Astronomy - USA (2022-02)

Mare

Erythraeum

Chryse

Aurorae

Sinus

South polar

cap (SPC)

Southern

cloud

Northern

cloud

Mare

Erythraeum

Aurorae

Sinus

Southern

cloud

Northern

cloud

SPC

clouded over

SPC

clouded over

Coalesced

clouds

Dark bar

WWW.ASTRONOMY.COM 15

SECRET SKY

The beauty of keeping

observing records —

whether images, a log-

book, or sketches — is that they

may be important in the future. A

recent investigation of Mars

proves this point.

In 1973, the McDonnell

Douglas Amateur Astronomers

Club of St. Louis, Missouri, orga-

nized an amateur observing

campaign of Mars, which led to a

curious finding. On the morning

of Sept. 2, Jim Melka and Rick

Melvin of St. Louis used an 8-inch

Newtonian ref lector to image

Mars with a 35mm camera. At

7:50 Universal Time (UT), they

recorded a peculiar pair of north-

south-oriented clouds, their

perimeters touching.

Visually, the clouds resembled a figure

eight and appeared as bright as the south

polar cap. Their timing was odd — during

early winter in Mars’ northern hemisphere,

when the planet’s atmosphere is normally

too dry to support discrete clouds. And they

were unlike any other known cloud features

on the Red Planet, such as frosts or fogs.

Melka later determined the clouds were

in the planet’s southwestern quadrant in the

Solis Lacus, the famous Eye of Mars. The

northern cloud was 430 miles (692 kilome-

ters) wide, and the southern one 378 miles

(608 km) wide.

Two other observers also imaged these

clouds: George Fiedler, also of St. Louis, 31 minutes

later; and Lionel Brown of Las Cruces, New Mexico, two

hours and 21 minutes after Fiedler.

Their images showed the clouds changing appear-

ance over a remarkably short period of time. In Fiedler’s

image, the two clouds appeared to be merging, and the

south polar cap had greatly dimmed in the half-hour

since Melka and Melvin’s observation. Brown’s image

also showed a veiled polar cap, with the two clouds now

coalesced into a single circular obscuration that spanned

24° of longitude and covered much of Solis Lacus.

In addition, the initial cloud pairing obscured a dark,

barlike surface feature in Daedalia and Phasis in the

northwest quadrant of the Solis

Lacus region that was once again

visible in Brown’s image. In a

2020 report for the Journal of the

Association of Lunar and

Planetary Observers, Mel k a

writes that as far as he knows,

“such rapid changes in clouds

and obscurations on Mars are

unique to these observations.”

These odd clouds might have

gone down as a historical foot-

note. But recently, Melka came

across maps produced by NASA’s

Mariner 9 probe while it orbited

Mars in 1971 and 1972 — before

the clouds were sighted. Melka

realized he could investigate a

hypothesis: The clouds were

caused by a comet smashing into

the Red Planet.

Melka compared Mariner 9’s

maps to images from the Viking

Orbiter in 1976 — three years

after the clouds were observed.

He found a cluster of six new

craters where he had spied the

northern cloud. These craters

have diameters ranging from

2.6 miles (4.2 km) up to 6.2 miles

(10 km). At that size, they should

have been visible in Mariner 9’s

survey — but they weren’t. Thus, they must

have been fresh when Viking spotted them.

Images from NASA’s Mars Reconnaissance

Orbiter back this up, showing the craters to

be very young and pristine in appearance.

Melka proposes that these craters and the

unusual phenomena that he and his fellow

observers witnessed nearly a half-century

ago were all caused by a comet impact. Ice

the comet deposited in the atmosphere could

explain the formation of the bright clouds.

And shock waves from the impacts could

have lifted a blanket of dust over the entire

southwestern quadrant of Mars, obscuring

all albedo features in that area. If he’s right,

it would be the earliest photographic detection of a

comet impacting a planet — predating the 1994 plunge

into Jupiter of Comet Shoemaker-Levy 9 (D/1993 F2).

Send thoughts and comments to Melka at jtmelka@

yahoo.com, and as always, if you’ve seen anything

unusual, send your reports to [email protected].

Analysis of historical imagery may have
uncovered a cometary impact on Mars.

A martian

comet crash?

Three images of Mars

taken by four different

observers — Jim Melka

and Rick Melvin (top),

George Fiedler

(center), and Lionel

Brown (bottom) —

on Sept. 2, 1973. The

images cover the

course of nearly three

hours. Note that the

pair of clouds in the

Melka/Melvin image

coalesce over time into

a single obscuration.

Note also the dimming

of the south polar cap.

TOP: JIM MELKA, RICK MELVIN;

CENTER: GEORGE FIEDLER; BOTTOM:

LIONEL BROWN; IMAGES COURTESY

OF JIM MELKA

Their images

showed the

clouds

changing

appearance

over a

remarkably

short period

of time.

BY STEPHEN
JAMES O’MEARA
Stephen is a globe-
trotting observer who
is always looking
for the next great
celestial event.

BROWSE THE “SECRET SKY” ARCHIVE AT

http://www.Astronomy.com/OMeara