What could alternate, alien forms of life look like?

All life as we know it relies on carbon and water. But researchers speculate this doesn't have to be the case.

What could alternate, alien forms of life look like?
Photo credit: JR Korpa on Unsplash
  • Life on Earth (and therefore all life we know) relies on carbon and water.
  • Carbon and water make for excellent ingredients when making life, but many other elements could serve in their place under the right conditions.
  • What are these alternative forms of life and under what conditions could they flourish?

All life on Earth, and thus, all life we've ever observed in the universe, shares a few basic characteristics. Its molecular structures are built using carbon, it relies on water to act as a solvent and facilitate chemical reactions, and it uses DNA or RNA as its blueprints.

These qualities seem so ubiquitous that most any compound we can find that contains carbon is called an organic compound. Carbon works very well as the basis for the chemistry of life. It can bond with many molecules, building structures large enough to be biologically relevant, and its bonds are strong and stable. Using water and DNA/RNA are also seemingly fine-tuned to enable life to exist.

But just because these properties of life are true on Earth doesn't mean they are true everywhere. In fact, we can readily imagine different environments where alternative forms of life can exist. Here are some of the major ways we think that life can vary from the standard we see on Earth.


An artists' rendering of organosilicon-based life. Organosilicon compounds contain carbon-silicon bonds.

Lei Chen and Yan Liang (BeautyOfScience.com) for Caltech

The same stuff that constitutes computer chips and electrical circuits may also constitute life somewhere in the universe. Carbon can form bonds with up to four other atoms at once, bind to oxygen, and form polymer chains, all of which make it ideal for the complex chemistry of life. Silicon, which lies just beneath carbon on the table of elements, also shares these characteristics.

Despite these qualities, silicon is still quite limited as a basis for life. It can only form stable bonds with a limited number of other elements; its polymers would be very monotonous, limiting its ability to form the complex compounds needed for life to occur; and silicon chemistry is not stable in aqueous, or watery, environments. Another issue is that when carbon oxidizes, it forms carbon dioxide, an easily expellable gas. When silicon oxidizes, it forms silicon dioxide, also known as silica, quartz, or sand. This solid waste would pose some serious mechanical challenges for any silicon-based life. Such a hypothetical lifeform would excrete bricks of sand every time it took a breath, which would make vacationing at the beach somewhat more horrifying.

Under certain conditions, silicon-based chemistry might be more favorable for life than carbon-based. Silicon chemistry would also be much more amenable to life in oceans of cold elements that we don't usually associate with life, such as liquid nitrogen, methane, ethane, neon, and argon. Places like these exist in the universe, notably in our own solar system: One of the major features of Saturn's largest moon, Titan, is its lakes of liquid ethane and methane.


An artist's depiction of a world with ammonia-based life. Ittiz [CC BY-SA 3.0]

Most of the chemical reactions that life relies on take place within a watery environment. Water dissolves many different molecules — it is a solvent, and having a good solvent is a prerequisite for the kind of chemistry that brings about life.

Like water, ammonia is also common throughout the galaxy. It's also capable of dissolving organic compounds like water, and, unlike water, it can also dissolve some metallic ones, opening up the possibility for some more interesting chemistry to be used in living things.

However, ammonia is also flammable in the presence of oxygen; has much lower surface tension than water, making it difficult to hold prebiotic molecules together for very long; and its melting and boiling points are much lower than water, at –78°C and –33.15°C, respectively. Thus, the chemistry of ammonia-based life would occur much more slowly, and commensurately, its metabolism and evolution would also be slower. An important caveat, though, is that these are the melting and boiling points that occur at Earth's atmospheric pressure. Under higher pressures, these values would rise.

One of the exciting features of ammonia-based life is that it could exist outside of the so-called habitability zone, or the range where liquid water can exist. Titan, for instance, may hold oceans of ammonia beneath its surface, and although it lies outside of our solar system's habitable zone, it could for this reason host life. Astrobiologists often point to Titan as a possible site of alternative life forms within our own solar system.

Alternate chirality

Just as a person can be left-handed or right-handed, so too can organic molecules. These molecules are mirror images of one another, but life, for whatever reason, wound up using one side or the other, which is called chirality. Amino acids, for instance, are "left-handed," while the sugars in RNA and DNA are "right-handed." For these molecules to interact with one another, they have to be of the correct kind of chirality; if protein chains are made with mixed-chirality amino acids, they simply don't work. But a protein chain constructed from right-handed amino acids, the opposite of what life on Earth uses, would work perfectly fine.

All of Earth's ecology depends on this convention. In order to eat, we need to consume food of the appropriate chirality. We can be infected and defend against infections of the appropriate chirality. Everything on Earth has the appropriate chirality, so this works just fine.

But alien life might evolve to use the opposite chirality as Earth. This life would be fundamentally quite similar to life on Earth — using carbon as its backbone and water as its solvent — but it would interact with us in one of two possible ways. First, it wouldn't be able to interact at all. Even if microbial life tried eat some other microbial life, the "reverse" sugars would be indigestible, and viruses wouldn't be able to bind to host cells. This would probably be a good thing, since we don't want to be infected with any alien diseases.

But there are critters on Earth that don't eat chiral nutrients, such as cyanobacteria. A comparable alien microbe would be able to eat as much as it wants, reproduce indefinitely, and would never be kept in check by predators since it itself would be of the wrong chirality. This would dramatically disrupt the food chain on an apocalyptic scale.

These alternative forms of life aren't the only ones that exist, but they're among the most likely. A lot of what we know about chemistry suggests that carbon- and water-based life will be the most common among the universe, but we've only ever had a sample of one to study: our own planet. If we find life on other worlds, we'll gain even greater insight into how living things come about.

A landslide is imminent and so is its tsunami

An open letter predicts that a massive wall of rock is about to plunge into Barry Arm Fjord in Alaska.

Image source: Christian Zimmerman/USGS/Big Think
Surprising Science
  • A remote area visited by tourists and cruises, and home to fishing villages, is about to be visited by a devastating tsunami.
  • A wall of rock exposed by a receding glacier is about crash into the waters below.
  • Glaciers hold such areas together — and when they're gone, bad stuff can be left behind.

The Barry Glacier gives its name to Alaska's Barry Arm Fjord, and a new open letter forecasts trouble ahead.

Thanks to global warming, the glacier has been retreating, so far removing two-thirds of its support for a steep mile-long slope, or scarp, containing perhaps 500 million cubic meters of material. (Think the Hoover Dam times several hundred.) The slope has been moving slowly since 1957, but scientists say it's become an avalanche waiting to happen, maybe within the next year, and likely within 20. When it does come crashing down into the fjord, it could set in motion a frightening tsunami overwhelming the fjord's normally peaceful waters .

"It could happen anytime, but the risk just goes way up as this glacier recedes," says hydrologist Anna Liljedahl of Woods Hole, one of the signatories to the letter.

The Barry Arm Fjord

Camping on the fjord's Black Sand Beach

Image source: Matt Zimmerman

The Barry Arm Fjord is a stretch of water between the Harriman Fjord and the Port Wills Fjord, located at the northwest corner of the well-known Prince William Sound. It's a beautiful area, home to a few hundred people supporting the local fishing industry, and it's also a popular destination for tourists — its Black Sand Beach is one of Alaska's most scenic — and cruise ships.

Not Alaska’s first watery rodeo, but likely the biggest

Image source: whrc.org

There have been at least two similar events in the state's recent history, though not on such a massive scale. On July 9, 1958, an earthquake nearby caused 40 million cubic yards of rock to suddenly slide 2,000 feet down into Lituya Bay, producing a tsunami whose peak waves reportedly reached 1,720 feet in height. By the time the wall of water reached the mouth of the bay, it was still 75 feet high. At Taan Fjord in 2015, a landslide caused a tsunami that crested at 600 feet. Both of these events thankfully occurred in sparsely populated areas, so few fatalities occurred.

The Barry Arm event will be larger than either of these by far.

"This is an enormous slope — the mass that could fail weighs over a billion tonnes," said geologist Dave Petley, speaking to Earther. "The internal structure of that rock mass, which will determine whether it collapses, is very complex. At the moment we don't know enough about it to be able to forecast its future behavior."

Outside of Alaska, on the west coast of Greenland, a landslide-produced tsunami towered 300 feet high, obliterating a fishing village in its path.

What the letter predicts for Barry Arm Fjord

Moving slowly at first...

Image source: whrc.org

"The effects would be especially severe near where the landslide enters the water at the head of Barry Arm. Additionally, areas of shallow water, or low-lying land near the shore, would be in danger even further from the source. A minor failure may not produce significant impacts beyond the inner parts of the fiord, while a complete failure could be destructive throughout Barry Arm, Harriman Fiord, and parts of Port Wells. Our initial results show complex impacts further from the landslide than Barry Arm, with over 30 foot waves in some distant bays, including Whittier."

The discovery of the impeding landslide began with an observation by the sister of geologist Hig Higman of Ground Truth, an organization in Seldovia, Alaska. Artist Valisa Higman was vacationing in the area and sent her brother some photos of worrying fractures she noticed in the slope, taken while she was on a boat cruising the fjord.

Higman confirmed his sister's hunch via available satellite imagery and, digging deeper, found that between 2009 and 2015 the slope had moved 600 feet downhill, leaving a prominent scar.

Ohio State's Chunli Dai unearthed a connection between the movement and the receding of the Barry Glacier. Comparison of the Barry Arm slope with other similar areas, combined with computer modeling of the possible resulting tsunamis, led to the publication of the group's letter.

While the full group of signatories from 14 organizations and institutions has only been working on the situation for a month, the implications were immediately clear. The signers include experts from Ohio State University, the University of Southern California, and the Anchorage and Fairbanks campuses of the University of Alaska.

Once informed of the open letter's contents, the Alaska's Department of Natural Resources immediately released a warning that "an increasingly likely landslide could generate a wave with devastating effects on fishermen and recreationalists."

How do you prepare for something like this?

Image source: whrc.org

The obvious question is what can be done to prepare for the landslide and tsunami? For one thing, there's more to understand about the upcoming event, and the researchers lay out their plan in the letter:

"To inform and refine hazard mitigation efforts, we would like to pursue several lines of investigation: Detect changes in the slope that might forewarn of a landslide, better understand what could trigger a landslide, and refine tsunami model projections. By mapping the landslide and nearby terrain, both above and below sea level, we can more accurately determine the basic physical dimensions of the landslide. This can be paired with GPS and seismic measurements made over time to see how the slope responds to changes in the glacier and to events like rainstorms and earthquakes. Field and satellite data can support near-real time hazard monitoring, while computer models of landslide and tsunami scenarios can help identify specific places that are most at risk."

In the letter, the authors reached out to those living in and visiting the area, asking, "What specific questions are most important to you?" and "What could be done to reduce the danger to people who want to visit or work in Barry Arm?" They also invited locals to let them know about any changes, including even small rock-falls and landslides.

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