NASA is testing an alien-hunting, upside-down underwater rover in Antarctica. It's one of several plans to explore 2 ocean worlds for signs of life.

NASA began testing an alien-hunting underwater rover in Antarctica this week.

It's called the Buoyant Rover for Under-Ice Exploration (BRUIE). That's because, instead of relying on its weight to keep it on the ground, BRUIE relies on buoyancy to keep it afloat.

The rover rolls across the bottom of the sheet of ice covering the polar ocean.

"Instead of a rover that drives on the ground, we'll have a rover that drives on the ceiling," Dan Berisford, a mechanical engineer on the BRUIE team, said in a 2015 NASA video.

BRUIE has already been tested in Alaska and the Arctic.

BRUIE is a prototype, but its unique approach could allow future rovers to explore alien oceans beneath the ice of distant worlds.

"We've found that life often lives at interfaces, both the sea bottom and the ice-water interface at the top," lead engineer Andy Klesh said in a press release.

BRUIE's buoyancy keeps it anchored to the ice, resistant to most ocean currents that could sweep it away.

The rover can also power down safely and turn back on when it's time to take measurements. Klesh said that this energy-saving method would allow BRUIE to spend months exploring underwater.

"BRUIE will carry several science instruments to measure parameters related to life, such as dissolved oxygen, water salinity, pressure and temperature," Berisford said.

With two rotating high-definition cameras, the rover can look down to the ocean floor and up at the ice above.

The team plans to keep testing and developing BRUIE until it can survive months below the ice.

NASA scientists hope this technology will one day search for alien life on two faraway moons that hide vast oceans below thick sheets of ice.

"The ice shells covering these distant oceans serve as a window into the oceans below, and the chemistry of the ice could help feed life within those oceans," Kevin Hand, the lead scientist working on BRUIE, said in the release. "Here on Earth, the ice covering our polar oceans serves a similar role, and our team is particularly interested in what is happening where the water meets the ice."

The first ocean world, Europa, is an icy moon of Jupiter.

Scientists have long suspected that Europa conceals an ocean below its ice surface — possibly with twice the volume of Earth's oceans.

Scientists measured water vapor above Europa's surface for the first time on Monday — further evidence of the liquid water that lies below.

In June, scientists spotted sodium chloride (also known as table salt) in Europa's ice, indicating that the ocean below could be similar to those on Earth.

Chemical reactions between this salt and rocks on the ocean floor could create nitrogen compounds, which are crucial in the formation of life.

The second world, Enceladus, is an icy moon of Saturn.

Like Europa, Enceladus hides a subsurface ocean of liquid water that could be habitable to alien life.

Much smaller than Europa, Enceladus is about as wide as the state of Arizona.

In 2005 the Cassini spacecraft, which explored Saturn and its moons, "tasted" the jets of ice and gas that shoot out of Enceladus's south pole. The jets seem to come from deep within the ocean below.

Both moons seem to contain the basic chemical ingredients for life.

Based on Cassini's data, NASA recently confirmed that water on Enceladus contains organic compounds — the building blocks of amino acids that make up DNA and formed the foundations of life on Earth.

As for Europa, scientists think it first formed with the necessary chemical elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Asteroid impacts may have delivered even more life-giving elements.

But there's one major problem: The sunlight that fuels life on Earth is 25 times fainter on Europa.

Enceladus, even further from the sun, is about -330 degrees Fahrenheit on its surface.

Even the most durable species on Earth, which have adapted to the most extreme conditions, would probably not survive on these moons.

That's because life requires heat and energy. On Earth, we get those things from our proximity to the sun.

But both moons' oceans are likely much warmer than their surfaces, thanks to their oval-shaped orbits.

Because it's tidally locked, like our own moon, the same side of Europa is always facing Jupiter.

As Europa follows its oval-shaped orbit, its distance from Jupiter changes, so the difference between the gravitational pull on Europa's two sides regularly grows and shrinks. These changes are called tides.

Enceladus has the same relationship to Saturn.

These tides stretch and relax Europa and Enceladus, cracking their surface ice and building friction that heats the moons from the inside.

That's what keeps the subsurface oceans from freezing solid.

These tides could crack the moons' mantle and give rise to deep-sea hydrothermal vents.

On Earth, such vents produce intense heat that rips apart molecules and sparks chemical reactions. They form where seawater seeps into the planet's rocky crust, meets volcanically active rock, and blasts back toward the surface.

Scientists first discovered these vents in 1977.

Life on Earth crops up around these vents. These ecosystems don't need sunlight to survive.

On land, food chains rely on plants to convert sunlight to sugar. But in the deep-sea food chain, microbes convert hydrogen to sugar. Rather than photosynthesis (which is fueled by light), this process of "chemosynthesis" uses chemical reactions.

That's why NASA is developing robots to explore hidden alien oceans — like this submarine, called Orpheus.

A drone like Orpheus could dive to the ocean floor in search of hydrothermal vents and deep-sea life.

"It's the great, great, great, great, great, great, great, great grandmother of the vehicle that may go to Europa," biologist Tim Shank, who leads the team that's sending this submarine to Earth's deep seas, previously told Business Insider.

First, scientists must learn how to recognize and observe forms of life that might thrive at deep-sea pressures.

"We only really know how to detect life similar to that on Earth," Berisford said. "So it's possible that very different microbes might be difficult to recognize."

That's why Orpheus is reaching for the unexplored depths of Earth's oceans. It could help to study life in these mysterious Earth environments that are most similar to Europa or Enceladus.

But NASA is still just testing those technologies. Before they take off, the agency plans to send a spacecraft to scout the surface of Europa.

The Europa Clipper spacecraft is slated to fly close to the icy moon 45 times. NASA plans to launch it in 2025.

The spacecraft is expected to fly through Europa's water vapor plumes to analyze what might be in the ocean.

Its radar tools will also measure the thickness of the ice and scan for subsurface water, as the spacecraft flies as close as 16 miles above Europa's surface.

That investigation could help scientists land a future spacecraft on Europa's surface and punch through the ice.

The future lander could search deep ice for signs of life in the ocean below, digging 4 inches below Europa's surface to extract samples for analysis in a mini, on-the-go laboratory.

Scientists estimate the ice around both moons is up to 16 miles thick, though in some parts of Europa it could be as thin as half a mile.

After Europa, Enceladus is about an extra 400 million miles away, but NASA scientists have proposed a mission to search for life there.

The mission, called Enceladus Life Finder (ELF), would fly past Enceladus 10 times to sample the material shooting out of its jets. Those jets are essentially a sample of what's in the moon's hidden ocean.

ELF would try to determine the temperature, pH, oxygen levels, and how energy is moving in the subsurface ocean.

NASA has twice chosen to fund other projects instead.

Hilary Brueck contributed reporting to this post.