THE 2018 PERIHELIC APPARITION OF MARS
By : Jeffrey D. Beish (11-20-2013)
 

INTRODUCTION

Mars appears more Earth-like to us than most of the other planets because we can observe its surface, atmospheric clouds and hazes, and its brilliant white polar caps.  The latter are composed of frozen CO2 and underlying water ice, and wax and wane during the Martian year. These aspects, along with the changing seasons and the possibility of life, have made Mars one of the most studied planets in our solar system.

The Red Planet Mars offers both casual and serious observers many challenges and delights, as well as providing astronomers a laboratory to study another planet's atmosphere and surface. Some Martian features even appear to shift position around the surface over extended periods of time.

There are several cooperating international Mars observing programs under way to assist both professional and amateur astronomers. These include the International Mars Patrol (I.M.P.) coordinated by the Mars Section of the Association of Lunar and Planetary Observers (A.L.P.O), the International MarsWatch and the Terrestrial Planets Section of the British Astronomical Association (B.A.A.).  

Information for observing Mars during a typical apparition is presented in a separate report titled, “General Information for Apparitions of Mars.”  Also, you can find gobs of information at this site.

With the advent of modern CCD camera technology the amateur can produce useful images of Mars when it is as small as 3.5 arcsec . Early in an apparition, Mars rises in the east or morning sky and sets with the rotation of the Earth in the western or evening sky.  During the past few apparitions (2001-2016), observers began to take CCD images when Mars was only 30 degrees away from the Sun.  Since Mars was only a visual magnitude of ~1.8 then the planet would have been difficult to locate bright twilight hours.

In the pre-apparition reports the observer will find the motion of Mars in our sky, the characteristics for that particular apparition, information pertaining to the polar cap(s) and any special events that may be seen during that particular apparition.  As usual a calendar of events will be included with each report that contains cardinal dates for seasonal activity and orbital information of Mars.

MOTION OF MARS IN OUR SKY

As a general rule, an "apparition" begins when a planet emerges from the glare of the Sun shortly after conjunction. Mars will be in conjunction with the Sun on July 26, 2017 (38.9° Ls); however, it will not be safe to observe Mars until after September 02, 2017 when it is at least 12 degrees away from the glare of the Sun.

The apparent declination of Mars begins at 13.3° in early September 2017 in the constellation Leo and during mid-October will descend south into the constellations Virgo. Two weeks later Mars will be south of the celestial equator throughout the apparition until January 2019. This is not good news for those observing in the Northern Hemispheres because Mars will be seen fairly low in their sky during the entire apparition. Good news for southern hemisphere observers however. The declination of Mars will continue to descend south of the celestial equator reaching -23.6° by March 26, 2018 and will remain below -20° until October 16, 2018.  After that Mars will cross from the constellation Caprcornus into Aquarius in early-November and ascend northward for the remainder of 2018 to cross north of the celestial equator in January 2019.

By March 24, 2018, a ‘0.4’ visual magnitude Mars will be seen rising early in the morning sky in the constellation Sagittarius and it will be at western quadrature with the phase defect or terminator of 40.2°.  NOTE: The Solar Elongation for Mars is the angle between the lines of sight from Earth to the Sun and from Earth to Mars.  When these lines of sight form a right triangle then Mars is at quadrature (eastern or western). For detailed definitions and graphics for the motion of Mars in our sky see these excellent web sites:  Planetary Aspects and Elongations and Configurations.

Figure 1. A heliographic chart of the orbits of Mars and the Earth showing the relative positions of both planets.  Quadrature is when Mars is directly east or west of Earth as shown.


The 2018 apparition of Mars begins retrogression, or retrograde motion against the background stars eleven months after on June 28, 2018 (201.1° Ls) and continues through August 27, 2018 (238.2° Ls).  Each night for this brief period of time; before, during and after opposition the Red Planet will appear to move backwards toward the western sky in the Caprcornus .

Since the Martian year is about 687 Earth days long -- nearly twice as long as ours, the Martian seasons are similarly extended. While the Earth's seasons are nearly equal in duration, the Martian seasons can vary by as much as 52 days from each other due to that planet's greater orbital eccentricity (see Figure 2).

Figure 2. A heliographic chart of the orbits of Mars and the Earth showing the relative seasons of both planets in the planetocentric longitude system Ls. Graphic Ephemeris for the 2018 Perihelic Apparition of Mars. Original graph prepared by C.F. Capen and modified by J.D. Beish.


2018 APPARITION CHARACTERISTICS

Another general rule for predicting oppositions of Mars is from the following: the planet has an approximate 15.8-year periodic opposition cycle, which consists of three or four Aphelic oppositions and three consecutive Perihelic oppositions. Perihelic oppositions are also called "favorable" because the Earth and Mars come closest to each other on those occasions. We sometimes refer to this as the seven Martian synodic periods. This cycle is repeated every 79 years (± 4 to 5 days) and, if one were to live long enough, one would see this cycle nearly replicated in approximately 284 years. The 2018 Mars apparition is considered Perihelic because the orbital longitude at opposition will be 31° from the longitude of perihelion (250° Ls).

NOTE: Ls is the planetocentric longitude of the Sun along the ecliptic of Mars' sky. 0° Ls is defined as that point where the Sun crosses the Martian celestial equator from south to north, that is the planet's northern hemisphere vernal equinox. The other Ls values that define the beginnings of Martian northern hemisphere seasons are: summer, 90° Ls; autumn, 180° Ls; and winter, 270° Ls. For Mars' southern hemisphere these values represent the opposite seasons. Distance (A.U.) - Distance from Earth to Mars in astronomical units, where one (1) A.U. equals 92,955,807.267 miles or 149,597,870.691 km.

Opposition occurs 12 months after conjunction when Mars is on the opposite side of the Earth from the Sun. At that time, the two planets will lie nearly in a straight line with respect to the Sun, or five weeks after retrogression begins. Opposition will occur at 0507 UT on July 27, 2018 (218.9° Ls) with an apparent planetary disk diameter of 24.3 arcsec. Mars will remain visible for more than 12 months after opposition and then become lost in the glare of the Sun around July 27, 2019 as it approaches the next conjunction (September 02, 2019). The cycle is complete in 780 Earth days. .

Closest approach occurs at 0751 UT on July 31, 2018 (221.4° Ls) with an apparent planetary disk diameter of 24.3'' at a distance of 0.384966506 astronomical units (AU) or 35,784,872 mi (57,590,170-km).  During closest approach in 2018 the apparent diameter of Mars will be 5.7 arcsec larger than it was at the same period in 2016 and it will be 4.4 degrees higher in the sky – not quite as good for observing the Red Planet for observers in the northern hemisphere.  It should also be noted that closest approach between Earth and Mars is not necessarily coincident with the time of opposition but varies by as much as two weeks.

Figure 3. A simulated view of the appearance of Mars during opposition at 0507 UT on July 27, 2018 (218.9° Ls, CM 143.9°)


The observable disk diameter of Mars will be greater than 6 arcsec from February 12, 2018 [-21.4° d ] (128.3° Ls) and will not fall below this value until February 06, 2019 (336.4° Ls), lasting 11.9 months or 208 degrees Ls.  Imaging by CCD devices may begin with a disk diameter of 4.3 arcsec or more, commencing on or about December 05, 2017.

The Sub-Earth (De) and Sub-Solar (Ds) points are graphically represented in Figures 4 and 5. The 2017-2019 Ephemeris of Mars is tabulated on Internet in this web site. A glossary of Terms appears at the end of this table.

Figure 4. As it approaches Earth, it will swell from a small apparent disk of 6" in February 12, 2018 to a maximum diameter on July 31, 2018, and then shrink as it moves away.  Closest approach occurs on July 31, 2018 (Opposition July 27, 2018).  Images shown at 0h UT.


Figure 5. Graphic Ephemeris of Mars during the 2018 apparition from February 13, 2018 through February 05, 2019.     Opposition and 6 arcsec apparent diameter range arc indicated.  Plot illustrates the Declination (black line),   the latitude of the Sub-Earth point (De) or the apparent tilt ( brown line ) in areocentric degrees, and the latitude of the Sub-Solar point ( green line ) in areocentric degrees.  The areocentric longitude (Ls) of the Sun, shown along the bottom edge of the graph defines the Martian seasonal date.  The value of Ls is 0° at the vernal equinox of the northern hemisphere, 70° when Mars is at aphelion, and 90° at the summer solstice of the northern hemisphere 250° when Mars is at perihelion, and 180° is northern autumn.


Figure 6. Graphic Ephemeris of Mars from February 13, 2018 through February 05, 2019.  Plot illustrates the apparent diameter of Mars in seconds of arc.  The areocentric longitude (Ls) of the Sun, shown along the bottom edge of the graph defines the Martian seasonal date.


THE SOUTH POLAR REGION

Astronomers will have a view of the Martian South Polar Region (SPR) will be tilted toward the Earth and will remain so throughout the apparition when it reaches 0° De on March 04, 2018 and should be free of its hood. For more detailed information on the the south polar cap click to this web site.

DUST STORMS

Observations of Mars indicate that major dust storms tend to be more frequent when Mars is closest to the Sun – during southern hemisphere spring and summer. While predicting these events is nearly impossible to make our studies show that the Martian dusty season should begin about the first week in April (241° Ls) throughout the middle of October 2018 (270° Ls). The highest probability of dust storms occurring will be on or about September 23, 2018 (255° Ls) and a sensitive area for the development of dust storms is in northwest Hellas.

Massive, planet-encircling storms usually occur in southern hemisphere summer and that will come by the end of December (315° Ls).  Observers should be alert for dust clouds in the northeast Hellas Basin, the Serpentis-Noachis region, and the Solis Lacus region. If the Red Planet behaves as it did in 2001 and 2003, then 2007 may be a time when Mars may be very dusty indeed!  For more detailed information on Martian dust storms on this web site.

SPECIAL EVENTS

For those interested in catching a glimpse of possible "flares" from the surface of Mars there will be a period when it will be possible to be seen when the De and Ds are coincident will be on or about July 14, 2018. The last time this event was recorded was during June 07, 2001 when the De = 1.8° and Ds = 2.5° and Mars was situated favorably in the sky for observers to record this rare event. The De and Ds will be very close together in this apparition; however, they will be slightly larger than in 2001 when the "flares" were seen. Detailed report of the 2001 event (Dobbins, Thomas A. and Sheehan, William (2001), "The Martian-Flares Mystery," Sky and Telescope, Vol. 101, No. 5, May, pp115-123) and "Solving the Martian Flares Mystery," Dobbins and Sheehan, ALPO Web Site.  See table below for dates when De - Ds +/- 1.0°:


First Period: EST = UT - 4 || Di = 22.8" – 23.4 "

Date

Rise

Edom Transit (UT)

De

Ds

De - Ds

i

July 12

0143

0950

-12.9

-12.3

-0.6

13.3

July 13

0138

1026

-12.8

-12.5

-0.3

12.5

July 14

0134

1102

-12.7

-12.7

0.0

11.8

July 15

0129

1140

-12.6

-13.0

0.4

11.0

July 16

0125

1216

-12.4

-13.2

0.8

10.2

NOTE: Di = apparent diameter of Mars
                De = Declination of Earth from Mars
              Ds = Declination of Sun from Mars
De - Ds = coincident
        i = phase defect in degrees



 
 

Table 2. CALENDAR OF EVENTS -- MARS, 2017 - 2019

DATE

PHYSICAL

REMARKS

2017 Jul 27

Ls 38.9°

Conjunction. Mars is behind the Sun ~2.655 AU.

2017 Oct 06

Ls 70° 
De 25.5° 
Ds 23.6° 
RA 11:20
Dec 5.5° 
A. Dia 3.7’’

Mars at Aphelion. Is NPC fairly static or entering rapid retreat phase. Watch for "Aphelic Chill" in NPR (usually between 60° and 70° Ls). NPC Rima Tenuis may appear.  Antarctic hazes, hood. South polar regions becoming difficult to observe. Any signs  NPC width ~31° ±3°

2017 Nov 20

Ls 90° 
De 23.0° 
Ds 25.2° 
RA 13:07 
Dec -6.0° 
A. Dia 4.1’’

Solstice - Northern Summer/Southern Winter. Orographic clouds over the Tharsis volcanoes – W-Cloud? Local seasonal clouds should wrap around Syrtis Major and be prominent in Lybia. Hellas white cloud and Ice-fog activity? Discrete clouds? NPC remnant? Lemuria (210° W, 82° N) detached from NPC? Any other detachments (projections at 135° W and 290° W) near NPC remnant, NPC Width ~18° ±4°.

2018 Feb 12

Ls 128.3° 
De  5.4° 
Ds  19.5° 
RA 16:34 
Dec -21.4° 
A.Dia   6’’

Apparition begins for observers using 4-inch to 8-inch apertures telescopes and up. Begin low-resolution CCD imaging. Views of surface details not well defined. Novus Mons reduced to a few bright patches and soon disappears. Is Mare Acidalium broad and dark? Bright spots in Tempe-Arcadia-Tharsis-Amazoins? "Domino effect" appears around 120° - 125° Ls. Topographic clouds increase. (NPC Width ~14° ±3°). 

2018 Mar 25

Ls 148.6° 
De  -4.8° 
Ds  12.8° 
RA 18:18 
Dec -23.5° 
A.Dia   8’’

Mars at quadrature. Are both polar hoods visible? SPH present and edge of NPH visible.  Hellas frost covered?  Are W-clouds present? Is Morpheos Lacus (228°W, 37°N) prominent? Are topographic clouds prominent in Libya, Aeria , Moab, Edom , and Candor?

2018 Apr 20

Ls 162.1° 
De -10.1° 
Ds 7.5° 
RA 19:18 
Dec -23.1° 
A. Dia  10’’

Is Mare Acidalium broad and dark? Bright spots in Tempe-Arcadia-Tharsis-Amazoins? "Domino effect" appears around 120° - 125° Ls. Topographic clouds increase. 

2018 May 09

Ls 172.4° 
De -13.0° 
Ds   3.2° 
RA 19:57 
Dec -22.4° 
A.Dia 12’’

Is the North Polar Hood present? Does SPH or frost cover Hellas? Hellas should begin to clear and darken. Are W-clouds present? South cap emerges from darkness of Winter. SPH thinning and forms "Life Saver Effect"? 

2018 May 23

Ls  180° 
De -14.4° 
Ds  -0.1° 
RA  20:21
Dec -21.9° 
A.Dia 13.9’’

Equinox - Northern Autumn/Southern Spring . South Polar Cap (SPC) maximum width.   Is the North Polar Hood present? Does SPH or frost cover Hellas? Hellas should begin to clear and darken. Are W-clouds present? South cap emerges from darkness of Winter. SPH thinning and forms "Life Saver Effect"? 

2018 Jun 26

Ls 200° 
De -14.4° 
Ds  -8.3° 
RA 20:51 
Dec -22.4° 
A.Dia 20.5’’

Retrogression Begins. Mars begins retrogression, or retrograde motion against the background stars 11 months after conjunction, when it appears to move backwards toward the west for a brief period before, during and after opposition.  SPC shrinking. Syrtis Major darkens and continues to shrink. W-clouds possible. Surface details increasing in contrast Hellas the features Zea Locus and Alpheus dark? SPC Novissima Thyle (300°-330°W) projection present?  (SPC width ~52° ±6°).

2018 Jul 27

Ls 218.7° 
De -11.1° 
Ds -15.2° 
RA 20:32 
Dec -25.5° 
A.Dia 24.3’’

Mars at Opposition. Bright SPC projection Novissima Thyle  (300°W - 330°W) Areographic longitude. Dark rift Rima Augusta connected from 60° to 270° longitude. Rima Australis visible in SPC (290°-350°W)? W-clouds possible. SPC bright projection Argenteus Mons (10°W-20°W). SPC Dust clouds in Serpentis-Hellespontus, in Hellas or Noachis?   (SPC width ~44° ±3°).

2018 Jul 31

Ls 221.2° 
De -10.7° 
Ds -16.3° 
RA 20:28 
Dec -25.5° 
A.Dia 24.3’’

Mars at Closest Approach. Bright SPC projection Novissima Thyle  (300°W - 330°W) Areographic longitude. Dark rift Rima Augusta connected from 60° to 270° longitude. Rima Australis visible in SPC (290°-350°W)? W-clouds possible. SPC bright projection Argenteus Mons (10°W-20°W). SPC Dust clouds in Serpentis-Hellespontus, in Hellas or Noachis?   (SPC width ~44° ±3°). 

2018 Aug 27 

Ls 238.2° 
De -9.6° 
Ds  -21.2° 
RA 20:08 
Dec -26.2° 
A.Dia 21.7’’

Retrogression Ends. Mars begins westward motion against the background stars. SPC rapid retreat. Novus Mons small, bright, and high-contrast. Rima Australis widens. SPC isolated bright spot at 155° longitude? Any white patches near -20° latitude may brighten. Atmosphere of Mars very clear during Ls 240°- 250°. Occasional morning limb hazes. Dust clouds? Note: Several "planet-encircling dust storms have been reported during this season at 24° Ls. (SPC width ~28° ±3°).

2018 Sep 15

Ls 250° 
De -11.5° 
Ds  -23.6° 
RA  20:16
Dec -24.7° 
A.Dia 18.4’’

Mars at Perihelion . SPC in rapid retreat. Novus Mons smaller. Dust clouds expected over Serpentis-Hellaspontus (Ls 250° - 270°). Syrtis Major beginning to narrow. Frost in bright deserts? Orographic clouds (W-clouds) possible. Elysium and Arsia Mons bright?    Note: Several "planet-encircling dust storms have been reported during this season.  High probability 255° Ls. (SPC width ~ 24° ±3°).

2018 Oct 16

Ls 270° 
De -17.1° 
Ds  -25.2° 
RA 21:04
Dec -20.0° 
A.Dia 13.6’’

Solstice - Northern Winter/Southern Summer. W-clouds present? NPH extends 50° N? Decreased number of White clouds. "Syrtis Blue Cloud"? White areas in deserts? Dust clouds in south until 270° Ls?  Watch for planetary system clouds bands. Orographic cloud over Arsia Mons? Syrtis Major is narrow.  (SPC width ~ 17° ±2°).

2018 Oct 31

Ls 279.2° 
De -20.0° 
Ds -24.9° 
RA  21:35
Dec -16.8° 
A.Dia 12’

NPH extends 50° N? White clouds rare. Dust storm? Frost patches? Dust storms visible at 285° Ls (Martin & Zurek). 
(SPC width ~ 12° ±2°).

2018 Nov 21

Ls 292.1° 
De -23.5° 
Ds -23.2° 
RA 22:22
Dec -11.7° 
A.Dia 10’’

Bright SPC projection Novissima Thyle  (300°W - 330°W) Areographic longitude. Dark rift Rima Augusta connected from 60° to 270° longitude. Rima Australis visible in SPC (290°-350°W)? W-clouds possible. SPC bright projection Argenteus Mons (10°W-20°W). SPC Dust clouds in Serpentis-Hellespontus, in Hellas or Noachis?   (SPC width ~10° ±2°).

2018 Dec 21

Ls 310.0° 
De -26.2° 
Ds -19.0°
RA 23:33
Dec -3.5° 
A.Dia 8’’

Edom bright? Is SPC remnant visible in mid-summer? High probability of dusty storm at 315° Ls. Orographic cloud over Arsia Mons? Topographic cloud over Libya ? (SPC width ~10° ±2°).

2019 Feb 06

Ls 336.4° 
De -23.0° 
Ds  -9.8° 
RA 01:29
Dec  9.8° 
A.Dia 6’’

Hellas Ice-fog activity? Topographic cloud over Libya ? Topographic cloud over Edom ?  NPC large hood present. W-Cloud? Orographic cloud over Arsia Mons? (SPC width ~10° ±2°).

2019 Mar 23

Ls 0° 
De -13.2° 
Ds  -0.1° 
RA 03:27
Dec  19.7° 
A.Dia 4.8’’’

Equinox - Northern Spring/Southern Autumn . North Polar Hood (NPH) breaking up, North Polar Cap (NPC) should be exposed. ("Areo-" is a prefix often employed when referring to Mars or "Ares.") 

2019 Sep 02

Ls 77.3°

Conjunction. Mars is behind the Sun ~2.67AU.