Impact cratering on Mars… Now

B. A. IvanovInstitute for Dynamics of Geospheres, RAS, 119334, Moscow, Russia, [email protected]

CTX_P05_003145_2050_XI_25N098W HiRISE PSP_6758_2050

October 2010 1MS3-1-3

No No Yes!

HiRISE resolution from 25 to 30 cm/pixel allows us recognize small craters with D≥ 1.5 m

Main attention is devoted to young small craters with the age of 10 years and younger.

19 of 20 young craters from Malin’s collection (Malin M.C. et al. ,2006, Science, 314, 1573-1577) are confirmed

Now the list of dated (“new”) small craters has more than 140 candidates

Data processing is finished for ~140 cases About 50% of impacts create clusters of craters


Martian atmosphere

Atmospheric density at 15 km altitude is the same as at 40 km above the Earth surface. Projectiles making bolides on Earth, are able to record themselves making craters on Mars


Welcome the Town of Young Craters!

Location: dusty areas on Mars (where easy to find) Population: from 1 to 3000 individual craters

in a cluster (clustered and single crater cases are ~50/50)

Altitude: from -6km to +18 km (in respect to the smoothed areoid surface)

Maximum size to date: D~35 m (ND>31.25 =3)


Location of new craters


Almost all new impact sites found over dusty areas


Largest new crater (54m)


Smallest new crater (~2.5 m) formed between April 2006 and February 2008 (ESP_016660_1860)

Not all craters are as simple …

Atmospheric breakup

0.5 vT

0.5 vT

Circle: 2 r = vT *tflight

Ellipse: 2a = r/sin , 2b=r

Z

Z/sin

PPT=Plane Perpendicular to Trajectory

Passey&Melosh, 1980, Icarus 42 (2), 211-233

Zvv

r T

sin2

v

PSP_09185_1770Ellipse 137x37 mApparent angle ~15o


Cluster at lowermost altitude (Hellas): h=-6.36 km (PSP_007596_1295)

<r>, m75.59

<r_w>,m79.81

Dmax ~ 5.2 m

m~3000 kgm-3

Z~ 3 km (+6 km below zero)10 ~11 bar

Minimum altitude: size 7 m, population N>49


New Impact Site Formed between November 2007 and February 2009 (ESP_013707_1915)

<r> =42 m, Dmax ~ 3.2 mMaximum altitude of 18 km: size 5 m, population N>23


Single fragmentation Shock wave interaction gives small transversal

velocity vT to fragments

CT ~ 0.9 (derived from the numerical modeling) corresponds to separation of craters on surface of the order of 30 to 50 m on Mars

Atmospheric density at the fragmentation point defines stagnation pressure at the breakup, proportional to the effective strength

a v2

The final separation radius in PPT is a function of Z with a maximum at ~22 km on Mars (at v=10 km/s ~ 2 bar – similar to Earth bolides)

0.5 vT0.5 vT

ArtemievaA&Shuvalov, JGR 106, 3297-3310, 2001

)/( 21 LLfvCvm

aiTT

m

aiT vv

45.05.0


Three levels of modeling

1. Geometrical modeling: assumes one craters-one fragment, takes care only about location of craters with a weak role of the fragment (crater) size. Gives quick estimates of break up parameters

2. Analytical solution to fragment deceleration with a constant ablation coefficients (recognizes ice, CC, OC, and iron meteoroids)

3. Direct 3D modeling of each fragment atmospheric passage with the ablation

##2 and 3 needs to assume a scaling law for crater sizes. Currently we use porous (dry sand) scaling with a possibility to have strength-to-gravity transition for D=1 to 10 m. Loose dust? How thick? What is the substrate?

Projectile sizes for our crater collection is below ~2 m in diameter


Cascade fragmentation

October 2010 1MS3-1-3ESP_016200_1740 altitude of 5.4 km

The first fragmentation event happened at stagnation pressure of 0.32 MPa (3.2 bar), while further fragmentation events occur at 0.42 to 0.49 MPa (4.2 to 4.9 bar).

1 10 100D ,m

0.1

1

10

100

1000

N bi

n

Type 3: largest D ~ 25 to 40 mN (1.5 m ) >200

D _eff=39 m , N _cascade=13


D eff=61 m , N _cascade=9


H iR ISE crater counts in c lusters:

PSP_003172 ("new ")

PSP_005942 ("new ")

PSP_3812 ("o ld")

PSP_2959, #4 ("o ld")

Cascade model: Each fragment is divided in two subfragments with mass mn= (1-r) m1

n+1 + r m2n+1 ; r is the random number from 0.5 to 1.

After Ncascade fragmentation events the shower includes 2Ncascade fragments. Largest counted clusters have Ncascade from 9 to 13.

Cascade model: Size-frequency distribution (SFD) as increment number of craters in diameter bins with diameter step 20.5

Black – modelColor - counted


The most populated claster: >3000 individual craters

PSP_003172_1970; formed between 13 December 2004 and 6 May 2006; Deq=36 m, Dmax=17 m

Assumed angle of incidence ~30o

Along trajectory Across trajectory

Number of craters D>2 per 20-m strips

2

2

*

*2 x

x

x exxxN

N

Cascade model

feT nvv ~*

Multiple fragmentation enhances crater dispersion in a clusterOctober 2010 1MS3-1-3

Other examples: PSP_003259, inclination ~45o

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400

N

Model

0

5

10

15

20

25

0 20 40 60 80 100 120

N

Model

PSP_005375, inclination ~40o


Tentative classification by meteoroid’s densityEntry velocity and strength can be estimated as a combination:(0 – is near-surface atmospheric density)

20v

Normalized to a single 2-fragment separation event cluster width (zero for single craters v.s. target surface altitude. Projectiles with the same density and should follow a straight line started at the disruption altitude (zero separation)

Low density (and low ablation!!!) or high cT projectiles???


Searching for the impact rate


Cratering rate seems well established for D>15 m as about 3 recognized impact per year

Dating of young areas – Opportunity area


Golombek, M., K. Robinson, A. McEwen, N. Bridges, B. Ivanov, L. L. Tornabene, and R. Sullivan (2010),Constraints on ripple migration at Meridiani Planum from Opportunity and HiRISE observations of fresh craters,J. Geophys. Res., doi:10.1029/2010JE003628, in press. (accepted 6 August 2010)

HiRISE gives orbital view Opportunity gives ground view

Dating of young surfaces


Hartmann’s isochrones HiRISE’s isochrones

Size frequency distribution

Observations fit Hartmann’s model in the first approximation (what is amazing!)

R

a) Estimates for an “effective” single craters

b) Integrated model – assumes our collection represent an average cratering style

Well cratered surface have a “flat” SFD in R-plot due to overlapped clusters.


Conclusions In contrast to early expectations tiny atmosphere of Mars

disrupts ~50% of small projectiles Some clusters have large dispersion at the surface. Low

density (comet)? Higher efficiency of separation (e.g. multiple, not binary fragmentation)?

Complex structure of “apparently fragile” projectiles – weakly bounded aggregates of stronger pebbles? Can they survive long enough in space?

Crater age estimates for D<30 to 50 m should take into account the clustering effect (one entry – hundreds of small craters)


Curving albedo patterns due to interaction of atmospheric shock waves generated by near-simultaneous impacts


Impact cratering on Mars… Now

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