Astronomy - February 2014

Up

quark

Up

antiquark

Electron

Antineutrino

Down

antiquark

Up quark

Bottom

quark

Bottom

antiquark

34 ASTRONOMY t FEBRUARY 2014

A: In early January 2005, on

the 339th martian day (each

day is a “sol”) of its mission, the

Mars Exploration Rover (MER)

Opportunity discovered a

nickel-iron meteorite on the

surface of Mars’ Meridiani

Planum. It was the first mete-

orite found on another planet.

Because the meteorite lies

near the rover’s discarded heat

shield, the MER team dubbed

it Heat Shield Rock. (Its formal

name is “Meridiani Planum,”

but many scientists use its

informal name.)

The meteorite is 12 inches

(31 centimeters) long and

shows the characteristic pits,

hollows, and voids common in

nickel-iron meteorites on Earth.

Scientists estimate Heat Shield

Rock’s mass is 88 pounds (40

kilograms).

Currently, the meteorite

is resting on a flat plain of

sulfate-rich sandstone, named

the Burns Formation. There’s

no sign of a crater near it, and

a small drift of wind-blown

sand has collected around the

meteorite. Scientists have stud-

ied the nearby rocky geography,

and they believe it erodes rela-

tively easily.

According to planetary

researchers, Heat Shield Rock

might have landed 3 billion

years ago or 300,000 years ago.

Scientists have no way to tell for

sure. It’s also unclear whether it

landed as an individual meteor-

ite, broke off from a larger

incoming mass, or survived a

crater-making impact.

Heat Shield Rock does have

some interesting features. It has

an oxide coating that the MER

Astronomy’s experts from around the globe answer your cosmic questions.

ALIEN MARTIAN

team thinks might be rust

instead of a fusion crust, which

we typically expect for a mete-

orite due to its flight through an

atmosphere. The meteorite also

displays sculpting from wind-

blown sand grains. Such details

offer glimpses of a harsh envi-

ronmental history for the

region. For example, Block

Island (a nickel-iron meteorite

about twice the size of Heat

Shield Rock discovered in 2009

on Mars) rests on a 2-inch-high

(5cm) pedestal of Burns Forma-

tion sandstone, which suggests

that much rock has eroded from

around and underneath the

meteorite since it landed.

So why isn’t there a crater at

Heat Shield Rock? Long story

short, the meteorite landed ages

and ages ago, and it’s a great

deal tougher than the rocks it’s

sitting on.

Robert Burnham

Mars Space Flight Facility,

Arizona State University

Q: Could the energy

produCed during

matter-antimatter

annihilation in the

early universe be dark

energy? if not, where is

that produCed energy

to day?

Michael Lynch

Dallas

A: Astronomers see galaxies

flying away from each other

faster than expected. Some sort

of energy — dubbed “dark

energy” because we cannot

identify what it is — must be

causing this repulsion. We

know that dark energy com-

poses an amazing 68 percent of

the universe, so it produces an

extremely big effect. Normal

matter — like stars, gas, and

planets — is only 5 percent of

the cosmos.

Scientists believe that the

laws of physics are constant

everywhere and at all times.

Decades of experiments have

tested this principle and shown

that it is valid. Therefore, we

can use our current theory to

predict what happened at the

Big Bang, even though no one

was around to observe the uni-

verse’s beginning.

We can study and measure

matter-antimatter annihilations

in high-energy accelerators.

For instance, when quarks

interact with antiquarks, we

can measure the newly pro-

duced particles that have energy

we can observe. Thus, the colli-

sions aren’t creating dark energy

(we can’t see dark energy; we

can only detect its effect). Accel-

erator experiments also show no

hint of dark matter — the mys-

terious mass that makes up 27

percent of the cosmos.

ASKASTR0

Q: WHAT IS THE MASS OF THE HEAT

SHIELD ROCK METEORITE ON MARS?

SHOULDN’T IT HAVE LEFT A CRATER WHEN

IT HIT? INSTEAD, IT LOOKS LIKE IT IS JUST

SITTING “PASSIVELY” ON THE SURFACE AS

IF GENTLY PLACED THERE.

Dennis Barnes, Seneca, South Carolina

The Mars Exploration

Rover Opportunity

captured this photo-

graph of Heat Shield

Rock on the Red

Planet; it is the first

meteorite found on

another world. NASA/

JPL/CORNELL

When a quark collides with its antiquark, the interaction produces energy in

the form of moving particles, antiparticles, and energy. Because scientists can

detect these particles and energy, they are not the mysterious dark energy.

ASTRONOMY

: ROEN KELLY

Quark and antiquark

annihilate each

other