Christopher Palma, Penn State

On April 8, 2024, millions across the U.S. will have the once-in-a-lifetime chance to view a total solar eclipse. Cities including Austin, Texas; Buffalo, New York; and Cleveland, Ohio, will have a direct view of this rare cosmic event that lasts for just a few hours.

While you can see many astronomical events, such as comets and meteor showers, from anywhere on Earth, eclipses are different. You need to travel to what’s called the path of totality to experience the full eclipse. Only certain places get an eclipse’s full show, and that’s because of scale.

The relatively small size of the Moon and its shadow make eclipses truly once-in-a-lifetime opportunities. On average, total solar eclipses are visible somewhere on Earth once every few years. But from any one location on Earth, it is roughly 375 years between solar eclipses.

I’m an astronomer, but I have never seen a total solar eclipse, so I plan to drive to Erie, Pennsylvania, in the path of totality, for this one. This is one of the few chances I have to see a total eclipse without making a much more expensive trip to someplace more remote. Many people have asked me why nearby eclipses are so rare, and the answer is related to the size of the Moon and its distance from the Sun.

Size and scale

You can observe a solar eclipse when the Moon passes in front of the Sun, blocking some or all of the Sun from view. For people on Earth to be able to see an eclipse, the Moon, while orbiting around the Earth, must lie exactly along the observer’s line of sight with the Sun. Only some observers will see an eclipse, though, because not everyone’s view of the Sun will be blocked by the Moon on the day of an eclipse.

The fact that solar eclipses happen at all is a bit of a numerical coincidence. It just so happens that the Sun is approximately 400 times larger than the Moon and also 400 times more distant from the Earth.

So, even though the Moon is much smaller than the Sun, it is just close enough to Earth to appear the same size as the Sun when seen from Earth.

For example, your pinky finger is much, much smaller than the Sun, but if you hold it up at arm’s length, it appears to your eye to be large enough to block out the Sun. The Moon can do the same thing – it can block out the Sun if it’s lined up perfectly with the Sun from your point of view.

Path of totality

When the Earth, Moon and Sun line up perfectly, the Moon casts a shadow onto the Earth. Since the Moon is round, its shadow is round as it lands on Earth. The only people who see the eclipse are those in the area on Earth where the shadow lands at a given moment.

The Moon is continuously orbiting around the Earth, so as time goes on during the eclipse, the Moon’s shadow moves over the face of the Earth. Its shadow ends up looking like a thick line that can cover hundreds of miles in length. Astronomers call that line the path of totality.

From any given location along the path of totality, an observer can see the Sun completely eclipsed for a few minutes. Then, the shadow moves away from that location and the Sun slowly becomes more and more visible.

A tilted orbit

Solar eclipses don’t happen every single time the Moon passes in between Earth and the Sun. If that were the case, there would be a solar eclipse every month.

If you could float above the Earth’s North Pole and see the Moon’s orbit from above, you would see the Moon line up with the Sun once every time it orbits around the Earth, which is approximately once per month. From this high point of view, it looks like the Moon’s shadow should land on Earth every orbit.

However, if you could shift your perspective to look at the Moon’s orbit from the orbital plane, you would see that the Moon’s orbit is tilted by about 5 degrees compared with Earth’s orbit around the Sun. This tilt means that sometimes the Moon is too high and its shadow passes above the Earth, and sometimes the Moon is too low and its shadow passes below the Earth. An eclipse happens only when the Moon is positioned just right and its shadow lands on the Earth.

As time goes on, the Earth and the Moon continue spinning, and eventually the Moon aligns with Earth’s orbit around the Sun at the same moment the Moon passes between the Sun and the Earth.

While only certain cities are in the path of totality for this April’s eclipse, the entire U.S. is still close enough to this path that observers outside of the path of totality will see a partial eclipse. In those locations, the Moon will appear to pass in front of part of the Sun, leaving a crescent shape of the Sun still visible at the moment of maximum eclipse.

Christopher Palma, Associate Dean for Undergraduate Students and Teaching Professor of Astronomy & Astrophysics, Penn State

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Amy J. Williams, University of Florida

Is or was there life on Mars? That profound question is so complex that it will not be fully answered by the two NASA rovers now exploring it.

But because of the literal groundwork the rovers are performing, scientists are finally investigating, in-depth and in unprecedented detail, the planet’s evidence for life, known as its “biosignatures.” This search is remarkably complicated, and in the case of Mars, it is spanning decades.

As a geologist, I have had the extraordinary opportunity to work on both the Curiosity and Perseverance rover missions. Yet as much as scientists are learning from them, it will take another robotic mission to figure out if Mars has ever hosted life. That mission will bring Martian rocks back to Earth for analysis. Then – hopefully – we will have an answer.

From habitable to uninhabitable

While so much remains mysterious about Mars, there is one thing I am confident about. Amid the thousands of pictures both rovers are taking, I’m quite sure no alien bears or meerkats will show up in any of them. Most scientists doubt the surface of Mars, or its near-surface, could currently sustain even single-celled organisms, much less complex forms of life.

Instead, the rovers are acting as extraterrestrial detectives, hunting for clues that life may have existed eons ago. That includes evidence of long-gone liquid surface water, life-sustaining minerals and organic molecules. To find this evidence, Curiosity and Perseverance are treading very different paths on Mars, more than 2,000 miles (3,200 kilometers) from each other.

These two rovers will help scientists answer some big questions: Did life ever exist on Mars? Could it exist today, perhaps deep under the surface? And would it be only microbial life, or is there any possibility it might be more complex?

The Mars of today is nothing like the Mars of several billion years ago. In its infancy, Mars was far more Earth-like, with a thicker atmosphere, rivers, lakes, maybe even oceans of water, and the essential elements needed for life. But this period was cut short when Mars lost its magnetic field and nearly all of its atmosphere – now only 1% as dense as the Earth’s.

The change from habitable to uninhabitable took time, perhaps hundreds of millions of years; if life ever existed on Mars, it likely died out a few billion years ago. Gradually, Mars became the cold and dry desert that it is today, with a landscape comparable to the dry valleys of Antarctica, without glaciers and plant or animal life. The average Martian temperature is minus 80 degrees Fahrenheit (minus 62 degrees Celsius), and its meager atmosphere is nearly all carbon dioxide.

Early exploration

Robotic exploration of the Martian surface began in the 1970s, when life-detection experiments on the Viking missions failed to find any conclusive evidence for life.

Sojourner, the first rover, landed in 1997 and demonstrated that a moving robot could perform experiments. In 2004, Spirit and Opportunity followed; both found evidence that liquid water once existed on the Martian surface.

The Curiosity rover landed in 2012 and began ascending Mount Sharp, the 18,000-foot-high mountain located inside Gale crater. There is a reason why NASA chose it as an exploration site: The mountain’s rock layers show a dramatic shift in climate, from one with abundant liquid water to the dry environment of today.

So far, Curiosity has found evidence in several locations of past liquid water, minerals that may provide chemical energy, and intriguingly, a variety of organic carbon molecules.

While organic carbon is not itself alive, it is a building block for all life as we know it. Does its presence mean that life once existed on Mars?

Not necessarily. Organic carbon can be abiotic – that is, unrelated to a living organism. For example, maybe the organic carbon came from a meteorite that crashed on Mars. And though the rovers carry wonderfully sophisticated instruments, they can’t definitively tell us if these organic molecules are related to past life on Mars.

But laboratories here on Earth likely can. By collecting rock and soil samples from the Martian surface, and then returning them to Earth for detailed analysis with our state-of-the-art instruments, scientists may finally have the answer to an age-old question.

Perseverance

Enter Perseverance, NASA’s newest flagship mission to Mars. For the past three years – it landed in February 2021 – Perseverance has been searching for signs of bygone microbial life in the rocks within Jezero crater, selected as the landing site because it once contained a large lake.

Perseverance is the first step of the Mars Sample Return mission, an international effort to collect Martian rock and soil for return to Earth.

The instrument suite onboard Perseverance will help the science team choose the rocks that seem to promise the most scientific return. This will be a careful process; after all, there would be only 30 seats on the ride back to Earth for these geological samples.

Budget woes

NASA’s original plan called for returning those samples to Earth by 2033. But work on the mission – now estimated to cost between US$8 billion to $11 billion – has slowed due to budget cuts and layoffs. The cuts are severe; a request for $949 million to fund the mission for fiscal 2024 was trimmed to $300 million, although efforts are underway to restore at least some of the funding.

The Mars Sample Return mission is critical to better understand the potential for life beyond Earth. The science and the technology that will enable it are both novel and expensive. But if NASA discovers life once existed on Mars – even if it’s by finding a microbe dead for a billion years – that will tell scientists that life is not a fluke one-time event that only happened on Earth, but a more common phenomenon that could occur on many planets.

That knowledge would revolutionize the way human beings see ourselves and our place in the universe. There is far more to this endeavor than just returning some rocks.

Amy J. Williams, Assistant Professor of Geology, University of Florida

This article is republished from The Conversation under a Creative Commons license. Read the original article.