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After Cassini: 14 Epic Planetary Science Missions to Get Excited About

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An artistic representation of the Mars2020 rover (operating the SuperCam remote sensing instrument) on the Red Planet.

NASA’s Cassini spacecraft ended its epic 13-year stint at Saturn on Sept. 15, but there are other upcoming planetary science missions to look forward to.

Currently, there are several active missions (led by NASA as well as other space agencies) exploring planets and other rocky objects in the solar system. For example, the Juno probe is studying Jupiter, and the Curiosity rover is exploring Mars. Looking ahead, NASA is reviewing mission proposals that may include returning to Saturn to search for signs of life on ocean worlds, like the planet’s moons Enceladus and Titan. Planned missions to Mars, Mercury, Jupiter and other celestial bodies in our solar system and beyond are also in the works.

Here’s a list of some of the orbiters, probes and rovers en route to new destinations or slated to launch in the next few years. [In Photos: Cassini Mission Ends with Epic Dive into Saturn]

NASA’s Mars 2020 rover will search for signs of past microbial life and possibly habitable conditions that may have once existed on the Red Planet. The rover’s basic design resembles that of NASA’s nuclear-powered Curiosity rover. The Mars 2020 rover will use a drill to collect core samples of rocks and soils, and then examine those samples on a microscopic level to search for biosignatures, or chemicals that could be indicative of ancient life on the Red Planet. (The Mars 2020 drill will probe much deeper into the Martian surface than the drill on Curiosity.) Samples collected by the Mars 2020 rover could potentially be returned to Earth in a future mission.

The rover is expected to launch in July or August of 2020 aboard a United Launch Alliance Atlas V rocket. Three potential landing sites have been selected and include an ancient lake bed called the Jezero crater, the edge of the Syrtis Major volcanoes and a hot-spring site called Columbia Hills. Mission scientists hope that, after it touches down on the Red Planet, the rover will explore the Martian surface for two years. This mission offers a unique opportunity to prepare for future human exploration of Mars, mission team members have said.

NASA’s InSight Mars lander is expected to launch in May 2018 and arrive at the Red Planet in November 2018. The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission will study the planet’s deep interior to gain a better understanding of the processes that helped shaped rocky planets like Mars and Earth.

Once it has landed on the Red Planet, the probe will spend a full Mars year (687 Earth days) surveying its surroundings. (Because it’s not a rover, it will have to stay in one spot.) InSight will be equipped with two specialized instruments, allowing it to probe deep beneath the Martian surface and study the planet’s internal geologic activity and temperature.

The European Space Agency (ESA) also has its eye on the Red Planet. The ExoMars rover mission is designed to search for signs of ancient life that may have once existed on Mars. The golf-cart-size rover will be equipped with a drill to collect samples, as well as a panoramic camera system for stereoscopic imaging and ground-penetrating radar to search for ice beneath the Martian surface.

The ExoMars rover is scheduled to launch in the spring of 2020. The rover will reach the Red Planet in 2021, joining the ExoMars Trace Gas Orbiter — the first phase of the ExoMars mission, which launched toward the Red Planet on March 14, 2016.

This artist’s rendering shows NASA’s Europa Clipper spacecraft, which is being developed for a launch sometime in the 2020s and will explore Jupiter’s icy moon.

Credit: NASA/JPL-Caltech

NASA’s Europa Clipper mission will study the possibly habitable Jovian moon Europa. The probe is expected to launch in 2022 and later settle into orbit around Jupiter in 2025. The solar-powered spacecraft will perform about 40 flybys of Europa to learn more about the ocean of liquid water that lies beneath the moon’s icy crust and perhaps determine if it is capable of supporting life as we know it on Earth.

ESA is also planning a mission to Jupiter in 2022. However, the Jupiter Icy Moons Explorer, also known as JUICE, won’t arrive at the Jovian giant until 2029. JUICE will study Jupiter’s atmosphere and magnetic environment, and it will also investigate three of the planet’s Galilean moons: Europa, Callisto and Ganymede.

ESA and the Japanese Aerospace Exploration Agency (JAXA) plan to launch a joint mission to Mercury in October 2018. The mission includes a carrier spacecraft called the Mercury Transfer Module (MTM) — which supplies electrical power during interplanetary cruising — and two separate orbiters: Europe’s Mercury Planet Orbiter and Japan’s Mercury Magnetospheric Orbiter.

The spacecraft will take about seven years to get into orbit around Mercury, using several gravity assists from Earth and Venus. The mission is designed to investigate how Mercury formed so close to a parent star, and to take a closer look at the planet’s interior structure, geology, composition and magnetic field.

This artist's rendering show the Parke Solar Probe, which will fly closer to the sun than any previous spacecraft.

This artist’s rendering show the Parke Solar Probe, which will fly closer to the sun than any previous spacecraft.

Credit: Johns Hopkins University Applied Physics Laboratory

NASA’s Parker Solar Probe, which is scheduled to launch on July 31, 2018, will travel closer to the sun than any spacecraft in history. The mission will perform 24 close flybys of the sun — some of which will bring the spacecraft within just 3.9 million miles (6.2 million kilometers) of the solar surface.

From this unique vantage point, the probe will be able to measure the sun’s electric and magnetic fields, photograph the solar structure and study the solar wind. These findings could help astronomers answer questions about the sun’s perplexing outer atmosphere, also known as the corona, and other long-standing mysteries.

China is planning to launch a sample-return mission to the moon at the end of November 2017. The mission, called Chang’e 5, will be the first to return lunar material to Earth in more than 40 years. The spacecraft will include an orbiter, a lander, an ascender and an Earth re-entry module. Chang’e 5 is one in a series of China’s moon exploration missions, which also include Chang’e 4 — a lunar probe set to launch around 2018 and make the first-ever soft landing on the farside of the moon.

The Google Lunar X Prize is an international challenge to land a robot on the lunar surface, have it travel at least 1,650 feet (500 meters), and send high-definition photos and videos back to Earth. There are five teams still competing for the $30 million prize: Florida-based Moon Express, Israel’s SpaceIL, Japan’s Hakuto, India-based TeamIndus and the international collaboration Synergy Moon. To qualify for the Lunar X Prize, teams must complete their lunar missions by March 31, 2018.

This artist's concept shows the Origins Spectral Interpretation Resource Identification Security - Regolith Explorer (OSIRIS-REx) spacecraft grabbing a sample of an asteroid for return to Earth.

This artist’s concept shows the Origins Spectral Interpretation Resource Identification Security – Regolith Explorer (OSIRIS-REx) spacecraft grabbing a sample of an asteroid for return to Earth.

Credit: NASA’s Goddard Space Flight Center

NASA’s OSIRIS-REx (short for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) mission will study the near-Earth asteroid Bennu. The mission launched on Sept. 8, 2016, and is slated to arrive at Bennu in 2018.

OSIRIS-REx will spend approximately two years studying the rocky body in great detail, before collecting a sample to bring home to Earth. Asteroids are leftovers from the formation of planets and carry blueprints of the early solar system. Samples collected from Bennu will therefore help astronomers learn more about the evolution of our solar system and how planets formed. If all goes according to plan, OSIRIS-REx will return to Earth in 2023, marking the first U.S. asteroid sample-return mission.

JAXA’s Hayabusa2 mission is another asteroid-sampling mission en route to its target destination. The spacecraft launched on Dec. 2, 2014, and is expected to arrive at asteroid 162173 Ryugu in 2018. Hayabusa2 follows JAXA’s historic 2003 Hayabusa mission, which brought the first pristine samples of an asteroid to Earth in 2010.

This time around, the mission will land a small probe on the surface of the asteroid, as well as a pair of rovers for exploring the asteroid’s surface. Hayabusa2 will spend a year studying the asteroid before collecting samples to return to Earth in December 2020.

NASA’s Psyche mission will launch in 2022 to study a bizarre metal asteroid up close. The asteroid, called 16 Psyche, is located in the belt between Mars and Jupiter. Whereas most asteroids are made of rock, Psyche is composed of metallic iron and nickel — the same material found in Earth’s core. It’s the only known object of its kind in the solar system, leading astronomers to believe that the asteroid is the remnant of what was once a protoplanet in the early solar system. Therefore, learning more about this asteroid will help scientists better understand the cores of Earth, Mars, Mercury and Venus.

NASA’s New Horizons probe visited Pluto in July 2015, completing a nearly decade-long journey to the distant dwarf planet. The mission provided the first-ever up-close view of Pluto, revealing new details about its icy surface and largest moon, Charon.

Since accomplishing this amazing feat, the probe is still going strong and is set on a new object deeper in the Kuiper Belt, located approximately 1 billion miles (1.6 billion km) beyond Pluto. On Jan. 1, 2019, the spacecraft will fly within just 2,175 miles (3,500 kilometers) of the distant object called 2014 MU69, allowing the probe to study the rocky body up close. This ancient object is also expected to help paint a clearer picture of what the early solar system was like.

This artist's concept depicts NASA's Voyager 1 spacecraft entering interstellar space in 2012. The probe and its twin, Voyager 2, are still in contact with Earth.

This artist’s concept depicts NASA’s Voyager 1 spacecraft entering interstellar space in 2012. The probe and its twin, Voyager 2, are still in contact with Earth.

Credit: NASA/JPL-Caltech

This year, NASA’s historic Voyager mission celebrated 40 years in space, and it’s not ready to quit anytime soon. The twin spacecraft launched two weeks apart in 1977 — Voyager 2 on Aug. 20 and Voyager 1 on Sept. 5 — with an initial goal to study the planets and explore the outer solar system.

Over the course of the mission, the Voyager probes have captured up-close views of Jupiter, Saturn, Uranus, Neptune and many of the moons of these giant planets. In August 2012, Voyager 1 became the first spacecraft ever to reach interstellar space, and Voyager 2 is currently flying through the bubble of solar material that marks the boundary between the solar system and interstellar space.

Follow Samantha Mathewson @Sam_Ashley13. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

Space.com is the premier source of space exploration, innovation and astronomy news, chronicling (and celebrating) humanity's ongoing expansion across the final frontier. We transport our visitors across the solar system and beyond through accessible, comprehensive coverage of the latest news and discoveries. For us, exploring space is as much about the journey as it is the destination. So from skywatching guides and stunning photos of the night sky to rocket launches and breaking news of robotic probes visiting other planets, at Space.com you’ll find something amazing every day.

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In Pictures: Sierra Nevada's Dream Chaser Aces Glide Test Flight

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Free-Flying

Credit: NASA

The glide test success indicates the program is one step closer to orbital exercises.

Ready for Testing

Ready for Testing

Credit: Ken Ulbrich/NASA

The spacecraft was moved from inside the facility by a transport to prepare for the test.

Gear in Place

Gear in Place

Credit: Ken Ulbrich/NASA

Apparatus attached atop the Dream Chaser enabled a helicopter to raise the craft for the release and flight.

Up and Away

Up and Away

Credit: Ken Ulbrich/NASA

The full scale Dream Chaser craft, shown here lifted by a Columbia Helicopters Model 234-UT Chinook helicopter, flew a pre-planned flight path after its release.

Safe and Secure

Safe and Secure

Credit: Ken Ulbrich/NASA

The successful test flight ended at Edwards Air Force Base on Runway 22L.

Big Plans

Big Plans

Credit: Ken Ulbrich/NASA

The first trip to the International Space Station for the Dream Chaser is planned for 2020.

Practicing for the Big Journey

Practicing for the Big Journey

Credit: Ken Ulbrich/NASA

The Dream Chaser is scheduled for at least six missions under NASA’s Commercial Resupply Services 2 contract, beginning as early as 2020.

Proof

Proof

Credit: NASA

This atmosphere Free-Flight test verified the craft has the design and capabilities to return and land safely.

Free-Flight

Free-Flight

Credit: NASA

Sierra Nevada Corporation’s Dream Chaser has displayed the ability to provide safe and reliable orbital flight, according to corporate vice president, Mark Sirangelo.

Approaching the Runway

Approaching the Runway

Credit: NASA

With NASA on board, Sierra Nevada Corporation will analyze the test data.

Wheels Down

Wheels Down

Credit: Ken Ulbrich/NASA

Using results from this Free-Flight test, engineers can perfect the aerodynamics of the Dream Chaser, making it even safer for future flights.

Catching Some Air

Catching Some Air

Credit: Ken Ulbrich/NASA

As the full-scale test vehicle is raised by the Chinook helicopter on Nov. 13, 2017, Sierra Nevada Corporation looks to the future of this spacecraft.

Rollout

Rollout

Credit: Ken Ulbrich/NASA

The Dream Chaser spacecraft is readied for the atmospheric Free-Flight test.

Suspended Suspense

Suspended Suspense

Credit: Ken Ulbrich/NASA

Rising high into the atmosphere, the Dream Chaser begins the atmospheric Free-Flight test.

Teamwork

Teamwork

Credit: Ken Ulbrich/NASA

The Flight Crew prepares Sierra Nevada Corporation’s full-scale Dream Chaser test vehicle for the upcoming Free-Flight test.

Last Minute Checks

Last Minute Checks

Credit: Ken Ulbrich/NASA

The Flight Crew for the Dream Chaser spacecraft completes preflight checks before the craft participates in the monumental test.

Future at Hand

Future at Hand

Credit: Ken Ulbrich/NASA

Sierra Nevada Corporation’s Dream Chaser spacecraft looks to be the next phase in NASA’s journey into space.

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Apollo 17 Astronaut Begins Releasing Diary 45 Years After Moon Mission

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Apollo 17 astronaut Harrison Schmitt, seen here during the first of his three moonwalks in December 1972, has begun to release his diary of the last lunar landing mission 45 years later.

Harrison Schmitt went for a walk on Dec. 11, 1972. Forty-five years later, he is almost ready to share his diary of that day.

The last of the twelve NASA astronauts to step foot onto the surface of the moon — and the only geologist to do so — Schmitt was the lunar module pilot on NASA’s Apollo 17 mission, the sixth, last, and as Schmitt puts it, “most recent human visit to the moon.” Now, on the 45th anniversary of his lunar journey, Schmitt is beginning to take the public on a stroll through history, his memories and the findings that came from exploring Taurus Littrow Valley on the moon.

“This project began 45 years ago,” explains Schmitt. “I am gradually getting to the point where the drafting, I think, is good enough that I can let other people share in what my impressions were during the mission, as well as what the whole operation was about.” [The Apollo Moon Landings: How They Worked (Infographic)]

Apollo 17: Diary of the Twelfth Man” quietly debuted as a new section of Schmitt’s website in early November when he uploaded the fourth chapter, “30 Days and Counting.” Although the first through third chapters still remain to be published, Schmitt chose the fourth to begin “because the interval between its online publication date and the launch date coincides with the chapter title,” he wrote in a note to accompanying the post.

On Friday (Dec. 7), 45 years to the day after he lifted off with Apollo 17 commander Eugene Cernan and command module pilot Ronald Evans, Schmitt posted the diary’s fifth chapter, “30 Seconds and Counting!” — an almost 49,000 word account of the Saturn V launch that follows Schmitt and his crewmates from the ground to leaving Earth orbit for the moon.

“[Seven and a half] 7.5 million pounds of thrust would lift us slowly at first and then faster and faster toward orbit on a trail of brilliant flame, not visible to us but splitting open the night for everyone below,” recounts Schmitt. “Pulsing waves of sound and searing streams of light buffeted the bodies and minds of onlookers, bringing spontaneous and unexpected hugs, cries and tears.”

The Apollo 17 launch marked the first time NASA had sent astronauts into space at night.

“Once again, a life-tipped pillar of fire, the Saturn rocket, a massive tribute to boldness and imagination, became a blazing symbol of human potential for greatness. Except for reflected light coming through the small window in front of Gene and another in the boost protective cover over the [crew] hatch, I had only a vague sense of the brilliant flame beneath us,” writes Schmitt.

Schmitt pulls from a wide variety of sources for the content of the diary. Amid his own observations, Schmitt cites from NASA air to ground radio transcripts, public affairs reports and the recollections of others. He has also formatted the journal to help readers keep track of the topics at hand.

“A complication to reading diaries is their instantaneous jump from subject to subject. In addition to the liberal use of endnotes, distinguishing between subjects and sources is aided by the consistent use of different font styles and colors in the text,” Schmitt explains in his preface.

Harrison Schmitt is releasing “Apollo 17: Diary of the Twelfth Man” in chapters on his website.

Credit: NASA

For example, Schmitt turns the text red when discussing anomalies, or problems, during the mission. He uses blue when writing of Earth observations and he uses purple for views about the moon. He reserves turquoise for “probable dialog” between he and his two crewmates, as he can best derive from his memory.

“On the horizon, bands of orange and blue lay below the black of space,” Schmitt writes, describing — in blue text — and captioning a photograph of his first view of Earth from space. “Outside, darkness finally had been broken by a spectacular sunrise that had provided what I described a few minutes later as ‘the biggest rainbow I’d ever seen,’ extending along the entire pre-sunrise horizon.”

“Like childhood’s home, we really see the Earth only as we prepare to leave,” waxes Schmitt.

“It [the diary] is both technical and philosophical, in some aspects,” Schmitt told collectSPACE.

Schmitt, who is now 82, logged a total of 75 hours on the lunar surface, including 22 hours out on three moonwalks. With Cernan’s death in January, Schmitt became the final living member of the Apollo 17 crew (Evans died in 1990).

Although 45 years have already passed for Schmitt to feel ready to prepare and share the diary, he still sees it as a “long-running project” that will not be completed within just the 12-day span of the mission.

“The 45th anniversary seemed like a good time to begin,” he said. “It is going to take a long time still. There is a lot to be said.”

To read “Apollo 17: Diary of the Twelfth Man,” see Harrison Schmitt’s website, americasuncommonsense.com.

Follow collectSPACE.com on Facebook and on Twitter at @collectSPACE. Copyright 2017 collectSPACE.com. All rights reserved.

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How Was Mars Made? | Formation of Mars

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The planet Mars was formed, along with the rest of the solar system, about 4.6 billion years ago. But exactly how the planets formed remains a subject of debate. Currently, two theories are duking it out for the role of champion.

The first and most widely accepted theory, core accretion, works well with the formation of the terrestrial planets like Mars but has problems with giant planets. The second, the disk instability method, may account for the creation of these giant planets. 

Artist’s conception of our solar system’s solar nebula, the cloud of gas and dust from which the planets formed.

Credit: Painting copyright William K. Hartmann, Planetary Science Institute, Tucson

Scientists are continuing to study planets in and out of the solar system in an effort to better understand which of these methods is most accurate. 

The leading theory, known as core accretion, is that the solar system began as a large, lumpy cloud of cold gas and dust, called the solar nebula. The nebula collapsed because of its own gravity and flattened into a spinning disk. Matter was drawn to the center of the disk, forming the sun.  

Other particles of matter stuck together to form clumps called planetesimals. Some of these combined to form asteroids, comets, moons and planets. The solar wind — charged particles streaming out from the sun — swept away the lighter elements, such as hydrogen and helium, leaving behind mostly small, rocky worlds. In the outer regions, however, gas giants made up of mostly hydrogen and helium formed because the solar wind was weaker.

Exoplanet observations seem to confirm core accretion as the dominant formation process. Stars with more “metals” — a term astronomers use for elements other than hydrogen and helium — in their cores have more giant planets than their metal-poor cousins. According to NASA, core accretion suggests that small, rocky worlds should be more common than the more massive gas giants.

The 2005 discovery of a giant planet with a massive core orbiting the sun-like star HD 149026 is an example of an exoplanet that helped strengthen the case for core accretion.

“This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance,” said Greg Henry in a press release. Henry, an astronomer at Tennessee State University, Nashville, detected the dimming of the star.

In 2018, the European Space Agency plans to launch the CHaracterising ExOPlanet Satellite (CHEOPS), which will study exoplanets ranging in sizes from super-Earths to Neptune. Studying these distant worlds may help determine how planets in the solar system formed.

“In the core accretion scenario, the core of a planet must reach a critical mass before it is able to accrete gas in a runaway fashion,” said the CHEOPS team

“This critical mass depends upon many physical variables, among the most important of which is the rate of planetesimals accretion.”

By studying how growing planets accrete material, CHEOPS will provide insight into how worlds grow.

Core accretion was first postulated in the late 18th century by Immanuel Kant and Pierre Laplace. Nebula theory helps explain how the planets in our solar system were formed. But with the discovery of “Super-Earth” planets orbiting other stars, a new theory, known as disk instability was proposed. 

Although the core accretion model works fine for terrestrial planets, gas giants would have needed to evolve rapidly to grab hold of the significant mass of lighter gases they contain. But simulations have not been able to account for this rapid formation. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.

According to a relatively new theory, disk instability, clumps of dust and gas are bound together early in the life of the solar system. Over time, these clumps slowly compact into a giant planet. These planets can form faster than their core accretion rivals, sometimes in as little as a thousand years, allowing them to trap the rapidly-vanishing lighter gases. They also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.

According to exoplanetary astronomer Paul Wilson, if disk instability dominates the formation of planets, it should produce a wide number of worlds at large orders. The four giant planets orbiting at significant distances around the star HD 9799 provides observational evidence for disk instability. Fomalhaut b, an exoplanet with a 2,000-year orbit around its star, could also be an example of a world formed through disk instability, though the planet could also have been ejected due to interactions with its neighbors.

The biggest challenge to core accretion is time — building massive gas giants fast enough to grab the lighter components of their atmosphere. Recent research on how smaller, pebble-sized objects fused together to build giant planets up to 1000 times faster than earlier studies.

“This is the first model that we know about that you start out with a pretty simple structure for the solar nebula from which planets form, and end up with the giant-planet system that we see,” study lead author Harold Levison, an astronomer at the Southwest Research Institute (SwRI) in Colorado, told Space.com in 2015.

In 2012, researchers Michiel Lambrechts and Anders Johansen from Lund University in Sweden proposed that tiny pebbles, once written off, held the key to rapidly building giant planets.

“They showed that the leftover pebbles from this formation process, which previously were thought to be unimportant, could actually be a huge solution to the planet-forming problem,” Levison said.

Levison and his team built on that research to model more precisely how the tiny pebbles could form planets seen in the galaxy today. While previous simulations, both large and medium-sized objects consumed their pebble-sized cousins at a relatively constant rate, Levison’s simulations suggest that the larger objects acted more like bullies, snatching away pebbles from the mid-sized masses to grow at a far faster rate.

“The larger objects now tend to scatter the smaller ones more than the smaller ones scatter them back, so the smaller ones end up getting scattered out of the pebble disk,” study co-author Katherine Kretke, also from SwRI, told Space.com. “The bigger guy basically bullies the smaller one so they can eat all the pebbles themselves, and they can continue to grow up to form the cores of the giant planets.”

In 2018, NASA will launch the InSight mission to Mars that will study the planet’s interior.

“But InSight is more than a Mars mission — it is a terrestrial planet explorer that will address one of the most fundamental issues of planetary and solar system science — understanding the processes that shaped the rocky planets of the inner solar system (including Earth) more than four billion years ago,” according to NASA.

“InSight seeks to answer one of science’s most fundamental questions: How did the terrestrial planets form?”

Whether Mars got its start through disk instability or core or pebble accretion, it continued to pack on the weight as it grew. Models suggest that the Red Planet should be about as large as Venus and Earth if gas and dust were smoothly spread through the solar system. Instead, Mars is only 10 percent as massive, suggesting that it formed in a region low on planetary building blocks.

Enter the Grand Tack model, the leading theory to explain the so-called “small Mars problem.” According to the model, Jupiter and Saturn migrated toward the sun shortly after their birth before tacking like a sailboat and returning to the outer solar system. Along the way, they would have swept up much of the debris that should have fed Mars’ formation.

The western scarp of Olympus Mons has both steep and gentle slopes with clear channels, some likely created by flowing liquid, perhaps water, and some apparently carved by glaciers.

The western scarp of Olympus Mons has both steep and gentle slopes with clear channels, some likely created by flowing liquid, perhaps water, and some apparently carved by glaciers.

Credit: Nature/ESA/G. Neukum

“Provided that Jupiter changed direction close to 1.5 AU, the growth of Mars would be successfully stunted while leaving enough material closer to the sun to form Earth and Venus,” John Chambers of the Carnegie Institution for Science wrote in a 2014 “Perspectives” piece published in the journal Nature.

Another possibility is that regions of low density formed naturally in the protoplanetary disk. 

“If this partial gap survived long enough, it could have been preserved in the distribution of planetesimals and planetary embryos that formed subsequently,” Chambers writes. “The simulations performed by Izidoro show that reducing the number of planetary building blocks near Mars’ current orbit by 50 to 75 percent favors the formation of a puny Red Planet.”

Another option is that Mars actually got its start in the asteroid belt, then migrated toward the sun because of its interaction with planetesimals.

“Since Mars is more massive than the planetesimals, it tends to lose energy when it scatters these planetesimals because it passes them to Jupiter, which then ejects them from the solar system,” Ramon Brasser, lead author and associate professor at the Tokyo Institute of Technology’s Earth-Life Science Institute, told Space.com.

Like all planets, Mars became hot as it formed because of the energy from these collisions. The planet’s interior melted and denser elements such as iron sank to the center, forming the core. Lighter silicates formed the mantle, and the least-dense silicates formed the crust. Mars probably had a magnetic field for a few hundred million years, but as the planet cooled, the field died. 

The young Mars had active volcanoes, which spewed lava across its surface, and water and carbon dioxide into the atmosphere. But there is no tectonic activity on Mars, so the volcanoes remained stationary and grew with each new eruption.

The volcanic activity also probably gave Mars a thicker atmosphere. Mars’ magnetic field protected the planet from radiation and solar wind. With a higher atmospheric pressure, water probably flowed on Mars’ surface, studies indicate. But about 3.5 billion years ago, Mars began to cool. Volcanoes erupted less and less and the magnetic field disappeared. The unprotected atmosphere was blown away by solar wind and the surface was bombarded by radiation.

Under these conditions, liquid water cannot exist on the surface. Studies suggest water is be trapped underground in both liquid and frozen forms and in the ice sheets of the polar ice caps.

All life as we know it requires liquid water, so there is much interest in finding evidence of it on Mars.

— Additional reporting by Nola Taylor Redd, Space.com Contributor

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