How did the earth come to be? NASA's new spacecraft will open secrets
Simulations can turn bits of dust into so-called pebbles up to a meter in size and transform larger, kilometer-sized objects into planetary cores, but they had a problem in the middle, going from small pebbles to rocks kilometers across . The computer models suggested collisions would blow apart growing planetary embryos in this size range. Known as the meter-sized barrier, the problem has plagued planetary formation for decades.
Researchers revealed their first results at the annual Lunar and Planetary Sciences conference in the Woodlands, TX, in March.† MU69 stretches roughly 35 kilometers (20 miles) from tip to tip and spins on its axis every 15.9 hours. Unlike most solar system inhabitants, which spin with their equator aimed at the sun, MU69 lies on its side, pole pointing sunward. The two lobes are roughly the same color, a reddish hue similar to other KBOs observed from Earth, which likely comes from materials known as tholins, formed when radiation from the sun modifies organic molecules.
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The autonomous craft spent 718 days in space. Unlike its previous three trips, when the Air Force provided few details about its mission, this time military spokespeople can point to two totally not nefarious at all science projects.
Because of the nature of Perseverance’s mission, many extra precautions had to be taken. Above all, the rover has to be incredibly clean to avoid any contamination of Mars. Engineers baked the rover at more than 300 degrees Fahrenheit (150 degrees Celsius), to completely sterilize it. “When we collect that sample, we have to make sure when we open it up and we interrogate it for possible ancient microbial life, that we don’t say, ‘Hey, big newsflash, we found life on Mars!’ And come to find out, it’s actually something that hitched a ride from Earth,” Moogega Stricker, the lead for planetary protection on Perseverance at NASA JPL, tells The Verge.
Entire teams of scientists and engineers will determine where and when Perseverance drills. They’re trying to plan as much as they can now, but it all depends on exactly where Perseverance lands within Jezero Crater. Once it’s on the ground, the rover can analyze its surroundings and highlight key areas of interest. The teams are on the hunt for certain types of rocks like carbonates, which have been seen from orbit surrounding the rim of Jezero Crater. They’re also hoping to spot bumpy mounds of layered dirt, known as stromatolites. Stromatolites form in shallow water environments on Earth when mats of cyanobacteria trap tiny bits of sediment, creating layered rock formations. Similar structures on Mars would provide enticing spots to drill.
A slow collision would mean that MU69’s interior probably remains loosely packed, a fluffy aggregate of ice and rock similar to the interior of comets . But had the lobes instead collided at high speed, their interiors would have been compacted.
“We are going to see what looks to us from orbit as a very, very habitable environment,” Ken Williford, the deputy project scientist for Perseverance at NASA’s Jet Propulsion Laboratory, tells The Verge. “If there were microbes in that lake, they had every opportunity to colonize, especially, say, on the edge of the lake where you often find pond scum on Earth or this sort of just green goo at the edge of a pond or a lake. That’s like our holy grail.”
Planet BuildingFully understanding the images of MU69 will take some time, but the first view is already helping shed some light on how planets formed. As rocky pieces move through the gas and dust around a newborn star, they should collide with one another to form larger and larger bodies. It is thought that they would have built up to objects like MU69, which then came together to form larger objects known as planetesimals, which in turn built the planets, from Mercury to Pluto . But there is a missing chapter to this story.Simulations can turn bits of dust into so-called pebbles up to a meter in size and transform larger, kilometer-sized objects into planetary cores, but they had a problem in the middle, going from small pebbles to rocks kilometers across . The computer models suggested collisions would blow apart growing planetary embryos in this size range. Known as the meter-sized barrier, the problem has plagued planetary formation for decades.In recent years, new theories have emerged to help with the problem. One idea, known as the streaming instability , suggests that gas in the disk creates drag, forcing these small objects to concentrate into clumps. Gravity then pulls the clumps together to create kilometer-sized objects in a matter of tens of thousands to hundreds of thousands of years, depending on the solar distance no midsized material is necessary. A popular extension of this theory, known as pebble accretion, suggests that smaller objects continue to fall onto the kilometer-sized objects to rapidly build the cores of giant planets.Both the pancake shape of MU69 and its dearth of midsized craters seem to validate these theories. If large objects form via the streaming instability, there will be few projectiles in the range of tens of meters to create those craters. And according to David Nesvorny, who models early solar system formation at SwRI, the flat lobes of MU69 are also telling. The shapes are too flat to have come from the initial spin of a cloud of material. Such a large cloud would have had too much angular momentum to hold together and, instead, split into two objects. A loose collection of material in the outer solar system could have gathered together, as proposed by the streaming instability. Gravity would have pulled most of it together, creating objects flatter than a sphere but not quite pancake-like, then pebble accretion would have piled most of the remaining pieces on top, creating the flattened lobes visible today. Eventually, Ultima and Thule would have drawn closer together, gently docking with one another.The researchers haven’t yet simulated the process for MU69 specifically, although they hope to. If correct it could solve the problem of how to build a planet. “That meter-sized barrier that we all stressed out about, we just jumped right over it,” says astronomer Kevin Walsh of SwRI. “We all suspected something like that but these observations from the Kuiper Belt really seem to clinch it.”Previous models have always worked with spherical bodies rather than pancakes, in part because of limited computer power, according to McKinnon. “ might have tracked angular momentum, but they certainly didn’t try to build shapes,” he says. The new results from MU69 have likely changed that. “Now I’m sure people will because we have the capabilities to do this sort of stuff.”MU69 may live at the edge of the solar system, but the processes that formed it probably worked throughout, Nesvorny says. Earth and other planets likely formed from similarly flattened objects. And the process is not limited to our planetary neighborhood. “Elsewhere in exoplanetary systems, it might be quite common to form pancake and contact binaries,” he says.New Horizons will be sending back data on MU69 until the end of 2020, and plenty of further study will be needed, but it already has provided a wealth of insight about both the Kuiper Belt and the entire solar system. “We had no right to expect that we would get so much from this one small body,” Stern says, “but we have.”
