Impact: Mars!

Bad Astronomy
The entire universe in blog form
Feb. 13 2014 7:45 AM

An Asteroid Impacted Mars, and Did This

Mars sits on the inner edge of the asteroid belt, which means it gets hit by rocks more often than Earth does. Not only that, its thinner atmosphere allows smaller asteroids to make it all the way to the ground, so even something the size of a basketball can slam into the surface intact. This probably happens a few times a month on the Red Planet.

That means new craters are excavated by impacts all the time on Mars! The result? Well, see for yourself:

crater on Mars
One of the youngest craters on Mars, and by young, I mean just a few years old! In this enhanced color photo, the blue tint is not real, but shows where redder material on the surface has been ejected by the impact.

Photo by NASA/JPL/University of Arizona


Bang! That is one of the newest craters on Mars: It’s about 30 meters (100 feet) across, and formed by the explosive impact of an asteroid no more than four years ago. We know that because it wasn’t there in an image of Mars taken by the Mars Reconnaissance Orbiter spacecraft in July 2010, but it was in one taken in May 2012.

How big was the rock that did this, and how big was the resulting kaboom? Well, I can guess. In my TV show Bad Universe we detonated 7,500 pounds of ANFO (ammonium nitrate fuel oil mix)—the explosive equivalent of 1.5 tons of TNT—in the Arizona desert to simulate an asteroid impact. The crater we blasted out of the sand was about 20 meters across. The relationship between the size of an explosion and the size of the crater it leaves behind is complicated, but it’s safe to say that the explosive impact on Mars to form this new crater was at least twice as large as the one we set off, or three tons of TNT.

That's me, standing in the bottom of the crater we blew out for the TV show, holding a beach ball. As one does.

Courtesy of Phil Plait

We figured the size of an impactor needed to make a 20 meter crater on Earth is about that of a beach ball: half a meter or so across. That means the asteroid that hit Mars was bigger. There are lots of other variables—impact speed, composition of the asteroid, composition of the surface, and so on—but it all adds up to something probably in the one-meter range. Say the size of an easy chair.

Mind you, the asteroid itself isn’t explosive like a bomb, but it’s moving at high speed, many times faster than a rifle bullet. That gives it a lot of energy, and that energy is released all at once during impact, making what is to all intents and purposes an explosion. It blows out a huge hole in the ground, ejecting material in all directions. Much of it is laid down as a blanket around the crater, and some in long, narrow rays that you can see clearly in the image. Scientists report that some material reached a distance of more than 15 kilometers (more than 9 miles) from the impact site!

An asteroid this size would almost certainly not make it to the ground on Earth because our much thicker atmosphere would tear it apart on its way in. The huge pressures involved would shred the rock into smaller and smaller pieces while it was still dozens of kilometers above the ground (and that would still likely happen even for an iron asteroid).

But bigger rocks are out there; we’re approaching the first anniversary of one 19 meters (more than 60 feet) across that blew up over Russia. While relatively rare, asteroid impacts are a real threat, and this fresh scar carved into the Martian surface is a stark reminder of that.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  



The Irritating Confidante

John Dickerson on Ben Bradlee’s fascinating relationship with John F. Kennedy.

My Father Invented Social Networking at a Girls’ Reform School in the 1930s

Renée Zellweger’s New Face Is Too Real

Sleater-Kinney Was Once America’s Best Rock Band

Can it be again?

The All The President’s Men Scene That Captured Ben Bradlee

Medical Examiner

Is It Better to Be a Hero Like Batman?

Or an altruist like Bruce Wayne?


Driving in Circles

The autonomous Google car may never actually happen.

The World’s Human Rights Violators Are Signatories on the World’s Human Rights Treaties

How Punctual Are Germans?

  News & Politics
The World
Oct. 21 2014 11:40 AM The U.S. Has Spent $7 Billion Fighting the War on Drugs in Afghanistan. It Hasn’t Worked. 
Oct. 21 2014 5:57 PM Soda and Fries Have Lost Their Charm for Both Consumers and Investors
The Vault
Oct. 21 2014 2:23 PM A Data-Packed Map of American Immigration in 1903
  Double X
The XX Factor
Oct. 21 2014 1:12 PM George Tiller’s Murderer Threatens Another Abortion Provider, Claims Right of Free Speech
  Slate Plus
Behind the Scenes
Oct. 21 2014 1:02 PM Where Are Slate Plus Members From? This Weird Cartogram Explains. A weird-looking cartogram of Slate Plus memberships by state.
Oct. 21 2014 12:05 PM Same-Sex Couples at Home With Themselves in 1980s America
Future Tense
Oct. 21 2014 4:14 PM Planet Money Uncovers One Surprising Reason the Internet Is Sexist
  Health & Science
Climate Desk
Oct. 21 2014 11:53 AM Taking Research for Granted Texas Republican Lamar Smith continues his crusade against independence in science.
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.