Joy Moses Hall

Joy Moses Hall

Gravity pulls a blue Ford from the top of Little Tonshi Mountain to the highway a mile away.

Gravity tugs on a single-engine airplane and keeps it from cruising off into the deep blue of space.

Gravity will grasp and guide a spacecraft on a journey from the surface of Florida, known for its oranges, to the surface of Mars, known for its iron-coated “blueberries.”

The blue Ford was driven by Mom. When the engine stalled a few yards out of the driveway in 1980, she didn’t have the strength to stop it without power brakes, and gravity pulled her, faster and faster, down the road, from mountain to valley, out of control and careening between trees, finally launching across the highway and safely up a grade that took back the speed.

The blue-sky airplane was piloted by Dad. A passion for flight and height took him to a gravitational sweet spot, soaring the sky suspended by speed yet compelled to the atmosphere. If he stopped moving ahead, or if the propeller stopped flinging air back at the wings, the lift forces from air running over the wingfoils would cease, and Earth’s gravity would pluck him down, toward the valleys, back to the solid surface of the Earth.

The spacecraft, expected to launch in July or August, carries Perseverance, a NASA rover intended to scoop and scratch the rusty, dusty, red rocks of Mars, and taste and test its wispy, twisty, weak winds. The seven-month space glide will ride the rails of gravity.

To go to another planet, one must navigate the gravity pulls of every object in the universe, although most are vanishingly small. All things, even blue Fords and blueberries, have gravity fields, but most are so small that nothing sticks. But here, Earth dominates in size and proximity and holds our feet to the floor.

Farther from Earth, its gravity weakens, so the first miles up require the most effort. To leave, one has to push against Earth faster than it pulls back. Once away, Earth fades and other gravities grow prominent, and, like banked roadways, steer everything from asteroids to spacecraft.

A million miles from Earth, the strongest tug out there in the nothing is from the sun, the gravity behemoth of the solar system that holds the planets and everything in between from proceeding off into space, including Mars-bent spaceships. To get to Mars, the Perseverance craft will glide along an arc of orbit highway that leads all the way around the Sun like a solar beltway. But with precision timing, Mars will cross that orbit just as the Perseverance-craft arrives, opening an offramp from the beltway into Mars’ gravity field. From there, Perseverance will parachute through the thin Martian atmosphere to the cold, dry surface, so cold and dry that carbon dioxide freezes in winter, and it snows dry ice.

There are no lakes or rivers.

But the Martian “blueberries” are evidence that there may have been liquid water flowing through Mars’ past. Blueberry rocks can only grow when watered.

And there are no rivers, but there are riverbeds.

Perseverance will rove around a crater that features a channel resembling a dried-up river bed flowing into a dried-up lake. It is not known for blueberries, but the little iron pebbles and the riverbeds speak to the same story that every Mars mission has chronicled: the history of water.

The blueberries on Mars are similar to little iron pebbles found in the fine reddish sands of Utah, and if they share the same blueprint, the blueberry balls were built by water below ground in the fine reddish sands of ancient Mars.

In Utah, the iron balls begin as desert sand, buried under layers of more sand. Gradually, it clumps into sandballs as more sand piles on top. Groundwater leaches iron from the overlying sand, trickles down to the sandballs, and coats them with layer upon layer of iron, leaving iron berryballs.

Sand for sandballs comes from weathering, from wind and water scrubbing specks off rocks. Today, Mars has lots of rocks but not much weather. But maybe it did, the same kind of weather that would rain water into rivers and lakes and plaster iron onto blueberries. Today, Mars has a tenuous, rarefied atmosphere. But billions of years ago, its air and gas may have had flair and mass, rippling out rasping rains that gouged the Martian grains, and the river could have flowed to the lake. And that same water could host life chemicals or even primitive life forms.

It’s a big story, and it depends on gravity to pull it loose from Mars.

The gravity of the situation can’t be overstated.

Joy Moses-Hall teaches physics and astronomy at Pitt Community College. She has a PhD in oceanography and is the author of the novel Wretched Refuge. Follow @jmoseshall on Facebook.