Scroll down to read this post.


Please consider supporting my work, as I take no advertisements nor accept any sponsors in order to keep the website clean, easy to read, and to avoid any accusations of conflict of interest. Your support leaves me entirely independent, able to say whatever I think while being free from censorship or reprisals.


You can support me either by giving a one-time contribution or a regular subscription. There are five ways of doing so:


1. Zelle: This is the only internet method that charges no fees. All you have to do is use the Zelle link at your internet bank and give my name and email address (zimmerman at nasw dot org). What you donate is what I get.


2. Patreon: Go to my website there and pick one of five monthly subscription amounts, or by making a one-time donation.

3. A Paypal Donation:

4. A Paypal subscription:

5. Donate by check, payable to Robert Zimmerman and mailed to
Behind The Black
c/o Robert Zimmerman
P.O.Box 1262
Cortaro, AZ 85652

Deciphering the strange geology of Mars — or anything!

Eroding Medusae Fossae Formation ash deposits
Click for full image.

Today’s cool image is for once not taken by the high resolution camera on Mars Reconnaissance Orbiter (MRO). Instead, the image to the right, cropped and reduced to post here, was taken by Mars Odyssey on April 5, 2020, and shows the scouring and erosion caused by winds over many eons in a region dubbed Zephyria Planum. (Note that the image might fool your eye. Sunlight is coming from the east, and the rough terrain at the top is higher than the smooth plain at the bottom.)

Years ago, when I first started to rummage through the archives of images from the various Mars orbiters, I would have seen this image and posted it because I was completely baffled by what I saw, and thought that mystery made it worth showing to the public. Since then my incessant probing of research papers as well as asking a lot of questions of scientists has taught me a lot more about what scientists now surmise of the Martian geology. This greater knowledge in turn makes it possible for me to look at an image like this and immediately make a reasonable guess as to an explanation. This photo, while still containing much that is mysterious, is no longer completely baffling to me.

This willingness to ask questions and dig deeper is fundamental to all things. To have a deeper understanding and not simply guess about any subject, you always have to recognize that your assumptions are likely wrong, and that to learn anything you have to repeatedly ask what I call “the next question.” The first answer will force you to recognize that your first guesses are wrong, raise more questions, which in turn will lead to more questions, and then more questions, and so forth.

Whether I am researching Mars or early space history or politics, this rule always applies. Don’t leap to a conclusion. Think it possible you could be wrong. Ask the next question. And the next. You will repeatedly find that what you thought you knew was not correct, and in the end you will gain a deeper understanding of what is actually known about any subject, as well as what is unknown. And knowing the unknowns is probably the most important thing you can learn.

To gain a better understanding of today’s particular image, our first questions must start with context. Where is this feature on Mars? What is the surrounding history of that location? And what is already known about this place?

The location immediately reveals a great deal, as shown in the overview map below.


The picture is located by the tiny white rectangle within the section of the Medusae Fossae Formation south of both the Athabasca Valles flood lava field, thought to be the youngest on Mars, and the giant shield volcano Elysium Mons. The Medusae Fossae Formation is one of the largest but not immediately obvious features on the Martian surface, a vast deposit of volcanic ash as large as either India or Kazakhstan. In fact, it is believed that a large bulk of the dust and sand we see across the entire surface of Mars comes from this giant ash deposit.

Thus, what we are looking at here is a thick ash deposit that is now being slowly eroded by the Martian winds. (See this May 5, 2020 post for a similar feature on the edge of Medusae more to the east.) In the process those winds are distributing that eroded sand across the face of Mars, producing the material for the many dunes found everywhere else.

What about the winds? Can we establish their prevailing direction from this image alone? The photo suggests they are blowing across from the southwest to the northeast, eating away at the edge of the ash deposit. Maybe, but once again having more information makes it possible not to guess.

Eroding ridges just to the west
Click for full image.

The photo to the right, cropped, reduced, and annotated by me, was taken by MRO’s high resolution camera on January 16, 2010 and shows the geology just to the west and north of the first Mars Odyssey image above. In fact, there is a slight overlap. The white arrows point to the same ridge seen in both images.

This second image clearly confirms the orientation of the winds, blowing from the southwest to northeast.

Of course, this raises more questions. Why do the prevailing winds here blow in this direction? How deep is the Medusae Fossae ash deposit? Are we looking at its base, or are there more layers of ash below the low flat areas?

And what volcanic events, from where, deposited this ash here?

Many of these questions have probably been addressed by scientists in other research. Several web searches using a variety of key words — like “Zephyria Planum,” “Mars,” “Martian winds,” “Martian weather,” “Medusae Fossae” — will likely reveal a variety of geology papers addressing these questions, though I can safely bet that not all the questions have been answered, or if some have, many additional questions will become immediately obvious.

I normally would do it, but today I’d rather leave it to my readers. Do some searching. Ask some next questions. And post what you learn in the comments below. We will all be both entertained and enlightened.

Genesis cover

On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.

The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.

The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.

"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News


  • Alex Andrite

    I once worked for a Japanese semiconductor equipment company.
    During my interface with my factory counterparts, it was not uncommon while brainstorming a particular (problem) that our team would ask themselves “why” after each answer, five times. By the fifth answer we were usually at the solution.
    It was a particular “problem” or “symptom” which we were troubleshooting, fairly easily when we could stand around the machine and observe, even when adding other layers to the original problem / symptom.
    Mars ? Well , I await to hear about what is next.

    Thanks for the Cool Image Times Mr.Z. Always my favorite time.

  • wayne

    Alex –
    good stuff!

  • Star Bird

    So what ever became of the face on Mars we was hearing about those years ago?

  • Star Bird: I very graciously answered this question, which you asked previously, in this comment on May 28th.

    If you are not going to read what people write, why are you bothering to ask the question?

Readers: the rules for commenting!


No registration is required. I welcome all opinions, even those that strongly criticize my commentary.


However, name-calling and obscenities will not be tolerated. First time offenders who are new to the site will be warned. Second time offenders or first time offenders who have been here awhile will be suspended for a week. After that, I will ban you. Period.


Note also that first time commenters as well as any comment with more than one link will be placed in moderation for my approval. Be patient, I will get to it.

Leave a Reply

Your email address will not be published. Required fields are marked *