as 22 percent, far more than could be caused by an orbiting planet passing in front of the star, Shostak said.

In short, the star was "really weird," Tabetha Boyajian, lead author of a study about the star and a researcher at Yale University, told the Atlantic in October of that year.

One possible explanation suggested by some experts was an "alien megastructure," an enormous array orbiting KIC 8462852, built by a hypothetical alien civilization advanced enough to possess technology capable of drawing power from a star. Such a construct could — in theory — periodically block visible light and make the star appear dramatically dimmer when seen from Earth, Space.com reported in 2015.

However, there is no data to actively support this hypothesis. In fact, on all fronts, evidence of any extraterrestrial presence — within our own solar system or beyond its boundaries — remains elusive. But scientists seeking life on other worlds are undaunted by the ongoing challenge, Shostak told Live Science.

"The search should continue, simply because it's a very interesting question," he said.

"Is Earth special? Is it the only place around with intelligent life? That would be remarkable — but it's just as remarkable to find you're not the only kid on the block. That's something that would change our view of ourselves forever," he said.

Original article on Live Science.

MESA, Arizona — Since the dawn of the space age NASA and other agencies have spent billions of dollars to reconnoiter Mars — assailing it with spacecraft flybys, photo-snapping orbiters and landers nose-diving onto its surface. The odds are good, many scientists say, for the Red Planet being an extraterrestrial address for alien life — good enough to sustain decades' worth of landing very expensive robots to ping it with radar, zap it with lasers, trundle across its terrain and scoop up its dirt. Yet against all odds (and researchers' hopes for a watershed discovery), Mars remains a poker-faced world that holds its cards tight. No convincing signs of life have emerged. But astrobiologists continue to, quite literally, chip away at finding the truth.

As the search becomes more heated (some would say more desperate), scientists are entertaining an ever-increasing number of possible explanations for Martian biology as a no-show. For example, could there be a "cover up" whereby the harsh Martian environment somehow obliterates all biosignatures—all signs of past or present life? Or perhaps life there is just so alien its biosignatures are simply unrecognizable to us, hidden in plain view.

Of course, the perplexing quest to find life on Mars may have a simple solution: It's not there, and never was. But as the proceedings of this year's Astrobiology Science Conference held here in April made clear, life-seeking scientists are not giving up yet. Instead, they are getting more creative, proposing new strategies and technologies to shape the next generation of Mars exploration.

A SLUMBERING BIOSPHERE?

Talk about looking for Martians and you inevitably talk about water, the almost-magical liquid that sustains all life on Earth and seems to have served as an indispensable kick-starter for biology in our planet's deepest past. "It all started out with ‘follow the water;' not necessarily ‘follow the life'…but ‘follow one of the basic requirements for living systems,'" says Arizona State University geologist Jack Farmer, referring to NASA's oft-repeated mantra for Martian exploration. "There are many indications of water on Mars in the past, perhaps reservoirs of water in the near subsurface as well," he says. "But what is the quality of that water? Is it really salty—too salty for life?"

Without liquid water, Farmer points out, one would naively think organisms cannot function. The reality may be more complex: on Earth, some resilient organisms such as tardigrades can enter a profound, almost indefinite state of hibernation when deprived of moisture, preserving their desiccated tissues but neither growing nor reproducing. It is possible, Farmer says, that Martian microbes could spend most of their time as inert spores "waiting for something good to happen," only springing to life given the right and very rare conditions. Certain varieties of Earthly "extremophiles"—microbes that live at extremes of temperature, pressure, salinity and so on—exhibit similar behavior.

Farmer says there is as yet no general consensus about the best way to go about life detection on the Red Planet. This is due in no small part to the runaway pace of progress in biotechnology, which has led to innovations such as chemistry labs shrunken down to fit on a computer chip. These technologies "have been revolutionizing the medical field, and have now started to enter into concepts for life detection on Mars," he explains. Things move so fast that today's best technology for finding Martian biology may be tomorrow's laughably obsolete dead-end.

But no matter how sophisticated a lab on a chip might be, it won't deliver results if it is not sent to the right place. Farmer suspects that seriously seeking traces of life requires deep drilling on Mars. "I basically think we're going to have to gain access to the subsurface and look for the fossil record," he explains. But discovering a clear, unambiguous fossil biosignature on Mars would also raise a red flag. "We probably would approach the future of Mars exploration—particularly accessing habitable zones of liquid water in the deep subsurface—more cautiously, because life could still be there. So planetary protection would be taken very seriously," he says. ("Planetary protection" is the term scientists commonly use for precautions to minimize the chance of biological contamination between worlds. Think of it not so much in terms of bug-eyed aliens running rampant on Earth but of billion-dollar robots finding "Martians" that prove to only be hardy bacterial hitchhikers imported from our own world).

THE MARTIAN UNDERGROUND

Like-minded about deep diving on Mars is Penelope Boston, director of the NASA Astrobiology Institute at the agency's Ames Research Center. "That's my bias," she says. "Given Mars' current state, with all the challenging surface manifestations of dryness, radiation and little atmosphere, the best hope for life still extant on Mars is subsurface." The subsurface, she says, might also offer better chances of preserving past life—that is, of fossils, even if only of single-celled organisms.

The planet's depths hold the potential for harboring liquid water under certain circumstances, Boston thinks. But how far down might that water be? "I suspect it's pretty far…and how we get to it, that's a whole other kettle of fish," she says. Over the years scientists have estimated the average depth of the planet's possible liquid reservoirs as anywhere between tens of meters to kilometers. Then again, recent observations from orbiters have revealed mysterious dark streaks that seasonally flow down the sunlit sides of some Martian hillsides and craters. These "recurring slope lineae" could conceivably be brines of liquid water fed by aquifers very close to the surface, some researchers say.

Such lingering uncertainties emerge from the indirect and scattered nature of our studies of Mars, and ensure that any argument for life there is based solely on circumstantial information, Boston notes. "Each individual piece of evidence is, on its own merits, weak," she says. Only by amassing a diverse suite of independent measurements can a well-built case for life on Mars be made, she says: "In my opinion, we can't make that strong case unless we push to do all of those measurements on exactly the same precise spot. We don't do that because it's very difficult, but it's something to aspire to." Despite decades of sending costly hardware to Mars, Boston believes that what is still missing is a sense of harmony between instruments, allowing them to work together to support a search for alien life. "I think that the precise requirements of a really robust claim of life at the microscopic scale require us to push on further," she notes.

Attendees at the astrobiology meeting in Arizona showcased an assortment of high-tech devices for next-generation exploration, ranging from microfluidic "life analyzers" and integrated nucleic acid extractors for studying "Martian metagenomics" to exquisitely sensitive, miniaturized organic chemistry labs for spotting tantalizing carbon compounds and minerals at microscopic scales. Missing from the mix, however, was any solid consensus on how these and other tools could all work together to provide a slam-dunk detection of life on Mars.

WHAT'S THE WEATHER?

Some scientists contend a new kind of focus is sorely needed. Perhaps the pathway to finding any Martians lurking in the planet's nooks and crannies is to learn where exactly on Mars those potentially life-nurturing niches exist, and how they change over the course of days, months and years rather than over eons of geologic time. That is, to find homes for extant life on Mars today, researchers should probably not just be studying the planet's long-term climate but also its day-to-day weather.

"Right now we're sort of shifting gears. Once you've found out that a planet is habitable, then the next question is, ‘Was there life?'—so it's a completely different ball game," says Nathalie Cabrol, director of the Carl Sagan Center at the SETI Institute. "On Mars you cannot look for life with the tools that have been looking for habitability of that planet," she argues. "We should be looking for habitats and not habitable environments. You are dealing on Mars with what I call extremophile extreme environments on steroids," she says, "and you don't look for microbial life with telescopes from Mars orbit."

Cabrol advocates making an unprecedentedly robust, high-resolution study of environmental variability on Mars by peppering its surface with weather stations. Sooner or later telltale signs of the possible whereabouts of extant life may emerge from the resulting torrents of data. "Today's environment on that planet is a reflection of something in the past," she says, and planting numbers of automated stations on Mars does not need to be expensive. "This is of interest not only to astrobiology but to human exploration. The first thing you want to know is what the weather is like," she says, adding, "Right now we're not equipped to do this and I'm not saying it's going to be easy to look for extant life. I'm not saying what we're doing now is wrong. Whatever we put on the ground we are learning. But there is variability on Mars. You go up or down one meter, things change. Habitats at a microscopic level can happen at the scale of a slope. It can happen at the scale of a rock!"

MIGHT WE BE MARTIANS?

"I think Mars offers us the highest chance of finding life" somewhere beyond Earth, says Dirk Schulze-Makuch, a planetary scientist at Technical University of Berlin in Germany. But, like Boston and others, he maintains confirmation of life will only come from multiple "layers of proof" that have to be consistent with one another. "We really need at least four different kinds of methods," he says. "My point is that there's no slam-dunk. We need several instruments. You have to build a case, and right now we can do better…unless the biosignature through a microscope is waving hello." The trouble, he adds, is that too-stringent planetary protection rules may preclude getting the evidence necessary for that proof. "We have the technology to go to places where there could be life," he says. "But we can't go to certain areas on Mars, like recurring slope lineae or…under patches of ice. It seems to be ridiculous."

Indeed, Schulze-Makuch speculates planetary protection may be a lost cause for Mars—or at least a misguided endeavor. It may even be that any Martian microbes are actually Earth's long-lost cousins. Or, conversely, Mars rather than Earth is really the sole site of biogenesis in our solar system. Both scenarios are possible, considering that single-celled organisms can likely survive world-shattering impacts and the subsequent interplanetary voyages if embedded in ejected shards of rock that could fall elsewhere as meteorites. Innumerable impacts of this scale battered the solar system billions of years ago, potentially blasting biological material between neighboring worlds. On balance, Schulze-Makuch says, "the chances are higher that we are Martians."

This article was first published at ScientificAmerican.com. © ScientificAmerican.com. All rights reserved.

Follow Scientific American on Twitter @SciAm and @SciamBlogs. Visit ScientificAmerican.com for the latest in science, health and technology news.

Paul Sutter is an astrophysicist at The Ohio State Universityand the chief scientist at COSI Science Center. Sutter is also host of Ask aSpaceman, RealSpace and COSI Science Now. 

I'm sure by now we've all heard the refrain: "The Higgs boson creates mass." And if you haven't heard it, well, now you have. This simple statement seems to pack a wallop of a concept: that every pound and gram of your body, down at the submicroscopic scale, is due to this invisible Higgs goo that fills up the universe.

Hence the nickname "The God Particle," which has frustratingly entered the public consciousness. Without the Higgs, there wouldn't be mass. Without the Higgs, all the physics that we know and love would come screeching to a halt, particles flying off at the speed of light, hardly ever having the chance to interact. [What Is the 'God Particle'? Higgs Boson Explained (Video)]